Utilization of hexagonal boron nitride as a solid acid–base bifunctional catalyst

Article abstract of DOI:10.1016/j.jcat.2017.09.013

This work explores the use of hexagonal boron nitride (h-BN), a graphite-like compound, as a novel catalyst with base and acid functionalities. For use as a solid catalyst, the layered structure of h-BN was disrupted by ball-milling, exposing boron and ni

Full text of DOI:10.1016/j.jcat.2017.09.013

Journal of Catalysis 355 (2017) 176–184
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Utilization of hexagonal boron nitride as a solid acid–base bifunctional
catalyst
a
b
b
a
a,
⇑
Shusaku Torii , Keiko Jimura , Shigenobu Hayashi , Ryuji Kikuchi , Atsushi Takagaki
a
Department of Chemical Systems Engineering, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Research Institute for Material and Chemical Measurement, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki
05-8565, Japan
b
3
a r t i c l e i n f o
a b s t r a c t
Article history:
This work explores the use of hexagonal boron nitride (h-BN), a graphite-like compound, as a novel cat-
alyst with base and acid functionalities. For use as a solid catalyst, the layered structure of h-BN was dis-
rupted by ball-milling, exposing boron and nitrogen edge sites as well as increasing the surface area from
Received 6 July 2017
Revised 4 September 2017
Accepted 15 September 2017
2
À1
3
to ca. 400 m g . Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and pro-
1
ton magic-angle spinning nuclear magnetic resonance spectroscopy ( H MAS NMR) indicated simultane-
ous and adjacent formation of amino and hydroxyl groups by milling, which function as Brønsted base
Keywords:
and acid sites, respectively. Analysis using color indicator reagents and pyrrole-adsorbed 1H MAS NMR
Solid acid–base catalyst
Hexagonal boron nitride
Ball milling
Nitroaldol reaction
Knoevenagel condensation
results revealed that the ball-milled h-BN had basic sites of strength +9.3 > H
À
ꢀ +7.2, comparable to
3
1
those of KY zeolite. Measurements of P MAS NMR of adsorbed trimethylphosphine oxide indicated that
the ball-milled h-BN had weak acid sites, comparable to those in HY zeolite. Despite its weak basicity, the
ball-milled h-BN showed high activity and selectivity toward b-nitroalkenes for the nitroaldol reaction
(Henry reaction) and the Knoevenagel condensation, whereas nontreated h-BN did not show activity.
The nitroaldol reaction was considered to proceed in two steps: the abstraction of a proton from nitro-
methane by the amino group and the formation of an imine followed by a nucleophilic attack of the
deprotonated nitromethane. Kinetic isotope effect experiments using
D
-substituted nitromethane
revealed that the first step was the rate-determining step. Several nitroaldol reactions using a variety
of monosubstituted benzaldehydes indicated that electron-donating groups enhanced the activity, sug-
gesting that the formation of adjacent base and acid sites is responsible for it. This study shows the high
catalytic activity of BN, a solid catalyst with moderate basicity and weak acidity.
Ó 2017 Elsevier Inc. All rights reserved.
1
. Introduction
Design of well-defined catalytic active sites and understanding
For heterogeneous acid–base catalysts, amine-tethered silica
materials such as bulk silica [5], mesoporous silica [6–8], and
silica-alumina [9] have been extensively studied as efficient
acid–base cooperative catalysts that catalyze carbon–carbon bond
formation reactions including aldol, nitroaldol, and Knoevenagel
condensations and the Michael addition. For aminosilica materials,
the coexistence of weak acid sites (e.g., silanols) with base sites
(alkylamines) in spatially appropriate positions is a key to realizing
high catalytic activity for these reactions. The neighboring weak
acid sites are important because they help to stabilize the carbonyl
groups of the ketone and aldehyde species, which efficiently pro-
motes the condensation reactions. To maximize the cooperativity,
there is an optimal distance between acid and base sites, which can
be controlled by adequate concentration of acid and base sites [10],
use of aminoalkyl silanes with appropriate alkyl linker lengths
of their catalytic behavior are of significance for both homogenous
and heterogeneous catalysts. The concept of frustrated Lewis pairs
in which Lewis acid and base sites are rationally positioned in one
molecule without neutralization is an excellent example of the
application of homogeneous acid–base cooperative catalysts for
the activation of small molecules, including hydrogen, alkynes,
and carbon dioxide [1–3]. Another good example is the molecular
catalyst proline, which is an amino acid consisting of a secondary
amine-containing pyrrolidine with an attached carboxylic acid
group [4]. The acid and base sites are appropriately positioned
and efficiently catalyze asymmetric aldol reactions.
[
11], and adjustment of pore size for mesoporous silica [8]. The
⇑
Corresponding author.
E-mail address: atakagak@chemsys.t.u-tokyo.ac.jp (A. Takagaki).
spatial positioning of the acid–base pair with high precision is
https://doi.org/10.1016/j.jcat.2017.09.013
021-9517/Ó 2017 Elsevier Inc. All rights reserved.
0

S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
177
necessary, but the active pairs formed on silica are average struc-
tures because of the amorphous nature of the silica, in which a
variety of surface silanols including isolated, vicinal, and geminal
silanols with slightly different bond lengths are present [12,13].
An alternative method of developing acid–base pairs on silica is
the use of strained siloxane bridges as a source of active pairs.
