Efficient Epimerization of Aldoses Using Layered Niobium Molybdates

Article abstract of DOI:10.1002/cssc.201501093

Both non-acidic LiNbMoO₆ and strongly acidic HNbMoO₆ efficiently catalyze the epimerization of sugars including glucose, mannose, xylose, and arabinose in water. The reactions over these oxides reached almost equilibrium within a few hours where yields of corresponding epimers from glucose, xylose, and arabinose were 24-29 %. The layered mixed oxides functioned as heterogeneous catalysts and could be reused without loss of activity, whereas bulk molybdenum oxide MoO₃ was completely dissolved during the reaction. A ¹³C substitution experiment showed that the reaction proceeds through a 1,2-rearrangement mechanism. The surface Mo octahedra were responsible for the activity. The layered HNbMoO₆ could also afford mannose from cellobiose through hydrolysis and successive epimerization.

Full text of DOI:10.1002/cssc.201501093

DOI: 10.1002/cssc.201501093
Communications
Efficient Epimerization of Aldoses Using Layered Niobium
Molybdates
Atsushi Takagaki,* Shogo Furusato, Ryuji Kikuchi, and S. Ted Oyama[a]
Both non-acidic LiNbMoO6 and strongly acidic HNbMoO6 effi-
ciently catalyze the epimerization of sugars including glucose,
mannose, xylose, and arabinose in water. The reactions over
these oxides reached almost equilibrium within a few hours
where yields of corresponding epimers from glucose, xylose,
and arabinose were 24–29%. The layered mixed oxides func-
tioned as heterogeneous catalysts and could be reused with-
out loss of activity, whereas bulk molybdenum oxide MoO3
was completely dissolved during the reaction. A 13C substitu-
tion experiment showed that the reaction proceeds through
a 1,2-rearrangement mechanism. The surface Mo octahedra
were responsible for the activity. The layered HNbMoO6 could
also afford mannose from cellobiose through hydrolysis and
successive epimerization.
glucose to d-mannose, d-xylose to d-lyxose, and l-arabinose
to l-ribose. These transformations can also be attained
through sequential isomerizations, such as glucose-to-fructose-
to-mannose;[20] however, the selectivity to epimers is usually
very low owing to the involvement of intermediate isomers
and equilibrium limitations.
This carbon rearrangement reaction is known as the Bílik re-
action, first discovered over homogeneous Mo-based catalysts
in acidic aqueous solution where a binuclear molybdate–glu-
cose complex was proposed as the transition state
(Scheme 1).[21,22] Homogeneous molybdate catalysts including
molybdic acid and heptamolybdate can catalyze the glucose–
mannose epimerization as well as the xylose–lyxose epimeriza-
tion in acidic aqueous solutions.[23] Recently, a polymer-sup-
ported Mo catalyst[24] and Mo-based polyoxometalates[25] were
The selective transformation of
sugars into desired products
using heterogeneous catalysts
has recently received much at-
tention because the catalysts
have high reaction rates and can
be readily separated from the
products for reuse.[1–5] To date,
a variety of solid acid and base
catalysts have been developed
for the selective transformation
of glucose into valuable chemi-
Scheme 1. Reaction mechanism of Mo-catalyzed epimerization of glucose. Adapted from Ref. [22].
cal intermediates such as levoglucosan,[6] 5-hydroxymethylfur-
fural,[7–9] and lactic acid[10–12] through dehydration, isomeriza-
tion, the retro-aldol reaction, and rehydration. The catalysts
also proved effective in the transformation of xylose[13–15] and
arabinose.[16,17]
found to exhibit high activity for epimerization of carbohy-
drates in water according to the same reaction mechanism.[22]
Another example of an efficient heterogeneous catalyst for
epimerization of carbohydrates is Sn-beta zeolite.[26] The selec-
tive formation of epimers from glucose, xylose, and arabinose
was achieved in sodium tetraborate-containing aqueous,[26,27]
or methanol solutions,[28] although aqueous isomerization into
fructose proceeded preferentially without any additives. The
reaction mechanism of epimerization over Sn-beta zeolite[29] as
well as metal oxalates including V, Mo, and W[30] was investigat-
ed using density functional theory. The selective epimerization
in the presence of sodium tetraborate is ascribed to the forma-
tion of a glucose–borate complex that inhibits the competitive
isomerization.
