Article

Advanced in conversion of hemicellulosic biomass to furfural and upgrading to biofuels. Catal Sci Tech 2:2025-2036

Catalysis Science & Technology (Impact Factor: 5.43). 10/2012; DOI: 10.1039/c2cy20235b
ABSTRACT
Recent approaches to furfural synthesis from hemicellulosic biomass and pentose sugars with both homogeneous and solid acidic catalysts have been summarized by addressing the associated sustainability issues. The features of deconstruction of hemicellulosic biomass by acid hydrolysis to produce pentose sugar feedstock for furfural have been discussed in brief. Several strategies including solvent extraction in a biphasic process, application of surface functionalized materials such as acidic resins, mesoporous solids and mechanistic insight in limited cases are discussed. The present status of the promising furfural platform in producing second generation biofuels (furanics and hydrocarbon) is reviewed. The performances of each catalytic system are assessed in terms of intrinsic reactivity and selectivity toward furfural production. Overall, this minireview attempts to highlight the scope of further developments for a sustainable furfural process and upgrading to fuels.

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Cite this: DOI: 10.1039/c2cy20235b
Advances in conversion of hemicellulosic biomass to furfural and
upgrading to biofuels
Saikat Dutta, Sudipta De, Basudeb Saha* and Md. Imteyaz Alam
Received 14th April 2012, Accepted 28th May 2012
DOI: 10.1039/c2cy20235b
Recent approaches to furfural synthesis from hemicellulosic biomass and pentose sugars with
both homogeneous and solid acidic catalysts have been summarized by addressing the associated
sustainability issues. The features of deconstruction of hemicellulosic biomass by acid hydrolysis
to produce pentose sugar feedstock for furfural have been discussed in brief. Several strategies
including solvent extraction in a biphasic process, application of surface functionalized materials
such as acidic resins, mesoporous solids and mechanistic insight in limited cases are discussed.
The present status of the promising furfural platform in producing second generation biofuels
(furanics and hydrocarbon) is reviewed. The performances of each catalytic system are assessed
in terms of intrinsic reactivity and selectivity toward furfural production. Overall, this minireview
attempts to highlight the scope of further developments for a sustainable furfural process and
upgrading to fuels.
1. Introduction
While the easily accessible oil fields are becoming depleted and
CO
2
emissions from fossil fuels are affecting the earth’s
climate, the most imminent result that awaits mankind is the
tremendous crisis of energy if we remain dependent on the
fossil resources. Hence, much research is being devoted to
exploring non-fossil carbon energy sources. Among these,
biofuels derived from cellulosic and hemicellulosic fractions
of the biomass are considered as a promising alternative for
transportation fuel under test. The driving factors for biofuels
derived from biorenewable sources are not only confined to
exploring new energy platform and CO
2
savings, but include
opportunities to secure the local supply of energy and support
agricultural economics.
1,2
The conversion of lignocellulosic
biomass into fuels and chemicals requires effective utilization
Laboratory of Catalysis, Department of Chemistry, North Campus,
University of Delhi, Delhi, India. E-mail: bsaha@chemistry.du.ac.in;
Fax: +91 2766 7794; Tel: +011-2766 6646
Saikat Dutta
Dr Saikat Dutta obtained
his PhD in organometallic
chemistry from Indian Insti-
tute of Science, Bengaluru, in
2008. After a couple of post-
doctoral appointments in
Taiwan and in India, he was
awarded a Fulbright–Nehru
Postdoctoral Fellowship in
2012. Dr Dutta is a co-author
of more than 20 research
publications in scientific jour-
nals. His research experience
includes transition metal
organometallics,polymerization
catalysis, materials develop-
ment for photophysical/catalytic applications, biomass conversion
for platform chemicals and fuels. His current research interest is
in the area of materials design for photochemical conversion of
CO
2
, degradation of non-biodegradable polymers and chemistry
of main-group elements.
Sudipta De
Sudipta De obtained his BSc
in chemistry from University
of Calcutta in 2008. After
receiving his MSc from
University of Calcutta in
2010, he enrolled in the PhD
Program at the University of
Delhi under supervision of
Professor Basudeb Saha.
Currently he is working in
the area of biomass conversion
to valued chemicals and liquid
fuels with a major focus in
design and development of
materials for catalytic appli-
cations. His research interests
also include the template directed synthesis of mesoporous
and nanocrystalline materials having various catalytic and
photophysical applications.
Catalysis
Science & Technology
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of the C
5
and C
6
sugars present in hemicellulose and cellulose,
respectively, by either processing these fractions together or
separating and processing them separately. The fractionation
of hemicellulose and cellulose allows the processing of each
fraction to be tailored to take advantage of the different
chemical and physical properties of these fractions, and
provides increased flexibility of operation. For example,
chemical processing methods can be employed to convert C
5
sugars in hemicelluloses into fuels/chemicals. The conversion
of cellulose to chemicals and liquid fuels has been demon-
strated through the formation of several platform molecules,
such as glucose, 5-hydroxymethylfurfural, and levulinic acid
(LA), utilizing chemical routes.
3–6
However, limited studies
addressed the conversion of hemicellulose into chemicals and
fuels.
7,8
Hemicellulosic fraction of biomass can be used as feed-
stock to produce many important chemicals, such as furfuryl
alcohol, furan, and THF.
9
This contribution is an account of most recent results in the
field of furfural synthesis from the biomass resources with a
variety of catalysts including metal salts and solid acidic
materials in aqueous, organic and biphasic media, highlighting
their respective catalytic performances/efficiencies and overall
advantages. Possible outlooks and scope of this work are also
included.
2. Furfural platform
Furfural is the most common industrial chemical derived from
lignocellulosic biomass, with an annual production volume of
more than 200 000 tonnes.
10,11
The commercial utility of furfural
was first discovered at the Quaker Oats Company in 1921.
12
Quaker Oats tested a variety of processes to valorize the hulls
and found that treating them with dilute sulfuric acid yields
useful amounts of furfural. Furfural also deserves attention as a
potential platform for biofuels. Furfural is produced by the
hydrolysis and dehydration of xylan contained in lignocellulose.
The value chains of furanic biofuels are realized in terms of
conversion of furfural to different components for example
2-methylfuran, 2-methyltetrahydrofuran etc. Furfural hydrogena-
tion and acid–base-catalyzed reactions applied to upgrade furfural
to fuels have recently been initiated commercially.