Opening of siloxane bridges by base molecules simultaneously
affords acid–base sites [14,15]. One example is ammonia treatment
at low temperature of a mesoporous silica that was pretreated at
high temperature [14]. The high-temperature calcination led to
formation of strained siloxane bridges, SiAO–Si. Ammonia disso-
ciatively adsorbed onto the siloxane bridges by the reaction –
(BELSORP-mini II, Microtrac-BEL). Samples were pretreated by
evacuation at 200 °C for 3 h prior to the measurements.
The crystal structure of the catalysts was determined by X-ray
diffraction (XRD) (RINT-2700, Rigaku with CuKa radiation) at a
voltage of 40 kV and a current of 100 mA with 2h values from
20° to 60°. Measurements were made on samples without
pretreatment.
Fourier transform infrared spectroscopy (FTIR) (FT/IR-6100,
JASCO) measurements were performed to identify functional
groups on the catalysts. Samples without pretreatment were
pressed into pellets with KBr for the measurements.
X-ray photoelectron spectroscopy (XPS) (JPS-9200, JEOL with
SiAO–Si– + NH
3
? -Si–NH
2
+ –Si–OH. The thus formed acid–base
MgKa radiation) measurements were carried out to investigate
pairs were well-defined and functioned as cooperative catalysts
for the Knoevenagel condensation. Very recently, it was reported
that mesoporous silica SBA15, which mostly has strained reactive
siloxane rings, reacted with aniline to produce N-phenylsilana
mine–silanol acid–base pairs, which were also effective for the
Knoevenagel condensation. The SBA-15 was pretreated at very
high temperature (1100 °C) under vacuum before reaction with
aniline [15].
the oxidation state of elements and their compositions over the
catalyst surface. The binding energies in each measurement were
referenced to the core level of the C1s peak (284.8 eV), and all
obtained peaks were fitted by Gaussian curves. The samples were
not pretreated.
Titrations using color indicator reagents were carried out to
evaluate the base strength and base amounts of the catalysts. A
quantity of 50 mg of catalyst was added to 1 mL of toluene solution
It is desirable to design such well-defined acid–base pairs for
other materials composed of different elements. Utilization of the
crystal structure of inorganic compounds is a promising approach.
In this study, hexagonal boron nitride (h-BN) was investigated as a
new bifunctional catalyst. h-BN is a layered compound isostruc-
tural with graphite and has been widely studied for diverse appli-
cations outside of catalysis, such as electronic and optical devices
using heterostructures linked by van der Waals forces [16,17],
coating materials with high-temperature oxidation resistance
containing 0.4 mg of bromothymol blue (pK
thalein (pK = 9.3) for 1 day. The base amounts of the catalyst were
determined by titration using 0.01 M benzoic acid in toluene.
a
= 7.2) or phenolph-
a
1
Proton magic-angle spinning nuclear magnetic resonance ( H
MAS NMR) measurements were conducted to quantify proton-
containing functional groups (hydroxyl and amino groups). Mea-
surements were carried out as previously reported [27]. The 1
H
MAS NMR spectra were recorded with a Bruker Avance III HD
600WD spectrometer operating at a frequency of 600.39 MHz. A
Bruker MAS probehead was used with a zirconia rotor of outer
diameter 4 mm. All measurements were performed at room tem-
[
[
18], adsorbents for organic compounds useful in water cleaning
19], and materials for hydrogen storage [20]. Unlike graphite, BN
1
has unshared electron pairs localized on the nitrogen atoms,
resulting in a polarized nature. Owing to strong chemical bonding
between the atoms and physical bonding between the layers, BN is
chemically and thermally stable, electrically insulating, and
mechanically robust. However, very recently, h-BN was found to
catalyze oxidative dehydrogenation of alkanes [21,22], and hydro-
genation [23]. The present study focuses on the chemically polar-
ized nature of h-BN. The nitrogen and boron composing h-BN are
expected to function as base and acid sites, respectively, under
appropriate treatment. A top-down ball-milling method was cho-
sen to disrupt the structure of bulk h-BN [24–26], resulting in
exposure of the edge sites of the planar h-BN. This study reveals
that amino and hydroxyl groups were simultaneously formed at
adjacent positions on the h-BN surface using the simple ball-
milling method, and these groups functioned as efficient coopera-
tive acid–base sites for the nitroaldol reaction.
perature with a spinning rate of 10 kHz. The H spectra were mea-
sured with an ordinary single pulse sequence. The flip angle of the
pulse and the recycle delay were
p
/2 and 3 s, respectively. We did
3
1
not measure the P spin–lattice relaxation times of the present
3
1
samples. The P spin–lattice relaxation times are 6.3 and 1.7 s
31
for crystalline and hydrated TMPO, respectively [28]. The P spins
are relaxed mainly by the rotation of the methyl group. We did not
quantitate the adsorbed TMPO, and thus full recovery was not nec-
essary. To improve the efficiency of the signal accumulation, we set
1
the recycle delay to 3 s. The H chemical shift was expressed with
respect to neat tetramethylsilane (TMS). Experimentally, sec-
ondary standard compounds were used, such as adamantane
1
(1.85 ppm at a spinning rate of 8 kHz). The H content in the sam-
ples was quantitated by comparing the integrated signal intensities
of the samples with that of adamantane.
