Single-atom catalysts for biomass-derived drop-in chemicals

Abstract

Conversion of biomass to fuel and drop-in chemicals is envisaged to solve the problem of depleting fossil fuel reserves while leveling-off the staggering CO2 concentration. By-passing the natural carbon cycle via the transformation of abundant lignocellulosic biomass into chemicals does not add any extra CO2 to the environment and the net CO2 concentration remains the same. The paradigm shifts from fossil fuel-based chemicals to biomass-derived products will rely on efficient and cost-effective catalysts that can compete with cheap and readily available fossil fuels. Existing transition and noble metal-based nanoparticle catalysts either in the supported or unsupported form are crippling due to poor activity/selectivity, deactivation of catalytically active sites, and the complex composition, recalcitrant nature, and high moisture content of biomass. Single-atom catalysts (SACs) possessing single-atom centers decorated on support have shown great promise in biomass conversion due to their unique geometric configuration, electronic properties, and ensemble effect. In contrast to traditional catalytic systems, SACs encompass the advantages of both heterogeneous and homogeneous catalysts with improved performance and easy recyclability. Because of the availability of each metal center for the reaction and unique geometrical configuration, SACs have displayed exceptional catalytic activity and selectivity (~95% in most cases). In addition, the SACs show increased thermal and chemical stability due to the stabilization of the metal center on the support. The present chapter highlights the various aspects of SACs for efficient and selective biomass conversion into drop-in chemicals.

Full text

4
Single-atom catalysts for biomass-
derived drop-in chemicals
Pawan Kumar, M.A. Khan, Jinguang Hu, Md. Golam Kibria
DEPARTMENT OF CHEMICAL AND PETROLEUM ENGINEERING, UNIVERSITY OF CALGARY,
CALGARY, AB, CANADA
4.1 Introduction
If there is anything the year 2020 has taught us is that, if we human beings do not take
responsibility for our actions and take corrective measures, then mother nature will take
matters into its own hands which could be catastrophic for us. There are a few numbers that
highlight how much human activities have impacted the environment around us. Since
1850, 950 gigatons of CO
2
from human activity went into the atmosphere. If the oceans did
not come to our rescue, the number would have been close to 2,400 gigatons of CO
2
[1].
This had led to an increase in the concentration of carbon dioxide in the Earth’s atmosphere
from around 275 parts per million (ppm) before the Industrial Revolution to over 419 ppm
in June 2021, causing global warming and resulting in a damaging effect on the environment
[1,2]. At the current pace of CO
2
emissions, the global temperatures are expected to rise by
B4.8C by the year 2100, which is a scary scenario [3]. The effect of global warming is
becoming more evident and visible in the form of melting of glaciers (thawing of the Arctic
permafrost), increased sea levels, frequent extreme events, and endangered biodiversity
[47]. In 2018, the world was shocked when an iceberg (B-46, size: 115 square miles), five
times the size of Manhattan was broken in Pine Island Glacier in West Antarctica [8]. In other
news, the Arctic’s strongest sea ice is breaking up for the first time and will have been
pushed away from the coast to an area where it will melt more quickly.
And what is more worrying is that we are still heading in the wrong direction. Despite all
the good intentions and the signing of the Paris Agreement in 2016, emissions are still going
up. In 2019, about 43.1 billion tons of CO
2
from anthropogenic activities were emitted into
the atmosphere. It was an all-time high, breaking the previous record from 2018 [1].
According to the UN Environment Program, emissions must fall by 25% before 2030 to keep
temperature increase within 2C by 2100 and 55% reductions are needed by 2030 to limit the
increase to 1.5C[1]. We need a huge shift in energy policies to maximize energy produc-
tion/use from renewables and work on technologies for carbon capture, conversion, and uti-
lization and storage (CCUS) [911]. In the last decade, there has been tremendous progress
63
Advanced Catalysis for Drop-in Chemicals. DOI: https://doi.org/10.1016/B978-0-12-823827-1.00009-2
Copyright ©2022 Elsevier Inc. All rights reserved.

in efficient harvesting renewable energy from sunlight and wind and CCUS technologies. It is
amazing to think that plants and trees have been doing this very job for us for millions of
years, that is, harvesting sunlight and capturing CO
2
to convert to energy (photosynthesis),
in what we call biomass energy. Human beings have been using this energy from biomass
for thousands of years via burning of forestry materials and agricultural waste biomass [12].
Indeed, energy from traditional biofuels remains the largest source of renewables, account-
ing for 60%70% of renewable energy use [12]. Bio-based renewables are carbon neutral
and can be easily integrated into current energy systems. Plants harvest solar energy in the
chemical bond of biomolecules which can be later used for energy production. The entire
human population consumes 12.2 billion tons (equivalent 101 million barrels) of crude oil in
a year (2019), while the Earth receives 90,000 billion tons of oil equivalent solar energy in
one year [1315]. So all the energy humanity needs in an entire year can be supplied by col-
lecting sunlight for an hour. Plants can capture almost 72 billion tons of oil-equivalent
energy in the form of biomass which means one-sixth of biomass can supply our entire
energy demand [16,17].
In the past few years, there has been a lot of focus on developing materials and technolo-
gies for sustainable conversion and the use of biomass energy by converting it into fuels and
value-added chemicals. A sustainable economy where we depend on biomass energy is
depicted in Fig. 41, where we need to make the transition from a traditional hydrocarbon-
based “oil-refinery”to a more sustainable biomass-based “bio-refinery”platform. Biomass
can be used to produce biofuel which currently represents only a total of 1.2%2.0% of
renewables [18]. Bio-derived chemicals are equivalent to their petrochemical molecules
regardless of their origin, however, they have lower carbon footprints and can be used as
FIGURE 4–1Sustainable conversion of biomass into fuel and value-added chemicals.
64 Advanced Catalysis for Drop-in Chemicals

drop-in chemicals and fuel additives [19]. The term “drop-in chemicals”is used for large vol-
ume commodity chemicals and polymers, and bio-derived drop-in chemicals represent bio-
versions of existing petrochemicals identical to existing fossil-based chemicals [20]. Current
bio-based drop-in chemicals are more expensive compared to their fossil counterparts due
to less mature catalytic and processing technologies. The conventional first generation of
biofuels and chemicals were produced from sugary, starchy, and fatty oil-rich crops [21]. For
instance, a wide variety of starchy crops such as sugarcane (Brazil), wheat, and sugar beets
(Europe), corn (United States), cassava, potato, rice, oats, grapes, and other fruits were used
for bioethanol production via fermentation. As per the regulations, it is mandatory to blend
10% ethanol into gasoline fuel. Similarly, biodiesel, a long-chain fatty acid ester prepared
by transesterification of fatty acids with alcohol (usually methanol), was produced from fatty
acid-rich biomass. The debate on “food versus fuel”criticizes the use of wide arable land
and food crops just for the production of fuel, which now widely accepted is that first-
generation biofuels cannot completely replace fossil fuels [22]. To overcome these issues,
researchers are looking to develop second-generation biofuels that can convert lignocellulose
biomass in nonedible plant matter, waste vegetable oils, and agricultural wastes to fuels and
chemicals [23,24].
Lignocellulose is composed of cellulose (by polymerization of glucose), hemicellulose (a
polymer of glucose and xylose), and lignin (cross-linked polymer built of substituted phe-
nols) [25]. Cellulose represents the largest fraction of renewable biomass produced via pho-
tosynthesis (22 310
9
tons annually); an actual estimated annual production of 7.5 310
10
tons [26,27]. Approximately 4 310
9
ton/year cellulose derived from wood (timbers), grass,
sugarcanes, etc., are readily available for the production of fuel and chemicals. Nonfood
crops such as switchgrass and giant miscanthus can be used as energy crops [28]. Therefore,
it is desirable to use lignocellulosic biomass as feedstock. The various reaction pathways
for renewable fuels and chemicals that can be obtained from lignocellulosic biomass are
summarized in Fig. 42. Currently, there are two major biomass valorization platforms,
employing either thermochemical or biological processes. Thermochemical processes such
as pyrolysis are the preferred conversion method for biomass to liquids [3032]. Thermal
pyrolysis and fractionation of lignocellulosic biomass allow easy processing of C5 and C6
sugars in biomass to convert them into important platform chemicals such as furfural,
hydroxy-methyl-furfural (HMF), and levulinic acid. A recently developed biphasic system
that includes acidic treatment followed by an organic solvent such as 2-sec-butylphenol has
increased yields of platform chemicals [29]. However, the requirement of harsh acidic/basic
condition to breakdown the recalcitrant cell walls of the lignocellulosic (polymer-lignin-
hemicellulose) biomass makes the process costly. Additionally, the major fraction of cellulose
present in the cell is highly crystalline such as cellulose nanocrystals and cellulose nanofibers
[33]. The biomass feedstocks are usually rich in oxygen and moisture contents that results
in the production of thermally unstable and oxygenated chemicals. The deoxygenation of
these oxygenated chemicals involve the removal of oxygen as water, producing high-value
chemical feedstocks that can substitute petroleum or petroleum-derived chemicals [34].
While thermochemical processes for biomass conversion can produce biofuels, chemicals,
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 65

