Efficient catalytic conversion of concentrated cellulose feeds to hexitols with heteropoly acids and Ru on carbon

Article abstract of DOI:10.1039/c001096k

A combination of heteropolyacids and Ru on carbon catalyzes the conversion of concentrated cellulose feeds into hexitols under H₂ pressure. Quantitative conversion of ball-milled cellulose was observed with remarkable hexitol volume productivity.

Full text of DOI:10.1039/c001096k

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Efficient catalytic conversion of concentrated cellulose feeds to hexitols
with heteropoly acids and Ru on carbonw
Jan Geboers, Stijn Van de Vyver, Kevin Carpentier, Kevin de Blochouse,
Pierre Jacobs and Bert Sels*
Received 18th January 2010, Accepted 8th March 2010
First published as an Advance Article on the web 7th April 2010
DOI: 10.1039/c001096k
A combination of heteropolyacids and Ru on carbon catalyzes
the conversion of concentrated cellulose feeds into hexitols under
H2 pressure. Quantitative conversion of ball-milled cellulose was
observed with remarkable hexitol volume productivity.
the highest activity and hexose selectivity for commercially
available H3PW12O40 and H4SiW12O40 HPAs (see ESIw).
Notably, the HPAs performed much better than an equinormal
aq. H2SO4 solution, an industrial standard for cellulose hydro-
lysis. This was recently also reported by Shimizu et al.6
A comparison of the performance of the combined hydro-
lysis and hydrogenation catalysts, viz. H4SiW12O40 and Ru/C
against H2SO4 and Ru/C, in the conversion of microcrystalline
cellulose to hexitols at 463 K and 5 MPa H2 is presented in
Fig. 1. The experiments were performed in a 100 ml stainless
steel autoclave, the product mixture being analysed after
derivatisation on a HP 5890 GC equipped with a HP
7673 autosampler, a 50 m CP-Sil-5CB column and a FID
(see ESIw). The conversion of cellulose was determined by
measuring the soluble carbon content in the reaction mixture
using TOC. The H4SiW12O40–Ru/C combination is clearly
effective and selective in producing hexitols with 82% cellulose
conversion (vs. 56% for H2SO4–Ru/C) after 24 h, and a
maximum hexitol yield of 49% (vs. 28% for H2SO4–Ru/C).
Within the fraction of hexitols, the sugar alcohols, viz.
sorbitol and mannitol, are the main products, followed by
sorbitan, the 1,4-isomer being analyzed most abundantly.
These cyclic polyols result from a HPA–catalyzed dehydration
of the sugar alcohols. Other minor byproducts include ethylene
glycol, 1,2-propanediol, glycerol, and C4 and C5 polyol isomers.
This second family of by-products results from the Ru/C
catalyzed hydrogenolysis of the primary hexitols, as reported
Limited supply of fossil fuels and the emission of greenhouse
gases associated with their consumption has prompted renewed
research and industrial interest in biomass as a sustainable
source of fuels and chemicals.1 Cellulose is the most abundant
source of biomass. Unlike starch, it is insoluble in water and
not digestible by humans because of the semicrystalline struc-
ture composed of b-1,4-glycoside bonded glucose monomers.
One of the most promising routes for cellulose valorisation is
inspired by reactions on starch, and relies on the one-pot dual
catalytic conversion of cellulose into useful chemicals.2 Herein,
cellulose is hydrolysed under the action of an acid catalyst,
followed by a fast metal–catalyzed hydrogenation of the
formed glucose to hexitols. Hexitols, viz. sorbitol, mannitol
and sorbitan, are important platform molecules3 for renewable
chemicals and fuels.4 A main advantage of the bifunctional
catalytic approach is the fast removal of unstable glucose
avoiding its degradation into caramel, tars, etc. Since the
formed hexitols have a higher thermal stability than the
corresponding hexoses, higher process temperatures and
cellulose concentrations are feasible than in classic cellulose
hydrolysis.
Several authors have recently reported on the bifunctional
catalytic conversion of cellulose diluted in water (0.8–2 wt%)
using catalyst loadings of 0.2 to 0.4 wt%,2,5 achieving hexitol
yields in the range of 30–73%, typically within 24 h. However,
the need remains for a catalytic system that is capable of
more rapidly and selectively transforming more concentrated
cellulose feeds into hexitols in high yields. We report here the
catalytic conversion of highly concentrated microcrystalline
and ball-milled cellulose into hexitols by combining a hetero-
poly acid (HPA) and Ru on carbon (Ru/C).