1
The solid base properties of the catalyst were investigated by H
MAS NMR using pyrrole (Wako Pure Chemical Industries, Ltd.) as a
probe molecule. In a typical adsorption experiment, an appropriate
volume of pyrrole (2–10 lL, 0.03–0.14 mmol) was injected into a
2
. Experimental
glass tube containing h-BN (0.20 g) under nitrogen. Samples were
heated to 100 °C for 3 h to ensure homogeneous distribution of
the probe molecules. Samples were carefully transferred into a zir-
conia rotor in nitrogen atmosphere and capped tightly to avoid
moisture.
2
.1. Preparation of ball-milled h-BN
A quantity of 0.8 g of h-BN (Wako Pure Chemical Industries,
Ltd.) was ball-milled at 400 rpm for 6–24 h using a planetary ball
mill (Pulverisette 7, Fritsch, zirconia vessel with six zirconia balls
The solid acid properties of the catalysts were examined by 31
P
(
diameter 10 mm)). Ball milling was conducted for 12–48 cycles,
MAS NMR using trimethylphosphine oxide (Wako Pure Chemical
Industries, Ltd) as a probe molecule. The NMR spectra were mea-
sured at room temperature using a Bruker Avance 400 spectrome-
ter at a Larmor frequency of 161.98 MHz. A single-pulse sequence
was employed with high-power proton decoupling at a sample
spinning rate of 8 kHz. The flip angle of the pulse and the recycle
in which each cycle was carried out for 30 min with 5 min inter-
vals. The rotation direction was reversed each cycle. The samples
prepared were denoted as h-BN bm6–24 h, respectively.
2
.2. Characterization
3
1
delay were
erenced to 85% H
ondary reference material at 1.33 ppm. The samples with adsorbed
p
/4 and 3 s, respectively. The P chemical shift was ref-
The specific surface area of the catalysts was evaluated by the
3
PO at 0.0 ppm, with (NH HPO used as a sec-
4
4
)
2
4
Brunauer–Emmett–Teller (BET) method using nitrogen adsorption

1
78
S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
TMPO were prepared by dehydration at 250 °C for 1 h under vac-
uum followed by immersion in a dichloromethane solution con-
taining TMPO at room temperature for 1 day in a glove bag filled
with N . An amount of 0.06 mmol of TMPO was exposed to 0.30
2
g of the BN sample. After evacuation to remove the dichloro-
methane solvent, the sample was packed in a rotor housed in a
Increase of both micropores and mesopores was found for the
ball-milled samples. The variations in surface area were mostly
influenced by emergence of micropores, and those in pore volume
by that of mesopores. It seems that both surface area and pore vol-
ume increased by exfoliation and successive disruption of bulk
structure. Further ball-milling treatment decreased the surface
2
À1
À1
glove box under N
2
.
area to 155 m
g
and the pore volume to 0.33 mL g for h-BN
bm24h, likely due to agglomeration [32]. It should be noted that
the reproducibility of the procedure was confirmed. For example,
2
.3. Nitroaldol reaction
the samples of h-BN bm12 h were prepared three times. The BET
surface areas of three samples were 404, 417, and 399 m2
g
À1
.
A typical reaction was conducted using 50 mg of catalyst in 2
mL of toluene solution with p-methoxybenzaldehyde (60.5 L,
.5 mmol), nitromethane (68.0 L, 1.25 mmol), and n-decane
19.5 L, 0.1 mmol) as an internal standard. The reaction was per-
Other physical properties determined from XRD and FTIR were
almost the same.
l
0
(
l
Fig. S1 in the Supplementary Material shows XRD patterns for
pristine h-BN and h-BN bm12h (see Supplementary information).
Both samples showed strong peaks centered at 26.6° (2h), corre-
sponding to the (0 0 2) interlayer reflection of h-BN along with
peaks attributed to in-plane structures. Ball milling resulted in
weakening of all peaks, indicating that the bulk structure of h-BN
was disrupted. Peaks corresponding to the layer structure ((0 0 2
and (0 0 4)) preferentially weakened compared with those of the
in-plane structure ((1 0 0), (1 0 1) and (1 0 2)) (Table S1). In addi-
tion, the (0 0 2) peak position remained unchanged, indicating that
the interlayer distance was maintained after ball milling.
Fig. 2 shows FTIR spectra for pristine h-BN and ball-milled h-BN
l
formed at 100 °C, and aliquots were taken by syringe and analyzed
by gas chromatography (GC-FID; GC-2014, Column DB-1MS, Shi-
madzu). n-Butylamine (Kanto Chemical Co., Inc.) and boric acid
(
Wako Pure Chemical Industries, Ltd.) were used for comparison.
The Hammett equation was used to analyze the results using
different substrates for the nitroaldol reaction [29,30]. The reaction
was conducted using monosubstituted benzaldehyde (0.50 mmol)
and nitromethane (1.25 mmol) as substrates under the same con-
ditions as above. The monosubstituted benzaldehydes used were
p-methoxylbenzaldehyde (Wako Pure Chemical Industries, Ltd.),
p-tolualdehyde (Tokyo Chemical Industry Co., Ltd.), benzaldehyde
(
h-BN bm12h). The IR spectrum of pristine h-BN exhibited only lat-
(Wako Pure Chemical Industries, Ltd.), pAchlorobenzaldehyde
(Tokyo Chemical Industry Co., Ltd.), and m-methoxybenzaldehyde
(Wako Pure Chemical Industries, Ltd.). The primary kinetic isotope
tice vibration modes derived from covalent bonds between nitro-
gen and boron atoms (Fig. 2, bottom). A strong absorption band
À1
centered at 1378 cm is assigned to an in-plane B–N bond stretch-
effect (KIE) was carried out using p-methoxybenzaldehyde with
nitromethane as well as d-substituted nitromethane (CD CN)
31] (see the Supplementary Material).
