A bifunctional cerium phosphate catalyst for chemoselective acetalization

Article abstract of DOI:10.1039/c6sc05642c

Acid-base solid catalysts synthesized with structurally controlled uniform active sites can lead to unique catalysis. In this study, a CePO₄ catalyst was synthesized using a hydrothermal method and found to exhibit high catalytic performance for the chemoselective acetalization of 5-hydroxymethylfurfural with alcohols, in sharp contrast to other homogeneous and heterogeneous acid and/or base catalysts. In the presence of CePO₄, various combinations of carbonyl compounds and alcohols are efficiently converted into the corresponding acetal derivatives in good to excellent yields. Mechanistic studies show that CePO₄ most likely acts as a bifunctional catalyst through the interaction of uniform Lewis acid and weak base sites with 5-hydroxymethylfurfural and alcohol molecules, respectively, which results in high catalytic performance.

Full text of DOI:10.1039/c6sc05642c

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A Bifunctional Cerium Phosphate Catalyst for Chemoselective
Acetalization
Shunsuke Kanai,a Ippei Nagahara,a Yusuke Kita,a Keigo Kamataa and Michikazu Hara*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
Acid‐base solid catalysts synthesized with structurally controlled uniform active sites can lead to unique catalysis. In this
study, a CePO4 catalyst was synthesized by the hydrothermal method and found to exhibit high catalytic performance for
the chemoselective acetalization of 5‐hydroxymethylfurfural with alcohols, in sharp contrast to other homogeneous and
heterogeneous acid and/or base catalysts. In the presence of CePO4, various combinations of carbonyl compounds and
alcohols are efficiently converted into the corresponding acetal derivatives in good to excellent yields. Mechanistic studies
show that CePO4 most likely acts as a bifunctional catalyst through the interaction of uniform Lewis acid and weak base sites
with 5‐hydroxymethylfurfural and alcohol molecules, respectively, which results in high catalytic performance.
DOI: 10.1039/x0xx00000x
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results in atom‐efficient reactions such as the chemical fixation
of CO2, regioselective N‐alkylation of indoles, and
chemoselective acylation of alcohols. On the other hand, rare
Introduction
Synergistic and cooperative activation by two or more
catalytically active sites with acid, base, and redox properties
allows high catalytic activity and specific selectivity.1 In
particular, acid‐base bifunctionality has received much
attention because such concepts are widely applied to catalyses
related to hydrocarbon conversion, atom‐efficient functional
group transformation, tandem reaction, and asymmetric
synthesis.2 In the fields of heterogeneous catalysis, the acid‐
base properties of metal oxide‐based materials have been
extensively studied, and various effective simple and mixed
oxide catalysts have been reported.3 However, difficulty in the
construction of uniform electrically and structurally controlled
acid‐base sites often leads to a problem where the fine‐tuning
of the catalyst structure and the reactivity are restrained. While
the modification of oxide surfaces with organic acids and/or
bases is a powerful method,4 the susceptibility of the organic
parts to oxidative/thermal degradation has limited the
usefulness of such catalysts. Therefore, the design and
development of new high‐performance all‐inorganic
heterogeneous acid‐base catalysts remains a strongly desired
and challenging subject of research.
earth (RE) metal species act as Lewis acid catalysts for various
carbon‐carbon bond forming reactions through the activation
of carbonyl compounds.6 Against this background, we
anticipated that RE orthophosphates, REPO4, would be good
candidates as bifunctional acid‐base catalysts that can work in
concert to promote electrophilicity and nucleophilicity in
reactive partners. In this communication, we report the highly
chemoselective acetalization of 5‐hydroxymethylfurfural (1a),
which has alcohol and aldehyde functionalities,7 with alcohols
using a monoclinic CePO4 catalyst synthesized by the
hydrothermal method (Fig. 1(a)). Compound 1a is a versatile
carbonyl compound with sensitive function groups. For
reactions of 1a with alcohols in the presence of acid catalysts,8,9
ethers or a complex mixture of products are typically obtained
due to the presence of Brønsted acid‐sensitive hydroxyl groups
in 1a 8
. The present system has the following significant
advantages: (i) high yields and chemoselectivity toward acetals,
even for substrates with hydroxyl groups, (ii) applicability to
various combinations of substrates and larger‐scale syntheses,
and (iii) reusability as a heterogeneous catalyst system. While
metal phosphates and related materials have been extensively
We have recently reported unique base catalysis with
oxoanions including [WO4]2– and [PO4]3–.5 Despite their weaker
basicities than those of inorganic and organic strong bases, their
specific activation of nucleophiles such as alcohols and amines
investigated for the conversion of carbohydrates into 1a 10
, acid‐
base catalysis over CePO4 has not been reported to date11 and
the present bifunctionality can lead to the chemoselective
acetalization of carbonyl compounds containing sensitive
functional groups, such as 1a
.
a. Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo
Institute of Technology, Nagatsuta‐cho 4259, Midori‐ku, Yokohama 226‐8503
Japan. E‐mail: hara.m.ae@m.titech.ac.jp
Results and discussion
Synthesis and Characterization of CePO4
b. Advanced Low Carbon Technology Research and Development Program (ALCA),
Japan Science and Technology Agency (JST), 4‐1‐8 Honcho, Kawaguchi 332‐0012
Japan.
Monoclinic CePO4 was synthesized through the hydrothermal
reaction of Ce(NO3)3 and (NH4)2HPO4 at 180 °C, followed by
†
Electronic Supplementary Informaꢀon (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Fig. 1 (a) Structure of CePO4. Green, gray, and red spheres represent Ce, P, and O atoms, respectively. (b) XRD patterns for CePO4
(upper) and monoclinic CePO4 (lower, ICSD79748). (c) XPS Ce 3d spectrum and (d) SEM image of CePO4.
calcination at 900 °C (see details in ESI). Fig. 1(b) shows the at 1445 cm−1 is assigned to pyridine species coordinated with
powder X‐ray diffraction (XRD) pattern measured for CePO4, Lewis acid sites, while no band due to pyridinium ions bonded
which is in good agreement with that reported for the to Brønsted acid sites was observed at ca. 1540 cm−1. The
monoclinic CePO4 structure, in which a Ce3+ ion connects seven amounts of Lewis acid sites on CePO4 were estimated from the
tetrahedral PO43– groups [space group P21/n].12 Impurity phases intensities of the band 1445 cm−1 to be 0.096 mmol g−1. The
of other cerium and phosphorus oxides were not observed. The difference IR spectrum for CePO4 with adsorbed CHCl3 is shown
infrared (IR) spectrum of CePO4 is shown in Fig. S1. Asymmetric in Fig. 2(b). The red‐shift of the original C–H stretching mode of
stretching vibrations of the PO43− groups were split into bands the CHCl3 molecule (from 3034 cm–1 to 3008 cm–1) indicates the
at 1091, 1062, 1028, 994, 956 cm–1 due to a decrease in the presence of base sites on the surface.16 In addition, a new broad
3–
symmetry of PO4 from Td to C1.13 The bands in the range of shoulder band appeared at 1250 cm−1, which was assigned as
500–700 cm–1 are assigned to the bending of P–O links in δ(ClC−H) for CHCl3 molecules due to interaction of the acidic
distorted PO43− tetrahedra, and these band positions are similar hydrogen and chlorine atoms with the basic oxygen and Lewis
to those previously reported for monoclinic CePO4.13 Elemental acid sites, respectively.16 Thus, the base sites on CePO4 could be
analysis of bulk CePO4 using energy dispersive X‐ray located in close proximity to the Lewis acid sites, in agreement
spectroscopy (EDX) revealed that the molar ratio of Ce:P is 1:1. with the structure of CePO4.
The valence state of surface Ce was investigated using X‐ray
photoelectron spectroscopy (XPS) (Fig. 1(c)). The Ce 3d3/2 5/2
,
Catalytic acetalization of 1a with methanol
spectra are composed of two multiplets (v and u), which
correspond to the spin‐orbit split 3d5/2 and 3d3/2 core holes.14
The Ce 3d spectrum of CePO4 exhibits four peaks with binding
energies of 904.9, 901.0, 806.5, and 883.0 eV, which correspond
to the u’, u0, v’, and v0 peaks, respectively, and are in good
agreement with the reported Ce 3d spectra for Ce(III) oxides.15
The specific surface area of CePO4 calculated from a Brunauer‐
Emmett‐Teller (BET) plot of the N2 adsorption isotherm (77 K)
was up to 37 m2 g–1. Fig. 1(d) shows a scanning electron
microscopy (SEM) image of CePO4 with rod‐like shaped particles
100–500 nm long and 20–50 nm wide.