and generate power, they are carbon and energy-intensive processes. Large scale processes
to convert biomass to fuels and commodity chemicals will rely on efficient catalytic systems
which can make the biorefinery concept sustainable. The key challenge for the bio-refinery
platform will be the development of efficient catalytic systems that can selectively transform
bio-derived organic molecules into valuable commodity chemicals.
Several catalytic systems have been widely investigated for biomass valorization and
upgrading biomass to renewable chemicals and fuels [3537]. A plethora of micro/mesopor-
ous material with uniform pore size distribution such as zeolites, MCM-41, MSU with mono-
and bifunctional catalytic sites have been studied for catalytic biomass conversion [38]. The
presence of acidic, basic, or bifunctional sites play a crucial role in product formation. For
example, H-ZSM-5 (a zeolite with the structure code MFI) promotes deoxygenation, decar-
boxylation, and decarbonylation, protolytic cracking or β-scission, alkylation, isomerization,
cyclization, oligomerization and aromatization reactions of bio-mass catalyzed by acidic sites
via a carbonium ion mechanism [39,40]. However, coke deposition and tar formation are
challenging issues which are temporarily overcome by burning coke at high temperatures in
FIGURE 4–2Reaction pathways for renewable fuels and chemicals from lignocellulosic biomass [29].Adapted with
permission from S.G. Wettstein, D.M. Alonso, E.I. Gürbüz, J.A. Dumesic, A roadmap for conversion of lignocellulosic
biomass to chemicals and fuels, Curr. Opin. Chem. Eng., 1 (2012) 218224. Copyright 2012 Elsevier.
66 Advanced Catalysis for Drop-in Chemicals

the air. As the conversion of biomass involves the removal of oxygen contents of bio-derived
products via deoxygenation, hydrodeoxygenation, hydrogenation, and hydrogenolysis/dehy-
droxylation, noble metal-catalyst envisages as promising catalysts. Noble metal, due to their
innate affinity to hydrogen adsorption and high specific surface energy, can promote a vari-
ety of reactions such as hydrogenation and deoxygenation. Furthermore, the activity can
be boosted by the deposition of noble and transition metals on micro/mesoporous hybrid
materials such as activated carbon, mesoporous clay, and zeolites. Unfortunately, the limited
abundance of noble metals in earth and high cost restrict their application. To subside the
noble metals, abundant transition metals such as nickel (Ni) and cobalt (Co) deposited on
zeolites (ZSM-5) are also used for upgrading biomass pyrolysis products [41]. Alloy and
bimetallic catalysts were also found to improve overall conversion efficiency. Some transition
metal carbides (an interstitial alloy of metal and carbon with C:N ,0.59) have shown noble
metal-like properties especially in hydrogenation reactions, which stimulated hope to develop
inexpensive catalysts with similar properties [42]. Indeed, a few metal carbides such as W
2
C
and Mo
2
C displayed platinum-like properties (sometimes better) and promote biomass con-
version by CaC, CaOaC, and CaOaH bonds cleavage more specifically for the hydrogena-
tion reactions [43]. However, due to the high oxygen content in biomass feedstocks, the
presence of multiple components requiring different reaction conditions, low selectivity
due to side reactions, and catalyst poisoning under operating conditions, most of the catalysts
display poor performance in biomass conversion [44,45]. The effective distribution of specific
catalytic centers and the maximization of the number of catalytic sites are the keys to design-
ing more active and product selective catalysts. During past decades, numerous catalytic sys-
tems with a fine distribution of transition metal on active supports have been reported,
however, controlling the size and nature of catalytic sites remains challenging. Single-atom
catalysts (SACs) are new frontiers in catalysis due to their exceptional selectivity, reactivity,
and recyclability. The availability of every single-atom and their specific stabilization on active
support enhance catalytic activity and selectivity. Combined with the two most significant
attributes: (1) high activity like in homogenous catalysts, and (2) better recyclability like in
heterogeneous catalysts, SACs can solve the aforementioned issues. The next sections are a
detail discussion on SACs and their applications in biomass conversion.
4.2 Single-atom catalysts
In recent years, single-atom catalysts (SACs) have emerged as a promising class of material
to target catalytic reactions that traditionally suffer from low selectivity such as biomass con-
version. SACs contain isolated individual atoms dispersed on the surface of appropriate sup-
ports and provide an alternative strategy to tune the activity and selectivity of catalytic
reactions. Noble metals such as Pd, Pt, Ru, Rh, Au, Ag, etc. are superior heterogeneous cata-
lysts that are traditionally used in the petrochemical industry due to their exceptional reactiv-
ity. However, limited abundance and high cost make it essential to downsize catalysts
economically. Transformation of bulk materials into nano-regime (1100 nm) increases the
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 67

performance of catalysts because the exposed catalytic site at the edge of nanoparticles
works as a catalytic center. The physical, chemical, mechanical, and optoelectronic proper-
ties of nanoparticles are highly size-dependent; in the size range of 110 nm (nanoclusters),
the nanoparticles (NPs) display entirely different properties than bulk and individual atoms.
Such changes are observed due to the change in the geometric and energic state due to the
discretization of orbitals.
Further, reduction of metal size below the sub-nano regime (in atomic-scale Å range)
tremendously alters the catalytic performance due to the low-coordination environment of
metal atoms, increased surface energy, high atom utilization, and improved strong metal-
support interactions. SACs bridge the gap between homogenous and heterogeneous catalysis
by offering ultimate atom utilization due to accessibility of each active site, like homoge-
neous catalysts, while preventing leaching of metal-like heterogeneous catalysts, resulting in
better selectivity and cost reduction [46,47]. The specific electronic and geometric configura-
tion, due to the absence of M-M bonds and presence of the cationic (or sometimes anionic)
catalytic sites, provides unique and tunable selectivity for an SAC [48,49]. For example,
Pt
1
/FeO
x
SACs promotes the chemo-selective reduction of 3-nitrostyrene while neither pure
Fe
2
O
3
support nor the Pt
1
/FeOOH-RT were active at all for the reaction [50]. Moreover, low-
ering the coordination number of Pt-O led to a lower oxidation state of Pt single-atoms and
a higher catalytic activity.
Additionally, by tuning the structural attributes of support, the nature and spatial distribu-
tion of metal centers can be controlled to achieve 100% selectivity like an enzyme [51]. Since
the first report of Tao Zhang in 2011 on the synthesis of isolated Pt single-atoms on FeO
x
nano-crystallites for CO oxidation, numerous SACs have been developed [52]. The metal
atom can catalyze a reaction either individually or coordinate with supporting material to get
activated. When a single-atom is stabilized on the second metal surface, single-atom alloy
(SAA) is the preferred terminology [53]. While mentioning SACs, It should be strictly under-
stood that they are comprised of isolated single atoms free of small organic ligands on a sup-
porting material and do not include gaseous metal atoms or homogeneous metal complexes
on any surface, such as silicate or zeolites. However, the removal of organic ligands can
afford monoatomic SACs. So the later homogenous metal complex supported on zeolites
are an example of a single-site heterogeneous catalyst (SSHC), which possess identical cata-
lytically active sites dispersed on a solid surface [54]. Contrary to SSHC, in SACs not all the
catalytic sites are identical due to surface heterogeneity of the supporting matrix. Despite the
observed performances for various reactions, significant challenges associated with the syn-
thesis, stabilization, and application of SACs exist as well. A major challenge in the synthesis
of SACs is to precisely control and determine the coordination environment of the single-
atoms. The coordination affects the electronic environment which concomitantly affects
reactivity and selectivity. Another key challenge in the application of SACs is the stabilization
of isolated metals on supports without compromising catalytic activity, especially at high
temperatures or under harsh reaction conditions. (Fig. 43)
Various methods, such as impregnation, photochemical reduction, high-temperature
trapping, atomic layer deposition, and pyrolysis of metal organic frameworks (MOFs) have
68 Advanced Catalysis for Drop-in Chemicals

been used to prepare SACs [5660]. Many supports such as metal oxides (TiO
2
, ZnO, ZrO
2
,
FeO
x
), metal nitrides (TiN), phosphomolybdic acid, etc., have been used for the fabrication
of SACs [6165]. The most commonly used supports for the fabrication of single-atom cata-
lysts are oxides due to a high specific surface area, the presence of plenty of oxygen/metal
vacancies, and surface oxygens (-OH) that stabilize the single metal atoms [58,66]. The strong
metal-support interaction helps in improving thermal stability at elevated operating tempera-
tures. In recent years, two-dimensional (2D) materials have also emerged as excellent sup-
porting materials to fabricate SACs because of their high surface area, excellent conductivity,
and the possibility of chemical functionalization [6769]. The incorporation of single-atoms
in the framework of 2D materials can lead to synergistically improved performance due to
the stabilization of single-atom via coordination and charge transfer between 2D materials
and a single-atom. Numerous 2D materials such as MoS
2
,WS
2
, and MXenes (i.e., Ti
3
C
2
)
have been used for the synthesis of SACs [7075]. Recently, graphitic carbon nitride (g-C
3
N
4
),
a 2D polymer, constituted of tris-s-triazine (heptazine) units linked together with tertiary
amine, has emerged as an ideal candidate to fabricate SACs due to its intriguing chemical,
physical, and optical properties [8,14,76]. Carbon nitride-based SACs can be synthesized at
low temperatures and have high metal loading due to the accommodation of metals in the
in-plane cavities via coordination with N atoms [7779]. Although many SACs fabricated on
various supports have already been gaining popularity for the synthesis of fine chemicals,
only limited reports are available on their applications in biomass transformations. In the
next sections, we will review the work done on developing SACs for important catalytic con-
version of biomass and its derived molecules.
FIGURE 4–3Geometric and electronic structures of single-atom, clusters, and nanoparticles [55].Reproduced with
permission from L. Liu, A. Corma, Metal catalysts for heterogeneous catalysis: from single-atoms to nanoclusters
and nanoparticles, Chem. Rev., 118 (2018) 49815079. Copyright 2012 American Chemical Society.
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 69

