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Recent Progress in the Steam Reforming of Bio-Oil for Hydrogen Production: A Review of Operating Parameters, Catalytic Systems and Technological Innovations

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The present review focuses on the production of renewable hydrogen through the catalytic steam reforming of bio-oil, the liquid product of the fast pyrolysis of biomass. Although in theory the process is capable of producing high yields of hydrogen, in practice, certain technological issues require radical improvements before its commercialization. Herein, we illustrate the fundamental knowledge behind the technology of the steam reforming of bio-oil and critically discuss the major factors influencing the reforming process such as the feedstock composition, the reactor design, the reaction temperature and pressure, the steam to carbon ratio and the hour space velocity. We also emphasize the latest research for the best suited reforming catalysts among the specific groups of noble metal, transition metal, bimetallic and perovskite type catalysts. The effect of the catalyst preparation method and the technological obstacle of catalytic deactivation due to coke deposition, metal sintering, metal oxidation and sulfur poisoning are addressed. Finally, various novel modified steam reforming techniques which are under development are discussed, such as the in-line two-stage pyrolysis and steam reforming, the sorption enhanced steam reforming (SESR) and the chemical looping steam reforming (CLSR). Moreover, we argue that while the majority of research studies examine hydrogen generation using different model compounds, much work must be done to optimally treat the raw or aqueous bio-oil mixtures for efficient practical use. Moreover, further research is also required on the reaction mechanisms and kinetics of the process, as these have not yet been fully understood.
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Catalysts 2021, 11, 1526. https://doi.org/10.3390/catal11121526 www.mdpi.com/journal/catalysts
Review
Recent Progress in the Steam Reforming of Bio-Oil for
Hydrogen Production: A Review of Operating Parameters,
Catalytic Systems and Technological Innovations
Anastasia Pafili 1, Nikolaos D. Charisiou 1,*, Savvas L. Douvartzides 1,2, Georgios I. Siakavelas 1,3, Wen Wang 4,
Guanqing Liu 4, Vagelis G. Papadakis 3 and Maria A. Goula 1,*
1 Laboratory of Alternative Fuels and Environmental Catalysis (LAFEC), Department of Chemical
Engineering, University of Western Macedonia, GR-50100 Kozani, Greece; aepafili@gmail.com (A.P.);
sdouvartzidis@uowm.gr (S.L.D.); giorgosiakavelas@gmail.com (G.I.S.)
2 Department of Mechanical Engineering, University of Western Macedonia, GR-50100 Kozani, Greece
3 Department of Environmental Engineering, University of Patras, GR-30100 Agrinio, Greece;
vgpapadakis@upatras.gr
4 Biomass Energy and Environmental Engineering Research Center, Beijing University of Chemical
Technology, Beijing 100029, China; wangwen@buct.edu.cn (W.W.); gqliu@mail.buct.edu.cn (G.L.)
* Correspondence: ncharisiou@uowm.gr (N.D.C.); mgoula@uowm.gr (M.A.G.)
Abstract: The present review focuses on the production of renewable hydrogen through the cata-
lytic steam reforming of bio-oil, the liquid product of the fast pyrolysis of biomass. Although in
theory the process is capable of producing high yields of hydrogen, in practice, certain technological
issues require radical improvements before its commercialization. Herein, we illustrate the funda-
mental knowledge behind the technology of the steam reforming of bio-oil and critically discuss the
major factors influencing the reforming process such as the feedstock composition, the reactor de-
sign, the reaction temperature and pressure, the steam to carbon ratio and the hour space velocity.
We also emphasize the latest research for the best suited reforming catalysts among the specific
groups of noble metal, transition metal, bimetallic and perovskite type catalysts. The effect of the
catalyst preparation method and the technological obstacle of catalytic deactivation due to coke
deposition, metal sintering, metal oxidation and sulfur poisoning are addressed. Finally, various
novel modified steam reforming techniques which are under development are discussed, such as
the in-line two-stage pyrolysis and steam reforming, the sorption enhanced steam reforming (SESR)
and the chemical looping steam reforming (CLSR). Moreover, we argue that while the majority of
research studies examine hydrogen generation using different model compounds, much work must
be done to optimally treat the raw or aqueous bio-oil mixtures for efficient practical use. Moreover,
further research is also required on the reaction mechanisms and kinetics of the process, as these
have not yet been fully understood.
Keywords: renewable hydrogen; bio-oil steam reforming; steam reforming catalysts;
two-stage in-line pyrolysis and reforming; sorption enhanced steam reforming;
chemical looping steam reforming
Citation: Pafili, A.; Charisiou, N.D.;
Douvartzides, S.L.; Siakavelas, G.I.;
Wang, W.; Liu, G.; Papadakis, V.G.;
Goula, M.A. Recent Progress in the
Steam Reforming of Bio-Oil for
Hydrogen Production: A Review of
Operating Parameters, Catalytic
Systems and Technological
Innovations. Catalysts 2021, 11, 1526.
https://doi.org/10.3390/catal11121526
Academic Editor: Ken-ichi Fujita
Received: 22 November 2021
Accepted: 11 December 2021
Published: 15 December 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional
claims in published maps and institu-
tional affiliations.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://cre-
ativecommons.org/licenses/by/4.0/).
Catalysts 2021, 11, 1526 2 of 36
1. Introduction
The decarbonization of our global energy market and the exploitation of renewable
energy sources (RES) are widely considered as the most important policies which can
bring our planet into a secure sustainable future [1–3]. Renewable energy, such as solar,
wind, tidal and geothermal, will inevitably play a decisive role in the next decades, but
clean and effective technologies are also necessary for the supply of alternative transpor-
tation biofuels [4–7].
Lignocellulosic biomass, such as agricultural residues and dedicated energy crops,
has a vast unused potential for continuous energy supply at a low price and with neutral
carbon dioxide (CO2) environmental impact [8–10]. The utilization of lignocelluloses can
open a renewable carbon-neutral roadmap [11,12] for the production of heat, electrical
power and biofuels. The conversion of biomass to renewable hydrogen (H2) is of major
interest as it can be used as a fuel in combustion engines and fuel cells or may be used for
the synthesis of useful chemicals and high energy density transportation biofuels [13,14].
Hydrogen is the cleanest fuel available since its utilization produces only steam vapors
and does not pollute the atmosphere with CO2, greenhouse gases or other emissions [15–
21].
An empirical chemical formula of biomass can be written as CnHmOk·xH2O [22,23]
and typical biomass compositions are given in Table 1. Depending on the characteristics
of the raw feedstock, lignocellulosic biomass can be treated for the production of high
value biofuels and bio-chemicals using several thermochemical or biochemical processes.
Table 1. Composition of woody, herbaceous and waste biomass. Adopted from ref. [24].
Feedstock Composition Woody Herbaceous Wastes
Proximate
Volatiles (%) 84.0 79.1 76.7
Ash (%)
1.3
5.5
6.6
Fixed carbon (%) 14.7 15.4 14.8
Ultimate
H (%) 6.0 5.8 5.9
C (%)
50.7
47.4
46.0
N (%) 0.32 0.75 1.3
O (%) 41.9 41.0 38.3
S (%) 0.03 0.10 0.15
Structural
Cellulose (%)
51.2
32.1
28.4
Hemicellulose (%) 21.0 18.6 16.4
Lignin (%) 26.1 16.3 12.5
Conversion technologies that rely on thermochemical methods, such as gasification
and pyrolysis, are able to convert the entire lignocellulosic matter into gaseous and liquid
products, which can be used directly as transport fuels or may serve the synthesis of up-
graded biofuels [25–29]. Conventional and catalytic fast pyrolysis technologies especially,
lead to the formation of a condensed liquid product known as bio-oil (tar or pyrolytic oil),
which serves as an intermediate for the generation of hydrogen and upgraded transpor-
tation biofuels.
The present article reviews the recent trends and research outputs in the technology
of hydrogen production through the catalytic steam reforming (SR) of bio-oil. The review
discusses the most important steam reforming processing parameters such as the reactor
feed composition, the reactor design and the reaction conditions and presents the recent
research findings on the development of effective catalysts. Information on noble, transi-
tion, bimetallic and perovskite type catalysts is critically presented, and specific attention
Catalysts 2021, 11, 1526 3 of 36
is paid to the effect of the catalyst preparation method and the recent progresses against
catalyst deactivation through coke formation, metal sintering, metal oxidation and sulfur
poisoning. The critical examination of the available literature reveals that while in theory
the process is capable of producing high yields of hydrogen, in practice certain technolog-
ical issues require further investigation and radical improvements before its commercial-
ization. A major challenge is the high chemical complexity of raw bio-oil, which does not
readily allow a systematic approach on the maximization of hydrogen productivity and
the alleviation of carbon deposition issues. Moreover, raw bio-oil cannot be completely
vaporized and when heated leads to the formation of residual solids which accelerate cat-
alyst poisoning at rates much higher than the usually examined model compounds. Ad-
ditional research is also required on the reaction mechanisms and kinetics, which have
not yet been fully understood and much work must be done to optimally treat the raw or
aqueous bio-oil mixtures for efficient practical use. Finally, further consideration must be
given to identify catalysts with low cost, high activity and stability, strong regenerative
ability and extensive operating lifetime for successful operation in industrial conditions.
Since catalyst deactivation is a major problem encountered during the steam reforming
process, the mechanisms of coke formation and metal sintering should also be further in-
vestigated. However, the challenges outlined herein must be met with increased vigor as
the efficient production of renewable hydrogen promises to help move away from the
current, fossil-based model of energy production.
2. Bio-Oil Properties and Composition
Biomass can be converted into hydrogen via two major thermochemical routes: (i)
gasification, to directly produce syngas, and (ii) pyrolysis, to obtain bio-oil, followed by
reforming. An excellent review of the major biomass to hydrogen production processes
has been provided by Martino et al. [30]. Biomass pyrolysis takes place between 220 and
900 °C, in the absence of O2. During the process, hemicellulose, cellulose, and lignin bi-
opolymers thermally decompose to form a solid residue (charcoal or biochar), condensa-
ble gases and non-condensable/permanent gases (e.g., CO2, CO, H2 and other light hydro-
carbons). After cooling at room temperature, the non-permanent gases are condensed to
form the bio-oil, a liquid phase product of higher energy density than biomass [25,31].
Bio-oil is a dark brown liquid of high viscosity, comprised of a plethora of heavy organic
and inorganic molecules and, hence, it is a suitable platform source for many upgraded
chemicals. The composition of bio-oil and its properties depend on the raw feedstock com-
position and the conditions at which pyrolysis was undertaken, such as temperature, heat-
ing rate, and residence time [32]. Compounds in bio-oil include carboxylic acids, alde-
hydes, alcohols, ketones, anhydrosugars and substituted furans derived from cellulose
and hemicellulose, and phenolics and cyclic oxygenates derived from lignin. The most
abundant species are acetic acid, acetone, acetol, glycolaldehyde, furanones, levogluco-
san, and phenol [33–37].
Water addition can help separate the bio-oil into two distinct fractions. The first frac-
tion is the hydrophilic carbohydrate or aqueous phase containing typically 20 wt.% or-
ganics, which can be catalytically steam reformed. The second fraction, often called pyro-
lytic lignin is the hydrophobic organic phase and contains furan and aromatic based spe-
cies; this fraction can be used for the development of a plethora of products [38,39]. As
raw bio-oil cannot be totally vaporized, the solids that remain as residuals can cause clog-
ging in the feeding lines and the reactor. In addition, despite the higher H2 yields obtained
from the reforming of the whole oil, the SR of the bio-oil aqueous fraction is preferred in
several studies [40–47]. Table 2 summarizes the characteristics of both the raw bio-oil and
its aqueous fraction, while Table 3 provides a typical composition of raw bio-oil.
Catalysts 2021, 11, 1526 4 of 36
Table 2. Raw bio-oil characteristics [48].
Parameter
Bio-Oil
pH 2.6 2.5
Water content (wt. %) 36 84
Ultimate analysis (wt. %)
Carbon 36.07 7.35
Hydrogen 8.45 10.82
Nitrogen 0.11 0.00
Oxygen 1 55.37 81.83
1 Determined by difference.
Table 3. Raw bio-oil composition, as derived from the fast pyrolysis of pine wood. Adopted from
[12,22,23,25,49].
