One-pot conversion of cellobiose to mannose using a hybrid phosphotungstic acid-cerium oxide catalyst

Article abstract of DOI:10.1039/c5ra02645h

A hybrid catalyst composed of phosphotungstic acid coated cerium oxide nanoparticles was demonstrated to catalyze the one-pot conversion of cellobiose, the disaccharide unit of cellulose, to a monosaccharide mixture of glucose and mannose. A high % conversion of cellobiose (up to 99%) was achieved resulting in a yield of mannose up to 15.8%. The yield of mannose from a glucose starting material was 22.8%, exceeding those of previous cerium-based glucose epimerization catalysts. The components of the hybrid material were revealed to function synergistically via a two-step process. Cellobiose was hypothesized to be first hydrolyzed to glucose, which was subsequently epimerized to mannose by the cerium ions leached from the catalyst. The ¹³C NMR spectroscopic study suggested that the epimerization likely occurred by way of a 1,2-carbon shift reaction mechanism. This journal is

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ARTICLE
One-Pot Conversion of Cellobiose to Mannose Using a
Hybrid Phosphotungstic Acid-Cerium Oxide Catalyst
Zane C. Gernhart,a Anuja Bhalkikar,a John J. Burke,a Kate O. Sonnenfeld, a Chris
M. Marin, a Richard Zbasnik, b and Chin Li Cheung a,*
A hybrid catalyst composed of phosphotungstic acid coated cerium oxide nanoparticles was
demonstrated to catalyze the one-pot conversion of cellobiose, the disaccharide unit of
cellulose, to a monosaccharide mixture of glucose and mannose. A high % conversion of
cellobiose (up to 99%) was achieved resulting in a yield of mannose up to 15.8%. The yield of
mannose from a glucose starting material was 22.8%, exceeding those of previous cerium-
based glucose epimerization catalysts. The components of the hybrid material were revealed to
function synergistically via a two-step process. Cellobiose was hypothesized to be first
hydrolyzed to glucose, which was subsequently epimerized to mannose by the cerium ions
leached from the catalyst. The 13C NMR spectroscopic study suggested that the epimerization
likely occurred by the way of a 1,2-carbon shift reaction mechanism.
Research on the enzymatic production of mannose has been
primarily focused on the conversion of fructose to mannose.9
1. Introduction
The negative environmental and economic impacts of using
petroleum sources to produce fuels and chemicals necessitate
the development of viable alternative feedstocks.1, 2 The use of
plant-based food stocks for this purpose has been widely
accepted, but concerns have been raised about the economic
impacts of using food for fuel and chemical production.3
Cellulose, a non-edible polymer which serves as a structural
framework for plants, is one of the most abundant bio-
renewable materials on earth and is a promising alternative to
petroleum feedstocks.4 The cellulose polymer is composed of
glucose monomer units linked by β-(1,4)-glycosidic bonds.
Enzymatic depolymerization of cellulose into glucose units is
While fructose is found in abundance in the edible portion of
many plants, it is present in only small amounts in the non-
edible biomass portion.11 The enzymatic production of mannose
from glucose, a major building block of non-edible biomass,
appears to be much less common. However, the successful use
of inorganic catalysts for this transformation has been
demonstrated through the Lobry de Bruyn-van Ekenstein
reaction, Bilik reaction, and similar 1,2-carbon shift reactions.8,
12
The possibility of combining an inorganic epimerization
catalyst with
a catalyst capable of causing cellulose
depolymerization raises the exciting prospect that a one-pot
hybrid catalyst system can be designed for the conversion of
cellulose to mannose.
6
already being utilized for commercial biofuels production.5,
However, the production of other monosaccharides from
cellulose sources has been investigated to a much lesser extent.
Many of these sugars are valuable on their own and they have
the potential to be converted into other high value products.7, 8
For example, mannose, a monosaccharide which is only found
as part of complex polysaccharides in nature, has many
important uses such as a precursor for sweeteners, vitamin
production, antitumor agents and immunoregulatory agents.9, 10
The first step in any one-pot catalytic conversion of
cellulose to mannose is the depolymerization of cellulose to
glucose. Solid acid catalysts are potential alternatives for
activating cellulose hydrolysis because of their capability to
function in a large range of temperatures and their general ease
of separation from products.13-16 Examples of solid acid
catalysts reported for cellulose processing include metal oxides,
sulfonated carbon-based particles, polymer-based acids,
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zeolites, supported inorganic catalysts, and polyoxometalates investigated to determine possible roles of leached cerium ions
(POMs).17 Of these catalysts, POMs are particularly appealing from the catalyst in the conversion of cellobiose to
due to their highly acidic nature and low corrosiveness.18 For monosaccharides.
example, phosphotungstic acid (PWA, H3PW12O40) has been
demonstrated to hydrolyze cellulose with yield of glucose
greater than 50% and glucose selectivity greater than 90%.19
Catalysts composed of POMs supported on or constructed in
2. Experimental Methods
2.1 Preparation of Catalysts
hybrid with a variety of metal oxide particles have also been
Phosphotungstic acid decorated cerium oxide nanoparticles
(7 mol. % PWA/CeO2-x) were synthesized using a previously
reported co-precipitation method.20 In a typical synthesis, 2.32
g of cerium(IV) ammonium nitrate ((NH4)2Ce(NO3)6, 99.5%,
Alfa-Aesar, Ward Hill, MA) were dissolved in a solution
containing 0.97 mL of acetic acid, 10 mL of ethanol, and 1 mL
of 18 MΩ·cm deionized water. The resulting solution (Solution
A) was stirred at room temperature for 1 h. Solution B was
made by dissolving 0.994 g of dodecatungstophosphoric acid
hydrate (H3O40PW12•xH2O, reagent grade, Sigma-Aldrich, St.
