Porous Boron Nitride as a Weak Solid Base Catalyst

Article abstract of DOI:10.1002/cctc.202001435

Porous boron nitride was synthesized by pyrolysis from boric acid and urea mixed in varying molar ratios. The boron nitride prepared had high surface areas ranging from 376 to 647 m² g^?1 with both microporous and mesoporous structures. The sample prepared with a urea-to-boric acid molar ratio of 5 exhibited the highest pore volume with the highest surface area of mesopores. Boron-K edge X-ray absorption fine structure spectroscopy revealed that the surface structure consisted of BN₃ sites along with BN₂O, BNO₂, and BO₃ sites. Fourier transform infrared (FTIR) spectroscopy indicated the formation of amino and hydroxyl groups on the surface. Analysis using color indicator reagents and deuterated chloroform-adsorbed FTIR results indicated that the porous boron nitride had very weak base sites of strength +7.2>H?≥+6.3. Porous boron nitride exhibited a high activity for the nitroaldol reaction with a high selectivity for nitroalkene (>97 %). A good correlation was observed between the catalytic activity of the boron nitride catalysts and their porous structures.

Full text of DOI:10.1002/cctc.202001435

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Porous Boron Nitride as a Weak Solid Base Catalyst
Authors: Shohei Nakamura, Atsushi Takagaki, Motonori Watanabe,
Kanta Yamada, Masaaki Yoshida, and Tatsumi Ishihara
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10.1002/cctc.202001435
ChemCatChem
FULL PAPER
Porous Boron Nitride as a Weak Solid Base Catalyst
Shohei Nakamura,[b] Atsushi Takagaki*[a,c], Motonori Watanabe,[a,b,c] Kanta Yamada,[d] Masaaki
Yoshida,[d,e] and Tatsumi Ishihara*[a]
[a]
Prof. A. Takagaki, Prof. M. Watanabe, Prof. T. Ishihara
Department of Applied Chemistry, Faculty of Engineering
Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
E-mail: atakagak@cstf.kyushu-u.ac.jp, ishihara@cstf.kyushu-u.ac.jp
S. Nakamura, Prof. M. Watanabe, Prof. T. Ishihara
Department of Automotive Science, Graduate School of Integrated Frontier Sciences
Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
Prof. A. Takagaki, Prof. M. Watanabe, Prof. T. Ishihara
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER)
Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
K. Yamada, Prof. M. Yoshida
Graduate School of Sciences and Technology for Innovation,
Yamaguchi University
[b]
[c]
[d]
[e]
Tokiwadai, Ube, Yamaguchi 755-8611, Japan
Prof. M. Yoshida
Blue Energy Center for SGE Technology (BEST)
Yamaguchi University
Tokiwadai, Ube, Yamaguchi 755-8611, Japan
Abstract: Porous boron nitride was synthesized by pyrolysis from
boric acid and urea mixed in varying molar ratios. The boron nitride
prepared had high surface areas ranging from 376 to 647 m2 g-1 with
both microporous and mesoporous structures. The sample prepared
with a urea-to-boric acid molar ratio of 5 exhibited the highest pore
volume with the highest surface area of mesopores. Boron-K edge
X-ray absorption fine structure spectroscopy revealed that the
surface structure consisted of BN3 sites along with BN2O, BNO2, and
BO3 sites. Fourier transform infrared (FTIR) spectroscopy indicated
the formation of amino and hydroxyl groups on the surface. Analysis
using color indicator reagents and deuterated chloroform-adsorbed
FTIR results indicated that the porous boron nitride had very weak
base sites of strength +7.2 > H_ ≥+6.3. Porous boron nitride
exhibited a high activity for the nitroaldol reaction with a high
selectivity for nitroalkene (>97%). A good correlation was observed
between the catalytic activity of the boron nitride catalysts and their
porous structures.
While boron nitride has so far been mainly regarded as an
inert material, it has recently attracted much attention as a novel
heterogeneous catalyst.[3-8] Hexagonal boron nitride and its
nanotubes and nanosheets were found to catalyze oxidative
dehydrogenation of light alkanes to the corresponding alkenes
with excellent selectivity, much superior to that of other catalysts
such as supported vanadium oxide and carbon nanotubes.[3]
Also, boron nitride could activate methane to give CO and C2
products with minor formation of CO2.[4] Moreover, boron nitride
was found to function as
production[5] and CO2 reduction.[6] Currently, boron nitride is
recognized to have variety of interesting catalytic
a photocatalyst for hydrogen
a
functionalities in addition to exhibiting excellent properties as
catalyst support.[9,10]
Here, we synthesized porous boron nitride for use as solid
base catalysts. Porous boron nitride was synthesized by
condensation of boric acid (H3BO3) and urea (CO(NH2)2),
followed by pyrolysis. The prepared materials had high surface
areas with both micro- and mesopores. The boron nitride
catalyst exhibited catalytic activity for the nitroaldol reaction and
selectively yielded nitroalkene, which possesses a high synthetic
potential in organic chemistry, whereas conventional strong
bases such as CaO and Mg-Al hydrotalcite did not. The
physicochemical and solid base properties of the synthesized
boron nitride were characterized through a variety of techniques.
