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
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
ChemSusChem 2012, 5, 2139 – 2142
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2139