13
Synthesis routes shown in Scheme 1 are ranked based on
their industrial potential by considering their manufacturing
footprint, investment cost and CO
2
emission of furfural upgrade.
For example, 2-methylfuran (2-MF) has been considered as a
promising liquid fuel candidate and an extensive road trial of
over 90 000 km with promising outcomes has been reported.
13
3. Hemicellulose structure
Hemicelluloses, a heterogeneous polymer constructed with C
5
and C
6
sugars (such as xylose, arabinose, glucose, galactose,
mannose, etc.), is typically the second-most-abundant component
of biomass, after cellulose.
14
In most grasses and hardwoods,
xylan, a polymer of xylose, is often found as the primary
hemicellulose. As a result, xylan conversion is critical for
utilization of important biomass feedstocks such as corn stover,
Miscanthus, switchgrass, and poplar. Major hemicelluloses are
Scheme 1 Furfural platform for biofuels (modified from Fig. 1 in
ref. 13).
Basudeb Saha
Dr Basudeb Saha, born in
Calcutta (India), graduated
in chemistry at Calcutta
University and received his
PhD from Indian Association
for the Cultivation of Science,
India. He did postdoctoral
research with Professor James
Espenson at Iowa State Uni-
versity (USA), jointly with
BP Chemical Company, on
removal of toxic by-products
in the manufacture of
terephthalic acid (PTA). In
2007, he joined the poly-
urethane business R&D
division of Dow Chemical Company, USA, where he led several
breakthrough and implementation research projects. Since 2009,
he has been an Associate Professor at Delhi University and has
been pursuing research on utilization of bio-renewable feed-
stocks for chemicals and fuels production via effective catalysis.
Md. Imteyaz Alam
Md. Imteyaz Alam obtained
his BSc in chemistry from Jai
Prakash University (India) in
2006, and MSc from Univer-
sity of Delhi (India) in 2009.
After a brief research experi-
ence in S. C. Johnson Pro-
ducts Pvt. Ltd. and Indian
Agricultural Research Insti-
tute (India), he enrolled in
the PhD program at the
University of Delhi in 2011
under supervision of Professor
Basudeb Saha. His research
interests include mesoporous
and nanocrystalline materials
synthesis and their catalytic
applications.
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mannans, xylan, arabinans, and galactans among which
most important in softwoods (coniferous trees) are galacto-
glucomannans (20%) and arabinoglucurunoxylans (10%)
15,16
(Fig. 1).
In the hydrolysis of hemicelluloses, selective cleavage of the
C–O bonds present between adjacent sugar units is very
important to yield intact monomer sugar molecules. Mineral
acids and enzymes are generally used as catalyst to hydrolyze
these polymeric carbohydrates.
17
Selective acid hydrolysis of
hemicellulose substrates produces xylose in good yield which
essentially depends on the hydrolysis kinetics. Furthermore,
softwoods contain arabinogalactan, xyloglucans, and other
glucans. Pine (Pinus sylvestris) and spruce (Picea abies) contain
about 20 wt% O-acetyl-galactoglucomannan and 510 wt%
arabino-4-O-methyl glucuronoxylan.
18
The amounts of different
hemicelluloses in wood are listed in Table 1.
19
The selective acid hydrolysis of hemicelluloses to produce
pentose sugars is an interesting process, especially for the
production of rare sugars (mannose, galactose, lactose), which
are value-added compounds in biorefinery. Acid hydrolysis of
hemicelluloses from biomass can be compared with cellulose
hydrolysis. Selective dilute acid catalyzed hydrolysis of hemi-
celluloses from both wood chips and agricultural wastes has
been investigated by many researchers in the past.
20
The rate
of acid hydrolysis of hemicellulose is partially determined by
the anhydrosugar structure, for example, whether it is an a-or
a b-anomer or it is furanose or pyranose form. It is known that
the b-anomers react faster than a-anomers.
21
Furthermore, the
rate of acid hydrolysis is faster for furanose compared with
pyranose, thus indicating that arabinose undergoes easier
hydrolysis than xylose.
22
The reason for the faster furanose
hydrolysis rate compared with that of pyranose is the higher
structural angle strains in the furanoside sugar units, whereas
pyranose rings are strain-free.
The acid hydrolysis rate of wood chips depends on the type
of tree; for example, softwoods, especially pine, are generally
more difficult to hydrolyze than hardwoods.
23
Acid hydrolysis
of hemicelluloses fraction of the lignocellulosic biomass produces
sugars like xylose, mannose, galactose etc. Selective dilute acid
hydrolysis of wood chips and agricultural wastes has been
investigated and very high mannose yields were a chieved from
balsam, whereas the yield was very low from switchgrass. The
mechanism of acid hydrolysis of hemicelluloses proceeds through
the cleavage of glycosidic bonds (Fig. 1) via protonation either of
the glycosidic bond or of pyranic oxygen.
24
Although the
formation of a cyclic intermediate via the conformational
changes of the tetrahydropyran is proposed, this route needs
more energy compared to the acyclic route.
25
From the mechanistic point of view, acid hydrolysis rate
of hemicelluloses varies depending on their structure. Both
random scission
26
and selective scission of the side chain have
been reported.
27
Furthermore, acid hydrolysis of xylan was
reported to be random,
27
whereas hydrolysis of the vegetable
fibers,
L-arabinose was selective when using dilute acids as
catalysts.
28,29
From these results, it is revealed that furanosides
hydrolyze faster than pyranosides
30
due to the fact that
hydrolysis rate is faster for glycosidic linkages exhibiting
nonreducing ends.
31
Sugars from hemicelluloses are easy to
separate almost quantitatively
32,33
due to their structures and
noncrystalline nature.
Acid hydrolysis of biomass starting from plant biomass
substrates for production of xylose has been intensively
studied (Table 2), for example, from sugar cane bagasse, wheat
straw, rice straw, cotton-seed, cotton stalk, sunflower stalk,
corn stover, and many more.
20
Different strategies of decon-
struction of hemicelluloses such as acid hydrolysis, enzymatic
hydrolysis, hot water extraction, and microwave treatment, to
prepare xylo-oligosaccharides have been reported.
20
Unlike
pure xylan, hemicelluloses in biomass serve as linkers of
cellulose fibers to microfibrils, and cross-linkers of cellulose
with lignin to create complex networks that provide structural
stability.
34
Such network in lignocellulosic biomass turns the
Fig. 1 The most important hemicelluloses of softwoods: (a) xylan,
(b) glucomannan.