4.3 Single-atom catalysts for biomass utilization
4.3.1 Hydrodeoxygenation of furfuryl alcohol to 2-methylfuran
The hydrodeoxygenation (HDO) of lignocellulosic biomass via hydrogenation and cleav-
age of specific CaO bonds is an attractive reaction to achieve a sustainable carbon-neutral
economy [80,81]. Noble metals are excellent hydrogenation catalysts, however, they promote
nonselective hydrogenation of CQC bonds resulting in ring hydrogenation and ring-opening
reactions (ring chemistry) of furan and aromatic rings of biomaterials [82,83]. On the other
hand, metal oxides are highly active and selective toward CQO hydrogenation and cleavage
of hydroxyls group attached to rings without ring chemistry [84]. The selective CaO bond
cleavage via the reverse Marsvan Krevelen mechanism takes place on hydrogen-generated
oxygen vacancies as active redox centers, and surface oxygen is regenerated via CaO bond
scission for the continuation of the catalytic cycle [85]. The rates of CaO bond cleavage for
oxygen regeneration and oxygen vacancy generation via H
2
should be equilibrated to get a
robust catalyst. The more reducible metal oxide enhances catalytic activity due to increased
numbers of vacancies; however, it renders the oxide less stable towards reduction. Previous
studies based on temperature programmed reduction (TPR) show a decrease of reduction
temperature on noble metal-metal oxides [86]. Further, noble metals and reducible metal
oxides can also work synergistically for improving the activity and selectivity for the deox-
ygenation of various biomass-derived substrates [87,88]. So a metal oxide surfaces doped
with noble metals should be an ideal catalyst to increase the surface activity and HDO
turnover number, by promoting the formation of vacancies and redox centers without
compromising bulk stability. Distinguishably, the use of a metal single-atom catalyst can fur-
ther enhance the turn over frequency (TOF) and selectivity of the reaction.
Fu et al. demonstrated that atomically dispersed Pt-doped TiO
2
with an ultralow-loading
of Pt (0.04%) could selectively transform furfuryl alcohol (FA) to 2-methylfuran (2MF) [89].
The FA was chosen as a model substrate for the reaction because of its structural similarity
with 5-hydroxymethylfurfural (HMF), a furan derivative produced from sugar valorization.
Based on DFT, the hydrogen-assisted Gibbs energy for vacancy formation for Pt doped TiO
2
was calculated by considering five and six oxygen atom coordinations for Pt. As (101) facet of
TiO
2
was predominated (97% surface) and doped Pt mainly catalyzes the reaction in compar-
ison to Pt-adsorbed catalyst, the energy profiles of various reaction intermediates were calcu-
lated on this model. A catalytic cycle constituted FA adsorption, CaO bond scission, with H
back donation from the leaving -OH to C, followed by 2MF desorption, was finally drawn. At
0.04% loading, Pt atoms were highly active and when the loading was increased to 1%, they
tended to aggregate to form nanoparticles. Extended in-situ x-ray absorption fine structure
70 Advanced Catalysis for Drop-in Chemicals

(EXAFS) spectras of 1% Pt nanoparticle-loaded TiO
2
show a sharp peak at 1.86 Å due to the
bond between Pt atoms and the TiO
2
matched with ɑ-PtO
2
(PtO bond .1.86 Å, the
nearest Pt, and the second-shell O 2.7 and 3.3 Å). A weak peak around 2.3 Å was observed
due to the Pt-Pt bond (2.3 Å and 2.9 Å) as present in Pt metal. For the lowest Pt loaded TiO
2
(0.04%Pt-TiO
2
) sample, the peaks due to the first-shell PtPt contribution and the second-
shell PtO contribution being absent, that validates the absence of Pt nanoparticles. Further,
x-ray absorption near edge structure (XANES) and x-ray photoelectron spectroscopy (XPS)
spectra confirm that Pt was cationic and remains oxidized even at elevated temperature.
Further, Fourier transform infrared (FTIR) spectrum of 1% Pt/TiO
2
showed a broad peak
from 2060 to 2089 cm
21
, assigned to CO adsorbed on well-coordinated (B2080 cm
21
) and
undercoordinated (2,060 cm
21
)Pt
0
metal sites. As the Pt loading was decreased to 0.04%, a
new peak at 2118 cm
21
could be assigned to CO adsorbed on isolated Pt
δ1
atoms, suggesting
the presence of isolated Pt atoms. To probe the synergistic role of atomic Pt and TiO
2
in
HDO reaction, pristine TiO
2
and silica-supported Pt nanoparticles (0.1% Pt/SiO
2
) were tested
for the HDO of FA. As expected, pure TiO
2
displayed low HDO activity without affecting the
ring chemistry while 0.1% Pt/SiO
2
showed nonselective HDO with FA ring hydrogenation.
When 0.04% Pt/TiO
2
is used as catalyst, no ring chemistry was observed and the HDO rate
was increased 31-fold in comparison to pristine TiO
2
ascribed to an increased number of
oxygen vacancies in the vicinity of Pt. Highly dispersed Pt stabilized on TiO
2
hampers the
ring chemistry due to the ensemble effect. Furthermore, adsorption of 2,5-dimethylfuran
(DMF) was weak on the atomically dispersed catalyst, and the bond length of the furan ring
remains the same as those in gaseous DMF (Fig. 44).
4.3.2 Cellulose to ethylene glycol and acetol conversion
Nickel catalysts are widely used for activated hydrogen-assisted biomass conversions such
as hydrogenation, hydrogenolysis, and hydrodeoxygenation reactions due to its inexpensive,
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 71

FIGURE 4–4(a) HADDF-STEM images of 0.04% Pt/TiO
2
, and (b) 1% Pt/TiO
2
catalysts after reduction. Arrows mark
isolated Pt atoms (Pt
iso
); (c) The k
2
-weighted EXAFS spectra of Pt/TiO
2
catalysts and Pt standards at the Pt L
3
edge.
χ(R) denotes the magnitude of the Fourier-transformed EXAFS spectra. Identical parameters are used for
background removal and forward Fourier transform: the frequency cut-off parameter R
bkg
51.1 Å, k range
Δk5210 Å
21
. The dashed lines indicate peaks corresponding to the PtPt first coordination shell, (d) FTIR-CO
spectra of Pt/TiO
2
. The dashed line on left indicates the position of the peak assigned to CO adsorbed on isolated
72 Advanced Catalysis for Drop-in Chemicals

earth abundance, and excellent activity [9092]. Unfortunately, their susceptibility to agglom-
erate, leach out and deactivation via dissolution under acidic conditions, restricts their appli-
cation. The encapsulation of Ni in porous supports and alloy formation with other metals is
envisaged to solve the problem, but it compromises activity due to mass transfer limitations
[93,94]. SACs with a composition MaNaC (M metals such as Fe, Co, Ni) containing a metal
center coordinated with N core-bonded with carbon skeleton has been proven an excellent
acid and heat resistant catalyst [9597]. Achieving high metal loading for NiaNaC SACs is
highly desirable to get practical hydrogenation rates for the application in biomass valoriza-
tion. A NiaNaC SAC with a high Ni content c.a. 7.5 wt.% (ICP) was prepared by pyrolysis of
a Ni(phen)
3
complex dispersed on MgO at 600CinN
2
for 2 h followed by etching of MgO
supports [98]. Among catalysts prepared at 500, 600, and 700C, the NiaNaCa600C shows
the highest surface area reaching up to 500 m
2
g
21
while for NiaNaCa700 the surface area
was decreased due to structural collapse of nanoparticles formation. XANES spectra of the
three NiaNaC SACs show distinct features. The order of E
0
value for the oxidation state was
found Ni foil ,NiaNaCa700 ,Ni
II
Pc ,NiaNaCa600 ,NiaNaCa500 ,Ni
2
O
3
,
suggesting the oxidation state of Ni single-atoms is between Ni
II
and Ni
III
. The XANES spec-
trum of the NiaNaCa600 catalyst displayed four distinct peaks A, B, C, and D at 8333 (pre-
edge), 8339 (pre-edge), 8351 (absorption-edge) and 8366 eV (post-edge). The pre-edge peaks
at 8333 and 8339 eV were assigned to 1 s!3d, 1 s!4p
z
electronic transition suggesting: (1)
Ni cations are in a distorted octahedral geometry originating from slight (p-d) orbital mixing,
and (2) square-planar M-N4 moiety in a distorted state as compared to Ni
II
Pc [99,100]. The
absorption edge peak C was corroborated to transition from 1 s to 4p continuum states while
peak D was originated from the multiple scattering. Density functional theory (DFT) calcula-
tions on various models were run; the corresponding XANES spectrum calculated show only
(NiaN4)UUUN configuration perfectly matched with the experimental spectrum of
NiaNaCa600 catalyst. In this model, four pyridinic N atoms were coordinated with the cen-
tral Ni to form a distorted quasi-planar Ni-N4 structure and the fifth pyridinic N atom weakly
coordinated to the Ni atom in the perpendicular direction, suggesting distorted octahedral
coordination geometry Ni-N5 structure with a ligand missing. XPS spectra also show a 4.4:1
ratio between pyridinic N and Ni that was in close agreement with XANES optimized Ni-N5
structure, validating a homogeneously distributed structure throughout the catalyst. EXAFS
results at the Ni K-edge showing a peak at 1.40 Å, similar to the feature of Ni
II
Pc and after
the EXAFS fitting exhibited two distinct Ni-N coordination shells, with a 3.4 and 1.7
L
Pt atoms, whereas the yline on right indicates a peak maximum at 2,089cm
21
for CO on 1% Pt/TiO
2
.; (e) deconvoluted
Pt4f, Cu3p in situ XPS spectra of 0.04% Pt/TiO
2
measured at room temperature and vacuum; and (f) 1.5 Torr H
2
,and
promotional effects of Pt dopant on oxygen vacancy formation and selective CO bond scission; (g) quantitative EPR
analysis at room temperature; (h) HDO and ring saturation (RS) rates over Pt/TiO
2
(squares and circles with boundry,
respectively) and Pt/SiO
2
(square and circle without boundry). The RS rate is below detection for #0.06% Pt/TiO
2
.
(i) DMF hydrogenation rate at various Pt loadings. Reaction conditions: 0.088 M FA (or DMF), 20 mL 2-propanol,
100 mg catalyst, reaction temperature at 160C, 100 psi H
2
, and 160 psi N
2
measured at room temperature [89].
Reproduced with permission from J. Fu, J. Lym, W. Zheng, K. Alexopoulos, A.V. Mironenko, N. Li, et al., CO bond
activation using ultralow loading of noble metal catalysts on moderately reducible oxides, Nat. Catal., 3 (2020)
446453. Copyright 2020 Nature Publishing Group.
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 73

