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Carbon-Based Nanomaterials: a Paradigm Shift in Biofuel Synthesis and Processing for a Sustainable Energy Future

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Energy Conversion and Management X 22

DOI:10.1016/j.ecmx.2024.100590

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Nazia Rodoshi Khan

Islamic University of Technology

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Carbon-Based Nanomaterials: a Paradigm Shift in Biofuel Synthesis and

Processing for a Sustainable Energy Future

Nazia Rodoshi Khan

, Adib Bin Rashid

Department of Industrial and Production Engineering, Military Institute of Science and Technology (MIST), Dhaka, Bangladesh

ARTICLE INFO

Keywords:

Carbon-based nanomaterials

Biofuels production

Renewable energy

Carbon nanotubes

Graphene

Carbon nanobers

ABSTRACT

The increased demand for energy has driven researchers to concentrate on generating secure energy sources for

humanity. One advantage of inventing a biological and chemical way to generate green and efcient electrical

energy is that it eliminates dependency on soon-to-extinct fossil fuels. Biofuels are one of many emerging sectors

that are receiving major attention, all of which are alternative fuels. Carbon-based nanomaterials, due to their

larger surface area, affordability, and ability to withstand high temperatures, have been engineered to serve as

efcient electrocatalysts without the need for metals. These nanomaterials can also provide a stable structure for

metal-based electrocatalysts, enabling the conversion of biofuels into environmentally friendly energy sources.

Thus, biofuel synthesis is “greener” using its catalyst. Graphite, carbon nanoparticles, nanotubes, nanohorns, and

nanorods can be used to construct and build a wide range of biofuels. This review gives insight into how carbon-

based nanomaterials are used across all different stages of biofuel production, along with corresponding issues

and possibilities. Moreover, it emphasizes the utilization of diverse carbon-based nanomaterials and nano-

biocatalysts to enhance biofuel production by increasing its yield. Given its many possible uses, the research

in concern exhibits a seemingly indenite scope.

1. Introduction

The need for energy on a global scale is continuously increasing as a

result of growing populations and rapid industrialization. The majority

of global energy needs are addressed through the utilization of nonre-

newable resources that account for a large fraction of the human-caused

increase in atmospheric CO

levels. The depletion of crude oil supplies

necessitates searching for non-conventional energy sources that are both

environmentally friendly and commercially feasible [1,2]. Biofuels are

an important part of the renewable energy sector since their energy is

generated through biological carbon xation. Therefore, signicant

technological investment is needed to create methods for mass-

producing biofuel. Nanotechnology provides a potentially helpful

novel approach for dealing with the problems that have plagued con-

ventional pre-treatment, hydrolysis, and fermentation methods for a

long time. Using nanoparticles during biofuel production boosts the

fermentation response rate and improves pre-treatment and enzymatic

hydrolysis effectiveness. As of 2016, over 60 countries had initiated

national nanotechnology programs, signaling a robust commitment to

this burgeoning eld. With the backing of both governmental bodies and

the private sector, the trajectory of nanotechnology is poised for

continued expansion. The anticipated global surge in nanotechnology’s

industrial applications underscores its profound and comprehensive

inuence on nearly every economic sector. A testament to its interna-

tional reach is the array of global companies engaged in producing

nanotechnology-enabled products, as detailed in Table 1. This expan-

sion is driven by the diverse applications of nanomaterials across various

industries, including biofuels, where they enhance production efciency

and sustainability [3]. Therefore, global utilization of nanomaterials has

seen signicant growth. In addition, nanoparticles include a high

adsorption limit, durability, stability, and durability, as well as a supe-

rior degree of catalytic activity, durability, and crystallinity. Owing to

their unique properties, nano-biocatalysts have emerged as potential

materials for enhancing biofuel generation. Typically employed as

reactant specialists, chelates contribute greatly to electron transfer, s

inhibitory combinations and promoting the activity of anaerobic groups

[4–6].

Carbon nanomaterials (CNMs), a burgeoning subeld of nanotech-

nology, are making strides in diverse use ranging from drug delivery and

tissue engineering to medical transplantation. Dened by their size,

* Corresponding author.

E-mail address: naziarodoshi@iut-dhaka.edu (N. Rodoshi Khan).

Contents lists available at ScienceDirect

Energy Conversion and Management: X

journal homepage: www.sciencedirect.com/journal/energy-conversion-and-management-x

https://doi.org/10.1016/j.ecmx.2024.100590

Received 15 February 2024; Received in revised form 7 April 2024; Accepted 10 April 2024

Energy Conversion and Management: X 22 (2024) 100590

which ranges between 1 and 100 nm, CNMs are integral in enhancing

various properties such as surface area, refractive index, multi-

functionality, pharmaceutical delivery, durability, and immunogenicity

[7,8]. This category encompasses a variety of forms, including nano-

carbon, graphene oxide, graphene nanosheets, graphene quantum dots,

nanodiamond, and single- and multi-walled carbon nanotubes, as well as

fullerene and carbon foam [9,10].

These CNMs are particularly notable for their potential in biological

sciences, attributed to their distinct features. Their intrinsic properties

are leveraged in biofuel manufacture, where they play a critical function

in improving the synthesis process. The global biofuel sector is experi-

encing a surge in the adoption of CNMs, with carbon nanotubes, nano-

bers, and nanosheets leading the charge due to their stability and

catalytic capabilities. These materials are essential in enzyme immobi-

lization, signicantly enhancing biofuel synthesis and contributing to

the production of biogas, biodiesel, and bioethanol. This trend un-

derscores a growing recognition of the efciency and cost-effectiveness

of nanomaterials, marking a shift towards more innovative and sus-

tainable energy solutions [11–13]. This review discusses the

manufacturing of biofuels and the many process methodologies, feed-

stocks, and obstacles involved. In the present context, biofuel generation

is crucial. The concerned review provides an overview of the processes

involved in making biofuels from a wide range of feedstocks utilizing

various technological approaches utilizing carbon-based nanomaterials.

The use of fossil fuels as energy sources has peaked due to industri-

alization and globalization, posing a signicant threat to the environ-

ment. Presently, there is a shift towards sustainable alternatives such as

biofuels to meet the needs of future generations. This review examines

the application of different carbon-based nanomaterials for eco-friendly

biofuel production. The review extensively covers the use of various

carbon-based nanomaterials in biofuel synthesis, discussing the syn-

thesis, functionalization, and the circular economy of green-synthesized

CNMs in detail. The insights presented in the review aim to assist the

scientic community in adopting studies on highly efcient CNMs to

enhance the biofuel production process.

2. Overview of biofuels production

Biofuels are a category of fuels made from biomass derived from

plant-based materials processed to create a sustainable energy source.

These biofuels come in various forms, such as liquid, solid, and gaseous,

providing a wide range of fuel types for applications like heat, elec-

tricity, chemicals, and other materials. Biofuels are seen as a promising

alternative energy source that shares similarities with petroleum fuels,

particularly in transportability. This characteristic gives biofuels an

advantage over other renewable energy sources, making them a po-

tential driving force for transportation. Due to their plant-based origin,

burning biofuels does not contribute to increased atmospheric carbon

dioxide levels, as the carbon stored in biofuels was initially absorbed

through photosynthesis from the atmosphere [14–16]. Bioethanol and

biobutanol, also known as biogasoline, are widely used biofuels.

Currently, there are numerous opportunities to harness bioenergy. For

over 50 years, the gasoline industry has been the primary energy source.

However, transitioning to a biobased sector could offer a dependable

alternative energy source, leading to widespread support for promoting

biofuels in different industries. [17,18].

The classication of biofuels into different generations is based on

the origin of the biomass used in their production. This categorization

helps understand the various types of biofuels available and their po-

tential applications in the energy sector. Biofuels are classied based on

the source material used for production, with categories including rst-

generation (1G), second-generation (2G), third-generation (3G), and

fourth-generation (4G), as depicted in Fig. 1 [4].

Researchers predict that 10–50 % of the world’s energy consumption

will be derived from biomass by 2050. This estimation underscores the

critical role of biomass as a primary sustainable energy source globally.

The direct burning of biomass for energy supply has been associated

with numerous conicts and problems. This practice has played a sig-

nicant role in the emission of greenhouse gases (GHGs) and the sub-

sequent impact on climate change. However, by utilizing modern

biofuel production technologies, energy recovery from biomass holds

immense potential in addressing global energy security concerns and

balancing trade decits, as depicted in Fig. 2 (data taken from the En-

ergy Institute (EI) Statistical Review of World Energy, 2023 [19]). Bio-

fuels, generated through various technologies, offer signicant

potential. While 1st-generation biofuels have limitations, 2nd, 3rd and

4th generation biofuels show promise. In 2018, global biofuel produc-

tion reached 154 billion liters, with an expected 25 % expansion by

2024. The bioenergy sector has also created approximately 2.8 million

jobs, emphasizing its role in employment opportunities. Biofuels

continue to evolve, playing a crucial role in meeting energy demands

while promoting sustainability [20].

2.1. Biodiesel

Bio-diesel is an alternative fuel from long-chain fatty acids in mono-

alkyl esters originating from vegetable oil and animal fat. The produc-

tion of biodiesel involves trans-esterication, a chemical reaction where

oils or fats are converted into fatty acid methyl esters (FAME),

commonly known as biodiesel. The process requires an alcohol (usually

methanol) and a catalyst (such as sodium hydroxide). The by-product of

this reaction is glycerol, which can be used in other industries, adding to

the sustainability of the process [21]. Therefore, it is crucial to track

down natural sources of inexpensive feedstock. The feedstock for bio-

diesel can be diverse. Edible oils like soybean or rapeseed oil are com-

mon, but there’s a growing interest in non-edible oils from plants like

Jatropha. Waste vegetable oil and animal fats are also used to improve

economic viability and reduce waste. The choice of feedstock affects the

quality and nancial feasibility of the biodiesel, as well as its environ-

mental impact [21]. Vegetable oils are trans-esteried with methanol (or

ethanol) to produce it. Biodiesel is a sustainable energy resource since it

is made from a renewable, domestic resource, which reduces the de-

mand for petroleum fuel. It also has the added benets of being biode-

gradable and harmless [22,23]. Additionally, biodiesel offers a more

appealing combustion emission prole than petroleum-based diesel,

including minimal carbon monoxide emissions, particle matter, and

unprocessed hydrocarbons. Unlike petroleum diesel, a greater ash

point (150 ◦C) is found in biodiesel, making it more stable and more

secure to handle or carry. In light of the presence of free fatty acids, it

can also act as a lubricant and help keep engines running smoothly for

Table 1

Some global nanotechnology companies [3].

Country Company Operations

UK Applied Graphene

Materials

Develops and applies graphene

nanoplatelet dispersions

UK Advanced Material

Development

Develops 2D nanotechnologies and

metamaterial systems

USA 3 M Manufactures numerous nanomaterials

Canada CelluForce Produces a form of cellulose nanocrystals

(CelluForce NCC™)

Luxembourg OCSiAl Luxembourg Produces graphene nanotubes

Japan Zeon Corporation Manufactures single-walled carbon

nanotube

USA Cerion Manufactures metal, metal oxide, and

ceramic nanomaterials

Portugal INNOVNANO Manufactures ultra-ne nanostructured

ceramic powders

Germany RAS AG Produces and distributes of

nanomaterials

France Superbranche Develops functionalized metallic oxide

nanoparticles

Spain Nanogap Manufactures novel nanomaterials from

atomic quantum clusters

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

longer. Numerous cutting-edge methods have been recognized in the

study of bioenergy that have the potential to raise biodiesel output

signicantly [24]. Economically viable and readily available feedstocks

and new fabrication and purication processes are the primary research

areas presently [25,26]. Advancements in technology play a vital role in

enhancing the efciency and cost-effectiveness of biodiesel production.

These advancements include catalyst improvement, novel feedstock

processing methods, and overall production process optimization.

Additionally, research efforts are directed towards scaling up production

to meet commercial demands while ensuring minimal impact on food

supply and land use. Biodiesel offers a substantial advantage over

traditional fossil fuels by signicantly reducing carbon emissions,

thereby contributing to the mitigation of greenhouse gases. Further-

more, biodiesel is biodegradable and non-toxic, reducing the risk of

environmental contamination. Its utilization also leads to a reduction in

harmful particulate emissions, which pose a threat to human health.

Continuous research and technological progress are crucial in address-

ing challenges related to production costs, scalability, and sustainability

[21].

2.2. Bioethanol

Bioethanol, also referred to as bio-alcohol, undergoes a series of four

primary stages during its production: pretreatment, hydrolysis,

fermentation, and distillation. The production of bioethanol involves the

fermentation of carbohydrates from biomass. This process typically in-

cludes the pretreatment of biomass to release fermentable sugars, fol-

lowed by the fermentation stage, where microorganisms, such as yeast,

convert these sugars into ethanol [21]. Pretreatment aims to decompose

the complex structure of lignocellulosic biomass and increase its

accessibility for enzymatic hydrolysis. Pretreatment methods include

physical, chemical, biological, and combined techniques. Following

that, hydrolysis converts the polysaccharides (cellulose and hemicellu-

lose) in the biomass into simple sugars (glucose and xylose) using en-

zymes or acids. The general equation for the fermentation process is

represented as (C

→ 2C

OH +2CO

), indicating the produc-

tion of two molecules of CO

and ethanol for each molecule of glucose

fermented. The industrial fermentation process usually stops when

ethanol concentration reaches about 9–10 %, and the yeasts can be

Fig. 1. The categorization of biofuel feedstocks.

Fig. 2. Production and consumption of biofuels globally (2012–2022).