Epimerization involves a carbon skeleton rearrangement re-
action, which consists of cleavage of the bond between car-
bons C2 and C3 and subsequent formation of a new bond be-
tween carbons C1 and C3 (Scheme 1). Epimerization is of sig-
nificance for the synthesis of rare sugars and pharmaceuti-
cals.[18,19] The reaction allows the direct transformation of d-
[a] Dr. A. Takagaki, S. Furusato, Prof. Dr. R. Kikuchi, Prof. Dr. S. T. Oyama
Department of Chemical Systems Engineering
School of Engineering
Herein, epimerization of sugars in water was examined using
two crystalline layered niobium–molybdenum mixed oxides,
LiNbMoO6 and HNbMoO6. This is the first example of epimeri-
zation of sugars in water using heterogeneous mixed oxides.
These NbMo oxides consist of layers formed by randomly
placed MO6 (M=Nb and Mo) octahedra with Li+ or H+ in the
interlayers (Figure 1). The protonated niobium molybdate can
The University of Tokyo
7-3-1 Hongo
Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: atakagak@chemsys.t.u-tokyo.ac.jp
Supporting Information and ORCID(s) from the author(s) for this article
are available on the WWW under http://dx.doi.org/10.1002/
cssc.201501093.
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Table 1. Transformation of monosaccharides over a variety of catalysts in
water.[a]
Entry Catalyst
Substrate Conv. [%]
Selectivity [%]
Epimer
Isomer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
HNbMoO6
LiNbMoO6
Glucose 33, 30,[b] 32[c] 88, 84,[b] 88[c]
0
0
0
0
0
0
0
0
0
0
0
0
0
Mannose
Xylose
Arabinose
Glucose
67
31
31[d]
26
19[e]
65[f]
33[d]
30[f]
32[e]
16[e]
96
>99
76[d]
91
98[e]
>99[f]
>99[d]
64[f]
86[e]
96[e]
0
Figure 1. Structure of layered niobium molybdate.
Mannose
Xylose
Arabinose
be obtained by proton-exchange of LiNbMoO6 where crystal
structure remains unchanged. The protonated HNbMoO6 is
highly acidic, and can function as a solid acid catalyst.[31] The
layered HNbMoO6 is easily prepared by proton exchange of
the precursor LiNbMoO6, which is obtained by calcination of
a stoichiometric mixture of Li2CO3, Nb2O5, and MoO3 at 853 K
(See the Supporting Information for details).
Li0.85Nb0.85Mo1.15O6 Glucose
Li1.15Nb1.15Mo0.85O6
LiNbWO6
Amberlyst-15
Mg-Al hydrotalci-
te[g]
<1
<1
0
21
4
72
15
16
17
18
19
Nb2O5·nH2O
SnO2
TiO2
WO3
MoO3
2
1
1
<1
49
0
0
0
0
55
42
0
0
0
0
Figure 2 shows the time courses of transformation of glu-
cose and mannose over layered LiNbMoO6 in water. The con-
centration of glucose decreased with the increase of mannose,
[a] Reaction conditions: substrate (300 mg, 1.67 mmol), catalyst (10 mg),
water (3 mL), 393 K, 1.5 h. Equilibrium conversions of glucose, mannose,
xylose, and arabinose are theoretically calculated to be 30, 70, 33, and
31%, respectively.[35] [b] 2nd run. [c] 3rd run. [d] 3 h. [e] Glucose (50 mg,
0.28 mmol), catalyst (50 mg), water (3 mL), 353 K, 0.5 h. [f] 4.5 h. [g] Mg/
Al=3.
molybdate, LiNbMoO6 also gave mannose as mentioned above
(entry 5). No fructose was formed over either HNbMoO6 or
LiNbMoO6. This indicates that the selective epimerization was
catalyzed over these niobium molybdates regardless of the
solid acidity. A sulfonated polystyrene-based ion exchange
resin, Amberlyst-15, which is a representative Brønsted acid
catalyst, did not convert glucose in water (entry 13). An anionic
clay, Mg–Al hydrotalcite [Mg6Al2(OH)16CO3·nH2O], which is
a common Brønsted base catalyst, selectively gave fructose
through isomerization (entry 14).[32] An amorphous niobium
oxide hydrate, Nb2O5·nH2O, which was recently reported as
a water-tolerant Lewis acid catalyst,[33] also afforded fructose
(entry 15). Other metal oxides such as TiO2, WO3, and SnO2
were inactive (entries 16–19). In contrast, molybdenum oxide
could convert glucose and yielded mannose, although the
oxide was completely dissolved during the reaction (entry 18).