coordination that matched well with XANES and DFT results. The reported catalyst demon-
strated excellent reduction performance for a wide variety of unsaturated functional groups
present in the biomass such as nitroarenes (aNO
2
), aldehyde (aCQO), styrene (aCQCa),
phenylacetylene (aCRCa), and quinoline (aCQNa). The control experiment using
pyrrole as a Lewis acid and SCN
2
ions as Ni cations poison (Lewis acid sites) showed a
remarkable drop in activity, confirming Ni cations (Lewis acid sites) and the neighboring
non-coordinated pyridinic N atoms (Lewis basic sites) work as a frustrated Lewis pair.
Additionally, the actual hydrolysis of cellulose using NiaNaCa600 and tungstic acid (pro-
motes CaC cleavage via a retro-aldol pathway) under harsh reaction condition (245 C,
60 bar H
2
, strongly acidic hydrothermal conditions) yielded EG (36.8%) and acetol (17.4%) as
main products (other products: hydroxyacetone, glycerol, isosorbitol, mannitol, 1,2-butane-
diol, erythritol 1,2-propylene glycol, etc.) without deactivation of the catalyst. The catalyst
showed a negligible reduction in activity after several reaction cycles and had identical
atomic distribution as in fresh catalyst (Fig. 45).
4.3.3 Hydrogenation of furfural to furfuryl alcohol
Furfural is an important biomass feedstock that can be used to produce a wide variety of
chemicals [101,102]. The selective chemical hydrogenation of furfural to furfuryl alcohol is an
important step to produce fine industry chemicals and biofuels. Current processes for hydro-
genation of furfural to furfuryl alcohol rely on the toxic chromium-based catalyst and require
harsh reaction conditions [103]. Noble metals are excellent hydrogenation catalysts, however,
due to poor selectivity, side products such as tetrahydrofurfural, tetrahydrofurfuryl alcohol,
furan, tetrahydrofuran, or levulinic acid are produced in large quantities [104106]. SACs of
noble metals have the potential to selectively reduce furfural to furfuryl alcohol and alleviate
the use of toxic chemicals. To check the performance for hydrogenation of furfural to furfuryl
alcohol on SACs, iridium single-atom catalysts were made on defective phase titanium diox-
ide nanosheets (Ir
1
/def-TiO
2
(B)) and mesoporous graphitic carbon nitride (Ir
1
/mpg-C
3
N
4
)
[107]. It was found that the as-prepared Ir
1
/def-TiO
2
(B) catalyst exhibited outstanding con-
version (99%), high selectivity (99%), and good stability in the hydrogenation of furfural to
furfuryl alcohol, which is superior to Ir
1
/mpg-C
3
N
4
and the Ir nanoparticles. The increased
peak intensity of def-TiO
2
(B) in the Ti K-region of XANES spectra and presence of EPR sig-
nals at a g-value of 2.003 in comparison to nondefected per-TiO
2
(B) suggest the presence of
oxygen point defects. The TEM and HAADF-STEM images of 0.10 wt.% Ir-containing catalyst
did not show the presence of any nanoparticles. Aberration-corrected HAADF-STEM which
differentiates the high atomic number Ir from Ti and O due to Z-contrast showed many small
74 Advanced Catalysis for Drop-in Chemicals

bright dots, validating the homogeneous distribution of Ir on the def-TiO
2
(B). The Ir L3-edge
XANES spectra and XPS spectra of the Ir
1
/def-TiO
2
(B) and the Ir
1
/mpg-C
3
N
4
catalyst reveals
oxidized Ir
1
species with a different positive charge for def-TiO
2
(B) and mpg-C
3
N
4
support.
The presence of peak due to Ir 2O shell (R B1.7 Å) and the absence of Ir-Ir peak in EXAFS
spectra further confirm atomically distributed Ir in an oxidized state. The first principle calcu-
lation demonstrates four types of oxygen vacancies among which (010) surface configuration
in which the Ir atom is bonded to three Ti atoms is energetically most favorable.
Further, the adsorption energies of furfural on Ir
1
/mpg-C
3
N
4
,Ir
1
/def-TiO
2
(B), and Ir
147
nanoparticle were found to be 0.60, 1.63, and 3.41 eV respectively, suggesting furfural bound
very weakly to Ir
1
/mpg-C
3
N
4
while very strongly to Ir
147
nanoparticles. The moderate bond-
ing of furfural with the Ir
1
/def-TiO
2
(B) sample was caused by the interaction of furfural
FIGURE 4–5(a) Synthetic scheme of the Ni-N-C catalyst; (b) HAADF-STEM image; (c) fast Fourier transform (FFT
image); (d) intensity profiles along the lines at positions 1 and 2 in the HAADF-STEM image; (e) comparison of
experimental and calculated XANES spectra of the NiaNaCa600, and the inset shows the structure model of Ni-
N5; and (f) the durability of the NiaNaCa600 in comparison with Ni/AC for the one-pot conversion of cellulose.
Reaction conditions: 0.3 g cellulose, 50 mg catalyst, 30 mg tungstic acid, and 30 mL water in 100 mL autoclave,
245 C, 6.0 MPa H
2
, 1 h 45 min, 1000 rpm. Product Code: EG: ethylene glycol; HA: hydroxyacetone; iso-Sor:
isosorbitol; Man: mannitol; Gly: glycerol; 1,2-PG: 1,2-propylene glycol; 1,2-BDO: 1,2-butanediol; Ery: erythritol and
others [98].Reproduced with permission from W. Liu, Y. Chen, H. Qi, L. Zhang, W. Yan, X. Liu, et al., A durable
nickel single-atom catalyst for hydrogenation reactions and cellulose valorization under harsh conditions, Angew.
Chem., 130 (2018) 71897193. Copyright 2018 Wiley-VCH.
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 75

molecules with Ir single-atom and an adjacent Ti atom, which increases the adsorption
energy resulting in the best catalytic performance. In contrast, furfural adsorbs on Ir
1
/mpg-
C
3
N
4
with an Ir single-atom and no neighboring atoms participate in bonding, resulting in
the weak interaction. Similarly, for the Ir
147
nanoparticles, a strong chemical interaction of
furfural with four Ir atoms resulted in the strongest interaction and least desorbed product.
An energy coordinate of furfural to FA was calculated that proceeds through adsorption of
furfural and H
2
to formaldehyde and a successive reduction to alcohol. The recycling experi-
ment shows that Ir
1
/def-TiO
2
(B) catalyst remains equally active as a fresh catalyst and shows
atomic dispersion even after five runs, demonstrating the stability of the catalyst (Fig. 46).
FIGURE 4–6(a) TEM image and (b) HAADF-STEM image of an Ir
1
/def-TiO
2
(B) nanosheet; (c) EDX elemental mapping
analysis for the distributions of Ti, O, and Ir; (d) AC HAADF-STEM image of the Ir
1
/def-TiO
2
(B) sample and X-ray
absorption analysis; (e) XANES spectra at the Ir L
3
- edge of the Ir power, Ir
1
/def-TiO
2
(B), Ir
1
/mpg-C
3
N
4
, and the IrO
2
samples; and (f) Fourier transform (FT) at the Ir L
3
-edge of the above four materials. (g) and (h) Corresponding fits
of the EXAFS spectrum of Ir
1
/def-TiO
2
(B) at the R space and the k space, respectively, (solid line: experimental data;
dots: fitting data). The inset of panel (g) shows the local structure of the Ir
1
species on def-TiO
2
(B) obtained from
the DFT simulations; (i) Adsorption configurations of a furfural molecule on the Ir
1
/ mpg-C
3
N
4
monolayer (left), the
Ir
1
/def-TiO
2
(B) substrate (middle), and the Ir
147
nanoparticle (right); and (j) Energy profile (unit: eV) and reaction
pathway for the catalytic hydrogenation of furfural to furfuryl alcohol at the Ir
1
active site on def-TiO
2
(B).
Structures of all the intermediate states are also shown on the energy profile [107].Reproduced with permission
from S. Tian, W. Gong, W. Chen, N. Lin, Y. Zhu, Q. Feng, et al., Regulating the catalytic performance of single-
atomic-site Ir catalyst for biomass conversion by metalsupport interactions, ACS Catal., 9 (2019) 52235230.
Copyright 2019 American Chemical Society.
76 Advanced Catalysis for Drop-in Chemicals