Table 1 The percentage of hemicellulose in wood
20
Hemicellulose Hardwood Softwood
Methylglucuronoxylans 80–90 5–15
Arabinomethylglucuronoxylans 0.1–1 15–30
Glucomannans 1–5 1–5
Galactoglucomannans 0.1–1 60–70
Arabinogalactans 0.1–1 1–15
Other galactans 0.1–1 0.1–1
Pectins 1–5 1–5
Table 2 The yield of xylose in acid hydrolysis of lignocellulosic
biomass and hemicelluloses (Table 2 in ref. 20)
Biomass Acid Temp/1C Time/min Yield (%)
Wheat straw TFA 99 420 80
Wheat straw TFA 99 1380 70
Wheat straw HCl 99 120 73
Wheat straw H
2
SO
4
90 720 97
Rice straw H
2
SO
4
145 20
Rice straw H
2
SO
4
121 27 77
Corncob/corn stover H
2
SO
4
140 50 81
Corn stover H
2
SO
4
180 0.67 80
Sugarcane bagasse H
2
SO
4
160 15 88
Sugarcane bagasse H
2
SO
4
140 20 83.3
Sugarcane bagasse H
2
SO
4
120 60 80
Eucalyptus chips H
2
SO
4
140 10 21.18
Poplar H
2
SO
4
180 1 80
Arpen wood H
2
SO
4
140 16 76.4
Oak hardwood H
2
SO
4
150 83
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hydrolysis of hemicellulose more difficult than that of pure
xylan. Although, cellulose hydrolysis with solid acid catalysts
has been reported,
35,36
methods for one-pot conversion of solid
hemicelluloses (without any pre-treatment) into xylose, arabinose
and furfural using solid acid catalysts (HZSM-5 and HUSY zeolites
with 0.5 to 0.74 nm pore diameters, layered clays, aluminum
incorporated mesoporous silica Al-MCM-41, Al-SBA-15) in
aqueous medium are also known.
37
As per the results and claim,
the method is also capable of selectively converting just the
hemicelluloses in lignocellulosic biomass using solid acid catalysts.
A reaction using 1 wt% sulphuric acid catalyst at 170 1C produced
50% xylose + arabinose and 10% furfural in 1 h, however, a
maximum of 41% xylose + arabinose was achieved at 170 1Cin
3 h with HUSY (Si/Al = 15) catalyst. In this solid catalyzed
process, maximum 12% furfural can be obtained when using
HUSY (Si/Al = 15) and K10 montmorillonite clay catalyst. The
higher yield of xylose + arabinose with sulfuric acid compared to
HUSY (Si/Al = 15) can be explained by the fact that while sulfuric
acid releases 12 mmol H
+
in the reaction mixture, HUSY releases
0.165 mmol H
+
under the reaction conditions.
4. Homogeneous catalytic strategy
Conversion of pentoses into furfural has been a well-explored
process.
38–40
The process invented by Quaker Oats employs a
dilute sulfuric acid catalyst and stream pressure, achieving
50% molar yields of furfural from xylan.
41
Most industrial
processes reported similar yields, likely limited by side reactions
such as homopolymerization and condensation with unreacted
xylose. Furfural can be produced from the dehydration of
xylose by using Brønsted acids, such as HCl and H
2
SO
4
.
42–44
However, these mineral acids are limited by the fact that they
cause corrosion, safety problems, and require critical reaction
conditions. Very recently, homogeneously catalyzed process of
furfural synthesis from the pentose sugars and hemicellulosic
biomass has been investigated and will be the subject of the
discussion herein.
Dehydration reactions play vital roles in liquid-phase catalytic
processing and aqueous phase reforming to produce jet and
diesel fuel range alkanes from biomass-derived oxygenated
hydrocarbons.
45,46
Furfural is a feedstock to make gasoline,
diesel, or jet fuel
47
and a kinetic model for the dehydration of
xylose in biphasic reaction using a homogeneous catalyst
(Scheme 2) depicts the overall scheme. Furfural and xylose can
react together to form undesired solid humins, highly polymerized
insoluble carbonaceous species. Self-reaction of furfural also can
result in solid humins.
It was found that a chromium-based process offers an
advantageous route from pentoses and pentosans to furfural.
Based on initial studies by Binder et al. xylose conversion into
furfural by using combination of chromium(
II) or chromium(III)
salts and HCl cocatalyst results in moderate yields via isomeriza-
tion and dehydration. This dual catalyst has been used for the
xylose and xylan conversion in N,N-dimethylacetamide containing
lithium chloride (DMALiCl) and related solvents.
48
Halide
additives (LiCl, LiBr etc.) were found to be effective for the
xylose conversion, affording a maximum yield of furfural
(56%) with CrCl
2
(6 mol%) in DMA containing 10 wt% LiBr
in 4 h at 100 1C. Analysis revealed a first-order dependence of
furfural formation on xylose concentration and half-order
dependence on Cr
II
concentration, indicating direct involvement
of Cr in the process. Kinetic analyses and deuterium-labeling
experiments supported hydride-shift mechanism involving
chromium for xylose isomerization through a 1,2-hydride
shift by forming xylulose, a reactive ketose intermediate that
dehydrates readily into furfural. More challenging xylan
conversion into furfural, however, afforded 25% furfural,
and 22% from corn stover even at the higher temperature of
140 1 C and with HCl as a cocatalyst.
Zhao et al. obtained furfural from xylan in 63% yield with
CrCl
3
catalyst in ionic liquids under microwave-assisted heating
at B200 1C and later extended this method for real biomass corn
stalk, rice straw, and pinewood.
49
Furfural yields from these
biomass variants were only 23–31%, including significant humin
formation. Solid residues (humins) have been a challenge for
Jones et al. reported acid-catalyzed production of furfural from
xylose in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) in
the presence of Brønsted acid H
2
SO
4
.
42
In search of a new catalyst that would potentially replace
the industrially used mineral acid catalysts, recent studies
in aqueous–organic biphasic media showed new directions.
Earlier, for sugar dehydration, metal chlorides (CrCl
2
, ZnCl
2
,
FeCl
3
) have been assessed in non-aqueous deep-eutectic
solvents such as chlorine chloride fructose mixtures
50
as well
as in monophasic aqueous media.
51,52
A biphasic medium
composed of aqueous solution of FeCl
3
6H
2
O and NaCl
combined with biomass derivable 2-methyltetrahydrofuran
(2-MTHF) phase has been demonstrated as an effective
biorefinery strategy for xylose dehydration (Fig. 2) by Leitner
and Maria et al.
