Depolymerization of cellulosic feedstocks using magnetically separable functionalized graphene oxide

Article abstract of DOI:10.1039/c3ra41025k

Hydrolysis of cellulose into saccharides using a magnetically separable functionalized graphene is reported for potential applications in the environmentally benign saccharification of cellulose. Crystalline pure cellulose is hydrolyzed by graphene bearing -SO₃H, -COOH and -OH functional groups in combination with iron nanoparticles. We observed nearly complete hydrolysis of cellulose into glucose and small (4-5 unit size) oligomers using low (1:1) catalyst to cellulose ratio. The apparent activation energy for the hydrolysis of cellulose into glucose using these catalysts is estimated to be 12 kJ mol^-1, several times smaller than that for sulfuric acid under optimal conditions (170 kJ mol^-1). The catalyst can be readily magnetically separated from the saccharide solution after the reaction for reuse in the reaction without loss of activity. Nearly complete hydrolysis of sugarcane bagasse into water soluble saccharides with repeated recycling was also possible. The catalytic performance of the graphene-based catalyst is attributed to the ability of the water soluble nanostructured material with a large concentration of polar groups (-OH, -COOH) which readily adsorb cellulose, while providing a large concentration of acidic functionality to hydrolyze the cellulose.

Full text of DOI:10.1039/c3ra41025k

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Depolymerization of cellulosic feedstocks using magnetically separable
functionalized graphene oxide
Deepak Verma, Rashmi Tiwari and Anil Kumar Sinhaa
aCSIRꢀIndian Institute of Petroleum, Mohkampur, 248005, INDIA. Fax: +91ꢀ135ꢀ2660202; Tel: 91ꢀ
135ꢀ2525842; Eꢀmail: asinha@iip.res.in
Abstract
Hydrolysis of cellulose into saccharides using a magnetically separable functionalized
graphene is reported for potential application in the environmentally benign saccharification
of cellulose. Crystalline pure cellulose is hydrolyzed by the graphene bearing ꢀSO3H, ꢀCOOH
and ꢀOH functions along with iron nanoparticles. We observed nearly complete cellulose
hydrolysis of cellulose into glucose and small (4ꢀ5 unit size) oligomers using low (1:1)
catalyst to cellulose ratio. The apparent activation energy for the hydrolysis of cellulose into
glucose using these catalysts is estimated to be 12 kJ molꢀ1, several times smaller than that for
sulfuric acid under optimal conditions (170 kJ molꢀ1). The catalyst can be readily
magnetically separated from the saccharide solution after reaction for reuse in the reaction
without loss of activity. Nearly complete hydrolysis of sugarcane bagasse into water soluble
sachharides with repeated recycling use was also possible. The catalytic performance of the
grapheneꢀbased catalyst is attributed to the ability of the water soluble nanostructured
material with large concentration of polar groups (ꢀOH, ꢀCOOH) to readily adsorb cellulose,
while providing large concentration of acidic functionality to hydrolyze cellulose.
1. Introduction
Limited fossil fuels and environmental concerns have posed immense challenge for
researchers to look at all possible alternatives, and biofuels have attracted much attention in
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this decade, as potentially viable renewable alternative.1 Production of fuels and chemicals
from biomass will reduce the total CO2 production as compared to fossil fuels, thereby
having much lower contribution to global warming.2,3 Carbohydrates (cellulose and
hemicelluloses) constitute about 50ꢀ80% of complete dry weight of biomass. The major
components of plant biomass ꢀ cellulose, hemicellulose and lignin ꢀ possess much higher O/C
molar contents than gas, coal and crude oil. Cellulose, which forms 40% of lignocelluloses, is
the most abundant, renewable natural polymer resource available today worldwide. Cellulose
is mainly made up by basic unit of Dꢀglucose through 1,4ꢀβꢀglycosidic linkages. Enzymatic
and catalytic transformation of small cellulose oligomers4 and glucose into liquid fuels, such
as ethanol or dimethylfuran5 is an efficient strategy to decrease the O/C molar ratio. But
hydrolysis of cellulose is not an easy task. Several papers have been published concerning
hydrolysis of cellulose by using ionic liquids,6 metal salts7 and acidic heterogeneous
catalysts, to replace mineral acids due to environmental concerns. Catalysts like sulfonated
activated carbon,8 layered niobium molybdate,9 mesoporous carbonꢀsupported Ru,10 are
reported to hydrolyze cellulose to glucose and water soluble products. It has been reported
11
that sulfonated silica/carbon cellulose hydrolysis catalysts afford 50% glucose yield while
sulfonated mesoporous carbon give a glucose yield of 75%12 during hydrolysis of cellulose.