A preliminary screening of the hydrolysis of cellobiose, a
b-1,4-glycoside bonded disaccharide, over several mineral
acids, HPAs and solid acids, viz. USY zeolites, demonstrated
Centre for Surface Science and Catalysis, Catholic University of Leuven,
Kasteelpark Arenberg 23, 3001 Heverlee, Belgium.
E-mail: bert.sels@biw.kuleuven.be; Fax: (+32) 1632 1998;
Tel: (+32)1632 1593
w Electronic supplementary information (ESI) available: Cellobiose
hydrolysis, experimental details of cellulose conversion, ball milling
pretreatment and characterisation cellulose. See DOI: 10.1039/
c001096k
Fig. 1 Conversion of microcrystalline cellulose by H4SiW12O40
&
Ru/C (filled) and H2SO4 & Ru/C (open); (square) conversion, (circle)
hexitol yield. Reaction conditions: cellulose 1 g, [H+] = 1.22 10À2 M,
Ru/C 0.25 g, water 50 ml, H2 5 MPa, 463 K.
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Fig. 2 Reaction scheme for cellulose conversion to hexitols.
shorter reaction times, thereby suppressing the Ru-catalyzed
cracking reaction while maintaining high cellulose conversion
and hexitol yield, we used a 24 h ball-milling pretreatment
(see ESIw) to reduce cellulose crystallinity before reaction
(Table 1). It is important to emphasize here that the present
catalytic concept is also applicable with cellulose obtained
from decrystallisation procedures, perhaps more practical than
ball-milling. It follows that compared to untreated cellulose,
the ball-milled form is fully converted after 1 h, while conver-
sion of microcrystalline cellulose is still incomplete, even after
24 h (compare entry 1 and 4, Table 1).
As a result of the shorter contact time with mechanically
treated cellulose, very high hexitol selectivities, up to 87%,
were obtained at full conversion. The reaction temperature
had no effect on the hexitol selectivity. For instance, an
increase from 453 to 463 K shows an enhanced cellulose
conversion rate (higher conversion or shorter time to reach
about 100% conversion) without changing the hexitol selecti-
vity at isoconversion (entries 1 and 2, Table 1). Only after
contacting the hexitol fraction for 24 h in the reaction con-
ditions at 463 K, can a significant decrease in yield be noticed,
thus pointing to a high stability of the hexitols in this reaction
medium. At 443 K, the hexitol fraction is stable far beyond the
24 h reaction time, but we noticed an incomplete cellulose
conversion at the lower temperature even after prolonged
reaction times. Possibly, to the hydrolysis power of the HPA
catalyst, that of overheated water4 should be added.
Fig. 3 Hexitol selectivity vs. cellulose conversion for (m) H4SiW12O40
& Ru/C and (,) H2SO4 & Ru/C. Reaction conditions as in Fig. 1.
e.g. in ref. 7. An overall reaction scheme for the HPA–Ru/C
system is advanced in Fig. 2.
Taking a closer look at the catalytic data further reveals a
selectivity maximum (of 68%) at intermediate cellulose con-
version in the H2SO4 experiment, while the hexitol selectivity
remains high and around 65% at very high cellulose conver-
sion with HPA and Ru/C (Fig. 3).
The drop in hexitol selectivity for high cellulose conversion
in the H2SO4 experiment is attributed to a slow formation of
glucose at longer contact times. As both hydrogenation
and hydrogenolysis are competitive surface reactions on Ru,
hydrogenolysis of the hexitol fraction will dominate if glucose
is offered too slowly. In contrast, contacting cellulose with the
HPA–Ru/C catalytic system provides a more balanced reac-
tion network in favor of hexitol formation. For clarity, the
low hexitol selectivity at low cellulose conversion for the
experiment with H2SO4–Ru/C is due to the formation of large
quantities of oligosaccharides, as measured by HPLC analysis.
Intriguingly, no such high amounts of oligomers were analyzed
with HPA–Ru/C, suggesting a selective hydrolysis mechanism,
in which glucose-by-glucose units are released from cellulose.
Chemical or mechanical lowering of the cellulose crystal-
linity is known to improve its degradability.6,8,9 To allow
As the presence of glucose is negligible in the reaction
medium of the various entries in Table 1, the hydrogenation
activity of the Ru/C catalyst is sufficiently high compared to
the hydrolysis rate of the acid catalyst. Thus, the subtle balance
of the catalytic functions, viz. hydrolysis and hydrogenation
needed for high yield of hexitols seems to be optimal in the
HPA–Ru/C, except for the small deficit in selectivity (B15%)
due to hydrogenolysis.