4.3.4 Hydrogenation of levulinic acid to γ-valerolactone
The valorization of cellulosic biomass also produces levulinic acid (LA) which serves
as an important precursor for green solvents (γ-valerolactone), plasticizers, biofuel, pharma-
ceutical, cosmetics (ethyl levulinate), biodegradable herbicides (aminolevulinic acid), fuel
and additives (2-methyl-THF), etc [109112]. Hydrogenation of levulinic acid (LA) to
γ-valerolactone (GVL) is of utmost importance as GVL is the platform molecule used in
many of those upgrading routes [113115]. Existing biomass valorization techniques operate
in the liquid phase under harsh reaction conditions such as high temperature and pressure,
and use corrosive acidic reaction media. Noble metals catalysts such as Ru, Pt, Ir, Pd, etc.,
supported on an inert substrate such as activated carbon, has shown great promise for
hydrogenation of LA to GVL, but leaching from the substrate, cessation of catalytic activity
in acidic conditions, and nonselective hydrogenation leading to by-product formations are
major issues [116118]. Ir-based catalysts, especially homogeneous metal complexes such as
polyazines, and pincer complexes, can reach up to 99% conversion efficiency with good
selectivity, but their difficult and tedious separation makes them inapplicable [119,120].
To overcome these problems, Cao et al. synthesized an ultra-stable 0.6 wt.% Ir@ZrO
2
@C
SAC via an in-situ grafting of Ir metal during assembly of UiO-66 MOF, followed by confined
pyrolysis [108]. The XPS spectra of 0.6 wt.% Ir@ZrO
2
@C SAC showed a single peak due to
Ir
δ1
from an isolated atomically dispersed Ir coordinated with the ZrO
2
matrix while Ir/C
and Ir/ZrO
2
reveals Ir
0
,Ir
δ1
, and Ir
41
peaks due to the presence of metallic Ir, isolated Ir
single-atoms, and IrO
x
/IrO
2
species. The H
2
-TPR analysis shows only one reduction peak for
Ir@ZrO
2
@C SAC at a relatively higher temperature (182C) compared to Ir/C nanocatalyst
and Ir/ZrO
2
which shows Ir oxide reduction peaks around 82C. This confirms the presence
of only single-atom sites in the catalyst. The synthesized Ir@ZrO
2
@C SAC displayed excellent
stability in the selective hydrogenation of LA to GVL without a sign of deactivation even after
six and seven consecutive runs at pH 53 and one respectively at 180C and 40 bar H
2
. The
performance of SAC was compared with standard 2.7 wt.% Ir/C and 0.6 wt.% Ir/ZrO
2
nanoca-
talysts, which showed significant deactivation and leaching under the same conditions. The
2.7 wt.% Ir/C catalyst show 5.8% leaching after the first recycle while 0.6 wt.% Ir/ZrO
2
catalyst
show 6.7% leaching and a GVL yield drop from B56% to 43% after three runs with pH-3.
The STEM, EDX mapping, and AC-HAADF-STEM measurements of 0.6 wt.% Ir@ZrO
2
@C
SAC show atomic dispersion of Ir and no aggregation even after seven reuses. The LA to
GVL conversion yield of Ir/C and Ir/ZrO
2
was 95% in 20 min and 98% in 80 min corre-
sponded to 0.67 s
21
and 0.32 s
21
, turn over frequency (TOF). The obtained GVL yield of
Ir@ZrO
2
@C SAC was much lower (98.8% after 10 h; TOF of 0.034 s
21
), however, an astonish-
ing .99% selectivity without a trace of any by-product.(Fig. 47)
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 77

4.3.5 Oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran
5-Hydroxymethylfurfural (HMF) is another platform molecule derived from the acid-
assisted valorization of biomass [121]. HMF can be oxidized to 2,5-diformylfuran (DFF) which
is an interesting substrate to produce biopolymers, drugs, antifungal, macrocyclic ligands, etc
[122,123]. The transformation of HMF to DFF is a challenging reaction from the chemistry
point of view because it requires controlled oxidation of primary alcohol while leaving the
aldehyde group intact. Many stoichiometric/nonstoichiometric catalytic systems such as pyri-
dinium chlorochromate, K
2
Cr
2
O
7
dimethylsulfoxide, 2,2,6,6-tetramethylpiperidine-1-oxide
(TEMPO), H
2
O
2
and sodium hypochlorite, have been reported for the oxidation of HMF to
DFF, but they afford poor yield and unselective oxidation [124,125]. Supported noble-metal
nanoparticle catalysts, specifically the Ru-based catalyst, have shown good performance for
FIGURE 4–7(a) Schematic illustration of the in-situ synthesis approach for the synthesis of 0.6 wt.% Ir@ZrO
2
@C,
STEM images of (b) Ir@UiO-66, (c) 0.6 wt.% Ir@ZrO
2
@C, (d) high-resolution STEM image and (e) EDX-mapping
analysis of the 0.6 wt.% Ir@ZrO
2
@C. Time profiles of catalytic conversion of 10 wt.% levulinic acid in water using
(f) 2.7 wt.% Ir/C, (g) 0.6 wt.% Ir/ZrO
2
, and (h) 0.6 wt.% Ir@ZrO
2
@C as the catalysts. The recycling experiments of (i)
2.7 wt.% Ir/C (10 min), (j) 0.6 wt.% Ir/ZrO
2
(20 min), and (k) 0.6 wt.% Ir@ZrO
2
@C (1 h) in a pH 53 aqueous solution.
Ir 4 f XPS of the (l) fresh and spent (three-times recycled) 2.7 wt.% Ir/C nanocatalyst in a pH 53 aqueous solution;
(m) fresh and spent (three-times recycled) 0.6 wt.% Ir/ZrO
2
nanocatalyst in a pH 53 aqueous solution; and (n) fresh
and spent (seven times recycled) 0.6 wt.% Ir@ZrO
2
@C SAC in a pH 53 aqueous solution [108].Reproduced with
permission from W. Cao, L. Lin, H. Qi, Q. He, Z. Wu, A. Wang, et al., In-situ synthesis of single-atom Ir by utilizing
metal-organic frameworks: An acid-resistant catalyst for hydrogenation of levulinic acid to γ-valerolactone, J.
Catal., 373 (2019) 161172. Copyright 2019 Elsevier.
78 Advanced Catalysis for Drop-in Chemicals

catalytic oxidation of HMF into DFF [122,126]. Ru-based SACs can further improve the perfor-
mance and selectivity of the reaction due to the ensemble effect (the combined synchronized
effect of groups/metal/compounds ameliorating the performance of catalyst than individual
components). Recently, Liu et al. used ball milling as a facile, low-cost synthetic strategy for
Ru single-atoms supported on NiO (Ru
1
/NiO) catalyst using ruthenium(III)-2,4-pentanedio-
nate and nickel(II) acetylacetonate (1:400) precursor followed by calcination at 400C[124].
The characterization of Ru
1
/NiO using TEM, STEM, and AC-HAADF shows the atomic distri-
bution of the Ru on the NiO surface. The absence of any Ru nanoparticle peak in XRD
and Ru3d XPS peak due to 12 and 13 state confirm atomically distributed Ru cation in NiO
lattice. Additionally, 0.14 wt.% Pt
1
/NiO and 0.29 wt.% Pd
1
/NiO SACs were also prepared and
tested for the oxidation of HMF into DFF. The Ru
1
/NiO SAC demonstrated the highest con-
version (91.1%) while selectivity reached up to 81.3% higher compared to Pt
1
/NiO (31.6%)
and Pd
1
/NiO (38.4%). Changing the support from NiO to ZnO and FeO
x
in 0.23 wt.%
Ru
1
/ZnO, and 0.26 wt.% Ru
1
/FeO
x
detrimentally reduced the conversion up to 40.6% and
48.4%, respectively, suggesting a synergistic interaction between NiO and Ru was key for
improved performance. The Ru
1
/NiO SAC shows almost unchanged activity after three recy-
cle runs; AC HAADF-STEM and XPS showed no change in the morphological and chemical
state (Fig. 48).
4.3.6 Selective hydrogenolysis of glycerol to 1,2-propanediol or
propylene glycol
Another important biomass-derived platform chemical is glycerol which is the smallest
representative of polyols. Glycerol is produced as a by-product during biodiesel production
from the transesterification of vegetable oils (triglycerides) and methanol [127,128]. A fraction
of glycerol is also produced as a by-product of bioethanol production from lignocellulose
[129]. The production of glycerol during biodiesel production is 10% w/w of the actual prod-
uct means for every 9 kg of biodiesel, 1 kilogram of crude glycerol is produced (Pachauri,
2006) [130]. Since biodiesel production has skyrocketed in the past few years, the price of
glycerol has dropped significantly. Except for some limited applications such as food, cos-
metic, coolant, and polymer industries, the glycerol has limited commercial utility. So it is
imperative to use glycerol for the production of value-added chemicals [19,131].
Various strategies have been proposed for the conversion of glycerol into commodity che-
micals, such as oxidation (oxalic acid, glyceraldehyde, etc), hydrogenolysis (1,2 propanediol,
1,3 propanediol, ethylene glycol), dehydration (acrolein), selective esterification (monogly-
cerides and glycerol dimethacrylate), and carboxylation (glycerol carbonates) [132139].
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 79