Sulfonic group functionalized magnetic SBAꢀ15 catalyst (Fe3O4–SBA–SO3H) is reported to
hydrolyze microcrystalline cellulose and amorphous cellulose with 26% and 50% yields,
respectively.13 Acidic resins with large pores, are also reported as suitable catalysts for the
depolymerization of cellulose in ionic liquids to form oligomers.6a, 14 It is highly desirable
that solid catalysts should be readily separable from the solid residues.
Hara and coꢀworkers8 used sulfonic group functionalized amorphous carbon for cellulose
hydrolysis and obtained very low yield of glucose, but high yield of only 1,4ꢀβꢀ glucan
(oligomers), at 1000C, using very high catalyst:cellulose ratio (12:1). Here, we first time
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report a new strategy to hydrolyze cellulose using functionalized graphene, rather than
functionalized amorphous carbon,8 which has several advantages: (1) low catalyst:cellulose
ratio (1:1) is used; (2) surprisingly high activity, with high yields of monoꢀ and diꢀ glycerides,
along with water soluble small (4ꢀ5 unit) oligomers is obtained; (3) easy magnetic separation
and main text of the article should appear here. Headings and reuse of catalyst is possible.
Graphene has received much attention of researchers in the past few years due to its some
exceptional properties like excellent electronic, mechanical, optical and thermal conductivity.
Graphene is an allotrope of carbon which includes graphite, diamond, fullerene, charcoal and
carbon nanotubes, and has two dimensional (2D) one atom thick planar sheet of sp2
hybridized carbon atom with honeycomb structure.15Many methods are used for the graphene
synthesis, and chemical exfoliation is one of them. Graphene production from graphite is a
two step process – (1) vigorous oxidation of graphite (into graphene oxide) which extensively
functionalizes the graphene sheets with groups like ꢀOH, ꢀCOOH and epoxide, which results
in increase in volume, polar solvent solubility and change of color as compared to bulk
graphite; (2) Reduction of graphene oxide to graphene to remove the functional groups using
reducing agent like hydrazine.16a
2. Experimental Section
2.1 Catalyst Preparation
2.1.1 Synthesis of graphene oxide (GO): GO is prepared from graphite (Acros) by using
Hummer’s method.16b
2.1.2 Synthesis of GO-SO3H: Functionalization of GO is carried by diazotization by pꢀ
aminobenzenesulphonic acid (Acros) with NaNO2 (Acros) followed by reduction using
NaBH4 (Acros). In this procedure, 1000mg of GO, 200ml of double distilled water were
taken in a 500ml of beaker and kept for sonication for 30min. pH 9ꢀ10 was adjusted by
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adding 5wt% Na2CO3 solution. 200mg of NaBH4 (5.2mmol) was added to the reaction
mixture at 800C for 1hr with vigorous stirring. During the reduction, the dispersion turned
from dark brown to black and after some time aggregation of graphene particle start and
resulting separation of partially graphene oxide. The rGO (reduced graphene oxide) was
separated through centrifuge with 3500rpm for 5min. and washed 3times with double
distilled water (3x30ml).The resulting rGO was reꢀdispersed in 100ml of water via mild
sonication. The aryl diazonium salt used for sulfonation was prepared from the reaction of
900 mg pꢀaminobenzenesulphonic acid (5mmol) and 360mg (5.2mmol) sodium nitrite in
100g water and 10 g 1N HCl solution in an ice bath. The diazonium salt solution was added
to the dispersion of partially reduced graphene oxide in an ice bath under stirring, and was
kept in ice bath for 2.5 h. After centrifuging, the material was washed with 2ꢀ3 times with
water and dried at 650C for 2h.