À1
ing vibration. A sharp peak centered at 816 cm is characteristic
3
of an out-of-plane B–N–B bending vibration [33,34]. The IR spec-
trum of the ball-milled h-BN sample is shown at the top. The B–
N in-plane vibrational mode broadened, suggesting a substantial
change in lattice vibrations due to ball milling. In contrast, the
out-of-plane B–N–B bending mode remained constant at 816
[
2
.4. Knoevenagel condensation
A typical reaction was conducted using about 50 mg of catalyst
in 2 mL of toluene with aldehyde (0.5 mmol), malononitrile (33 mg,
.5 mmol, Sigma-Aldrich, Co.), and n-decane (19.5 L, 0.1 mmol).
À1
cm , indicating retention of the specific honeycomb structure of
h-BN. However, a decrement of the absorbance of B–N–B bending
mode was observed from that of the B–N in-plane vibrational
mode, indicating that B–N–B bonds in ball-milled h-BN were
cleaved by the ball-milling treatment. A significant difference
between before and after ball-milling was the appearance of bands
0
l
The reaction was performed at room temperature, and aliquots
were taken by syringe and analyzed by GC using n-decane as an
internal standard. Aldehydes used were p-methoxybenzaldehyde,
benzaldehyde, p-nitrobenzaldehyde, furfural (Tokyo Chemical
Industry Co., Ltd.), propionaldehyde (Wako Pure Chemical Indus-
tries, Ltd.), and pivalaldehyde (Wako Pure Chemical Industries,
Ltd.).
À1
centered at 3400 and 3200 cm , which can be assigned to O–H
and N–H stretching bands, respectively [35]. This clearly indicates
formation of hydroxyl and amino groups [36].
From the results of the FTIR measurements, the surface struc-
ture of ball-milled h-BN is suggested to be as shown in Scheme 1,
which involves cleavage of B–N–B bonds and simultaneous forma-
3
. Results and discussion
2
tion of B–OH and –NH groups. The cleavage of B–N–B bonds is
3
.1. Structure and physicochemical properties of ball-milled h-BN
expected to proceed by reaction with moisture in air. This oxida-
tive cleavage of B–N–B bonds is in good agreement with the FTIR
results previously reported [34].
Table 1 shows BET surface areas and pore volumes of pristine
and ball-milled h-BN samples. Ball milling increased the BET sur-
Measurements of XPS spectra were performed to further inves-
tigate the chemical states of B, N, and O in the h-BN catalysts. XPS
survey spectra of pristine h-BN and h-BN bm12h are shown in
Fig. S2. The O1s peak of the samples was located at a binding
energy of around 533 eV. In the spectrum of h-BN bm12h, a large
increase in the signal intensity of the O1s peak was observed,
whereas no significant changes in peak position or signal intensity
were observed for N1s and B1s peaks. C1s detected was attributed
to surface contamination. In addition, pristine h-BN has no oxygen
2
À1
face area of h-BN from 3 to 227 m
g
for h-BN bm6h and 404
2
À1
m
g
for h-BN bm12h. Ball milling also increased pore volumes
À1
À1
from 0.02 to 0.46 mL g for h-BN bm6h and 0.50 mL g for h-
BN bm12h. Fig. 1 shows nitrogen sorption isotherms and pore size
distributions obtained from Barrett–Joyner–Halenda plots.
Table 1
BET surface areas and pore volumes of pristine and ball-milled h-BN.
1
species in its structure and no hydroxyl groups, as confirmed by H
À1
À1
Sample
S
BET/m2
g
Pore volume/mL g
MAS NMR measurements (vide infra); hence the O1s peak detected
in the pristine h-BN can also be attributed to surface
contamination.
Fig. 3 shows core-level spectra of B1s and N1s of h-BN samples
before and after ball milling. The XPS spectrum of pristine h-BN
h-BN
3
0.02
0.46
0.50
0.33
h-BN bm6h
h-BN bm12h
h-BN bm24h
227
404
155

S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
179
(
A) 3
(B) 0.16
00
50
00
50
00
bm12h
0
0
0
.14
.12
.10
2
2
1
1
bm12h
bm6h
0.08
bm6h
0
0
0
0
.06
.04
bm24h
5
0
.02 bm24h
BN
.00
BN
1.0
0
0
.0
0.2
0.4
0.6
0.8
1
10
P/P0
Pore size r /nm
p
2
Fig. 1. N sorption isotherms and pore size distributions of pristine and ball-milled h-BN.
ture, it is expected that it will show single peaks for each element.
Analysis of the B/N atomic ratio indicated an elemental formula of
BN0.97 for pristine h-BN.