Components wt. %
Acetic acid 15.0-15.5
Acetone 5.0-5.5
Alcohols 12.0-12.5
Ethers
0.5-1.0
Hydroxyacetaldehyde 10.5-11.0
Levoglucosane 3.5-4.0
Other acids and esters 10.5-11.0
Other aldehydes 9.5-10.0
Other ketones 21.5-22.0
Others 1.0-1.5
Phenols
6.5-7.0
Unidentified 1.0-1.5
Generally, bio-oil consists of organic components rich in oxygen (30–40 wt. %), tars
(e.g., naphthalene, toluene and benzene), and water (approximately 25 wt.%). The pres-
ence of organic acids decreases the bio-oil pH at 2.5–3.0 and causes corrosion and storage
issues. Compared with the fossil petroleum distillates, crude bio-oil has a very large oxy-
gen content and its heating value is only around 16 MJ/kg, i.e., almost 2.7 times lower than
that of typical fossil diesel fuel [31,50]. Therefore, before final use the stability and com-
bustion properties of bio-oil need to be enhanced; this is achieved by reducing its water
and oxygen content. Kumar and Strezov [51] and Lian et al. [52] provide information on
the methods currently employed for the upgrading of bio-oil regarding the production of
valorized fuels. Catalytic fast pyrolysis is regarded as the most encouraging method for
the efficient production of bio-oil (Figure 1). The atmosphere in which the reaction takes
place needs to be inert, while the temperatures can be between 400 and 600 °C. High heat-
ing and cooling rates (1000–10,000 K/s) and short residence times (1–2 s) are also neces-
sary. This process helps obtain liquid with a yield of up to about 75 wt.%; the yield for the
gas and for the char products is typically between 10 and 20 wt. % and 10 and 15 wt. %,
respectively [53]. In recent years, the coupling of the fast pyrolysis of biomass with the
catalytic SR of the pyrolytic oils has received considerable attention [26,31,54].
Catalysts 2021, 11, 1526 5 of 36
Figure 1. Schematic representation of the fast pyrolysis of biomass for H2 production.
Due to the complexity of bio-oil, previous studies usually focus on the use of model
compounds. The most studied of these compounds is acetic acid as it is one of the main
constituents of bio-oil [39,55–82]. Numerous literature reports can also be found concern-
ing the steam reforming of bio-alcohols such as methanol [14,56,58,72], ethanol
[14,58,70,83–88] and glycerol [14,89–94]. The reforming of acetone has also attracted con-
siderable attention [56,58,63,69,70,77,95–97]. Other model compounds systematically
tested include hydroxyacetaldehyde, cellulose and lignin [72], phenol [70,74,98], acetol
[38,74,99], m-cresol, furfural and guaiacol [58,89]. The use of model compounds, i.e.,
knowing exactly the composition of the feed entering the reactor, also provides the ad-
vantage that it allows the comparison of different catalytic systems, provided that similar
experimental conditions are used. The analysis of the liquid products is also less complex
as the number of products is limited [23,53].
3. Mechanism of Bio-Oil Steam Reforming
A major advantage of the production of hydrogen through catalytic SR is that bio-oil
dehydration, a rather expensive process, can be avoided [100]. Steam reforming (SR) is an
endothermic equilibrium reaction between an organic compound and steam in the pres-
ence of a catalyst. It results in the formation of a mixture of hydrogen and carbon monox-
ide (Equation (1)) and, usually, it is accompanied by the water gas shift reaction (Equation
(2)) [101,102]:
CnHmOk + (n − k) H2O nCO + (n + m/2 − k) H2 (1)
nCO + nH2O nCO2 + nH2 (2)
The general chemical reaction for the steam reforming of bio-oil is as follows [15,95]:
CnHmOk + (2n − k) H2O nCO2 + (2n + m/2 − k) H2 (3)
Generally, other undesirable reactions take place when oxygenates react over a metal
surface and so the hydrogen yield is lower than the stoichiometric yield. Carbon monox-
ide and dioxide methanation (Equations (4) and (5)), methane reforming (Equations (6)
and (7)), and C2 steam reforming (Equation (8)) are amongst the secondary reactions that
occur during bio-oil steam reforming [44]. A major issue is carbon deposition, which can
lower hydrogen production and shorten the catalyst life expectancy. The partial thermal
decomposition of oxygenates (Equation (9)) and the Boudouard reaction (Equation (10))
are the main solid carbon forming reactions [53].
Catalysts 2021, 11, 1526 6 of 36
CO + 3H2 CH4 + H2O (4)
CO2 + 4H2 CH4 + 2H2O (5)
CH4 + 2H2O CO2 + 4H2 (6)
CH4 + CO2 2CO + 2H2 (7)
C2Hn + 2H2O 2CO + (n/2 + 2) H2 (8)
CnHmOk CxHyOz + gases (H2, CO, CO2, CH4) + coke (9)
2CO C + CO2 (10)
Carbon deposits are also produced from methane decomposition (Equation (11)), car-
bon monoxide and dioxide decomposition (Equations (12) and (13)), and ethene polymer-
ization (Equation (14)) [12].
CH4 C + 2H2 (11)
CO + H2 C + H2O (12)
CO2 + 2H2 C + 2H2O (13)
nC2H4 Carbon (14)
Bio-oil oxygenates conversion is given as the ratio of the moles of carbon converted
to products (gaseous and liquids) to the moles of carbon in the feed, as shown in Equation
(15). Alternatively, conversion can be calculated by the quantity of organic feed that re-
mains unconverted in the liquid effluents [63]. Catalytic activity may also be gauged by
the calculation of H2 selectivity, which is the percentage molar or mass concentration of
hydrogen in the product stream. The selectivity (Equation (16)) of a product is in relation
to the other competing products, while the yield (Equation (17)) calculation is based on
the quantity of feed [23].
% Conversion =
      
      × 100 %
(15)
% X Selectivity =
  
    () × 100 %
(16)
% X Yield =
   
     × 100 %
(17)
In Equations (16) and (17) X represents the products found at the outlet of the reactor
(e.g., H2, CO, CO2, CH4). The yield of hydrogen cannot be equal to the stoichiometric max-
imum because of the undesirable production of CO and CH4 which are formed during
reforming via reverse water gas shift and methanation reactions [63].
Catalysts 2021, 11, 1526 7 of 36
4. Operating Parameters Affecting Bio-Oil Steam Reforming
The operating parameters affecting the SR of bio-oil include the composition of the
feedstock, the reactor type, the reaction temperature, the space velocity and the steam to
carbon (S/C) ratio, which means that in the effort to approach stoichiometric yields, a wide
range of combinations have been tested. Generally, a higher bio-oil conversion is favored
at higher reforming temperatures, low pressures and higher steam to carbon ratios.
4.1. Effect of the Feed Composition
Comparative studies on the SR of different organic molecules derived from bio-oil
(e.g., furfural, formic acid, methanol, acetic acid, ethanol, acetaldehyde, guaiacol, acetone)
have demonstrated that the molecular structures have a large influence on the reactivity
and tendency to coking. Bimbela et al. [38,39] investigated acetic acid, acetol and n-butanol
steam reforming using co-precipitated Ni/Al2O3 catalysts of different Ni contents (23, 28
and 33%) and concluded that reactivity followed the order: acetic acid > acetol > n-butanol
(Figure 2); the rate of catalyst deactivation due to coke deposition followed the opposite
trend. Similarly, Baviskar and Vaidya [103] reported that the conversion of oxygenates
with different functional groups was butyraldehyde > ethyl acetate > 1-methoxy-2-propa-
nol > 2-butanone; coke formation followed the opposite trend. These results suggest that
carbonyls are easier to convert in comparison with hydroxyls.
Figure 2. Comparison of the dependence of carbon conversion with time for acetic acid, acetol and
n-butanol tested at GHSV around 30,000 h−1, 650 °C and 33%Ni/Al2O3 catalyst. Reproduced with
permission from [38]. Copyright Elsevier, 2021.
The effect that molecular structures have on the conversion of formic, acetic, propi-
onic and butyric acids (i.e., carboxylic acids) during steam reforming, was examined by
Li et al. [55]. It is noted that carboxylic acids are usually found at around 5 wt. % in bio-
oil, which means that the clarification of their reaction behavior is essential for the opti-
mization of the SR process. The authors concluded that an increase in the length of the
aliphatic chain led to a decrease in the conversion rate (Figure 3).
Catalysts 2021, 11, 1526 8 of 36
Figure 3. Comparison of the dependence of carbon conversion with temperature for four carboxylic
acids tested at S/C = 5, LHSV = 12.7 h−1, P = 1 atm and 20% Ni/Al2O3 catalyst. Reproduced with
permission from [48]. Copyright Elsevier, 2021.
Zhang et al. [58] carried out a study on the steam reforming of methanol, formic acid,
acetone, acetic acid, acetaldehyde, ethanol, furfural and guaiacol and concluded that the
structure of the feed molecules has a significant impact on reactivity, with methanol and
formic acid reformed at low temperatures due to the absence of the cracking of C-C bonds
involved in their conversion. Li et al. [56] supported the above finding as they concluded
that the reforming of acetic acid is relatively easier in comparison to that of acetone, as the
former has a lower molecular size. Ortiz-Toral et al. [40] also observed that aqueous bio-
oil fractions with higher concentrations of lower molecular-weight oxygenates, such as
acetic acid and acetol, converted more effectively into H2, whereas the existence of heavier
molecules, such as levoglucosan, furfural and phenolics compounds significantly im-
pacted the time-on-stream catalytic stability. Moreover, the large oxygenate compounds
contained in bio-oil do not vaporize easily upon entering the reactor, which means that
there is a risk of blockage in the feeding line and/or the reactor by residual solids [72].
Table 4 summarizes the effect of different model compounds as feedstock in the steam
reforming process.
Catalysts 2021, 11, 1526 9 of 36
Table 4. Summary of the literature on the effect of different model compounds as feedstocks in the steam reforming pro-
cess.
Type of Feed 1 Catalyst 2 Experimental Conditions Comments Ref.
acetic acid, acetol
and n-butanol,
separately
23, 28 and 33%
Ni/Al2O3
fixed bed quartz reactor; 0–
8.70
g catalyst min/g model com-
pound; feeding rate: 0.15, 0.17,
0.23 mL/min; GHSV = 28,500,
20,000, 57,000 h−1; S/C = 5.58,
14.70; P = 1 atm; T = 550–750 °C
28% Ni provide the highest H2
yield at
650 °C. Increasing temperature en-
hanced the yields to H2, CO and CO2.
[38,39]
2-butanone, 1-
methoxy-2-propa-
nol, ethyl acetate,
butyraldehyde,
separately
20% Ni/Al2O3
fixed bed quartz reactor;
1.5 g of
catalyst; flow rate: 0.25–1
mL/min; S/C = 15–25
; P = 1 atm;
T = 350–500 °C
45.4% H2 yield at 500 °C with 2-buta-
none; 51.1% H2 yield at 500 °C with 1-
methoxy-2- propanol; 52.8% H2
yield
at 500 °C with ethyl acetate; 54.2% H2
yield at 500 °C with butyraldehyde
[103]
formic acid, acetic
acid, propionic
acid and butyric
acid, separately
20% Ni/Al2O3
fixed bed quartz reactor;
500 mg
of catalyst LHSV = 12.7 h−1;
GHSV = 49,317 h−1 at 300 °C and
79,848 h−1 at 700 °C;
S/C = 5; P =
1 atm
The increase of the length of the ali-
phatic carbon chain inhibited reform-
ing reactions, led to lower yields of H2
and to increased coking.
[55]
methanol, ethanol,
formic acid, acetic
acid, acetaldehyde,
acetone, furfural,
guaiacol, sepa-
rately
15% Ni-5%
La/Al2O3
fixed bed quartz reactor;
0.5 g of
catalyst; flow rate: 0.12 mL/min
;
LHSV = 12.7 h−1, S/C = 5; P = 1
atm; T = 300–600 °C
The reforming of methanol and formic
acid was achieved at a low tempera-
ture; coking was minimized
. Ethanol,
acetic acid, acetaldehyde or acetone
required higher reforming tempera-
tures; significant coke deposition, es-
pecially for acetone or acetaldehyde.
The coke derived during the SR of fur-
fural and guaiacol was more graphite-
like (difficult to oxidize)
[58]
methanol, acetic
acid, acetone sepa-
rately
15% Mn/Al2O3,
15% Fe/Al2O3,
15% Co/Al2O3,
15% Ni/Al2O3,
15% Cu/Al2O3,
15% Zn/Al2O3
and
unsupported Mn,
Fe, Co, Ni, Cu, Zn
fixed bed quartz reactor;
0.50 g
of catalyst; flow rate: 0.12
mL/min; LHSV = 12.7 h−1;
S/C =
5; P = 1 atm
Ni and Co catalysts were more active
than Mn, Fe or Zn. Alumina helped
enhance metal dispersion. Coke
formed during the SR of acetic acid
was more aromatic than that formed
during the SR of acetone.
[56]
bio-oil aqueous
fraction 11% Ni/Al2O3
fixed bed quartz reactor;
WHSV
= 0.87 h−1; flow rate: 4.0 mL/h;
S/C = 4, 8, 12,18; T = 500–700 °C
H
2
production was enhanced when
low MW species (acetic acid and ace-
tol) were used and declined when
higher MW species (
levoglucosan and
furfural) were used.
