Renewable alkanes by aqueous-phase reforming of biomass-derived oxygenates

Article abstract of DOI:10.1002/anie.200353050

A clean stream of alkanes from renewable biomass resources is obtained through aqueous-phase reforming in a single reactor. Alkanes are produced from biomass-derived sorbitol through a bifunctional pathway (see scheme) that involves the dehydration of sorbitol on acid sites (SiO₂/Al ₂O₃) and hydrogenation of intermediates on a metal catalyst under a H₂ atmosphere. Hydrogen is produced from sorbitol and water on the metal catalyst in the same reactor.

Full text of DOI:10.1002/anie.200353050

Angewandte
Chemie
Alkane Production
These initial steps are followed by hydrogenation of the
dehydrated reaction intermediates on the metal catalyst.
When these steps are balanced properly, the hydrogen
produced in the first step is fully consumed by hydrogenation
of the dehydrated reaction intermediates, which leads to the
overall conversion of sorbitol to alkanes plus CO2 and water.
Figure 1 shows the essential features of the bifunctional
reaction pathway for the production of alkanes from sorbitol.
Renewable Alkanes by Aqueous-Phase
Reforming of Biomass-Derived Oxygenates**
George W. Huber, Randy D. Cortright, and
James A. Dumesic*
À
Hydrogen is produced on the metal by cleavage of C C bonds
Research is being conducted world-wide to develop new
technologies for the generation of energy from renewable
resources. In this respect, it has recently been stated that
“biomass is the only practical source of renewable liquid
fuel”.[1] Current technologies to produce liquid fuels from
biomass are typically multistep and energy-intensive proc-
esses, including the production of ethanol by fermentation of
biomass derived glucose,[2] bio-oils by pyrolysis or high
pressure liquefaction of biomass,[3,4] polyols from hydro-
genolysis of biomass derived sorbitol,[5] and biodiesel from
vegetable oils.[6] Biomass can also be gasified to produce CO
and H2 (synthesis gas), which can be further processed to
produce methanol or liquid alkanes by Fischer–Tropsch
synthesis.[3] In addition, liquid hydrocarbons can be formed
from biomass-derived carbohydrates over zeolite catalysts at
temperatures from 570 to 920 K, accompanied by formation
of carbonaceous residues on the catalyst.[7]
We have recently demonstrated that it is possible to
produce hydrogen and light alkanes (primarily methane) by
aqueous-phase reforming (APR) of biomass-derived oxygen-
ates, including glucose, sorbitol, and glycerol.[8,9] The APR
process offers a simple route for the production of renewable
fuels from biomass, since this process takes place in a single
reaction vessel. Herein, we show how aqueous-phase reform-
ing of sorbitol (the sugar alcohol obtained by hydrogenation
of glucose) can be tailored to selectively produce a clean
stream of heavier alkanes consisting primarily of butane,
pentane, and hexane. The conversion of sorbitol to alkanes
plus CO2 and water is an exothermic process that retains
approximately 95% of the heating value and only 30% of the
mass of the biomass-derived reactant.
Figure 1. Reaction pathways for the production of alkanes from sorbi-
tol over catalysts with metal and acidic components.
followed by the water–gas shift reaction. Dehydrated species
such as ring compounds like isosorbide or enolic species are
formed on acid sites,[11] which migrate to metal sites where
they undergo hydrogenation reactions. Repeated cycling of
dehydration and hydrogenation reactions in the presence of
hydrogen leads to heavier alkanes (such as hexane) from
sorbitol. Formation of lighter alkanes takes place by more
Production of alkanes by aqueous-phase reforming of
sorbitol takes place by a bifunctional reaction pathway that
involves first the formation of hydrogen and CO2 on the
appropriate metal catalyst (such as Pt) and the dehydration of
sorbitol on a solid acid catalyst (such as silica-alumina[10]).