3
400
3
200
8
16
For all the ball-milled h-BN samples, the shape of the B1s spec-
tra became asymmetric with a shoulder at higher binding energy
(192.9 eV), indicating the appearance of oxidized boron on the sur-
face. The main peaks of the ball-milled samples (h-BN bm6h,
bm12h, and bm24h) were centered at 191.0, 191.1, and 191.1 eV,
respectively, corresponding to that of pristine h-BN at 191 eV. A
new peak, which appeared at 192.9 eV, for all ball-milled samples
1
378
h-BN bm12h
h-BN
0
.5
3
+
could be assigned to fully oxidized boron (B ) on the edge sites,
because this peak position was in good agreement with the previ-
ously reported binding energy for oxidized boron [38–40]. It is
noteworthy that the ratio of oxidized boron increased with
increasing ball-milling time. The ratios of oxidized boron to h-BN
3
600 3200 2800 2400 2000 1600 1200
800
-1
Wavenumber /cm
Fig. 2. FTIR spectra for pristine h-BN and h-BN bm12h.
3
+
+
structural boron, B /B , were 0.16 for h-BN bm6h, 0.19 for h-BN
bm12h, and 0.33 for h-BN bm24h. For h-BN bm12h, analysis of
the B/N/O atomic ratio indicated an elemental formula of
3
+
H O
H O
2
2
BN0.94O0.27. The O/B ratio was calculated to be around 1.7 for h-
BN bm12h. This indicates that the surface structure for h-BN
bm12h consists of a mixture of mono- and di-hydrolyzed boron
with hydroxyl and amino groups, as shown in Scheme 1. The N1s
spectrum of h-BN bm12h was broad and can be deconvoluted to
two peaks with the same width (Fig. 3B). The peak centered at
Scheme 1. Simultaneous formation of hydroxyl and amino groups via cleavage of
B–N bonds.
3
98.9 eV can be attributed to amino groups, which were formed
by cleavage of B–N bonds.
1
showed single peaks for the core-level B1s at 191.1 eV and N1s at
Measurements of H MAS NMR were conducted to estimate the
1
3
98.9 eV. All peak positions were in close agreement with those
concentrations of hydroxyl and amino groups. Fig. 4 shows H MAS
reported for characterization of synthesized h-BN and modified
h-BN substances [35,37,38]. Because BN has a delocalized struc-
NMR spectra for pristine h-BN and h-BN bm12h. The spectrum of
pristine h-BN (Fig. 4b) was very similar to that of the empty rotor
(
A) B 1s
(
B) N 1s
1
91.1
398.9
1
92.9
h-BN bm24h
3
99.8
h-BN bm12h
h-BN bm6h
h-BN bm12h
3+
B
+
B
(Bh-BN)
h-BN
h-BN
1
96 194 192 190 188
Binding energy /eV
404 402 400 398 396 394
Binding energy /eV
Fig. 3. B1s and N1s core levels of pristine and ball-milled h-BN.

1
80
S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
was estimated by titration using benzoic acid. The h-BN bm12h
À1
sample had the greatest number of basic sites, 0.37 mmol g
.
OH 6.7
3
.7 NH2
The basic site number in h-BN bm6h was 0.21 mmol g and that
À1
À1
in bm24h was 0.29 mmol g
.
The solid base properties of ball-milled h-BN were also investi-
SSB
SSB
SSB
1
SSB
gated by H MAS NMR using pyrrole as a probe molecule. Fig. 5
(
d) (c) - (a)
1
shows the H MAS NMR spectra of h-BN bm12h before and after
adsorption of pyrrole. There are two signals with peak positions
at 6.7 and 3.7 ppm before pyrrole adsorption, as mentioned above
(
1
Fig. 4d). After adsorption of pyrrole, two peaks appeared at 6.1 and
.2 ppm along with a small peak at ca. 9.5 ppm. The signal at 6.1
ppm can be ascribed to a mixture of and b protons of the pyrro-
a
(
c) h-BN bm12h
b) h-BN
late anion [44], and the signal at 1.2 ppm can be attributed to new
proton species that are formed by dissociative adsorption of pyr-
role [30]. These results imply that ball-milled h-BN has strong base
sites that can dissociate pyrrole, although color indicator reagents
composed of large molecules cannot interact with such sites
because of steric hindrance. A small peak at around 9.5 ppm can
be attributed to the N–H proton of pyrrole adsorbed on moderately
basic sites of h-BN [45]. The signal for the N–H proton moves
toward higher frequency after adsorption of pyrrole, with the
chemical shift indicative of the base strength of the solids. For
example, for LiY, KY, and KX zeolites, the signals for the N–H pro-
ton were observed at 8.4, ca. 9.7, and 11.5 ppm, respectively [45].
Corresponding signals appeared at 11.3 and 11.2 ppm for calcined
(
(
a) vacant rotor
5
0
40 30 20 10
0 -10 -20 -30
δ /ppm
Fig. 4. 1H MAS NMR spectra for (a) empty rotor, (b) pristine h-BN, (c) h-BN bm12h,
and (d) h-BN bm 12h minus the empty rotor background. SSB indicates spinning
sidebands.
