Organocatalytic conversion of cellulose into a platform chemical

Article abstract of DOI:10.1039/c2sc21403b

The search for a source of fuels and chemicals that is both abundant and renewable has become of paramount importance. The polysaccharide cellulose meets both criteria, and methods have been developed for its transformation into the platform chemical 5-(hydroxymethyl)furfural (HMF). These methods typically employ harsh reaction conditions or toxic heavy metal catalysts, deterring large-scale implementation. Here, we describe a low-temperature, one-pot route that uses ortho-carboxyl-substituted phenylboronic acids as organocatalysts in conjunction with hydrated magnesium chloride and mineral acids to convert cellulose and cellulose-rich municipal waste to HMF in yields comparable to processes that use toxic heavy metal catalysts. Isotopic labeling studies indicate that the key aldose-to-ketose transformation occurs via an enediol intermediate. The route, which also allows for facile catalyst recovery and recycling, provides a green prototype for cellulose conversion.

Full text of DOI:10.1039/c2sc21403b

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Organocatalytic Conversion of Cellulose into a Platform Chemical†
Benjamin R. Caes,a Michael J. Palte,b and Ronald T. Rainesa,c,*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
The search for a source of fuels and chemicals that is both abundant and renewable has become of paramount importance. The
polysaccharide cellulose meets both criteria, and methods have been developed for its transformation into the platform chemical
5ꢀ(hydroxymethyl)furfural (HMF). These methods employ harsh reaction conditions or toxic heavy metal catalysts, deterring largeꢀscale
implementation. Here, we describe a lowꢀtemperature, oneꢀpot route that uses orthoꢀcarboxylꢀsubstituted phenylboronic acids as
organocatalysts in conjunction with hydrated magnesium chloride and mineral acids to convert cellulose and celluloseꢀrich municipal
waste to HMF in yields comparable to processes that use toxic heavy metal catalysts. Isotopic labeling studies indicate that the key
aldoseꢀtoꢀketose transformation occurs via an enediol intermediate. The route, which also allows for facile catalyst recovery and
recycling, provides a green prototype for cellulose conversion.
10
Introduction
The dependence on fossil fuels for energy and chemicals has
15 become unsustainable.1 Abundant, renewable biomass resources
could meet the fuel and chemical demands of the future,2 but
must recapitulate the wide array of products now derived from
fossil fuels. The sixꢀcarbon furanic 5ꢀ(hydroxymethyl)furfural
(HMF) has the potential to meet this challenge.3 HMF can be
20 transformed into a variety of useful products, such as common
polyester building blocks4 and the promising liquid fuel 2,5ꢀ
dimethylfuran.5 The carbon skeleton of HMF is identical to those
of the hexose sugars that are the primary components of cellulose
and hemicelluloses in biomass. Still, accessing this resource
25 requires a process that efficiently transforms these carbohydrates
into HMF.
The conversion of cellulose to HMF can proceed via three
steps: hydrolysis of cellulose to glucose, isomerization of glucose
to fructose, and dehydration of fructose to HMF (Fig. 1).
30 Although several methods exist for transforming glucose and
fructose into HMF,6 few are capable of producing HMF in high
yields directly from cellulose.3 Solid acids7 and heavy metals6b,8
are the best extant catalysts for this conversion. The inefficiency
of solid acid catalysts and the toxicity of heavy metals could
35 diminish their impact.9 Considering the everꢀgrowing need for
green chemistry, we sought a conversion process that uses benign
and recyclable reagents, catalysts, and solvents, as well as mild
reaction conditions. Here, we report on our discovery of such a
process.
40 Results and discussion
Our initial experiments focused on discovering an alternative to
heavy metals for accessing HMF from glucose. As a solvent, we
choose dimethylacetamide (DMA), a polar aprotic solvent that
has served as the medium for other biomass conversions.8b,10 We
Fig. 1. Putative route for the conversion of cellulose to 5ꢀ
50 (hydroxymethyl)furfural, mediated by the depicted boronic acids. Labeled
hydrogens in glucose (green) and water (blue) have the indicated fates.