Louis, MO) in a solution of 10 mL of ethanol and 1 mL of 18
MΩ·cm deionized water. Solution B was added drop-wise via a
glass burette to Solution A while being stirred. The resulting
mixture was left stirring overnight, followed by aging at 40 °C
for 6 h. and then 100 °C for 2 h. The obtained solid was
transferred into a crucible and calcined at 400 °C for 4 h.
Cerium oxide (CeO2-x) nanoparticles which were used as a
control comparison to the PWA/CeO2-x particles were
synthesized using a similar precipitation method but without the
addition of Solution B.
21
previously reported,20,
with some of the earliest work
focusing on the PWA/silica system.22 POM supported on silica
was demonstrated to effectively depolymerize cellulose in the
conversion
of
carbohydrate-rich
weeds
to
5-
hydroxymethylfurfural.21 However, the production of mannose
was not observed or expected for this reaction because of the
lack of an epimerization catalyst.
An essential second step in a one-pot catalytic conversion of
cellulose to mannose is the epimerization of glucose to
mannose. Only a handful of inorganic catalyst systems have
been reported for this transformation. Typical examples include
homogeneous catalysts such as metal ions complexed with
amines12, 23 and solid acid catalysts such as tin beta (Sn-beta)
25
zeolites.24,
The use of Sn-beta zeolites with sodium
tetraborate salts has proven quite successful for the
epimerization of glucose with yield of mannose reaching as
high as 21% when the glucose to sodium tetraborate reactant
molar ratio was 10:1.24 Lanthanide ions, including cerium, have
also been demonstrated to effectively catalyze the
epimerization of glucose to mannose via a 1,2-carbon shift.12, 23
The use of Ce3+ in the presence of diamines was shown to
effectively epimerize glucose with yields of mannose as high as
14%.23 Previous research has suggested that understanding
homogeneous catalyst systems can allow for the design of
improved solid catalysts.26
2.2 Materials Characterization
The crystal structure of the as-synthesized samples was
investigated using powder X-ray diffraction (XRD)
(PANalytical Empyrean, Westborough, MA). The average
wavelength of the Cu Kα X-ray source was 1.544 Å.
Diffraction peak assignments corresponding to fluorite-
structured cerium(IV) oxide (CeO2) in the XRD data were
indexed using the ICDD data card #04-013-4361. Further
analysis of the crystal structures and chemical properties of the
samples were made using high resolution transmission electron
microscopy (HRTEM) operated at 200 kV and energy-
dispersive X-ray spectroscopy (EDX) (Tecnai F-20, FEI,
Hillsboro, OR).
Herein we report the effectiveness of a hybrid catalyst
composed of PWA coated on cerium oxide nanoparticles
(hereafter referred to as PWA/CeO2-x) towards the conversion
of cellobiose into glucose and mannose. CeO2-x, 0<x≤0.5, is a
widely used support material which has previously been used to
prepare cerium oxide supported PWA. This material was
demonstrated to catalyze the oxidation of >99 % of sulfur
compounds from model oil systems with high recyclability.20
The presence of Ce3+ and Ce4+ valence sites on the surface of
27
nanostructured CeO2-x in conjunction with its demonstrated
2.3 Activity Evaluation of Catalysts
ability to support PWA led to the hypothesis that it would be an
effective catalyst for the production of mannose from cellulose.
Cellobiose was used to evaluate this catalyst system because it
is a dimer of glucose molecules connected by a β-(1,4)-
glycosidic bond. Its molecular similarity to cellulose, which is a
polymer of glucose monomer units connected by β-(1,4)-
glycosidic bonds, has allowed it to be used as a model cellulose
system.28-31 Our investigations of the roles of each catalyst
component revealed that the PWA/CeO2-x catalyst demonstrates
a synergistic ability to convert cellobiose to monosaccharides.
This synergistic effect was attributed to the epimerization of
glucose to mannose. 13C NMR was utilized to investigate the
mechanism behind this conversion. Catalyst leaching was
The catalytic activity of 7 mol.% PWA/CeO2-x towards the
conversion of cellobiose into glucose and mannose was
investigated using a MARS 6 Microwave Reaction System
(CEM, Matthews, NC). In a typical reaction, 0.021 g of 7
mol.% PWA/CeO2-x was transferred into a GlassChem reaction
vessel (CEM, Matthews, NC). 10 mL of a 50 mM solution of
D-(+)-cellobiose (C12H22O11, 98+%, Alfa-Aesar, Ward Hill,
MA) was then added to the reaction vessel. Water served as the
solvent for all reactions reported in this study. The vessel was
sealed and placed inside the microwave where it was heated to
the pre-determined reaction temperature over a 10 min.-period.
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[푐푒푙푙표푏푖표푠푒]푟−[푐푒푙푙표푏푖표푠푒]푝
[푐푒푙푙표푏푖표푠푒]푟
The reaction mixture was stirred using a Teflon stir bar and the
reaction temperature was monitored using a fiber optic probe.