53
This method exhibited a maximum of 71%
furfural yield in the presence of 20 wt% of NaCl additive.
When 2-methyltetrahydrofuran (2-MTHF) was used as an
extractant, the authors reported an extraction of 98% furfural
by enhancing the furfural production in aqueous phase.
Gratifyingly, in this case the direct use of seawater comprising
different salts with FeCl
3
6H
2
O also resulted in an improved
furfural production rate. Conversion of nonpurified xylose
effluents e.g. beech wood (particle size 0.5 to 0.1 mm) in
biphasic water/2-MTHF using oxalic acid as catalyst to
furfural that has been performed also emphasizes the potential
of FeCl
3
6H
2
O as catalyst.
54
Similar to the iron system, AlCl
3
6H
2
O was employed as catalyst
in biphasic water–THF medium at 140 1C giving 499% xylose
conversion with the formation of xylulose as intermediate with
maximum 30% furfural yield in 5 min.
55
AlCl
3
6H
2
O was
also effective for the conversion of lignocellulosic biomass
Scheme 2 Xylose dehydration in aqueous phase.
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(corn stover, pinewood, switchgrass, and poplar), affording
different furfural yields due to species-dependent hemicellulo-
sic recalcitrance.
For commercial purpose, it is realized that the production of
furfural (FuAl) from C
5
sugars (i.e. xylose) suffers from the
low concentrations of FuAl in the product stream due to
the low xylose concentrations (12 wt%) obtained from
hemicelluloses deconstruction.
7,8,53
A new biorefining strategy
for converting the hemicellulose fraction of lignocellulosic
biomass to FuAl by utilizing biphasic systems that consist of
an extractive organic layer (2-sec-butylphenol) and an aqueous
layer that contains a mineral acid has been demonstrated by
Dumesic and co-workers.
56
This biphasic system achieved
high concentrations of FuAl with maximum 75% yield and
82% selectivity. The use of alkylphenol solvents (Fig. 3) was
advantageous because of (i) high partition coefficients for
extraction of FuAl; (ii) not extracting significant amounts of
mineral acids from aqueous phase; (iii) higher boiling points
than the final product; and (iv) the fact that they can be
derived from biomass (i.e., lignin). We envisaged the potential
of similar solvents (e.g. eugenol) which can also be extracted
from certain essential oil (mainly clove oil) and found them to
be an efficient extracting agent when used in a biphasic
aqueous–organic system in the presence of cellulose-derived
solid acid for xylose conversion.
A recent study demonstrated an efficient xylose and xylan
conversion process with 72% furfural yield using maleic acid
as catalyst in an aqueous medium at 200 1C.
54,57
A kinetic
study also revealed that xylose degradation rates are lower in
aqueous medium which may be due to the reason that furfural
plays the role of a Brønsted base, which reacts with H
3
O
+
,
thus decreasing the total acid concentration of the aqueous
system and slowing the degradation of xylose as proposed by
Antel et al.
58
It was demonstrated that xylose reaction
rate increases by the addition of potassium halides in the
order Cl
4 Br
4 I
; on the other hand, selectivity, and
thus furfural yield, are also improved by following the opposite
order I
4 Br
4 Cl
in aqueous acidic solution.
59
Highest
yield (87.5%) and selectivity (95.3%) of furfural was achieved
using a combination of KCl and KI due to the synergistic effect.
5. Solid catalysts
Unlike in the homogeneous regime, several factors such as
preparation conditions of the catalyst, structural properties,
and accessibility of acid sites are associated with the efficiency
of solid acid catalyzed conversion of xylose to furfural (Fig. 4).
For example, sulfonic acid functionalized ordered mesoporous
silica SBA-15 (Santa Barbara amorphous) with hexagonal array
or pores were prepared by both co-condensation and grafting
methods, respectively. However, the grafting SBA-15-SO
3
H(G)
exhibits slightly weaker catalytic activity than the co-condensatio n
SBA-15-SO
3
H(C), possibly due to the less uniformly distributed
sulfonic acid sites on the surface and within pore walls.
60
Valente and co-workers have extensively studied the catalytic
activity of various solid acid catalysts for the dehydration of xylose.
These include modified mesoporous silicas,
61,62
exfoliated aggre-
gated nanosheets of metal oxides,
63
sulfonated metal oxides,
64
and
microporous silicoaluminophosphates (SAPO).
65
Their studies
revealed that a delaminated zeolite (Si/Al = 29) prepared from
lamellar precursor (Nu-6(1)) can be an efficient catalyst in
water–toluene biphasic media at 170 1C affording 47% furfural
yield.
66
It is revealed that del-Nu-6(1) material could be a
promising alternative to conventional zeolites or mesoporous
materials for the production of furfural probably due to
the easier accessibility of its active sites. Possibly, catalyst
performance can be further improved by optimizing the Si/Al
ratio and the delamination procedure.
Advantage was taken of the extraction solvent for the
conversion of xylose to furfural catalyzed by mesoporous
molecular sieve MCM-41 in biphasic water/1-butanol media
and the strategy was further enhanced by addition of NaCl as
an auxiliary catalyst.
67
The ability to efficiently execute the
dehydration step with solid catalysts in a biphasic regime could
be beneficial from both economical and ecological points of view.
That being said, there are still many unanswered questions
pertaining to the behavior of solid catalysts in aqueous medium.
An area of particular focus is the interfacial interactions between
Fig. 2 Conversion of xylose to furfural in aqueous-2-MTHF biphasic
medium using iron catalyst starting from beech wood fractionation.
(modified from Fig. 3 in ref. 53).
Fig. 3 Effective extracting agents derived from lignin biomass.
Fig. 4 Solid acid catalysts employed for the conversion of hemicellulose
biomass to furfural.
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aqueous solutions and metal oxides. The metal oxide–water
interface is reactive due to a range of chemistries, including
acid–base, ligand exchange, and/or redox.
68
In general, the
exposure of solid oxides to water gives rise to electrical charges
on the solid surface due to hydration effects that can involve
H
+
and OH
:
ions from the bulk aqueous phase. Incomplete
coordination of the exposed metal or oxide ions at the solid
surface is the cause of this phenomenon. As a result, positive
and negative sites are present on the solid surface, and the
excess of one type of site determines the net charge. Contrary
to biomass conversion, solid catalysts in the oil and petro-
chemical industries are typically used in gas phase or in liquid
phase, where the reaction medium is usually non-polar.