À
rapid cleavage of C C bonds compared to hydrogenation of
dehydrated reaction intermediates. Lighter alkanes can also
be formed by hydrogenation of CO and/or CO2 on metals
such as Ni and Ru.[12] Accordingly, the selectivities for
production of various alkanes by aqueous-phase reforming
[*] G. W. Huber, R. D. Cortright, Prof. J. A. Dumesic
Chemical and Biological Engineering Department
University of Wisconsin
À
depend on the relative rates of C C bond cleavage, dehy-
Madison, WI 53706 (USA)
Fax: (+1)608-262-5434
E-mail: dumesic@engr.wisc.edu
dration and hydrogenation reactions. The selectivities for the
production of alkanes can be varied by changing the catalyst
composition, the reaction conditions, and by modifying the
reactor design. In addition, these selectivities can be modified
by co-feeding hydrogen with the aqueous sorbitol feed,
leading to a process in which sorbitol can be converted to
alkanes and water without the formation of CO2 (since
hydrogen is supplied externally and need not be produced as
an intermediate in the process). As another variation, the
production of alkanes can be accomplished by replacing the
[**] This work was supported by U.S. Department of Energy (DOE),
Office of Basic Energy Sciences, Chemical Sciences Division, by the
National Science Foundation (NSF) through a STTR grant, and by
Conoco-Phillips. We thank John W. Shabaker, Marco Sanchez-
Castillo, Bret Wagner, Juben Chheda and Rupali R. Davdi for
technical assistance.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 1549 –1551
DOI: 10.1002/anie.200353050
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1549

Communications
solid acid with a mineral acid (such as HCl) that is co-fed with
the aqueous sorbitol reactant.
Table 1 summarizes the experimental results for aqueous-
phase reforming of sorbitol at various reaction conditions.
These data were collected over catalysts consisting of 4 wt%
Pt/SiO2–Al2O3 (Pt–SiAl), 3 wt% Pd/SiO2–Al2O3 (Pd–SiAl),
À
3 wt% Pt/Al2O3 (Pt–Al) and physical mixtures of Pt Al with
SiO2–Al2O3 (SiAl). The Al2O3 content of the SiO2–Al2O3
support was 25 wt%. All data were collected after 20 h on-
stream. Experimental details are given in the Supporting
Information.
The Pt–SiAl catalyst showed no deactivation after 6 days
on-stream at a conversion of 92% and 498 K. As shown in
Table 1, the carbon selectivity for production of butane,
pentane and hexane over Pt–SiAl varies from 58 to 89%,
depending on the reaction conditions. The selectivity to
hexane increases for the Pt–SiAl catalyst at 498 K as the
pressure increases from 25.8 to 39.6 bar, and slightly decreases
as the pressure is increased further to 52.7 bar. The alkanes
are formed by a combination of metal and acidic components,
since they are formed in only small amounts on the Pt–Al
catalyst. The alkanes formed are straight-chain compounds
with only minor amounts of branched isomers (less than 5%).
Alkanes heavier than hexane were not formed under the
reaction conditions of this study.
Figure 2. Carbon selectivities for aqueous-phase reforming of 5 wt%
sorbitol over Pt–SiAl: white, 498 K and 34.8 bar; gray, 538 K and
60.7 bar, black; co-feeding H2 at 498 K and 34.8 bar.
distribution as the temperature is increased from 498 to
538 K (Figure 2).
Figure 3a shows that the selectivities to heavier alkanes
increase as more solid acid sites are added to a non-acidic Pt–
Al catalyst by making physical mixtures of Pt–Al and SiAl.
The alkane selectivities for an acidic Pt–SiAl catalyst and for a
mixture of Pt–Al and SiAl components are similar, both
having the same ratio of Pt to acid sites. The alkane
distribution also shifts to heavier alkanes for the non-acidic
Pt–Al catalyst when the pH of the aqueous sorbitol feed is
lowered by the addition of HCl, as shown in Figure 3b.