[40]
1 aqueous solution of every model compound is used as feedstock; 2 wt. %.
Catalysts 2021, 11, 1526 10 of 36
4.2. Effect of the Reactor Type
As mentioned above, coke deposition is a major obstacle in the SR of bio-oil or its
model compounds as it leads to the deactivation of the catalyst and the fouling of the
reactor. To avoid reactor fouling, a variety of specially designed reactors have been pro-
posed. These include two-stage pyrolysis-reforming, separate fixed bed and fluidized bed
reactors, micro-reactors, and membrane, spouted bed, and nozzle-fed reactors [23,26,104].
Fixed bed reactors are more commonly used and, as shown in Figure 4, they are typically
made up of a cylindrical vessel packed with catalyst pellets. However, fixed bed reactors
are susceptible to coke deposition over the catalyst surface, limiting the operating time
and hydrogen yield due to the large amount of residue formed, especially when reforming
larger model compounds or crude bio-oil [105]. Thus, this type of reactor is preferred for
the reforming of lighter model compounds, such as acetic acid and ethanol. In contrast,
fluidized bed reactors with continuous operation tackle the reactor blockage by the gasi-
fication of carbonaceous deposits [104,106,107]. Comparing these two types of reactors,
fixed bed reactors are easy to design, control, and operate and have lower maintenance
costs, but coke formation is a challenging issue.
Figure 4. Schematic diagram of a typical fixed-bed reactor. Reproduced with permission from [79].
Copyright Elsevier, 2021.
A triple-nozzle spraying system has also been employed for the aqueous bio-oil frac-
tion [108,109] with Basagiannis et al. [109] arguing that carbon deposition is minimized to
a great extent when the liquid is fed into the reactor using high flow rate nozzles. Moreo-
ver, Kechagiopoulos et al. [110,111] examined the SR of ethylene glycol and the aqueous
phase of bio-oil in a pilot scale spouted bed reactor and argued that surface carbon can be
minimized if the hot particles are rapidly mixed with cold reactants along and the constant
cyclical movement of solid particles.
Catalysts 2021, 11, 1526 11 of 36
4.3. Effect of Temperature
Being an endothermic process, the SR of bio-oil is carried out at high temperatures.
Thermodynamics suggest that the yield to hydrogen is maximized around 550 °C, above
this temperature, the yield gradually declines due to coke formation and the competing
thermal cracking reactions of the organic compounds [78]. Lower temperatures should be
avoided as they favor the formation of unstable by-products through decomposition and
dehydration of the feed molecules. The presence of high amounts of steam in the process
can favor the water-gas shift reaction and the hydrogen produced can combine with CO
to generate CH4. The water-gas shift reaction and methanation are both exothermic reac-
tions and take place at low temperatures. When the reaction temperature is below 500 °C,
CH4 is the thermodynamically favored product. With a rise in temperature, H2 and CO2
formations increase [51]. The reactivity of the organic molecules is also affected by their
molecular structures. For example, methanol and formic acid require lower temperatures
for their reforming, while ethanol, acetic acid, acetaldehyde, or acetone require higher
temperatures to crack the C-C bonds in the aliphatic carbon chain [55,58].
4.4. Effect of Pressure
Vagia and Lemonidou [112,113] have conducted thermodynamic studies and inves-
tigated the effect of pressure on the steam reforming process. According to the authors,
the equilibrium shifts in favor of the lighter chemical species, such as hydrogen, when the
pressure is lowered. Thus, the SR of bio-oil is usually carried out at atmospheric pressure,
which enhances H2 selectivity and ensures the optimum yield. Higher pressures lead to a
drop in hydrogen production.
4.5. Effect of Space Time and Liquid Feed Rate
Space time is another important variable that affects the SR of bio-oil. A high space
time promotes the reforming and the WGS reactions, and hence improves the hydrogen
yield. At low space velocity values, the RWGS reaction is favored, which leads to an in-
crease in CO concentration and a decrease in CO2. Meanwhile CH4 and light hydrocarbons
(secondary products produced mainly from cracking reactions) disappear [22,54].
The effect of the rate at which liquid is fed has also been investigated. It is known
that the partial pressure in the reaction bed is increased when a higher liquid feed rate is
used and since the rate of the reaction depends directly on the concentration of reactants,
the hydrogen yield is increased [102]. In general, the steam reforming performance will
approach the thermodynamic equilibrium if sufficient contact time is allowed between
the feedstock and the catalyst.
4.6. Effect of the Steam-to-Carbon Ratio
The steam to carbon (S/C) ratio is a critical process parameter that determines the
distribution of products. This is because the feedstock used and the steam contest for the
active sites exist on the surface of the catalyst [12]. The S/C ratio (Equation (18)) is given
by dividing the amount of steam by the total amount of carbon in the feedstock (taking
into account the H2O content) [23].
S/C =
  
       ( )
(18)
The H2 yield can be maximized by promoting the WGS reaction, which requires the
use of high steam partial pressure (high S/C ratio) as it favors the adsorption of steam on
the catalyst active sites. On the other hand, the use of low S/C values favors the decompo-
sition of the feedstock, which promotes the formation of CH4 and CO and diminishes the
yield to H2 [102]. Moreover, low S/C ratios also promote the deposition of carbon. On the
contrary, high S/C ratios can promote the gasification of solid carbon [114]. However, the
drawback of using a high S/C ratio is that additional energy and heat are required to sep-
arate the steam from the products [115,116].
Catalysts 2021, 11, 1526 12 of 36
5. Catalyst Developments in the Bio-Oil Steam Reforming
As is well understood, the ideal bio-oil reforming catalyst should exhibit: (i) high
reforming activity, (ii) high selectivity towards hydrogen generation, (iii) resilience to-
wards deactivation by carbon deposition and/or metal particle sintering, and (iv) the abil-
ity to cope with the presence of O2-containing functional groups. The following subsec-
tions provide an overview of the state of the art and the recent advances made on the
utilization and development of bio-oil steam reforming catalysts.
5.1. Noble Metal-Based Catalysts
Noble metals such as Ru, Pt, Pd, Rh and Ir are known to exhibit high catalytic activity
and hydrogen selectivity during the SR of bio-oil, as they have exceptional ability to cleave
the C-C bonds. Moreover, noble metals show low propensity to form coke [64,68–
70,95,109]. Jeong and co-workers [117] proved that the activity during the SR of acetic,
propionic and butyric acid, decreased in the order of Ru > Pd ~ Rh > Pt > Ni. Vagia et al.
[63] investigated the steam reforming of acetic acid and acetone using catalytic systems
with 5 wt. % Ni and 0.5 wt. % Rh that were based on CaO·2Al2O3 and 12CaO·7Al2O3. The
authors concluded that the Rh-based catalyst was more active at higher temperatures than
Ni in the SR of acetic acid but provided only a slightly higher H2 yield in the SR of acetone.
The catalysts that included noble metals as part of the active phase also showed the lowest
coke deposition.
The nature of the support also plays a major role in the reforming reactions and basic
oxides, such as La2O3, CeO2, MgO and CaO, have generally been shown to enhance the SR
activity. Moreover, the addition of alkali species can modify the interaction between the
adsorbed species and the active metal. Basagiannis and Verykios [81] compared the per-
formances of Pd, Pt, Ru, Rh and Ni-based catalytic systems supported on Al2O3 modified
with La, Mg and Ce in the SR of acetic acid and showed that the most active systems were
those based on Ni, Rh and Ru. Moreover, the authors were able to prove that the catalytic
systems based on La2O3/Al2O3 and MgO/Al2O3 remained active during long-term stability
tests.
Vagia and Lemonidou [64] used CeO2–ZrO2-mixed oxides as supporting material
and prepared catalytic systems with Ni and Rh as active phase. The authors then tested
the performance of these systems in the SR of acetic acid and observed low carbon depo-
sition after stability tests. The authors concluded that this was due to the synergy of the
support and metal, which enhanced the oxygen exchange reactions. In another interesting
work, Rioche et al. [70] reported the highest H2 yield (75%) at 800 °C using Rh, Pd and Pt
catalysts supported on CeZrO2, which was significantly higher in comparison to Al2O3-
based catalysts. Takanabe et al. [68,69,95] suggested a bifunctional mechanism for the SR
of acetic acid over Pt/ZrO2 catalysts. Specifically, the authors argued that the activation of
acetic acid takes place on the metal sites and that steam gets activated on the support. The
acetic acid conversion was 100% during the entire experiment, but the H2 yield dropped
drastically after only 25 min of reaction time (though the activity of Pt supported by ZrO2
was maintained for longer).
Table 5 provides a summary of works that have utilized noble metal-based catalysts
for the SR of bio-oil. As evidenced, the loading of these metals on supports is very low
leading to low availability of total metal sites. Therefore, in order to boost feed conversion,
a higher reaction temperature (>600 °C) is required. Furthermore, a well-known disad-
vantage of noble metal catalysts is their high cost which limits practical utilization and
industrial implementation. The combination of noble and transition metals could amelio-
rate this limiting factor and also help to augment the resistivity towards coking.
Catalysts 2021, 11, 1526 13 of 36
Table 5. Summary of the literature on bio-oil steam reforming using noble metal-based catalysts.
Type of Feed 1 Catalyst Prep. Method
Experimental Conditions Comments Ref.
acetic acid, ace-
tone, separately
5% Ni, 0.5% Rh
supported on
CaO·2Al2O3 and
12CaO·7Al2O3
wet impreg-
nation
fixed bed quartz reactor; 0.05
g catalyst diluted with 0.10 g
quartz particles; GHSV =
34,500 h−1 for acetic acid &
28,500 h−1
for acetone; S/C = 3;
P = 1 atm; T = 550–750 °C
Ni/CaO·2Al2O3 showed
highest H2 yield and
Rh/CaO·2Al2O3 showed
highest coking resistance.
[63]
acetic, propionic,
butyric acid, sepa-
rately and mixture
(HAc:HPr:HBu
6:1:3)
5% Ru, Pd, Rh, Pt,
Ni supported on
Al2O3
incipient wet-
ness impreg-
nation
fixed bed quartz
reactor; 200
mg catalyst; GHSV = 25,000
h−1; S/C = 9; T = 300–600 °C
Activity decreased in the
order of Ru > Pd ~ Rh > Pt >
Ni.
[117]
acetic acid
1% Pt, 1% Pd, 0.5%
Rh, 1 and 5% Ru,
17% Ni supported
on Al2O3, 15%
La2O3/Al2O3,
15%
MgO/Al2O3 and
30% CeO2/Al2O3
wet impreg-
nation
fixed bed micro-reactor; 100
mg catalyst; flow rate: 290
cm3
/min; S/C = 4; P = 1 atm; T
= 550–800 °C
Ni- and Ru-
based catalysts
present higher activity and
selectivity (approximately
100% at 750 °C). Ru cata-
lysts show long-term stabil-
ity (for ~35 h).
[81]
acetic acid, phenol,
acetone, ethanol,
separately and
raw bio-oil
1% Pt, Pd and
Rh/Al2O3 and
CeO2–ZrO2
(15/85%)
incipient wet-
ness impreg-
nation
fixed bed quartz reactor; 100–
200 mg catalyst; GHSV = 3090
h−1; S/C = 5–10.8; T = 650–
950
°C
Order of activity: 1% Pd-
Al2O3 < 1% Pt-Al2O3 < 1%
Pd-CeZrO2 < 1% Rh-Al2O3
<
1% Pt-CeZrO2 < 1% Rh-
CeZrO2.
[70]
acetic acid
5% Ni, 0.5%
Rh/CeO2–ZrO2
(15/85)
wet impreg-
nation
fixed bed quartz reactor; 50
mg of catalyst diluted with
100 mg quartz; GHSV = 34.500
h−1; S/C = 3; P = 1 atm; T = 550–
750 °C
Ni and Rh metals enable
the reforming reactions to
proceed with high rates
even at 650 °C
. Lowest coke
deposition for the Rh cata-
lysts.
[64]
acetic acid 0.5% Pt/ZrO2 wet impreg-
nation
fixed-bed reactor; 50 mg cata-
lysts; WHSV = 9.0 h−1
; GHSV =
160,000 h−1; S/C = 5; T = 500–
700 °C
Pt was essential for the SR
to proceed. ZrO2 helped ac-
tivate steam.
[68]
acetic acid 0.5% Pt/ZrO2 wet impreg-
nation
fixed bed reactor; 50–200
mg
catalysts; SV = 40,000–
160,000
h−1; S/C = 5; T = 600–800 °C
Pt/ZrO2
was initially active
but then deactivated rap-
idly due to the blockage of
the Pt-related active sites.