69
On
the catalyst surface, Brønsted acid sites (proton donors) can be
generated from highly polarized hydroxyl groups. Alternatively,
Lewis acid sites form coordinately unsaturated cationic sites,
which leave M
n+
exposed to interact with guest molecules as an
acceptor of an electron pair. Exposure of the catalyst to a polar
solvent such as water can potentially alter the intrinsic nature of
the surface due to solvation effects. For instance, the hydroxyl
ion from the water molecule (Lewis base) can react with a Lewis
acid site (M
n+
) on the surface to generate Brønsted sites.
70
Poisoning of the acid sites by water may also occur depending
on the surface hydrophilicity/hydrophobicity of the catalyst.
71
With the objective of investigating the role of Lewis and
Brønsted sites in solid acid catalysts for the dehydration of
carbohydrates in aqueous media, it is desirable to maintain a
high ratio of Brønsted to Lewis acid sites. This conclusion was
based on results of comparative catalytic activity for a series of
catalysts Zr–P, SiO
2
–Al
2
O
3
,WO
x
/ZrO
2
, g–Al
2
O
3
and HY
zeolite for aqueous phase dehydration of xylose.
72a
Lewis acid
sites decrease furfural selectivity by catalyzing side reaction
between xylose and furfural to form insoluble humins, e.g. HY
zeolite due to strong irreversible adsorption of the furfural in
the pores, causing an increase in the rate of humin formation.
Analysis also suggests that the catalyst with the highest
number of Lewis acid sites was the most active. The catalyst
pore confinement was found to have an adverse effect on furfural
selectivity. Adsorption–desorption studies in the aqueous phase
and decomposition experiments with furfural in aqueous
solutions have confirmed that HY zeolite causes furfural to
irreversibly adsorb in the zeolite pores and polymerize to form
humic substances. Therefore, it can be concluded that a
micropore containing catalyst may not be suitable for xylose
dehydration due to strong adsorption of the product in
the catalyst pore. Dehydration of xylose using ion-exchange
polymer resins (Naflon SAC-13 and Amberlyst 70) with strong
Brønsted acidic sites showed similar furfural selectivity to
Zr–P and HCl. This confirms that furfural selectivity is a direct
function of the Brønsted acid sites concentration. Ebitani et al.
have reported high yield of furfural and 5-hydroxymethylfurfural
(HMF) from xylose and polysaccharides, respectively, in their
one-pot synthetic approach using Amberlyst-15 and hydrotalcite
catalysts.
72bc
These furfurals are also efficiently synthesized
using tin–tungsten mixed oxide catalyst.
72d
Ion-exchange membrane Nafion 117 (Fig. 5), a sulfonated
tetrafluroethylene based fluropolymer-copolymer, as robust
and reusable catalyst is promising in terms of economical
furfural production as this possesses excellent chemical and
thermal stability under the xylose dehydration conditions.
73
After 15 consecutive runs under the optimized reaction conditions,
the robust Nafion membrane remained intact with furfural yields
rangingfrom58to62%in2hat1501C in DMSO. Deprotona-
tion of the sulfonic acid groups of Nafion 117 would deactivate the
catalyst by reducing the number of available acid sites for xylose
dehydration. Nanoparticulate-sized organic residue deposits are
also responsible for covering up the smooth surface of the Naflon
as revealed from AFM study (Fig. 5(b)).
73
Useofsulfonicacid
functionalized resin Amberlyst 70 as catalyst afforded 65% xylose
conversion with B100% furfural selectivity, however, this process
challenges economics due to require ment of high xylose loadings.
74
In another approach, hydrothermally stable porous siliceous
materials containing solid silica core and porous silica shell were
investigated for the dehydration of xylose in aqueous media
(Scheme 3). The modified mesoporous core–shell structured
silica (MSHS) spheres (260 nm diameter, solid core and shell)
functionalized with sulfonic acid acted as an efficient catalyst
for the dehydration of xylose to furfural with higher selectivity
than the aluminosilicate.
75
Sulfated tin oxide (SO
4
2
/SnO
2
), with the SO
4
2
group on
SnO
2
exhibited superior catalytic activity in producing furfural
from xylose.
76
An aqueous phase cyclodehydration of xylose
was carried out using a composite material consisting of
zeolite Beta (BEA) nanocrystals (Si/Al = 12) embedded in a
pure siliceous TUD-1 mesoporous matrix (BEATUD) at 170 1C.
A significantly higher xylose conversion to furfural was noted
with the BEATUD catalyst than that with the BEA catalyst as
the former contained a lower amount of carbonaceous matter
and hence favourable for an efficient adsorption caused by the
surrounding silica matrix.
77
Fine tuning of the Si/Al ratio
might change the total amount of acid sites and surface
polarity which may affect the dispersion and the total number
of accessible acid sites of the zeolite.
Carbonaceous materials are promising catalysts due to their
high surface area and hence provide adequate catalytic active
sites. Sulfonated graphene oxide (SGO) has been demonstrated
Fig. 5 Naflon 117 with Brønsted and Lewis acidic sites and AFM
image showing the smooth surface of the material before the reaction
(a) and after reaction (b). (AFM image is reproduced from ref. 73 with
permission, Copyright (2010) Wiley-VCH).
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to be a rapid and water-tolerant carbocatalyst at a low catalyst
loading at 200 1C with an average yield of 61% furfural from
xylose.
78
Surface area analysis and reaction results suggested
that the aryl sulfonic acid groups were the key active sites for
high temperature production of furfural in water. In all four
cases (grapheme, grapheme oxide, sulfonated grapheme, and
sulfonate grapheme oxide), the materials exhibited sheet-like
appearances (TEM study), despite the presence of oxygen-
bearing and SO
3
H functional groups that might disrupt the sp
2
carbon network in GO, SGO and SG among which SGO contains
large surface area (680 m
2
g
1
). High stability of the C–C bond
anchori ng ar ylSO
3
H group s is re sponsibl e for the catalytic
activity, and remains active after repeated reactions at 200 1C.
Direct evidence of isomerization of xylose to xylulose followed
by dehydration to furfural parallels the conversion of hexoses,
for example, the isomerization of glucose to fructose followed by
dehydration to HMF.
79a
Moliner et al. have investigated the
isomerization of glucose to fructo se using the Sn-beta zeolite with
yields comparable to biological catalysis (Scheme 4).