By changing the nature of the metal component in the
As shown in Figure 2, the selectivity for production of
hexane increases when hydrogen is co-fed with the aqueous
sorbitol stream. Thus, increasing the hydrogen partial pres-
sure in the reactor increases the rate of hydrogenation
À
catalyst, one can vary the relative rates of C C bond cleavage
À
compared to C C bond cleavage on the metal catalyst
versus hydrogenation, thereby controlling the selectivities for
the production of different alkanes. Reaction-kinetics studies
surface. No major difference is observed in the alkane
Table 1: Experimental data for aqueous phase reforming of 5 wt% sorbitol
Catalysts and
additives to feed
Pt–SiAl
Pt–
Pt–
Pd–
SiAl
H2
Mix 1[c] Mix 2[c] Pt–Al Pt-Al
Pt–Al
HCl
pH 2
Pt–Al
SiAl
SiAl
HCl
pH 3
Void[d]
[b]
[b]
[b]
H2
H2
WHSV [hÀ1 [a]
]
1.3
498
1.3
498
1.3
498
1.3
498
1.3
498
1.3
538
1.3
538
1.3
498
1.3
498
34.8
0.29
538
1.4
538
57.5
10
0.46
538
57.5
0.75
538
57.5
0.75
538
57.5
0.75
538
57.5
2.4
538
57.5
16
Temperature [K]
Pressure [bar]
Carbon in liquid
phase effluent [%]
Carbon in gas
phase effluent [%]
Carbon selectivi-
ty [%][f]
25.8 33.1 34.8 39.6 52.7 55.5 60.7 29.3
16
58.2
13
10
10
27
8
8
12
11
28[e]
5
10
8
11
81
89
93
92
76
95
95
84
85
67
91
94
86
88
89
80
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
15
13
14
20
17
21
11
9
7
11
10
17
19
36
63
6
8
8
17
21
40
64
10
10
9
15
19
37
53
11
9
7
10
11
16
21
35
60
0
3
6
11
25
55
91
0
4
7
11
24
54
91
2
2
4
7
9
35
20
16
12
10
7
42
25
17
8
5
3
24
19
21
15
12
9
9
7
17
9
10
18
19
33
58
11
17
21
31
58
13
18
23
30
53
14
22
25
23
47
12
17
20
25
53
8
28
56
98
Alkane to total gas 46
31
27
29
phase carbon [%][g]
H2 selectivity [%][h]
21
2
2
1
1
7
4
–
–
–
11
46
43
31
6
16
[a] WHSV=weight hourly space velocity; mass of sorbitol solution per mass of catalyst per hour. [b] H2 co-fed at a gas-hourly space velocity of 120–
900 hÀ1. Densities of SiO2-Al2O3 catalysts are 0.54 gcmÀ3. [c] Mix 1 contains Pt-Al (1.45 g) and SiAl (1.11 g). Mix 2 contains Pt-Al (3.30 g) and SiAl
(0.83 g). [d] Empty void space representing 70% of the total reactor volume located upstream from the catalyst bed. [e] Approximately 25% of the
carbon was in a volatile organic liquid phase product and 3% of the carbon was in the aqueous phase product. The carbon selectivities are based on
gas phase products. [f] Carbon selectivity=(moles alkane”number of carbon atoms in alkane)/(total moles of carbon atoms in alkane products)”
100. [g] The gas phase carbon consisted of alkanes and carbon dioxide. [h] % H2 selectivity=(molecules H2 produced/C atoms in gas phase)”(6/13)”
100.
1550
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 1549 –1551

Angewandte
Chemie
selectivities for the production of alkanes over Pt–Al, since
the intermediates formed by homogeneous reactions in the
liquid phase are hydrogenated to alkanes over Pt.
The general conclusion from this work is that a clean
stream of alkanes can be formed by aqueous-phase reforming
of sorbitol over bifunctional catalyst systems in which sorbitol
is repeatedly dehydrated by an acid catalyst (e.g., a solid acid
or an aqueous mineral acid) and then hydrogenated on a
metal catalyst (e.g., Pt or Pd). Hydrogen, which is needed for
the hydrogenation reaction, can be produced in situ by
aqueous-phase reforming of sorbitol over a catalyst (such as
À
Pt) that facilitates C C bond cleavage and water–gas shift
reactions, or it can be co-fed to the reactor with the aqueous
sorbitol reactant. The selectivities for production of heavier
alkanes can be controlled by the choice of reaction conditions
and by co-feeding hydrogen to the reactor. It is likely that
advances in the understanding of new types of solid-acid
catalysts[14] as well as new metal alloy catalysts[8] will lead to
further process improvements.