2.1.3 Synthesis of Fe-GO-SO3H: 200g of rGOꢀSO3H was dispersed in 80 ml water through
sonication for 15 min. The mixture of 0.5 g of ferrous sulphate (1.7mmol), 1g of PEGꢀ4000
and 5ml water was added slowly in to the rGOꢀSO3H solution with vigorous stirring for 24h.
Total volume of solution was made up to approx. 1l by adding water. After 1hr, 1g of NaBH4
(26mmol) added to this reaction mixture at 800C and kept it for next 2h.The black color
magnetically separable FeꢀGOꢀSO3H was separated by filtration and washed with water and
dried at 650C.
2.1.4 Synthesis of G-PhSO3H: Synthesis of functionalized graphene was done from post
reduction of rGOꢀSO3H by using hydrazine hydrate. In this step, 400mg of GOꢀSO3H was
dispersed in 100mg of water with 12g of hydrazine hydrate (80%) and kept the reaction
mixture at 1000C for 24h.The GꢀSO3H as black precipitate was separated through filtration
and washed with water and dried at 650C.
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2.1.5 Synthesis of Iron nanoparticle on G-SO3H (Fe-G-SO3H): 1g of rGOꢀSO3H was dispersed
in 200 ml water through sonication for 15 min. The mixture of 0.5 g of ferrous sulphate
(1.7mmol), 1g of PEGꢀ4000 and 5ml water was added slowly in to the GOꢀSO3H solution
with vigorous stirring for 24h. 15g of hydrazine hydrate (80%) was added to the mixture at
1000C for 24 h. The black color magnetically separable FeꢀGꢀSO3H was separated by
filtration and washed with water and dried at 650C.
2.1.5 Synthesis of Iron nanoparticle on GO (Fe-GO): In the typical synthesis, 200mg of GO
was dispersed in 80 ml water through sonication for 30 min. The mixture of 0.5 g of ferrous
sulphate (1.7mmol), 1g of PEG 4000 and 5ml double distilled water was added slowly in to
the GO solution with vigorous stirring for 24h. Total volume of solution was made up to
approx. 1l by adding water. After 1hr, 1g of NaBH4 (26mmol) added to this reaction mixture
at 800C and kept it for next 2h. The black color FeꢀGO was separated by filtration and
washed with water and dried at 650C.
2.2 Catalyst Characterization
Structural information for the prepared functionalized and Feꢀgraphene oxide was obtained
SEM, TEM (120 KV JEOL 1210), Raman (Horiba, Lab RAM HR), FTꢀIR analysis. The IR
spectra were recorded on a Perkin Elmer FTꢀIR X 1760 instrument using KBr pellet
technique. FEꢀSEM analyses were done on FEI, Netherland equipment with samples
supported on holeyꢀcarbon grids from an ethanol suspension. C, H, N, and S elemental
analysis was carried out using CHNS Elemental Analyzer (PerkinElmer). , the densities of
SO3H groups were estimated based on the sulfur content determined from sample
compositions obtained by elemental analysis. The total SO3H + COOH and SO3H + COOH +
OH contents were estimated from the exchange of Na+ in aqueous NaCl and NaOH solutions,
respectively,8c to afford the proportions of each functional group. Structure analysis by SEM
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and TEM was done for the prepared graphene material obtained by sulfonation and after
deposition of iron nanoparticles.
2.3 Depolymerization of Cellulose and Baggase
Hydrolysis of cellulose was carried out at atmospheric pressure and at different temperature
like 750C, 1000C and 1250C for different time. Cellulose (300DP, 50mg), catalyst (50mg),
and water (HPLC, 5ml) were taken in a 50ml beaker and heated at 750C for 24h. After cooled
at room temperature, the catalyst was separated through magnet from the product and washed
2 times with double distilled water (2.5ml x 2).
