Applied Catalysis A: General 391 (2011) 436–442
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Applied Catalysis A: General
j ou rn a l h om epa g e: www. el sevi er. com /l oca te/a pca ta
Accelerated cellulose depolymerization catalyzed by paired metal
chlorides in ionic liquid solvent
Yu Su, Heather M. Brown, Guosheng Li, Xiao-dong Zhou, James E. Amonette,
John L. Fulton, Donald M. Camaioni, Z. Conrad Zhang *
Pacif i c Northwest National Laboratory, Institute for Interfacial Catalysis, P.O. Box 999, Richland, WA 99352, United States
a r t i c l e i n f o
Article history:
Received 2 March 2010
Received in revised form 24 August 2010
Accepted 23 September 2010
Available online 1 November 2010
Keywords:
Ionic liquid
1-Alkyl-3-methylimidazolium chloride
1-Ethyl-3-methyl-imidazolium chloride
Cellulose
Biomass
Depolymerization
Catalysis
Bioenergy
Hydrolysis
Glucose
Cellobiose
Maltose
Cellulose conversion
Catalyst
Paired metal chlorides
CuCl 2
PdCl 2
a b s t r a c t
Eff i cient hydrolytic depolymerization of crystalline cellulose to sugars is a critical step and has been a
major barrier for improved economics in the utilization of cellulosic biomass. A novel catalytic system
involving CuCl 2 (primary metal chloride) paired with a second metal chloride, such as CrCl 2 , PdCl 2 , CrCl 3
or FeCl 3 in 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) ionic liquid solvent has been found to
substantially accelerate the rate ofcellulose depolymerization under mild conditions. These paired metal
chlorides are particularly active forthe hydrolytic cleavage of1,4-glucosidic bonds when compared to the
rates ofacid-catalyzed hydrolysis at similar temperatures (80–120 o C). In contrast, single metal chlorides
with the same total molar loading showed much lower activity under similar conditions. Experimental
results illustrate the dramatic effect of the second metal chloride in the paired catalytic system. An
array of characterization techniques, including electron paramagnetic resonance (EPR) spectroscopy,
differential scanning calorimetry (DSC), X-ray absorption f i ne structure (XAFS) spectroscopy, and X-ray
absorption near edge structure (XANES) spectroscopy, in combination with theoretical calculations at
the DFT level, was used to reveal a preliminary understanding of possible mechanisms involved in the
paired CuCl 2 /PdCl 2 catalytic system. We discovered that Cu(II) was reduced during the course of the
reaction to Cu(I) only in the presence of a second metal chloride and a carbohydrate source such as
cellulose in the ionic liquid system. Our results suggest that Cu(II) generates protons by hydrolysis of
water to catalyze the depolymerization step, and serves to regenerate Pd(II) reduced to Pd(0) by side
reactions. Pd(II) likely facilitates the depolymerization step by coordinating the catalytic protons, and
also promotes the formation ofhydroxymethylfurfural (HMF). Ourresults also suggest that the C2-proton
ofthe imidazolium ring is not activated by the paired metal-chloride catalysts.
(c) 2010 Elsevier B.V. All rights reserved.
1. Introduction
Cellulose is nature’s most abundant polymer that stores energy
in chemical bonds derived from CO 2 and H 2 O by photosynthesis. Its
chemical structure is made ofanhydro-glucose linearlylinked by-
1,4-glucosidic bonds. An extensive network ofsuch polymer chains
via hydrogen-bonding [1] and van der Waals forces [2] arranges in
ordered alignment, resulting in a supramolecular cellulose struc-
ture ofvarious size, crystallinity, and complexity, depending on the
type ofbiomass. Decrystallization and hydrolytic cleavage ofcellu-
lose polymers to glucose has been a bottleneck in the path toward
energy-eff i cient and economical utilization of cellulosic biomass.
Considerable research has focused on improving the rate of cellu-
lose hydrolysis to produce the constituent sugars. The two most
*
Corresponding author. Tel.: +1 713 299 4165; fax: +1 281 334 2832.
E-mail addresses: zczhang@yahoo.com, conrad.zhang@kior.com (Z.C. Zhang).
studied cellulose depolymerization processes in aqueous systems
involve either multiple enzymes [3] or strong mineral acids [4] as
catalysts. However progress has been limited partly due to the lack
ofsolubility ofcellulose in water. Enzymatic hydrolysis ofcellulose
is typically slow and the rate is also inhibited by contaminants orig-
inating from other biomass components. Pretreatment ofcellulose,
for example by ammonia in a high-pressure process or by mechan-
ical milling, is typically required to increase the accessible surface
area of cellulose for a reasonable rate of enzymatic hydrolysis [5].
Mineral acids have been used to catalyze hydrolysis at a variety
of acid concentrations and temperatures. However, a rather high
temperature (180–230 o C) is needed to obtain an acceptable rate of
cellulose hydrolysis using only a dilute acid [6]. Degradation ofthe
resulting glucose becomes an issue at this temperature.
Taking advantage ofthe ability of1-alkyl-3-methylimidazolium
chloride, [AMIM]Cl, ionic liquids to dissolve cellulose [7,8], we eval-
uated a large number of metal chlorides, in catalytic amounts,
dissolved in the same media [9]. We recently reported that, in
0926-860X/$ – see front matter (c) 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.09.021