From these results, it can be deduced that Lewis acids and
Brønsted bases do not produce mannose in water. As will be
discussed, the reaction also cannot be attributed to the Brønst-
ed acid sites within the interlayers of HNbMoO6. Rather the
surface Mo octahedra in the niobium molybdates are the likely
active sites for the reaction. The niobium tungstate, LiNbWO6,
that is isostructural to LiNbMoO6 showed no activity (entry 12).
The niobium molybdates with different amounts of Mo,
Li0.85Nb0.85Mo1.15O6, LiNbMoO6, and Li1.15Nb1.15Mo0.85O6, were syn-
thesized[34] and tested for the glucose epimerization (entries 6,
10, and 11). It was found that the activity increased with in-
crease of the Mo content.
Figure 2. Time courses of epimerization of (a) glucose and (b) mannose over
layered LiNbMoO6. Reaction conditions: glucose (50 mg, 0.28 mmol), catalyst
(50 mg), H2O (3 mL), 373 K.
and after 0.5 h, the concentrations of each sugar reached a pla-
teau (Figure 2a). A similar trend was observed for the transfor-
mation of mannose, in which mannose was rapidly converted
to glucose (Figure 2b). For both cases, the final concentrations
of glucose and mannose were almost the same, indicating that
the reaction was reversible and proceeded to completion in
the presence of LiNbMoO6. In addition, no formation of fruc-
tose was observed. Thus, layered LiNbMoO6 was found to cata-
lyze the glucose–mannose epimerization efficiently.
Table 1 lists the results of the transformation of glucose over
a variety of catalysts in water. The reaction was carried out in
3 mL of 1.67m sugar solution with a slight amount of catalyst
(10 mg) at 393 K. The protonated niobium molybdate,
HNbMoO6, selectively afforded mannose (Table 1, entry 1). The
yield of mannose was 29%, which is almost the equilibrium
yield. The catalyst could be reused at least three times without
loss of activity (entry 1). No apparent leaching of Mo was con-
firmed by inductively coupled plasma atomic emission spec-
trometry (ICP-AES) (<0.01 ppm). The lithium form of niobium
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The transformation of other monosaccharides, such as man-
nose, xylose, and arabinose, was also investigated using
HNbMoO6 and LiNbMoO6 (entries 2–4, 7–9). Both niobium mo-
lybdates afforded the corresponding epimers selectively. The
conversions of glucose, xylose, and arabinose over HNbMoO6
were around 30% with remarkable selectivity toward the cor-
responding epimers. The theoretical equilibrium product ratios
from Gibbs free energy calculations were estimated to be
70:30, 67:33, and 69:31 for glucose/mannose, xylose/lyxose,
and arabinose/ribose epimerizations, respectively,[35] indicating
that these reactions reached equilibrium with HNbMoO6. The
epimerization reactions over LiNbMoO6 also reached equilibri-
um by prolonging the reaction time from 0.5 h to 3 or 4.5 h
(entries 7–9).
of HNbMoO6 as shown in a previous study.[37] In contrast, the
(001) peak remained unchanged for LiNbMoO6. Considering
that both layered oxides catalyzed the epimerization in a similar
manner, the active sites of LiNbMoO6 for the reaction are at-
tributable to the Mo octahedra at the surface, not the Mo octa-
hedra within the interlayer. The unit cell parameters of LiNb-
MoO6 were reported as a=b=0.4785 nm and c=0.925 nm.[38]
One Mo atom is located in the ab-plane in the unit cell, and
the surface density of Mo octahedra is calculated to be
7.6 mmolmÀ2. The BET surface area of LiNbMoO6 used in this
study was 5 m2 gÀ1. Thus, the amount of surface Mo octahedra
in 50 mg of LiNbMoO6 was 1.9 mmol, much lower than
170 mmol calculated by the molecular formula. This value was
used for calculation of the turnover frequency (TOF).