Hydrolysis of sugarcane bagasse was carried out with baggase washed two times with
distilled water (10ml x 2) before reaction. The round bottom flask containing the catalyst and
reactant was connected with reflux condenser (FeꢀGO, 0.300 g; distilled water, 15 g; bagasse;
0.300 g) was placed in an oil bath at 750C for 44 h and then cooled to room temperature. The
catalyst was separated through magnet from the product and washed 2 times with double
distilled water (5ml x 2).
2.4 Product characterization
The water soluble products were analyzed by HPLC (Shimadzu LCꢀ2010HT). An Agilent Hiꢀ
Plex H column (300 × 7.7 mm, 8 ꢁm) was used with water as eluent with flow rate of 0.4 ml
min−1 keeping column oven temperature at 700 C with RI detectors. The column used in
HPLC analysis was Agilent, USA (PLꢀHiꢀPlex H) with HPLC water as eluent with flow rate
of 0.4 ml min−1 keeping column oven temperature at 700 C with RI detectors.
MALDI/TOF (Matrix Assisted Laser Desorption Ionization/Time of Flight) analyses were
recorded on TofSpec 2E MALDIꢀTOF mass spectrometer (Micromass) using 2, 5ꢀ
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dihydroxybenzoic acid (DHB) as matrix. 1H NMR spectra were recorded on a Bruker
Avance iii spectrometer operating at 500 MHz for proton observation.
The water soluble products were analyzed by HPLC (representative HPLC Chromatogram
shown in Supporting Fig. S1). The yields (in terms of C6H10O5 unit for monoꢀ, diꢀ and
oligosaccharides) of water soluble organic compounds in resultant solutions after reaction
were calculated as follows:
Glucose (monosaccharide) yield (%) = (moles of glucose in water soluble products x 1)/ (mol
of C6H10O5 unit in charged cellulose) x 100.
Xylose (monosaccharide) yield (%) = (moles of xylose in water soluble products x 1)/ (mol
of C6H10O5 unit in charged cellulose) x 100.
Cellobiose (disaccharide) yield (%) = (moles of cellobiose in water soluble products x 2)/
(mol of C6H10O5 unit in charged cellulose) x 100.
Oligosaccharides yield (%) = (total moles of water soluble products as C6H10O5 unitꢀ total
moles of glucose, xylose and cellobiose as C6H10O5 unit)/ (mol of C6H10O5 unit in charged
cellulose) x 100.
The conversion (%) of cellulose hydrolysis was calculated by using following formula:
(A) Using dry wt. of cellulose
Conversion (%) = (AꢀB) x 100/A
where, A = Dry weight (g) of Cellulose before reaction, B = Dry weight (g) of cellulose after
reaction,
The results were crossꢀchecked using following formula:
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(B) Using moles of cellulose
Conversion (%) = (aꢀb) x 100/a
Where, a = moles of cellulose taken before reaction, b = moles of unreacted cellulose left
after reaction,
Here we assume the cellulose in moles considering the repeating anhydroglucose units in
cellulose. (25 mg of cellulose is approximately equal to 0.15 mmol of anhydroglucose).17
3. Result and Discussion
3.1 Catalyst Characterization
Free standing wavy graphene sheets are observed in SEM images (Fig. 1a). SEM images
revealed that the reduced GO material consists of randomly aggregated, thin, crumpled sheets
closely associated with each other and forming a disordered solid. The folded regions of the
sheets were found to have widths of 10ꢀ20 nm by highꢀresolution SEM. The surface of the
sheets is fairly smooth. The particle size of graphene oxide and functionalized graphene oxide
were estimated to be 10ꢀ40 ꢁm and 10ꢀ30 ꢁm respectively by SEM, and a layered structure is
observed at the edge of the agglomerates. TEM (Fig. 1b) of functionalized graphene material
shows wellꢀexfoliated separated 1ꢀ2 layer sheets randomly oriented. Fe functionalized
graphene clearly shows 15 nm and lower sized nanoparticles on a graphene sheet (Fig. 1c).