[69]
acetic acid 0.5% Pt/ZrO2 wet impreg-
nation
fixed bed reactor; 10–50 mg
catalysts; GHSV = 320,000 or
1,600,000 h−1; S/C = 5; T = 400–
700 °C
H2O activated on ZrO2 to
create additional surface
hydroxyl groups.
[95]
1 aqueous solution of every model compound is used as feedstock.
Catalysts 2021, 11, 1526 14 of 36
5.2. Transition Metal-Based Catalysts
Transition metals have attracted great interest as catalysts for steam reforming reac-
tions due to their low cost and good catalytic activity. As is well understood, the choice of
support has a crucial role on the properties of the catalytic system, as it affects the disper-
sion of the active phase over the surface of the carrier, the stability shown by the catalytic
system through the degree of interaction achieved between metal and support. Moreover,
the support can influence the reaction pathway and the deposition of carbon [118]. For the
SR of bio-oil in particular, the organic molecules are dissociatively adsorbed on metal
sites, whereas H2O molecules are adsorbed on the supporting metal oxide surface (e.g.,
Al2O3, MgO).
5.2.1. Ni-Based Catalysts
Ni is known to exhibit great capacity to break C–C and C–H bonds. For this reason,
nickel is regarded as highly efficient for the SR of raw bio-oil, its aqueous fraction and
oxygenate model compounds. This is demonstrated by numerous studies which have re-
ported very high values for bio-oil conversion and H2 selectivity [77,79,100]. Al2O3 is fre-
quently used to support Ni catalysts due to its high surface area and high thermal and
chemical stability [76,77,79]. It can also have a large influence on the stability of the cata-
lyst, by enhancing the dispersion of the metals and providing active sites that are more
accessible to the reactants. Chornet and co-workers [41,42,72,108,119] extensively studied
the catalytic SR of bio-oil over various research and commercial Ni-based catalysts. Ex-
periments on the steam reforming of bio-oil aqueous fraction over Ni/α-Αl2O3 at 825 °C,
S/C = 4.92 and HSV = 126,000 h−1 have shown an initial hydrogen yield of about 90% (the
remaining 10% being CH4 and coke) which dropped by about 30% after 25 min of experi-
ments due to partial deactivation by coking [41]. Support modified Ni/MgO-Al2O3 and
Ni/MgO-La2O3-Al2O3 catalysts exhibited superior performance with higher hydrogen
yields and significantly slower deactivation. Czernik et al. [120] compared the perfor-
mance of commercial and laboratory prepared Ni-based catalysts during the SR of raw
bio-oil and reported H2 yields up to 80% of the stoichiometry. This result was achieved at
850 °C, S/C = 5.8 and a CH4 equivalent space velocity of 920 h−1. The commercial naphtha
reforming Ni-based C11-NK (Sud-Chemie) catalyst showed somewhat higher activity
than four NREL prepared Ni-based catalysts and a remarkable stability on the yield of H2
over a 18 h time-on-stream experiment. Despite this fact, catalyst deactivation due to the
methanation of CO and the thermal cracking of complex bio-oil molecules was still a de-
tectable problem. Generally, an increase of the Ni content in the catalytic system up to 10–
15% increases the conversion of oxygenates, but attention should be paid, as very high Ni
loadings lead to extensive sintering [121].
Another common feature in the catalyst design is the incorporation of additives or
promoters. Examples include the addition of an alkali or alkaline earth metals, often in
high surface area supports, such as Al2O3, in order to combine with adjacent catalytically
active metal sites for SR [122,123]. As CaO has good chemical and thermodynamic prop-
erties, it is considered an efficient solid sorbent that can aid enhanced activity [63,65,66,84].
A number of research works have also reported that the incorporation of K and La into
the support, in small concentrations, can help prevent sintering and the formation of car-
bon [124]. In addition, the use of La as a promoter is known to aid the dispersion of the
active metal and to promote the production of H2 [72,74,80,125]. Galdamez et al. [126]
synthesized Ni/Al2O3 catalysts using co-precipitation and investigated the extent to which
the loading of La2O3 onto the catalysts affected the H2 yield. They observed that during
the non-catalytic SR, the yields of hydrogen and carbon dioxide were very low. Valle et
al. [43] worked in a similar vein to the Galdamez group and concluded that the good per-
formance of Ni/Al2O3 modified with La was due to its capacity for water adsorption, and
thus enhancement of WGS reaction. Alternatively, MgO may be used to promote Al2O3 as
Catalysts 2021, 11, 1526 15 of 36
the formation of a magnesium-aluminum spinel phase is thought to enhance the adsorp-
tion of steam [44,98,127]. Garcia et al. [41] studied the SR of the aqueous fraction of bio-oil
using La and Mg modified alumina. The authors concluded that the presence of these
modifiers enhanced steam adsorption and helped the gasification of surface carbon. Ca,
Ce, Mg, Mn and Zn were also used as modifiers on Ni/Al2O3 catalysts by Yao et al. [128]
in the SR of the aqueous fraction of bio-oil. Ni/MgO-Al2O3 catalyst showed the highest H2
yield, equal to 56.46%, followed by Ni/CeO2-Al2O3 (55.30%) and Ni/ZnO-Al2O3 (52.01%).
5.2.2. Other Transition Metal-Based Catalysts
Although Ni has been by far the most investigated metal [53,129,130], other transition
metals can also provide high activity at moderate temperatures. Li et al. [49] examined the
performance of mono, unsupported Mn, Fe, Co, Ni, Cu and Zn, and also, mono Mn, Fe,
Co, Ni, Cu and Zn supported on Al2O3, in the SR of methanol, acetic acid and acetone. The
authors showed that Ni- and Co-based materials had enhanced activity in comparison to
Mn, Fe or Zn, due to the low capacity of the latter to break the chemical bonds of the
organics or to activate steam. They also showed that the unsupported Cu-based catalyst
was significantly less stable than Cu/Al2O3. However, the authors also observed that the
unsupported Ni-based catalyst was more resistant towards coking in comparison with
Ni/Al2O3.
Additionally, mesostructured materials, such as SBA-15 silica, are thought to consti-
tute promising supporting materials because they can help to improve the dispersion of
the metallic phase. This property is the result of their mesoporous structure, which allows
metal particles to diffuse through the available channels. Thus, sintering may be avoided
through the increased interaction achieved between the support and the active [131]. Viz-
caíno et al. [84] investigated a novel application of Co-based catalysts supported on SBA-
15 and Mg or Ca modified SBA-15 in the SR of ethanol. The authors were able to show
that the use of these modifiers resulted in an improvement of the dispersion of the active
phase, increased metal-support interaction, and enhanced the materials’ basicity. Despite
these improvements, ethanol conversion and H2 production were clearly lower compared
to analogous Ni catalysts. This was attributed to the higher reduction temperatures re-
quired for their activation. Megía et al. [65,66] also analyzed the effect of Ca addition to
Co/SBA-15 in the SR of acetic acid. The authors observed that higher temperatures were
necessary to activate the catalysts, which they attributed either to the stronger interaction
between the active phase and the support or to the formation of a new Ca-Co compound.
Additionally, the authors investigated the effect on the catalytic activity of the addition of
Cu, Ag, Ce and Cr, i.e., Co-M/CaSBA-15 (M:Cu, Ag, Ce, Cr). These promoters not only
helped obtain increased metallic dispersion and stronger metal-support interaction, but
also increased the reducibility leading to higher acetic acid conversion. The Co-Ce/CaSBA-
15 catalyst showed the highest conversion value and the highest reducibility. The addition
of CeO2 in the catalyst enhanced the population of oxygen vacancies and aided the dis-
persion of metal over the support. As a result, an improvement in the H2 yield was ob-
served and coke gasification and WGS reaction were promoted. However, it was also ob-
served that Cu could not improve the yield of H2, as it favored the decarboxylation of
acetic acid rather than its SR. Regarding the role of Ag, it was found to improve catalytic
performance, but it also promoted the formation of carbonaceous species. The perfor-
mance of Co-Cr/SBA-did not differ much in comparison to the performance of Co-
Ce/CaSBA-15, but the presence of Cr raises questions about the material’s toxicity [132].
Generally, the characteristics of Co-based catalysts are similar to Ni-based systems, how-
ever, Co is susceptible to particle agglomeration [65,66,77,84,96].
Fe is one of the most commonly used metals due to its capacity to break the C―C
bond however, it also shows poor reactivity and weak reducibility [98]. Cu-based catalysts
can also break the C―H bond and are considered active for the SR of methanol, but do
not have the capacity to scissor the C-C bond in acetic acid or acetone [56] Table 6 sum-
marizes the experimental conditions and the catalysts used in the above studies.
Catalysts 2021, 11, 1526 16 of 36
Table 6. Summary of the literature on bio-oil steam reforming using transition metal-based catalysts.
Type of
Feed 1 Catalyst Preparation
Method Experimental Conditions
Comments Ref.
acetic acid,
acetone,
separately
25% Ni/Al2O3 incipient
wetness
impregnation
fixed bed quartz reactor;
0.2 g catalyst with equal
amount of quartz; flow
rate = 0.2 mL/
min; LHSV =
12.1 h−1; S/C = 6; P = 1
atm;T = 450–700 °C
Ni/Al
2
O
3
was highly selective
and stable after
the suppression
of the presence of the Ni species
which strongly interacted with
alumina resulting in the for-
mation of by-products and cok-
ing.
[77]
acetic acid
18% Ni/Al2O3 commercial cata-
lyst
fixed bed quartz reactor; 2
g catalyst; flow rate: 0.033
6
mL/min; S/C = 2–3; P = 1
atm; T = 550–750 °C
H2 yield was 76.4% of equilib-
rium. HAc conversion was
88.97% at 750 °C. Chemical loop-
ing reforming technology.
[79]
acetic acid
Ni/Al2O3
(Ni/Al 1:2),
Ni/La2O3-Al2O3 (8
and 12% La)
co-p
recipitation
fluidized bed stainless
steel reactor; flow rate:
1.84–2.94 g/min; GHSV =
13,000 h−1; S/C = 5.58; T =
450–700 °C
yield of H2 was 0.029 g/g acetic
acid at 650 °C. [126]
bio-oil
aqueous
fraction
10% Ni/a-Al2O3
, 10%
Ni/La2O3-a-Al2O3
(10% La)
wet impregna-
tion
fluidized bed reactor; 0.10–
0.45 g catalyst h/g bio-
oil;
GHSV = 8100–8140,300 h−1
;
S/C = 12; P = 1 atm; T =
600–800 °C
La2O3 improves the H2
yield and
selectivity. [43]
bio-oil
aqueous
fraction
Ni-based catalyst
with dolomite (CaO-
MgO) sorbent
commercial cata-
lyst (Z417)
fixed bed quartz reactor; P
= 1 atm; T = 550–650 °C
75% H2
yield at 600 °C. Chemical
looping technology (CO2 cap-
ture).
[52]
acetic acid
17% Ni/γ-Al2O3 pro-
moted with 15%
Mg, La, Cu, and K
incipient w
etness
impregnation
fixed bed reactor system;
0.044 mL catalyst volume;
S/C = 5.3; P = 1 atm; T =
450–600 °C
With Mg promoter ~100% of H2
and carbon selectivity, even at
450 °C.
[80]
acetic acid
phenol
15% Ni/Ash (SiO2,
Al2O3, Fe2O3, CaO,
MgO, K2O, and
Na2O)
wet impregna-
tion
fixed-bed reactor; 1 g cata-
lyst; WHSV = 4 h−1; S/C =
9.2, 7.5, 5, 2.5, 1; T = 500–
800 °C
98.4% acetic acid conversion and
83.5% phenol conversion, 85.6%
H2
yield from acetic acid SR and
79.1% H2 yield from phenol SR,
at 700 °C.
[125]
acetic acid
15% Ni-MgO/γ-
Al2O3 (1,5,10% Mg)
wet impregna-
tion
fixed bed quartz reactor;
100 mg of catalyst; flow
rate = 0.25 mL/h; T = 500–
600 °C
15% Ni-5% Mg/Al2O3 more selec-
tive for H2
production with high
stability and sintering resistance
ability.
[127]
bio-oil
aqueous
fraction
Ni/Al2O3, Ni/MgO-
Al2O3 Ni/MgO-
La2O3-Al2O3 (15%
Ni, and mole ratios
of Mg/Ni = 1, Ni/La
= 8)
wet impregna-
tion
fixed bed microreac-
tor/
molecular beam mass
spectrometer; 3 cm high
catalyst with quartz chips
;
GHSV up to 126,000 h−1;
S/C = 4.92–11; T = 825–875
°C
Mg and La
promoters enhanced
steam adsorption. [41]
raw bio-oil
Ni-based naphtha commercial cata-
lyst (C11-NK)
fluidized bed reactor;
flow
rate = 120–300 g/h;
GHSV =
Yields approached the theoreti-
cally possible for stoichiometric
conversion at 850 °C.