(
Fig. 4a), indicating that there were almost no protons in pristine h-
BN. On the other hand, the spectrum of h-BN bm12h consisted of
two distinct peaks centered at 6.7 and 3.7 ppm. The peaks on each
side of the main peaks (40, 24, À11, and À27 ppm) are spinning
sidebands (SSB). Subtraction of the empty rotor background from
the h-BN bm12h signal gives the spectrum shown in Fig. 4d. The
two main peaks at 6.7 and 3.7 ppm can be tentatively attributed
to hydroxyl and amino groups [41] formed by the ball-milling
treatment, as discussed in the IR and XPS sections. From the quan-
2 3
hydrotalcite and c-Al O [30]. Thus, the peak position of the ball-
milled h-BN is comparable to that of KY zeolite [45]. The amount
À1
of adsorbed pyrrole (0.14 mmol g ) was smaller than the amount
À1
of base sites (0.37 mmol g ), suggesting that pyrrole molecules
were preferentially adsorbed on relatively stronger base sites.
The solid acid properties of the ball-milled h-BN were also
3
1
investigated by P MAS NMR using trimethylphosphine oxide
(TMPO) as a probe molecule. On the acidic sites of solids, the signal
of TMPO moves toward higher frequency by adsorption to form a
1
titative analysis of H content in the sample, the amount of hydro-
À1
xyl groups was estimated to be 3.91 mmol g and of amino groups
À1
+
was 1.67 mmol g . The ratio of hydroxyl groups to amino groups
was 2.34, which is close to 2, as shown in Scheme 1.
To further investigate chemical bonding in these proton species,
use was made of the spin–echo double resonance (SEDOR) method.
By irradiating the B nucleus and observing the H nucleus, it is
protonated species, TMPOH . Thus, the chemical shift is indicative
of the acid strength of the solids [46,47]. Fig. 6 shows the 31P MAS
NMR spectra for pristine h-BN and h-BN bm12h after adsorption of
1
1
1
11
possible to determine whether B is present in the vicinity of
1
H. Fig. S3a shows the 1H MAS NMR spectrum obtained by the
Hahn echo method (90°–s–180°–s–acquisition). Fig. S3b shows
11
6.1
the spectrum obtained by irradiating a B pulse (192.62 MHz)
1
during the interval between the 90° and 180° H pulses. Compared
with Fig. S3a, the signal intensity decreased, indicating that only
1
11
the H signal in the vicinity of B decreased. Fig. S3c is the SEDOR
spectrum, which was obtained by subtraction of Fig. S3b from
Fig. S3a. Both characteristic signals still remained unchanged. This
clearly indicates that both protons are present equally near boron.
Therefore, it is reasonable that these two proton species can be
1
.2
9
.5
After
adsorption
2
assigned to B-OH and B-NH .
6
.7
3
.7
B-OH
B-NH2
3.2. Solid acid–base properties of ball-milled h-BN
Solid base strength was evaluated using color indicator
reagents. Treatment of the three ball-milled h-BN samples (h-BN
bm6h, 12 h, and 24 h) with a solution of toluene containing bro-
mothymol blue (pKa = 7.2) resulted in a green color, whereas pris-
tine h-BN gave a yellow color. However, these ball-milled h-BN
samples did not become pink in phenolphthalein (pKa = 9.3). This
indicates that pristine h-BN has no basic sites and ball-milled h-
Before
adsorption
1
5
10
5
0
-5
BN samples have weak basic sites in the range of + 9.3 > H
7.2, which are comparable to those in Mg(OH) and NaX zeolite,
but weaker than those in hydroxyapatite and stronger than those
in CaCO [42,43]. The number of basic sites in ball-milled h-BN
À
ꢀ
δ
/ppm
+
2
Fig. 5. 1H MAS NMR spectra of h-BN bm12h before and after adsorption of pyrrole.
Amount of pyrrole adsorbed: 0.14 mmol of pyrrole per gram of h-BN bm12h.
3

S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
181
ball-milled h-BN catalyzed the reaction; the pristine h-BN showed
negligible activity (entry 1). The ball-milled h-BN catalysts
afforded high selectivity (76–79%) toward p-methoxy-b-
nitrostyrene (3a) (entries 2, 3, and 6). For the three ball-milled cat-
alysts, h-BN bm12h showed the highest activity (84% conv.) with
high selectivity (77%) for 3a (entry 3). These results indicated that
exposure of the side planes of h-BN resulted in the formation of
reaction sites of the catalysts and led to significant improvement
of the catalytic activity. In addition, the reaction using h-BN
bm12h was not decelerated in the presence of water (0.11 mmol)
CH3
43 physisorbed
CH3
CH3
H C
P
3
H C P = O
6
3
3
O
H C
3
h-BN bm12h
h-BN
4
1
crystalline
TMPO
9
0
(
entry 4). The main reason for the selectivity losses is moderate
SSB
carbon balance (ꢁ90%). Some products were adsorbed onto the
catalyst surface. Lack of mass transfer limitations was verified by
the Weisz–Prater criterion (CWP) [51], which is based on the ratio
of reaction rate to diffusion rate, and the calculations are summa-
rized in Table S2. If the value of CWP is less than 0.3, the internal
mass transfer effects can be neglected. The value of CWP for the case
of the highest reaction rate (entry 4) was 0.02, much less than 0.3.
The turnover frequencies (TOF) are also shown in Table 2. For
1
00
80
60
40
20
0
δ
/ppm
Fig. 6. 31P MAS NMR spectra of trimethylphosphine oxide adsorbed onto pristine h-
BN and h-BN bm12h.