In an attempt to suppress such C–C cracking reaction, while
retaining the high hexitol yields and fast cellulose conversion
obtained at higher temperatures, the influence of the H2
Table 1 Temperature dependence of ball-milled cellulose conversion in presence of H4SiW12O40 and Ru/C catalystsa
Hexitol yield/%
Entry Reaction T/K Reaction time/h Conversion/% Hexitol selectivity/%b
Sugar alcohol yield/% Sorbitan yield/% Glucose yield/%
1
2
463
453
443
463
1
2
6
24
99
99
83
83
87 (72)
86 (82)
84
68
68
57
32
19
17
11
17
1
1
1
3
4c
59
o0.5
a
Reaction conditions: Ball-milled cellulose 1 g, [H+] = 1.22 Â 10À2 M (assuming full dissociation of acid catalyst), Ru/C 0.25 g, water 50 ml,
b
H2 5 Mpa. Values in parentheses refer to a reaction time of 24 h. Untreated cellulose.
c
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Table 2 Influence of H2 pressure and water concentration on cellulose conversion by H4SiW12O40 and Ru/C catalysts
Hexitol yield/%
Sugar
alcohol
yield/%
Sorbitan
yield/%
Glucose
yield/%
Pressure/ Reaction Conversion/ Productivity/
g LÀ1 hÀ1d
Hexitol
selectivity/%
Entry Substrate
MPa
time/h
%
1
2
3
4
5
Ball-milled cellulosea
9.5
1
5
0.33
4
1.5
100
77
96
79
19
2
279
15
83
100
72
95
80
85
36
65
27
38
15
19
27
34
22
o0.5
1
o0.5
o0.5
o0.5
Microcrystalline cellulosea 9.5
Ball-milled celluloseb
5
5
5
Microcrystalline celluloseb
Microcrystalline cellulosec
65
91
a
b
Reaction conditions: substrate 1 g, [H+] = 1.22 Â 10À2 M, Ru/C 0.25 g, water 50 ml, 463 K Reaction conditions: substrate 5 g, [H+] = 6.1 Â
10À2 M, Ru/C 1.25 g, water 50 ml, 463 K. Reaction conditions: substrate 10 g, [H+] = 12.2 Â 10À2 M, Ru/C 2.5 g, water 50 ml,
c
d
463 K Calculated at the given reaction time.
pressure was investigated. Hydrogenolysis reactions are usually
negative order in H2 concentration, which is attributed to
competitive adsorption between H2 and the substrate, viz.
hexitol. An increase of H2 pressure from 5 to 9.5 MPa
eliminates hydrogenolysis, resulting in an unprecedented
quantitative conversion of cellulose into hexitols. A similar
pressure effect was noticed with microcrystalline cellulose
(compare Fig. 1 with Table 2, entry 2).
This work was performed in the framework of IAP (Belspo),
IDECAT and Methusalem projects. J. Geboers and S. Van de
Vyver thank IWT-Vlaanderen and FWO-Vlaanderen respec-
tively for a doctoral fellowship.
Notes and references
1 Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411–2502;
G. Centi and R. A. van Santen, Catalysis for Renewables, Wiley,
Chichester, 2007; B. Kamm, Angew. Chem., Int. Ed., 2007, 46,
Next to the high hexitol yield, we here communicate fast
cellulose conversion and excellent hexitol volume producti-
¨
5056–5058; P. Maki-Arvela, B. Holmbom, T. Salmi and
D. Y. Murzin, Catal. Rev. Sci. Eng., 2007, 49, 197–340;
J. H. Clark and F. E. I. Deswarte, Introduction to Chemicals from
Biomass, Wiley VCH, Weinheim, 2008; D. A. Simonetti and
J. A. Dumesic, Catal. Rev. Sci. Eng., 2009, 51, 441–484; E. Arceo,
P. Marsden, R. G. Bergman and J. A. Ellman, Chem. Commun.,
2009, 3357; M. A. Harmer, A. Fan, A. Liauw and R. K. Kumar,
Chem. Commun., 2009, 6610–6012; A. Takagaki, C. Tagusagawa
and K. Domen, Chem. Commun., 2008, 5363–5365; Y. Zhu,
Z. Kong, L. Stubbs, H. Lin, S. Shen, E. Anslyn and J. Maguire,
ChemSusChem, 2010, 3, 67–70; E. D’Hondt, S. van de Vyver,
B. F. Sels and P. A. Jacobs, Chem. Commun., 2008, 6011–6012.
2 A. Fukuoka and P. L. Dhepe, Angew. Chem., Int. Ed., 2006, 45,
5161–5163; P. Jacobs and H. Hinnekens (Synfina-Oleofina),
EP0329923, 1989.