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

reused in subsequent cycles [21]. Lastly, distillation separates the

ethanol from the fermentation broth and puries it to the desired con-

centration. Distillation is an energy-intensive process that constitutes a

considerable proportion of the production expense. The ethanol yield

from glucose is between 88–95 %, with byproducts like glycerin (3–5 %)

and acetic acid. The fermented mash is distilled to increase ethanol

concentration to 94 %, making it marketable. The resultant bioethanol

can be puried through dehydration [21]. Bioethanol production is a

complex and challenging process that requires optimization of various

factors, such as feedstock selection, pretreatment method, enzyme

loading, fermentation conditions, and distillation efciency [27,28].

Bioethanol distinguishes itself from other biofuels due to its rapid

combustion rate, lower boiling point, and increased heat vaporization.

Its superiority over traditional fuels is evident, offering a multitude of

advantages. Bioethanol, in both hydrated and dehydrated forms, is used

as fuel either in pure form or blended with gasoline. It offers signicant

economic and environmental benets, such as a higher-octane number

and improved combustion efciency. Additionally, the use of ethanol in

fuels generally leads to reduced emissions of carbon monoxide (CO),

volatile organic compounds, and sulfur oxides compared to typical fossil

fuels [21]. The advantages of bioethanol over petroleum-based fuels

stem from its higher compression ratio, shorter burn time, and smaller

engine size. Bioethanol, an oxygenated fuel, contains 35 % oxygen,

reducing the emissions of nitrogen oxides (NOx) and sulfur oxides (SOx)

during combustion. Additionally, the improved ignition of bioethanol

results in a cleaner exhaust in terms of hydrocarbons and particulate

matter [4].

2.3. Biogas

Biogas, a type of renewable energy, is created through anaerobic

digestion, a process where microorganisms convert organic matter into

methane and carbon dioxide. This process occurs in different environ-

ments like marshes, landlls, and the digestive tracts of ruminants [29].

The main components of biogas are methane and carbon dioxide,

accompanied by small amounts of other gases. It can be produced from

various biomass, including carbohydrates, proteins, and fats. Biogas

production involves the decomposition of organic matter, like agricul-

tural waste, sewage sludge, and animal waste, by specic microorgan-

isms in the absence of oxygen. These microorganisms break down the

organic matter and transform it into biogas. Critical factors in biogas

production are the composition and potential of the organic matter, the

absence of harmful pathogens, and the quality of biogas for its intended

uses. The leftover residues from fermentation should also be suitable as

fertilizer for agricultural purposes. The technology for biogas production

is well-established and allows for recycling minerals and nutrients into

the soil, promoting sustainable development. Biogas can be used directly

for energy or processed in combustion engines or cars [21]. Its appli-

cations include electricity generation, heating, and cooking fuel. One of

the signicant advantages of biogas is its ability to reduce greenhouse

gas emissions since it captures and utilizes methane that would other-

wise be released into the atmosphere. Furthermore, the byproduct of

biogas production, called digestate, is a natural fertilizer that is nutrient-

rich and can enhance soil fertility. As the world strives for sustainable

and cleaner energy alternatives, biogas is vital in the transition towards

a more carbon–neutral future [30].

2.4. Bio-Hydrogen

Biohydrogens are gaining increasing attention as potential fuels for

the future due to their unique attributes in clean energy generation,

waste management, and high energy content. Biohydrogen represents a

signicant step towards achieving a sustainable and eco-friendly soci-

ety. Hydrogen has a high energy yield per mass, making it an efcient

energy carrier. The production of biohydrogens can utilize organic

waste, thus contributing to waste reduction and management [31,32].

Unlike traditional fossil fuel-based hydrogen production, biohydrogen

production, especially through biological processes, is considered

cleaner and more sustainable. It can be easily manufactured from spe-

cic biomasses, although there are several challenges in its bio-

production, such as non-condensable gas storage and transportation.

Recently, biohydrogen biosynthesis has become a signicant focus as a

fuel for the next generation, thanks to its sustainable characteristics

[33]. Overall, biohydrogen can be obtained through various biological

processes, including dark fermentation, photofermentation, and the

utilization of algal biomass. Photofermentation, carried out by photo-

synthetic bacteria (PSB), utilizes light energy to produce hydrogen by

consuming various substrates in the presence of light. On the other hand,

cyanobacteria and microalgae undergo direct and indirect biophotolysis

to produce hydrogen, using inorganic CO₂, sunlight, water, and various

substrates, including organic acids. Dark fermentation, in contrast, oc-

curs in the absence of light, where anaerobic bacteria decompose com-

plex substrate molecules into basic monomers to produce hydrogen.

Dark fermentation is a widely used method for producing biohydrogen,

with glucose, glycerol, and low organic acids being the most common

substrates. Researchers have found that combining dark fermentation

with photofermentation can create a more efcient process known as

dark-photo co-fermentation [34]. This technique is cost-effective and

environmentally friendly and utilizes renewable feedstock, making it

highly popular. In addition to traditional substrates, scientists have also

explored the use of algal biomass and residual algal biomass to enhance

the efciency of direct hydrogen production from microalgae [35,36].

3. Overview of Carbon-Based nanomaterials

Different types of nanomaterials fall under the category of “nano-

materials,” including nanosheets, nanoparticles, nanocomposites,

nanotubes, nanocrystalline materials, metal-based nanomaterials, and

carbon-based nanomaterials. These nanomaterials have been exten-

sively used in biofuel production research due to their benecial prop-

erties, such as low density, high surface area, recycling potential,

insolubility, and stability [37]. Consequently, developing nanocatalysts

for biodiesel synthesis is highly anticipated [38,39]. The ndings indi-

cate that nanocatalysts have the potential to enhance transesterication

processes and improve productivity. Consequently, numerous studies

have focused on investigating the capabilities of various nanocatalysts,

including those derived from metal oxides, supported metal oxides,

zeolites/nano-zeolites, and magnetic materials, in producing biofuels

[40–43]. Additionally, it has been discovered that heterogeneous cata-

lysts and carbon-based support materials are commonly used in catalysis

[44].

Numerous research studies have established that carbon nano-

particles, such as acid- and base-activated carbon, supported carbon

materials, and sulfonated CNTs, have signicant potential as catalysts

for producing biofuels [45,46]. Carbon-based nanomaterials mainly

consist of carbon atoms arranged in unique structures with nanoscale

dimensions that confer exceptional properties benecial in various ap-

plications. Furthermore, these catalysts offer advantages, including

excellent thermal stability, lack of metals, and ease of synthesis [39].

Nanosized carbon-based catalysts show even more tremendous promise

for biofuel synthesis due to their cost-effectiveness, wide availability,

stability at high temperatures and different chemical environments,

ability to modify hydrophobicity and polarity, and easy extraction from

reaction mixtures [37]. Consequently, these nanomaterials play a

crucial role in efciently converting biomass sources like agricultural

residues, algae, and waste into biofuels, thereby enhancing the overall

processing efciency and reducing energy consumption.

Carbon-based nanomaterials can enhance biofuel production’s

catalysis by being integrated as starting or supporting materials in the

production process. Using agricultural byproducts like fruit peels, rice

husk, and de-oiled seed cake can create catalysts through sulfonation

and high-temperature calcination. These catalysts not only make use of

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

the byproducts from bioreneries but also offer a partial solution to the

waste management problem. Table 2 provides a concise overview of the

research conducted on carbon-based catalysts and their application in

the biodiesel sector [47].

3.1. Synthesis and functionalization of Carbon-Based nanomaterials for

biofuels

The advancement in Carbon-based nanomaterials (CBNs) research

has led to the development of a wide range of applications, spanning

from industrial purposes such as creating strong compounds and nano-

scale electronic devices to scientic applications like enhancing tissue

scaffolds, drug delivery systems, and biosensors. Due to their excep-

tional combination of mechanical, electrical, thermal, and optical

properties, CBNs have gained popularity due to their versatility in

various elds [54]. There are diverse classications of Carbon-based

nanomaterials (CNMs), including nanocarbon, graphene quantum dots

(GQDs), graphene oxide, graphene nanosheets, nanodiamond, fullerene,

single-walled carbon nanotubes (SWCNTs), multiwalled carbon nano-

tubes (MWCNTs), and carbon foam. These CNMs exhibit unique char-

acteristics that are benecial for several biological applications.

Moreover, introducing nitrogen doping in CNMs can enhance their

properties by promoting a porous structure, increased catalytic activity,

and improved biocompatibility. Such modications allow CNMs to

adhere more effectively to cellular surfaces, ultimately leading to higher

metabolic product yields. These improved CNMs possess intrinsic fea-

tures, making them highly sought after [9]. Some CNMs have already

been extensively utilized and show promise for potential biofuel appli-

cations, as illustrated in Fig. 3 [55]. The widespread use of these CNMs

in various industries highlights their signicance and the growing in-

terest in exploring their capabilities for future innovations and ad-

vancements in the eld of biofuels.

3.1.1. Fullerenes

Fullerene, a 0-dimensional nanomaterial, may be formed into

spheres (Buckyballs), ellipsoids (Bucky tubes or CNTs), and tubes. It

wasn’t until 1970 that it was effectively synthesized in a vacuum using a

laser vaporization method. Fig. 4 depicts an analytical representation of

fullerene C

, which was discovered to be made of 12 pentagonal and 20

hexagonal rings with their symmetry planes aligned, similar to graphite.

Initially considered inert, their solubility in organic solvents opened up

possibilities for functionalization via addition and redox reactions. The

molecule (which has 60 carbon atoms) is popularly referred to as a

“Buckyball” (because it resembles a football) and takes its name from

Buckminster Fuller’s geodesic dome design. Fullerenes have the

following properties: (i) Fullerenes like C

, C

, and C

are

fundamentally chiral due to D

-symmetry; (ii) C

lacks super-

aromaticity; (iii) fullerenes have high stability; (iv) fullerenes have

varying solubility; and (v) fullerenes’ contamination varies based on the

category, solvent functional group, and synthesis technique [8].

Following are the synthesis techniques for fullerene-.

3.1.1.1. Arc discharge. Under conditions of high temperature, the arc

discharge method is utilized as a synthesis technique. This methodology

encompasses a range of equipment such as a water-cooled trap, a

heating furnace, carbon electrodes, a high voltage pulsed-power input,

and a quartz tube. Within a temperature range of approximately 25 to

1000 ◦C, carbon clusters undergo annealing while a buffer gas is passed

through the quartz tube. To maintain a pressure of around 500 Torr, a

ow rate of 300 cm

−1

is set. To detect the presence of fullerenes,

including at low concentrations, the Performance Liquid Chromatog-

raphy (HPLC) technique is employed [56].

3.1.1.2. Laser ablation. When a high-energy laser beam is directed to-

wards a solid target material, typically graphite or a graphite rod, the

focused beam causes the target to heat rapidly, leading to vaporization.

As a result of the intense laser pulse, the target material is transformed

into vaporized carbon atoms, which subsequently condense and cool

down. These condensed carbon atoms then form clusters, eventually

evolving into structures known as fullerenes. The entire process occurs

in a controlled setting, commonly within an environment lled with

inert gases like helium or argon, which helps regulate the conditions for

forming fullerenes [57].

3.1.1.3. Chemical vapor deposition (CVD). A precursor gas with carbon

atoms is fed into a reaction chamber, which breaks down at high tem-

peratures to release carbon atoms. These carbon atoms accumulate on a

substrate surface, creating a thin lm or deposit. For fullerenes, the

precursor gas consists of carbon atoms that eventually form fullerene

structures. Various methods can be employed for chemical vapour

deposition (CVD), such as hot lament CVD, plasma-enhanced CVD, and

thermal CVD. In hot lament CVD, a heated tungsten lament dissoci-

ates the precursor gas. The temperature of the lament is carefully

regulated to aid in fullerene formation. Soot containing fullerenes is

produced as a by-product on the edges and rear surface of the substrate

holder, where diamond deposition is not favoured due to temperature

conditions. This technique enables the controlled production of fuller-

enes in a specic environment. Besides synthesis, CVD can also dope

fullerenes by introducing foreign atoms like nitrogen into the fullerene

structure. Nitrogen-doped fullerenes possess unique properties and are

applied in various elds, including optoelectronics and catalysis [58].

The free fullerene molecule’s limited solubility and hydrophobic

nature hinder its widespread use in different applications, leading re-

searchers to investigate modications that can improve its reactivity and

versatility for specic purposes. Functionalization of fullerenes can be

achieved through covalent or non-covalent methods, with covalent

changes capitalizing on C

′

s electrophilic properties for nucleophilic

addition. In contrast, non-covalent alterations involve hydrogen

bonding, pi-pi interactions, and coordination with metal ions. Adjusting

the fundamental characteristics of fullerene derivatives can be custom-

ized to exhibit increased lipophilicity, water solubility, and amphiphi-

licity, broadening their potential uses across various elds. An example

Table 2

Synopsis of research on carbon-based catalysts for biodiesel production.

Support/starting material Preparation method Application Yield References

D-glucose A sequence of incomplete

carbonization and sulfonation

Transesterication of used oil and

esterication of oleic acid

90 % for transesterication; 95 % for esterication [48]

Rice husk A string of carbonization and

sulfonation

Glycerol esterication with acetic acid Conversion of glycerol to triglycerides at a 90 % rate [49]

Rubber de-oiled cake Calcination promptly following direct

sulfonation

Synthetic transesterication of a non-

edible oil combination

Single-step biodiesel production yields 91.2 %,

while two-step production yields 93.7 %.

[50]

Microcrystalline cellulose

powder

A sequence of incomplete

carbonization and sulfonation

Oleic acid esterication and triolein

transesterication.

Transesterication at 98 % and esterication at 100

[51]

Activated carbon Wet impregnation with

Tungstophosphoric acid

Jatropha oil transesterication from

crude

87.33 % biodiesel yield [52]

Vegetable oil and

petroleum asphalt

Sulfonation of carbonized petroleum

asphalt/vegetable oil

Transesterication of used oil and

esterication of oleic acid

>80 % for both esterication and transesterication [53]

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

of this is the successful functionalization of Buckminsterfullerene (C

)

with Polyvinyl pyrrolidone (PVP) to produce a water-soluble derivative

[59].

