Strong Br?nsted acid-modified chromium oxide as an efficient catalyst for the selective oxidation of methacrolein to methacrylic acid

Article abstract of DOI:10.1016/j.catcom.2019.03.020

Gas–phase oxidation of methacrolein to methacrylic acid was carried out over an acid-modified Cr₂O₃/SiO₂ catalyst. While only total oxidation occurred over bare Cr₂O₃/SiO₂, the acid-modified Cr₂O₃/SiO₂ showed catalytic activity for the formation of methacrylic acid. In particular, H₃PW₁₂O₄₀ strong Br?nsted acid was the most effective modifier for improving both activity and selectivity. The interface between Cr₂O₃ and H₃PW₁₂O₄₀ particles on SiO₂ appears to be responsible for the formation of active sites for the selective formation of methacrylic acid. The strong Br?nsted acid would help the activation of methacrolein through rendering it more electrophilic, which is a key step for the formation of methacrylic acid over the present catalyst.

Full text of DOI:10.1016/j.catcom.2019.03.020

CatalysisCommunications125(2019)43–47
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Strong Brønsted acid-modified chromium oxide as an efficient catalyst for
the selective oxidation of methacrolein to methacrylic acid
Shuhei Yasudaa, Atsuki Iwakuraa, Jun Hiratab, Mitsuru Kannob, Wataru Ninomiyab,
⁎
Ryoichi Otomoc, Yuichi Kamiyac,
a Graduate School of Environmental Science, Hokkaido University, Kita 10 Nishi 5, Sapporo 060-0810, Japan
b Otake R&D Center, Mitsubishi Chemical Corporation, 20-1, Miyuki-cho, Otake-shi, Hiroshima 739-0693, Japan
c Faculty of Environmental Earth Science, Hokkaido University, Kita 10 Nishi 5, Sapporo 060-0810, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Gas–phase oxidation of methacrolein to methacrylic acid was carried out over an acid-modified Cr2O3/SiO2
catalyst. While only total oxidation occurred over bare Cr2O3/SiO2, the acid-modified Cr2O3/SiO2 showed
catalytic activity for the formation of methacrylic acid. In particular, H3PW12O40 strong Brønsted acid was the
most effective modifier for improving both activity and selectivity. The interface between Cr2O3 and H3PW12O40
particles on SiO2 appears to be responsible for the formation of active sites for the selective formation of me-
thacrylic acid. The strong Brønsted acid would help the activation of methacrolein through rendering it more
electrophilic, which is a key step for the formation of methacrylic acid over the present catalyst.
Methacrolein oxidation
Acid modification
Chromium oxide
Keggin-type heteropoly acid
Gas phase oxidation
1. Introduction
oxides as a heterogeneous catalyst. While hexavalent chromium oxide
(CrO3) has very strong oxidizing ability, it is unstable at a relatively
Methacrylic acid (MAA) is an important chemical for the production
of methyl methacrylate and other derivatives and is commercially
produced by gas–phase catalytic oxidation of methacrolein (MAL) (Eq.
(1)) [1–3].
high temperature and spontaneously releases oxygen to be reduced into
Cr2O3 (non toxic) at around 473 K or higher even in an oxidizing at-
mosphere [18]. Thus, Cr2O3 and Cr3+-containing materials are more
suitable in oxidation catalysts. So far, oxidation catalysis of Cr2O3 and
Cr3+-containing materials for organic synthesis has been extensively
investigated. While some efficient oxidation reactions catalyzed by
them with organic peroxides including tert-butyl hydroperoxide have
been developed [19–23], utilization of O2 or air as an oxidizing agent is
more advantageous for cost economy and environmental impact. In
1940's, the pioneering work on the oxidation of para-substituted
ethylbenzene into corresponding acetophenone with air in the presence
of Cr2O3 at 403–433 K was reported [24]. Recently, it was found that
supported Cr2O3 [25,26] and Cr3+-containing mesoporous materials
[27–29] and metal-organic framework (Cr-MIL-101, Ref. [30]) effec-
tively promoted oxidation of benzyl alcohol to benzaldehyde [25], cy-
cloalkanes to α,β-unsaturated ketones [28,30], monoterpenic alkenes
to epoxide and allylic derivatives [27,30], and autooxidation of cyclo-
hexane into cyclohexanone and cyclohexanol [26,29] in an O2 or air
atmosphere. However, to the best of our knowledge, there is no report
on the selective oxidation of aldehyde into carboxylic acid with O2 over
Cr2O3-based catalysts.