45 screened a variety of metal chlorides (Table S1) and found that
magnesium, nickel, zinc, cerium, or lanthanum, like chromium,
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Table 1 Conversion of glucose to HMF in DMA a
production, we added water to reaction mixtures containing the
35 anhydrous salt (Table S5 in the Supporting Information) and
found that HMF yields increase steadily up to an H2O:Mg(II)
ratio of ~6 (Fig. S1). Other anhydrous metal chlorides that work
synergistically with 2ꢀcarboxyphenylboronic also have a similar
reliance on water (Table S5).
HMF
time yield
40
Encouraged by the successful conversion of glucose to HMF,
we next attempted to convert cellulose to HMF using boronic
acids in 1ꢀethylꢀ3ꢀmethylimidazolium chloride ([EMIM]Cl), an
ionic liquid that dissolves cellulose17 and enables a mineral acid
to catalyze its hydrolysis.18 We observed that phenylboronic acids
T
metal chloride, mol%
—
boronic acid, wt%
(°C) (h)
(%)
0
—
100
100
120
120
120
120
120
120
120
120
120
120
105
105
120
120
6
6
6
3
6
3
6
6
4
6
6
6
4
4
3
3
6
4
6
4
CrCl2, 25
—
58
29
32
21
22
22
2
54
48
46
31
24
18
51
49
22
57
15
52
MgCl2ꢁ6H2O, 200
NiCl2ꢁ6H2O, 300
ZnCl2, 300
—
—
—
45 with an ortho carboxylic acid or ester promote the conversion of
cellulose to HMF (Tables S6 and S7 in the Supporting
Information), and that MgCl2ꢁ6H2O is necessary to maximize
HMF production (Table 2). The observed yields of HMF (e.g.,
41% in 1 h at 105 °C with 2ꢀmethoxycarbonylphenyl boronic
50 acid) are comparable to those attainable with a heavy metal.8
Boronic acid/MgCl2ꢁ6H2O also promote the conversion of cotton,
paper towels, and newspaper (Table 2), which are celluloseꢀrich
components of municipal waste. The HMF yields correlate with
the purity of the cellulose in these materials. Thus, HMF can be
55 produced efficiently from cellulosic material in a single, oneꢀpot
process that is devoid of heavy metals.
CeCl3ꢁ7H2O, 300
LaCl3ꢁ7H2O, 300
—
—
—
2ꢀcarboxyphenyl, 100
2ꢀcarboxyphenyl, 100
2ꢀcarboxyphenyl, 50
2ꢀcarboxyphenyl, 25
2ꢀcarboxyphenyl, 10
2ꢀcarboxyphenyl, 100
2ꢀcarboxyphenyl, 100
2ꢀcarboxyphenyl, 100
2ꢀcarboxyphenyl, 100
MgCl2ꢁ6H2O, 200
MgCl2ꢁ6H2O, 200
MgCl2ꢁ6H2O, 200
MgCl2ꢁ6H2O, 200
NiCl2ꢁ6H2O, 300
ZnCl2 + 6H2O, 300
CeCl3ꢁ7H2O, 300
LaCl3ꢁ7H2O, 300
—
2ꢀmethoxycarbonylphenyl, 100 100
2ꢀmethoxycarbonylphenyl, 100 120
2ꢀethoxycarbonylphenyl, 100
2ꢀethoxycarbonylphenyl, 100
MgCl2ꢁ6H2O, 200
—
MgCl2ꢁ6H2O, 200
100
120
The high boronate loadings necessary to turnover cellulose
rapidly (≤2 h) led us to search for a recovery method that enables
boronate recovery. Our initial separation strategy focused on
60 isolating 2ꢀcarboxyphenylboronic acid from a reaction mixture
using an anionꢀexchange resin. The reaction mixture was diluted
with water, filtered to remove any humins (which are insoluble
byproducts from aldol addition and condensation19), extracted
with ethyl acetate to remove HMF, and passed through a column
65 of resin. The anionic boronate was retained on the resin and
eluted with aqueous NH4HCO3 (1 M) to yield purified boronate,
which retained catalytic activity (Fig. S2). This strategy was,
however, inapplicable to the oꢀesterphenylboronic acids, which
have low solubility in water and high solubility in ethyl acetate.