FTIR spectra (Fig. 2) confirmed the surface functionalization, reduction of functional
group and incorporation of Fe nanoparticles on graphene oxide. The spectra also shows the
presence of CꢀO (νCꢀO at 1060 cmꢀ1), CꢀOꢀC (νCꢀOꢀC at 1250 cmꢀ1), CꢀOH (νCꢀOH at 1365
cmꢀ1), and C=O in carboxylic acid and carbonyl moieties (νC=O at 1720 cmꢀ1) on the surface
of graphene oxide.16a The very broad intense peak at 3400 cmꢀ1 is seen for OꢀH stretching
frequency. But the intensity of OꢀH group on GO is much more than functionalized
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graphene, which clearly shows the decreasing of ꢀOH groups on the surface of graphene.16a, 18
The peaks at 1060 cmꢀ1, 1250 cmꢀ1 and 1365 cmꢀ1 are attenuated after sulfonation followed by
reduction in the sulfonated graphene oxide (GOꢀSO3H) spectrum as compared to GO. The
presence of sulfonic group (ꢀPhSO3H) is confirmed by peaks at 1175 cmꢀ1, 1125 cmꢀ1, and
1042 cmꢀ1 (two νSꢀO and one νSꢀphenyl, respectively), and the peaks at 1007 cmꢀ1 (νCꢀH inꢀ
plane bending) shows the characteristic vibrations of a paraꢀdisubstituted phenyl group. The
absence of the peaks at 1365 cmꢀ1, 1250 cmꢀ1, 1060 cmꢀ1 and 1760cmꢀ1 indicates that the
epoxide, hydroxyl and carbonyl groups respectively have been removed from the surface of
basal graphene layer after the final reduction by hydrazine. CHOS analysis was used to
confirm complete absence of adsorbed sulfonic acid groups from the graphene oxide.
Elemental analysis revealed that the sample composition was CH0.15O0.08 in the GO. S: C
atomic ratio was 1:20 for the sulfonic group (ꢀSO3H) functionalized graphene oxide, with 0.8
mmol/g sulfonic groups.
Raman spectroscopy was used to characterize the materials and to obtain information about
the quality and number of layers in a given sample (Fig. 3). All the samples show the Dꢀband
around 1336 cmꢀ1 arising from disorder and the Gꢀband around 1600 cmꢀ1 due to singleꢀlayer
graphene. The Raman spectra of the reduced and functionalized samples show an increased
D/G intensity ratio compared to that in GO. This change may be attributed to creation of new
smaller size graphitic domains resulting in a decrease in the average size of the sp2 domains
upon reduction of the exfoliated GO.19
3.2 Cellulose hydrolysis
Scheme 1 gives a schematic representation of the sequential steps in the hydrolysis of
cellulose into water soluble carbohydrates. In comparison to functionalized amorphous
carbon reported earlier,8 we have used a grapheneꢀbased catalyst which is easily magnetically
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separable. Small size of graphene nanosheets, their high adsorption capacity for cellulose due to
large number of OH and COOH functionality, and further adsorption assisted rapid hydrolysis due to
high concentration of acid sites (ꢀCOOH, ꢀSO3H), is expected to give high reaction rates (lower
activation energy). The water soluble carbohydrate product formed after hydrolysis of cellulose
contains oligomers, disaccharides and monosaccharides. Water was used as the reaction
medium, and the separation of water soluble products from the catalyst was carried out
magnetically.