The correlation between the catalytic activities of the boron
nitride catalysts and their other properties was investigated.
Introduction
Acid and base catalysts are essential for the production of fuels
and chemicals in both the established petrochemical industry
and emerging biorefineries. Solid acid and base catalysts are
easily separated from the reaction solution and can be used for
further reactions without neutralization so that the replacement
of liquid acids and bases with their solid counterparts offers
environmental and economic benefits. While many types of solid
acids such as zeolites, metal oxides, and sulfonated ion-
exchange resins have been developed, and some of them are
commercially available,[1] the use of solid bases such as alkaline
earth metal oxides has been limited. This is mainly owing to the
poisoning of the catalyst with water and CO2 and the difficulty in
controlling base-catalyzed reactions in which side reactions
typically occur in series and parallel, resulting in insufficient
selectivity to the desired products.[2] Thus, the development of
solid base catalysts is still required.
Results and Discussion
Porous boron nitride was synthesized as follows. 10 mmol of
boric acid and 50 mmol of urea were dissolved in 100 mL of
water. The solution was then evaporated to dryness. The
precursor powder obtained (ca. 3 g) was transferred to an
alumina boat and heated at 1273 K for 3 h in an NH3 flow (100
cm3 (NTP) min-1). The final product was a white powder.
1
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10.1002/cctc.202001435
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Synthesis was also carried out by using varying molar ratios of
boric acid to urea, namely, 1:2, 1:5, 1:10, and 1:15. The
synthesized boron nitride is denoted as BN(1:n).
Figure 2 shows scanning electron microscopy (SEM)
images of boron nitride samples. Aggregates of very small
particles were observed for BN(1:2) (Fig. 2(a)). With increasing
urea content, the surface morphology changed drastically,
leading to the formation of mesopores. Pores less than 20 nm in
size were observed in BN(1:5) (Fig. 2(b)). Similar morphologies
were obtained in the case of BN(1:10) and BN(1:15) (Figs.
2(c,d)). It seems that the pores in BN(1:10) were larger than
those in BN(1:5). These pores could have been formed as a
result of the evaporation of the excess urea that could not be
condensed with boric acid during synthesis.
Figure 1 shows the X-ray diffraction (XRD) patterns for
boron nitride samples synthesized from boric acid and urea
mixed at different molar ratios. All samples exhibited two
diffraction peaks corresponding to the (002) and (100)
reflections of turbostratic boron nitride (t-BN), which was in good
agreement with previous reports.[11] An increase in the molar
ratio of urea broadened the (002) diffraction peak, indicating
lower crystallinity. Table 1 shows the crystallite sizes calculated
from the (002) and (100) reflections using the Scherrer equation.
The crystallite sizes for BN(1:2) were 1.1 and 2.8 nm from the
(002) and (100) diffractions, respectively. The difference
between the two crystallite sizes is due to the anisotropy of the
layered materials. With increasing urea content, the crystallite
size, as determined from the (002) diffraction, decreased
monotonically from 1.1 to 0.7 nm. The average numbers of
layers were roughly estimated to be 3 for BN(1:2) and 2 for
BN(1:15). The decrease in the thickness of layers was in good
agreement with the results of a previous study.[12]
(a)
(b)
100 nm
100 nm
Table 1 lists the elemental compositions of the boron
nitride samples determined by energy-dispersive X-ray
spectroscopy (EDX). As desired, the synthesized samples
consisted of boron and nitrogen with a small amount of carbon
and oxygen. The elemental compositions of the synthesized
samples were roughly the same except for BN (1:2).
(c)
(d)
100 nm
100 nm
Figure 2. SEM images of boron nitride synthesized from boric acid and urea at
various molar ratios. (a) BN(1:2), (b) BN(1:5), (c) BN(1:10), and (d) BN(1:15).
The porous structure of the synthesized samples was
investigated by nitrogen adsorption measurements. Figure 3
shows nitrogen sorption isotherms and pore size distributions
obtained from Barrett-Joyner-Halenda (BJH) plots. Table 2
shows the BET surface areas and pore volumes of the
synthesized samples. The surface areas of the synthesized BN
samples were significantly higher than that of commercial BN (6
m2 g-1). The BET surface area of BN(1:2) was 376 m2 g-1, and
the surface areas of BN(1:5) and BN(1:10) were even higher:
496 and 647 m2 g-1, respectively. The BET surface area
decreased to 483 m2 g-1 for BN(1:15). The nitrogen sorption
isotherms indicated that the synthesized BN had micropores.