The reaction temperatures evaluated were 140 °C, 160 °C, and
180 °C. The reaction pressure was estimated using an EasyPrep
autoclave vessel (CEM, Matthews, NC) with reaction volume
scaled to match those carried out previously. The pressures
were found to be 225, 446, and 784 kPa at 140 ºC, 160 ºC, and
180 ºC, respectively. The reaction mixture was held at the pre-
determined temperature for 30, 60, 90, or 120 minutes, after
which it was then allowed to cool to room temperature. A 1.5-
mL product sample was centrifuged and syringe filtered with a
0.2-µm pore-size polytetrafluoroethylene membrane (VWR
International, Radnor, PA) before its analysis. To investigate
the effect of each catalyst component on the reaction, the
foregoing procedure was repeated with cerium oxide
nanoparticles (CeO2-x) and PWA hydrate of masses 0.0093 g
and 0.0127 g, respectively. These masses were chosen so that
they were equal to the maximum weight amounts of each
catalyst component which could have been present in 0.021 g
of the 7 mol.% PWA/CeO2-x catalyst. In order to determine the
effect of microwave heating, a series of reactions was
performed in a convection oven at 160 ºC using Teflon-lined
stainless steel autoclaves (Parr, Moline, IL). The maximum %
conversion of cellobiose and yields of mannose were similar to
those obtained using microwave heating but required longer
reaction times. These results are provided for comparison in the
ESI.
% 퐶표푛푣푒푟푠푖표푛 =
× 100%
Eq. 1
[푠]푝
% 푆푒푙푒푐푡푖푣푖푡푦 =
) ×2 × 100% Eq. 2
]
푝
[
([푐푒푙푙표푏푖표푠푒]푟− 푐푒푙푙표푏푖표푠푒
[푆]푝
% 푌푖푒푙푑 = [푐푒푙푙표푏푖표푠푒] ×2 × 100%
Eq. 3
푟
where S represents glucose or mannose, and subscripts p and r
represent product and reactant respectively.
2.5 Saccharide Epimerization Study
To evaluate the effect of the hybrid catalyst and individual
components on glucose epimerization, catalysis experiments at
160 ºC were repeated with a 25 mM solution of D-(+)-glucose
(C6H12O6, ≥99.5%, Sigma-Aldrich, St. Louis, MO). In a typical
reaction, 0.021 g of 7 mol.% PWA/CeO2-x was transferred into
a GlassChem reaction vessel (CEM, Matthews, NC). 10 mL of
a 25 mM solution of D-(+)-glucose was then added to the
reaction vessel. The vessel was sealed and placed inside the
microwave where it was heated to 160 ºC over a 10 min.-
period. A standard 30-min. reaction duration was used for all
trials. The reaction mixture was stirred using a Teflon stir bar
and the reaction temperature was monitored using a fiber optic
probe. A 1.5-mL product sample was centrifuged and syringe
filtered with
a 0.2-µm pore-size polytetrafluoroethylene
membrane before its analysis by HPLC. To investigate the
effect of each catalyst component on the reaction, the foregoing
procedure was repeated with cerium oxide nanoparticles (CeO2-
x) and PWA hydrate of masses 0.0093 g and 0.0127 g,
respectively.
2.4 Reaction Product Analysis by HPLC
To investigate the conversion of glucose to mannose by the
PWA/CeO2-x catalyst, a 5 mg/mL solution of glucose with a 13C
label at the C1 position (D-glucose, 1-13C, 98-99%, Cambridge
Isotope Laboratories, Inc., Andover, MA), was prepared with
18 MΩ·cm deionized water. 10 mL of this solution was added
to a GlassChem reaction vessel containing the same weight
amount of catalyst as described in the above cellobiose
conversion study. The solution underwent the standard 30-min.
microwave-heated reaction at 160 ºC. Both the starting solution
and reaction products were analyzed by 13C NMR using a
Bruker Avance III-HD 400 MHz Spectrometer (Bruker
BioSpin, Billerica, MA). One drop of deuterated water (D2O)
was added to each NMR sample. Thirty two scans of standard
13C {1H} NMR with a pulse width of 30º were performed to
obtain each NMR spectrum.
The saccharide content in the reaction products was
analyzed for each trial in our study using a method developed
by the National Renewable Energy Laboratory.32 Briefly, a 20-
μL product sample solution was injected into
a high
performance liquid chromatography (HPLC) instrument
(Waters, Milford, MA) with a 0.005 M sulfuric acid (H2SO4,
99.999%, Sigma-Aldrich, St. Louis, MO) mobile phase flowing
at 0.6 mL/min. An Aminex HPX-87H column (Bio-Rad
Laboratories Inc., Hercules, CA) heated at 55 °C was used to
separate the product sample solution into different saccharide
components. The separated saccharide components were
detected using
a Waters 410 Differential Refractometer
(Waters, Milford, MA) heated at 35 °C. Confirmation of
mannose in the sample was made using a Dionex ICS-3000
HPLC (ThermoScientific, Sunnyvale, CA) equipped with a
Dionex Carbopac PA20 column (ThermoScientific, Sunnyvale,
CA). A second confirmation was made using an Aminex HPX-
87P column (Bio-Rad Laboratories Inc., Hercules, CA).