The reaction of 1a with methanol was examined first in the
presence of various catalysts that have been reported as
effective for acetalization,17 and the results are summarized in
Table 1. Three products of 5‐(dimethoxymethyl)‐2‐
furanmethanol (2a), 5‐methoxymethylfurfural (3a), and 2‐
(dimethoxymethyl)‐5‐(methoxymethyl)furan (4a) were mainly
formed. The reaction did not proceed in the absence of a
catalyst (entry 27). Among the catalysts tested, CePO4 exhibited
the highest activity for the acetalization of 1a to give 2a in 78%
yield (entry 1). In this case, the selectivity toward 2a reached
96% without the formation of 3a or 4a. In the presence of
homogeneous Brønsted acid catalysts (H2SO4, p‐toluenesulfonic
acid (TsOH), and H3PW12O40) and Lewis acid catalysts (scandium
trifluoromethanesulfonate (Sc(OTf)3) and Ce(OTf)3), only 3a
and/or 4a were obtained in low to moderate yields (entries 2–
The acid properties of CePO4 were evaluated by IR
spectroscopy for a sample with adsorbed pyridine as a probe
base (see details in ESI).3,10,16 Differential IR spectra of CePO4
with adsorbed pyridine are shown in Figs. 2(a) and S2. The band
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DOI: 10.1039/C6SC05642C
1699
1445
(a)
(c)
N
O(H)
O
O
O
O
P
Abs 0.2
Ce
Abs 0.2
O
O
O
O
CePO4
1700
1600
1500
1400
)
1300
1800
1700
Wavenumber (cm–1
1600
1500
Wavenumber (cm–1
)
1220
(b)
(d)
2952
2849
Cl
3008
Cl
C
H
Cl
Abs
0.05
O(H)
O
O
O
P
Ce
O
O
Abs
0.02
O
Abs
0.05
O
O
CePO4
1250
4000
3500
3000
2500
3100 3050 3000 2950 130012751250122512001175
Wavenumber (cm–1 Wavenumber (cm–1
Wavenumber (cm–1
)
)
)
Fig. 2 Difference IR spectra for (a) pyridine‐, (b) chloroform‐, (c) acetone‐, and (d) methanol‐adsorbed CePO4 at 25 °C.
Table 1 Effect of catalysts on the reaction of 1a with methanola –6). The reaction of 1a with methanol in the presence of these
homogeneous acid catalysts was carried out by reducing the
catalyst amounts to the surface Ce content (i.e., 9.6 μmol) with
Lewis acid sites measured by pyridine‐IR (Table S1). While the
Entry
Catalyst
Conv. (%)
Yield (%)
conversions of these homogeneous acid catalysts were higher
than that of CePO4, 2a was not formed under the reaction
conditions. In addition, no product was obtained with a
homogeneous base catalysts of K3PO4, K2HPO4, and KH2PO4
2a
78
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
31
1
9
<1
3
<1
3
1
<1
9
1
20
39
55
2
3a
<1
24
54
4
4a
<1
2
2
<1
5
1
CePO4
H2SO4
TsOH
H3PW12O40
Sc(OTf)3
Ce(OTf)3
K3PO4
K2HPO4
KH2PO4
Ce(NO3)3·6H2O
(NH4)2HPO4
Ce(NO3)3·6H2O + (NH4)2HPO4
Nb2O5
SiO2
ZrO2
CeO2
Al2O3
MgO
TiO2
81
>99
>99
>99
>99
74
81
<1
1
68
43
85
38
12
9
2b
3b
4b
(entries 7–9). Thus, homogeneous acid or base catalysts
5b
49
27
<1
<1
<1
12
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
2
6b
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
15
43
21
13
43
23
<1
themselves are not deemed as effective for the chemoselective
acetalization of 1a. Acetalization over Nb2O5 was less effective
than CePO4, and other metal oxide catalysts including SiO2, ZrO2,
CeO2, Al2O3, MgO, TiO2, and SnO2 were almost inactive (entries
13–20). Typical solid acid catalysts such as sulfated zirconia,
sulfonated carbon, Nafion, mordenite, and montmorillonite
7
8
9
10b
11b
12b
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
gave complex mixtures of 2a, 3a, and 4a (entries 21–26). The
catalyst precursors of Ce(NO3)3, (NH4)2HPO4, and a mixture of
Ce(NO3)3 and (NH4)2HPO4 were not effective for acetalization
(entries 10–12), which indicates that CePO4 plays an important
role in the acetalization reaction.18
5
3
3
2
SnO2
2
To verify whether the observed catalysis is derived from
solid CePO4 or from leached cerium or phosphorus species, the
reaction of 1a with methanol was conducted under the
conditions described in entry 1 of Table 1, and CePO4 was
removed from the reaction mixture by hot filtration at ca. 30%
conversion of 1a (at t = 15 min). The filtrate was then heated
again under the same reaction conditions. In this case, no
further production of 2a was observed, as shown in Fig. 3. No
leaching of cerium or phosphorus species into the filtrate was
observed using inductively coupled plasma atomic emission
spectroscopy (ICP‐AES, detection limits for Ce and P atoms of ca.