The micropore structure was characterized further by using the
t-plot method. The surface area of micropores (Smicro) was 370
m2 g-1 for BN(1:2), which was close to the BET surface area, 376
m2 g-1, indicating that most of the pores of BN(1:2) were inside
the particles as corroborated by the SEM image (Fig. 2). BN(1:5)
had both micro- and mesopores, resulting in the highest pore
volume (1.24 mL g-1) and highest external surface area (Sext, 160
m2 g-1) among the samples prepared. The pore size distribution
for BN(1:5) clearly showed evidence of mesopores centered at
12 nm, which is in good agreement with the SEM image. The
BN(1:10) sample had the highest surface area of micropores,
which led to the highest total surface area but a lower pore
volume than BN(1:5).
Figure 1. XRD patterns for boron nitride samples synthesized from boric acid
and urea at various molar ratios.
Table 1. Crystallite sizes and elemental compositions of the synthesized boron
nitride.
Sample
Crystallite size /nm[a]
Atomic conc./% [b]
(002)
1.1
(100)
2.8
B
C
N
O
10
5
BN(1:2)
BN(1:5)
BN(1:10)
BN(1:15)
36
45
44
45
14
13
13
13
40
38
35
39
0.9
3.1
0.8
3.0
8
0.7
2.6
4
[a] Calculated by Scherrer equation. [b] From EDX.
2
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10.1002/cctc.202001435
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Figure 5 shows the B-K edge XAFS spectra for the
synthesized boron nitride. The spectra for commercial boron
nitride and boron oxide (B2O3) are shown for reference. The B-K
edge XAFS spectra were measured by the surface-sensitive
total electron yield method. The spectrum of commercial
hexagonal boron nitride had a single peak at 192.0 eV, which is
attributable to the excitation of B1s electrons to π*(BN3),
whereas the spectrum of boron oxide showed a peak at 194.0
eV, ascribed to excitation to π*(BO3).[18-21] For BN(1:5), BN(1:10),
and BN(1:15), in addition to the strong excitation of B1s
electrons to π*(BN3), weak excitations to π*(BN2O), π*(BNO2),
and π*(BO3) were observed.[22,23] This indicated that the surface
of these materials had BN3 sites along with BN2O, BNO2, and
BO3 sites, which was related to the formation of amino and
hydroxyl groups on the surface, as revealed by FTIR
spectroscopy. Note that the spectra were not influenced by the
presence of hydrogen. For example, the BN2O sites could
comprise N-B(N)-OH and NH2-B(N)-OH. In contrast, for BN(1:2),
a strong peak for BO3 sites was observed, indicating that the
surface of BN(1:2) was mostly hydroxylated, even though XRD
indicated a boron nitride structure. The difference of oxygen
contents observed between EDX and B-K edge XAFS is due to
the surface sensitivity of the XAFS spectra. The detection
method used in this measurement probes to a depth of about
0.6 nm from the surface.[24,25]
(a)
(b)
Borid acid : Urea
1 : 5
0.12
800
600
400
200
0
0.10
0.08
0.06
0.04
0.02
0.00
1 : 10
1 : 10
1 : 5
1 : 15
1 : 15
1 : 2
1.0
1 : 2
0.0
0.2
S
0.4
P / P0
0.6
0.8
1
10
Pore size rp/ nm
100
(c)
(d)
700
1.2
1.0
0.8
0.6
0.4
0.2
0.0
micro
600
500
400
300
200
100
0
S
ext
1:2
1:5
1:10
1:15
1:2
1:5
1:10
1:15
Boric acid : Urea
Boric acid : Urea
Figure 3. (a) N2 sorption isotherms, (b) pore size distributions, (c) surface
areas, and (d) pore volumes of the synthesized boron nitride.
Table 2. Surface areas, pore sizes, and pore volumes of the synthesized
boron nitride.
Sample
SBET
/m2 g-1
Smicro
/m2 g-1
Sext
/m2 g-1
Pore volume
/mL g-1
BN(1:2)
BN(1:5)
BN(1:10)
BN(1:15)
376
496
647
483
370
336
556
392
6
0.18
1.24
0.89
0.85
*(BN3)
*(BN2O)
*(BNO )
160
91
*(BO32)
Boric acid : Urea
1 : 15
91
1 : 10
1 : 5
Figure 4 shows the FTIR spectra of the synthesized
samples. The characteristic absorption for boron nitride was
occurred at 1372 and 816 cm-1. The former strong absorption
corresponds to B-N stretching, and the latter to B-N-B
bending.[12-14] All samples produced these bands, indicating the
formation of boron nitride, as confirmed by XRD. In addition, a
broad band around 3400 cm-1 with a shoulder around 3200 cm-1
was observed. These are attributed to O-H stretching vibrations
and N-H stretching vibrations, respectively, indicating the
formation of amino and hydroxyl groups.[12, 15-17]
1 : 2
B2O3
Commercial BN
190
195
200
205
210
Photon energy /eV
Figure 5. B K-edge XAFS spectra for synthesized boron nitride.