Some other approaches are pyrolysis to produce syn-gas and microorganisms assisted con-
version to citric acid, 1,3 propanediol, poly(hydroxyalkanoates) (PHA), etc [140,141]. Among
all these processes selective hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) or pro-
pylene glycol is attractive because of its usage in the synthesis of wide varieties of commodity
chemicals such as unsaturated polyester resin, food additives, antifreeze, and cosmetics. The
main challenge associated with the transformation of glycerol to 1,2-PDO is selective cleav-
age of terminal CO bond without breaking another CaO/CaC bond of the carbon chain.
Alloyed catalyst has shown great promise over single component noble metal (Pd, Ru, and
Pt, etc.) or nonnoble metal catalysts (Cu, Ni, and Co) catalyst due to synergistic interaction
[137,142146]. The performance of bimetallic catalysts can be further maximized by dispers-
ing a single-atom on another active metal to form a single-atom alloy (SAA) catalyst, utilizing
each atom in the surface of the catalyst. The Cu-based catalyst shows high selectivity toward
CaO bond cleavage without scissoring CaC bond, but deactivation at high temperatures
and poor activity are limiting parameters [147]. On the other hand, Pt-based catalysts are
FIGURE 4–8(a) Elemental mapping images Ru
1
/NiO and AC HAADF-STEM image of (b) Ru
1
/NiO. (c) Pd
1
/NiO, (d) Pt
1
/
NiO, (e) Ru
1
/ZnO, and (f) Ru
1
/FeO
x
, (g) Ru3d XPS spectrum of Ru
1
/NiO and (h) Catalyst recycling experiments for
Ru
1
/NiO [124].Reproduced with permission from Y. Liu, T. Gan, Q. He, H. Zhang, X. He, H. Ji, Catalytic oxidation of
5-hydroxymethylfurfural to 2, 5-diformylfuran over atomically dispersed ruthenium catalysts, Ind. Eng. Chem. Res.,
59 (2020) 43334337. Copyright 2020 American Chemical Society.
80 Advanced Catalysis for Drop-in Chemicals

extremely active but have less selectivity for CaO bond cleavage [148]. So for taking advan-
tage of the high selectivity of Cu and high activity of Pt, Zhang et al. prepared a PtCu single-
atom alloy (SAA) catalyst by decorating a single Pt atom on Cu nanoclusters using a hydrotal-
cite layered double hydroxide (CuMgAlLDH) precursor and sequential oxidation at 500Cto
form mixed metal oxides (MMO); the reduction under H
2
followed by a galvanic replacement
reaction [149]. The developed PtCuSAA exhibited excellent catalytic performance reaching a
99.6% conversion and 99.2% selectivity with a turnover frequency (TOF) of up to 2.6 310
3
mol
glycerol
mol
PtCuSAA
21
h
21
for hydrogenolysis of glycerol to 1,2-propanediol under mild
reaction conditions (200C, 2 MPa). HR-TEM images show crystal planes of Cu(111) while
aberration-correction high-angle annular dark-field scanning transmission electron micros-
copy (ACHAADFSTEM) imaging confirmed the atomic distribution of Pt atoms sur-
rounded by Cu atoms. In situ CODRIFTS (diffuse reflectance infrared Fourier transform
spectroscopy) experiment on PtCuNPs and Pt/ MMO samples shows two bands at
20102060 and 18101880 cm
21
due to linear and bridged carbonyl between two adjacent Pt
atoms. PtCuSAA catalyst shows a broad absorption band in between 2060 and 2140 cm
21
which after deconvolution gave two peaks at 2097 cm
21
and 2116 cm
21
due to CO chemi-
sorbed on Cu
0
and Cu
1
species, while the third peak centered at 2088 cm
21
was corroborated
to isolated single Pt atoms linearly bounded to CO. EXAFS of PtCuSAA at the Pt-L3 edge dis-
played a single peak at B2.22.3 Å centered between PtO
2
and Pt peaks, due to different
coordination environment of Pt in SAA. The coordination of PtCuSAA sample was found to
be B7.4 PtCu that did not match with PtPt or PtO coordination, validating an isolated
single Pt atom was surrounded by Cu atoms. In comparison to Pt/MMO which displays no
peak and Cu/MMO which displays a sharp intense peak at 146C, the PtCuSAA sample
shows an asymmetrical peak around 106CinH
2
-TPD, suggesting atomically distributed Pt
enhances hydrogen dissociation and spillover capacity. In situ XANES spectra of PtCuSAA at
CuK edge and Pt-L3 edge show that after the addition of glycerol to fresh catalyst, a fraction
of Cu
0
gets converted to Cu
1
while Pt was transformed into a low oxidation state. Based on in
situ glycerol-DRIFTS measurement, it was found that, for Pt/MMO and Cu/MMO catalysts,
dehydrogenation proceeds with glyceraldehyde as an intermediate and for PtCuSAA acetol
(PtCu) and glyceraldehyde (CuaCu) were the intermediates (Fig. 49).
4.3.7 Temperature controlled selective transformation of vanillin to
vanillyl alcohol and 2-methoxy-4-methylphenol
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 81

Lignin is a main component of biomass, comprised of cross-linked phenolic polymers
and about 30% of wood waste. This lignin-based waste can be pyrolyzed followed by hydro-
genation to get biofuel. Vanillin (4-Hydroxy-3-methoxybenzaldehyde) is a common compo-
nent of lignin pyrolysis oil which can be hydrogenated/hydro-deoxygenated (HDO) to
vanillyl alcohol and 2-methoxy-4-methylphenol, a potential future biofuel [150153]. The
hydrogenation of carbonyl groups into hydroxyl groups is easy. However, selective reduction
of the hydroxyl group via deoxygenation to get alkyl derivative is the main challenge. The
selective hydrodeoxygenation of these phenolic components will play an important role in
upgrading biofuel and the production of value-added products. Tian et al. synthesized meso-
porous carbon nitride, supporting a isolated single-atom Ru catalyst (Ru
1
/mpg-C
3
N
4
)bya
simple wet impregnation method [154]. The absence of any XRD peak for Ru NPs, HAADF-
STEM, and EDX mapping confirms the atomic distribution of Ru in mesoporous structure.
FIGURE 4–9(a) A schematic illustration for the preparation of PtCuSAA, (b) ACHAADFSTEM image of
PtCuSAA, (c) corresponding enlarged images at 2 nm scale bar, (d) In situ CO-DRIFTS spectra of PtCuSAA, and
(e) in situ CO-DRIFTS spectra of Cu/MMO and PtCuSAA at 25C (solid line: experimental data; dotted line: fitting
curve). (f) Surface H
2
TPR spectra of various samples after N
2
Ooxidation pretreatment: Pt/MMO, PtCuSAA, and
Cu/MMO. (g) Catalytic evaluation of PtCu-SAA and monometallic catalysts (Pt/MMO and Cu/MMO) toward glycerol
hydrogenolysis to 1,2-PDO; the TOF value determined as moles of initial glycerol converted per mole of exposed
active sites per hour in the catalytic dynamic range. [149] Reproduced with permission from X. Zhang, G. Cui, H.
Feng, L. Chen, H. Wang, B. Wang, et al., Platinumcopper single-atom alloy catalysts with high performance
towards glycerol hydrogenolysis, Nat. Commun., 10 (2019) 5812. Copyright 2019 Nature Publishing Group.
82 Advanced Catalysis for Drop-in Chemicals

The Ru content was found to be at 0.10 wt.% as determined from inductively coupled plasma
optical emission spectrometry (ICP-OES). Ru K-edge XANES spectra of Ru
1
/mpg-C
3
N
4
show
an absorption edge of single-atom Ru in between Ru and RuO
2
, due to partly positive Ru
interacting with C
3
N
4
support. The presence of a single peak in Fourier transformed (FT)
k3-weighted EXAFS of Ru
1
/mpg-C
3
N
4
due to RuN contribution, and absence of any peak at
B2.6 Å due RuRu contribution, validate the presence of a single-atom in the matrix. The
developed material demonstrated excellent conversion and selectivity for the hydrogenation
and hydro-deoxygenation of vanillin to vanillyl alcohol and 2-methoxy-p-cresol or methoxy-
phenol. Interestingly, hydrogenation of vanillin into vanillyl alcohol at 60C showed B100%
selectivity and 95% conversion. At 120C a mixture of vanillyl alcohol and 2-methoxy-p-
cresol was obtained and further increase in temperature at 140C only 2-methoxy-p-cresol
was observed with B100% conversion and selectivity. The first principle calculation was
used for investigating the reason for this unprecedented selectivity at different temperatures.
Among four possible optimized structures of Ru
1
/mpg-C
3
N
4
, the structure where Ru coordi-
nated with three N atoms and one C atom show the highest adsorption energy for vanillin
and H
2
. The reaction proceeds with adsorption of vanillin on the Ru site, and the free energy
of adsorption was obtained 20.42 eV and 20.21 eV at 60C and 140C, respectively. The
energetics of the intermediate state clearly show that hydrogenated 2-methoxy-p-cresol is
thermodynamically more stable than the partially hydrogenated vanillyl alcohol and the high
energy of IM2 hydrogenation makes vanillyl alcohol as deoxygenation prohibited product
and 2-methoxyl-p-cresol as deoxygenation allowed product (Fig. 410).
4.3.8 2,5-Furandicarboxylic acid via hydroxy-methyl-furfural oxidation
and CO
2
-assisted furoate carboxylation
Transformation of biomass to oxygenates chemicals such as 2,5-furandicarboxylic acid
(FDCA) which serves as a monomer to produce polyethylene furandicarboxylate (PEF) bio-
plastics can remove the dependency on petroleum-derived terephthalic acid [155]. The cur-
rent route utilizes C6 sugar-derived HMF as a substrate for the synthesis of FDCA and
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 83

is inefficient due to poor conversion of lignocellulose-to-HMF and the instability of HMF
[156,157]. Recently, the C5 route that uses a furfural derivative called 2-furoic acid (FA) and
CO
2
as feedstocks to produce FDCA has attracted attention [158160]. The lignin part of bio-
mass is tedious to transform into chemicals due to phenolic structure and processability.
The phenolic structure coordinates with metal ions and can be used for the synthesis of
highly dispersed carbon-based catalysts. Zhou et al. synthesized cobalt/zinclignin (Co/ZnaL)
complexes with lignin by taking advantage of metalphenol coordination, which upon carbon-
ization using dicyandiamide as a source of nitrogen produces N-doped carbon-supported Co
single-atom catalysts (Co SACs/N@C) [161]. HR-TEM images show carbonaceous sheets and
AC HAADF-STEM validates the distinct presence of isolated Co single-atoms. Furthermore,
the XANES absorption edge between CoO and Co
3
O
4
and the presence of a single peak
FIGURE 4–10 (a) EDX element maps of Ru
1
/mpg-C
3
N
4
showing the distributions of Ru, C, N, and their overlap (b)
AC-HAADF-STEM images of the Ru
1
/mpg-C
3
N
4
sample, (c) XANES spectra at the Ru K-edge of Ru
1
/mpg-C
3
N
4
, RuO
2
,
and Ru foil. (d) Fourier transform (FT) at the Ru K-edge of Ru
1
/mpg-C
3
N
4
, RuO
2
, and Ru foil. (e) and (f)
Corresponding fits of the EXAFS spectrum of Ru
1
/mpg-C
3
N
4
in R space and K space, respectively. (gj) Geometries
of four different single-atom Ru catalyst structures on carbon nitride C, N, and Ru atoms are shown in gray, blue,
and green, respectively. (kn) Geometries of vanillin adsorption on the four single-atom Ru sites. (o) Free energy
diagrams of vanillin hydrogenation to vanillyl alcohol and 2-methoxy-2-methoxy-p-cresol at 60C (blue) and 140C
(red). The structures of reactant and products are shown above the energy profiles, while the intermediate state
structures are shown below the energy profiles. The geometry of Ru
1
/mpg-C
3
N
4
is shown in the upper right. [154]
Reproduced with permission from S. Tian, Z. Wang, W. Gong, W. Chen, Q. Feng, Q. Xu, et al., Temperature-
controlled selectivity of hydrogenation and hydrodeoxygenation in the conversion of biomass molecule by the
Ru
1
/mpg-C
3
N
4
catalyst, J. Am. Chem. Soc., 140 (2018) 1116111164. Copyright 2018 American Chemical Society.
84 Advanced Catalysis for Drop-in Chemicals