The concentrations of cellobiose, glucose and mannose in
the product samples were determined by integrating the areas of
corresponding refractive index peaks and comparing these areas
with those of the respective standard. Percentage (%)
conversion of cellobiose, % product yields for glucose and
mannose, and % product selectivity for each monosaccharide
were reported according to equations 1, 2 and 3 below:
2.6 Catalyst Leaching Study
Metal ion leaching has been demonstrated to have a large
impact on the catalytic activity of many reported catalyst
systems.33 To investigate the presence and role of leaching
during the course of our reaction, a 25 mM solution of D-(+)-
glucose (C6H12O6, ≥99.5%, Sigma-Aldrich, St. Louis, MO) was
heated to 160 ºC in the presence of 7 mol.% PWA/CeO2-x. Once
the reaction mixture reached 160 ºC, the heating was stopped
and the reaction vessel was allowed to cool to room
temperature. To remove the catalyst particles, the product
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Fig. 1 XRD of (top) PWA/CeO2-x, (middle) CeO2-x
nanoparticles, and (bottom) PWA.
mixtures were centrifuged and filtered using a 0.2 µm
polytetrafluoroethylene filter. The filtered solution was frozen
at -14 ºC. After thawing, the solution was centrifuged and
filtered using a 0.02 µm Whatman Anotop filter (GE Healthcare
Bio-Sciences, Pittsburgh, PA). A 1.0 mL sample of filtered
solution was removed for analysis by HPLC. It was then placed
back in the microwave and heated at 160 ºC for an additional
30 minutes. The reaction products after the second microwave
heating were analyzed by HPLC to determine the effect of
filtration on the conversion of cellobiose and by inductively
coupled plasma mass spectrometry (ICP-MS, ThermoScientific,
Waltham, MA) to determine the concentration of cerium in the
catalyst products.
3. Results and Discussion
3.1 Structural and Chemical Characterization of the Catalysts
Fig. 2 (a) HRTEM image of the 7 mol. % PWA/CeO2-x catalyst
with zoomed out image (top inset) and measured lattice spacing
(bottom inset); (b) corresponding EDX spectrum.
Both powder XRD patterns of the CeO2-x and PWA/CeO2-x
demonstrate the presence of a fluorite-phase (Fm-3m) cerium
oxide. (Figure 1) The XRD pattern of the 7 mol.% PWA/CeO2-x
samples calcined at 400 ºC is consistent with a previously
reported pattern.20 The presence of tungsten oxide (WO3) is
indicated by a peak at a 2 theta of 24.39º. These features in the
XRD pattern are consistent with the partial decomposition of
PWA during the calcination process.20, 34
HRTEM images of the CeO2-x and PWA/CeO2-x samples
also reveal a fluorite-structured cerium oxide crystal lattice for
both materials. The HRTEM image of a PWA/CeO2-x sample in
Figure 2a displays lattice spacing of 3.24 Å, which corresponds
to the distance between the (-111) and (1-11) planes of fluorite-
structured cerium dioxide when viewed down the [110]
direction. An observed 3% increase in lattice spacing for the
PWA/CeO2-x could be explained by possible increased lattice
strain in this hybrid material. The HRTEM image of the CeO2-x
sample shows characteristic lattice spacings for cerium dioxide.
(Figure 3a) The identified lattice spacing of 3.15 Å corresponds
to the distance between adjacent (111) planes. Another set of
identified lattice spacings of 1.95 Å corresponds to one half of
the distance between (011) planes. This matches well with the
location of a second plane of cerium atoms.
Elemental analysis of the PWA/CeO2-x by EDX confirmed
the presence of both cerium and tungsten in the PWA/CeO2-x
samples. (Figure 2b) In contrast, no peaks indicating the
presence of tungsten were observed in the EDX data of the
CeO2-x samples, in spite of a longer integration time being
allowed for the CeO2-x sample than for the PWA/CeO2-x sample.
(Figure 3b)
3.2 Effects of Reaction Temperature and Duration on Cellobiose
Conversion
The effects of reaction temperature and duration on the
catalytic cellobiose conversion were investigated using a range
of microwave heating temperatures (Figure 4) and reaction
periods. (Figure 5) The primary reaction products were
identified by HPLC as glucose and mannose. For reactions
performed with durations of 30 min., when the reaction
temperature was increased from 140 ºC to 180 ºC, the
conversion of cellobiose increased from 15.8% to 99.1%.
However, the resulting product selectivity for glucose
decreased from 75.1% to 27.7%. The % yield of glucose was
observed to reach a maximum value of 32.6% at 160 ºC and
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Fig. 4 Effect of reaction temperature on 30-minute cellobiose
hydrolysis reactions with the 7 mol.% PWA/CeO2-x catalyst.
Fig. 3 (a) HRTEM image of CeO2-x nanoparticle clusters with
zoomed out image (top inset) and measured lattice spacing
(bottom inset); (b) corresponding EDX spectrum.
decrease at higher reaction temperatures. The % yield of
mannose was observed to increase with an increase in reaction
temperature over the evaluated range, and it reached 15.4% at a
reaction temperature of 180 ºC.
The effect of reaction durations from 30 minutes to 2 hours
was investigated at a fixed reaction temperature of 160 ºC.
(Figure 5) The conversion of cellobiose was found to increase
with an increase in reaction duration. The % yield of glucose
was observed to increase as the reaction time was increased
from 30 minutes to 1 hour. However, there was little change to
the % yield of glucose for reaction durations from 1 hour to 90
minutes. As the reaction duration was increased to 2 hours, the
% yield of glucose was observed to decrease. The only
evaluated reaction duration within the evaluated temperature
range where both the % conversion of cellobiose and % yield of
glucose were above 50% was at 30 minutes.