Boric acid : Urea
1 : 15
Solid base properties of the synthesized boron nitride
samples were examined using color indicator reagents and
CDCl3-adsorbed FTIR measurements. Two color indicator
reagents, bromocresol purple (pKa = +6.3) and bromothymol
blue (pKa = +7.2), were used. The coloration by bromocresol
purple was observed for all samples, revealing that the
synthesized boron nitride samples had weak base sites with H_
≥ +6.3. When treated with a solution containing bromothymol
blue, BN(1:5) turned green, whereas the other samples turned
yellow, indicating that BN(1:5) had base sites of strength H_ ≥
+7.2 while the other BN samples had very weak base sites of
strength +7.2 > H_ ≥ +6.3. The amounts of base sites, estimated
by titration with benzoic acid, were 0.10, 0.22, 0.21, and 0.12
1 : 10
1 : 5
1 : 2
B-N-B
816
N-H
3200
O-H
3400
B-N
1372
4000
3000
2000
1000
Wavenumber /cm-1
Figure 4. FTIR spectra for synthesized boron nitride.
3
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10.1002/cctc.202001435
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mmol g-1 for BN(1:2), BN (1:5), BN (1:10), and BN (1:15),
respectively.
Nitroaldol
reaction
Michael addition
reaction
The base strength was also determined by using CDCl3-
adsorbed FTIR measurements. Figure 6 shows the difference
FTIR spectrum for CDCl3-adsorbed boron nitride. The C-D
stretching band was observed at 2252 cm-1. It has been reported
that the observed frequency of the C-D band for CDCl3 adsorbed
on silica was 2265 cm-1;[26] the corresponding values were 2253
cm-1 on γ-Al2O3,[27] and 2245 and 2220 cm-1 on MgO.[28] Thus,
BN had weak base sites, stronger than SiO2, comparable to
Al2O3, but much weaker than MgO.
1
2
3
4
1a R = OCH3 , R’ = H
1b R = H , R’ = CH3
1c R = H , R’ = H
1d R = CH3 , R’ = H
1e R = Cl, R’ = H
Scheme 1. Nitroaldol reaction of substituted benzaldehyde and nitromethane,
and the subsequent Michael addition reaction.
Table 3. Results of nitroaldol reaction of nitromethane and p-
methoxybenzaldehyde using synthesized boron nitride.[a]
2252
0.01
Entry
Catalyst
1a
Selectivity /%
Conv. /%
3a
52
97
97
99
3
4a
0
1
2
3
4
5
6
7
8
BN(1:2)
16
83
77
58
26
22
28
43
BN(1:5)
0
2300
2250
Wavenumber /cm-1
2200
BN(1:10)
BN(1:15)
h-BN[b]
1
0
Figure 6. Difference FTIR spectrum of CDCl3-adsorbed boron nitride.
3
CaO[c]
4
12
63
4
The base catalytic activity of boron nitride was determined
through the nitroaldol reaction (Scheme 1). Table 3 lists the
results of the nitroaldol reaction for the synthesized boron nitride.
The synthesized BN showed high activities (entries 1-4), while
commercial BN (h-BN) was inactive for the reaction (entry 5).
Notably, BN(1:5) gave the highest conversion of p-
methoxybenzaldehyde (1a) with excellent selectivity to the
corresponding nitroalkene (3a) (97%) (entry 2). The BN(1:5)
sample was also synthesized in Ar atmosphere instead of NH3,
and used for the reaction. The sample also exhibited a high 1a
conversion of 93% with a selectivity to 3a (71%). BN(1:10) and
BN(1:15) also exhibited similarly high selectivities to the
nitroalkene (97 and 99%) (entries 3 and 4). In contrast, BN(1:2)
showed a lower conversion and selectivity than other boron
nitride catalysts. These results suggest that the surface amino
groups functioned as base sites for the reaction. Conventional
solid bases such as CaO, Mg-Al hydrotalcite, and γ-Al2O3 were
used for comparison. The strong solid bases CaO and
Mg-Al hydrotalcite[d]
20
77
[e]
γ-Al2O3
[a] Reaction conditions: 1a (0.5 mmol), 2a (1.25 mmol), catalyst (50 mg), n-
decane (0.1 mmol), toluene (2 mL), 393 K, 30 h. [b] Commercial boron nitride
(Wako). SBET; 6 m2 g-1. [c] Pretreated at 973 K for 3 h. SBET; 8 m2 g-1. [d] Mg/Al
= 3. Pretreated at 773 K for 3h. SBET; 219 m2 g-1. [e] SBET; 88 m2 g-1.