at 1.4 Å in EXAFS also evidenced the presence of atomically distributed cobalt atoms. Using
Co SACs/N@C as catalyst and Cs
2
CO
3
as a base, 90.5% conversion and 99.8% selectivity
for furfural to FA oxidation was achieved at 65C; while under identical conditions, Co
nanoparticle catalysts (CoNPs/N@C) afforded only 48.6% conversion and 43.3% selectivity.
The calculated activation energy (E
a
) of Co SACs/N@C was found only at 20.8 kJ mol
21
,
much lower than Co NPs/N@C (37.9 kJ mol
21
) and other reported noble metal catalysts,
such as AuPd/Mg(OH)
2
(30.4 kJ mol
21
). The catalyst demonstrates extreme resiliency and
98.4% conversion, and 89.8% selectivity was observed even after six reuse experiments. An
isotopic labeling experiment using
18
O labeled H
218
O and
18
O
2
reveals that H
2
O reacts with
furfural to form geminal diol intermediates via base facilitation reversible hydration, while
O
2
facilitates removal of protons from the active center of the catalyst due via accepting elec-
trons. The reaction proceeds via an aldehyde-water shift (AWS) reaction pathway, reversible
hydration reaction, and subsequent oxidative dehydrogenation. Furoate carboxylation using
CO
2
in the presence of Cs
2
CO
3
affords a 71.1% conversion and 82.9% selectivity for FDCA
production (C5 route). Interestingly, the catalyst also demonstrated unprecedented activity
for HMF oxidation (C6 route) to FDCA under a mild condition that illustrates the potential of
a catalyst to catalyze a wide variety of biomass conversion reactions (Fig. 411).
FIGURE 4–11 (a) Synthesis methodology of the Co SAs/N@C catalyst, catalytic performances of Co SAs/N@C for the
oxidation of furfural to furoate, (b) conversion, and (c) selectivity of furfural oxidation at various temperatures
over Co SAs/N@C and Co NPs/N@C; reaction conditions: 1 mmol furfural, 1.5 equiv. Cs
2
CO
3
, catalyst
(
n
Co/
n
furfural 50.03), 10 mL H
2
O, 1 bar O
2
, 11 h, (d) The Arrhenius plots of Co SAs/N@C and Co NPs/N@C. (e)
Recycling of the Co SAs/N@C catalyst. Convergent production of FDCA from biomass and CO
2
. (f) Synthesis of FDCA
from furoate. (g) Synthesis of FDCA from HMF. (h) Conceptual diagram of convergent FDCA production from
lignocellulosic biomass and CO
2
.[161] Reproduced with permission from H. Zhou, H. Xu, X. Wang, Y. Liu,
Convergent production of 2, 5-furandicarboxylic acid from biomass and CO
2
, Green. Chem., 21 (2019) 29232927.
Copyright 2019 Royal Society of Chemistry.
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 85

4.3.9 Synthesis of cellulosic ethanol by facilitated methyl glycolate
reduction
Ethanol is mostly produced from the fermentation of starch biomass from sugarcane,
potato, corn, wheat, etc. Ethanol is widely used to produce chemicals, solvent, brewery, and
fuel. Water-free ethanol also called “gasohol”with high octane numbers is blended with gas-
oline to reduce environmental impact. Since the 1980s, most gasoline-powered vehicles are
compatible to use ethanol-blended gasoline and, in some countries/provinces (Brazil and
United States), it is mandatory to blend 10% ethanol in gasoline [162]. The ethanol-gasoline
blend called E85 or flex-fuel can have an ethanol percentage of 51% to 83% depending on
geography and season [163,164]. The production of ethanol from biomass is advantageous
as it reduced greenhouse gas emission but soaring food prices due to the usage of arable
land are concerning points. Lignocellulosic biomass derived from agricultural wastes such as
straw, corn stover, wood chips, and energy crops, such as switchgrass, can be used as feed-
stocks for producing ethanol without impacting the food chain supply [165].
Tao Zhang’s group has reported that the cellulose can be converted to ethanol by a
chemo-catalytic route by converting cellulose to methyl glycolate (MG) using methanol on a
W-based catalyst followed by hydrogenation to ethanol over a supported copper catalyst
[166]. However, the total yield of cellulose to ethanol conversion remains as low as 30%. Due
to the high reaction temperature conversion of cellulose to methyl glycolate conversion
remains low for this step. On the other hand, the yield of MG hydrogenation which proceeds
at relatively low temperatures can be further improved by using a more efficient catalyst.
Dimethyl oxalate (DMO) hydrogenation which simulates methyl glycolate hydrogenation has
been widely investigated using a copper-based catalyst, but the formation of ethylene glycol
is thermodynamically favored in comparison to DMO hydrogenation, as EG forms at 200C,
whereas ethanol forms at 260C[167]. To improve the selectivity, platinum single-atoms iso-
lated by copper on silica support were prepared to overcome the disadvantages of Pt nano-
particles that lead to CaC bond cleavage [168]. The PtCu/SiO
2
single-atom alloy catalyst
which has 0.1% Pt shows the increased intensity of a copper phyllosilicate peak in XRD and
FTIR suggesting better interaction between Cu and SiO
2
. Further, 0.1% Pt shows the highest
dispersion of copper, confirmed by N
2
O chemisorption (oxidize Cu to Cu
2
O). Furthermore,
H
2
-TPD of 0.1PtCu/SiO
2
shows a 20C decrease in the Cu
2
O reduction peak, suggesting Pt
promotes the reduction of CuO. The CO adsorption followed by measurement of DRIFT
spectra and XPS spectra including Cu LMM x-ray excited Auger spectroscopy (XAES) con-
firms that the incorporation of Pt increased the surface site density of Cu
1
. In comparison
to the Cu/SiO
2
catalyst which shows a maximum 68.8% selectivity at 250C, the selectivity of
86 Advanced Catalysis for Drop-in Chemicals

0.1PtCu/SiO
2
was found at 76.7% at 230C (approximately 20C lower). It was speculated
that MG dissociates at the Cu
1
sites while H
2
is dissociated as Pt atoms spillover to the
neighboring Cu
0
atoms for subsequent hydrogenation. The 0.1PtCu/SiO
2
catalyst showed a
7.7x higher TOF and stable activity up to 700 h of reaction with any deactivation (Fig. 412).
FIGURE 4–12 (a) Production of ethanol from lignocellulosic biomass through a two-step chemocatalytic approach;
(b) production of ethanol from coal through dimethyl oxalate hydrogenation. Catalytic performances of (c) Cu/
SiO
2
, and (d) 0.1PtCu/SiO
2
at different reaction temperatures. Reaction conditions: weight hour space velocity
(WHSV) 52.0 h
21
,H
2
/MG 5120, and P(H
2
)53 MPa. Catalytic performances versus time on stream over (e) Cu/SiO
2
(f) 0.1PtCu/SiO
2
catalysts at 503 K. Reaction conditions: P(H
2
)53 MPa and H
2
/MG 5120 [168].Reproduced with
permission from C. Yang, Z. Miao, F. Zhang, L. Li, Y. Liu, A. Wang, et al., Hydrogenolysis of methyl glycolate to
ethanol over a PtCu/SiO
2
single-atom alloy catalyst: a further step from cellulose to ethanol, Green. Chem., 20
(2018) 21422150. Copyright 2018 Royal Society of Chemistry.
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 87

