^1H and ^(13)C NMR spectroscopy on isotopically labeled glucose reveals that in the presence of tin-containing zeolite Sn-Beta, the isomerization reaction of glucose in water proceeds by way of an intramolecular hydride shift (see scheme) rather than proton transfer. This is the first mechanistic demonstration of Sn-Beta acting as a Lewis acid in a purely aqueous environment

Davis, Mark E.

Labinger, Jay A.

Moliner, Manuel

Román-Leshkov, Yuriy

English

Caltech Authors - Main

Biomass Conversion
DOI: 10.1002/anie.201004689
Mechanism of Glucose Isomerization Using a Solid Lewis Acid
Catalyst in Water**
Yuriy Romn-Leshkov, Manuel Moliner, Jay A. Labinger, and Mark E. Davis*
The conversion of glucose into fructose for the production of
high-fructose corn syrups (HFCS) is the largest biocatalytic
process in the world, and it recently has been considered as a
key intermediate step in the conversion of biomass to fuels
and chemicals.[1] This reaction is typically catalyzed by an
immobilized enzyme, xylose isomerase, that generates an
equilibrium mixture of 42 wt% fructose, 50 wt% glucose, and
8 wt% other saccharides. The enzymatic process is highly
selective, but it has several drawbacks that increase process-
ing costs, including the use of buffering solutions to maintain
pH, narrow operating temperatures, strict feed purification
requirements, and periodic replacement of the enzyme due to
irreversible deactivation.[1a] Inorganic bases can also catalyze
this reaction, albeit with low yields due to the reduced
stability of monosaccharides in the presence of basic cata-
lysts.[2] The mechanism of this aldose–ketose isomerization
involves hydrogen transfer from C-2 to C-1 and from O-2 to
O-1 of an a-hydroxy aldehyde to create the related a-hydroxy
ketone (Scheme 1).[3] This mechanism can occur either by a
proton transfer (Scheme 1A) or by an intramolecular hydride
shift (Scheme 1B). Several studies have shown that base-
catalyzed isomerizations take place by a proton transfer
mechanism through a series of enolate intermediates gener-
ated after the deprotonation of the a-carbonyl carbon in
water.[4] The xylose isomerase-catalyzed process is also
thought to be mediated by enolate intermediates generated
by histidine-directed base catalysis;[5] however, recent studies
have shown that metal centers in the enzyme are responsible
for the stabilization of the sugars open-chain form and the
subsequent aldose–ketose isomerization by way of an intra-
molecular hydride shift.[6, 7]
Recently, we reported on a tin-containing zeolite (tin in
the framework of a pure-silica analog of zeolite beta, denoted
Sn-Beta) that is a highly active material for the isomerization
of glucose into fructose in aqueous media.[8] This catalyst was
shown to be active over a wide temperature range (343–
413 K), in acidic solutions, and with glucose feeds as high as 45
wt% to give product yields equivalent to those obtained with
xylose isomerase (at its preferred reaction conditions). Here,
using 1H and 13C NMR spectroscopy on isotopically labeled
glucose, we demonstrate that in the presence of Sn-Beta, the
isomerization reaction in water proceeds by way of an
intramolecular hydride shift. Although Lewis acidity is
usually suppressed by the presence of water, verification of
this mechanistic pathway confirms that framework tin centers
in Sn-Beta act as Lewis acids in aqueous media. Previous
reports have shown strong interactions between Lewis acid
centers in zeolites and hydroxy/carbonyl moieties in reactants
dissolved in organic solvents.[9] For example, Corma et al.
used Sn-Beta to catalyze the Meerwein–Ponndorf–Verley
(MPV) reduction of carbonyl compounds in methanol,[10] and
Taarning et al. used Sn-Beta for the production of lactate
derivatives from monosaccharides in methanol.[11] The MPV
reaction mechanism involves the simultaneous coordination
Scheme 1. Glucose isomerization mechanisms by way of A) proton
transfer and B) intramolecular hydride shift.
[*] Dr. Y. Romn-Leshkov,[+] Dr. M. Moliner,[+] Dr. J. A. Labinger,
Prof. M. E. Davis
Division of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena, CA 91125 (USA)
Fax: (+1)626-568-8743
E-mail: mdavis@cheme.caltech.edu
[+] These authors contributed equally to this work.
[**] This work was financially supported as part of the Catalysis Center
for Energy Innovation, an Energy Frontier Research Center funded
by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC00010004. M.M.
acknowledges Fundacin Ramn Areces Postdoctoral Research
Fellowship Program for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004689.
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8954  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2010, 49, 8954 –8957
of the carbonyl group of a ketone and the hydroxy group of an