79b,80
By
combining the Sn-beta zeolite (Lewis acid) with a Brønsted acid
(Amberlyst-15), furfural can be prepared from xylose via the
xylulose intermediate at 120 1C in aqueous medium.
81
A 60% conversion of xylose with a 27% xylulose yield at
100 1C prompted us to investigate the role of Sn-beta zeolite. It
was found that xylose does not react with Amberlyst-15
or HCl at low temperatures; however, when xylulose is
the reactant, conversion is B66%, and furfural yield is 24%.
This result supports a reaction network in which xylulose
dehydrates rapidly to furfural via Brønsted acid catalysis and
xylose isomerizes to xylulose with a Lewis acid catalyst
advocating for dual acidic sites of a catalyst. Formation of
xylulose is a key step to furfural and requires either functional
group rearrangement or a configurational change around the
C1 and C2 carbon atoms. Structural studies using an X-ray
absorption fine structure (EXAFS) technique reveals that Sn is
substituted in pairs on opposite sides of six-membered rings,
i.e. uniform crystallographic location of Sn in the b crystal
structure that leads to sites with uniform catalytic activity and
high chemical selectivity (Fig. 6).
82
The results of the Sn-beta
zeolite catalyzed process indicate that the active site of the
catalyst interacts with the carbonyl group of C1 and the adjacent
hydroxyl group on C2. Kinetic studies of isomerization reactions
indicate that certain acids and metals are able to transfer the
hydrogen directly through a hydride shift between C-2 and C-1.
83
Lewis acidity in the catalyst is essential to polarize the carbonyl
group in the ketone while also coordinating both the alcohol and
the ketone to facilitate a hydride shift between them.
84
It is
therefore plausible that Sn in zeolite Beta performs the isomer-
ization reaction followed by an intramolecular hydride shift
between the carbonyl-containing C-1 and the hydroxyl-bearing
C-2 of glucose by way of a 5-member complex.
Important factors in the Sn-beta isomerization of glucose in
aqueous media include the role of the solvent, the confinement
and polarity effects within the micropores of the zeolite,
and the impact of the coordination state of the Sn atom on
the framework as either partially hydrolyzed framework
Sn centers (–Si–O–)
3
Sn–OH or fully framework coordinated
Sn atoms Sn(–Si–O–)
4
.
It was shown that in the presence of organic solvent, para-
xylene, the aqueous phase hydrolysis of hemicelluloses with
H-Beta (Si/Al = 19) and HUSY (Si/Al = 15) catalysts
increased the furfural yield from 18% to 56%.
85
Acomparative
analysis of the catalytic performance indicates that high surface
and easy accessibility of the acidic sites are the key factors for
efficient xylose to furfural conversions. Despite new develop-
ments described above, the cost and energy expense of furfural
production and recovery requires significant improvement by the
use of efficient solid catalyst and superior extracting media.
6. Furfural upgrade to fuels
Furfural is considered as a platform chemical for the produc-
tion of liquid hydrocarbons
86
and gasoline additives such as
2-methyltetrahydrofuran (2-MTHF).
87
Hydrogenation remains
Scheme 3 Mesoporous silica bead with solid core and mesoporous
shell for catalytic cyclodehydration of xylose to furfural (reproduced
from ref. 75, Copyright (2011) Elsevier).
Scheme 4 Sn-beta zeolite catalyzed furfural synthesis via xylose
intermediate.
Fig. 6 (a) Sn-beta-zeolite structure derived from EXAFS in which
the pair of Sn (red) atoms occupies opposite vertexes of the six-
member rings. (Reproduced from ref. 82 with permission, Copyright
(2005) American Chemical Society.) (b) Proposed active site of Sn-
beta-zeolite. (c) Proposed intermediate 1,2-hydride shift at the active
site.
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the most versatile reaction to upgrade furanic components to
biofuels. For instance, it can lead to promising gasoline blends
including 2-methylfuran (2-MF), 2,5-dimethylfuran (DMF)
and 2-methyltetrahydrofuran (2-MTHF).
88,89
Synthesis of
biofuel components, including DMF from biomass and
biomass derived carbohydrates via HMF platform chemical
has received significant attention in the recent years.
87,90a,b
The
literature report suggested that selective conversion of furfural
(95%) to 2-MF can be achieved by using Cu catalysts at high
temperature (200–300 1C) under the condition of H
2
/furfural
molar ratio of 5–8.
90c
The reaction proceeds through
the formation of furfuryl alcohol (FAlc) as intermediate.
Raney-Cu, Cu/Al
2
O
3
and Cu-chromite showed similar
behavior, although the latter was more active and stable.
However, the catalysts deactivated rapidly and regeneration
process at 400 1C was energy expensive. Carbon-supported
Cu-chromite was found to be selective for 2-MF, however,
deactivated within a few days.
91
In a recent study, furfural was
hydrogenated over a Cu/Fe catalyst in gas phase with 99%
conversion and 98% selectivity in 2-MTHF.
92
2-MTHF was
also obtained via hydrogenation of furfural over the Cu-based
catalyst in the vapor phase.
93
In the case of 2-MF synthesis, a
rapid deactivation of the catalyst was observed in gas-phase
hydrogenation which triggered the necessity of a process at
milder temperature and in liquid phase. Nudelman et al.
94
described the hydrogenolysis of furfural to 2-MF and of
5-methylfurfural to DMF, using Pd supported on carbon
(Pd/C) at room temperature and 0.2 MPa H
2
. Sun et al.
95
reported a polymer-supported Pd
II
complex that catalyzed the
hydrogenolysis of furfural to MF.
Ethyl levulinate (EL), being known as a potential fuel
additive with a boiling point of 206 1C, is a novel diesel
miscible biofuel usually produced by esterification of levulinic
acid (LA) in ethanol.
96,97
Traditionally, the formation of EL
depends on the yield of levulinic acid obtained from biomass
by the treatment with aqueous mineral acid (H
2
SO
4
and HCl)
at 100 1C which provided maximum 40% yield of LA.
98
The
same was improved to 60–70% by continuous flow conditions
at higher temperatures and pressure using H
2
SO
4
as catalyst
associated with complicated work-up during the separation.
99
However, furfural to ethyl levulinate conversion via hydro-
genation to furfuryl alcohol over copper-based catalysts and
subsequent ethanolysis in the presence of strong acids has been
reported (Scheme 5).
100
Furfuryl alcohol derived from furfural was then converted
to EL by the use of several strong acidic resins on a sulphonated
polystyrene framework (Amberlyst) and zeolites as solid acid
catalysts since these are known to sustain cock burnt-off during
regeneration.