Graphene oxide (GO) which is water soluble showed highest activity and glucose yield
(Table 1, Fig. 4). Soluble –COOH groups for hydrolysis, and ꢀOH groups for ready cellulose
adsorption to the graphene surface, are synergetically responsible for high activity. Catalyst
separation was not possible, but addition of a salt, FeCl3 to the solution resulted in the
aggregation of the soluble graphene, which could then be separated out. But a more easily
separable catalyst system is required, so we modified the graphene oxide surface with
magnetically separable iron nanoparticles. Complete conversion of cellulose into water
soluble products was possible, and waterꢀsoluble oligomers as well as cellobiose, glucose,
xylose were formed (FeꢀGO, Table 1, Figs. 4 and 5). The magnetically separable catalyst
could be recycled and reused for several cycles without any observable loss in performance
(Fig. 6). Functionalization of the graphene oxide surface with ꢀPhSO3H group also resulted in
high activity and selectivity for the glucose (GOꢀSO3H, Table 1, Figs. 4 and 7). But reduced
graphene with ꢀPhSO3H functional group (GꢀSO3H, Table 1, Figs. 4 and 7) showed much
lower activity, and cellobiose was the predominant product. This observation clearly
indicates the very indispensable role of ꢀOH and ꢀCOOH functionalities in the cellulose
hydrolysis, which is to readily adsorb the cellulose on graphene surface and to hydrolyze it.
Magnetically separable catalysts were prepared by depositing ironꢀnanoparticles on the
surface of ꢀPhSO3H functionalized graphene (FeꢀGꢀSO3H). This catalyst showed nearly 95%
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conversion with 50% glucose selectivity which increased to 55% selectivity and 98%
conversion with increase in reaction temperature by 250C(FeꢀGꢀSO3H, Table 1, Figure 7).
The catalyst could be easily separated magnetically and reused. The reaction over this FeꢀGꢀ
SO3H catalyst is slow, which is indicated by the longer reaction time (44 h) required to
achieve 95% conversion. In comparison, catalysts prepared by depositing ironꢀnanoparticles
on the surface of ꢀPhSO3H functionalized graphene oxide (FeꢀGOꢀSO3H, Table 1), showed
96% conversion after 24 h of reaction.
The yields of oligomers, cellobiose, glucose and xylose depend on the reaction time from 24
to 44h. Many papers6 have reported the formation of 5ꢀhydroxymethylfurfural, and levulinic
acid (byꢀproducts formed by the decomposition of glucose) during the hydrolysis of cellulose
under very acidic condition which are not observed in our system. Here we report the
formation of only monoꢀ, diꢀ and oligosaccharides from cellulose hydrolysis using
magnetically separable functionalized graphene oxide and there is no observable deactivation
of catalyst, even after testing five times. This process of depolymerization of microcrystalline
cellulose using functionalized Feꢀgraphene oxide as catalyst, is more attractive and promising
than other processes such as those using ionic liquids, acid or alkaline solution due to lower
reaction temperature and easily separable as well as recyclable catalyst.
1H NMR (Supporting Fig. S2) shows a multiplet in spectra which clearly indicate the
presence of water soluble components (monoꢀ, diꢀ, and oligomers) in the product and size of
the oligomeric product components was estimated to be C6H11O5ꢀ(ꢀOꢀC6H10O4ꢀ)2ꢀ3ꢀOꢀ
C6H11O5 by MALDIꢀTOFꢀMASS spectral analysis of the reaction product solution
(Supporting Fig. S3).
Smaller atomicꢀsize range graphene thickness compared to the hydrogenꢀbonded cellulose
interlayer distance (7ꢀ8Å) would favour the penetration of graphene into the interlayer space.
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The hydrolysis would be initiated preferably in the accessible interlinking regions between
the crystallites of the elementary fibrils or their aggregation. The splitting of glycosidic links
in these accessible areas would be followed by hydrolysis of those inside the wellꢀordered
crystallites (supporting scheme 1). Thus, high hydrolysis activity can be attributed to the
nanoꢀsheet structure of soluble graphene oxide with large number of polar acidic and
hydroxyl functional groups readily adsorbing and interacting with the surface of cellulose
through nonꢀpolar and polar interactions which is further assisted by the polarꢀpolar
interactions with water medium, resulting in the separation of layers of cellulose from
cellulose bunch and the formation of sandwich structure (Scheme 1). This is confirmed by the
partial homogenous reaction mixture obtained, just after few hours of reaction. There are
more functional groups present in graphene oxide (GO) than reduced graphene oxide (G) and
hence the former is more active as well as more selective for glucose formation. ꢀCOOH
concentration was very high (1.6 mmol/g) and OH concentration was 0.6 mmol/g, on GO.