The manner in which the epimerization of glucose over
HNbMoO6 proceeded was further elucidated by using 13C nu-
clear magnetic resonance (NMR) spectroscopy (Figure 3) using
a method previously reported.[22] Using d-(1-13C)glucose and
D2O as the solvent, the shift in position of the C1 carbon was
followed. Before the reaction, two signals were observed at
95.8 and 92.0 ppm that corresponded to the C1 carbon of the
b-pyranose and a-pyranose configurations of glucose.[36] After
the reaction two new signals appeared at 71.1 and 70.6 ppm,
which were assigned to the C2 carbon of the b-pyranose and
a-pyranose configurations of mannose. No signals for the C1
carbon of mannose (94.6 and 95.1 ppm) were observed. These
results demonstrated that a 1,2-rearrangement occurred for
epimerization of glucose over HNbMoO6 in water.
Figure 4a shows the reaction rate versus the initial concen-
tration of glucose for the epimerization over LiNbMoO6. The re-
action rate gradually levelled off with increase of initial concen-
The XRD patterns of LiNbMoO6 and HNbMoO6 before and
after immersion in the glucose-containing aqueous solutions
are shown in Figure S1 in the Supporting Information. A shift
of the (001) peak, which corresponds to expansion of the basal
spacing, was observed for HNbMoO6 after the immersion,
which is a result of intercalation of glucose into the interlayers
Figure 4. (a) Reaction rate versus initial glucose concentrations using LiNb-
MoO6 at 373 K. Reaction conditions: glucose (0.28–2.8 mmol), LiNbMoO6
(50 mg), water (3 mL), 373 K. (b) Arrhenius plots for epimerization of glucose
over LiNbMoO6.
tration of glucose. This behavior is typical of Langmuir–Hin-
shelwood kinetics, r=(S)kKC0/(1+KC0) where r is the reaction
rate (molLÀ1 sÀ1), (S) is the concentration of active sites
(molLÀ1), k is the rate constant (sÀ1), K is the adsorption equi-
librium constant (LmolÀ1), and C0 is the initial glucose concen-
tration (molLÀ1). On the basis of mole of active sites, the reac-
tion rate r’ [molsÀ1 (mol-active sites)À1]=r (molLÀ1 sÀ1)”(3”
10À3 L)/(1.9”10À6 mol) is also shown in Figure S2. Based on the
kinetics, the rate constants over LiNbMoO6 at 353, 373, and
393 K were estimated to be 0.09, 0.27, and 1.1 sÀ1. These
values are identical to TOFs at saturation coverage, KC0 @1,
since TOF=r/(S) is equal to k. The TOFs were higher than
those over Mo-based polyoxometalates, such as H3PMo12O40,
Sn0.75PMo12O40, and Ag3PMo12O40.[25] Figure 4b also shows Ar-
rhenius plots for the epimerization. The apparent activation
energy, Ea was 73 kJmolÀ1, which is much lower than that of
a homogeneous molybdate catalyst (126 kJmolÀ1 [23]
and Mo-
)
based polyoxometalates (96–99 kJmolÀ1),[25] and comparable to
that of Sn-beta zeolite in methanol (70 kJmolÀ1).[28] The reac-
tion mechanism is similar to Scheme 1 because Mo is crucial
for the activity. However, MoÀOÀNb bonding is dominant for
the layered NbMo oxides. Thus, one possible explanation for
the lower activation energy of the layered NbMo oxide than
Figure 3. 13C NMR spectra taken before and after epimerization of d-(1-
13C)glucose using HNbMoO6 in D2O. Reaction conditions: d-(1-13C)glucose
(300 mg, 1.67 mmol), HNbMoO6 (10 mg), D2O (3 mL), 393 K, 1.5 h.
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Amberlyst-15.[a]
Catalyst
Acid amount Conv. Selectivity [%]
TOF
[mmolgÀ1
]
[%]
Glu[b] Man[c] HMF[d] [hÀ1
]
HNbMoO6
LiNbMoO6
Amberlyst-15
1.9
0
4.8
32
0
6
47
0
>99
29
0
0
8
0
0
0.25
0
0.03
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This work was supported by a Grant-in-Aid for Young Scientists
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Keywords: biomass
catalysis · layered compounds · renewable resources
·
carbohydrates
·
heterogeneous
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Published online on October 23, 2015
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