The production of mannose was also influenced by reaction
duration. The % yield of mannose followed a similar trend as
that of glucose up to 90 minutes. After a 90-min. reaction,
while the yield of glucose began to fall, the yield of mannose
began to increase slightly. The highest yield of mannose in our
Fig. 5 Effect of reaction duration on cellobiose hydrolysis
reactions at 160 ºC using the 7 mol.% PWA/ CeO2-x catalyst.
reaction duration study was 15.8%. This was the result of a
120-min. reaction at 160 ºC. This % yield of mannose was
similar to the 14% yield previously reported for a homogeneous
cerium catalyst. However, while the previously reported
catalyst system utilized glucose as the starting reactant, our
catalyst was able to achieve similar yields with
a
cellooligosaccharide chemical in a one-pot reaction.23 This
yield is also similar to those achieved using 1%Ru-
5%Ni/mesoporous carbon (MC) and 1%Ir-5%Ni/MC catalysts
with microcrystalline cellulose.35 However, it should be noted
that these reactions were performed at 245 ºC in the presence of
6 MPa of hydrogen. A noteworthy advantage of this 7 mol.%
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Table 1 Comparison of reaction yields from 30-min. reactions in 50 mM cellobiose
solutions at 160 ºC. Masses of PWA hydrate and CeO2-x used correspond to the
amounts present in 0.021 g of 7 mol.% PWA/CeO2-x
.
PWA/CeO2-x catalyst is its ability to produce mannose under where %CHeating is the % conversion due to heating, %CCeO2-x is
acidic conditions which are more similar to those used for the % conversion observed for a solution containing CeO2-x as a
cellulose processing than the basic conditions which were catalyst, %CPWA is the % conversion observed for a solution
previously reported.12, 36
with PWA as a catalyst, and %CPWA/CeO2-x is the % conversion
observed for a solution containing the hybrid catalyst system.
Interestingly, when the analyzed % conversions attributed to
each component (Equations 4 and 5) are added together, the
total conversion is 10.3% less than the %conversion caused by
the hybrid catalyst system. (Equation 6) This suggests a
possible synergistic effect for the system.
The effects of a hybrid catalyst of PWA and CeO2-x on the
conversion yield were compared to that of a reaction containing
the two catalyst components added separately in the same
solution. For the reaction with separate catalyst components not
in hybrid form, it was found that the % conversion of cellobiose
was 56.7%, the yield of glucose was 37.6% and the yield of
mannose was 7.9%. Hence, the effect of having the two
components in a hybrid form appears negligible in terms of %
cellobiose conversion. However, a decreased yield of glucose
for the hybrid material and slightly higher yield of mannose
suggest that the hybrid material could have a positive effect on
this step of the conversion.
3.3 Evaluation of the Catalytic Activity of Individual Catalyst
Components on Cellobiose Conversion
To determine the effect of each component of the
PWA/CeO2-x catalyst system on the cellobiose conversion
reaction, a series of control experiments were performed.
(Table 1) The effects of heating a cellobiose solution without
any catalyst and with each catalyst component added separately
were compared to the effects of heating a cellobiose solution
with 7 mol.% PWA/CeO2-x. When the 50 mM cellobiose
solution was heated at 160 ºC for 30 minutes without any
catalyst in the solution, the obtained conversion of cellobiose
was 8.9%. When an amount of CeO2-x equal to the amount
found in 0.021g of 7 mol.% PWA/CeO2-x was added to a 50
mM cellobiose solution, the corresponding conversion of
cellobiose was determined to be 9.0 %. It is worth noting that
the presence of CeO2-x by itself appears to have little effect on
the conversion of cellobiose. When an amount of PWA equal to
the amount in 0.021g of 7 mol.% PWA/CeO2-x was added to a
50 mM cellobiose solution, a 47.1 % conversion was obtained.
A zeroth-order approximation was made that a base
conversion of 8.9% was achieved just by heating the solution.
By subtracting this base conversion from the total percent
conversion of a reaction with one catalyst component, the
resulting number can be designated as the % conversion caused
by that component. (Equations 4 and 5) Likewise, the %
conversion attributed to the hybrid catalyst system can be
obtained when the base conversion achieved by heating is
subtracted from results produced by the 7 mol.% PWA/CeO2-x
catalyst. (Equation 6)
3.4 Epimerization of Glucose to Mannose
In the preceding sections, it was noted that the % yield of
mannose increased as that of glucose decreased. This
observation led to the hypothesis that glucose was being
converted to mannose after the hydrolysis of cellobiose. To
verify this hypothesis, a 25 mM glucose solution mixed with
the 7 mol.% PWA/CeO2-x catalyst was heated at 160 ºC for 30
minutes. All other reaction conditions were identical to those
used for the conversion of cellobiose. Under these conditions, a
54.3% conversion of glucose and a 22.8% yield of mannose
were obtained. (Table 2) When a glucose solution with no
catalyst was heated to 160 ºC and held at this reaction
temperature for 30 min., the resulting conversion was 11.0%
(variable CHeating in Equations 4, 5, and 6). Almost no change
was observed between the conversion for a glucose solution
heated without catalyst and that for a glucose solution heated
with CeO2-x. The conversion caused by an amount of PWA
equal to the molar amount which could be present in 7 mol.%
PWA/CeO2-x was 12.2%. When the 7 mol.% PWA/CeO2-x was
% Conversion by CeO2−x = % CCeO − % CHeating
Eq. 4
Eq.5
2−x
% Conversion by PWA = % CPWA − % CHeating
% Conversion by PWA/CeO2−x
= % C
− % CHeating
PWA/CeO2−x
Eq. 6
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used as a catalyst, the conversion was observed to increase to synthesis. The activity of these catalysts depends on the form
54.3%. However, when the conversions resulting from each and chemical identities of ions leached from the metal particles
component are independently assessed to account for the effect into the solution.38 Understanding this relationship has allowed
researchers to make modifications to the reaction mixture or
catalyst system to better control reaction products.33
In order to investigate the effect of catalyst leaching on
mannose production from glucose, the catalyst material was
filtered from the reaction once the mixture had reached 160 ºC.
