Copper-based catalysts for efficient valorization of cellulose

Article abstract of DOI:10.1002/cssc.201200482

Noble causes: Cellulose is effectively converted into methanol, propylene, and ethylene glycol over Cu-based catalysts. Overall yields of above 93 %, together with 63 % yield of C₁-C₃ compounds, can be reached over simple noble-metal

Full text of DOI:10.1002/cssc.201200482

DOI: 10.1002/cssc.201200482
Copper-Based Catalysts for Efficient Valorization of Cellulose
Kameh Tajvidi,[b] Kristina Pupovac,[b] Murhat Kꢀkrek,[b] and Regina Palkovits*[a]
The combination of diminishing fossil fuel resources and an in-
creasing energy demand has increased the focus on the possi-
ble transformations of renewable feedstocks. Biomass is
a promising alternative source of carbon for implementing
a potentially fossil-free chemical industry. When employed in
fuel production, biomass reduces the carbon dioxide footprint
of combustion. This has led to an increased use of biofuels
such as biodiesel and bioethanol in recent years; however,
their production is based on food crops, which induces com-
petition between food and fuel production. This challenge can
be remedied by using lignocellulose as feedstock.[1–3] Lignocel-
lulose consists of cellulose, hemicellulose, and lignin, with cel-
lulose as the major constituent in amounts of up to 50%.
Therefore, efficient chemical transformations of cellulose into
platform chemicals will play a potentially crucial role in the
transition to biorefinery schemes.
Nevertheless, when considering large-scale applications the
high costs and limited availability of noble metals encourages
the development of noble-metal-free catalysts. Recently, Zhang
et al. presented first studies on cellulose degradation over
nickel-promoted tungsten carbide, yielding up to 75% ethyl-
ene glycol.[10] However, the recyclability of the catalysts re-
mained challenging. Simple Cu-based catalysts present a prom-
ising alternative, as they are known for their high activity
toward hydrodeoxygenation of CÀO bonds and have already
reached high activity in the hydrogenolysis of glycerol.[11] More-
over, Gallezot et al. investigated Cu-based catalysts in the hy-
drogenolysis of sugar alcohols such as sorbitol with 63% selec-
tivity to deoxyhexitols.[12] Interestingly, in a recent study a Cu-
doped porous metal oxide was utilized to transform wood bio-
mass into liquid and gaseous products.[13] Liquefaction oc-
curred in supercritical methanol at temperatures above 3008C
and pressures of 160–220 bar, resulting in a mixture of various
aliphatic alcohols and CO2. In contrast, we present herein the
controlled hydrolytic hydrogenation of cellulose over Cu-based
catalysts into C1–C3 compounds that are already utilized in
today’s chemical industry, including methanol, ethylene glycol
(EG), 1,2- and 1,3-propanediol (PD), as well as glycerol.
Acid treatment of cellulose depolymerizes this abundant
biopolymer to glucose, and depending on the reaction condi-
tions dehydration to products such as 5-hydroxymethylfurfural
(HMF) occurs.[3,4] Combining hydrolysis with hydrogenation en-
ables the direct transformation of cellulose into highly valuable
intermediates such as C6 sugar alcohols and short-chain alco-
hols. The first studies on hydrolytic hydrogenation were per-
formed in the 1960s by Sharkov, using noble-metal catalysts
together with diluted mineral acids.[5] Recently, Fukuoka et al.
reported the aqueous-phase conversion of cellulose over Pt/
Al2O3 at 1908C within 24 h, reaching up to 30% yield of hexi-
tols.[6] In addition, Lu et al. demonstrated the hydrolytic hydro-
genation of cellulose over Ru clusters at higher temperature in
significantly shorter times, reaching yields of C6 polyols of up
to 40%.[7] Previously, we investigated the conversion of cellu-
lose by combining diluted mineral acids and noble-metal cata-
lysts, such as Ru, Pt, and Pd, supported on activated carbon.