4.3.10 Selective decarboxylation of furfural to furan
The United States Department of Energy (US DoE) has ranked furfural and its derivatives
among the top 30 biomass-derived platform chemicals [169]. The acid-catalyzed production
of furfural from hemicellulose derived xylose is used at an industrial scale. Furfural serves as
a substrate for the synthesis of high-volume products, such as polyols, which have direct
applications in the polymer industry (polyesters, polyurethanes, and polyamides) [101,170].
However, a major fraction of furfural (B62%) is consumed to produce furfuryl alcohol, a pre-
cursor molecule for the synthesis of several chemicals, such as lubricants, ascorbic acid,
furan-based resins, anticorrosive coatings, perfumes, and flavorings [171,172]. Existing tech-
nologies for the hydrogenation of furfural to furfuryl alcohol utilized toxic copper chromate
catalysts at intense reaction temperature (200C) and pressure (30 bar) which pose a disposal
problem at the industrial scale [173]. Metal catalysts (Cu, Ru, Pt, etc.)on various supports
have also been investigated as an alternative. Metals supported on MgO, CeO
2
, and γ-Al
2
O
3
were found to perform optimally for the selective hydrogenation of furfural to furfuryl alco-
hol [174176]. DFT calculations suggest that furfural binds to copper via η
1
(O)-aldehyde
mode, which leads to a 100% selectivity of furfural to FA over a monometallic Cu/Al
2
O
3
at
90C[177]. To investigate the effect of metal size on product selectivity Islam et al. prepared
a Cu/Al
2
O
3
catalyst with 1% and 5% Cu loading, having high Cu dispersion and Cu nanopar-
ticles, respectively [178]. XPS spectra showed Cu matched with 11 oxidation state while the
L
3
VV Auger spectrum displayed a peak shift to lower kinetic energies, demonstrating the
unique electronic character of Cu. Cu K-edge XANES and EXAFS validate the isolated atom-
ically dispersed octahedral Cu
21
(O, OH)
6
. The isolated and dimer Cu atoms in 1.0 wt.% Cu/
Al
2
O
3
catalysts promote the decarbonylation of furfural to furan because of the distinct inter-
action of atomic copper with furan ring. Cu nanoparticles catalyst in 5.0 wt.% Cu/Al
2
O
3
leads
to the formation of furfuryl alcohol. Interestingly, the introduction of sulfur in a catalyst using
sulfate precursor has a detrimental effect on furfuryl alcohol selectivity, decreasing up to
0.8% from 94.6% and switching o an acetalization pathway and generating 2-furaldehyde
dimethyl acetal (FDMA). (Fig. 413)
4.3.11 Selective production of γ-butyrolactone from succinic acid
hydrogenation
Succinic acid (SA) is one of the most demanding sugar-derived C4 dicarboxylate chemicals
and is in the list of the top 12 bioderived chemicals [169]. It is a precursor of many industrial
value chemicals and is commercially derived from the fermentation of sugars. Hydrogenation
of succinic acid yields γ-butyrolactone (GBL), an important chemical intermediate and
88 Advanced Catalysis for Drop-in Chemicals

solvent in fine chemicals, such as N-methyl-2-pyrrolidone (NMP) and 2-pyrrolidone. GBL can
be further hydrogenated to 1,4-butanediol (BDO), or tetrahydrofuran (THF) [179181]. BDO
is a precursor to industrially important polymers such as polyurethane, polybutylene succi-
nate (PBS), and polybutylene terephthalate (PBT), while THF is a versatile polar solvent [182].
On an industrial-scale γ-butyrolactone (GBL) is generally produced by direct hydrogenation
of maleic anhydride [183]. Maleic anhydride is produced from fossil fuel feedstock, so repla-
cing GBL production using succinic acid might provide an alternative route to use biomass
for the production of drop-in chemicals at a low price and minimal environmental impact.
Several Pd-based catalysts especially supported on mesoporous carbon and alumina have
been found as an excellent catalyst for the hydrogenation of SA to GBL [184,185]. The hydro-
genation of SA to GBL proceeds via the formation of succinic anhydride following CO bond
reduction and dehydration on acidic sites. In the second step, the oxidative hydrogenation of
the CO bond leads to the formation of GBL on novel metal sites [186]. Alumina (Al
2
O
3
) pos-
sessing plenty of an acidic catalytic site has been used as support for Pd, but size heterogene-
ity leads to side reactions [186]. To overcome this drawback, Pd was atomically dispersed on
γ-AlOOH nanosheets (Pd/γ-AlOOH) with 0.1, 0.5, and 1.0 wt.% Pd loading using a wet-
impregnation method [187]. The SA to GBL conversion for 0.1 Pd/γ-AlOOH, 0.5 Pd/γ-AlOOH,
and 1.0 Pd/γ-AlOOH was found to be 50.3%, 90.7% and 97.1% after 4 h while the corresponding
selectivity was found to be 98.7%, 96.9%, and 93.8%, respectively. The TOF of 0.1 Pd/γ-AlOOH
was obtained 305.4 cm
21
, which was much higher than 0.5 Pd/γ-AlOOH (175.7 cm
21
)and
1.0 Pd/γ-AlOOH (86.7 cm
21
). Based on the wt.% of Pd loading, the 0.1 Pd/γ-AlOOH displayed
1.9- and 10-times higher conversion than 0.5 Pd/γ-AlOOH and 1.0 Pd/γ-AlOOH, respectively.
By considering the magic numbers for one atom (Pd
1
), 13 atoms (Pd
13
), and 55 atoms (Pd
55
),
FIGURE 4–13 (a) A general outline of furfural decarboxylation to furan on a single-atom and furfuryl alcohol on
multiatomic nanoparticles and (b) Cu L
3
VV XAES spectra for the six Cu/Al
2
O
3
catalysts and Cu, CuO and Cu
2
O. The
reaction profiles of furfural conversion and furfuryl alcohol selectivity using 1.0 wt.% Cu/Al
2
O
3
catalysts selectivity
at (c) 1.5 bar and (d) 10 bar of H
2
, Conditions: temperature: 50C, 600 rpm, and 30 mg of catalyst. Solid and dashed
lines represent the conversion and selectivity to furfuryl alcohol, respectively [178].Reproduced with permission
from M.J. Islam, M. Granollers Mesa, A. Osatiashtiani, M.J. Taylor, J.C. Manayil, C.M. Parlett, et al., The effect of
metal precursor on copper phase dispersion and nanoparticle formation for the catalytic transformations of
furfural, Appl. Catal. B: Environ., 273 (2020) 119062. Copyright 2020 Elsevier.
Chapter 4 •Single-atom catalysts for biomass-derived drop-in chemicals 89

the catalytic activity of supported single-atom was calculated to be 305.4 h
21
, 9.2 h
21
and
0.27 h
21
for SA hydrogenation, demonstrating the significance of a single-atom site. The
kinetic tests reveal that the catalyst with low Pd loading and single-atom distribution (0.1 Pd/
γ-AlOOH, 0.2 Pd/γ-AlOOH, and 0.5 Pd/γ-AlOOH) have almost same activation energies (Ea)
of 83.5 60.5 kJ mol
21
, while 1.0 Pd/γ-AlOOH with 28.2% single-atom contribution show
an Ea 5132.5 kJ mol
21
, suggesting reaction process is driven by metallic Pd active sites. Using
DFT calculations of Pd
n
models (n 51, 3, 8, 10, 13, and 55) supported on γ-AlOOH (010), the
adsorption energies were calculated: the E
ad
/n value for single-atom was 20.90 eV while for
Pd
55
cluster the value was 23.00 eV. The more negative value of Pd clusters suggests that in
comparison to clusters, the group of individual atoms was more unstable, resulting in higher
activity of the atomically dispersed catalyst (Fig. 414).
4.4 Conclusions
The abundant and renewable nature of biomass makes it an attractive alternative energy
resource that can alleviate the problem of climate change and depleting fossil fuels. Currently,
only a tiny fraction of biomass, specifically starch-rich grains/crops, is being utilized for alco-
hol/organic acid generation, but food security and the loss of biodiversity remain serious con-
cerns. On the other hand, lignocellulosic biomass is widely available that has the potential to
produce fuels and drop-in chemicals to replace fossil fuels. Conventional thermal valorization
requires a high temperature and relies on stoichiometric hazardous catalysts to convert ligno-
cellulosic biomass into chemicals. Additionally, the oxygen-rich nature of biomass and high
moisture content affords a wide variety of products. With the advent of the modern catalytic
system, the transformation of complex biomass and platform chemicals (derived from
FIGURE 4–14 (a) The catalytic activity of SA hydrogenation and the Arrhenius plot of reaction rates over Pd/
γ-AlOOH catalysts with different Pd loadings (b) Atom activity and adsorption energy for each Pd atom
contribution in clusters. [187].Reproduced with permission from C. Zhang, L. Chen, H. Cheng, X. Zhu, Z. Qi,
Atomically dispersed Pd catalysts for the selective hydrogenation of succinic acid to γ-butyrolactone, Catal. Today,
276 (2016) 5561. Copyright 2016 Elsevier.
90 Advanced Catalysis for Drop-in Chemicals

valorization) to industrially important chemicals have become feasible. Unfortunately, most of
the existing catalysts are facing the serious issue of poor selectivity and catalytic performance
limiting their deployment of the system on a large scale. Size reduction of transition metal cat-
alysts followed by anchoring on acidic or basic support was found to improve the perfor-
mance; however, due to the presence of variable exposed sites and unequal size distribution,
the problem of poor selectivity persists. The emergence of single-atom catalysts (SACs) has
reciprocated the complete scenario as it offers ultimate selectivity due to the uniform distribu-
tion of single-atom active sites. The availability of each atomic site and interaction of single-
atoms with support lead to a specific geometrical distribution that synergistically improves the
performance of SACs. Designing single-atom catalysts using noble metals enhance the perfor-
mance while at the same time make them recoverable and increase their stability under harsh
conditions. In the present chapter we have revisited synthesis and application of noble and
nonnoble metal SACs for biomass transformation into valuable chemicals. Only a handful of
SACs fabricated on various supports, such as inorganic oxides, sulfides, and 2D materials,
such as graphene and carbon nitrides using various metal centers, have been reported for the
transformation of various biomass-derived platform chemicals into drop-in chemicals with
unprecedented selectivity. Metal centers, such as Pt, Ru, Pd, Ir, Ni Cu, etc. supported on
Al
2
O
3
, TiO
2
, LDHs, graphene, and carbon nitride have been studied in detail for the conver-
sion of platform chemicals that demonstrating .95% selectivity in almost every case. The
entirely new properties, astonishing performance, selectivity, and amalgamated attributes of
homogenous and heterogeneous catalysts make SACs the catalyst for the future which can
overcome the issues of conventional catalyst systems.
Acknowledgments
The authors would like to thank the University of Calgary's Department of Chemical & Petroleum
Engineering/Schulich School of Engineering and the University of Calgary CFREF fund for financial assistance.
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