OH
H
1
2
+
O2
O
O
(1)
Heteropolyacids with a Keggin structure composed of P, Mo and V
have been utilized as industrial catalysts [1–3]. So far, partial sub-
stitution of H+ in H3PMo12O40 with other cations, substitution of some
Mo atoms with V and supporting them on carriers have been in-
vestigated to improve the catalytic performance [4–17]. However,
further improvement is still desired. A new catalyst not consisting of Mo
and V must trigger diverse researches, and accordingly open the way for
the development of more effective catalysts for the MAL oxidation.
It is well-known that hexavalent chromium compounds including
CrO3 and Na2Cr2O7 are useful oxidizing agents in organic synthesis,
especially for oxidations of alcohols and aldehydes into corresponding
carboxylic acids, known as Jones oxidation. However, considering cost
and environmental factors, it would be advantageous to use chromium
Here, we report that a strong Brønsted acid (H3PW12O40)-modified
⁎ Corresponding author.
E-mail address: kamiya@ees.hokudai.ac.jp (Y. Kamiya).
https://doi.org/10.1016/j.catcom.2019.03.020
Received 22 November 2018; Received in revised form 24 March 2019; Accepted 24 March 2019
Availableonline25March2019
1566-7367/©2019ElsevierB.V.Allrightsreserved.

S. Yasuda, et al.
CatalysisCommunications125(2019)43–47
Cr2O3/SiO2 is a highly active and selective catalyst for the gas-phase
oxidation of MAL to MAA. The loading amounts of Cr2O3 and
H3PW12O40, and the kind of acidic component used were systematically
investigated. Based on the structural analysis of the catalyst and results
on transient responses of reaction, the active sites formed on the cata-
lyst surface and the reaction mechanism are discussed.
2.2. Catalysts characterization
Powder X-ray diffraction (XRD) patterns of the various solid cata-
lysts were recorded on an X-ray diffractometer (MiniFlex, Rigaku)
equipped with a Cu Kα radiation (λ = 0.154 nm). Infrared (IR) spectra
of the samples were recorded for self-supporting disks on an IR spec-
trometer (FT-IR/230, JASCO). X-ray photoelectron (XPS) spectra were
taken by using a ULVAC-PHI Quantera II (Al Kα radiation). Binding
energy was calibrated with respect to C 1 s peak of a carbon tape at
284.6 eV.
2. Experimental
2.1. Catalysts preparation
2.3. Catalytic oxidation of MAL
Silica-supported chromium oxide (Cr2O3/SiO2) was prepared by
using the wet impregnation method. Cr(NO3)3∙9H2O (9.895 g,
24.7 mmol, Sigma-Aldrich) was dissolved in Milli-Q water (100 mL). To
the solution, SiO2 (3 g, AEROSIL 300, EVONIK) was added and the
suspension was vigorously stirred at room temperature for 1 h. The
suspension was then evaporated to dryness by using a rotary evaporator
at 323 K. The resulting solid was dried in air at 373 K overnight and
calcined in air at 823 K for 5 h. The loading amount of Cr2O3 on SiO2
was 38.5 wt%.
Catalytic oxidation of MAL was performed in a continuous flow
reactor under atmospheric pressure. After pretreatment of the catalyst
(1 g) at 593 K with a mixture of O2 (10.7 vol%), H2O (17.9 vol%) and
N2 (balance) at the total flow rate of 28 mL min−1 for 1 h, the tem-
perature was decreased to 573 K and the reaction mixture of MAL (3 vol
%), O2 (6 vol%), H2O (13 vol%) and N2 (balance) at the total flow rate
of 72 mL min−1 was fed into the reactor. The reaction products at the
outlet of reactor were analyzed by using on-line gas chromatography.
Details of the reaction products analysis is given in the Electronic
Supporting Information (ESI).
Modification of Cr2O3/SiO2 with H3PW12O40 was carried out by also
using the incipient wetness impregnation method. 12–Tungstophosphoric
acid (H3PW12O40∙nH2O, Nippon Inorganic Colour and Chemical Co., Ltd.)
was purified by extraction with diethyl ether and recrystallized from its
aqueous solution in advance. Cr2O3/SiO2 (1.8 g) was added to 100 mL
Milli-Q water and impregnated with an aqueous solution of H3PW12O40
(0.08 mol L−1, 3.35 mL) at room temperature. Then the resulting wet solid
after impregnation was dried in air at 373 K overnight and calcined in air
at 523 K for 2 h. The loading amount of H3PW12O40 on Cr2O3/SiO2 was
30 wt%. The resulting supported catalyst is denoted as H3PW12O40-Cr2O3/
SiO2. As a reference, H3PW12O40/SiO2 without Cr2O3 was prepared by a
similar manner to that of H3PW12O40-Cr2O3/SiO2 but using SiO2 instead of
Cr2O3/SiO2.