À1
example, the TOF of h-BN bm12h was 7.8 h (entry 4). In the case
À1
of benzaldehyde as a reactant at 100 °C, the TOF was 2.4 h (entry
TMPO. For pristine h-BN, a sharp signal was observed at 41 ppm,
along with a spinning side band at 90 ppm. The signal at 41 ppm
is ascribed to crystallized TMPO [48], indicating that there was
no interaction between pristine h-BN and TMPO because there
were no acid sites on pristine h-BN. In contrast, for h-BN bm12h,
two different signals were obtained at 63 and 43 ppm. The former
can be attributed to TMPO adsorbed onto low-acid-strength sites
of h-BN bm12h, which are comparable to those in HY zeolite
5), which was roughly comparable to that of a primary amine-
immobilized silica-alumina catalyst (2 h ) and much lower than
À1
that of a primary and tertiary amines-immobilized silica-alumina
À1
catalyst (ca. 70 h ), though the reactant concentrations were dif-
ferent [9].
The homogeneous counterpart boric acid, a Brønsted acid,
showed no activity (entry 9), and n-butylamine, a Brønsted base,
showed low selectivity toward b-nitrostyrene (entry 10). The main
product with n-butylamine (73% selectivity) was N-benzylidene
butylamine, an imine obtained by nucleophilic addition of the
amine to the aldehyde group of benzaldehyde (Fig. S4). The coex-
istence of boric acid and n-buthylamine did not improve the activ-
ity (entry 11). This suggests that cooperative catalysis needs
appropriate positioning of acid and base sites without neutraliza-
tion. Conventional solid-base catalysts such as calcined MgO and
calcined hydrotalcite were used for comparison and showed low
conversion but high selectivity toward 4a, which is obtained via
the formation of 3a followed by a Michael addition reaction with
[
[
48]. The latter can be ascribed to TMPO physisorbed on the sample
49,50].
3.3. Nitroaldol reaction
The catalytic activity of the pristine h-BN and the ball-milled h-
BN catalysts was evaluated for the nitroaldol reaction using p-
methoxybenzaldehyde (1a) and nitromethane (2a) as substrates
(Table 2). Boric acid and an amine, which are homogenous counter-
parts, were also used for comparison. It is noteworthy that only
Table 2
a
Results of nitroaldol reaction using h-BN catalysts.
À1b
À1c
Entry
Catalyst
Base amount /mmol g
1a Conv. /%
Selec. /%
TOF /h
3a
4a
1
2
3
h-BN
0
9
<1
79
77
79
72
76
38
31
0
0
–
h-BN bm6h
h-BN bm12h
h-BN bm12h
h-BN bm12h
h-BN bm24h
MgOf
0.21
0.37
0.37
0.37
0.29
0.31
2.03
–
25
84
90
51
45
13
16
6
<1
<1
4
<1
<1
62
69
0
1.7
6.2
7.8
2.4
2.6
0.6
0.1
–
d
4
5
e
6
7
8
9
g
Hydrotalcite
h
H
3
BO
n-Butylamine
BO
+ n-Butylamineh
3
h
1
1
0
1
–
–
9
9
27
8
0
0
0.1
0.1
H
3
3
a
b
c
Reaction conditions: 1a (0.5 mmol), 2a (1.25 mmol), catalyst (50 mg), toluene (2 mL), 100 °C, 8 h.
Determined by titration of benzoic acid for bromothymol blue-adsorbed h-BN. Determined by CO
Turnover frequency.
2
-TPD for MgO and hydrotalcite.
d
e
f
Added water (0.11 mmol).
Reaction conditions: benzaldehyde (0.5 mmol), 2a (0.5 mmol), catalyst (50 mg), toluene (2 mL), 100 °C, 12 h.
Pretreated at 700 °C for 3 h.
Pretreated at 500 °C for 3 h.
g
h
0
.05 mmol.

1
82
S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
nitromethane, reactions favored by the strong basicity of the solids
entries 7 and 8) [9]. It is reported that the activity over MgO was
not influenced by CO and H O in the nitroaldol reaction because
nitromethane is so acidic that it preferentially adsorbs on the cat-
alyst when CO and H O are preadsorbed on the catalyst [52].
tion. The first and second steps are expected to be interchanged
on the catalyst. The final step is a nucleophilic attack by the depro-
tonated nitromethane. In this sequence, both imine formation [56]
and imine protonation to form iminum ions [55] are promoted by
the acid–base bifunctionality.
(
2
2
2
2
The ball-milled h-BN could catalyze the nitroaldol reaction.