Figure 7 shows time-course of the reaction over BN(1:5)
and the results of its recycle use. The reaction smoothly
proceeded, indicating that both adsorption of the reactant and
desorption of the products satisfactorily occurred. For the
recyclability test, the catalysts were collected by centrifugation at
3000 rpm, washed with 10 mL of toluene three times, dried
overnight, and reused for further reactions. It was found that the
activity gradually decreased. The 3a yield after 3rd reuse was
58%.
hydrotalcite yielded
a dinitro compound 4a, rather than
(a)
(b)
90
80
70
60
50
40
30
20
10
0
nitroalkene 3a because of the subsequent Michael addition
reaction (entries 6 and 7). The difference between conversion
and sum of two products is probably due to adsorption of the
reactant on the catalyst and degradation of the reactant. A
weaker solid base, γ-Al2O3, could yield nitroalkene 3a, but the
conversion and selectivity were much lower than those for boron
nitride. It can be concluded that weak base sites of boron nitride
were effective for selective formation of nitroalkene. It is reported
that strong basic sites are not necessary for the nitroaldol
reaction because the abstraction of a proton from nitromethane
is easy.[29]
80
60
40
20
0
0
5
10
15
20
25
30
Fresh
1st reuse 2nd reuse 3rd reuse
Run
Reaction time /h
Figure 7. (a) Time course of nitroaldol reaction over BN(1:5) and (b) recycle
use of BN(1:5) Reaction conditions: 1a (0.5 mmol), 2a (1.25 mmol), BN(1:5)
(50 mg), n-decane (0.1 mmol), toluene (2 mL), 393 K, 30 h.
4
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The scope of the reaction was investigated by using
0.3
0.2
0.1
0.0
various substituted benzaldehydes. Table 4 lists the results of
reaction rate of the nitroaldol reaction. It was found that the
substrates with electron-withdrawing groups such as m-methoxy
benzaldehyde (1b) and p-chloro benzaldehyde (1e) exhibited
higher activity than benzaldehyde (1c). This trend is the same as
that over alkaline-cation exchanged zeolite.[30] Surprisingly, it
was found that the substrates with electron-donating groups
including p-methoxy benzaldehyde (1a) and p-tolualdehyde (1d)
also showed higher reaction rate than benzaldehyde (1c). Figure
8 shows the Hammett plot for the nitroaldol reaction over
BN(1:5) catalyst. As the results, the V-shaped plot was observed
over the porous boron nitride catalysts while the plot with a
positive value of slope was obtained over basic zeolites.[30] The
concave upward curve indicates a change in the reaction
mechanism.[31] Scheme 2 shows proposed two different reaction
mechanisms, base mechanism, and acid-base cooperative
mechanism. The base catalyzed mechanism is a general one
which involves a proton abstraction of nitromethane by base
sites, amino groups on the boron nitride in this case, a
nucleophilic attack of the deprotonated nitromethane to
benzaldehyde, and subsequent dehydration to form nitroalkene.
The substrates with electron-withdrawing groups which have
positive values of the Hammett constants would follow this
mechanism because they are susceptible to the nucleophilic
attack, resulting in the positive value of slope in the Hammett
plot.
p-OMe
p-Cl
p-CH3
m-OMe
y = 0.670x +0.011
H
y = -0.894x -0.010
-0.3 -0.2 -0.1
0.0
0.1
0.2
0.3
Hammett constant 
Figure 8. Results of the Hammett plot for the nitroaldol reaction
As for the substrates with electron-donating groups which
showed the negative value of slope in the Hammett plot, the
acid-base cooperative mechanism is proposed. We have
previously investigated a solid base activity of a ball-milled boron
nitride and obtained a negative value of slope for the Hammett
plot.[32] It is considered that electron-donating groups stimulated
the polarization of the carbonyl group of benzaldehyde by the
resonance effect (Scheme 2(c)) and enhanced the interaction
with the hydroxyl group of the catalyst. The benzaldehyde is
subject to the nucleophilic attack by the base sites on the boron
nitride to form an imine intermediate. The imine intermediate
reacts with deprotonated nitromethane, resulting in the formation
of nitroalkene. Originally, this acid-base bifunctional mechanism
was proposed for aminosilica catalysts,[33,34] and the same
mechanism can be adopted for the boron nitride catalyst having
amino- and hydroxyl groups.[30] Owing to the acid-base
bifunctionality, the boron nitride catalyst exhibited higher activity
than γ-Al2O3 though both had comparable basicity.
Table 4. Nitroaldol reaction of nitromethane and various substituted
benzaldehyde using BN(1:5) catalyst [a]
Substrate
Hammett
Reaction rate /mmol g-1 h-1
constant σ
p-Methoxy benzaldehyde (1a)
m-Methoxy benzaldehyde (1b)
Benzaldehyde (1c)
-0.268
0.115
0
0.27
0.20
0.15
0.20
0.22
p-Tolualdehyde (1d)
-0.17
0.227
p-Chloro benzaldehyde (1e)
Scheme 2 Proposed reaction mechanism of nitroaldol reaction. (a) base
[a] Reaction conditions: 1 (0.5 mmol), 2 (1.25 mmol), BN(1:5) (50 mg), n-
mechanism, (b) acid-base cooperative mechanism, and (c) resonance effect.
decane (0.1 mmol), toluene (2 mL), 393 K.