alcohol to a metal center. Adequate Lewis acidity in the metal
center allows the polarization of the carbonyl group of the
ketone and promotes a hydride shift from the hydroxy group
of the alcohol to the carbonyl group of the ketone (see
Figure S1 in the Supporting Information).[10] Corma and co-
workers have shown that metal centers (Nb, Ta, and Sn)
incorporated into zeolites can act as Lewis acids when used in
organic solvents containing a small amount of water.[12]
Christensen et al. recently reported on the isomerization of
dihydroxyacetone using Sn-Beta in methanol and in water,
where it was observed that no loss in activity occurred in
methanol, but severe catalyst deactivation occurred in water
due to the formation of large carbonaceous deposits. The
mechanism of reaction in water was not explained.[13] Thus,
the work reported herein is the first to describe the
mechanism whereby Sn-Beta acts as a Lewis acid in a
purely aqueous environment.
Glucose deuterated at the C-2 position (glucose-D2,
Figure S2A) was used to perform NMR studies on the
isomerization reaction. As shown in Scheme 1, the proton at
the C-2 position plays a fundamental role in the reaction
mechanism regardless of which pathway is followed. Indeed,
isotopic substitution at the C-2 position resulted in a two-fold
decrease in the initial reaction rate (kH/kD= 1.98, Figure S3),
revealing a considerable kinetic isotopic effect. Importantly,
13C NMR spectra of glucose-D2 heated in water at the
reaction temperature (383 K) in the absence of catalyst
reveal that no isotopic scrambling occurs (Figure S2B). This
result is important because it demonstrates the inertness of
the CD bond in water at reaction conditions and indicates
that any isotopic rearrangement within the molecule during
the reaction is entirely due to the actions of the catalyst.
13C and 1H NMR spectroscopy were used to study the
isomerization of glucose-D2 using Sn-Beta as the catalyst. A
comparison of 13C NMR spectra of unlabeled glucose and
glucose-D2 feed solutions reveals that resonances observed at
d= 74.1 and 71.3 ppm in the unlabeled glucose solution
appear as low-intensity 1:1:1 triplets in the glucose-D2
solution (Figure 1a,b). This effect is related to the disruption
of the nuclear Overhauser enhancement (NOE) by the
deuterium atoms in the C-2 positions of the two configura-
tions of glucose-D2 in solution (b-pyranose and a-pyranose,
present in a 64:36 ratio). During NOE, 13C resonance
intensities are enhanced up to 200% for directly bonded
13C–1H pairs when 1H broad-band decoupling is used to
suppress C,H couplings;[14] however, this resonance amplifi-
cation is not observed for 13C–2H pairs and resonances
associated with C-2 in glucose-D2 are thus substantially
diminished. After reacting a 10 wt% solution of glucose-D2 in
water at 383 K for 15 min in the presence of Sn-Beta, the
glucose and fructose fractions were separated using high-
performance liquid chromatography (HPLC). Analysis of
13C NMR spectrum for each fraction shows that after
reaction, glucose-D2 remains unchanged (Figure 1b,c),
while the fructose product has significant differences com-
pared to an unlabeled fructose standard (Figure 1e,f). Specif-
ically, resonances at d= 63.8 and 62.6 ppm assigned to the C-1
position of the b-pyranose and b-furanose configurations in
the unlabeled fructose standard appear as low-intensity
triplets for the fructose product recovered after reaction.
1H NMR spectra for both sugars further confirm these results.
The 1H NMR spectra of glucose-D2 before and after reaction
remain constant (Figure 2b,c), while the fructose spectrum
shows the disappearance of the resonance at d= 3.45 ppm due
to the presence of a deuterium atom in the C-1 position
Figure 1. 13C NMR spectra of a) unlabeled glucose, b) labeled glucose-D2, c) glucose fraction obtained after reacting glucose-D2 with Sn-Beta,
d) glucose fraction obtained after reacting labeled glucose-D2 with NaOH, e) unlabeled fructose, f) fructose fraction obtained after reacting
labeled glucose-D2 with Sn-Beta, and g) fructose fraction obtained after reacting labeled glucose-D2 with NaOH.
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(Figure 2 f). Integration of the areas in these spectra confirms
the presence of six C–H pairs for the glucose and fructose
fractions in contrast to the seven C–H pairs found in
unlabeled glucose or fructose. These results clearly indicate
that the deuterium atom located in the C-2 position of
glucose-D2 has moved to the C-1 position of fructose, thereby
unequivocally demonstrating that the glucose isomerization
reaction with a solid Lewis acid catalyst in pure water
proceeds by means of an intramolecular hydride shift.
A similar spectroscopic study was performed using