1 and 3 ppb, respectively). Therefore, there was no contribution
sulfated zirconia
sulfonated carbon
Nafion NR-50
Nafion SAC-13
mordenite
montmorillonite K10
Blank
>99
83
95
90
86
85
2
15
42
2
9
4
<1
a Reaction conditions: Catalyst (0.1 g), 1a (1.0 mmol), methanol (5 mL), reflux, 1
h. Conversion and yield were determined by GC analysis. Conversion (%) =
converted 1a (mol)/initial 1a (mol) × 100. Yield (%) = product (mol)/initial 1a (mol)
× 100. b Catalyst (0.43 mmol; i.e., equivalent to the Ce content in CePO4 (0.1 g)).
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to the observed catalysis from cerium or phosphorus species stretching modes, respectively. The appearance of such broad
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leached into the reaction solution, and the nature of the bands and the band positions of v(CH3) iDnOdiIc: a10t.e10t3h9a/Ct 6mSCe0th56a4n2oCl
observed catalysis is confirmed as truly heterogeneous.19 The is adsorbed molecularly on CePO4 via hydrogen bonds, which is
used CePO4 catalyst could be readily recovered from the consistent with previous reports for the non‐dissociative
reaction mixture by simple filtration (96% recovery). The adsorption of methanol on metal oxides.23 On the other hand,
recovered CePO4 catalyst could then be reused without a the IR spectrum for methanol adsorbed on CeO2 had bands at
significant decrease in the yield of 2a or the selectivity: 78% 2911 and 2805 cm–1 that were assigned to vas(CH3) and vs(CH3)
yield of 2a at 81% conversion (fresh) and 78% yield of 2a at 80% of methoxide species, respectively (Fig. S4). Therefore, CePO4
conversion (reused). There was no significant difference in the most likely acts as a bifunctional catalyst through interaction of
XRD patterns of the fresh and reused CePO4 catalysts, which the uniform Lewis acid sites and weak base sites with 1a and
indicates the high durability of CePO4 (Fig. S3).
alcohol molecules, respectively, which results in highly efficient
and chemoselective acetalization.24 The present acetalization of
1a with methanol possibly proceeds as follows (Fig. 4). First, the
activation of both 1a and methanol by CePO4 facilitates
nucleophilic attack of the OH group in methanol on the carbon
atom of the carbonyl group in 1a to give the corresponding
hemiacetal derivative. Further reaction of the hemiacetal with
methanol then occurs, most likely with the assistance of the
CePO4 catalyst, to give the corresponding acetal derivative.
90
80
70
60
50
Removal of catalyst
40
O
CH3
OH
+
HO
O
O
30
20
10
0
O
O
R
(= CHO R)
O(H)
O
H
OH
O
O
O
P
Ce
CH3OH
O
O
O
O(H)
O
O
O
O
P
Ce
O
CePO4
O
O
CH3OH
H2O
O
O
CH3OH
H2O
O
R
CHO
CePO4
+
CH3OH
0
5
10
15
20
25
O
O
Time (h)
O
OH
Fig. 3 Effect of CePO4 removal on the acetalization of 1a with
methanol. Without removal of CePO4 (●), and where the arrow
indicates the point of CePO4 removal (□). Reaction conditions:
CePO4 (0.1 g), 1a (1.0 mmol), methanol (5 mL), reflux.