[42]
Catalysts 2021, 11, 1526 17 of 36
700–1000 h−1;
S/C = 7, 9; T =
800–850 °C
raw bio-oil
Ni/ZrO2, Ni/Al2O3
(0, 5.6, 10.7, 14.1,
18% Ni)
wet impregna-
tion
fixed bed stainless steel re-
actor; 0.2 g catalyst;
WHSV
= 13 h−1; S/C = 5; T = 850 °C
61% H2 yield with 5.6% and
10.7% Ni/ZrO2 at 850 °C, 65% H2
yield with 14.1% Ni/Al2O3
at 850
°C.
[133]
bio-oil
aqueous
fraction
Ni/CeO2-ZrO2 (5,
7,5, 10, 12% Ni and
5, 7,5, 10% Ce)
co-p
recipitation
and wet impreg-
nation
fixed bed quartz reactor;
S/C = 4.9; P = 1 atm;T =
450–800 °C
69.7% H2 yield with 12% Ni/
7.5%
Ce-Zr-O at 800 °C. [45]
acetic acid
Ni/Ce-Zr-O (0, 2.5,
5, 7.5, 10, 12% Ni
and 0, 2.5, 5, 7.5,
10% Ce)
co-precipitation
and wet impreg-
nation
fixed bed quartz reactor;
3
g catalyst; LHSV = 3–11,5
h−1; S/C = 0–
3.5; P = 1 atm;
T = 500–900 °C
83.4% H2 selectivity and 0.39%
CH4
selectivity with 12% Ni/7.5%
Ce-Zr-O at 650 °C; S/C = 3;
LHSV
= 2.8 h−1.
[59]
acetic acid
15% Ni/CeO
2
-ZrO
2
-
CaO with diff
erent
Ce/Zr/Ca ratios of
0.2:1:5, 1:1:5, 1.2:1:5,
and1.5:1:5
sol−gel and wet
impregnation
fixed bed reactor; 2 g cata-
lyst; flow rate = 0.96 mL/h
;
LHSV = 0.48 mLg−1h−1;
S/C
= 4; T = 550–750 °C
83% H2 yield with Ni/Ce1.2Zr1Ca
5
catalyst at 550 °C; Sorption en-
hanced steam reforming.
[57]
acetic acid
Ni/ATC (Atta-
pulgite Clay)
precipitation,
wet impregna-
tion, and me-
chanical blend-
ing
fixed bed stainless steel re-
actor; 3 g catalyst; flow
rate = 14 mL/h; P = 1 atm;
T = 550–650 °C
83% H2 yield with precipitation
method synthesized catalysts at
650 °C.
[75]
ethanol
7% Ni, 7% Co sup-
ported on bare SBA-
15 and on Mg or Ca-
modified SBA-15
hydrothermal
method for SBA-
15, incipient wet-
ness impregna-
tion
fixed bed reactor;
100 mg
of catalyst;
flow rate: 0.075
mL/min (WHSVEtOH =
16.8h−1); GHSV = 22,300
h−1; P = 1 atm; T = 600–700
°C
100% EtOH conversion, 90.3 mol
% Η2 selectivity and 6.7 wt. %
coke deposition at 700 oCwith
Ni/Ca/SBA-15 catalyst. Mg and
Ca in Co/SBA-
15 promote metal
properties (dispersion and inter-
action) in greater degree
than in
Ni/SBA-15.
[84]
acetic acid
Co-M/SBA-15 (Co:
7%; M: 2% of Cu,
Ag, Ce and Cr)
hydrothermal
method for SBA-
15, incipient wet-
ness impregna-
tion
fixed bed stainless steel re-
actor; WHSV = 30.1 h−1;
GHSV = 11,000 h−1;
S/C = 2;
P = 1 atm; T = 600 °C
70 mol % H2 selectivity at 600 °C
with Co-Cr/SBA-15. [65]
acetic acid
Co/SBA-15 and Co-
M/CaSBA-15 cata-
lysts (Co: 7%; M: 2%
of Cu, Ag or Ce)
hydrothermal
method for SBA-
15, incipient wet-
ness impregna-
tion
fixed bed reactor;
WHSV =
30.1 h−1;
S/C = 2; P = 1 atm;
T = 600 °C
71.8% H
2
yield and 99% conver-
sion at 600 °C with Co-
Ce/CaSBA-15.
Cu improved the
decarboxylation reaction of acetic
acid and did not improve H2 pro-
duction. Ag enhanced catalytic
performance and decreased
coke
deposition
. Ce improved further
Co dispersion.
[66]
phenol Fe/50Mg-50Ce-
O (1,
2.5, 5,10% Fe)
sol–gel and in-
cipient wetness
impregnation
0.15 g catalyst in 0.15 g of
SiO2
GHSV = 80,000 h−1
T = 600–700 °C
5% Fe/50Mg-50Ce-O catalyst the
most active in terms of H2 yield
at 700 °C. Coke deposition in-
creased with increasing Fe load-
ing
[98]
1 aqueous solution of every model compound is used as feedstock.
Catalysts 2021, 11, 1526 18 of 36
5.3. Bimetallic Catalysts
Bimetallic systems are an effective way of combining the advantages of different ac-
tive metals [65,66,97,99]. For instance, the incorporation of Ru or Rh as promoters in Ni-
based systems has been shown to positively affect activity by helping the reducibility of
Ni species [89,91]. Salehi et al. [134] tested Ru-Ni/Al2O3 and Ni/Al2O3 catalysts with dif-
ferent Ni contents for the SR of acetic acid. The maximum H2 yield (85%, T = 950 °C) was
obtained with the doped Ru-Ni catalyst.
The synergistic interaction between Ni and Co also shows high reforming activity
and H2 selectivity since Co favors the WGS reaction where Ni is less active [135]. Assaf et
al. [76], investigated Ni-Co bimetallic catalysts with varied Ni and Co loadings and argued
that Co helped reduce coke formation and enhance catalytic performance. Garcia et al.
[41] carried out experimental work using the aqueous fraction of bio-oil. The catalytic sys-
tems tested comprised of Ni-Al2O3 modified with La and Mg and bimetallic Ni-Cr and Ni-
Co. The best results were recorded for Cr-promoted and Co-promoted catalysts based on
MgO-La2O3-Al2O3, because Cr and Co formed alloys with Ni and led to a lowering of the
crystallite size. Cr and Co promoters were also shown to inhibit the coke formation reac-
tions. Likewise, Pant et al. [78] synthesized Ni-Co catalysts, and showed that these were
more active than conventional monometallic systems, because they did not favor the
methanation or reverse WGS reactions and promoted the SR.
Wang et al. [73] studied Co-Fe unsupported catalysts with varied Co/Fe ratios in the
SR process. The authors concluded that increasing the amount of Fe in the system nega-
tively affected catalytic activity and stability, owing to the unstable adsorption of water
on Fe surface, which inhibited the SR and WGS reactions. Mohanty et al. [136] carried out
a detailed study on Cu-Zn catalysts and concluded that the incorporation of Zn improved
the hydrogen yield and minimized the deactivation of the active sites on the catalyst. Fi-
nally, it has also been shown that the use of Ni-Cu alloys leads to enhanced catalytic ac-
tivity and improved time-on-stream stability, in comparison to monometallic Ni-based
catalysts [137,138]. Table 7 provides a summary of the literature on bio-oil steam reform-
ing using bimetallic catalysts. As concluded from the above results, the formation of an
alloy of two metals enhances the catalytic performance and coke resistance. The synergy
of two metals strengthens metal support interaction and improves metal distribution as
well as the entire properties of the catalyst.
Table 7. Summary of the literature on bio-oil steam reforming using bimetallic catalysts.
Type of
Feed
1 Catalyst Preparation
Method Experimental Conditions Comments Ref.
glycerol, sy-
ringol, n-bu-
tanol, m-xy-
lene, m-cre-
sol,furfural
mixture
(1:1:1:1:1:1)
14% Ni/25% CeO2-
Al2O3; 1% Me-
14%
Ni/25% CeO2-
Al2O3 (Me = Rh,
Ru)
wet impregna-
tion
fixed bed reactor; 400 mg of cata-
lyst in 3.6 g of SiC; WHSV = 21.15
h−1; S/C = 5; P = 1 atm; T = 700–800
°C
Ru or Rh promoters en-
hanced the activity of the
Ni/CeO2-Al2O3
catalysts by
aiding the reducibility of
Ni.
[89]
acetone
12% Ni/15%
La2O3-Al2O3; 1%
M-12% Ni/15%
La2O3-Al2O3
(M =
Pt or Cu)
wet impregna-
tion
fixed bed quartz reactor; 0.5 cm3
of
catalyst diluted with SiC at a vol-
ume ratio of 3:1; GHSV= 10,180 h−1
;
P = 1 atm; T = 500–700 °C
The activity order of H2-
rich syngas Pt-Ni/La2O3-
Al2O3 > Cu-Ni/La2O3-Al2O3.
[97]
acetol
Ni/Al2O3
(Ni/Al =
½), Ni/La2O3-
Al2O3
(4, 8 and 12
wt. % La), Ni-
co-precipita-
tion
fluidized-bed stainless steel reactor
;
2.27 to 8.52 g catalyst min/g acetol
;
GHSV = 22,323 to 5947 h−1; flow
The activity order of syngas
Ni/Al2O3 = Ni-Co/Al2O3 >
Ni/La2O3-Al2O3.
[99]
Catalysts 2021, 11, 1526 19 of 36
Co/Al2O3
(Co/Ni =
0.025 and 0.25)
rates up to 5 mL/min,
S/C = 4.6; P =
1 atm; T = 450–650 °C
acetic acid
20% Ni–10%Co/γ-
Al2O3, 25%Ni–
5%Co/γ-Al2O3
fixed bed reactor; 100 mg catalyst
;
flow rate = 0.25 mLh−1;
P = 1 atm;T =
500–600 °C
Co led to an inhibition of
carbon deposition. [76]
bio-oil aque-
ous fraction
Ni-Cr/MgO-
La2O3-Al2O3; Ni-
Co/MgO-La2O3-
Al2O3 (15% Ni,
and mole ratios of
Mg/Ni = 1, Ni/La =
8, Ni/Cr = 3, and
Ni/Co = 3)
impregnatio
n
fixed bed micro-
reactor/molecular
beam mass spectrometer system;
3
cm high catalyst with quartz chips
;
GHSV up to 126,000 h−1; S/C = 4.92–
11; T = 825–875 °C
Co and Cr additives reduce
coke formation.
Ni-Cr/MgO-La2O3-Al2O3
show the best results.
[41]
acetone
8% Ni/MgAl2O4,
4% Co-4%,
Ni/MgAl2O4, 8%
Co/MgAl2O4
incipient wet-
ness impregna-
tion
fixed bed reactor; 100 mg catalyst
;
W/F = 70.6 gcat min gacetone−1; T = 550–
750 °C
Coke oxidation was fa-
vored on Co-containing cat-
alysts.
[96]
acetic acid
Ni-Co (20:80%),
Ni–Co/CeO2-ZrO2
(15:60:10:15%),
Ni/La2O3-Al2O3
(17% Ni,15% La)
co-precipita-
tion and im-
pregnation
fixed bed quartz reactor; 3 g cata-
lyst; flow rate: 0.5–1.12 mL/min;
GHSV = 79.6 g-cat h/mole acetic
acid; P = 1 atm; T = 550–700 °C
The unsupported Ni–Co
exhibited the highest activ-
ity and H2 yield.
[78]
acetic acid
Ni and Co (range
from 1:0 to 0:1)
co-precipita-
tion
fixed bed quartz reactor; 1 mL cata-
lyst with equal amount of quartz;
LHSV = 5,1 h−1;
S/C = 7.5; P = 1 atm;
T = 250–550 °C
Catalytic activity improved
by increasing the content of
Co. The best results were
achieved when
the Ni to Co
ratio was 0.25:1.
[7]
acetic acid
Co-Fe (pure Co,
Co/
Fe = 0.5, Co/Fe
= 2, and pure Fe)
co-precipita-
tion
fixed-bed reactor; 0.3 g catalyst
with quartz sand; LHSV = 4 h−1;
S/C
= 9.2; P = 1 atm; T = 350–600 °C
Catalyst activity increased
with increasing Co content
.
The conversion of acetic
acid using the pure Co cata-
lyst was 100%; the H2
yield
was 96%. These values
were achieved at 400 °C.
[73]
1 aqueous solution of every model compound is used as feedstock.