3.1.2. Carbon nanotubes

The development of novel nanomaterials, such as carbon nanotubes,

that can be employed in various applications has recently gained much

attention. Nanoscale carbon tubes (or CNTs) are cylinders made of

carbon. CNTs are well suited for biofuel use due to their biocompati-

bility, antifouling qualities, high specic surface area, and superior

conductivity. Single-walled carbon nanotubes (SWCNTs) are nanowires

of a single enrolled graphene layer of a specic diameter and length. In

contrast, multi-walled carbon nanotubes (MWCNTs) are nanowires of

many enrolled graphene layers of varying diameters, lengths, and

chirality [60]. Carbon nanotubes can be utilized as accelerators and

catalysts because of their outstanding capabilities, superior chemical

stability, minimal toxicity, substantial surface area, and effective

structure. Several research organizations have identied carbon nano-

tubes as an essential component in various catalytic processes, including

the production of biofuels [37].

The synthesis of carbon nanotubes (CNTs) through laser ablation, arc

discharge, and chemical vapour deposition (CVD) shares similarities

with fullerene production. However, plasma-enhanced chemical vapor

deposition (PE-CVD) differs as it sources carbon from gaseous feedstocks

like methane (CH₄) and carbon monoxide (CO), eliminating the need for

solid graphite. These gases decompose into reactive carbon species

(RCS) such as C₂ and CH in the presence of an argon plasma, facilitating

product formation at lower temperatures and pressures. Thermal

Fig. 3. Carbon nanoparticles with demonstrated and potential applications in biofuels.

Fig. 4. Synthesis of Fullerene.

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

synthesis encompasses various chemical deposition methods, including

ame synthesis and CO₂ synthesis. Unlike plasma-based laser ablation

and arc discharge, thermal synthesis relies solely on a hot reaction zone

(approximately 1200 ◦C) and thermal energy. Metal catalysts like nickel,

cobalt, iron, and the carbon feedstock are often used to produce CNTs

[56].

Carbon nanotubes (CNTs) can be chemically functionalized as cata-

lysts in biodiesel production. Multi-walled carbon nanotubes (MWCNTs)

are preferred due to their cost-effectiveness compared to single-walled

carbon nanotubes (SWCNTs). For transesterication catalysts,

MWCNTs undergo specic functionalization processes [61].

3.1.2.1. Purication of CNTs. For optimal performance, CNTs must be

puried to eliminate undesirable byproducts like graphite and amor-

phous carbon, fullerenes, and metal particles. Before functionalizing

them to create catalyst supports for transesterication, MWCNTs are

puried via liquid phase oxidation and resolved in an acidic solution.

This process typically involves immersing the MWCNTs in concentrated

nitric acid (HNO

), a 1:1 mixture of HNO

and hydrochloric acid (HCl),

or a 3:1 mixture of concentrated sulfuric acid (H

) and HNO

. The

treatment is carried out for a duration ranging from 3 to 24 h at tem-

peratures between 50 and 80 ◦C. The initial step involves sonicating the

MWCNTs in the acid solution for an hour, followed by a ltration pro-

cess where water is added until neutrality is achieved. Subsequently, the

MWCNTs are dried for 12 h at 120 ◦C. Post-treatment, the puried

MWCNTs are denoted as MWCNTs-COOH, indicating the introduction of

carboxylic functional groups to their structure [61].

3.1.2.2. Utilization of the amino groups for functionalization. The ability

of amino groups to bind onto MWCNTs has been observed, potentially

making them useful as base catalysts in transesterication reactions.

Grafting amino groups onto MWCNTs can be done in two ways: either by

thermally mixing the long-chain amine-modied MWCNTs or by intro-

ducing the amino groups into surface gaps and then reacting with carbon

monoxide in a C-C coupling reaction. The three-step direct thermal

mixing process begins with acid oxidation to generate carboxylic groups,

continues with acylation (often with thionyl chloride (SOCl

)), and ends

with amidation [61].

3.1.2.3. Catalytic acid functionalization. Sulfonation refers to adding a

sulfonic group (SO

H) to CNTs to modify their properties. Trans-

esterication using SO

H is effective, with substantial quantities of

biodiesel being obtained. To catalyze transesterication/esterication,

H appears to be an excellent option for grafting onto the surface of

MWCNTs. Functionalizing MWCNTs with SO

H groups has been re-

ported using seven distinct sulfonation techniques, as shown in Fig. 5

[61]. Within this gure, the initial method entails functionalizing

MWCNTs using concentrated H

via thermal treatment, representing

the exclusive reported sulfonation technique for transforming MWCNTs

into catalysts for biodiesel production. By treating MWCNTs with

concentrated H2SO4 at elevated temperatures, sulfonated MWCNTs are

obtained from a substantial quantity of SO

H groups. The second

approach involves in situ polymerization of acetic anhydride and HSO4

to generate sulfonated MWCNTs by incorporating MWCNTs-COOH

groups into a blend of H

and acetic anhydride ((CH

-CO)

O). The

third tactic employs in situ polymerization of 4-styrenesulfonate to afx

H groups onto the MWCNTs’ surface. The fourth method encom-

passes the sulfonation of MWCNTs through the thermal processing of p-

toluenesulfonic acid (TsOH) with D glucose, culminating in the creation

of carbohydrate-derived solid acid catalysts known as sulfonated

MWCNTs. The fth technique integrates the sulfonation of MWCNTs via

the thermal breakdown of (NH

)

. Eventually, the sixth sulfonation

technique involves initiating a reaction between MWCNTs and

aminomethane-sulfonic/aminobenzenesulfonic compounds. The sev-

enth technique entails oxidizing thiol groups with hydrogen peroxide

) to introduce sulfonic acid groups onto MWCNTs [61].

3.1.3. Sulfonated carbon nanotubes

Diminution of catalytic surface area and aggregation are the most

prominent causes of inactivation in conventional CNTs [62]. Sulfonation

is a viable option for xing this problem. Sulfonated multiwalled S-

MWCNTs, a form of carbon nanotubes, are heterogeneous acid catalysts

created for use in transesterication reactions. Numerous research

studies have been undertaken employing CNTs with sulfonic acid

functionality for the production of biodiesel from various feedstocks,

including cottonseed oil, PFAD (Palm Fatty Acid Distillate) bio-oil, tri-

laurin and PFAD, due to the benets of functionalized carbon materials

[63]. Table 3 depicts some notable research on sulfonated carbon

nanotubes and their key ndings.

Fig. 5. Sequential sulfonation techniques used in catalytic acid sulfonation.

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

3.1.4. Graphene/Graphene Oxide-Based nanocatalysts

The exploration of graphene nanocomposites has become a pivotal

area of research. Graphene, characterized by a single atomic layer of

graphite, is the fundamental unit for fullerenes and carbon nanotubes

(CNTs), boasting a spectrum of properties including optical, mechanical,

thermal, electrical, and chemical attributes. Structurally, it is a two-

dimensional molecule composed of a tightly interlinked honeycomb

lattice of sp

-hybridized carbon atoms. Graphene oxide (GO), noted for

its mechanical robustness and chemical stability, draws signicant sci-

entic interest. While commonly derived from graphite and graphite

oxide, it can also be synthesized from diverse materials such as recycled

plastics and organic waste [37]. Graphene oxide (GO) is synthesized

through the exfoliation and oxidation of layered crystalline graphite in

protonated solvents. It is composed of a single layer of graphite oxide,

which can be conceptualized as individual graphene sheets adorned

with oxygen-containing functional groups such as carbonyl (C =O),

hydroxyl (–OH), carboxylic acid (–COOH), and alkoxy (C-O-C). These

oxygen functionalities make GO more soluble in water and organic

solvents, making it a valuable component in biodiesel production.

Additionally, the hydrophilic nature of GO facilitates the integration of

Brønsted acid sites during sulfonation [69].

Graphene, isolated by Geim and Novoselov in 2007, is another solid

carbon compound (SCC) utilized in biodiesel production. It features sp

hybridized carbon atoms arranged in a unique one-atom-thick, two-

dimensional honeycomb lattice, offering remarkable mechanical

strength and a high specic surface area of 2630 m

/g. The processing of

raw graphene presents challenges due to its intricate synthesis, limited

solubility, and propensity for aggregation through van der Waals forces

[70]. Nongbe et al. demonstrated the synthesis of cost-effective sulfo-

nated graphene from graphite, showcasing its remarkable efcacy and

durability as a catalyst for biodiesel conversion from palm oil, which can

be reused multiple times without degradation [71]. The production

process of sulfonated graphene involves two stages: the oxidation of

graphite to GO and its conversion to graphene through hydrazine-

mediated ultrasonic exfoliation, as depicted in Fig. 6 [69].

3.1.5. Carbon nanobers (CNFs)

Since their emergence in the 1960 s, carbon nanobers (CNFs) have

become pivotal in modern science and technology. Initially produced

through melt spinning from carbon precursors, polyacrylonitrile (PAN)

was the primary precursor, and it has undergone various modications

like additive incorporation, low-temperature oxidation stabilization,

and stretching during stabilization and carbonization. Vapor grown

carbon bers (VGCFs) are crafted using a catalytic chemical vapor

deposition (CVD) process. Two principal methods for synthesizing CNFs

are Catalytic decomposition of carbon precursors and Carbonization and

electrospinning of polymers. The choice of method depends on the

desired properties of the synthetic CNFs. CNFs synthesized via CVD

involve decomposing carbon-containing gases like acetylene, ethylene,

methane, and propylene, catalyzed by metal particles. CNFs’ surface

area can increase signicantly, from 300 to 400 to 1700 m2/g, when

activated with dissolved KOH, as shown in Fig. 7 [72]. Functionalization

involves modifying the surface of CNFs to introduce specic chemical

groups that can enhance their interaction with biofuel precursors. This

process consists of the introduction of functional groups to the CNFs’

surface, improving their dispersibility, reactivity, and binding capacity.

Surface characterization is pivotal in understanding the unique structure

and chemistry of CNFs, which is achieved through various techniques

like scanning probe microscopy, infrared and electron spectroscopies,

and electron microscopy. Covalent functionalization, mainly through [2

+1] cycloaddition reactions, is another method that maintains the

graphitic structure’s integrity while allowing for the attachment of

diverse functional groups [73,74]. Stellwagen et al. [75] employed a

novel method to synthesize CNFs functionalized with aryl sulfonic acid

groups using diazonium coupling reactions. These CNFs, designated as

CNF-Ar-SO

H, exhibited a high afnity for reactant molecules at the aryl

sulfonic acid sites, achieving a loading of 0.62 mmol/g. Utilizing these

CNFs as a catalyst in transesterifying triolein, they achieved a 72 %

conversion to methyl oleate within four hours at 120 ◦C and a molar

ratio of 1:10, with a catalyst amount of 0.75 g. Remarkably, the catalyst

maintained its efcacy for up to four cycles of transesterication,

requiring only minimal treatment for reuse. Iron (Fe), cobalt (Co), and

nickel (Ni) are the most common catalysts for CNF growth, with chro-

mium (Cr), vanadium (V), and molybdenum (Mo) also being explored.

Table 3

Overview of several investigation methodologies involving Sulfonated Carbon

Nanotubes.

Reference Methodology Key Findings

Guan et al.

[62]

An efcient S-MWCNT was

synthesized, demonstrating a

Brunauer–Emmett–Teller (BET)

surface area of 198.9 m

/g with

pore diameters between 5 and 35

nm. The catalytic efciency was

further investigated by studying its

ability to transesterify trilaurin in

ethanol.

•Produced 97.8 % biodiesel

from triglycerides under

optimal reaction conditions

of 20:1 ethanol to trilaurin

mass ratio, 150 ◦C reaction

temperature, 3.7 wt%

catalyst loading, and one-

hour reaction duration.

Also, transesteried tri-

glycerides are more efcient

than hydrothermal

carbonization-synthesized

sulfonated carbon and metal

oxide catalyst WO

/ZrO

Shuit et al.

[64]

Exhibited the way to make S-

MWCNTs by thermally

decomposing ammonium sulfate

((NH

)

) with ultrasonication.

Through the esterication of

PFAD with methanol, S-

MWCNTs showed 84.9 %

biodiesel output.

Shu et al.

[65]

Investigated MWCNT sulfonation

with a 210 ◦C concentration of

sulfuric acid.

Found that the S-MWCNT

catalyst had an acid site density

of 3.09 mmol/g and an average

pore size of 7.48 nm.

Shuit et al.

[66]

Examined various sulfonation

procedures on MWCNTs used to

esterify PFAD in biodiesel

synthesis.

•In situ polymerization of 4-

styrene sulfonate gave 93.4

% FAME (Fatty Acid Methyl

Ester) under the following

conditions: 20:1 methanol to

oil molar ratio, 2 wt% cata-

lyst loading, 170 ◦C reaction

temperature, and 3 h reac-

tion duration.).

85.8 % was obtained

through in situ polymeriza-

tion of acetic anhydride and

sulfuric acid, while 88.0 %

was obtained through ther-

mal breakdown of ammo-

nium sulfate.

Fan et al.

[67]

This study utilized transmission

electron microscopy (TEM), X-ray

powder diffraction (XRD), Fourier-

transform infrared spectroscopy

(FT-IR), and Confocal laser

scanning microscopy (CLSM) to

examine Burkholderia cepacia

lipase (BCL)-derived MWCNTs

supported by magnetic iron oxide

and polyamidoamine dendrimers

(PAMAM).

•Encapsulated lipase was

shown to be the most

effective catalyst for

biodiesel transesterication,

yielding 92.8 %.