3 US Department of Energy, Energy Efficiency and Renewable
Energy, Top Value Added Chemicals From Biomass, Vol. 1:
‘‘Results of Screening for Potential Candidates from Sugars and
Synthesis Gas’’ http://eereweb.ee.doe.gov/biomass/pdfs/35523.pdf,
2004; J. Lehmann and K. Rapp, ‘Ullmann’s Encyclopedia of
Industrial Chemistry, 6th Ed.’, 2002, electronic release.
vities (in g LÀ1
h
À1) with the HPA–Ru/C catalytic system
(Table 2). Yamaguchi et al.9 recently proposed that the rate of
cellulose hydrolysis over a solid acid is dependent on the water
concentration. As cellulose hydrolysis in the present study is
rate limiting, more concentrated cellulose feeds were examined,
while maintaining the cellulose/catalyst ratio (Table 2, entries
3 & 4). The data on both ball-milled (compare Table 2, entry 3
with Table 1, entry 1) and microcrystalline cellulose (compare
Table 2, entry 4 with Fig. 1) indeed show significantly faster
reactions. For instance, a hexitol yield of 92% was reached
within 20 min using 10 wt% ball-milled cellulose, corresponding
to a record volume productivity of 279 g of hexitol fraction per
liter liquid phase in one hour (Table 2, entry 3). Catalytic
conversion of 20 wt% microcystalline cellulose with HPA–Ru/C
gives an hexitol volume productivity of 83 g LÀ1 hÀ1 with a
hexitol selectivity of 91% (Table 2, entry 5).
4 G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic,
Science, 2005, 308, 1446–1450.
In summary, the commercial heteropoly acids H3PW12O40
and H4SiW12O40 were demonstrated to be very effective acid
catalysts in combination with Ru/C to directly produce hexitols
from cellulose. The strength of HPAs seems the fast and selective
hydrolysis of cellulose to glucose. Immediate hydrogenation of
glucose at elevated H2 pressure with Ru/C provides (almost)
quantitative amounts of hexitols when ball-milled cellulose is
used. Record volume productivities were presented for concen-
trated treated and untreated cellulose feeds. As the recovery of
heteropoly acid catalysts from aqueous solutions has been
reported via recrystallisation6 or ether extraction,10 the develop-
ment of recyclable H3PW12O40–Ru/C and H4SiW12O40–Ru/C
systems is within reach.
5 Luo, S. Wang and H. Liu, Angew. Chem., Int. Ed., 2007, 46,
7636–7639; W. Deng, X. Tan, W. Fang, Q. Zhang and Y. Wang,
Catal. Lett., 2009, 133, 167–174; N. Yan, C. Zhao, C. Luo, P. J. Dyson,
H. Liu and Y. Kou, J. Am. Chem. Soc., 2006, 128, 8714–8715.
6 K. Shimizu, H. Furukawa, N. Kobayashi, Y. Itaya and
A. Satsuma, Green Chem., 2009, 11, 1627–1632.
7 D. Ariono, C. Moraes de Abreu, A. Roesyadi, G. Declercq and
A. Zoulalian, Bull. Soc. Chim. Fr., 1986, 5, 703–710.
8 Onda, T. Ochi and K. Yanagisawa, Green Chem., 2008, 10,
1033–1037; S. Zhu, Y. Wu, Q. Chen, Z. Yu, C. Wang, S. Jin,
Y. Ding and G. Wu, Green Chem., 2006, 8, 325–327; H. Zhao,
J. H. Kwak, Y. Wang, J. A. Franz, J. M. White and J. E. Holladay,
Energy Fuels, 2006, 20, 807–811.
9 Yamaguchi, M. Kitano, S. Suganuma, K. Nakajima, H. Kato and
M. Hara, J. Phys. Chem. C, 2009, 113, 3181–3188.
10 N. Dorokhova and I. P. Alimarin, Russ. Chem. Rev., 1979, 48, 502–516.
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Chem. Commun., 2010, 46, 3577–3579 | 3579