5.4. Perovskite Type Catalysts
Perovskite type oxides (Figure 5) have recently attracted considerable attention as
potential catalysts for reforming reactions. Perovskites are mixed oxides with distinctive
structural features and high redox properties. Their general formula isABO3, in which A
is a metal such as an alkali, alkaline, lanthanide or rare earth acting as a skeleton support,
while B is generally a transition metal such as Ni, Fe, Co, Cu, or Mn. The B site particularly,
is a cation with a coordination number of six and it is the central site of structure [62]. Due
to the great flexibility of the crystal lattice structure, partial substitution of cations in A
and B position by other elements of a similar size can be achieved, resulting in significant
changes to the catalyst’s properties [61,71]. In addition, perovskites combine both high
loadings of metals and high dispersion, preventing the agglomeration of metal ions incor-
porated in their lattice. In addition to the small metal particle size, perovskites can retain
their structure even at high temperatures, leading to good activity and thermal stability.
As a result, the structure of perovskites has more active sites, increased mobility of oxygen
ion vacancies and resistance to coke deposition [139].
Catalysts 2021, 11, 1526 20 of 36
Figure 5. Perovskite structure ABO3.
Liu et al. [62] compared the activity of LaNiO3 and LaNi0.8M0.2O3 perovskites substi-
tuted in the B site with Fe, Co, Mn, and Cu on the SR of acetic acid and showed that activity
followed the order: LaNi0.8Fe0.2O3 > LaNi0.8Co0.2O3 > LaNiO3 > LaNi0.8Mn0.2O3 >
LaNi0.8Cu0.2O3. In another work [61] the same authors also studied a series of Fe-doped
LaNiO3 perovskites with different Ni/Fe ratios and concluded that despite LaNiO3 show-
ing higher activity for hydrogen production, the Ni-Fe bimetallic perovskites were more
stable during the SR process. Among the partial substituted perovskites that have been
examined, LaNi0.8Fe0.2O3 demonstrated the best synergy between Ni and Fe. The coking
resistance of the perovskite was also effectively improved due to Fe-doping. In a more
recent work, Liu et al. [139] used La0.8M0.2Ni0.8Fe0.2O3 perovskites substituted in the A site
with Ca, Ce and Zr and showed that the La0.8Ce0.2Ni0.8Fe0.2O3 having stronger surface ba-
sicity and increased oxygen adsorption capacity was more active and stable.
Resende et al. [67] examined LaNiO3, LaPrNiO3 and LaSmNiO3 perovskites as pre-
cursors for catalysts in the SR of acetic acid. The products formed on the tested catalysts
differed only in terms of selectivity. The substitution with Pr and Sm only marginally af-
fected the catalytic performance. Li et al. [71] also concluded that Ce substitution en-
hanced the interaction between metal and support, promoted the WGS reaction and im-
proved the resistance to the deposition of coke. Transmission electron microscopy (TEM)
showed that the precursors were successfully synthesized (Figure 6).
Figure 6. TEM images of different perovskites: (a) LaNiO3; (b) La0.95Ni0.05NiO3; (c) La0.9Ce0.1NiO3; (d),
(f) La0.8Ce0.2NiO3; and (e) La0.7Ce0.3NiO3. Reproduced with permission from [71]. Copyright Elsevier,
2021.
Catalysts 2021, 11, 1526 21 of 36
The average H2 yield and acetic acid conversion were 90 and 95%, respectively, when
the La0.9Ce0.1NiO3 perovskite was used. Similarly, Junior et al. [82] evaluated the effect of
Ca content on the activity and hydrogen production in the SR of acetic acid. The results
showed that the presence of Ca in the perovskite enhanced hydrogen yield by promoting
the WGS reaction and limiting the ketonization reaction. Chen et al. [47] investigated the
effects of the K substitution on Mn-based perovskite type catalysts and compared them to
commercial Ni/ZrO2. The results showed that the La0.8K0.2MnO3 catalyst had a higher cat-
alytic activity, with a hydrogen yield of 72.5%, however deactivation was an issue. A sum-
mary of the literature on bio-oil steam reforming using perovskite type catalysts is illus-
trated in Table 8.
Table 8. Summary of the literature on bio-oil steam reforming using perovskite type catalysts.
Type of
Feed Catalyst
Prep.
Method Experimental Conditions
Comments Ref.
acetic acid
LaNiO
3
and
LaNi0.8M0.2O3
(M = Fe, Co,
Mn, Cu)
sol-gel
fixed bed reactor; 0.2 g cat-
alyst; GHSV = 34,736 g of
feed/(g catalyst h);
S/C = 2;
P = 1 atm; T = 650 °C
Activity, during the chemical looping SR fol-
lowed the order: LaNi0.8Fe0.2O3 >
LaNi0.8Co0.2O3 > LaNiO3 > LaNi0.8Mn0.2O3 >
LaNi0.8Cu0.2O3.
[62]
acetic acid
LaNixFe1-xO3
(x
= 0, 0.2,
0.4, 0.6,
0.8 and 1)
sol-gel
fixed bed reactor; 0.2 g cat-
alyst; flow rate = 35
mL/min;
S/C = 3; P = 1 atm;
T = 600 °C
Perovskites doped with Fe contained more
lattice oxygen withLaNi0.8Fe0.2O3
exhibits the
best synergistic effect and achieves the high-
est H2/CO for H2-rich syngas production.
Chemical looping steam reforming process.
[61]
acetic acid
LaNi0.8Fe0.2O,
La0.8M0.2Ni0.8Fe0.
2O3 (M =
Ca, Ce
and Zr)
sol-gel
fixed bed reactor; 0.25 g
catalyst;
S/C = 2; P = 1 atm;
T = 600 °C
Doping on A-
site with basic metals improves
redox properties of the perovskite. Ce-
doped
oxygen carriers showed improved catalytic
performance.
[139]
acetic acid
LaNiO3
LaPrNiO3
LaSmNiO3
precipita-
tion
fixed bed reactor
10 mg of catalyst diluted
with 150 mg of SiC; flow
rate = 400 mL/min;
S/C = 3;
P = 1 atm; T = 600 °C
Catalytic performance was affected only
marginally by the addition of Pr and Sm. [67]
acetic acid
La1-xCexNiO3
(x
= 0, 0.05, 0.1,
0.2, and 0.3)
citrate
fixed bed reactor; 500 mg
catalyst; flow rate = 4
mL/h;
S/C = 3; P = 1 atm; T
= 650, 700, 750 °C
Ce substitution of La affects the properties of
perovskites. La0.9Ce0.1NiO3 showed improved
performance with H2 yield of 90% and
acetic
acid conversion of 95%.
[71]
acetic acid
La1−xCaxNiO3
(x
= 0, 0.15, 0.30
and 0.50)
citrate
fixed bed reactor;
10 mg of
catalyst diluted with 150
mg of SiC; flow rate = 0.25
mL/min;
S/C = 3; P = 1 atm;
T = 400–700 °C for LaNiO3
and 600 °C for Ca-contain-
ing catalysts
The presence of CaO promoted the H2 pro-
duction and the WGS reaction. [82]
bio-oil
aqueous
fraction
La1-xKxMnO3
(x
= 0, 0.1, 0.2, 0.3)
sol-gel
fixed bed reactor;
WHSV =
12 h−1;
S/C = 3; P = 1 atm; T
= 600–800 °C
K substitution helped obtain a
higher surface
area for LaMnO3. H2 yield of 72.5% was rec-
orded for La0.8K0.2MnO3.
[47]
Catalysts 2021, 11, 1526 22 of 36
5.5. Effect of Catalyst Synthesis Methods
As is well understood, the choice of the catalyst synthesis method can have a major
impact on performance, as it regulates the dispersion of the active phase and the interac-
tion between metal and support. In addition, it can also affect the carbon formation and,
thus, the stability of the system [101,140]. As a consequence, a plethora of different meth-
ods have been employed to synthesize catalysts in an effort to enhance their properties
and increase their activity. Wet or dry (incipient wetness) impregnation, is commonly
used to load the metal species onto the supporting materials, owing to their simplicity.
Nabgan et al. [141] investigated the synergetic effects between Ni and Co, in the SR of
acetic acid using La2O3 as support; the catalysts were synthesized using the wet impreg-
nation technique. However, although the dispersion of the active phases was high, carbon
deposition was also heavy. Similarly, Valle et al. [142] attributed the deactivation of a
Ni/La2O3-Al2O3 in the SR of raw bio-oil, prepared by the incipient wetness impregnation
method, to the formation of encapsulating and filamentous coke. The smaller formation
of encapsulating coke was attributed to the oxygen present in bio-oil, which was absorbed
on the Ni sites, though the filamentous coke deactivating effect was the blockage of the
catalyst pores.
Wang et al. [75] prepared Ni catalysts supported on attapulgite (ATC) using the pre-
cipitation, wet impregnation and mechanical blending techniques and studied the cata-
lytic SR of acetic acid. The authors concluded that the interaction between the Ni species
and the ATC support was strong (for the samples prepared via precipitation), which was
beneficial for catalytic activity and stability. Similarly, Zhang et al. [143] examined the SR
of acetic acid using Ni-Co/MgO catalysts, synthesized using co-precipitation and wet im-
pregnation. The catalyst prepared via wet impregnation showed decreased H2 yield and
were less stable due to significant coke deposition.
Catalyst synthesis via the sol-gel process is also commonly employed as it provides
the means to control the surface and textural properties [47,62]. As the process is a wet
chemical technique, it is also known as chemical solution deposition [61,139].
The hydrothermal method has the significant advantage of helping the self-assembly
of products by taking advantage of the solubility of precursors in hot water (or organic
solvent) under increased pressure [101]. Bizkarra et al. [90] investigated Zeolite L as cata-
lyst support, during the SR of a mixture of bio-oil and bio-glycerol. The catalysts showed
improved catalytic performance, high H2 yields and resistance to deactivation during the
steam reforming process.
In recent years ultrasonic agitation has also been employed during catalyst prepara-
tion. Wu and co-workers [144] argued that this method can produce catalysts where the
active phase is highly and homogeneously dispersed, with a strong degree of interaction
between metal particles. These characteristics contributed to superior catalytic perfor-
mance with enhanced stability during time-on-stream.
6. Catalyst Deactivation and Regeneration
6.1. Coke Formation
Bio-oil contains a range of oxygenated organic compounds which lead to the for-
mation of carbon. This effect is more pronounced when reforming raw bio-oil. An excel-
lent review, presenting issues related to coking in the processes of bio-oil upgrading, the
properties of coke formed, the mechanism for coking and the methods developed for tack-
ling it has been provided by Hu et al. [105].
Coke deposition is a rather complicated issue as it can arise from a combination of
the polymerization, dehydration, and cracking reactions [114,145]. To study the effect that
the molecular structure of different oxygenated compounds has on coking, model com-
pounds are employed. The coke formed also has different properties, depending on the
structures of the different feedstock [85,105]. Figure 7 depicts encapsulating coke, with
Catalysts 2021, 11, 1526 23 of 36
aliphatic and higher aromatic nature, placed in the most superficial and inner layers, re-
spectively, and filamentous coke with more carbonized structure and/or polyaromatic
with low oxygenates content [146,147]. As is well understood, the nature of the deposited
carbon has a significant influence on catalytic performance, as amorphous carbon is easier
to combust during reaction [105,148,149].
It is also known that increased unsaturation, molecular weight, and aromaticity of
the feed lead to increased carbon deposition [106,150]. As known, lower C/H ratios indi-
cate coke that is more aliphatic, while higher C/H ratios indicate coke that is more aro-
matic. For example, carbon formed during the SR of acetone has been shown to be less
aromatic than that formed during the SR of acetic acid [105]. Li et al. [55] also examined
the reforming of carboxylic acids and concluded that the nature and number of C-C bonds
not only affected their reactivity, but also their propensity towards coke formation and its
properties. Furthermore, the authors argued that the longer aliphatic chains increased the
tendency towards coking; for example, the SR of acetic acid led to the formation of pre-
dominantly amorphous coke, while the reforming of heavier carboxylic acids resulted in
the formation of fibrous carbon. Zhang et al. [58] also showed that the particular molecular
structures had an important effect on coking. For example, the authors showed that meth-
anol and formic acid had low propensity to form coke during their SR, as they lack ali-
phatic carbon chains. On the other hand, the significant, graphite-like, coke deposition
observed during the SR of furfural and guaiacol comes about because of their π-conju-
gated ring structures [114]. In general, the presence of phenols and its derivatives (e.g.,
catechols, guaiacols and syringols), in raw bio-oil is undesirable, as these compounds pol-
ymerize into complex carbonaceous structures. Thus, such compounds are considered the
main responsible substances for catalyst deactivation, but clogging of the reactor, pipe-
lines and filters has also been observed [151].
Figure 7. SEM images of coke deposits at 550 °C: (a) and 700 °C (b) under the same operating con-
ditions. At 550 °C an amorphous carbon can be identified, and at 700 °C structures that are more
filamentous can be identified. Reproduced with permission from [147]. Copyright Elsevier, 2021.