This catalyst can be reused

up to 20 times and retains

90 % catalytic capacity.The

catalyst (BCL-mMWCNTs-

G3)

reduced the coagulation

limitation of carbon

nanotubes in magnetic iron

oxide.

Boshagh

et al.

[68]

To observe the catalytic

performance of functionalized

multi-walled carbon nanotubes

(MWCNT-COOH) during the

transesterication process of

biohydrogen production, FT-IR

and FESEM techniques were

employed.

•In comparison to a control,

MWCNT-COOH can boost

hydrogen production rate

(HPR) by 47 % and hydrogen

yield (HY) by 5 %.

It also exhibited a high

glucose degradation capa-

bility with an efciency of

96.20 % and good bio-

catalytic and stability

properties.

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

Fig. 6. Processing graphite into sulfonated graphene. Adapted with permission [70].

Fig. 7. (a) Typical electron image of CNF obtained by a CVD method from acetylene on the Fe/C/SiO

catalyst. (b) An electron micrograph shows that CNF was

treated with melted KOH modied with a 2% weight of copper. (c) Curve 1 representing the XRD patterns of CNF, while curve 2 representing the XRD patterns of

CNF activated with melted KOH. Reproduced from Ref. [73] with permission from the Royal Society of Chemistry.

Fig. 8. (a) Schematic diagram of Carbon Dot (CD) preparation via “top-down” and “bottom-up” approaches and modication, including functionalization, doping

and nanohybrids. (b) Various methods of synthesis of CD. Reproduced from Ref. [73] with permission from the Royal Society of Chemistry.

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

Compared to other carbon nanostructures like single-walled carbon

nanotubes (CNTs), CNFs offer a cost-effective advantage for mass pro-

duction, making them an economically favourable option in producing

biofuels [72].

3.1.6. Carbon dot (CD)

The synthesis of carbon dots (CDs) encompasses a variety of

methods, including hydrothermal/solvothermal, microwave, electro-

chemical, microplasma, and chemical oxidation. Each method imparts

distinct properties to the CDs. Broadly, these techniques fall into two

categories: “top-down” and “bottom-up” approaches, as depicted in

Fig. 8(a). The top-down approach involves fragmenting bulk carbona-

ceous materials into CDs through electrochemical synthesis, chemical

oxidation, and solvent heat treatment. Conversely, the bottom-up

approach constructs CDs from molecular precursors using hydrother-

mal treatment, microwave-assisted synthesis, thermal decomposition, or

carbonization, as illustrated in Fig. 8(b). Based on their structure,

properties, and synthesis methods, CDs are classied into carbon

quantum dots (CQDs), graphene quantum dots (GQDs), and carbonized

polymer dots (CPDs). The synthesis of carbon dots (CDs) can be opti-

mized by adjusting conditions such as time, temperature, and voltage.

Following synthesis, CDs are typically puried using methods like

centrifugation, dialysis, and ltration or more complex techniques such

as electrophoresis and chromatography. The hydrothermal/sol-

vothermal method is commonly used for CD synthesis, involving the

dissolution of raw materials in a solvent, heating in a Teon-lined

autoclave, and producing CDs with high yield and the potential for

element diversication [72]. In a study by Macina et al. [45], a novel

heterogeneous carbon dot catalyst was synthesized through glycine and

citric acid hydrothermal interaction. The catalytic potential of this

catalyst was then examined for the transesterication of canola oil to

generate biodiesel. The researchers attained a biodiesel conversion rate

of 97 % under the conditions of 150 ◦C and a catalyst loading of 1 %.

Furthermore, at least ve reaction cycles have demonstrated that this

nanocatalyst maintains its catalytic efcacy without deteriorating.

Although information is scarce on these nanocatalysts, previous

research suggests they have catalytic potential in the transesterication

of diverse feedstocks. Extensive investigation into these parameters is

required to determine how effective these nanocatalysts are as catalysts

for different feedstocks and reaction circumstances [76]. Su et al. syn-

thesized multifunctional CDs that can sense temperature, detect Ni(II),

and indicate doxycycline through uorescence changes. Meanwhile,

Zhang et al. employed electrochemical carbonization to convert alcohols

into CDs, observing increased size and graphitization with higher

applied potentials. This method showcases the versatility and adapt-

ability of CD synthesis techniques [72].

3.1.7. Carbon nanohorns

Carbon nanohorns (CNHs), also known as CNHs, are enclosed

structures composed of carbon atoms bound in a sp

conguration.

These nanohorns typically exhibit diameters ranging from 2 to 5 nm and

lengths between 40 and 50 nm. Due to their enclosed cage construction,

they are related to a class of fullerenes with a high aspect ratio. They can,

however, be extended by oxidation to disclose more of their surface

while providing access to their internal cavity. Because of their molec-

ular similarities, tiny single-walled carbon nanotubes serve as a struc-

tural analogue despite their elongated appearance. Nanohorns are

currently under investigation as prospective substitutes for nanotubes in

various applications, such as energy conversion, gas storage, medicine

delivery, and supercapacitors [77]. Unlike nanotube synthesis, nano-

horn production does not require metal catalysts and typically yields

high-purity samples with minimal byproducts like micrometric graphite

particles, fullerenes, and carbon onions. Current methods for synthe-

sizing carbon nanohorns involve energizing a carbon source, often

graphite, to the point of vaporization and restructuring, followed by

rapid cooling in an inert gas. The purity depends on the synthesis

method, and certain impurities can be reduced by preheating the carbon

source. Many applications do not necessitate further purication beyond

a possible thermal anneal. [77]. Synthesis processes are generally

categorized into three types concerning the methodology utilized to

infuse energy into the carbon, as depicted in Fig. 9.

CNHs were rst discovered in soot byproducts from arc-discharge

fullerene production. Heating can transform them into single-walled

and multiwalled nanohorns. Pulsed arc-discharge in atmospheric air

has been used to produce high-purity CNHs directly. Submerged arc-

discharge methods, including those in liquid nitrogen or argon, have

shown promise for large-scale production. An ‘arc in water’ method

using nitrogen gas has also been efcient for producing high-purity

CNHs. Laser ablation is another standard method for synthesizing car-

bon nanohorns (CNHs). This process involves the irradiation of pure

graphite with a high-powered laser in an inert atmosphere, such as

argon, without the need for a metal catalyst. It’s known for its high

production rate and yield, typically resulting in CNHs that form radial

aggregates. The size distribution and purity of the CNHs can be

controlled by adjusting the synthesis parameters, like temperature,

pressure, and laser power. This method is valued for producing CNHs

with high purity, which are helpful in various applications, including

biofuel production [78]. Induction heating is an alternative method for

producing carbon nanohorns, which involves generating high-frequency

eddy currents in graphite rods to create Joule heating. This process heats

the rods to over 3200 ◦C, forming a carbon plasma on the surface. The

resulting nanohorn soot, carried away by argon or helium gas, forms

dahlias or bud-like structures. This technique can produce approxi-

mately 0.1 kg of dahlias per hour and has the potential for signicant

scale-up. Although not explicitly stated, the purity of the product

Fig. 9. Methods for the synthesis of Carbon nanohorns.

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

includes some graphene sheets and minimal amorphous carbon, with

less than 4 % organic residuals [77]. The chemical functionalization of

carbon nanohorns (CNHs) is crucial for enhancing their solubility and

ease of handling in solvents, similar to other carbon nanoforms like

CNTs and graphene. There are two primary methods for modifying

CNHs chemically −covalent attachment of organic groups to the CNH

structure, creating stable bonds and noncovalent assembly of functional

molecules via

interactions, electrostatic forces, or attaching inor-

ganic nanoparticles. Furthermore, covalent functionalization is catego-

rized into two groups – i) Oxidizing the tips of CNHs to add oxygen-

containing groups, particularly carboxylic acids, which facilitate sub-

sequent chemical modications and ii) Directly adding functional

groups to the CNH sidewalls for additional properties or reactivity.

These modications allow for better manipulation of CNHs in various

applications [77].

3.1.8. Other Carbon-Based nanocatalysts

Nanocatalysts that are supported, doped, impregnated, or blended

all fall within the realm of possibility as catalysts. Under ideal condi-

tions, they outperform conventional heterogeneous catalysts, leading to

higher conversion and yield. As a result, nanocomposites with a large

surface area compared to their volume and a wide pore dispersion may

be a viable choice for the industrial-scale production of biodiesel.

Table 4 depicts an overview of some of the notable research involving

carbon-based nanocatalysts and their key ndings.

3.2. Properties and unique characteristics of carbon-based nanomaterials

Carbon nanomaterials are gaining prominence for their exceptional

properties that greatly benet biofuel production. Their small size pro-

vides a vast surface area that enhances catalytic efciency and optimizes

chemical reactions. These materials’ distinctive structural and electronic

properties make them powerful catalysts, thus expediting the biofuel

production process. They are resilient against the demanding conditions

of biofuel synthesis, ensuring reliable and consistent performance. The

robustness of carbon nanomaterials also allows for their reuse, which is

both cost-effective and environmentally friendly. Additionally, they play

a crucial role in improving the kinetics of biofuel production, leading to

faster and more efcient processes. In some biofuel production methods,

carbon nanomaterials are essential for electron transfer. These charac-

teristics make carbon nanomaterials an essential research focus for

developing more effective and sustainable biofuel production

technologies.

3.2.1. Surface Area, pore Size, and acid site density

The efcacy of catalytic supports in biofuel production is intrinsi-

cally linked to their surface area, which is determined by the size and

concentration of pores [83]. Supports with a high density of ultrane

micropores exhibit extensive surface areas, enhancing the catalyst’s

accessibility to reactants. However, relying on micropores alone can

introduce mass transfer limitations in liquid-phase reactions. The pres-

ence of large molecules, such as triglycerides and long-chain fatty acids,

can be particularly problematic, as they may not easily penetrate the

micropores, leading to reduced catalytic efciency. With its larger sur-

face area compared to multi-walled carbon nanotubes (MWCNTs),

activated carbon presents a viable alternative for catalyst support. Yet,

its predominant microporosity could potentially hinder its effectiveness

in transesterication reactions involving large molecules. Therefore,

selecting a catalyst support that boasts an optimal pore size distribution,

coupled with a signicant surface area, is crucial for maximizing cata-

lytic performance and ensuring a streamlined biofuel production process

[61].

3.2.2. Superior catalyst stability

Enzyme immobilization on carbon-based nanomaterials is a crucial

strategy in biofuel production. These materials, including carbon

nanotubes (CNTs), fullerenes, graphene, and carbon nanobers, provide

a robust platform for enzymes, enhancing their stability and activity.

CNTs, with their tubular structure and high surface area, allow for a high

enzyme loading, signicantly improving reaction kinetics and bio-

catalytic efciency. Functionalizing CNTs with specic functional

groups on their surfaces will enable them to be tuned to be catalytically

active, distinguishing them from other conventional transesterication

catalysts created via precipitation or penetration. In liquid-phase re-

actions, active species can leak off the solid support because they are not

bound to the solid via covalent bonds. CNTs are an excellent option for

catalyst support in transesterication/esterication because they can be

chemically altered so that functional groups can establish covalent

bonds. Strong covalent bonds between the active species and the CNTs

prevent the active species from leaching into the reaction medium, even

at the high temperatures experienced during the reaction. There have

been reports of tests in which covalently modied CNTs were utilized

without leaching issues. CNTs provide excellent replacement catalyst

support for overcoming the experienced difculty with low stability

when employing traditional transesterication catalysts; due to covalent

bonding, the catalysts are provided with superior stability, mobility, and

specicity [61]. Fullerenes are spherical molecules that can create a

unique microenvironment for enzymes, potentially protecting them

from denaturation and extending their operational life. As a one-atom-

thick planar sheet of carbon atoms, graphene offers a large surface for

enzyme attachment, which can facilitate faster reaction rates. Carbon

Nanobers can be used to immobilize multiple enzymes, which is

Table 4

Overview of several investigation methodologies involving carbon-based other

nanocatalysts.

Reference Methodology Key Findings

Ballotin

et al.[79]

Oleic acid was esteried with

methanol using sulfonated

carbon nanostructures

embedded in a bio-oil-derived

amorphous carbon matrix.

These substances have sulfonic

surface groups, and their

acidity is relatively high (up to

0.2 mmol/g).

Findings showed that 95 % oleic

acid could be converted via

esterication at 100 ◦C, a 1:10 M

oleic acid to methanol ratio, and

2.5 wt% of catalyst loading.

Dehkhoda

et al.[80]

Two novel carbon-based

catalysts (called “biochar”)

were created via sulfonating

pyrolysis with concentrated

sulfuric acid and fuming

sulfuric acid. After producing

both catalysts, they tested them

by transesterifying vegetable

oils and by esterifying FFAs

(Free fatty acids), respectively.

A sulfonated catalyst with a high

sulfuric acid concentration had

high esterication and low

transesterication activity,

indicating that it successfully

converted the feedstock in FFA.

Janaun et al.

[81]

Provided step-by-step

instructions for preparing

sulfonated carbon catalyst by

partially carbonizing D-glucose

and then sulfonating the

product with concentrated

sulfuric acid.

Obtained a 52 % increase in oleic

acid conversion by employing a

catalyst composed of sulfonated

granulated sugar.

Fu et al.

[82]

Presented the successful

production of an inexpensive

and remarkably durable

sulfonated solid catalyst. This

catalyst exhibited a substantial

concentration of active sites

(4.25 mmol/g) and a surface

area of 1 m

/g. The synthesis

process involved the utilization

of microalgae residue through

in situ hydrothermal

carbonization with the aid of

sulfuric acid.

The catalyst exhibited

remarkable stability and

reusability, maintaining its

activity over six consecutive

reaction cycles without any

discernible loss.