The change in the yield of mannose was then monitored for the
remainder of the reaction. It was expected that if no leaching
had occurred, little to no change in the yield of mannose would
be observed after filtering. The percent conversion of glucose
after heating to 160 ºC and cooling was 14.8% and the yield of
Table 2 Comparison of reaction yields from 30-min. reactions in
mannose was 8.6%. After the reaction mixture was filtered to
remove catalyst nanoparticles, it was placed back in the
microwave for a further 30 minutes of reaction at 160 ºC. The
resulting percent conversion of glucose increased to 56.2% and
the yield of mannose increased to 22.2%. This conversion is
remarkably close to the conversion achieved in the presence of
the hybrid catalyst system without any filtration treatments.
(Table 2) The concentration of cerium in the reactants after
filtering was measured to be ca. 50 ppm by ICP-MS.
The recyclability of the PWA/CeO2-x catalyst was studied to
further investigate the role of leaching in the catalyst system.
After a standard 30-minute reaction with cellobiose at 160 ºC,
the catalyst was collected by centrifugation and rinsed with
water. The catalyst was then placed back in a reaction vessel
with a new cellobiose solution and heated for another 30
minutes at 160 ºC. The % conversion of cellobiose with a
recycled catalyst was 23.3% and the % glucose yield was
15.5%. Neither value reached 50% of the original value
achieved before recycling. (Table 1) However, the value of
mannose yield showed the greatest relative change with a yield
of only 0.67% after recycling compared to a yield of 10.5%
before. The decreased yields from the recycled catalyst were
attributed to the leaching of cerium ions and PWA from the
catalyst. The recycled catalyst likely did not contain enough
acidic materials to cause the leaching of the cerium ions, which
are required for epimerization to occur. This hypothesis was
further evaluated by adding a mass of PWA equal to that in the
original catalyst to the recycled catalyst material. After
repeating the reaction under the same conditions, it was found
that the yield of mannose produced reached 83% of the yield
before recycling with additional PWA.
25 mM glucose solutions at 160 ºC. Masses of PWA hydrate and
CeO2-x used correspond to the amounts present in 0.021 g of 7
mol.% PWA/CeO2-x
.
of heating (using Equations 4 and 5), the sum of the
components is ca. 42% less than the conversion caused by 7
mol.% PWA/CeO2-x. (Equation 6)
The 22.8% yield of mannose achieved using this hybrid
catalyst with glucose is higher than the 14% yield previously
reported for reactions catalyzed using cerium ions with amines
instead.23 This yield is comparable to the 21% yield achieved
with recently reported high-performance Sn-β epimerization
catalysts in the presence of sodium tetraborate.24 A possible
explanation for this increase in yield when compared to the
previous use of cerium ion catalyst is that the acidic reaction
conditions minimized the formation of undesirable 2-ketoses
through the Lobry de Bruyn-van Ekenstein reaction, which
commonly proceeds under basic conditions.12
The synergistic effect observed in the conversion of
cellobiose using the PWA/CeO2-x catalyst system is likely due
to the conversion of glucose produced from the hydrolysis of
cellobiose into mannose. As the glucose was removed from the
reaction mixture, through conversion to mannose, the
conversion of cellobiose to glucose increased. The
epimerization of glucose to mannose catalyzed by cerium ions
has previously been reported.23 However, to the best of our
knowledge, no reports have been made of this epimerization in
the presence of CeO2-x. This raises the possibility that the
synergistic effect observed in the catalyst system is a result of
homogeneous catalytic reactions involving leached cerium ions
from the hybrid catalyst. Due to the important role of this effect
in other catalyst systems which were once thought to be entirely
heterogeneous,37, 38 it is critical that this effect be investigated
Previous studies on epimerization using cerium ions suggest
that the conversion of glucose to mannose occurs via a 1,2-
carbon shift.23 A recent study of Sn-beta zeolites has
demonstrated a reliable method to investigate this 1,2-carbon
shift using 13C NMR and D–(1-13C) glucose as the starting
for PWA/CeO2-x
.
3.5 Investigation of Catalyst Leaching
Understanding the effect of leaching in this catalyst system chemical.24 To determine whether the same mechanism was
is a critical step towards utilizing and improving supported responsible for the conversion using our 7 mol.% PWA/CeO2-x
PWA catalyst design. In the past, leaching was often considered catalyst, we applied the catalyst to convert a D–(1-13C) glucose
detrimental.39 However, recently, researchers have begun to solution at 160 ºC for 30 minutes and performed 13C NMR
view leaching as another tool to be harnessed in designing new experiments to analyze the reaction products. The NMR
catalyst systems.33 This effect has been studied extensively for spectrum of our initial D–(1-13C) glucose solution contained
platinum metal catalysts which are often used in organic peaks at δ = 97.0 and 93.2 ppm, which correspond to the 13C at
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the C1 position of the β-pyranose and α-pyranose forms of cellobiose hydrolysis was illustrated over a range of reaction
glucose. Their locations are consistent with NMR features in temperatures and durations to primarily yield glucose and
previous reports for D–(1-13C) glucose.24 (Figure 6a) After the mannose. When the catalytic activity of each catalyst
Fig. 7 Two step conversion of cellobiose to mannose by the
PWA/CeO2-x catalyst. In the first step, cellobiose is hydrolyzed
to form glucose monomers. In the second step, the glucose
monomers are converted to mannose by the leached cerium ions.