Large concentration of polar groups (ꢀCOOH, ꢀOH) on the surface of GO interact through
hydrogen bonding, with the polar groups (ꢀOH) on the surface of microcrystalline cellulose. ꢀ
COOH groups on GO nanosheets can penetrate the polymeric cellulose, and readily catalyzes
the depolymerization reaction, into monoꢀ, diꢀ and oligoꢀ saccharides (Supporting Scheme 1).
Organic acids (such as maleic acid)20 are reported to hydrolyze cellulose, and addition of salt
to organic acids is reported to enhance cellulose hydrolysis.21 Synergic effect of the ꢀSO3H
and ꢀCOOH groups in the polymer chain is reported to enhance cellulose hydrolysis.22
The hydrolysis of cellulose into monoꢀ, diꢀ and oligoꢀ saccharides (Scheme 1) would
involve three stages: (1) polar (ꢀOH, ꢀCOOH) groups of graphene oxide loosen the affinity of
hydrogen bond between cellulose layers; (2) H+ ions from Brönsted acid sites (ꢀCOOH) of
graphene oxide attack on easily accessible βꢀ1,4ꢀglycosidic linkage of cellulose to form
oligosaccharides and create a gap between those layer; (3) βꢀ1,4ꢀglycosidic bonds in the
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oligosaccharides are further, gradually hydrolyzed to form disaccharides and
monosaccharides (glucose and xylose). The role of synergetic effect of ꢀPhSO3H and ꢀCOOH
groups, and of large concentrations of ꢀCOOH and ꢀOH groups in hydrolysis, is further
evidenced by low conversion and very low glucose yield over ꢀSO3H functionalized reduced
graphene (GꢀSO3H, Table 1) which has little ꢀCOOH and ꢀOH functionalities. In case of Feꢀ
GO sample where iron nanoparticles are dispersed on the surface of graphene oxide, the
acidic functional groups would be partially blocked and reduced leading to its lower
monosaccharide selectivity than GO. The apparent activation energy for the hydrolysis of
cellulose into glucose using these GO catalysts is estimated to be 12 kJ molꢀ1, several times
smaller than that for sulfuric acid under optimal conditions (170 kJ molꢀ1). The low
activation energy may be due to the highly effective collisions during the reaction of cellulose
which is very effectively adsorbed on water dispersible graphene nanosheet catalysts which
can then be effectively hydrolyzed by the acid functionality of the catalyst.
Hydrolysis of real biomass feedstock (sugarcane bagasse) into water soluble sachharides,
was also successfully carried out, with repeated recycling use. The product consisted of
monosaccharides (glucose and xylose) and oligosaccharides (cellobiose and water soluble
higher oligomers). Conversion was 98% (after 44h) and the yield of monosaccharides was
42% (39% Xylose and 3% glucose), while the yield of oligomers was 51%. This further
shows the efficacy of our catalytic system.
4. Conclusion
In summary, hydrolysis of cellulosic feedstocks by graphene bearing ꢀSO3H, ꢀCOOH and ꢀ
OH functional groups along with iron nanoparticles are reported, to develop an easily
magnetically separable catalyst for the process. We have obtained high yield (up to 98%) of
monosaccharides and disaccharides at lower reaction temperature (750C), while only
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oligomers were reported in the earlier work using functionalized carbon.8 The catalyst:
cellulose ratio used (1:1) in the reactions is very low, 12 times lower than the earlier report.8
The higher activity in our work is due to nanosheet structure, and (4ꢀtimes) higher
concentration of ꢀCOOH (1.6 mmol/g) on GO than that in the carbon material reported
earlier.8 0.8 mmol/g sulfonic groups in the catalyst further enhance their acivity (by 3ꢀ6
times).
Acknowledgement
Director, IIP is acknowledged for approving the research; DV acknowledges UGC, India for
research fellowship.
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Scheme 1. Schematic of sequential steps for the hydrolysis of cellulose into water soluble carbohydrates.