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

benecial for cascade reactions often required in biofuel production

processes. The immobilization of these nanomaterials improves the en-

zymes’ performance by providing extra physical support and speeds up

the reaction, making the process more efcient and sustainable. Carbon

nitride-based nanomaterials are also highlighted for their sustainable

catalyst design, which can make biodiesel production more cost-

effective and environmentally friendly [84–86].

4. Applications of CNMs

4.1. Carbon-Based nanomaterials as catalysts for biodiesel conversion

Tremendous possibilities prevail for carbon nanomaterials to in-

crease the biofuel production rate by boosting reaction kinetics due to

reductions in material cost and processing time. Utilizing nanomaterials

would help the biofuel production procedure become more viable by

lowering expenses and improving environmental outcomes. Lipase

could be used in transesterication to reduce product contamination and

enhance glycerol recovery. Commercialization of lipase-catalyzed bio-

diesel synthesis is limited owing to the higher cost and decomposition of

enzyme byproducts and substrates. Considering methanol is an acyl

acceptor, the enzyme’s biocatalytic activity diminishes when it is pre-

sent. Immobilization of enzymes on nanocarriers and other porous

substances could be a valuable technique for tackling the issues above.

Lipase-like enzymes and support materials are vital in immobilizing

enzymes for biodiesel production. SWCNT aids in boosting enzyme ac-

tivity retention. MWCNT allows for improved resolution efciency and

stability. It also promotes increased enzymatic activity and thermal

stability [87,88]. It was found that nanocatalysts with an acidic nature,

high porosity, and a large surface area have superior catalytic activity.

Table 5 shows how carbon-based nanocatalysts boost biodiesel synthesis

[89].

Moreover, Sulfonated carbon-based catalysts (SCCs) are emerging as

a superior choice for biodiesel production due to their heterogeneous

acid nature. These catalysts are known for their high surface area,

elevated acid site density, and enhanced catalytic activity. They are

particularly effective in esterication and transesterication reactions,

which are crucial for converting biomass into biodiesel, as depicted in

Table 6 [69]. Using SCCs in biodiesel production offers improved cata-

lyst activity and operation stability and aligns with environmental sus-

tainability and economic affordability [94].

4.2. Carbon-Based nanomaterials for enhancing biogas production

An attractive approach to dealing with organic waste is anaerobic

digestion (AD), conserving the environment and producing biomethane

[29,101]. Nanoparticles (NPs) have been widely studied to improve this

process’s both quantitative and qualitative performance. Carbon-based

nanomaterials, such as carbon nanotubes and graphene, have emerged

as promising agents for enhancing biogas production through their

unique properties [102]. These nanomaterials offer increased surface

area, facilitating microbial attachment and biodegradation processes,

ultimately leading to higher biogas yields. Moreover, their exceptional

electron transfer properties and ability to immobilize enzymes further

optimize anaerobic digestion, promoting stable and efcient biogas

production. Additionally, carbon-based nanomaterials can mitigate

inhibitory compounds, stabilize pH levels, and create a favourable

microenvironment for methanogens, contributing to enhanced metha-

nation efciency. As ongoing research explores their potential impact

and scalability, these nanomaterials hold the key to sustainable and

renewable energy solutions through improved biogas production tech-

niques [103].

The utilization of single-wall carbon nanotubes (SWCNTs) has the

potential to enable direct interspecies electron transfer (DIET) in the

anaerobic digestion (AD) process. The study conducted by Zhang et el.

[102] observed that introducing SWCNTs resulted in an acceleration of

feedstock consumption and methane production rates when contrasted

with the control group. However, it is important to note that the

methane production yield remained constant at approximately 14 mL

after 180 h. Indeed, the process of DIET facilitates the direct exchange of

electrons among microorganisms, bypassing the need for formate or

hydrogen transfer [101]. Considering this, it is plausible that materials

possessing electrical conduction, such as single-walled carbon nano-

tubes (SWCNTs), have the potential to function as pathways connecting

electron-accepting and electron-donating microorganisms, specically

exoelectrogenic bacteria and methanogenic archaea, in the process of

DIET [104]. Interestingly, the promotion of methane production is more

efcient when conductive materials that allow DIET are present,

compared to the consumption of H

or alternative electron transport

mechanisms for intraspecies exchange, as demonstrated by Park et al.

[104].

Multi-wall carbon nanotubes (MWCNT) are an additional type of

electrically conductive material that has been found to have the possi-

bility of expanding biogas and methane production rates. This is ach-

ieved through the transfer of electrons towards methanogens, as

demonstrated in the study conducted by Ambuchi et al. in 2017. The

presence of CNTs has been widely documented. To signicantly enhance

the presence of electroactive bacteria, such as Caloramator sp. or Geo-

bacter sp., as well as other bacteria or methanogenic archaea, such as

Methanosarcina, Methanosaeta, and others [105]. Ash-based nano-

materials, derived from municipal solid waste incineration, have been

identied as effective carbon-based additives for biogas production.

Research by Lo et al. demonstrated that micro/nano y ash and bottom

ash could signicantly boost biogas yields over a 90-day anaerobic

digestion period at 35 ◦C. This enhancement is likely due to the

increased microbial habitat provided by the agglomerative properties of

these nanocompounds [106]. Conversely, Nyberg et al. (2008) found

that fullerene (C60) nanoparticles did not yield any benecial effects on

biogas production when applied long-term [107].

4.3. Carbon-Based nano-additives in Diesel/Biodiesel blends

CNTs have recently piqued the interest of scientists because of the

free-metal compounds they are composed of. Compared to metal-based

nanoparticles, carbon nanotubes possess enhanced thermal and chemi-

cal properties. CNTs increase the thermal characteristics of nanouids

by acting as a catalyst, increasing the cetane number and burning rate of

the fuel [108–110]. The tramp metal ions contained in fuels are

Table 5

Summary of the involvement of carbon-based nanocatalysts in increased bio-

diesel production.

Nanocatalyst

and

Catalyst

amount

(wt%)

Feedstock

Methanol

to oil

ratio

Reaction

Time (h)

and

reaction

temp.

(◦C)

Biodiesel

yield

(%)

References

Sulfonated

biochar and

activated

carbon (AC)

(2–8)

Vegetable

oil

6:1 6/55–60 97 Kastner

et al. [90]

s-MWCNT

(0.20)

Oleic acid 5.8:1 1.5/135 95.46 Zhang et al.

[91]

O/CNT(3) Used

cooking oil

20:1 3/65 97 Ibrahim

et al. [44]

SiC-NaOH-GO

(5)

Rapeseed

oil oleic

acid

48:1 0.1/65 96 Loy et al.

[92]

KOH loaded

MWCNTs

(3.02)

Canola oil 13.56:1 3.34/60 94.23 Omraei

et al. [93]

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

successfully sequestered by carbon brils in CNTs, which can then be

removed. CNTs at higher concentrations in diesel, biodiesel, or

biodiesel-diesel blends improve BTE (Brake thermal efciency)

compared to ordinary diesel. Carbon nanotubes have potential as cata-

lysts due to their higher chemical reactivity and reactive surface area.

CNT addition improves BTE even in fuels based on a water-diesel

combination compared to ordinary diesel. Since the boiling point of

water is less than that of diesel, any CNT water in the blend will quickly

evaporate when the combustion heats up. Because CNT has a greater

surface area to volume ratio, it can readily combine with diesel and air,

and the micro-explosion principle causes these CNT water droplets to

erupt on the surrounding oil layers. When CNT-combined diesel is

compared to ordinary diesel, the former has a lower BSFC (Brake-spe-

cic fuel consumption) and drops even more as the CNT dose increases.

A prospective reason for this discovery is that CNT evaporates faster

than expected owing to its catalytic potential [111,112].

Furthermore, using CNT minimizes the ignition time for the CNT

blended fuel inside the combustion chamber, avoiding a buildup of

waste gas and consequent combustion during the premixed state.

Overall, BSFC values are heavily inuenced by the viscosity and caloric

content of the mixed fuel. CNTs, on the other hand, assist in reducing

BSFC in biodiesel/biodiesel-diesel blends by increasing combustion and

carbon oxidation. It has been discovered that CNT blended diesel has

higher in-cylinder pressure than regular diesel due to the enhanced

igniting characteristics of the carbon nanotubes. The maximal HRR of

fuels coupled with CNT increases with increasing carbon nanotube dose,

and the HRR of blended fuels is greater than that of standard diesel. Due

to its high surface area-to-volume ratio and improved thermal conduc-

tivity, CNT can achieve a higher maximum HRR [113].

CNT-blended fuel is claimed to emit less CO (Carbon monoxide) than

ordinary diesel, which can be related to the secondary atomizing

properties of the entrapped CNT droplets. As a result of higher surface

area to volume ratio, uniform dispersion and mixing are facilitated

[113]. Table 7 and Table 8 provide information on the many function-

alities of employing carbon nano-additives in manufacturing biodiesels

and their inuence on engine efciency.

Table 6

Overview of catalytic performance of Sulfonated Carbon-based catalyst for biodiesel production [94].

Sulfonated Carbon-based

catalyst

Alcohol Oil Oil: alcohol

ratio

Catalyst loading

[wt%]

t [h] T

[℃]

Conversion

[%]

Biodiesel yield

[%]

Reference

Esterication

Sulfonated carbon catalyst Methanol Waste cooking oil 1:20 10 1 60 87 −[95]

Ordered mesoporous carbons Methanol Oleic acid 1:10 0.5 16 160 96.25 Selectivity: 94 [96]

Single-walled carbon

nanohorns

Methanol Palmitic acid 1:33 3 5 64 −72 [97]

Activated carbon Methanol Palmitic acid 1:33 3 5 64 −28 [97]

Carbon black Methanol Palmitic acid 1:33 3 5 64 −55 [97]

Monolithic porous carbons Ethanol Oleic acid 1:10 −10 75 71.0 −[98]

Monolithic porous carbons Ethanol Acetic acid 1:10 −10 75 94.0 −[98]

Graphitic carbon nitride Methanol Oleic acid 1:5 10 4 27 99 −[99]

Transesterication

Graphene oxide Methanol Wet microalgae

cells

1:3 5 0.67 90 84.6 −[100]

Graphene Methanol Wet microalgae

cells

1:3 5 0.67 90 50 −[100]

Active Carbon Methanol Wet microalgae

cells

1:3 5 0.67 90 0 −[100]

Monolithic porous carbons Ethanol Sunower oil 1:20 − − 75 −63 [98]

Multiwalled carbon

nanotubes

Ethanol Trilaurin 1:20 3.7 1 150 97.8 −[62]

Graphene Methanol Palm oil 1:8 10 14 100 −98 [71]

Table 7

The primary function of carbon nano-additives in diesel/biodiesel blends.

Diesel combination Blended

percentage

Nanoparticle NP’s dosage

and size

Main effect Ref

Oenothera

Lamarckian

biodiesel

20 % GO 30–90 ppm Power and EGT (Exhaust gas temperature) increased, whereas CO and UHC (Unburned

hydrocarbon) emissions decreased. However, carbon dioxide and nitrogen oxide emissions

rose marginally.

[114]

Biodiesel– ethanol 30 % CeO

and

CNT

25–100 ppm A 22.2 % rise in CO emissions was accompanied by 7.2 % and 47.6 % decreases in HC and

smog emissions, respectively.

[115]

Jatropha-n-Butanol 50 % GNP-

MWCNT

50 ppm NOx, CO, and UHC were reduced by 45 %, 55 %, and 50 %, respectively. [116]

Lemon and orange

peel oil

20 % CNT, CeO

50–100 ppm Increased BTE and decreased BSFC with reduced CO

and hydrocarbon emissions. [117]

Dairy scum oil

methyl ester

50 % GO 23–27 nm Signicant improvements were made, with BSFC reduced by 8.34 %, BTE elevated by 11.56

%, unburned HC reduced by 21.68 %, and smoke reduced by 24.88 %.

[118]

Table 8

Carbon nano-additives and their effect on vehicle efciency.

Diesel

combination

Nanoparticle BTE BFSC Power Reference

Cooking oil CNTs +8.12 % −7.12 % +3.67

[114]

Waste cooking

oil

CNT and

silver

− − 7.08 % +2% [119]

Dairy scum oil

graphene

oxide

GO +11.56

−8.34 % −[118]

Ailanthus

altissima

GO − − 14.48

+14.3

[120]

Cooking oil MWCNT − − 4.5 % +7.81

[121]

Jatropha Methyl

Ester

GO +17 % −20 % −[122]

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

4.4. Photocatalytic hydrogen production using various CNMs

The process of generating hydrogen through the photocatalysis of

water has been extensively researched and is currently attracting sig-

nicant interest due to its environmentally benecial characteristics

[123]. Photocatalytic hydrogen evolution involves a complex and multi-

layered combination of chemical and physical processes. Photocatalytic

solar water splitting uses a redox reaction to separate water molecules

into oxygen (O

) and hydrogen (H

). Photocatalysis begins with an

event of photo adsorption. Semiconductors must contain an acceptable

band gap to utilize solar energy efciently. According to the Nernst

equation, the minimum energy required for the water-splitting process is

1.23 electron volts (eV). Semiconductors with a band gap of around 2

electron volts (eV) are considered suitable for enhancing photocatalysis

processes. Despite several semiconductors in this eld, only a few have

been found to be suitable options for practical application in photo-

catalytic hydrogen generation. To achieve maximum efciency, it is

crucial to tackle the inherent limitations of present semiconductor

technology, such as inadequate light absorption and increased recom-

bination rates. Contemporary studies emphasize the necessity of nding

a cost-effective substitute that improves the range of light absorption

and redox potential to maximize the efciency of the photocatalytic

process [124].

Carbon-based nanomaterials (CNMs) possess distinctive character-

istics that make them highly efcient catalysts in various applications.