Fig. 4 Proposed reaction mechanism for the CePO4‐catalyzed
acetalization of 1a with methanol.
Substrate scope for the CePO4‐catalyzed acetalization
To investigate the effectiveness of the bifunctional
properties of CePO4, CePO4‐catalyzed acetalization was
explored with various substrates. In the presence of CePO4,
various combinations of carbonyl compounds and alcohols
were efficiently converted into the corresponding acetal
Reaction mechanism for the CePO4‐catalyzed acetalization
While it has been reported that other metal oxides (e.g.,
CeO2) can function as effective acid‐base catalysts,3,19d only
CePO4 exhibited high catalytic activity and chemoselectivity for
acetalization. Thus, the activation mode of substrates with
CePO4 and CeO2 was confirmed by IR measurements for
samples with adsorbed acetone and methanol. As shown in Fig.
2(c), one strong C=O stretching band of acetone adsorbed on
CePO4 was observed at lower wavenumber (1699 cm–1) than
that of acetone in the gas phase (1731 cm–1).20 In addition, the
IR spectrum of acetone adsorbed on CeO2 exhibited a shoulder
at 1700 cm–1 and a strong band at 1673 cm–1 due to acetone
molecules coordinated to different types of Lewis acid sites, and
absorptions at 1627 cm–1 and 1570–1550 cm–1 assignable to
condensated species (Fig. S4).20 These results indicate
interaction between the carbonyl oxygen of the ketone and the
uniform Lewis acid sites on CePO4 without the promotion of
aldol condensation.21,22
d
e
r
i
v
a
t
i
v
e
s
70
60
50
40
30
20
10
0
5
10
15
Fig. 2(d) shows the IR spectrum of methanol adsorbed on
CePO4. In the v(O–H) region, negative OH bands were observed
with the appearance of broad bands between 3000 and 3500
cm–1. In addition, the IR spectrum showed bands at 2952 and
2849 cm–1 that were assigned to asymmetric and symmetric CH3
Pyridine added (μmol)
Fig. 5 Yields of 2a against amounts of pyridine added. Reaction
conditions: CePO4 (0.1 g), 1a (1.0 mmol), methanol (5 mL), reflux,
30 min.
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Table 2. Acetalization of carbonyl compounds with alcohols strongly inhibited the reaction of 1a with methanol (Fig. 5). The
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a
DOI: 10.1039/C6SC05642C
yield of 2a decreased with an increase in the small amount of
catalyzed by CePO4
pyridine added (3–12 μmol; ca. 0.3–1.3 equivalents with respect
to the Lewis acid sites on CePO4). The nitrogen atom of pyridine
would strongly coordinate to the cerium metal center, which
would inhibit the reaction. The reactions of benzaldehydes with
electron‐donating and electron‐withdrawing para‐substituents
Entry Carbonyl compound
Alcohol
CH3OH
Time (h)
1
Product (Yield (%))
O
O
2a
(78)
O
O
1
OH
O
O
OH
OH
1a
O
2b
3b
4
1
2
6
6
6
6
6
6
HO
OH
2a'
(80)
1a
1a
O
(
1e–1g) proceeded to afford the corresponding dimethyl acetals
O
OH OH
O
2a''
in high yields (entries 7–9). Even a bulkier aldehyde of 1‐
naphthaldehyde (1h) could be efficiently acetalized when the
reaction time was prolonged to 20 h (entry 10). The
acetalization of cinnamaldehyde (1i) with methanol proceeded
smoothly without influence on the C=C double bond (entry 11).
Not only were aromatic and α,β‐unsaturated aldehydes
O
OH
(79)
O
O
O
S
O
2b
CH3OH
CH3OH
(79)
1b
1c
O
O
O
S
N
2c (70)
O
O
5
converted but also 3‐phenylpropionaldehyde
(
1j
)
and
not detected
O
6
1d
1e
CH3OH
CH3OH
CH3OH
cyclohexanecarboxaldehyde (1k) were efficiently converted
into the corresponding acetals in high yields (entries 12 and 13).
Furthermore, the present system could effectively catalyze
aliphatic and aromatic ketones with ethylene glycol.