3. Results and discussion
Table 1 summarizes the catalytic performance results for the oxi-
dation of MAL. While the unmodified Cr2O3/SiO2 showed no MAA
formation (Entry 1), the modification with acid components dramati-
cally increased the selectivity to MAA and simultaneously decreased
that to COx. It should be noted that H3PW12O40-Cr2O3/SiO2 showed the
highest activity and highest selectivity to MAA (Entry 2) among the
catalysts tested. Since H3PW12O40/SiO2 (Entry 3) as well as Cr2O3/SiO2
did not form MAA at all, combination of Cr2O3 with H3PW12O40 was
indispensable to form MAA. However, the physical mixture of Cr2O3/
SiO2 and H3PW12O40/SiO2 were less active and less selective (Entry 4)
than H3PW12O40-Cr2O3/SiO2, indicating that adjacency of the two solid
components was important for the selective formation of MAA.
H2SO4-Cr2O3/SiO2 and H3PO4-Cr2O3/SiO2 were also prepared by
using the wet impregnation procedure similar to that of H3PW12O40
-
Cr2O3/SiO2 now using dilute sulfuric acid (Wako Pure Chem. Ind., Ltd.,
0.08 mol L−1) and aqueous solution of phosphoric acid (Wako Pure
Chem. Ind., Ltd., 0.08 mol L−1). The loading amounts of H2SO4 and
H3PO4 were 1.5 and 1.0 wt%, respectively, which correspond to the
same number of H+ (0.31 mmol g−1) as that in H3PW12O40-Cr2O3/
SiO2. Preparation procedures for other catalysts are described in the
Electronic Supporting Information (ESI).
Next, we optimized the loading amount of H3PW12O40 and Cr2O3 to
improve the catalytic performance of H3PW12O40-Cr2O3/SiO2 (Figs. S1
and S2, ESI). The loading amount of both Cr2O3 and H3PW12O40 had
great impact on the catalytic performance. It was found that the loading
amount of 38.5 and 30 wt% for Cr2O3 and H3PW12O40, respectively,
Table 1
Catalytic performance results for gas–phase oxidation of MAL over various catalytic systems.a
Entry
Catalyst
Reaction rate/mmol h−1 g−1
Selectivity/ %
MAAb
MAL consumption
MAA formation
AcOHb
COx
1
Cr2O3/SiO2
0.42
0.60
0.54
0.30
0.06
0.36
0.30
0.24
0.36
0.12
0.12
2.52
0.00
0.44
0.00
0.12
0.04
0.19
0.17
0.09
0.16
0.08
0.01
< 0.01
0
3
4
0
7
5
6
6
7
7
4
12
2
97
24
100
54
21
41
39
56
48
28
79
98
2
H3PW12O40-Cr2O3/SiO2
72
0
3
H3PW12O40/SiO2
c
4
Cr2O3/SiO2 + H3PW12O40/SiO2
H3PW12O40/Cr2O3
39
74
52
55
36
45
68
9
5
6
H2SO4-Cr2O3/SiO2
d
7
H2SO4-Cr2O3/SiO2
8
H3PO4-Cr2O3/SiO2
d
9
H3PO4-Cr2O3/SiO2
10
11
12
Na3PW12O40-Cr2O3/SiO2
H3PW12
H3PW12
O
40
O
40
-Mn2O3/SiO2
-CuO/SiO2
< 1
a
Reaction conditions: MAL: O2: H2O: N2 = 3: 6: 13: 78; temperature, 573 K; catalyst weight, 1.0 g; total flow rate, 72 mL min–1; total pressure = 0.1 MPa. The
data in the table were taken after 3 h or more from the beginning of the reaction. Activity and selectivity were estimated from the data with conversion less than 16
%.
b
c
d
MAA and AcOH represent methacrylic acid and acetic acid, respectively.
Physical mixture of Cr2O3/SiO2 (1.0 g) and H3PW12O40/SiO2 (1.0 g). Two catalysts were mixed well in a mortar and afforded to the reaction.