sodium hydroxide (NaOH) as a basic catalyst. A 10 wt%
aqueous solution of glucose-D2 was reacted in the presence of
NaOH (0.1m) at 383 K for 2 min. 13C NMR and 1H NMR
spectra of the glucose and fructose fractions show consid-
erable differences when compared to the results obtained
with Sn-Beta. First, the 13C and 1H NMR spectra for the
unlabeled fructose and for the fructose fraction isolated after
reaction show no differences, indicating that the fructose
fraction does not contain deuterium atoms (Figure 1e,g and
Figure 2e,g). Second, the 13C NMR spectrum of the glucose
fraction shows glucose-D2 mixed with a small amount of
regular glucose, as indicated by the presence of a small
resonance at d= 74.1 ppm (see inset in Figure 1d). The
presence of unlabeled glucose is corroborated by the appear-
ance of a resonance at d= 3.1 ppm in the 1H NMR spectrum,
which is assigned to a proton in the C-2 position (Figure 2d).
These results clearly indicate that the basic catalyst operates
by a proton-transfer mechanism whereby the deuterium atom
is removed from the a-carbonyl carbon of glucose-D2 to form
the corresponding enolate, and a proton from solution is
subsequently re-incorporated into the molecule, yielding
some unlabeled glucose along with the unlabeled fructose
(Scheme 1A).
This study proves that Sn-Beta can act as a Lewis acid
capable of catalyzing the isomerization of glucose in a pure
aqueous medium. Further studies are necessary in order to
understand the nature of the interactions amongst the active
site in Sn-Beta, the sugar and the solvent during the isomer-
ization reaction. Previous reports by Corma and co-workers
have shown (using 119Sn-NMR) that isolated framework tin
centers in zeolites are responsible for drastically enhancing
the rates of certain reactions.[15] For glucose isomerization in
water, we observed that framework tin centers are necessary
for Sn-Beta to catalyze the reaction. Sn-Beta synthesized by a
procedure known to fully incorporate tin into the framework
(SnCl4 as the tin source) is highly active for the isomerization
reaction, whereas Sn-Beta synthesized using SnO2 as the tin
source is completely inactive (Figure S4A). The diffuse
reflectance UV/Vis spectrum for the active material shows a
single band centered at 220 nm, assigned to tetrahedrally
coordinated metal, while the spectrum for the non-active
material shows a band centered at 300 nm, assigned to
octahedrally coordinated metal in extra-framework positions
(Figure S4B). Indeed, SnO2 is known to be highly stable in
acidic and basic media and probably does not participate in
the zeolite crystallization process (thereby providing extra-
framework SnO2 nanoparticles within the zeolite pores). It
has been suggested that the actual active site in Sn-Beta is a
partially hydrolyzed framework tin species (i.e. a
(SiO)3Sn(OH) center).
[16] Interestingly, Kovalevsky et al.
show that analogous hydrolyzed metal species in xylose
isomerase play a fundamental role in protonation–deproto-
nation sequences with water and glucose molecules during the
isomerization process.[7] Identifying similarities between the
active sites in the biological and the Sn-Beta catalyst systems
is one of our current objectives.
Figure 2. 1H NMR spectra of a) unlabeled glucose, b) labeled glucose-D2, c) glucose fraction obtained after reacting glucose-D2 with Sn-Beta,
d) glucose fraction obtained after reacting labeled glucose-D2 with NaOH, e) unlabeled fructose, f) fructose fraction obtained after reacting
labeled glucose-D2 with Sn-Beta, and g) fructose fraction obtained after reacting labeled glucose-D2 with NaOH.
Communications
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Experimental Section
Sn-Beta was synthesized following procedures previously reported in
the literature (see Supporting Information for details).[15] Isomer-
ization experiments were carried out in 10 mL thick-walled glass
reactors (VWR) heated in a temperature-controlled oil bath. 1.5 g of
a 10 wt% glucose-D2 (Cambridge Isotopes Corp.) solution and the
corresponding catalyst amount to achieve a 1:50 metal:glucose molar
ratio were added to the reactor, sealed, and heated. Further
information about the catalytic tests and characterization techniques
can be found in the Supporting Information.
Received: July 29, 2010
Published online: October 20, 2010
.Keywords: biomass conversion · glucose isomerization ·
NMR spectroscopy · solid Lewis acids · zeolites
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Mechanism of Glucose Isomerization Using a Solid Lewis Acid Catalyst in Water

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Mechanism of Glucose Isomerization Using a Solid Lewis Acid Catalyst in Water

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