Despite its weak basicity, the h-BN exhibited much higher activity
than calcined MgO and hydrotalcite. The suggested reaction steps
for the nitroaldol reaction using the ball-milled h-BN catalysts
are shown in Scheme 2 [5,9]. The reaction steps involve formation
Two different kinetic isotope effect (KIE) experiments were car-
ried out using p-methoxybenzaldehyde with nitromethane (CH
NO ) as well as -substituted nitromethane (CD NO ) [31]. The
first KIE experiment was conducted by measuring the two individ-
ual rate constants of the reaction using CH NO and CD NO , k
and k . Fig. S5 shows the results of the nitroaldol reaction using
the two nitromethanes at 100 and 80 °C. The KIE values calculated
from the relative ratio of rate constants (k /k ) were 3.1 and 3.5 for
3
-
2
D
3
2
3
2
3
2
H
of imine intermediates with surface NH
steps in the suggested sequence. The first step is the abstraction
of a proton from nitromethane by an –NH group of the ball-
milled h-BN catalyst. The second step is formation of an imine by
nucleophilic attack by an –NH group of the catalyst, but this time
2
groups. There are three
D
2
H
D
measurements conducted at 100 and 80 °C, respectively. These
were greater than 1, indicating that abstraction of a proton from
nitromethane is the rate-determining step. The second KIE experi-
ment was conducted by measuring intermolecular competition
2
on the aldehyde group of the benzaldehyde. The coexisting –OH
groups on the catalyst polarize the carbonyl group of the aldehyde
to facilitate the formation of the imine intermediate [53,54]. For-
mation of an iminum ion creates a carbon center that is signifi-
cantly more electrophilic than the carbonyl compound [55]. This
between CH
KIE value was calculated from the relative amounts of products
formed by CH NO and CD NO ([P ]/[P ]), as shown in Table S3.
3 2 3 2
NO and CD NO in the same reaction vessel. The
3
2
3
2
H
D
facilitates attack by the carbon nucleophile (CH
NO
2 2
) in the reac-
The KIE value was found to be almost constant at 3.4 for reaction
times 0.5 to 1.5 h, which is in good agreement with the first KIE
experiment. This demonstrates that abstraction of a proton from
nitromethane is the rate-determining step for the reaction
(Scheme 2, Step1).
The reaction mechanism of the nitroaldol reaction over an
active h-BN catalyst was further investigated by applying the Ham-
mett equation to data obtained using various monosubstituted
6 4 3
benzaldehyde substrates (R-C H CHO, R = p-OMe, p-CH , H, p-Cl,
and m-OMe). Fig. 7 shows the results of a Hammett plot for the
nitroaldol reaction using h-BN bm12h at 100 °C. A negative
value of slope against the Hammett constant [29], which is the
substituent constant for aromatic substitution, was obtained.
Electron-donating groups (EDG) in the para-position of benzalde-
3
hyde (p-OMe, p-CH ) enhanced reactivity with nitromethane, and
the corresponding b-nitrostyrene was formed in excellent yields.
On the other hand, electron-withdrawing groups (EWG) in the
para-position of benzaldehyde (p-Cl) and electron-donating groups
in the meta-position (m-OMe) decreased reactivity, and the corre-
sponding b-nitrostyrene was obtained in low yields. These results
indicate that electron-donating groups stimulated polarization of
the carbonyl group of benzaldehyde, leading to enhanced interac-
tion with the –OH group of the catalyst (Scheme 2 Step 2). These
were different from those of aminosilica catalysts [57] and alkaline
cation-exchanged zeolites [58]. While a negative value of slope,
À1.8, was obtained for h-BN catalyst in this study, a slightly posi-
tive value of slope, 0.18, was observed for the aminosilica catalyst
0.6
0.4
0.2
0.0
p-OMe
p-CH3
Slope = -1.8
H
-
-
-
0.2
0.4
0.6
m-OMe
p-Cl
0.2
-0.3
-0.2
-0.1
0.0
0.1
0.3
Hammett constant σ
Scheme 2. Proposed overall mechanism of nitroaldol reaction on h-BN catalyst
surface.
Fig. 7. Results of the Hammett plot for the nitroaldol reaction.

S. Torii et al. / Journal of Catalysis 355 (2017) 176–184
183
Table 3
could respectively function as acid and base sites. The results of
solid state NMR of adsorbed pyrrole and trimethylphosphine oxide
indicated that the ball-milled h-BN had basic sites comparable to
those of KY zeolite, as well as weak acid sites comparable to those
in HY zeolite. The ball-milled h-BN showed high activity and selec-
tivity for the nitroaldol reaction, whereas nontreated h-BN did not
show activity. The abstraction of a proton from nitromethane was
the rate-determining step, and the reaction mechanism involves
formation of imine intermediates with surface amino groups. The
coexistence of acid and base sites on the catalyst enhanced its
activity. Further investigation of the catalyst may open up new
possibilities for design of new acid–base pairs based on metal
nitride compounds.
a
Results of the Knoevenagel reaction with h-BN bm12h.
Entry Substrate 1b
1
Product 3b
3b Yield /%
92
1ba
3
ba
2
3
100
38
1
bb
3bb
Acknowledgments
1
bc
3bc
The authors thank Professor S. Ted Oyama for valuable discus-
sions. This work was supported by a Grant-in-Aid for Challenging
Exploratory Research (No. 16 K14474) and Young Scientists (A)
4
100
70
1bd
3
bd
be
bf
(
No. 25709077) of JSPS, Japan. A part of this work was supported
5
1be
by the AIST Nanocharacterization Facility (ANCF) platform as a pro-
gram of the Nanotechnology Platform of the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
3
6b
34
1bf
3
a
Appendix A. Supplementary material
Reaction conditions: 1b (0.5 mmol), 2b (0.5 mmol), h-BN bm12h (50 mg),
toluene (2 mL), r.t., 90 min. Yields were determined by GC using n-decane as an
internal standard.
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2017.09.013.
b
1
20 min.
[
57]. Acceleration of the reaction rate by electron-donating groups
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