The loading amounts of H2SO4 and H3PO4 were twice as much as those for the catalysts in entries 6 and 8.
44

S. Yasuda, et al.
CatalysisCommunications125(2019)43–47
Cr 2p
W 4f
580
578
576
574
40
38
36
34
Binding energy/eV
Binding energy/eV
Fig. 3. XPS spectra of Cr 2p and W 4f for (e) H3PW12O40-Cr2O3/SiO2, (
Cr2O3/SiO2 and ( ) H3PW12O40/SiO2 catalysts.
)
Fig. 1. Time courses for the oxidation of MAL over H3PW12O40-Cr2O3/SiO2.
(●) MAL conversion and selectivities to (♦) MAA, (△) acetic acid, and (□)
COx. Reaction conditions: MAL: O2: H2O: N2 = 3: 6: 13: balance; catalyst
weight = 1.0 g; W F−1 = 230 gcat h−1 mol−1; and total pressure = 0.1 MPa.
H3PW12O40-CuO/SiO2 (Entry 12). These results demonstrate that the
combination of Cr2O3 with strong Brønsted acid like H3PW12O40 is
absolutely necessary for the selective and effective formation of MAA.
The powder XRD pattern of H3PW12O40-Cr2O3/SiO2 (Fig. 2) clearly
demonstrates that well-crystallized Cr2O3 and H3PW12O40 were present
and the mean crystallite size of Cr2O3 and H3PW12O40 estimated from
Scherrer's equation were 22 and 14 nm, respectively. There was no
were the best for both the activity and selectivity to MAA.
Subsequently, we further examined the stability of catalyst for the se-
lective oxidation of MAL (Fig. 1). While the activity was slightly de-
creased for the initial 100 min of reaction, the catalyst showed stable
activity and selectivity for at least 360 min (6 h).
evidence to prove the formation of
a mixed metal oxide from
H3PW12O40-modified unsupported Cr2O3 showed high selectivity to
MAA comparable to H3PW12O40-Cr2O3/SiO2, but the activity was very
low (Entry 5). This was due to the low surface area of Cr2O3 (2 m2 g−1).
In other words, utilization of SiO2 support was effective to give small-
sized Cr2O3 particles deposited on it. By the modification of Cr2O3/SiO2
with H2SO4 (Entry 6) and H3PO4 (Entry 8), the activities of catalysts for
MAA formation were brought about, but the catalysts were less active
and less selective than H3PW12O40-Cr2O3/SiO2, while H2SO4 was better
than H3PO4 for the modification. We investigated the modification of
Cr2O3/SiO2 with twice amounts of H2SO4 and H3PO4, but improve-
ments in the activity and selectivity were not so significant (Entries 7
and 9). Hammett acidity functions (H0) of H3PO4, H2SO4 and
H3PW12O40 are −5, −12 and −13, respectively, meaning that the acid
strength increases in this order. The order of the MAA formation rate as
well as the selectivity to MAA was correlated well with that of the acid
strength. This fact suggests that the strong Brønsted acid is required for
the MAA formation. In fact, Cr2O3/SiO2 modified with neutral sodium
salt of H3PW12O40 (Na3PW12O40) showed only little activity for the
MAA formation (Entry 10).
H3PW12O40 and Cr2O3 based on the powder XRD pattern and IR spec-
trum recorded on H3PW12O40-Cr2O3/SiO2 (Fig. 1). The structure of
H3PW12O40-Cr2O3/SiO2 after the oxidation of MAL was basically the
same as that before the reaction (Fig. S3). The XPS spectra indicated
that Cr3+ and W6+ were predominantly present on the surface of
H3PW12O40-Cr2O3/SiO2 (Fig. 3), as expected from their crystalline
phases. The peak due to Cr3+ was shifted toward high binding energies
and conversely that of W6+ moved toward low BEs compared with
those of the unmodified Cr2O3/SiO2 and H3PW12O40/SiO2 solids, re-
spectively. These peak shifts suggest that Cr2O3 partly interacts with
H3PW12O40 with the charge transfer from Cr3+ to W6+. Based on these
results, we could conclude that the interface formed between particles
of Cr2O3 and H3PW12O40 provided active sites for the MAA formation.
Further evidence about the interaction between Cr2O3 and H3PW12O40
was derived by TPD measurements of benzonitrile (basic probe mole-
cule), where the alteration of the acid strength of H3PW12O40 was
probed (Fig. S4, ESI).