The nature of the active metal not only affects the quantity of carbon deposition, but
also its quality and location [56], and a large number of works show that the latter factors
play a more significant role in catalyst deactivation than the amount [100,105]. Vagia et al.
[64], studied the SR of acetic acid, using Ni- and Rh- based catalysts supported on CeO2-
ZrO2, and showed that the almost negligible carbonaceous deposition on Rh catalysts can
be attributed to the minimal affinity of Rh to coking and to the fast supply of oxygen to
the metal interface. In contrast, the identification of coke even at very high temperatures
(750 °C) over the Ni catalyst indicated that the quantity of oxygen transferred through the
support vacancies at the perimeter of the metal crystallites was not sufficient to fully oxi-
dize the coke deposits.
Thus, the physicochemical properties of the catalyst play a fundamental role in the
coke formation mechanisms. Importantly, the catalyst should be able to provide ample
adsorbed H2O-derived species (i.e., OH and H), to minimize the impact of the reactions
Catalysts 2021, 11, 1526 24 of 36
that lead to coke formation. This means that the adsorbed OH and H should possess sur-
face mobility capable of reaching and reacting with the adsorbed hydrocarbon-derived
species [105]. Thus, the use of appropriate supports is key for this process. For example, a
number of works have shown that the SR ability of typical Ni-Al2O3 or Ni-SiO2/Al2O3 sys-
tems can be significantly improved when Ca and/or K are used as promoters
[65,66,120,123]. Incorporating MgO to Al2O3 can also improve the adsorption and H2O
dissociation capacity of the catalytic system [72]. Vizcaíno et al. [84], in their studies on
the SR of ethanol, used Mg- and Ca-modified Co or Ni/SBA-15 formulations, and were
able to show that these promoters (i.e., Ca and Mg) helped lower coke deposition. It is
noted that for Co-based catalysts this effect was more evident using Mg, and for the Ni-
based catalysts this effect was more evident by the addition of Ca.
Moreover, a key strategy for coke minimization is the enhancement of the adsorption
of steam which facilitates the gasification of coke precursors. Additionally, slowing down
or minimizing cracking, deoxygenation, and dehydration of adsorbed intermediate, i.e.,
the surface reactions leading to the formation of the coke precursors, is also crucial
[152,153].
The use of different reactor designs has also been studied in the attempt to eliminate
coke with reports suggesting that the use of fluidized bed reactors can enhance coke gas-
ification [104–106].
Temperature also plays an important role in the SR of bio-oil. When the reaction takes
place at low temperatures, the incomplete cracking of the organics favors their polymeri-
zation to form carbon. High reaction temperatures, on the other hand, helps the cracking
of high molecule-mass organics, and their ensuing SR. Moreover, higher temperatures
also favor the gasification of coke precursors with steam or carbon dioxide [83,154].
6.2. Active Metal Sintering
In addition to coke formation, sintering, caused by the high temperatures and high
pressures of steam used in the process, is another important cause for catalyst deactivation
(Figure 8). Sintering occurs when the metallic particles (active phase) are enlarged during
the reaction. As is well understood, sintering occurs through two basic mechanisms. The
first involves the relocation of entire particles over the support and their conjugation with
other, nearby particles. The other mechanism takes place through the migration of atoms
over the support from one crystallite to a neighboring crystallite (Ostwald ripening) [47].
In effect, sintering lowers the number of active sites available to the reactants but also
stipulates the formation of carbon (favored over larger metal particles).
Figure 8. HRTEM image of 15% Ni/Al2O3 catalyst representing the sintering process. Reproduced
with permission from [127]. Copyright Elsevier, 2021.
Catalysts 2021, 11, 1526 25 of 36
Sehested et al. [145,155–159] extensively studied the sintering of Ni-based catalysts
in H2O/H2 model atmospheres and observed that the partial pressures of steam and hy-
drogen are of the utmost importance, as they probably control the rate at which sintering
occurs. In particular, Ni supported in Al2O3 catalysts usually sinter when the environment
is very hydrothermal, due to the extensive loading of Ni and the formation of NiAl2O4.
However, the addition of small amounts of Mg, K, Ce or La can inhibit sintering by aiding
the dispersion of the active metal, and by preventing the formation of the less active
NiAl2O4 [88,128,160,161]. Zhao et al. [162] using a porous silica coated Ni/CeO2-ZrO2 cat-
alyst and Pu et al. [163] using a series of Ni core-shell catalysts (with different shell species
SiO2, Al2O3, CeO2, and TiO2) were able to avoid sintering even at high reaction tempera-
tures.
6.3. Active Metal Oxidation
The presence of O2 in bio-oil may bring about the oxidation of the catalyst metallic
species during steam reforming, which is subsequently detrimental to the catalytic activ-
ity and stability [150,164]. Nevertheless, the deactivation caused by active metal oxidation
is not considered as serious an issue as coke formation or active metal sintering because
of the use of inert carrier gases during the reaction.
6.4. Sulfur Poisoning
Sulfur, if present in the feed, is a severe poison which reduces the activity of the cat-
alysts. All sulfur-containing compounds in the feed are converted into hydrogen sulfide
(H2S) at reforming conditions and then the sulfur atom in H2S binds strongly to the metal
(Equation (19)) (either transition or noble). As a result, even traces of sulfur in the feed
lead to severe deactivation.
H2S + Ni(surface) Ni(surface)-S + H2 (19)
Given this information, it is important to take into consideration the sulfur adsorp-
tion capacity of SR catalysts. Azad et al. [165] showed that the binding of sulfur to a com-
pound such as CuO, added to noble metal-based catalysts, is more thermodynamically
stable. Sato et al. [166] doped a Ni/MgO–CaO catalyst with WO3 and showed good per-
formance for reforming of naphthalene. Ce0.8Gd0.2O1.9 has also been used to remove the
sulfur on the catalyst as H2S; this was achieved via a redox reaction [167]. Interestingly,
sulfur poisoning can be used beneficially in order to decrease the coke formation. H2S, in
ppm levels, can be used to block the most active step sites, which are very active in
whisker formation, and then the S-bonding can be reversed by treatment with H2. This
process allows the operation at a low S/C ratio [168,169].
Generally, a variety of physicochemical methods have been applied for the removal
of the H2S generated from industrial processes such as petroleum refining, natural gases,
biogas processing and coal gasification [170–172]. Compared to naphtha which contains
about 1.5% sulfur, bio-oil derived from the fast pyrolysis of biomass has a sulfur content
in the range of 0.01–0.2% [142]. Thus, a desulfurization unit prior to the reformer may not
be necessary for bio-oil feedstocks.
Catalysts 2021, 11, 1526 26 of 36
6.5. Catalyst Regeneration
A decrease in the cost of the reforming process can be achieved through catalyst re-
generation and reuse. As stated above, carbon deposition constitutes the main deactiva-
tion mechanism in bio-oil reforming. Therefore, catalyst regeneration can be achieved if
the coke can be removed. Combustion is a simply operated and highly efficient method
which is commonly used for this purpose. In addition, combustion not only can eliminate
the coke on the catalyst surface, but can also provide heat for the SR.
Wu et al. [173] reported that a regenerated Ni/MgO-Al2O3 catalyst had similar per-
formance to the fresh catalyst. Ochoa et al. [146] showed that the carbonization structure
and oxygen content affects the temperature at which combustion occurs, but also the heat-
ing value of coke. Filamentous coke, which has higher structure of carbonization and
lower oxygen content than encapsulating coke, requires high combustion temperature.
Montero et al. [87] showed that the Ni/La2O3-Al2O3 catalyst undergoes partial Ni sintering
in an ethanol reforming reaction-regeneration cycle system. Due to this, Oar-Arteta et al.
[174] attempted to synthesize catalysts with metal spinel structure in order to obviate loss
of metal activity in the regeneration process.
Coke can also be removed through gasification with air, oxygen or steam. However,
more energy is needed and the coke removal rate is very slow compared with combustion
[173,175].
7. Other Modified Reforming Techniques
As stated above, a higher S/C ratio is beneficial in attenuating coke formation during
bio-oil steam reforming. However, this makes the process costly for the large-scale gener-
ation of hydrogen. For this reason, modified reforming methods for H2 production from
bio-oil have also been investigated.
7.1. Pyrolysis and in-Line Steam Reforming
Two-stage pyrolysis steam reforming has recently been proposed as an advanced
technology for hydrogen generation. This process allows the valorization of both the
whole bio-oil and gases from the pyrolysis step, avoiding the additional costs of trans-
porting the bio-oil and also the bio-oil vaporization operational problems that occur dur-
ing the one step bio-oil reforming process [176,177]. Incomplete vaporization and re-
polymerization are some significant disadvantages of indirect bio-oil reforming that re-
duce its efficiency. As shown in Figure 9, pyrolysis and in-line reforming are carried out
in a different reactor. The pyrolyzed derived product is directly fed into the reforming
reactor and its thermal energy is utilized during reforming. The operating temperature is
a key factor of the system as it affects the hydrogen regeneration significantly. Both pro-
cesses have been optimized at different temperatures [42,178]. Therefore, pyrolysis pro-
cess accompanied with in-line reforming has attracted much attention due to its consid-
erable advantage over the biomass gasification, pyrolysis and bio-oil reforming.
Ma et al. [179] and Chen et al. [180] proposed a novel process for hydrogen produc-
tion through a gas-solid simultaneous gasification process which was integrated into the
two-stage pyrolysis SR process. This integrated process showed greatly increased H2 yield
and carbon conversion efficiency. The hydrogen obtained during this process was almost
tar free.
Catalysts 2021, 11, 1526 27 of 36
Figure 9. Schematic pathway of pyrolysis and in-line steam reforming process. Reproduced with
permission from [26]. Copyright Elsevier, 2021.
7.2. Sorption Enhanced Steam Reforming (SESR)
Sorption enhanced steam reforming (SESR) has been proposed as a means to improve
the purity of the hydrogen stream. Compared to the conventional SR process, SESR in-
volves a CO2 sorption reaction which shifts the reaction equilibrium of the WGS reaction
(Equation (2)) towards hydrogen, based on the Le Chatelier’s principle. This also helps
the removal of the produced CO. In general, a hydrogen feed of high CO2 content is diffi-
cult to use for energy generation in fuel cells [181,182]. Thus, a sorbent such as CaO is
added to the reacting system and reacts reversibly with CO2 to reduce its concentration in
the product stream according to the stoichiometry represented in Equation (20):
CaO + CO2 CaCO3 (20)
The CO2 removal by sorbents is an exothermic reaction. Therefore, the in-situ CO2
capture is included in the process not only to clean the product steam of the non-combus-
tible by-product, but also to decrease the whole reforming reaction temperature. The
sorbent stability can be increased and the reforming operation becomes simpler by com-
bining the sorbent and the catalyst in one catalytic system [57,183]. In addition to synthetic
sorbents, natural sorbents such as dolomite (mainly MgO and CaO) and hydrotalcite have
also been used [98,100]. The sorbent can also be regenerated and the high-purity CO2
which is released in the regenerator can be reused. Thus, the SESR process is a promising
pathway allowing low hydrogen production costs and a lower negative CO2 output [44].
7.3. Chemical Looping Steam Reforming (CLSR)
Chemical looping steam reforming (CLSR) is an advanced auto-thermal reforming
technology which has received appreciable attention during recent years. It has the abili-
ties to reduce the hydrogen production costs, to utilize waste energy and to decrease the
environmental impact. However, the complex reaction between an oxygen carrier and
bio-oil may constrain its development [61]. CLSR couples the endothermic steam reform-
ing and the exothermic partial oxidation of the reforming fuel (Equation (21)) by alternat-
ing fuel feed and oxidant feed, usually air. In partial oxidation, the substrate is oxidized
with oxygen and releases heat, which in turn balances the energy required for the steam
reforming process.
(Substrate) CnHmOk + air Carbonoxides + H2 + N2 (21)
In the fuel reactor, the bio-oil is partially oxidized into syngas by an oxygen carrier
which is circulated between the fuel and air reactors. The oxygen carrier has a double role
as it provides heat to the reactants within the reactor by oxidation reactions and also cat-
alyzes the steam reforming and WGS reactions for hydrogen rich syngas formation.
Hence, it is reduced from MxOy to MxOy−δ with less oxygen content. Then, the reduced
oxygen carrier can be re-oxidized by air in an air reactor [139]. In general, a large amount
Catalysts 2021, 11, 1526 28 of 36
of heat is transferred to the fuel reactor from the air reactor due to the exothermic nature
of oxygen carrier oxidation process. Oxygen carriers can be simple metal oxides, mixed
metal oxides and structured materials such as hydrotalcites and perovskites. Transition
metals, such as Ni, Fe, Co and Mn have been widely studied owing to their higher natural
abundance and higher sintering resistance than noble metals [62]. A well-designed CLSR
process enhances the energy efficiency and may also produce a non N2-diluted syngas
with low heating demand [79].