Cyclohexanone (1l) was quantitatively converted into 2,2‐
O
7
2e (91)
O
O
O
1f
8
2f
(75)
O
OMe
CN
pentamethylene‐1,3‐dioxolane (2l), and acetophenone (1m
)
OMe
2g (89)
CN
O
O
gave the ketal (2m) in moderate yield (entries 14 and 15). Even
in the presence of hydroxyl groups in the substrate (5‐hydroxy‐
2‐adamantanone (1n)), the corresponding ketal (2n) was
obtained in good yield (entries 16).
9
1g
CH3OH
CH3OH
O
O
2h(91)
1h
10
20
O
O
The present catalytic system was applicable to a gram‐scale
reaction of 1a (10.5 mmol scale) with methanol and 1.46 g of
analytically pure 2a could be isolated (eqn (1)). In this case, the
turnover number (TON) based on surface Lewis acid sites
reached 177 and the corresponding turnover frequency (TOF)
was 44 h–1. In addition, CePO4 efficiently catalyzed the gram‐
scale regioselective acetalization of acetone (1o) with glycerol
into an industrially important chemical, 2,2‐dimethyl‐1,3‐
dioxolan‐5‐ol (solketal (2o)).25,65 Solketal has been used as a
highly soluble additive to increase the octane number of fuel,
and as such many catalyst systems have been reported.25,26
O
O
1i
CH3OH
CH3OH
11
12
6
O
2i
(82)
O
2j
O
1j
O
20
(95)
O
O
O
O
2k
(76)
1k
CH3OH
13
14c
15
16
20
6
O
O
HO
OH
OH
2l
(99)
1l
O
CePO4 exhibited high regioselectivity (2o/2o’ = 98/2) and 1.29 g
O
O
HO
2m
2n
1m
(46)
(85)
6
of analytically pure 2o was successfully isolated (eqn (2)), while
the condensation of glycerol with 1o under acidic conditions
sometimes affords a mixture of five‐ and six‐membered acetals
O
O
O
6
1n
CH3OH
(
2o and 2o’, respectively).25
HO
HO
a
Reaction conditions: CePO4 (0.1 g),
1 (1.0 mmol), alcohol (5
mL), reflux. Yields were isolated yields. b Yield determined using
nuclear magnetic resonance spectroscopy (NMR). c 120 °C.
in good to excellent yields (Table 2). The acetalization of 1a with
methanol and with diols such as ethylene glycol and 1,3‐
propanediol proceeded selectively (entries 1–3). CePO4
effectively catalyzed the acetalization of other aldehydes
containing heteroatoms, such as furfural
(1b) and 2‐
thiophenecarboxaldehyde 1c), into the corresponding
(
dimethyl acetals, while the acetalization of 2‐
pyridinecarboxaldehyde (1d) did not proceed (entries 4–6). The
catalytic reactivity of CePO4 in the presence of pyridine was
investigated. It was confirmed that the presence of pyridine
Conclusions
In conclusion, CePO4 efficiently catalyzes the acetalization of
various aryl and aliphatic carbonyl compounds containing
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2015, 330, 558–568; (e) W. Hao, W. Li, X. Tang, XV.iZewenArgti,cYle.OSnulinne,
S. Liu, and L. Lin, Green Chem., 2016, 1D8O, 1I:0108.010–3190/8C86.SC05642C
There are only two examples of beta zeolite‐based catalysts
for chemoselective acetalization of HMF with alcohols: (a) K.
S. Arias, S. I. Al‐Resayes, M. J. Climent, A. Corma, and S. Iborra,
hydroxyl groups, C=C bonds, and heteroatoms with alcohols.
This study suggests the development of bifunctional solid
catalysts with uniform active sites is of particular importance.
This approach is a promising strategy for the development of
highly efficient heterogeneously‐catalyzed reactions through
the non‐dissociative activation of both electrophiles and
nucleophiles under very mild conditions.
9
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This work was supported in part by a Kakenhi Grant‐in‐Aid (No.
15K13802) from the Japan Society for the Promotion of Science
(JSPS), the ALCA and CREST programs of the Japan Science and
Technology Agency (JST), the Novel Cheap and Abundant
Materials for Catalytic Biomass Conversion (NOVACAM)
program of JST, and the European Commission Directorate‐
General for Research and Innovation.
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structure because of the stability.22 The amounts surface Ce
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View Article Online
DOI: 10.1039/C6SC05642C
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