For H3PW12O40-Cr2O3/SiO2, the change in the electronic states of Cr
and W may lead to the decrease in the activity of the unmodified Cr2O3/
SiO2, as well as of H3PW12O40/SiO2 for the total oxidation of MAL,
resulting in low selectivity to COx over the H3PW12O40-Cr2O3/SiO2
catalytic system.
Manganese oxides are well-known oxidizing agents commonly used
in organic synthesis as the chromium oxides ones. However,
H3PW12O40-Mn2O3/SiO2 gave mainly COx and the selectivity to MAA
was only 9% (Entry 11). In addition, there was no MAA formation over
To investigate some important issues of the reaction mechanism, the
XRD
IR
Fig. 2. Powder XRD patterns and IR spectra of (a) Cr2O3/SiO2 (b) H3PW12O40/SiO2 and (c) H3PW12O40-Cr2O3/SiO2. (●) Cr2O3 and (◇) H3PW12O40·14H2O.
45

S. Yasuda, et al.
CatalysisCommunications125(2019)43–47
OH
+
H
Cr Cr
O
O2
H
OH
H3PW O40
12
O
O
Cr Cr
+
H
Cr Cr
Cr2O3
SiO2
H
H
+
O
O
O
Cr Cr
H
Scheme 1. Plausible reaction mechanism for the selective oxidation of methacrolein over H3PW12O40-Cr2O3/SiO2 catalyst.
influence of steam and O2 in the reaction feed gas stream on the cata-
lytic performance of H3PW12O40-Cr2O3/SiO2 was studied. After the
reaction over H3PW12O40-Cr2O3/SiO2 reached near steady-state, the
supply of O2 to the reactor was stopped while the supply of MAL con-
tinued (Fig. S5). The obtained results showed that MAA formation
continued up to 3 h after the stop of O2 supply, suggesting that reaction
proceeds with Mars–Van Krevelen mechanism, in which lattice oxygen
of the catalyst can oxidize MAL. In addition, there was only little
change in the catalytic performance of H3PW12O40-Cr2O3/SiO2 without
steam in the reaction feed gas stream (Fig. S6, ESI). The non-necessity
of steam suggests that MAA formation is not passing through 1,1-diol
formed by hydration of MAL, unlike conventional oxidation of alde-
hydes with hexavalent chromium compounds, including CrO3 and
Na2Cr2O7, in which aldehyde is hydrated in the first step and the
formed 1,1-diol reacts with chromium compounds to give chromium
ester as a reaction intermediate. Based on these results, we propose that
the reaction mechanism for the MAL oxidation over H3PW12O40-Cr2O3/
SiO2 proceeds as follows (Scheme 1). The reaction occurs on the in-
terface between Cr2O3 and H3PW12O40 particles. Firstly, MAL is acti-
vated through protonation and the activated MAL is then oxidized with
lattice oxygen of Cr2O3 to form MAA. Finally, the reduced chromium is
re-oxidized with gas-phase O2 along with the regeneration of lattice
oxygen. Since strong acid was preferable, the activation of MAL on H+
would increase the electrophilicity of carbonyl carbon in MAL, facil-
itating nucleophilic addition of a lattice oxygen to it. However, there
are several unclear points in the reaction mechanism, that is, how
chromium species forms a redox cycle, what active oxygen species is,
and why the activity of Cr2O3 and H3PW12O40 for total oxidation is
suppressed by their combination. Therefore, further investigations are
absolutely necessary to clarify the reaction mechanism. In addition,
successive oxidation of MAA to COx might be another problem for
H3PW12O40-Cr2O3/SiO2 (Fig. S7) and this should be suppressed by the
addition of additives, choice of support and the control of reaction
conditions, subject of a future work.
Cr2O3/SiO2, and H3PW12O40-Cr2O3/SiO2 showed the highest activity
(0.44 mmol h−1 g−1 of MAA formation rate at 573 K) and selectivity to
MAA (72%). In addition, H3PW12O40-Cr2O3/SiO2 exhibited stable ac-
tivity and selectivity at least for 6 h. The reaction proceeds through the
Mars–Van Krevelen mechanism over H3PW12O40-Cr2O3/SiO2. The pre-
sence of steam in the feed gas stream was not necessary, unlike con-
ventional oxidation of aldehydes with hexavalent chromium com-
pounds. The interface formed between Cr2O3 and H3PW12O40 particles
on SiO2 provides active sites for the MAA formation and the strong
Brønsted acids would increase the electrophilicity of carbonyl carbon in
MAL, thus promoting MAA formation.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2019.03.020.
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