The photocatalytic efcacy of these materials can be ascribed to their

large surface area, suitable morphology, and chemical and thermal

durability. CNMs can be utilized for a multitude of objectives. Cao and

Yu [125] have claimed that carbon materials have numerous applica-

tions, such as supporting materials, photocatalysts, or co-catalysts

(Fig. 10). Furthermore, they can enhance the absorption capacity, in-

crease the number of active sites, electron acceptors, and transporter

channels, accelerate photosensitization, and ensure the effectiveness of

band gap narrowing. Carbon-based semiconductors offer numerous

benets that enhance the efciency of photocatalysis. Additionally, it is

feasible to optimize the effectiveness of carbon-based materials by

modications. Several modication approaches have been successfully

employed, such as doping, surface functionalization, and interface en-

gineering. Carbon-based nanomaterials, including carbon nanotubes,

graphene oxide, graphene, fullerenes, and carbon quantum dots, have

demonstrated signicant efcacy in enhancing hydrogen generation.

The current objective is to improve hydrogen production by utilizing

carbon nanomaterials (CNMs) for a more ecologically sustainable result.

Considerable studies have focused on using carbon-based materials to

facilitate the widespread application of photocatalytic hydrogen pro-

duction. Much research has been conducted on creating hydrogen (H

)

using carbon-based nanomaterials (CNMs) to achieve cost efciency,

identify desired features, and reduce environmental impact. Table 9

presents a thorough overview of the rates at which distinct carbon-based

nanomaterials have achieved photocatalytic hydrogen generation

[124].

4.5. Carbon-Based nanomaterials for biofuels separation and purication

Separating the biodiesel from the glycerol is the primary step in

many renement stages that follow the transesterication technique.

Because of their different densities, biodiesel and its waste glycerol may

be easily distinguished. In general, this separation is accomplished

through decanting or centrifugation. Other processes that can result in

separation include gravity settling, purication, and deposition.

Fig. 10. Carbon materials’ functions in developing enhanced photocatalytic activity for hydrogen production.

Table 9

Rate of photocatalytic hydrogen production utilizing various nanomaterials

based on carbon.

CNMs Semiconductor

/Composite

Co-catalyst H

Production

Rate

Reference

CNTs CdS Pt (Pt-Af-

CNT/CdS)

120.1 mmol g

−1

[126]

g-C

CNCNT 1208 µmol g

−1

[127]

Graphene WO

Graphene 288/µmol g

−1

[128]

TiO

Graphene 100 mmol g

−1

[129]

CdS Pt 2310 µmol g

−1

[130]

Graphene

Oxide

CdS Ni 8866 µmol g

−1

[131]

Carbon

Quantum

Dots

TiO

Pt 472 µmol g

−1

[132]

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

Following the elimination of glycerol, biodiesel undergoes additional

processing to eliminate contaminants such as tri-, di-, and mono-

glycerides, catalysts, soap, and alcohol residues. Biodiesel is typically

puried by ash evaporation or vacuum distillation to eliminate

alcohol, followed by wet or dry washings to separate triglycerides,

catalysts, and soap. Different variations on the wet washing approach

include using only distilled water, rst washing with acids and then

water, and last washing with an organic solvent and then water [133].

Separation technologies more generalized for biodiesel purication

include equilibrium-based, afnity-based, membrane-based, solid-

–liquid, and reaction-based processes, as depicted in Fig. 11 [134].

Thermochemical processes like combustion, pyrolysis, and gasica-

tion convert biomass into fuels with higher energy content. One specic

process, pyrolysis, involves the distillation of biomaterials in an oxygen-

free environment, resulting in compounds like biochar and bio-oil. Bio-

oil, in particular, is a valuable product obtained from biomass pyrolysis

using materials like rice straw, pine wood, rice husk, corn cob, wheat

shell, mesquite sawdust, and jatropha seed shell. These materials can be

further rened and modied to create high-value products such as

biofuels. Advanced rening methods like extraction and fractionation

are used to separate different bio-materials and their constituent

chemicals in rening natural oils. These methods aim to create a sus-

tainable technique with maximum productivity for producing high-

value bio-based materials, like the biorenery based on palm oil mills.

Presently, a wide range of extraction techniques are being employed to

purify and separate biofuels [135]. Table 10 offers a glimpse into the

diverse methods and outlines the advantages and challenges associated

with each [135].

4.6. Carbon-based micro and nanomaterials for enzymatic

immobilization

Enzyme immobilization increases the enzyme’s resistance to heat,

severe conditions, and chemicals and its ease of recovery from the re-

action mixture, making it more practical to utilize in continuous oper-

ations. Immobilizing the enzyme can typically enhance its heat stability

by maintaining its intact tertiary structure and active site. Cellulases

have extensive global usage, and their potential in biofuel production is

crucial due to the growing demand and production of biofuels. Enzyme

adsorption onto the biomass substrate is necessary for the enzymatic

hydrolysis process, wherein cellulase facilitates the breakdown of cel-

lulose. Once the enzyme moves from the liquid to the solid biomass

particles, the enzyme molecules will attach to the active sites on the

biomass particles sequentially [139,140].

Carbon nanotubes are the most often utilized carbon-based nano-

structures. Carbon nanotubes (CNTs) are well-suited for immobilizing

enzymes, particularly in biofuel synthesis, because of their improved

electrical characteristics. In the context of enzyme immobilization on

Fig. 11. Purication methods for biodiesel, categorized by their respective techniques. Adapted with permission [1 3 5].

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

carbon nanotubes (CNTs), noncovalent techniques are favoured over

covalent techniques due to their propensity to lessen enzyme denatur-

ation. Two forms of carbon nanotubes can be used for enzyme immo-

bilization: single-wall carbon nanotubes and multi-wall carbon

nanotubes. MWCNTs are commonly chosen for cellulase immobilization

due to their multilayered graphite composition, which allows for easier

dispersal and cost-effectiveness. Furthermore, various chemicals in

carbon-based composite nanomaterials contribute to distinct factors

that facilitate the immobilization of the enzyme or increase its activity.

Most composite nanomaterials incorporate a magnetic nanoparticle

(MNP) due to its facilitation of enzyme recycling. An enzyme’s immo-

bilization often occurs in the outside layer of a composite nanomaterial.

In contrast, the inner layers serve various purposes, such as giving the

composite functional groups, improving stability, sensitivity to tem-

perature changes, etc. Table 11 describes the numerous types of cellu-

lases and their carbon-based micro and nano supports in biofuel

production [139].

Complex polysaccharides, including lignin, cellulose, and hemicel-

lulose, comprise most of the non-edible biomass utilized to produce

biofuels. Such lignocellulosic wastes are pre-treated using various

Table 10

Methods for biofuel extraction [135].

Technique Advantages Challenges References

Water Extraction ▪ Inexpensive solvent

Sustainable solvent

Effortlessness in executing the technique

▪ Enhanced water consumption

Depending on the raw material type, the water to oil ratio can

differ.

[136]

Atmospheric

distillation

▪ The current plants have been merged effectively.

Separation is a common practice in the orthodox oil/

gas process.

▪ Greater energy consumption

High temperature

Does not apply to biomolecules that are sensitive

[137]

Organic solvent

Extraction

▪ Comprehensive and essential information through

extensive research.

Relatively well-established technique on a commer-

cial scale

Relatively Inexpensive and economical

▪ Widespread application of toxic and hazardous solvents leading

to considerable environmental damage.

Additional complexities rising from multi-stage extraction

techniques

Considerable water wastage

[137]

Supercritical

uid Extraction

▪ Simplied methodology

Environmentally sustainable

Modifying the temperature and pressure conditions

allows for better control of the properties of a

supercritical uid.

▪ Unavailability of fundamental data

Not economical

Safety risks arising from high-pressure operations posing a

signicant threat to individuals and equipment.

[138]

Steam distillation ▪ Temperature conditions below atmospheric distillation

are mandatory.

Orthodox oil and gas processing commonly involves

the separation of different components.

Substantial expenditure. [137]

Vacuum distillation ▪ Temperature conditions below atmospheric distillation

are mandatory.

Operating under reduced pressure.

Suitable for biomolecules susceptible to elevated

temperatures.

▪ Exorbitant capital as well as equipment cost

Recovery of distillate is hindered by the rapid evaporation of

vapours from the product gas, resulting in decreased yield.

High vacuum

[137]

Adsorption ▪ Inexpensive adsorbent

Simplied methodology

Lack of attentiveness towards toxic pollutants

▪ Greater energy consumption

Lack of environmental sustainability

Accumulation of secondary waste

Challenges arise when attempting to enlarge the scale of a

process, such as the inverse correlation between owrate and

adsorption capability.

Nature of amplied complexity of the technique

[137]

Molecular

distillation

▪ Temperature conditions below atmospheric distillation

are mandatory.

Suitable for biomolecules susceptible to elevated

temperatures.

Enhanced rate of distillate recovery

▪ High vacuum

Exorbitant capital as well as equipment cost

[137]

Chromatography ▪ Sorting materials based on their category.

The separated material exhibits an exceptional level of

purity.

▪ Time-intensive technique

Usage is conned to only small-scale applications

[137]

Membrane ▪ Minimal energy consumption

Simplied methodology

Seamless scalability

No requirement for extra chemicals

▪ Membrane fouling

High cost

[137]

Electrosorption ▪ Minimal energy consumption

Reversible process

No requirement for extra chemicals

Usage is conned to only small-scale applications [137]

Ionic liquid

Extraction

▪ Greater efciency

Superior thermal stability

▪ High cost

Lack of literature on ionic liquids toxicity

[137]

Table 11

Immobilization of cellulases on carbon-based nanomaterials and composites for biofuel conversion [139].

Source of Cellulase Carbon-based nanomaterial Amount of Enzyme Immobilized Reusability References

T. reesei RUT C-30 Functionalized MWCNTs 26.61 U/mL 98 % [141]

Aspergillus niger Functionalized MWCNTs 85 % 75 % [142]

Aspergillus niger Activated carbon 90.2 % 70 % [143]

Aspergillus nidulansSU04 Modied activated carbon (MAC) 4.935 mg cellulase/mg MAC 70 % [144]

Commercial Fe

/GO/CS 90 % 80 % [145]

Trichoderma viride GO-Fe

- 4arm-PEG-NH

132 mg/g 45 % [146]

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

techniques, such as physical, chemical, physicochemical, biological, or a

mix of techniques, to produce biofuels. Subsequently, the polymeric

compounds derived from pre-treatment processes, such as cellulose and

hemicellulose, undergo enzymatic hydrolysis.

5. Circular economy of green synthesized carbon-based

nanomaterials

The use of nanomaterials presents a potential risk of contamination

and exacerbates environmental pollution. As an example, carbon

nanotubes (CNTs) worsen the process of environmental deterioration

and contribute to the phenomenon of global warming [146]. Additional

negative consequences of the production and implementation of nano-

particles in energy studies encompass intricate fabrication processes,

heightened energy requirements and usage, toxicity and environmental

pollution, inadequate disposal and recyclability, and low rates of

recovering used nanomaterials [147]. The effect of heavy metals in most

nanoparticles on human health and environmental pollution is inu-

enced by their disposal, management, and biodegradability. Prolonged

contact with carbon nanotubes (CNTs) may result in skin irritation,

while inhalation or inadvertent ingestion of nanoparticles can lead to

lung inammation, heart problems, and disruption of the digestive

system. The disposal of used nanomaterials has the capacity to pollute

the soil, surface, and subsurface water sources [34]. Extensive research

has been conducted on using biomass pyrolysis to produce eco-friendly

carbon nanomaterials, which can be used in biofuel and bioenergy

generation. This proposed approach effectively utilizes the thermal

waste and residual gases generated during biomass pyrolysis to syn-

thesize nanomaterials, enhancing this process’s economic and environ-

mental advantages. The resulting products exhibit exceptional

performance in energy storage and various eco-friendly applications

[147].

5.1. Production of Three-Dimensional graphene Bubbles (3DGBs)

The production of Three-Dimensional Graphene Bubbles (3DGB)

from biomass waste requires several procedures and the use of special-

ized equipment known as the “Atmospheric Pressure Chemical Vapour

Deposition (APCVD)-catalytic purication” method. The primary input

for 3DGB production is biomass waste, which includes lignin, cellulose,

wheat straw, sawdust, and other types of organic waste. Pre-treatment

processes, such as drying and size reduction, could render it more

accessible to process biomass waste later. The rapid pyrolysis reactor is

used to rapidly heat biomass waste at extreme temperatures, often 400

to 600 ◦C, under oxygen-depleted conditions, resulting in bio-oil, syn-

gas, and charcoal. Through biomass gasication, biomass is converted

into syngas, also known as synthesis gas, which contains carbon mon-

oxide (CO), hydrogen (H₂), and other contaminants. Catalytic purica-

tion, specically Atmospheric Pressure Chemical Vapour Deposition

(APCVD)-catalytic purication, is used to clean the syngas. The puried

syngas is directed into a CVD reactor, an APCVD reactor. The syngas

decomposes at high temperatures within the CVD reactor, forming

carbon atoms on a substrate. The substrate can serve as a catalyst or a

specialized material that aids in forming 3DGB structures. The Forma-

tion of 3DGB Carbon atoms accumulates on the substrate, forming

connected graphene sheets that evolve into a three-dimensional gra-

phene foam structure. These structures offer excellent catalytic support

in the production of biofuels [148].

5.2. Production of carbon nanotubes and nanobers through microwave

pyrolysis

Microwave Pyrolysis (MAP) rapidly releases a volatile substance,

forming a highly porous biochar with a high temperature [149,150].