8. Prospects, Directions and Conclusions
As the necessity for sustainable energy sources becomes increasingly important, the
efficient production of renewable hydrogen becomes a challenge worth pursuing. The bio-
oil produced by the fast pyrolysis of lignocelluloses is a product of higher energy density
than the parent biomass which, being in liquid phase, can be transported in long distances
easily and economically, allowing large-scale hydrogen production or the subsequent
chemical synthesis of upgraded high density transportation biofuels (i.e., Fischer–Trospch
synthesis) in central facilities.
The production of hydrogen or syngas through the catalytic SR of bio-oil is a key
process in these large-scale energy scenarios and, as a result, has become a subject of ex-
tensive research during recent years. While in theory the steam reforming of bio-oil is
entirely feasible and capable of producing high yields of hydrogen, in practice certain
technological issues require further investigation and radical improvements before the
commercialization of the process. Steam reforming is generally performed at middle to
high reaction temperature, high S/C and low space velocity in order to maximize the for-
mation of hydrogen and minimize the by-products. However, these parameters should be
further investigated to simulate the conditions met in industrial operation. Also, the high
chemical complexity of raw bio-oil does not readily allow a systematic approach on the
maximization of hydrogen productivity while alleviating carbon deposition issues. Raw
bio-oil cannot be completely vaporized and when heated leads to the formation of resid-
ual solids which accelerate catalyst poisoning at rates much higher than the usually ex-
amined model compounds. Aqueous phase bio-oil reforming suffers also from low H2
yields and high coking rates and, as a result, model compound studies are used to simplify
the catalytic reforming process, to determine how reactive the bio-oil components are, and
to optimize H2 production with the least catalyst coking. While some researchers exam-
ined hydrogen generation using a mixture of model compounds, further research is re-
quired on the reaction mechanisms and kinetics, which have not been yet fully understood
and much work must be done to optimally treat the raw or aqueous bio-oil mixtures for
efficient practical use.
The purpose of the present investigation is to provide the fundamental knowledge
behind the technology of bio-oil steam reforming and to review the latest research out-
comes and the recent progress on the development of the best suited catalysts and the
most appropriate modified reforming techniques. Among the different catalysts which
are being investigated, the most preferable seem to be the Ni-based due to their low cost,
high abundance and good catalytic performance. For these, proper support modifications
with basic oxides and active metal additions with alkali and alkaline earth metals have
been reported to increase both the overall catalyst activity and the resistivity against cok-
ing. The properties of-and the interactions between the metal-based catalysts, the active
phase additives and the support materials also require further study and clarification. Ac-
cordingly, further emphasis must be given to the research for catalysts with low cost, high
activity and stability, strong regenerative ability and extensive operating lifetime for suc-
cessful operation in the industrial conditions. Since catalyst deactivation is a major prob-
lem encountered during the steam reforming process, the mechanisms of coke formation
and metal sintering should also be further investigated.
Catalysts 2021, 11, 1526 29 of 36
Author Contributions: A.P.: Investigation, Writing—Original Draft; N.D.C.: Conceptualization,
Methodology, Writing—Review & Editing, Project administration; S.L.D.: Writing—Review & Ed-
iting; G.I.S.: Writing—Review & Editing; W.W.: Investigation, Writing—Review & Editing, Funding
acquisition; G.L.: Writing—Review & Editing, Funding acquisition; V.G.P.: Writing—Review & Ed-
iting, Funding acquisition; M.A.G.: Writing - Review & Editing, Supervision, Project administration,
Funding acquisition. All authors have read and agreed to the published version of the manuscript.
Funding: The authors are grateful to the project SYNAGRON, a joint RT&D project under Greece–
China Call for Proposals launched under the auspices of the Ministry of Science and Technology
(MOST) of the People’s Republic of China and the Ministry of Development & Investments/General
Secretariat of Research and Technology (GSRT) of the Hellenic Republic. China: National Key Re-
search and Development Program, project code: 2017YFE0133300. Greece: European Regional De-
velopment Fund (ERDF) and National Resources (GSRT), project code: T7ΔKI-00388.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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... Ni-and Co-based materials were found to have an enhanced activity on bio-oil steam reforming in comparison with other transition metal-based catalysts. Bio-oil contains a range of oxygenated organic compounds which promote the formation of coke [47]. Megía et al. [48] investigated the deactivation of a Co/ SBA-15 catalyst during the steam reforming of the aqueous fraction of bio-oil and observed that cause of deactivation of the catalyst is due to the amount of coke deposited and the C/H ratio of hydrocarbons. ...
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Due to its characteristics, hydrogen is considered the energy carrier of the future. Its use as a fuel generates reduced pollution, as if burned it almost exclusively produces water vapor. Hydrogen can be produced from numerous sources, both of fossil and renewable origin, and with as many production processes, which can use renewable or non-renewable energy sources. To achieve carbon neutrality, the sources must necessarily be renewable, and the production processes themselves must use renewable energy sources. In this review article the main characteristics of the most used hydrogen production methods are summarized, mainly focusing on renewable feedstocks, furthermore a series of relevant articles published in the last year, are reviewed. The production methods are grouped according to the type of energy they use; and at the end of each section the strengths and limitations of the processes are highlighted. The conclusions compare the main characteristics of the production processes studied and contextualize their possible use.
Article
Greece and China are two agricultural countries in which a significant percentage of their biomass consists of agricultural and livestock wastes. In Greece, the management practice that is still applied is burning the majority of the agricultural residues in the field, while in China large amount of untreated agricultural waste causes serious adverse effects on the rural living environment. Residual biomass could arise from the agricultural sector in the form of crop residues and of animal wastes. A comprehensive literature review was performed for both countries on primary agricultural and livestock wastes that includes: (a) crop residues that are left in the fields, (b) animal manures and (c) secondary agricultural wastes consisted of liquid and solid wastes from agricultural products processing plants. The average annual quantity of agricultural residues, animal manure and agro-industrial residues in Greece and China (a small and a large country, respectively) is 10 Mt/y and 860 Mt/y, 26 Mt/y and 216.5 Mt/y, 13.2 Mt/y and 213.7 Mt/y, for each corresponding category. According to the amount of the above residues generated per country, the theoretical estimated energy can reach up to 77 TWh and 5500 TWh for Greece and China, respectively, which suggests that both countries can cover a significant part of their energy needs (approximately 135% and 99%, respectively) only from their agricultural and livestock wastes. Τhese observations constitute strong recommendations mainly addressed to policy makers to substantially strengthen the efforts of research, technological and economic optimization of the applications of energy use of agricultural residual biomass for the benefit of the environment and humanity.
Article
In this study, chemical looping reforming coupled with water splitting (CLRWS) process for coproduction of syngas and hydrogen using bio-oil model compound as fuel was investigated. The process simulation results indicated that the mass ratio of fuel to Fe2O3 (F/O) of 0.3 at 900 °C was suitable for syngas and hydrogen coproduction. Under these conditions, the CLRWS experiments were conducted in the fixed bed reactor using Ni-Fe bimetallic oxygen carriers (OCs). The Ni-Fe bimetallic oxygen carrier contained 5 wt% of NiO, 60 wt% of Fe2O3 and 35 wt% of support. Six metal oxides, Al2O3, CeO2, La2O3, MgO, TiO2 and ZrO2, were used as supports, and corresponding OCs were termed as NFA, NFC, NFL, NFM, NFT and NFZ, respectively. The interaction between different components had significant influence on coproduction of syngas and hydrogen. The formation NiAl2O4, FeAl2O4, MgFe2O4, and Fe2TiO5 were unreadily reduced and unfavorable for the high purity hydrogen production in SR. The NFL, NFZ and NFC presented the better performance than the NFA, NFM and NFT. The catalytic reforming reactions were enhanced significantly by introducing NiO and the supports, while the presence of Ni readily produced higher carbon deposition, which could be alleviated by the addition of steam in FR. The hydrogen purity of all OCs increased to more than 95% in the SR with S/C = 1.4. The top two hydrogen purity in SR for NFZ and NFC were 99.73% and 99.66%, the corresponding hydrogen yield were 1.132 Nm³/kg and 1.165 Nm³/kg, respectively. The NFL exhibited excellent catalytic performance and benefited to produce syngas. The NFZ had the highest oxygen transfer capacity and the lowest carbon deposition. The cyclic stability of NFL and NFZ decreased with the increasing of number of cycles. The NFC is a promising OC in the CLRWS process with the presence of steam, maintained high hydrogen purity and good stability performance in the multiple cycle tests.
Article
In the study presented herein, nickel catalysts supported on CeO2 and, for the first time in the literature, on La2O3-Sm2O3-CeO2, La2O3-Pr2O3-CeO2 and La2O3-MgO-CeO2 were prepared and evaluated for the reaction of CO2 methanation. The carriers were prepared through a sol-gel microwave assisted method and the catalysts were obtained following wet impregnation. The physicochemical properties of the catalysts prior to reaction were determined through H2-TPR, H2-TPD, Raman spectroscopy, XRD, CO2-TPD, N2 physisorption-desorption, XPS and TEM. The spent catalysts, after the time-on-stream experiments were further characterised using TEM and TGA. It was shown that the simultaneous incorporation of La3+, Pr3+ and La3+, Sm3+ into the crystal structure of cerium oxide created higher population of oxygen vacant sites. Moreover, the co-presence of La3+, Mg2+ and La3+, Pr3+ into the CeO2 increased the plethos of moderate basic sites. These physicochemical properties increased the rate of CO2 methanation reaction at relatively low temperatures. Furthermore, it is argued that the addition of La3+ stabilized the Ni active sites via the probable formation of a new compound (La-O-Ni) on the catalyst surface or synergetic catalytic centers at the interfacial area improving the catalytic properties (activity and stability). Finally, the catalytic performance tests revealed that the addition of La3+ mainly improved the conversion of CO2 and yield of CH4 for the Ni/La-Mg-Ce and Ni/La-Sm-Ce samples. The rCO2 and XCO2 values at 300 °C followed the order Ni/La-Sm-Ce >> Ni/La-Mg-Ce > Ni/La-Pr-Ce > Ni/Ce.
Article
The steam reforming of bio-oil is a promising and economically feasible technology for the sustainable H2 production, yet with the main challenge of designing highly active and stable catalysts. This work aimed to study the deactivation mechanism of a NiAl2O4 spinel derived catalyst, the role of Ni and alumina sites in this mechanism and the appropriate reaction conditions to attenuate deactivation. The reaction tests were carried out in a fluidized bed reactor with prior separation of the pyrolytic lignin. The fresh or used catalysts were characterized using X-ray diffraction, temperature-programmed oxidation, X-ray photoelectron spectroscopy, scanning electron microscopy combined with energy dispersive X-ray spectroscopy, and Raman spectroscopy. For steam/carbon ratios > 3.0, space time above 0.075 h and at temperature between 600-700 °C, high initial hydrogen yield is obtained (in the 85-90% range) with CO yield near 20%, CH4 yield below 5% and negligible initial yield of hydrocarbons. The catalyst is more stable at 600 °C, with coke formation preferentially located on Ni sites inside the catalyst particle. Increasing the temperature favors the coke development and consequent deposition on the alumina support, leading to a rapid catalyst deactivation because the limited availability of Ni and alumina sites. These results contribute to understand the phenomenon of catalyst deactivation in the steam reforming of bio-oil and set appropriate reaction conditions to mitigate this problem with a NiAl2O4 spinel derived catalyst.
Article
The yield of H2 and by-products and the heat duties in the reforming of different oxygenates in bio-oil (acids, ketones, aldehydes, phenols and saccharides) have been compared by means of thermodynamic analysis, focusing on steam reforming (SR), oxidative steam reforming (OSR) and autothermal reforming (ATR) as alternatives. The study is performed by minimization of Gibb's energy method with ProII-Simsci® 10.1 software and has been extended for a wide range of conditions (temperature, steam/carbon (S/C) and O2/C ratios, and inert gas addition). The results of products yield and heat duties of SR are slightly different with oxygenates of different nature, and S/C = 5 and 610–644 °C range are the most suitable conditions for attaining high equilibrium H2 yield (90–92%) without excessive penalty in energy requirements. At S/C = 1.5, the inert addition increases slightly hydrogen yield and decreases coke formation, but this effect is not significant for S/C ratios above 5. Original correlations are proposed to predict the maximum yield of H2 and byproducts and the optimum temperature for the SR of oxygenates, from the values of the C/S ratio in the feed and the H/C, O/C and O/H ratios characteristic of the oxygenates composition. At 630 °C, ATR regime is achieved with a different O2/C ratio for the oxygenates, in the 0.12–0.22 range (aldehydes < acids ≈ phenols < ketones), and with H2 yield of 83–77%.