Received: October 9, 2003 [Z53050]
Keywords: acidity · alkanes · basicity · heterogeneous catalysis ·
hydrogenation
.
Figure 3. Carbon selectivities for aqueous-phase reforming of 5 wt%
sorbitol at 538 K and 57.6 bar. A) Addition of solid acid to Pt–Al: black,
Pt–Al; gray, mixture 2: Pt–Al (3.30 g) and SiAl (0.83 g); white, mix-
ture 1: Pt–Al (1.45 g) and SiAl (1.11 g). B) Addition of mineral acid
(HCl) in feed over Pt–Al: black, pHfeed =7; gray, pHfeed =3; white,
pHfeed =2.
[1]T. E. Bull, Science 1999, 285, 1209.
[2]R. Katzen, G. T. Tsao, Adv. Biochem. Eng./Biotechnol. 2000, 70,
77.
[3]D. L. Klass, Biomass for Renewable Energy, Fuels, and Chem-
icals, Academic Press, San Diego, 1998.
[4]D. C. Elliott, D. Beckman, A. V. Bridgwater, J. P. Diebold, S. B.
Gevert, Y. Solantausta, Energy Fuels 1991, 5, 399.
[5]T. A. Werpy, J. G. Frye, Jr., A. H. Zacher, D. J. Miller, US 2003/
0119952A1, 2003.
[6]F. Ma, M. A. Hanna, Bioresour. Technol. 1999, 70, 1.
[7]N. Y. Chen, L. R. Koenig, US 4503278, 1985.
[8]G. W. Huber, J. W. Shabaker, J. A. Dumesic, Science 2003, 300,
2078.
for aqueous-phase reforming of ethylene glycol over silica-
À
supported Pd and Pt catalysts indicate that the rate of C C
bond cleavage is approximately one order of magnitude lower
for Pd than for Pt,[12] whereas Pd catalysts are used as
hydrogenation catalysts for various reactions.[13] As shown in
Table 1, a Pd–SiAl catalyst exhibits very high selectivity for
the production of alkanes versus CO2 when hydrogen is co-fed
À
[9]R. D. Cortright, R. R. Davda, J. A. Dumesic, Nature 2002, 418,
964.
[10]G. M. Kramer, G. B. McVicker, J. J. Ziemiak, J. Catal. 1985, 92,
355.
[11]P. Stoss, R. Hemmer, Adv. Carbohydr. Chem. Biochem. 1991, 49,
93.
[12]R. R. Davda, J. W. Shabaker, G. W. Huber, R. D. Cortright, J. A.
Dumesic, Appl. Catal. B 2003, 43, 1.
with aqueous sorbitol into the reactor, as the rate of C C
bond cleavage on Pd is slower than the rates of dehydration
on acid sites and hydrogenation reactions on Pd sites.
Furthermore, this catalyst system shows very high selectivity
for the production of hexane. This example shows that
effective bifunctional catalysts can be formulated by using
metals that, by themselves on non-acidic supports, show low
activities for hydrogen production by aqueous-phase reform-
ing reactions. The hydrogen used for this process can be
obtained by APR of sorbitol in a separate reactor over
nonprecious metal catalysts.[8]
[13]G. M. R. van Druten, V. Ponec, Appl. Catal. A 2000, 191, 163.
[14]D. G. Barton, S. L. Soled, G. D. Meitzner, G. A. Fuentes, E.
Iglesia, J. Catal. 1999, 181, 57.
Another factor that affects the selectivities for the
production of alkanes is the presence of void space in the
reactor, in which sorbitol can undergo homogenous liquid-
phase reactions. Products such as isosorbide and organic acids
(ꢀ 100–500 ppm) were observed in the liquid products from
an empty reactor with a feed of 5 wt% sorbitol heated at
538 K and 57.5 bar, whereas negligible amounts of liquid
products were observed at 498 K and 36.5 bar (both experi-
ments conducted with similar liquid residence times as used
for studies of supported Pt catalysts). As seen in Table 1, the
presence of void space in the reactor at 538 K leads to higher
Angew. Chem. Int. Ed. 2004, 43, 1549 –1551
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1551