5
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Characterization. The crystal structures of the boron nitride samples
were determined by X-ray diffraction (XRD, RINT-2500HLR+, Rigaku)
with Cu Kα radiation (λ = 0.15418 nm) at a voltage of 40 kV and a current
of 80 mA. Scans were obtained at a rate of 5º min-1 with a step width of
The correlation between the activities of the catalysts and
their characteristics was investigated. Figure 9(a) shows the
conversion of 1a over various boron nitride catalysts. The
conversion increased from BN(1:2) to BN(1:5) and then
decreased. This trend is close to that for the pore volume of the
catalyst, shown in Figure 3(d). A good correlation was obtained
between conversion and pore volume (Figure 9(b)). Having a
high porosity is important for increasing the catalytic activity. In
addition, the correlation between the solid base amount and the
catalytic activity was examined. It was found that the conversion
of 1a increased with increasing the amount of base sites. The
correlation among the amount of base sites, the pore volume,
and the catalytic activity indicates that all boron nitrides tested
have similar active sites within the pores. While the BN(1:5)
showed the stronger base sites than other BN materials, its
strong basicity seems to be not closely related to the catalytic
activity because BN(1:5) and BN(1:10) showed the same
turnover number, 38 for BN(1:5) and 37 for BN(1:10). In addition,
it is considered that a change in the oxygen content was not
directly correlated to the catalytic activity because amino groups
are attributable to the main active base sites.
0.05º for 2θ values from 10º to 80º
.
The surface morphologies of the samples were observed by scanning
electron microscopy (SEM, JSM-7900F, JEOL). The elemental
compositions of the samples were analyzed by energy dispersive X-ray
spectroscopy (EDX, X-MaxN, Oxford). The surface functional groups of
the samples were characterized by Fourier transform infrared
spectroscopy (FTIR, FT/IR-6600, JASCO). The samples were pressed
into pellets with KBr for the measurements. The solid base strength of
the sample was determined by CDCl3-adsorbed FTIR. For the adsorption
measurement, a mercury cadmium telluride (MCT) detector with a
resolution of 4 cm-1 was used. Sample BN(1:5) was pressed into a disk
with a radius of 1.0 cm without KBr and pretreated in the measurement
cell at 473 K in vacuum for 1 h. The sample was cooled to 303 K, and a
spectrum was recorded. CDCl3 gas was introduced to the cell, and the
sample was exposed to saturated CDCl3 gas for 30 min. After CDCl3 was
pumped out for 30 min, a spectrum of the CDCl3-adsorbed sample was
measured. A difference spectrum was obtained by subtracting the
spectrum of the dehydrated sample from the spectrum of the adsorbed
sample.
The surface area was determined by the Brunauer-Emmett-Teller (BET)
method using nitrogen adsorption (BELSORP-max-32-N-VP-CM and
BELLSORP mini-II, Microtrac-BEL). The sample was pretreated by
evacuation at 473 K for 2 h before the measurement. The pore size
distribution of the sample was obtained by analyzing the adsorption
branches of the isotherms through the Barrett, Joyner, and Halenda
(a)
(b)
80
60
40
20
0
80
60
40
20
0
(BJH) method.
A
t-plot was used for micropore analysis.
The B-K edge XAFS spectra were measured at beamline BL7A of the
Photon Factory at the Institute of Materials Structure Science, High-
Energy Accelerator Research Organization (KEK-IMSS-PF). The spectra
were collected by the total electron yield method. Samples were mounted
on carbon tape. All spectra were calibrated with reference to the B2O3
peak at 194.0 eV.
1:2
1:5
1:10
1:15
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Pore volume /mL g-1
Boric acid : Urea
Figure 9. (a) conversion of 1a over boron nitride and correlations between
conversion of 1a and (b) pore volume of boron nitride catalysts.
Nitroaldol reaction. The base catalytic activity of the sample was
determined through the nitroaldol reaction. The nitroaldol reaction was
conducted with the catalyst (50 mg) in 2 mL toluene solution containing
p-methoxybenzaldehyde (60.5 μL, 0.5 mmol), nitromethane (68.0 μL,
1.25 mmol) and n-decane (19.5 μL, 0.1 mmol) as an internal standard.
The reaction was performed at 393 K, and aliquots were taken by syringe
and analyzed by gas chromatography (GC-FID; GC-2014, column DB-
1MS, Shimadzu).
