Selective synthesis of benzoquinones over Cu(ii)-containing propylsalicylaldimine functionalized mesoporous solid catalysts

Article abstract of DOI:10.1039/c8dt01936c

The major product, 2,3,5-trimethyl-1,4-benzophenone (TMBO), was synthesised by an eco-friendly liquid-phase oxidation of 2,3,6-trimethylphenol (TMP-OH) over Cu(ii)-containing propylsalicylaldimine (CSA) functionalized mesoporous solid catalysts, namely, CSASBA-15(0.2), CSASBA-15(0.1) and CSAMCM-41(0.2), synthesized using various amounts of copper in a simple post-grafting method using different mesoporous silica materials, e.g., thick-silica walled SBA-15 and thin-silica walled MCM-41. The benzoquinones, i.e., 2,6-disubstituted p-benzoquinones (DSBQs), were also synthesised by the slurry-phase oxidation of di/tri-substituted phenols using the prepared catalysts. A promising chemical treatment was used for the removal of extra-framework copper species from the active surface of CSASBA-15(0.2), and the catalytic activity of the recovered catalyst, i.e. green mesoporous CSASBA-15(0.2) or W-CSASBA-15(0.2), was evaluated. Various reaction parameters, oxidants and solvents were used in this catalytic oxidation. To confirm the catalytic stability, recyclability and hot-catalytic filtration experiments were performed. On the basis of all catalytic results, it is worth noting that the mesoporous CSASBA-15(0.2) is an outstanding and a promising heterogeneous catalyst for the selective synthesis of TMBO and DSBQs, and produces 98% TMBO selectivity with 100% TMP-OH conversion at 353 K for 40 min and 97-99% DSBQ selectivity with 98-100% di/tri-substituted phenols conversion at 330 K for 1-3 h. The green mesoporous catalyst has unprecedented catalytic activity compared with that of other CSA functionalized mesoporous solid catalysts.