a Glucose was at 10 wt% relative to DMA. Mol% and HMF yield (HPLC)
are relative to glucose.
provides appreciable yields of HMF (Table 1). We chose to
employ magnesium due to its being one of the most abundant
elements in Earth’s crust11 and the human body.12
Next, we attempted to increase the efficiency of the conversion
while maintaining environmental benignancy. To do so, we
turned to organocatalysis,13 which had not been employed
previously in a biomass conversion process.2,3 We were aware
that boronic acids readily form boronate esters with 1,2ꢀ and 1,3ꢀ
5
10 diols, and bind to furanose sugars with greater affinity than to
pyranose sugars.14 We reasoned that these organoboranes would
alter the equilibrium between glucose and fructose (which exist
primarily in the pyranose and furanose forms, respectively), and
thereby enhance HMF production. The precedents were
15 discouraging, however, as phenylboronic acid had been reported
to inhibit the hydrolysis of cellulose and the dehydration of sugar
monomers.15,16
Phenylboronic acids have an affinity for carbohydrates that is
tunable based on substituents.14 We assessed the ability of twenty
20 derivatives of phenylboronic acid to promote the conversion of
glucose to HMF (Tables S2–S4 in the Supporting Information).
Our data corroborated the earlier work with phenylboronic acid
itself,15 which is not a catalyst in our hands, but revealed that
adding an ortho carboxyl group engenders catalysis, especially in
25 the presence of MgCl2ꢁ6H2O (Table 1). Further optimization
revealed prospects for catalytic turnover. For example, adding
25 mol% of 2ꢀcarboxyphenylboronic acid to the magnesium
increases the HMF yield from 29% to 46%.
70
Boronic acid moieties become anionic at high pH and partition
completely into an aqueous phase. Hence, we added basic water
to reaction mixtures containing 2ꢀethoxycarbonylphenylboronic
acid, filtered to remove any humins, and extracted with ethyl
acetate (Fig. S3). Removing solvent from the organic phase
75 provided HMF. Evaporating water from the aqueous phase
recovered the boronic acid and MgCl2. The recovered catalysts
provided HMF in comparable yield through four reaction cycles.
Additional data provided insight on the chemical mechanism of
our conversion (Fig. 1). Chromium is known to mediate the
80 isomerization of aldose to ketose sugars by a 1,2ꢀhydride
shift.10,20 To probe the mechanism of our conversion, we
performed two deuteriumꢀlabeling experiments. In the first,
glucoseꢀ2ꢀd
was
converted
into
HMF
by
1
2ꢀethoxycarbonylphenylboronic acid and MgCl2ꢁ6H2O. H NMR
85 spectroscopy revealed that virtually no deuterium was retained in
the HMF product. In the second, unlabeled glucose was
converted in the presence of D2O, and a substantial quantity of
deuterium was found at Cꢀ1 of HMF (Fig. S4). These results are
compatible with a mechanism that avails an enediol intermediate
90 (Fig. 1). A similar mechanism is used by the enzyme
Water can counter the dehydration of fructose (Fig. 1).