Conclusion
Boron nitride solid base catalysts were synthesized by a
pyrolysis method using boric acid and urea at various molar
ratios under NH3 atmosphere. The samples prepared were
highly porous turbostratic boron nitride and exhibited surface
amino and hydroxyl groups with weak basicity. While BN(1:2),
which had only micropores, gave a moderate activity for the
nitroaldol reaction, BN(1:5), which possessed both micropores
and mesopores, showed high activity with a high selectivity to
nitroalkene, superior to those of conventional solid bases such
as CaO and hydrotalcite. The porous structure of the boron
nitride catalysts correlated strongly with their catalytic activity.
Acknowledgements
This work was supported by
a Grant-in-Aid for Science
Research (B) (No. 18H01785) of JSPS, Japan. A part of this
work was performed at the KEK-IMSS-PF (2017G529 and
2019G674).
Keywords: Nitrides • Nitroaldol reaction • Porous materials •
Solid base catalyst • X-ray absorption spectroscopy
Experimental Section
[1]
[2]
K. Tanabe, W.F. Hölderich, Appl. Catal. A Gen. 1999, 181, 399-434.
Y. Ono, H. Hattori, Solid Base Catalysis, Springer-Verlag, Berlin
Heidelberg, 2011.
Catalyst Preparation. The samples were synthesized by using boric
acid (Kishida, 99.5%) and urea (Wako, 99.0%). 10 mmol of boric acid
and 50 mmol of urea were dissolved in 100 mL of DI water, and the
solution was evaporated to dryness. The obtained precursor powder (ca.
3 g) was transferred to an alumina boat and heated at 1273 K for 3 h in
an NH3 flow (100 cm3 (NTP) min-1). Synthesis was also carried out using
varying molar ratio of boric acid to urea, namely 1:2, 1:5, 1:10 and 1:15.
The synthesized boron nitride is denoted as BN(1:n).
[3]
a) J. T. Grant, C. A. Carrero, F. Goeltl, J. Venegas, P. Mueller, S. P.
Burt, S. E. Specht, W. P. McDermott, A. Chieregato, I. Hermans,
Science 2016, 354, 1570-1573; b) L. Shi, Y. Wang, B. Yan, W. Song. D.
Shao, A.-H. Lu, Chem. Commun. 2018, 54, 10936-10946; c) L. Shi, D.
Wang, W. Song, D. Shao, W.-P. Zhang, A.-H. Lu, ChemCatChem 2017,
9, 1788-1793; d) J.M. Venegas, J.T. Grant, W.P. McDermott, S.P. Burt,
J. Micka, C.A. Carrero, I. Hermans, ChemCatChem 2017, 9, 2118-
6
This article is protected by copyright. All rights reserved.

10.1002/cctc.202001435
ChemCatChem
FULL PAPER
2127;.e) R. Huang, B. Zhang, J. Wang, K.-H. Wu, W. Shi, Y. Zhang, Y.
[34] K. Motokura, M. Tada, Y. Iwasawa, Angew. Chem. Int. Ed. 2008, 47,
Liu, A. Zheng, R. Schlögl, D.S. Su, ChemCatChem 2017, 9, 3293-3297.
Y. Wang, L. Zhao, L. Shi, J. Sheng, W. Zhang, X.-M. Cao, P. Hu, A.-H.
Lu, Catal. Sci. Technol. 2018, 8, 2051-2055.
9230-9235.
[4]
[5]
a) C. Huang, C. Chen, M. Zhang, L. Lin, X. Ye, S. Lin, M. Antonietti, X.
Wang, Nat. Commun. 2015, 6, 7698; b) L. Chen, M. Zhou, Z. Luo, M.
Wakeel, A.M. Asiri, X. Wang, Appl. Catal. B Environ. 2019, 241, 246-
255.
[6]
a) R. Shankar, M. Sachs, L. Francàs, D. Lubert-Perquel, G. Kerherve,
A. Regoutz, C. Petit, J. Mater. Chem. A 2019, 7, 23931-23940; b) Y.
Cao, R. Zhang, T. Zhou, S. Jin, J. Huang, J. Ye, Z. Huang, F. Wang, Y.
Zhou, ACS Appl. Mater. Interfaces 2020, 12, 9935-9943.
L. Dai, Y. Wei, X. Xu, P. Wu, M. Zhang, C. Wang, H. Li, Q. Zhang, H. Li,
W. Zhu, ChamCatChem 2020, 12, 1734-1742.
[7]
[8]
a) X. Li, B. Lin, H. Li, Q. Yu, Y. Ge, X. Jin, X. Liu, Y. Zhou, J. Xiao, Appl.
Catal. B Environ. 2018, 239, 254-259; c) J. Zhao, B. Lin, Y. Zhu, Y.
Zhou, H. Liu, Catal. Sci. Technol. 2018, 8, 5900-5905; c) A. Takagaki,
Catal. Today 2020, 352, 279-286.
[9]
W. Zhu, Z. Wu, G.S. Foo, X. Gao, M. Zhou, B. Liu, G.M. Veith, P. Wu,
K.L. Browning, H.N. Lee, H. Li, S. Dai, H. Zhu, Nature Commun. 2017,
8, 15291.