This process also produces Carbon nanotubes (CNTs). The production of

CNTs is inuenced by factors such as microwave irradiation, hotspot

development, volatile matter composition, and inorganic materials. The

nucleation of CNTs begins at the hotspots in the biomass-derived char

during MAP. In the initial stage of MAP, biomass particles containing

volatile organic compounds are heated, causing the volatile materials to

be expelled from the core to the surface through the particle’s pores. The

squeezed volatile matter solidies on the particle’s surface due to the

temperature difference, forming hollow-shaped CNTs. Microwave-

mediated pyrolysis is one of the methods used to synthesize Carbon

nanobers (CNF), which is a catalyst-free process that yields efcient

results. Hollow carbon nanobers (HCNF) are produced in a shorter

reaction time during MAP, which can be further utilized in the pro-

duction of biofuels [151].

5.3. Production of carbon quantum dots (CQDs)

Apart from carbon, lignocellulosic waste also consists of nitrogen

(N), hydrogen (H), and oxygen (O). These elements are renewable but

also cost-effective and eco-friendly compared to other carbon sources.

The conversion of LCB waste into CQDs through synthesis helps trans-

form worthless biomass waste into valuable and practical commodities

[152]. Zhou et al. [89] were the rst to propose an environmentally

friendly synthesis method using watermelon peel as the carbon pre-

cursor, which initiated a new trend of utilizing biomass waste for CQD

production. Subsequently, researchers further explored the use of

diverse forms of agricultural, animal, fruit, and vegetable waste in this

process. CQDs are preferred over heavy-metal-based QDs for applica-

tions involving uorescence because of their low toxicity, excellent

biocompatibility, simple manufacturing, and great uorescence stability

[153].

6. Challenges and future directives

Carbon nanomaterials have several advantages, including superior

surface characteristics, minimal size, biocompatibility (mostly), ease of

synthesis, and evident usage in various industries. However, studying

nanoparticles’ possibly hazardous effects on living beings is currently in

the limelight. However, there is still cause for concern because the long-

term effects of widespread nanoparticle use or leakage into the envi-

ronment must be adequately examined. This has raised signicant

concerns about the benets and drawbacks of employing such tiny

particles, particularly in industrial contexts such as biofuel-biodiesel

synthesis. These opposing viewpoints and prospective consequences

present a once-in-a-lifetime chance for scientists worldwide to suggest

and analyze the long-term ramications of nanomaterials and strategies

to mitigate their potential downsides [154].

The use of biofuels may adversely impact the environment and so-

ciety. Some of the signicant repercussions include air pollution,

greenhouse gas emissions, deforestation, excessive water consumption

and pollution, loss of biodiversity, threats to food security, challenges in

rural development, instability in energy supply, social disputes, and

hazards to public health. One major drawback of biofuels is that they are

obtained from biomass, which is also used as a source of food for humans

[155]. This becomes an issue when there is inadequate food to support

everyone. Biofuels need substantial quantities of fertilizer, water, and

land area, leading to competition for land utilization and a poor net

energy ratio [156]. This process may also result in ecological degrada-

tion as a consequence of adverse impacts on biodiversity.

Biofuels provide a more cost-effective and viable alternative to fossil

fuels, which are becoming more costly. As a result, biofuels are being

seen as a feasible and economically efcient substitute [157,158]. The

manufacturing of biofuel necessitates the use of organic biomass derived

from agricultural waste, municipal trash, and other sources. Neverthe-

less, more studies are required to enhance biofuel output. Efcient pre-

treatment solutions that provide good value for money are needed to

strengthen the use of lignocellulosic biomass [159,160]. Algal biomass is

used to manufacture biodiesel due to its rapid growth, carbon neutrality,

N. Rodoshi Khan and A. Bin Rashid

Energy Conversion and Management: X 22 (2024) 100590

and high oil content. In the future, there is a potential for it to replace

fossil fuels in the manufacture of biodiesel ultimately. Cultivating algal

biomass is exceptionally costly due to the high energy demand for lipid

extraction [161].

Further research, in the context of the study, has the potential to

surpass the current constraints on biodiesel production greatly. Devel-

oping nanocatalysts with stable active sites that can perform many

functions during the trans-esterication reaction and have low leaching

effects and establishing a strategy for the industrial use of oils derived

from non-edible feedstocks. Lu et al. [162] investigated the efciency of

multi-functionalized carbon nanotubes combined with nitrate (NH

) and

carboxylic acid (COOH) in the transesterication of microalgal oil

(Chlorella), followed by hydrothermal liquefaction. The experiment was

conducted in a batch reactor with a 1:1 mass ratio of CNTs-NH

and CNTs-

COOH at 280 ◦C. Carbon nanotubes were discovered to play an essential

role in enhancing hydrogen output while lowering oxygen, sulphur, and

nitrogen levels. At 340 ◦C, the utilization of these CNTs created a steady

condition, resulting in a considerable proportion of hydrocarbon (47 %)

in bio-oil compared to the control (28 %). Also, carbonaceous and zeolite-

based catalysts are functionalized using nanomaterials and polymeric

materials to increase their effectiveness, specicity, and longevity. Opti-

mizing the rate of the chemical reactions required to make biodiesel by

structural modication depends on the size and form of nano-based ma-

terials. For instance, nanomaterials made of graphene and graphene oxide

have lately acquired favour for capturing microalgae biomass in a sus-

tainable manner. Graphene-based nanomaterials are lipophilic materials

with unique characteristics that have numerous applications in environ-

mental and biological sciences. Graphene-based nanomaterials are

benecial in various domains, including electrochemical sensors, elec-

troanalytical chemistry, and enzyme and biomolecule immobilization,

due to their unique physiochemical properties, including electrical, me-

chanical, optical, and thermal strength. An enzyme-based biofuel cell

creates electricity through electrochemical reactions involving biomass-

derived energy carriers such as ethanol, glucose, and oils. Elahi et al.

[118] studied the effects of adding graphene oxide nanoparticles to

various quantities of dairy scum oil biodiesel. Adding graphene oxide

nanoparticles to dairy scum oil biodiesel blends signicantly reduced CO

emissions (around 38.62 % for DSOME2040) and unburned hydrocarbons

(around 21.68 % for DSOME2040) while also increasing brake thermal

efciency (11.56 % for DSOME2040) and decreasing brake specic fuel

consumption (8.34 % for DSOME2040). Further improvement is crucial to

boost reaction throughput while minimizing mass transfer resistance,

energy expenditure, and waste. Increasing the possibility of sustaining

optimum reaction conditions and, as a result, maximizing biodiesel

output [163,164].

7. Conclusion

Even though the prospect for biofuels became apparent throughout

the twentieth century, the onset of the petroleum age was widely

recognized at the turn of the century. Other options for petroleum have

sparked much debate; they could aid in sustainability worldwide by

ensuring energy access and accomplishing GHG targets (at the same

time remaining protable and cost-competitive, if achievable) while not

affecting future cultures, economies, communities, and ecosystems.

Carbon NMs and their offspring are currently the centre of extensive

research. This is reinforced by the mountain of literature devoted to

exploring this intriguing area. The optical, mechanical, chemical, and

electrical properties they exhibit make them one of the most signicant

accessible and cannot be equalled by more commonplace materials. Yet,

substantial amounts of these elements still need to be produced for the

industry. However, the strong and consistent use of these materials

across different technologies has been hindered by the absence of uni-

formity in their structures and the irregularity of their production.

Numerous studies have shown that many rst-generation biofuels

are not as environmentally friendly as their petroleum-based

counterparts regarding greenhouse gas emissions and environmental

damage. However, it is essential not to let these challenges discourage us

from exploring the potential of biofuels and biomass consumption.

Instead, we should be motivated to nd more sustainable ways to

convert low-value and waste biomass into biofuels using reliable tech-

nologies and with a thorough understanding of their environmental

impacts. Future research should prioritize optimizing their qualities,

improving existing methods, and developing new synthesis approaches.

To fully benet from nanotechnology in biofuel research, it is crucial to

support interdisciplinary and collaborative investigations into cost-

effective and sustainable production techniques. Fuel industry stake-

holders must embrace research ndings to enhance the biofuel pro-

duction value chain. The outcomes of innovative studies on

nanotechnology applications, such as biofuel production and purica-

tion, development of efcient and affordable catalysts, etc., have the

potential to revolutionize biofuel research and production. Therefore,

emphasis should be placed on advancing research and development of

carbon NMs and related compounds across various industries. To opti-

mize biofuel production, it is imperative to deploy advanced technolo-

gies such as machine learning, articial intelligence, robotics, smart

metering, modelling and simulation software, numerical and mathe-

matical apparatuses, statistical techniques, and optimization tools.

These technologies can improve production parameters, reduce pro-

duction time, minimize material wastage, and advance biofuels pro-

duction. Given the increasing demand for sustainable energy solutions,

technologies like nanotechnology that accelerate biofuel production will

continue to receive attention from researchers and stakeholders.

Without a doubt, biofuels will remain positioned as critical contributors

to the renewable energy sector and vital sustainable energy solutions.

CRediT authorship contribution statement

Nazia Rodoshi Khan: Writing – original draft, Conceptualization.

Adib Bin Rashid: Writing – original draft, Supervision,

Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing nancial

interests or personal relationships that could have appeared to inuence

the work reported in this paper.

Data availability

Data will be made available on request.

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N. Rodoshi Khan and A. Bin Rashid

Current scenario of nanomaterials: The treasure

Article

Mar 2025

Anamika Roy

Nanoparticle (NPs) becomes a treasure for scientific community. The size related properties offers ample opportunities for amazing discoveries. The unpredictable and exceptional nature of nanoparticles contributes their prospective utilization in many technological applications. At the same time it also creates challenges for scientific society. In this review, we have covered applications of nanoparticles research and highlight their current development in various fields. The parameter includes the role of nanomaterials in catalysis and in food packaging applications. The industrial contributions of nanomaterials in architecture and construction, in cosmetics and beauty, in textile and in refinery have been discussed. Association as in defence appliances, in agriculture, in biomedical research, and in energy production also explained. The chosen parameters are not directly related to each other, but explain broadness of nanotechnology in commercial applied science relevance.

Valorization of Corncob through Integrated Acid and Enzymatic Pre-Treatment for Maximization Yield of Fermentable Sugar

Article

Full-text available

Mar 2025

The valorization of agricultural residues such as corncobs for bioethanol production offers a sustainable solution to global energy demands and environmental concerns. Present study focused on maximizing fermentable sugar yields from corncobs through an integrated acid and enzymatic pre-treatment process. Corncobs, which are abundant and rich in cellulose, were subjected to dilute sulfuric acid pre-treatment followed by enzymatic hydrolysis. A response surface methodology (RSM) using a Box-Behnken design was employed to optimize the pre-treatment process parameters acid concentration, temperature, and residence time. The obtained results revealed that optimal conditions for pre-treatment involved a 0.5% sulfuric acid concentration, a reaction temperature of 120 °C, and duration of 40 min. Under these conditions, the cellulose content of the pre-treated corncob increased to 66.10%, while the hemicellulose content significantly decreased. The enzymatic hydrolysis of pre-treated biomass resulted in a maximum saccharification yield of 41.1%, corresponding to a sugar yield of 185 mg/g of corncob. The study also includes the thermogravimetric analysis of raw corncob and BET surface area analysis that assessed thermal behaviour and the creation of pore space due to pre-treatment for improving enzyme accessibility. The findings highlighted that combining acid and enzymatic pre-treatment enhanced corncob into fermentable sugar. Aside from providing an efficient method of producing bioethanol, this process also contributes to the management of agricultural crop residue in a sustainable manner. Graphical Abstract

Eco-Friendly Additives: Performance and Emissions Analysis of a Diesel Engine With Jojoba Methyl Ester and CNT Blends

Article

Full-text available

Feb 2025
INT J ENERG RES

Diesel engines have extensive use in vehicles, power plants, and agriculture, but they pose significant challenges due to both regulated and unregulated emissions. As fossil fuel supplies diminish and pollution levels increase, it has become crucial to shift from conventional fuels to biodiesel and alternative additives. This experimental study investigates the emission and performance characteristics of a water-cooled, three-cylinder, four-stroke compression ignition engine operating on blends of Jojoba methyl ester (JME) combined with additives such as carbon nanotube (CNT). JME was mixed with pure diesel in concentrations of 5% and 15% by volume, creating blends referred to as J5 and J15. CNT was added to these blends at a concentration of 20, 40, and 60 ppm. Emission and performance metrics were evaluated across engine speeds varying from 1200 to 2200 rpm. The findings revealed significant improvements, with brake thermal efficiency (BTE), brake power (BP), and torque increasing by 6.7%, 3.8%, and 3.8% respectively. A 2.6% reduction in brake-specific fuel consumption (bsfc) was observed at 1600 rpm with the J5CNT60 blend. The introduction of Jojoba biodiesel into neat diesel resulted in reductions of 12.8%, 14.56%, 15.25%, and 7.27% in nitrogen oxide (NOx), carbon monoxide (CO), carbon dioxide (CO2), and unburned hydrocarbon (UHC) emissions, respectively. Although Jojoba biodiesel resulted in a slight increase in smoke opacity (SO) emissions by less than 1%, the addition of CNT led to a 2% decrease in SO emissions at 2000 rpm.

Nanocatalysts in Biodiesel Production: Advancements, Challenges, and Sustainable Solutions

Article

Full-text available

Feb 2025

The application of nanocatalysts for the conversion of crude oil into biodiesel represents a promising strategy to mitigate environmental issues and diminish dependence on fossil fuels. This review scrutinizes contemporary developments in nanocatalyst technology pertaining to biodiesel synthesis, emphasizing the effectiveness, specificity, and ecological sustainability of various nanocatalysts, which include metal oxides, magnetic solid catalysts, and an array of other nanostructured materials. These catalysts have exhibited substantial potential in augmenting the efficiency and selectivity of biodiesel production, attributable to their enhanced catalytic performance, extensive surface area, and resilience to saponification, rendering them appropriate for diverse feedstocks such as recycled oils, animal fats, and plant oils. Notwithstanding the obstacles associated with production costs and recycling procedures, recent progress in economically viable manufacturing techniques and sustainable recovery methods, including magnetic separation approaches, offers significant promise. A thorough lifecycle assessment (LCA) remains imperative to appraise the environmental and economic viability of employing nanocatalysts, taking into account parameters, such as raw material acquisition, energy utilization, and waste generation. The review accentuates the superior efficacy of nanocatalysts in comparison to traditional heterogeneous catalysts, underscoring their capacity to expedite biodiesel synthesis and facilitate a transition toward a more environmentally sustainable energy paradigm.