Interestingly, rather different product distributions were ob-
served over these catalysts: C5–C6 polyols were the main prod-
ucts in the case of Ru, while Pd and Pt yielded short-chain al-
cohols and gaseous products.[8] Optimization of the reaction
conditions and acid-to-metal ratio allowed the selective forma-
tion of C6 sugar alcohols,[9] or up to 80% yields of C4–C6 sugar
alcohols at only 1608C.[8]
First investigations concentrated on CuO/ZnO/Al2O3, a cata-
lyst usually utilized in industrial methanol synthesis.[14] At a re-
action temperature of 2458C, using water as solvent, CÀC and
CÀO bond cleavage was facilitated, resulting in C1–C3 com-
pounds as main products. In line, reactions at this temperature
over Ru- and Pt/Al2O3 delivered significant amounts of the de-
scribed C1–C3 compounds. Interestingly, a comparable product
distribution could be reached by applying simple Cu-based
materials, allowing direct conversion of cellulose (Figure 1a
and Supporting Information Table S1). These results could be
further optimized by increasing the metal content of the cata-
lyst, reaching overall yields of liquid-phase products of up to
95%, with 67.4% C1–C3 compounds of which 15.4% 1,2-PD
and 27.1% methanol, respectively (Figure 1b). In addition, the
high reaction temperature initiates not only the formation of
C1–C3 compounds but also dehydroxylation to products such
as 1,2,6-hexanetriol (1,2,6-HT) or 1,2-butanediol (1,2-BD; Sup-
porting Information Table S2). Interestingly, these results can
be transferred to spruce as feedstock, achieving 87.6% conver-
sion and 38.6% yield emphasizing EG and 1,2-PD as main
products (Supporting Information Table S3). Based on these re-
sults CuO/ZnO/Al2O3 catalysts can be classified as promising al-
ternatives for supported-noble-metal catalysts in the direct
transformation of cellulose.
[a] Prof. Dr. R. Palkovits
Institut fꢀr Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 1, 52074 Aachen (Germany)
Fax: (+49)2418022177
E-mail: palkovits@itmc.rwth-aachen.de
From an industrial and environmental point of view, catalyst
recyclability is of major importance considering continuous
production processes. To facilitate recycling experiments, cello-
biose was used as substrate in a first step (Supporting Informa-
tion Figure S1). The investigated Cu-based catalyst could be
[b] K. Tajvidi, K. Pupovac, M. Kꢀkrek
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470, Mꢀlheim an der Ruhr (Germany)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200482.
ChemSusChem 2012, 5, 2139 – 2142
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2139

lution for hydrogen pressures above 30 bar. However, such
high pressures place high demands on technical implementa-
tion. Consequently, further experiments were carried out at
30 bar H2 pressure, which proved sufficient to achieve efficient
transformation of cellulose (Supporting Information, Figure S5).
Based on the findings described above achieved with com-
mercial CuO/ZnO/Al2O3, a set of Cu-based catalysts was synthe-
sized and subsequently tested in the transformation of cellu-
lose to allow first insight into the influence of catalyst compo-
sition on activity and selectivity (Table 1). CuO/ZnO/Al2O3 and
CuO/NiO/Al2O3 catalysts were prepared by co-precipitation at
pH 5–6.5 and 35–708C.[14] The catalysts are denoted as CuO/
ZnO/Al2O3-cp-x, where x represents the precipitation tempera-
ture. Depending on the synthesis conditions the metal content
as well as specific surface area of the materials varied. Energy-
dispersive X-ray (EDX) analysis revealed a low zinc content
(<1%) for CuO/ZnO/Al2O3-cp-35 prepared at 358C and pH 5
(63 m2 gÀ1), whereas a zinc content of up to 12 wt% was ob-
served for materials prepared at neutral or only slightly acidic
pH values and 708C (CuO/ZnO/Al2O3-cp-70, 40 m2 gÀ1). Interest-
ingly, in catalysis experiments both materials exhibited results
comparable to the commercial system (98 m2 gÀ1; entry 1).
Consequently, the presence of zinc is not necessary to achieve
significant activity in the transformation of cellulose. Likewise,
the presence of zinc does not seem to have a major effect on
product distribution. This conclusion is further emphasized by
the comparable activities and product distributions for CuO/
Al2O3-cp-35 and CuO/NiO/Al2O3-cp-35 (entries 4 and 5).
Figure 1. Catalytic activity of CuO/ZnO/Al2O3 in the transformation of cellu-
lose to sugar alcohols, a) in comparison with supported noble metal cata-
lysts and b) utilizing different amounts of catalyst. Reaction conditions:
2458C, 50 bar H2 (258C), 10 mL H2O, 0.5 g cellulose.