component was discretely investigated, the combined hybrid
catalyst was inferred to display a synergistic effect for the
Fig. 6 13C NMR spectra of D–(1-13C) glucose solution (a) before
conversion of cellobiose. Our findings suggest that this effect
the reaction and (b) after the reaction. Peaks corresponding to
D–(1-13C) glucose are highlighted by an orange dotted rectangle.
indirectly stemmed from the epimerization of glucose to
mannose. Using 13C NMR, we further demonstrated that this
epimerization proceeded by the way of a 1,2-carbon shift
mechanism similar to that previously reported for several metal
ion catalyst systems.12 Upon closer investigation, the hybrid
catalyst’s primary method for causing this epimerization was
found to be likely through homogeneous catalysis of leached
cerium ions. The ability of the hybrid catalyst to cause
epimerization under acidic conditions allows it to avoid the
formation of 2-ketoses which are typically formed under basic
conditions. The understanding gained in this study is expected
to promote the development of improved catalysts for rare
sugar production from cellulose. Although this catalyst has
been demonstrated to be effective for cleaving β-(1,4)-
glycosidic bonds, its application towards cellulose will likely
require the use of other solvent systems which can more readily
solubilize cellulose crystals. We propose the investigation of
ionic liquids for this purpose due to their successful application
to many other solid acid catalyst systems.40, 41 Finally, our study
demonstrates that as more supported PWA catalysts are
developed, the presence and effect of support leaching on
catalytic activity should be closely investigated.
Peaks corresponding to D–(2-13C) mannose are highlighted by a
black dashed rectangle.
standard microwave reaction, a new additional set of peaks at δ
= 71.8 and 72.4 ppm was observed. (Figure 6b) Previous
reports have demonstrated that these new peaks correspond to
the presence of 13C at the C2 position of D–(2-13C) mannose.24
This finding is in agreement with our HPLC results which
demonstrated that mannose was the primary product of this
reaction. Since no peaks at δ = 94.6 and 95.1 ppm
corresponding to D–(1-13C) mannose were observed, our NMR
results suggest that the hybrid catalyst converts D–(1-13C)
glucose to D–(2-13C) mannose, demonstrating the catalyzed
epimerization proceeded by a 1,2-carbon shift mechanism.
3.6 Reaction Mechanism for Mannose Production
The results of the above catalytic activity studies lead us to
hypothesize a two-step mechanism for the production of
mannose from cellobiose catalyzed by the hybrid PWA/CeO2-x
catalyst. (Figure 7) In the first step, cellobiose is hydrolyzed
into glucose molecules by the supported PWA material. In the
second step, the resulting glucose molecules are then
epimerized to mannose via a 1,2-carbon shift catalyzed by
cerium ions, which may have been leached from the cerium
oxide support. This epimerization process allows for promoting
the cellobiose conversion by converting a portion of the glucose
products to mannose.
Acknowledgements
The authors acknowledge funding support from the
Nebraska Center for Energy Sciences and Army Research
Office (W911NF-10-2-0099). JJB and KOS acknowledge the
support of the UCARE program at the University of Nebraska-
Lincoln (UNL). JJB is also grateful for the NASA Nebraska
Space Grant Fellowship support. We thank the Undergraduate
Instrumentation Center and Research Instrumentation Facility
in the Department of Chemistry, Nebraska Center for Materials
and Nanoscience, and the Morrison Microscopy Core Research
4. Conclusions
A hybrid PWA/CeO2-x catalyst material was demonstrated
to effectively convert cellobiose to mannose and epimerize
glucose to mannose. The catalytic activity of the material for
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Facility at UNL for the use of their facilities. The authors also 18. W. P. Deng, Q. H. Zhang and Y. Wang, Dalton Trans., 2012, 41
acknowledge Michael Tai and Deepak Keshwani for help with 9817-9831.
HPLC verification of mannose in our samples and Martha 19. J. Tian, J. H. Wang, S. Zhao, C. Y. Jiang, X. Zhang and X. H. Wang,
Morton for help with NMR analysis. We thank Joseph Brewer Cellulose, 2010, 17, 587-594.
and Josh Beaudoin for help with ICP-MS measurements. In 20. M. Zhang, W. S. Zhu, S. H. Xun, H. M. Li, Q. Q. Gu, Z. Zhao and Q.
addition, we thank Johnny Goodwin at the University of Wang, Chem. Eng. J., 2013, 220, 328-336.
Alabama Central Analytical Facility for the high resolution 21. M. I. Alam, S. De, S. Dutta and B. Saha, RSC Adv., 2012,
,
2
, 6890-
electron microscopy work.
6896.
22. Á. Kukovecz, Z. Balogi, Z. Kónya, M. Toba, P. Lentz, S. I. Niwa, F.
Mizukami, Á. Molnár, J. B. Nagy and I. Kiricsi, Appl. Catal., A,
2002, 228, 83-94.
Notes and references
a Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE
68588 (USA)
23. T. Tanase, T. Takei, M. Hidai and S. Yano, Carbohydr. Res., 2001,
333, 303-312.
b
Department of Food Science and Technology, University of Nebraska-
Lincoln, Lincoln, NE 68583 (USA)
24. W. R. Gunther, Y. Wang, Y. Ji, V. K. Michaelis, S. T. Hunt, R. G.
*Author to whom correspondence should be addressed
Griffin and Y. Román-Leshkov, Nat. Commun., 2012, 3, 1109.
