Angewandte
Chemie
Abstract: The direct hydrolysis of crystalline cellulose to
glucose in water without prior pretreatment enables the
transformation of biomass into fuels and chemicals. To
understand which features of a solid catalyst are most
important for this transformation, the nanoporous carbon
material MSC-30 was post-synthetically functionalized by
oxidation. The most active catalyst depolymerized crystalline
cellulose without prior pretreatment in water, providing
glucose in an unprecedented 70% yield. In comparison,
virtually no reaction was observed with MSC-30, even when
the reaction was conducted in aqueous solution at pH 2. As no
direct correlations between the activity of this solid–solid
reaction and internal-site characteristics, such as the b-glu
adsorption capacity and the rate of catalytic hydrolysis of
adsorbed b-glu strands, were observed, contacts of the external
surface with the cellulose crystal are thought to be key for the
overall efficiency.
density of weak-acid sites, which are thought to activate
glycosidic oxygen atoms along the b-glu strand for hydrol-
ysis.
[15,18]
Catalyst synthesis was performed by the post-synthetic
surface functionalization of MSC-30, a high-surface-area
commercially available mesoporous carbon material. Simple
treatment of this material with aqueous NaOCl for various
durations at controlled pH values increases the density of
weak-acid sites on the surface by a factor of three to four, as
shown by catalysts T1–T4 in Table 1. Furthermore, similar
enhancements in the site density were also achieved for
catalyst T5 by oxidizing MSC-30 in 1m HNO solution at
3
1058C. Compared to the latter treatment, which is known to
result mainly in carboxylic acids as the surface functional
[19,20]
groups,
oxidation using NaOCl occurs more mildly and
results in nearly equal amounts of carboxylic acid, lactone,
and phenol functional groups on the surface (Supporting
Information, Figure S3). When NaOCl was used as the
oxidant, increasing the treatment time under acidic conditions
C
rystalline cellulose is the largest component of lignocellu-
[20]
losic biomass—the most abundant form of biomass on earth.
Its direct catalytic hydrolysis to glucose has been historically
recognized as a central bottleneck in the transformation of
(where HOCl is invoked as the main oxidizing agent) led to
a higher total acid-site density on the surface (T3 vs. T4 in
Table 1). On the other hand, under basic conditions (where
[
1–3]
À
biomass into value-added fuels and chemicals.
There is
OCl is thought to act as a weak oxidizing agent and fragment
[
20]
great interest in the development of solid synthetic catalysts
that depolymerize crystalline cellulose through solid–solid
CÀC bonds),
the treatment time had no effect on this
density (T1 vs. T2 in Table 1). Although such oxidative
[
4–11]
interactions.
However, to date, its recalcitrance has
treatments are known to slightly decrease the surface
[4–7,11]
[14,15]
required pretreatment, such as using either ball milling
or mixed milling,
area,
which is consistent with the data in Table 1,
[
9,10]
which adds to the number of processing
a comparison of catalysts T3 and T4 shows that an extended
oxidation time does not per se lead to a decrease in surface
area. An analysis of the pore-size distribution in catalysts
MSC-30 and T4 shows the formation of a small fraction of
larger mesopores with diameters of approximately 4 nm at the
expense of those smaller than 3 nm (Figure S2).
Crucially, the acid sites in catalysts T1–T5 are weak-acid
sites, as are those in the original MSC-30 material, as
confirmed by a nearly complete lack of neutralization upon
treatment with a pH 4 acetate buffer. This is in stark contrast
with strong-acid sites, such as surface sulfonic acid groups,
which are neutralized to more than 90% after a similar
treatment (Table S1).
steps and the energy and environmental footprint of sugar
[12,13]
release. Indeed, thus far, only enzymes (i.e., cellulases)
are used in practice for catalyzing the hydrolysis of crystalline
cellulose to glucose in water without pretreatment. Indeed,
the maximum reported yield for a synthetic chemical catalyst
[
8]
is below 20%, with the upper limit for a carbon catalyst
[
5]
being 4%. Herein, borrowing crudely from the concept of
weak-acid sites that are thought to be responsible for the
activity of enzymes in related catalytic hydrolysis reactions,
we synthesized a carbon catalyst that overcomes prior
limitations and catalyzes the hydrolysis of crystalline cellulose
(
i.e., Avicel) without pretreatment in water, providing soluble
sugars in 70% yield, of which 96% are glucose.
To further characterize the internal sites of the carbon
catalysts, we assessed their ability to both adsorb b-glu strands
derived from cellulose as well as, in a separate experiment, to
catalyze the hydrolysis of these adsorbed b-glu strands to
glucose in water. This was performed by first treating each
catalyst with a solution of cellulose dissolved in concentrated
aqueous HCl. Such treatments have previously been shown to
result in the adsorption of dissolved b-glu strands onto the
Our catalyst design incorporates weak-acid sites on the
carbon surface, which have previously been shown to be
hydrothermally stable even in the presence of high salt
[14,15]
concentrations.
Our approach leverages two previously
demonstrated aspects: 1) interactions between long-chain
poly(1!4)-b-glucan (b-glu) strands derived from crystalline
cellulose and the surface of the mesoporous carbon material,
as driven in part enthalpically through a multitude of CH–p
[14,16]
internal carbon surface.
The adsorption capacities of each
[
16,17]
interactions;
and 2) post-synthetic surface functionaliza-
catalyst after equilibration of 100 mg of the catalyst with
À1
tion for the synthesis of a carbon catalyst surface with a high
2.5 mL of b-glu solution (20 gl ) in concentrated aqueous
acid are listed in Table 1. MSC-30 adsorbs 65% of the total
b-glu amount in solution under the aforementioned condi-
tions, leading to an adsorbed b-glu coverage of 33% (wt%)
on the carbon surface. When MSC-30 and MCN, a mesopo-
rous carbon material that previously held the record for the
highest b-glu adsorption capacity, were compared under
identical conditions (Figure S4), MSC-30 had a two-fold
higher adsorption capacity than MCN. This is likely due to
[
*] Dr. A. T. To, Dr. P. W. Chung, Prof. Dr. A. Katz
Department of Chemical and Biomolecular Engineering
University of California, Berkeley
Berkeley, CA 94720 (USA)
E-mail: askatz@berkeley.edu
Angew. Chem. Int. Ed. 2015, 54, 11050 –11053
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
To, Anh The