Interestingly, the preparation procedure of the catalyst did
have a clear effect on activity and selectivity. Cu/Al2O3 and Cu/
C catalysts were prepared by impregnation using copper ni-
trate as a precursor. CuO/Al2O3, irrespective of the synthesis
method, as well as Cu/C achieved comparable results with
regard to the conversion of cellulose, but the yield of liquid-
phase products changed significantly. While CuO/Al2O3 pre-
pared via precipitation or Cu/Al2O3 based on high-surface-area
alumina (Table 1, entries 5 and 8; 116 and 157 m2 gÀ1) gave
yields of liquid-phase products of 79.4 and 65.9%, respectively,
a Cu/Al2O3 catalyst based on impregnation of a commercial
support only reached a yield of 24% (entry 7; <5 m2 gÀ1) indi-
cating further CÀC and CÀO cleavage to gaseous products
such as CO2.[8] This result points towards a positive effect of
a high specific surface area for Al2O3-based catalysts. In con-
trast, Cu/C (entry 6) only reached a yield of 22.7% liquid-phase
products; a result that is currently difficult to rationalize but
may be related to the use of activated carbon as support.
With regard to the overall product distribution, little metha-
nol and 1,2-PD were formed over catalysts prepared via im-
pregnation (Table 1, entries 6–10), while catalysts prepared via
precipitation, such as commercial CuO/ZnO/Al2O3, deliver
yields of liquid-phase products of up to 95%, with 14.7% 1,2-
PD and 22% methanol (entry 1). Mechanistic investigations
concerning the hydrogenolysis of sorbitol suggest a 1,5-cleav-
age via a retro-aldol reaction to release methanol.[11] Hence, dif-
ferent acid–base properties of the catalyst may play an impor-
tant role, and will be the focus of future studies. Moreover,
some commercial Cu-based catalysts, such as Raney copper,
successfully recycled for at least 4 runs, while elemental analy-
sis of the reaction solution indicated the absence of Cu leach-
ing. Surprisingly, recycling tests with cellulose as substrate indi-
cated deactivation of the catalysts (Supporting Information Fig-
ure S2, Table S3), but again no evidence for Cu leaching was
found. Thermogravimetric analysis (TGA) showed weight loss
of the catalyst after recycling at around 3008C, and extraction
with DMSO after intense washing of the catalyst with water
and MS analysis of the solution revealed oligomeric species
(Supporting Information Figures S3 and S4). These results can
be attributed to adsorption of polymeric species on the cata-
lyst surface, inducing deactivation and explaining the observed
formation of HMF upon recycling caused by coverage of the
active metal sites with carbon deposits (Supporting Informa-
tion Table S4). Future studies will focus on methods to circum-
vent deactivation or implement regeneration strategies.
To set the baseline for a knowledge-driven optimization of
catalyst and reaction system, the influence of reaction condi-
tions as well as catalyst composition was investigated. Temper-
ature variations clearly illustrated that a minimum temperature
of 2458C is required to achieve significant conversion of cellu-
lose, most probably due to the high activation energy of cellu-
lose hydrolysis (Supporting Information Table S2). Increasing
the hydrogen pressure from 20 to 50 and up to 80 bar caused
no increase in conversion and only a slight increase of the
amount of formed polyols, indicating a minor impact of gas–
liquid transfer and hydrogen concentration in the reaction so-
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ChemSusChem 2012, 5, 2139 – 2142

Table 1. Hydrogenolysis of cellulose over different Cu-based Catalysts
Entry Catalyst
Conv.[a] Total
[%]
Individual yields[c] [%]
yield[b] [%] sorbi- sorbi- isosor- xyli- anhydro- ery-
glyce- EG
thritol rol
1,3-PD 1,2-PD MeOH 1,2-BD,
1,2,6-HT
tol
tan
bide
tol
xylitol
Precipitation catalysts
[d]
1
2
3
4
5
CuO/ZnO/Al2O3
100
95.1
76.9
58.8
63.9
79.4
5.1
3.5
8.9
2.5
2.9
1.2
0.6
1.4
0.5
0.5
10.3
11.5
4.5
5.9
8.8
1.0
1.1
0.6
1.2
0.4
2.1
1.4
0.8
0.0
0.0
1.3
1.1
1.2
0.5
0.8
7.7
6.8
3.9
2.7
4.5
13.6 5.2
10.9 3.6
11.0 1.3
14.2 3.0
13.3 4.6
14.7
11.4
13.3
14.3
13.2
22.2
15.4
2.8
10.4
20.6
10.1
9.3
8.6
8.4
9.4
CuO/ZnO/Al2O3-cp-35 100