Principles, applications and future prospects in photodegradation systems

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Full-text available

Apr 2025

Photocatalytic materials have emerged as pivotal in addressing global challenges such as environmental pollution, energy scarcity, and industrial sustainability. This review delves into the principles, mechanisms, and applications of photocatalytic systems, emphasizing their roles in photodegradation and renewable energy production. United Nations (UN) specified a guideline for sustainable development strategies. UN determined 17 goals of sustainable development and the services of photocatalytic materials underwent 4 of these goals to reflect the distinguishable interest and importance of different photocatalytic materials in many fields for various purposes. Advances in material design, nanotechnology, and system integration have significantly advanced this field, aligning it with sustainable development goals. Key materials like TiO2, ZnO, g-C3N4, and quantum dots are highlighted for their unique properties and enhanced photocatalytic activity through modifications such as doping, heterostructure formation, and biopolymer-supported photocatalysts. Practical applications in wastewater treatment, hydrogen production, air purification, and carbon dioxide reduction are comprehensively analyzed, with case studies demonstrating the successful photodegradation of industrial pollutants. The review also explores the integration of photocatalysis with renewable energy sources, addressing challenges like charge carrier recombination and photocatalyst stability. Interdisciplinary approaches, including computational modeling and machine learning, are discussed for designing next-generation photocatalysts, aligning innovations with global sustainability initiatives.

Recent progress of carbon-based quantum dots and nanotubes for cancer targeting and drug delivery applications

Article

Apr 2025
J DRUG DELIV SCI TEC

Cancer remains a major global health challenge, driving extensive research into developing advanced nano systems for precise and effective drug delivery. Carbon-based nanomaterials, particularly carbon nanotubes and quantum dots, have gained prominence due to their exceptional structural and physicochemical properties. These nanomaterials offer significant advantages, including high drug-loading capacity, enhanced biocompatibility, and reduced immunogenicity, making them highly promising candidates for cancer therapy. This review provides a detailed examination of the synthesis and functionalization of carbon nanostructures, highlighting recent advancements in their use for cancer drug delivery. Also, it analyzes their potential to improve therapeutic outcomes through targeted delivery mechanisms, while addressing the challenges associated with their clinical translation. Furthermore, the review explores the prospects, challenges, and future directions of carbon nanotubes and quantum dots, focusing on their integration into clinical applications for cancer treatment.

Recent Progress in Carbon Nanotubes Production via Catalytic Chemical Vapour Deposition

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Mar 2025
J ANAL APPL PYROL

Carbon nanomaterials based catalytic conversion of biomass: An overview

Article

Mar 2025

Microbial Fuel Cells for Sustainable Biofuel Production: Cutting-edge Technology

Article

Full-text available

Feb 2025

As of 2023, coal, oil, and natural gas, which are non-renewable fossil fuels, account for about 80% of the world’s energy consumption. This underscores the pressing necessity for alternative energy sources in light of the worsening climate crisis. Apart from the problems, this review evaluates the potential of microbial fuel cells for biofuel production, which contributes to just 10–20% of the total energy consumption due to its relatively low environmental impact. The aim of this systematic review is to elucidate microbial fuel cells and their role in producing a range of bio-based fuels, including biogas, biodiesel, bioethanol, and biobutanol. The findings are categorized into ten main areas: biomass conversion techniques, substrates and microbial strains, design components of microbial fuel cells, strain improvement through metabolic engineering, enhancements of nanomaterials for biofuel production, advantages and disadvantages of biofuels, integration in biorefineries, potential applications, the challenges and limitations, and future trends in microbial fuel cell technology. This review also underscores the promise of microbial fuel cells as a sustainable and eco-friendly option for bioenergy production, emphasizing the need for advancements in design and efficiency to compete with conventional fossil fuels. Harnessing innovative strategies pertaining to the technology of microbial fuel cells (MFCs) can potentially transform energy generation into a more sustainable and commercially viable practice, positively impacting environmental conservation and public health.

Revolutionizing biodiesel with novel MWCNT-COOH@TiO2-ZnO nanocatalysts for enhanced transesterification and tribocorrosion resistance

Article

Feb 2025
RENEW ENERG

Lignocellulosic biofuel properties and reactivity analyzed by thermogravimetric analysis (TGA) toward zero carbon scheme - A critical review

Article

Full-text available

Apr 2024

Biomass is an organic substance widely available in nature as a fresh or a waste material considered renewable energy that aligns with the zero-carbon scheme to reduce the dependency on fossil fuels. However, after conversion, biomass’s physical or chemical properties highly affect biofuel characteristics. A variety of instruments can be used to figure out biofuel reactivity. Considering commonly adopted instruments, thermogravimetric analysis (TGA) is a simple, fast, and efficient way to determine biofuel properties and reactivity. The TGA method has the capability to analyze the biofuel properties (proximate analysis: moisture, volatile matter, fixed carbon, and ash) and combustion features of biomass (such as ignition, reactivity, etc). Most importantly, the TG curvatures (TGA and DTG) reveal the behavior of the biofuel during the thermodegradation process. As a consequence, the quality and quantity analyses on the biofuel properties and reactivity can be investigated comprehensively. Moreover, some TGA integration with artificial intelligence (AI) has been studied to better understand biofuel management and technology for future development. The outcome for the TGA-AI integration may obtain an excellent result with the fit quality value R2 >95 %. This study aims to comprehensively review relevant research using TGA to analyze the lignocellulosic biofuel properties and reactivity. Moreover, the discussion in this study is extended to perspective, challenges, and future work.

Generation of Green Carbon Nanomaterials from Biomass Pyrolysis for the Production of Biofuel and Bioenergy

Article

Full-text available

Dec 2023

The need to find a Renewable Energy Resource (RER) with significant potential has been brought to light by the fact that petroleum and natural gas are being used up to their full capacity, reducing the amount of energy produced globally. The issues relating to the preparation, hydrolysis with enzymes, and biomass culture processes that must be completed before bioenergy may be produced are still being addressed via ongoing initiatives. Because nanotechnology offers unique active areas for multiple responses and operations, it can overcome the difficulties posed by these biomass sources. Pyrolysis may be used to produce chemicals and RER from biomass sustainably. However, the process's high production expenses prevent it from being widely used. This approach's long-term reliability and financial viability may be greatly increased by fabricating high-quality, active carbon nanoparticles using waste heat and renewable progenitors. This paper suggests using Biomass Pyrolysis to Generate Green Carbon Nanomaterials (BP-GGCN) for biofuel and bioenergy production. The suggested method makes the most of biomass pyrolysis's financial gains and sustainability by producing superior Three-Dimensional Graphene Bubbles (3DGBs) using residual pyrolysis gases and thermal waste. The resultant 3DGBs work well in energy storage and ecologically sensitive applications. According to a life-cycle study, the current approach has less overall effect than the traditional Chemical Vapour Deposition (CVD) technique on human well-being, environmental systems, and resources. The specific qualities of this GGCN help biofuels, biodiesel, enzymes, and microbial fuel cells work better.

Impact of nanoparticle-based fuel additives on biodiesel combustion: An analysis of fuel properties, engine performance, emissions, and combustion characteristics

Article

Full-text available

Dec 2023

Nanoparticles (NPs) are becoming increasingly crucial in academic as well as industrial applications. Nanoparticles’ addition to biodiesel shortens the time it takes for the fuel to ignite, allowing combustion to begin earlier and reducing the amount of heat released and the pressure in the cylinders under full load. Recent review studies have focused on nanoparticle additives used in biodiesel and diesel engines, performance, combustion behavior, and emission properties of biodiesel-powered diesel engines, and stability and combustion characteristics of metal nanoparticles (NPs) and their additive impact on compression ignition engines powered by biodiesel and diesel. However, nanoparticle effects on either biodiesel properties, engine performance, emissions, or combustion have not been comprehensively investigated in these studies. This paper addresses this gap by focusing on cost-effective and sustainable strategies for the development of fuel additives for biodiesel combustion. The literature has demonstrated that the incorporation of NP mixes (CeO2+Al2O3) with biodiesel fuel improved the overall performance, emission characteristics, and combustion efficiency of the engine. For instance, the addition of TiO2 nanoparticles reduced smoke emission by 32.98%, carbon monoxide (CO) by 30% and unburned hydrocarbons (HC) by 28.68%. Emissions of nitrogen oxide (NOx), CO, HC, and smoke were reduced by 30%, 60%, 44%, and 38%, respectively, while brake power (Bp) and brake thermal efficiency (BTE) went up by 12%. This study will show advances and potential areas for nanoparticle-enhanced biodiesel engine improvement, leading to cost-effective and sustainable renewable energy solutions.

Lignocellulosic Biomass‐Based Carbon Dots: Synthesis Processes, Properties, and Applications

Article

Full-text available

Aug 2023
SMALL

Carbon dots (CDs), a new type of carbon‐based fluorescent nanomaterial, have attracted widespread attention because of their numerous excellent properties. Lignocellulosic biomass is the most abundant renewable natural resource and possesses broad potential to manufacture different composite and smart materials. Numerous studies have explored the potential of using the components (such as cellulose, hemicellulose, and lignin) in lignocellulosic biomass to produce CDs. There are few papers systemically aiming in the review of the state‐of‐the‐art works related to lignocellulosic biomass‐derived CDs. In this review, the significant advances in synthesis processes, formation mechanisms, structural characteristics, optical properties, and applications of lignocellulosic biomass‐based CDs such as cellulose‐based CDs, hemicellulose‐based CDs and lignin‐based CDs in latest research are reviewed. In addition, future research directions on the improvement of the synthesis technology of CDs using lignocellulosic biomass as raw materials to enhance the properties of CDs are proposed. This review will serve as a road map for scientists engaged in research and exploring more applications of CDs in different science fields to achieve the highest material performance goals of CDs.

A critical review on nanotechnological advancement in biogas production from organic waste

Article

Full-text available

Jun 2023

The fast depletion of fossil fuel reserves (coal, oil, natural gas) with an increase in environmental pollution has enforced the need for an alternate source of energy. A novel sustainable approach with the implementation of a zero waste discharge policy is an urgent need of the hour. Organic wastes with a higher fraction of biodegradable components can be considered an ideal source for sustainable bioenergy production. Biogas with a calorific value of 21–25 MJ/m³, produced by anaerobic digestion (AD) of organic waste, is considered an excellent replacement for fossil-derived fuels and natural gas while reducing more than 80% of greenhouse gases (GHGs) emissions to the environment. However, reduction in the rate of biomass hydrolysis and methane formation due to the accumulation of ammonia are some of the major limitations of the process that impede its industrial applicability. Recently, the performance of AD and methane production are improved with the usage of nanoparticles owing to their unique physical and chemical properties. In view of this, the present review summarizes various reports on nanotechnological advancement in biogas production while highlighting the knowledge gaps. The incorporation of nanoparticles in AD aids in the enhancement of biogas yield, increment in methane content, improvement of effluent quality, and reduction in H2S production. These nanoparticles act as the supplier of essential nutrients for the synthesis of enzymes and co-factors in anaerobic microorganisms which further stimulate their activity for higher biogas yield. Further, the mechanism of nanoparticle interaction during the AD process is elaborated. Graphical Abstract

Recent advances in the application of nanomaterials for improved biodiesel, biogas, biohydrogen, and bioethanol production

Article

Feb 2024
FUEL

Recent advances of carbon-based additives in anaerobic digestion: A review

Article

Sep 2023
RENEW SUST ENERG REV

A Review on Analysis of Biochar Produced from Microwave-Assisted Pyrolysis of Agricultural Waste Biomass

Article

Jul 2023
J ANAL APPL PYROL

Biohydrogen production, storage, and delivery: A comprehensive overview of current strategies and limitations

Article

Jul 2023
CHEM ENG J

Performance and emission characteristics of CI engine using hydrogen enrichment in biodiesel blend with additives—A review

Article

May 2023

It is an irresistible argument that there is a requirement for sustainable resources for energy production as there is an enormous rise in demand for energy. Various sectors involve a wide range of energy consumption shares. Conventional fuels are exhaustible and can be replaced with sustainable substitutes, i.e., biofuels including bio-ethanol, biogas, and biodiesel. Recently, biodiesel has gained popularity due to the availability and affordability of feedstock. To extract biodiesel from vegetable oils, the transesterification process is widely used. Biodiesel blends can be used as a direct substitute in diesel engines, which may result in a slight reduction in hydrocarbons, carbon monoxide, and an increase in oxides of nitrogen. The performance parameters may increase or decrease depending on the blend type and calorific value. The objective of this review is to analyze the performance and emission characteristics of second and third-generation biodiesel blends enriched with hydrogen and to explore techniques for improvement such as the addition of additives. Research has shown that the addition of hydrogen improves combustion, resulting in increased brake thermal efficiency and reduced brake specific fuel consumption. Similarly, the inclusion of additives can reduce the production of oxides of nitrogen during combustion. In conclusion, biodiesel blends enriched with hydrogen and additives can offer improved performance, combustion, and emission characteristics. To meet energy demand sustainably and reduce reliance on conventional fuels, further research is necessary to ensure the long-term viability of biodiesel as a sustainable energy source.

Carbon-Based Nanomaterials: a Paradigm Shift in Biofuel Synthesis and Processing for a Sustainable Energy Future

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