101
The optimum result was achieved by balancing
the number of acid sites with their accessibility in the resin. The
data presented by authors showed that the efficiency of the acid
catalysts decreases in the following order: H
2
SO
4
4 macro-
reticular resins 4 gel resins 4 zeolites. This ranking, however,
appears to result from two critical catalysts parameters,
namely, the acidity of the catalyst and the accessibility of its
acid sites. Good accessibility of the acid sites, for example,
through surface sulfonation as in the case of Amberlyst 46,
seems therefore favorable. Furfural platform has been further
upgraded to alkyl levulinate by the use of a novel hybrid
solid catalyst methylimidazolebutylsulfate phosphotungstate
([MIMBS]
2
PW
12
O
40
) affording a high yield of n-butyl levulinate
(93%).
102
As revealed, the mechanistic route of the alcoholysis of
the furfuryl alcohol involves the formation of a-angelica lactone
and oxonium ion which then turns into alkyl levulinates.
103
Sen and Yang have demonstrated that pentose sugars and
lignocellulosic biomass (e.g. corn stover) can be converted into
2-methyltetrahydrofuran (2-MTHF) by employing a soluble
robust rhodium catalyst and HI/HCl + NaI additive in the
presence of H
2
.
87,104
Using corn stover (glucan 40.1% and xylan
24.1%) as feed, maximum 63% 2-MTHF yield was achieved. Even
though the process is uneconomical due to the use of expensive
rhodium salt, corrosive acids, and dihydrogen, lignocellulosic
pretreatment, enzymatic hydrolysis of cellulose/hemicelluloses,
to obtain sugars is simplified to obtain 2-MTHF as the final
product. Previously, 2-MTHF was synthesized by the coupling
of the dehydrogenation of cyclohexanol and the hydrogenation
of furfural over the Cu–Zn–Al catalyst with optimal hydrogen
utilization.
105
In vapor phase hydrogenation of furfural to
2-methylfuran (2-MF), unselective formation of several furan
products including 2-MTHF was recorded.
106
Similarly, 2-MF
and 2-MTHF were obtained as a mixture of products from
super critical carbon dioxide (scCO
2
) mediated continuous-flow
hydrogenation using a commercial catalyst containing copper
chromite and Pd/activated C.
107
Liquid phase hydrogenation of
furfural was also attempted with the NiMoB/g–Al
2
O
3
catalyst
affording furfuryl alcohol as a major product.
108
In a recent study, the vapor phase conversion of furfural
with SiO
2
-supported Ni and Ni–Fe bimetallic catalysts in the
presence of H
2
(1 bar) demonstrated a significant deviation in
activity. When monometallic Ni catalyst favors formation of
furfuryl alcohol and furan as primary products via hydrogena-
tion and decarbonylation, the Ni–Fe bimetallic catalyst
formed 2-MF as a major product via C–O hydrogenolysis of
furfuryl alcohol.
109
In this case, addition of Fe suppresses the
decarbonylation activity of Ni while promoting the CQO
hydrogenation (at low temperatures) and the C–O hydro-
genolysis (at high temperatures). DFT analysis of the possible
surface species on the mono- and bimetallic surfaces suggests
that the differences in selectivity displayed by these catalysts
can be attributed to the stability of the Z
2
-(C,O) surface
species, which is higher on the NiFe than on pure Ni. As
a result, Z
2
-(C,O) species can be readily hydrogenated to
furfuryl alcohol and subsequently hydrogenolyzed to 2-MF
on the bimetallic alloy due to a strong interaction between the
carbonyl O and the oxyphilic Fe atoms. On the pure Ni
surface, Z
2
-(C,O) species can be converted into a surface acyl
species, which can be decomposed to produce furan and CO.
DFT calculations for geometries and relative stabilities of the
possible furfural species on the catalyst surface showed the
difference in heats of adsorption and bond lengths of furfuryl
Scheme 5 Conversion of furfural into EL by catalytic hydrogenation
and ethanolysis in acid conditions.
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Catal. Sci. Technol.
alcohol adsorbate by considering both upright and planar
configuration of furfural on Ni(111) and bimetallic NiFe(111)
alloy surface. Consistent lengthening of the C1O1 bond of the
furfural species on the Ni–Fe(111) bimetallic surfaces compared
to that on pure Ni (1.433 vs. 1.377 A
˚
) afforded higher CO
hydrogenolysis rate. DFT analysis on Ni(111) and bimetallic
NiFe(111) by considering a hydroxyalkyl intermediate
(C
5
H
4
O(OH)) that is expected to result from the dissociative
adsorption of furfuryl alcohol (C
5
H
5
O(OH)) (Fig. 7) indicates
that the CO hydrogenolysis is much faster with furfuryl
alcohol than with furfural; so, it is possible that the formation
of 2-MF goes through an alcohol intermediate.
Inspired by this method, we have investigated the synthesis
of 2-MF via liquid phase hydrogenation of furfural and
furfuryl alcohol using the Ru/C–formic acid catalytic system
under mild conditions in tetrahydrofuran (THF). The process
is also clubbed with a dehydration step of pentose sugar (xylose) in
thepresenceofBrønstedacidicionicliquid[DMA]
+
[CH
3
SO
3
]
.
110
Similar strategy has also been extended for a bimetallic catalyst
(Pd–Ru/C) with promising outcomes and would be the subject
of upcoming contribution. This attempt is the extension of our
recent efforts for a sustainable one-pot synthetic protocol for
hydrogenation–hydrogenolysis of HMF to gasoline blendstock,
DMF, using the Ru/C–formic acid catalytic system.
111
A remarkable synthesis strategy to derive branched hydro-
carbons with ten to eighteen carbon atoms within the diesel
fraction was recently developed by Corma et al. by using the
furfural platform derived 2-MF as a building block.
112
2-MF
is derivable from hemicelluloses and available as raw material
on an industrial scale. An oxygenated C
13
fuel precursor was
derived by condensation of 2-MF with acetone, which was
then hydrodeoxygenated into a C
12
/C
13
mixture (Scheme 6).
113
However, in the presence of Brønsted acid catalysts the
reaction medium became sufficiently acidic that it produced
ring-opening of 2-MF, allowing a trimerization to produce a
C
15
diesel precursor. Trimerization is possible in the case of
2-MF because one of the two reactive carbons (2-positions) is
blocked by a methyl group preventing the polymerization of
2-MF with aldehydes. Hydrodeoxygenation of a C
15
diesel
precursor with a mixture of Pt/C and Pt/TiO
2
catalysts gave
6-butylundecane as the main product (Scheme 6), which can be
blended directly with fossil-derived commercial diesel.