[10] P. Wu, Y. Wu, L. Chen. J. He, M. Hua, F. Zhu, X. Chu, J. Xiong, M. He,
W. Zhu, H. Li, Chem. Eng. J. 2020, 380, 122526.
[11] a) A. Nag, N. Raidongia, K.P.S.S. Hembram, R. Datta, U.V. Waghmare,
C.N.R. Rao, ACS Nano 2010, 4, 1539-1544; b) S. Marchesini, C.M.
McGilvery, J. Bailey, C. Petit, ACS Nano 2017, 11, 10003-10011.
[12] X. Li, X. Wang, J. Zhang, N. Hanagata, X. Wang, Q. Weng, A. Ito, Y.
Bando, D. Golberg, Nat. Commun. 2017, 8, 13936.
[13] J. Li, K. Gao, J. Mater. Chem. 2003, 13, 628-630.
[14] M.I. Baraton, T. Merle, P. Quintard, V. Lorenzelli, Langmuir 1993, 9,
1486-1491.
[15] T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y.K. Gun’ko, J.
Coleman, J. Am. Chem. Soc. 2012, 134, 18758-18771.
[16] C. Huang, C. Chen, X. Ye, W, Ye, J. Hu, C. Xu, X, Qiu, J. Mater. Chem.
A 2013, 1, 12192-12197.
[17] Y. Chao, J. Zhang, H. Li, P. Wu, X. Li, H. Chang, J. He, H. Wu, H. Li, W.
Zhu, Chem. Eng. J. 2020, 387, 124138
[18] I. Jiménez, A.F. Jankowski, L.J. Terminello, D.G.J. Sutherland, J.A.
Carlisle, G.L. Doll, W.M. Tong, D.K. Shuh, F.J. Himpsel, Phys. Rev. B
1997, 55, 12025.
[19] M.E. Fleet, S. Muthupari, J. Non-Cryst. Solids 1999, 255, 233-241.
[20] A.M. Love, B. Thomas, S.E. Specht, M.P. Hanrahan, J.M. Venegas,
S.P. Burt, J.T. Grant, M.C. Cendejas, W.P. McDermott, A.J. Rossini, I.
Hermans, J. Am. Chem. Soc. 2019, 141, 182-190.
[21] W. Zhu, X. Gao, Q. Li, H. Li, Y. Chao, M. Li, S.M. Mahurin, H. Li, H. Zhu,
S. Dai, Angew. Chem. Int. Ed. 2016, 55, 10766-10770.
[22] S.P. Huber, E. Gullikson, R.W.E. van de Kruijs, F. Bijkerk, D.
Prendergast, Phys. Rev. B 2015, 92, 245310.
[23] Q. Weng, D.G. Kvashnin, X. Wang, O. Cretu, Y. Yang, M. Zhou, C.
Zhang, D.-M. Tang, R.B. Sorokin, Y. Bando, D. Golberg, Adv. Mater.
2017, 29, 1700695.
[24] M. Kasrai, M.E. Fleet, S. Muthupari, D. Li, G.M. Bancroft, Phys. Chem.
Minerals 1998, 25, 268-272.
[25] D. Li, G.M. Bancroft, M.E. Fleet, P.C. Hess, Z.F. Yin, Am. Mineral. 1995,
80, 873-877.
[26] E.P. Paukshtis, N.S. Kostsarenko L.G. Karakchev, React. Kinet. Catal.
Lett. 1979, 12, 315-319.
[27] P. Barteau, M.-A. Kellens, B. Delmon, J. Chem. Soc., Faraday Trans.
1991, 87, 1425-1431.
[28] S. Huber, H. Knözinger, J. Mol. Catal. A Chem. 1999, 141, 117-127.
[29] K. Akutsu, H. Kabashima, T. Seki, H. Hattori, Appl. Catal. A 2003, 247,
65-74.
[30] R. Devi, R. Borah, R.C. Deka, Appl. Catal. A 2012, 433-434, 122-127.
[31] J.O. Schreck, J. Chem. Edu. 1971, 48, 103-107.
[32] S. Torii, K. Jimura, S. Hayashi, R. Kikuchi, A. Takagaki, J. Catal. 2017,
355, 176-184.
[33] J.D. Bass, A. Solovyov, A.J. Pascall, A. Katz, J. Am. Chem. Soc. 2006,
128, 3737-3747.
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Entry for the Table of Contents
Boron nitride is recognized to have a variety of interesting catalytic functionalities. Here, we synthesized highly porous turbostratic
boron nitride for use as a solid base catalyst. The boron nitride had very weak base sites and exhibited a high activity for the
nitroaldol reaction with a high selectivity to nitroalkene. The porous structure of the boron nitride catalysts correlated strongly with
their catalytic activity.
8
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