CuO/ZnO/Al2O3-cp-70 100
CuO/NiO/Al2O3-cp-35 100
CuO/Al2O3-cp-35
100
Supported catalysts
6
7
8
9
25% Cu/C
25%-Cu/Al2O3
25%-Cu/Al2O3
92.8
100
91.0
91.6
22.7
24.0
65.9
37.8
0.8
2.7
17.7
0.8
1.7
0.8
0.0
2.0
0.3
0.0
7.6
0.5
1.0
0.7
5.8
0.8
5.6
0.0
0.0
0.0
3.4
0.6
1.8
0.6
2.3
0.5
5.0
2.7
3.9
9.4
6.2
9.3
0.0
0.0
0.0
4.1
0.2
4.7
9.6
9.2
1.4
2.9
5.1
3.3
1.8
1.4
6.8
4.2
[e]
[f]
[g]
5%-Cu/Al2O3
Full catalysts
10
11
12
13
Raney copper
copper chromite
Cu2O
96.0
98.0
96.6
96.8
77.2
46.7
26.3
48.1
6.5
7.1
3.8
5.3
0.0
0.0
1.1
0.5
11.2
3.7
0.0
1.8
1.9
2.0
1.5
1.1
0.0
0.0
0.0
0.0
1.3
0.4
0.7
0.8
8.1
2.7
2.0
3.3
9.9
6.0
4.9
6.0
0.0
0.0
0.0
0.0
15.8
13.2
3.4
14.0
5.4
7.3
8.3
6.0
0.9
5.8
CuO
11.4
11.7
[a] Based on the recovered amount of cellulose. [b] Based on the yields of compounds 1–13. [c] Neither glucose nor its dehydration products were formed.
[d] Commercial catalyst (ICI). [e] Commercial alumina. [f] Alumina was prepared by hard templating. [g] Alumina was synthetized by sol–gel technique.
copper chromite, and copper oxides that exhibited promising
results for glycerol hydrogenolysis, were investigated for the
direct transformation of cellulose. Surprisingly, all materials al-
lowed almost complete conversion of cellulose while without
catalyst only 64.4% was reached (Supporting Information
Table S4). Raney copper reached a yield of liquid-phase prod-
ucts of up to 77%, with up to 48% of C1–C3 compounds (14%
methanol), and even for pure CuO 48% overall yield including
32.3% of C1–C3 compounds with 11.7% methanol could be
achieved (entries 10 and 13).
ethylene glycol, 1,2-PD, and methanol as main products. Addi-
tionally, variation of the catalyst composition and reaction pa-
rameters indicates a significant impact of preparation method
and support material on activity and selectivity of the catalyst.
Cu0 seems to be essential for a high catalytic activity, but the
presence of CuI can hardly be excluded. Future investigations
will focus on rationalization of the Cu oxidation state and
acid–base properties of the catalysts, as well as possibilities to
enhance catalyst selectivity and recyclability.
XPS measurements were performed to investigate the
nature of the active copper species (Supporting Information
Figures S6 and S7). The Cu2p spectra of commercial CuO/ZnO/
Al2O3 were characteristic for copper in the 2+ oxidation state.
The main peak of the catalyst after reaction was an overlap of
two copper species: Cu2+, with a binding energy value of
933.9 eV, and a lower oxidation state with a peak at 931.5 eV
that can be assigned to either Cu0 or Cu+. Usually, Cu+ and
Cu0 can be distinguished by their Auger peaks, but in this case
the copper Auger peak overlaps with the zinc Auger lines. For
the impregnated and reduced Cu on Al2O3, the main photo
peak at 931.5 eV and the Auger peak at 918.1 eV can be clearly
assigned to Cu0. The reduction of CuII under the employed re-
action conditions is further emphasized by the rose color of
the catalyst after reaction. Therefore, we suggest that Cu0 acts
as active species under reaction conditions. In line, experi-
ments with Raney copper and Cu/Al2O3 fully reduced to Cu0
before reaction (Table 1, entries 8 and 10) allowed an efficient
transformation of cellulose, further emphasizing the role of Cu0
in the reaction. However, further investigations to clarify the
nature of the catalytically active sites under the employed re-
action conditions are certainly necessary.
Experimental Section
Experimental and catalyst preparation procedures are described in
the Supporting Information.
Acknowledgements
We thank Dr. C. Weidenthaler for XPS analyses and valuable dis-
cussions, and A. Deege, H. Hinrichs, and N. Avraham for HPLC
analysis. This work was performed as part of the Cluster of Excel-
lence “Tailor-Made Fuels from Biomass” funded by the Excellence
Initiative by the German federal and state governments to pro-
mote science and research at German universities.
Keywords: biomass
· copper · heterogeneous catalysis ·
hydrogenolysis · methanol
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Published online on October 11, 2012
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