30 Accordingly, we attempted to increase our HMF yields by using
anhydrous MgCl2. We found, however, that anhydrous MgCl2
provides lower HMF yields than does the hydrated salt (Table
S4). To determine how much water is beneficial for HMF
2
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Table 2 Conversion of cellulosic substrates into HMF in ionic liquidsa
T
time
(h)
2
3
2
2
1
1
2
1
2
2
2
HMF yield
substrate
cellulose
cellulose
cellulose
cellulose
cellulose
cellulose
cellulose
cellulose
cotton
ionic liquid
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[EMIM]Cl
[BMIM]Cl
[EMIM]Cl
[BMIM]Cl
acid, wt%
HCl, 0.61
HCl, 0.61
HCl, 0.61
HCl, 0.61
H2SO4, 0.88
HCl, 0.61
HCl, 0.61
H2SO4, 0.88
—
metal chloride, mol%
—
MgCl2ꢁ6H2O, 300
—
MgCl2ꢁ6H2O, 300
MgCl2ꢁ6H2O, 300
—
MgCl2ꢁ6H2O, 300
MgCl2ꢁ6H2O, 300
MgCl2ꢁ6H2O, 300
MgCl2ꢁ6H2O, 300
MgCl2ꢁ6H2O, 300
boronic acid, mol%
—
(°C)
105
105
105
105
105
105
105
105
105
105
105
(%)
10
15
12
39
41
21
38
36
40
31
18
—
2ꢀmethoxycarbonylphenyl, 120
2ꢀmethoxycarbonylphenyl, 120
2ꢀmethoxycarbonylphenyl, 120
2ꢀethoxycarbonylphenyl, 160
2ꢀethoxycarbonylphenyl, 160
2ꢀethoxycarbonylphenyl, 160
2ꢀethoxycarbonylphenyl, 160
2ꢀethoxycarbonylphenyl, 160
2ꢀethoxycarbonylphenyl, 160
paper towel
newspaper
HCl, 0.61
—
a Substrates were at 5 wt% relative to the ionic liquid. Wt% is relative to the ionic liquid. Mol% and HMF yield (HPLC) are relative to glucose monomers
within the substrate, which was assumed to be pure cellulose.
(RR02781, RR08438), the NSF (DMBꢀ8415048, OIAꢀ9977486,
BIRꢀ9214394), and the USDA.
phosphoglucose isomerase, which catalyzes the interconversion
of glucose 6ꢀphosphate and fructose 6ꢀphosphate.21
Boronate esterꢀformation is known to be more favorable with
fructose than glucose.14 We found that boronic acids also serve
by catalyzing fructose dehydration (Table S8). We suspect that
the organocatalyst relies on an ortho carboxyl group because its
oxygen can donate electron density into the empty pꢀorbital of
boron, thereby decreasing the strength of the fructose–boronate
40 Notes and references
5
aDepartment of Chemistry, University of Wisconsin-Madison, 1101
University Avenue, Madison, WI 53706, USA
bMedical Scientist Training Program and Molecular & Cellular
Pharmacology Graduate Training Program, University of Wisconsin-
45 Madison, 1300 University Avenue, Madison, WI 53706, USA
cDepartment of Biochemistry, University of Wisconsin-Madison, 433
Babcock Drive, Madison, WI 53706, USA. E-mail: rtraines@wisc.edu
† Electronic Supplementary Information (ESI) available: Experimental
procedures, Tables S1–S8, and Figs. S1–S4. See DOI: 10.1039/b000000x/
50
10 complex in the nearly waterꢀfree medium.14b Finally, we propose
that water attenuates the reactivity of Mg(II), allowing its
participation in catalysis, but deterring reaction pathways that can
lead to humins.22
1
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about Energy, Washington, DC, 2007.
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15 In summary, we have discovered novel reactivity mediated by a
simple boronic acid composed of hydrogen, boron, carbon, and
oxygen—four of the first eight elements in the periodic table.
This organocatalyst mediates the transformation of fructose,
glucose, and cellulose into the platform chemical HMF. Although
20 boronate loadings for rapid (≤2ꢀh) conversions are high, this
liability is overcome by the facility of its recovery (or,
potentially, by its immobilization). This discovery, which is the
first to link two highly active areas of research: organocatalysis
and biomass conversion, could facilitate the transition from
25 fossilꢀbased fuels and chemicals to a more sustainable biomassꢀ
based society.
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Acknowledgements
We are grateful to Prof. S. S. Stahl and Dr. N. A. McGrath for
contributive discussions. This work was supported by the DOE
30 Great Lakes Bioenergy Research Center (DOE BER Office of
Science DEꢀFC02ꢀ07ER64494), a DOE Bioenergy Research
Center. M.J.P. was supported by preꢀdoctoral fellowship
09PRE2260125 (American Heart Association). This work made
use of the National Magnetic Resonance Facility at Madison,
35 which is supported by NIH grants P41RR02301 (BRTP/NCRR)
and P41GM66326 (NIGMS). Additional equipment was
purchased with funds from the University of Wisconsin, the NIH
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