25. R. Bermejo-Deval, R. Gounder and M. E. Davis, ACS Catal., 2012, 2,
Electronic Supplementary Information (ESI) available:
A Table
2705-2713.
comparing the % conversion and yields of product from reactions carried
out at 160 °C for 60, 120, and 360 minutes in Teflon-lined stainless steel
autoclaves heated by convection oven.
26. V. Choudhary, A. B. Pinar, R. F. Lobo, D. G. Vlachos and S. I.
Sandler, ChemSusChem, 2013, , 2369-2376.
27. C. Sun, H. Li and L. Chen, Energy Environ. Sci., 2012,
6
5
, 8475-8505.
28. L. Vanoye, M. Fanselow, J. D. Holbrey, M. P. Atkins and K. R.
Seddon, Green Chem., 2009, 11, 390-396.
1. M. S. Singhvi, S. Chaudhari and D. V. Gokhale, RSC Adv., 2014, 4,
8271-8277.
29. L. Negahdar, J. U. Oltmanns, S. Palkovits and R. Palkovits, Appl.
Catal., B, 2014, 147, 677-683.
2. L. Sunggyu and W. Barbara, in Handbook of Alternative Fuel
Technologies, Second Edition, CRC Press, 2014, pp. 1-18.
30. K. Li, L. Bai, P. N. Amaniampong, X. Jia, J.-M. Lee and Y. Yang,
3. M. Balat and H. Balat, Appl. Energy, 2009, 86, 2273-2282.
4. Y. H. P. Zhang, J. B. Cui, L. R. Lynd and L. R. Kuang,
ChemSusChem, 2014, 7, 2670-2677.
31. X. Peng, X.-G. Meng, C. Mi and X.-H. Liao, RSC Adv., 2015, 5,
Biomacromolecules, 2006, 7, 644-648.
9348-9353.
5. D. Klemm, B. Heublein, H. P. Fink and A. Bohn, Angew. Chem. Int.
Ed., 2005, 44, 3358-3393.
32. A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter and D.
Templeton, Determination of Sugars, Byproducts, and Degradation
Products in Liquid Fraction Process Samples, U.S. Department of
Energy, Golden, 2006.
6. P. Alvira, E. Tomas-Pejo, M. Ballesteros and M. J. Negro, Bioresour.
Technol., 2010, 101, 4851-4861.
7. C. Chatterjee, F. Pong and A. Sen, Green Chem., 2015, 17, 40-71.
8. S. Angyal, in Glycoscience, ed. A. Stütz, Springer Berlin Heidelberg,
2001, vol. 215, pp. 1-14.
33. V. P. Ananikov, K. A. Gayduk, I. P. Beletskaya, V. N. Khrustalev
and M. Y. Antipin, Eur. J. Inorg. Chem., 2009, 2009, 1149-1161.
34. E. Caliman, J. A. Dias, S. C. L. Dias, F. A. C. Garcia, J. L. de
Macedo and L. S. Almeida, Microporous Mesoporous Mater., 2010,
132, 103-111.
9. C.-S. Park, H.-J. Kwon, S.-J. Yeom and D.-K. Oh, Biotechnol. Lett,
2010, 32, 1305-1309.
10. A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411-2502.
11. S. Nguyen, S. Sophonputtanaphoca, E. Kim and M. Penner, Appl.
Biochem. Biotechnol., 2009, 158, 352-361.
35. J. Pang, A. Wang, M. Zheng, Y. Zhang, Y. Huang, X. Chen and T.
Zhang, Green Chem., 2012, 14, 614-617.
36. R. Rinaldi and F. Schuth, Chemsuschem, 2009, 2, 1096-1107.
12. L. Petruš, M. Petrušová and Z. Hricovíniová, in Glycoscience, ed. A.
Stütz, Springer Berlin Heidelberg, 2001, vol. 215, pp. 15-41.
13. Y. B. Huang and Y. Fu, Green Chem., 2013, 15, 1095-1111.
37. S. S. Pröckl, W. Kleist, M. A. Gruber and K. Köhler, Angew. Chem.
Int. Ed., 2004, 43, 1881-1882.
38. N. T. S. Phan, M. Van Der Sluys and C. W. Jones, Adv. Synth. Catal.,
2006, 348, 609-679.
14. R. Rinaldi and F. Schuth, Energy Environ. Sci., 2009, 2, 610-626.
15. Y. Zheng, Z. Pan and R. Zhang, Int. J. Agric. & Biol. Eng., 2009, 2,
39. A. Ignatyev Igor, V. Doorslaer Charlie, G. N. Mertens Pascal, K.
Binnemans and E. d. Vos Dirk, in Holzforschung, 2012, vol. 66, p.
417.
51-68.
16. Z. H. Zhang and Z. B. K. Zhao, Carbohydr. Res., 2009, 344, 2069-
2072.
40. Z. Zhang and Z. K. Zhao, Carbohydr. Res., 2009, 344, 2069-2072.
41. S.-J. Kim, A. A. Dwiatmoko, J. W. Choi, Y.-W. Suh, D. J. Suh and
M. Oh, Bioresour. Technol., 2010, 101, 8273-8279.
17. A. Wang, C. Li, M. Zheng and T. Zhang, in The Role of Green
Chemistry in Biomass Processing and Conversion, John Wiley &
Sons, Inc., 2012, pp. 313-348.
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