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Fig. 1 SEM (a) and TEM (b) image of functionalized graphene,GꢀSO3H; (c) image of FeꢀGrapheneꢀSO3H
(inset shows ED pattern)
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Fig. 2 FTIR of graphene oxide (GO), functionalized graphene oxide (GOꢀSO3H), functionalized graphene
(GꢀSO3H), magnetic FeꢀGO.
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Fig. 3 Raman spectra of graphene oxide (GO) functionalized graphene (GꢀSO3H), magnetic FeꢀGO.
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Table 1. Catalytic activity for hydrolysis of crystalline cellulose.
Catalyst
Functional groups
(conc. in mmol/g)
T
R.T Conv.
Yield (%)
(0C) (h)
(%)
Oligo
Cello
Xyl
Glu
Graphene oxide (GO)
ꢀOH(0.6), epoxide,
ꢀCOOH(1.6),
75
24
100
n.d
n.d
n.d
98
FeꢀGraphene oxide (Feꢀ ꢀOH(0.4), epoxide,
GO)
75
44
44
44
96
94
99
70
50
58
14
4.0
7.5
6.0
3.0
20
22
ꢀCOOH(1.2),
FeꢀGraphene oxide (Feꢀ ꢀOH(0.4), epoxide,
GO) ꢀCOOH(1.2),
100
125
12.9
12.5
FeꢀGraphene oxide (Feꢀ ꢀOH(0.4), epoxide,
ꢀCOOH(1.2),
GO)
FeꢀGraphene (FeꢀG)
ꢀꢀꢀ
75
75
24
24
4.0
89
3.0
10
n.d
n.d
n.d
n.d
n.d
78
Functionalized
graphene oxide (GOꢀ
SO3H)
ꢀOH(0.6), epoxide,
ꢀCOOH(1.5), ꢀ
PhSO3H(1.9)
Functionalized
PhSO3H(1.8)
75
24
50
2.1
46
0.3
0.2
graphene (GꢀSO3H)
Fe-Graphene -
SO3H(Fe-G-SO3H)
-PhSO3H(1.7)
75
44
44
24
95
98
96
40
21
19
4.3
14
14
n.d
5.0
n.d
50
55
58
FeꢀGrapheneꢀSO3H(Feꢀ ꢀPhSO3H(1.7)
GꢀSO3H)
100
75
FunctionalizedꢀFeꢀ
Graphene oxide (Feꢀ
GOꢀ SO3H)
OH(0.6), epoxide,
ꢀCOOH(1.5), ꢀ
PhSO3H(1.8)
Cellulose: 50mg; water: 5ml; catalyst: 50mg; oligo = 4ꢀ5unit oligomeric products, cello = cellobiose, xyl =
xylose, glu = glucose.
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Fig. 4 Catalytic activity for hydrolysis of crystalline cellulose.
3.0
Cellobiose
Glucose
Xylose
2.5
2.0
1.5
1.0
0.5
0.0
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Fig. 5 Catalytic activity for the hydrolysis of crystalline cellulose with magnetic graphene oxide as
a function of reaction time.
100
80
60
Fe-GO
Fe-GO ( at 1000C)
Fe-GO ( at 1250C)
40
20
10
20
30
40
Reaction Time (h)
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Fig. 6 Recycling activity for the hydrolysis of crystalline cellulose with magnetic graphene oxide Feꢀ
GO (reaction time=44h).
100
Oligomers
Cellobiose
90
Xylose
Glucose
80
70
60
50
40
30
20
10
0
5th
2nd
4th
1st
3rd
Repeat Time
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Fig. 7 Catalytic activity for the hydrolysis of crystalline cellulose with functionalized graphene as a
function of reaction time.
100
80
GO-SO3H
G-SO3H
60
Fe-G-SO3H
Fe-G-SO3H (at 1000C)
40
Glucose yield >50%
except G-SO3H
20
0
5
10
15
20
25
30
35
40
45
Time(h)
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Cellulosic feedstocks are hydrolyzed by the graphene bearing SO3H, COOH
and OH functions along with iron nanoparticles to develop a magnetically
separable catalyst.