7. Summary and outlook
So far in this contribution, we have been engaged in a
short discussion on hemicellulose structure and results of the
hydrolysis of lignocellulosic biomass to obtain pentose sugars,
e.g. xylose, a feedstock for furfural on industrial scale. Strategies
for the homogeneously catalyzed process have been explored for
xylose conversion using an acid catalyst and a powerful extrac-
tion solvent. However, significant development has been made
with the materials design, synthesis and their application for the
xylose dehydration to furfural in aqueous and organic media.
Typical time scales for many solid catalysts are in the order of
hours, leading to limited space time yields. In most cases, large
scale utilization of lignocellulosic biomass containing significant
hemicellulosic components will need to be addressed in
the following years since there is limited development in
this direction with both homogeneous and solid catalysts.
Xylose dehydration into furfural has an activation barrier of
B30 kcal mol
1
,
114
and hence it is carried out at high
temperatures (4150 1C) in aqueous medium. Under these
conditions, the furfural yield is B30% when carried out in a
single-phase system, and the same has been further improved
(71–78%) by developing continuous extraction processes,
115,116
using extracting solvents such as alkylphenol and 2-methyl-
tetrahydrofuran which can be derived from biomass feedstocks.
Recently homogeneously catalyzed processes have been
developed with chloride salts including (FeCl
6
6H
2
O and
AlCl
3
6H
2
O) in aqueous–organic biphasic medium in line with
the fact that Cl
promotes the formation of 1,2-enediol from
acyclic form of xylose and subsequent dehydration from an
aqueous acidic solution.
117
However, processes that are more in
line with green chemistry principles and are of higher furfural
selectivity are still needed. Towards the application of furfural
platform for the synthesis of hydrocarbon and furanic fuels,
combined efforts for efficient production of furfural via both
homogeneous and solid catalytic method must be improved.
Fig. 7 Optimized structures of furfuryl alcohol dissociatively adsorbed
on the Ni(111) surface (a) and the NiFe(111) surface (b). Side view of
surface and gas-phase hydroxyalkyl intermediate structures are shown
in (c) and (d), respectively. (Reproduced from ref. 109 with permission,
Copyright (2011) Elsevier.)
Scheme 6 Sylvan diesel process. First, one molecule of 2-MF is
hydroxyalkylated by an aldehyde and the corresponding product
alkylated a second sylvan molecule resulting a precursor which on
subsequent hydrodeoxygenation produces alkanes.
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1. The mineral acid catalyzed industrial process of furfural
must be replaced with an environmentally sustainable process
towards which limited developments have been made. Ionic
liquid catalyzed xylose dehydration to furfural and subsequent
molecular level monitoring of the process using
1
H NMR
spectroscopy is something yet to be investigated. In spite of
tremendous interest in this topic, mechanistic studies on ionic
liquids are rather complicated due to the lack of a powerful
analytic tool. Though many investigations have claimed to
reveal the mechanistic nature of hexose to 5-hydroxymethyl-
fural conversion via cyclic and acyclic routes in aqueous and
organic solvents,
118,119
such studies are missing for the pentose
sugars to xylose conversion except in limited cases where
xylulose intermediate has been proposed and estimated.
51,80
A knowledge of mechanistic pathways of xylose conversion in
aqueous or organic media is of principle importance to create
efficient procedures, control of selectivity, and to restrict side
reactions leading to insoluble and soluble humins.
120,121
The
goal of our future research is to understand the mechanistic
nature of xylose to furfural at the molecular level. We believe
in an NMR scale reaction, running the process in the NMR
tube by stirring and recording the reaction mixture in situ
would provide better insight into the reaction and would trace
the conformational changes that the sugar units have under-
gone during the process. It is revealed that the xylose to
furfural conversion goes through the formation of xylulose
and the steps need Lewis and Brønsted acid catalyst. We hope
to develop and apply dual acidic ionic liquid which can
operate the process under much lower temperature than the
temperature required for an aqueous phase version.
2. An efficient solid catalyst with dual acidic functionality
(Lewis and Brønsted) which can initiate the dehydration in
aqueous medium without much loss of activity must be
designed. Toward this direction we envisaged the application
of immobilized sulfonic acid functionalized ionic liquids,
mesoporous materials with sulfonated surface etc. Using such
dual functional material as catalyst, we hope to extract direct
evidence in favor of the isomerization process, including the
characterization of the xylulose intermediate. Catalytic strategies
may also involve the application of mesoporous carbonaceous
materials (e.g. Starbon) with surfaces ranging from hydrophilic
to hydrophobic based on the degree of carbonization. Such
material with ordered porous structure can then be modified
post-synthetically by incorporating Lewis and Brønsted acidic
sites (SnCl
4
and SO
3
H) to develop a one-pot process. Recently
Sn-beta zeolite has been successful in catalyzing xylose to
xylulose isomerization process.
80
This advocates the requirement
of a Lewis acidic site in the catalyst which promotes the
isomerization of xylose and subsequent dehydration catalyzed
by a Brønsted acidic site in the catalyst may lead to an efficient
conversion.
3. A liquid phase process of furfural hydrogenation–hydro-
genolysis with high selectivity for 2-MF is something yet to be
developed. Using a supported monometallic or bimetallic
catalyst (Pd/C or Pd–Ru/C) in the presence of formic acid as
a hydrogen source, we hope to develop a simple synthesis
protocol for potential fuels and solvents starting from furfural.
Deconstruction of biomass (celluloses and hemicelluloses)
has experienced a new development cycle, in which this process
is carried out over homogeneous and solid catalysts and coupled
to other reactions for a better utilization of the feedstock.
Hemicellulosic fraction of lignocellulosic biomass is the best
source for pentose sugars and it does not compete with food
supply. However, these materials are resistant to chemicals
transformation. Current practice has demonstrated that there is
indeed promise. Transformation of sugars into transportation
fuels and chemical commodities has received much more
attention. In the long term, however, the success of the
biorefinery concept also depends on the development of
energetically efficient processes to convert lignocellulosic
biomass directly into biofuels.
Acknowledgements
The authors gratefully acknowledge financial support from the
University Grant Commission (UGC), India. SD thanks UGC,
India, for a DS Kothari Postdoctoral Research Fellowship.
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