Full text of DOI:10.1039/c8dt01936c

Dalton
Transactions
View Article Online
View Journal
PAPER
Selective synthesis of benzoquinones over
Cu(II)-containing propylsalicylaldimine
Cite this: DOI: 10.1039/c8dt01936c
functionalized mesoporous solid catalysts†
Manickam Selvaraj * and Mohammed A. Assiri
The major product, 2,3,5-trimethyl-1,4-benzophenone (TMBvO), was synthesised by an eco-friendly
liquid-phase oxidation of 2,3,6-trimethylphenol (TMP-OH) over Cu(II)-containing propylsalicylaldimine
(CSA) functionalized mesoporous solid catalysts, namely, CSASBA-15(0.2), CSASBA-15(0.1) and
CSAMCM-41(0.2), synthesized using various amounts of copper in a simple post-grafting method using
different mesoporous silica materials, e.g., thick-silica walled SBA-15 and thin-silica walled MCM-41. The
benzoquinones, i.e., 2,6-disubstituted p-benzoquinones (DSBQs), were also synthesised by the slurry-phase
oxidation of di/tri-substituted phenols using the prepared catalysts. A promising chemical treatment was
used for the removal of extra-framework copper species from the active surface of CSASBA-15(0.2), and
the catalytic activity of the recovered catalyst, i.e. green mesoporous CSASBA-15(0.2) or W-CSASBA-15(0.2),
was evaluated. Various reaction parameters, oxidants and solvents were used in this catalytic oxidation. To
confirm the catalytic stability, recyclability and hot-catalytic filtration experiments were performed. On the
basis of all catalytic results, it is worth noting that the mesoporous CSASBA-15(0.2) is an outstanding and a
promising heterogeneous catalyst for the selective synthesis of TMBvO and DSBQs, and produces 98%
TMBvO selectivity with 100% TMP-OH conversion at 353 K for 40 min and 97–99% DSBQ selectivity with
Received 14th May 2018,
Accepted 24th January 2019
DOI: 10.1039/c8dt01936c
9
8–100% di/tri-substituted phenols conversion at 330 K for 1–3 h. The green mesoporous catalyst has un-
rsc.li/dalton
precedented catalytic activity compared with that of other CSA functionalized mesoporous solid catalysts.
2
stoichiometric amount of Cu(II) salt in dimethylformamide.
1
. Introduction
The yield of the corresponding p-benzoquinone was 80–87% at
Trimethyl-1,4-benzoquinone (TMBvO) is synthesized by the full conversion. BASF AG has prepared TMBvO with 98.5%
multi-step oxidation of 2,3,6-trimethyl-phenol (TMP-OH) using yield using copper(II) halide as a catalyst in an alcohol/water
4
traditional procedures, i.e., sulfonation, oxidation and two-phase medium. The oxidation of TMP-OH to TMBvO is
1
,2
reduction. To produce TMBvO, TMP-OH is sulfonated with catalyzed over copper(II) chloride by applying co-catalysts, such
sulfuric acid to produce 4-hydroxytrimethylbenzenesulfonic as lithium chloride, magnesium chloride, and hydroxylamine
4
,5
acid, which is then oxidized by manganese dioxide, as an in- hydrochloride. But all the studies have many disadvantages
organic metal-oxidant. The traditional method used for this technically. In some of the reports a high stoichiometric
oxidation system generates a great deal of solid and liquid amount of catalyst is required for a complete and selective
wastes because of the use of sulfuric acid and stoichiometric transformation from TMP-OH to TMBvO. In other cases, the
solid oxidants. TMBvO is an important precursor in the syn- co-catalysts, such as hydroxylamine hydrochloride, are partly
thesis of vitamin E, which is the best fat-soluble biological consumed during the oxidation. In a homogeneous catalyst
3
antioxidant. The demand for vitamin E is more than 30 000 system, the main drawbacks, e.g., separation, recycling, and
tons per year.
product purification, are apparent after the reaction has
In a patent, the alkyl-substituted p-benzoquinone, which is completed. The main product can still be contaminated
a major product, is synthesized by the oxidation of alkyl-substi- with traces of transition metals even though two solvent
tuted phenols under an oxygen atmosphere (30 bar) using a methods are used because of metal complexation by organics
and subsequent extraction into the organic phase. In addition,
the chlorine-contaminated by-products with a significant
amount of copper-containing waste can be formed when high
Department of Chemistry, Faculty of Science, King Khalid University, Abha 61413,
Saudi Arabia. E-mail: mselvaraj@kku.edu.sa; Fax: +966 17 241 7637
amounts of copper chloride are used as one of the catalytic
materials. This catalytic system not only produces lots of chlo-
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt01936c
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
rine- and copper-contaminated by-products with a main and 0.198 wt%, respectively, were synthesized by the post-syn-
product but a special catalytic apparatus made with corrosion- thetic grafting method using amino-functionalized SBA-15
resistant coatings might also be required.
(APTES-SBA-15), salicylaldehyde and copper(II) nitrate tri-
For the production of TMBvO, several industrial routes are hydrate. CSAMCM-41(0.2), which has the copper content of
used for the oxidation of TMP-OH with manganese dioxide, 0.192 wt%, was synthesized using the previously published
9
,10
nitric acid, or other toxic reagents as the catalysts but they are procedure.
The results of elemental analysis of CSASBA-15
6
not eco-friendly. As a challenging goal, eco-friendly catalytic (0.2), which were obtained by using a ICP-AES and CHNS/O
methods are developed for selective oxidation of organic com- analyser, confirm that the molar ratios of N : Cu and C : Cu are
pounds. For the synthesis of TMBvO, a few catalytic systems ≈3.5 and 23.8, respectively. The CSA functionalized meso-
have been applied using ‘green’ and inexpensive oxidants, porous silica catalysts were extensively used in an eco-friendly
such as molecular oxygen and hydrogen peroxide (HP), over oxidation of TMP-OH. Initially, the catalytic oxidation of
homogeneous catalysts such as Schiff-base cobalt complexes, TMP-OH was investigated using the reaction conditions listed
7
9
7
ruthenium salts and heteropoly compounds. These catalytic in Table 1. In this catalytic reaction, TMBvO, as a major
systems also remain problematic for catalyst separation, as the product, as well as 2,3,5-trimethylhydroquinone (TMHQ) and
pure target products could be contaminated with hazardous 2,2′,3,3′,5,5′-hexamethyl-4,4′-biphenol (HMBP-OH), as by-pro-
transition metal compounds.
ducts, were formed, through a hydroxyl-radical oxidation
1
1
Recently, copper(II) chloride in the ionic liquid medium has mechanism, which is confirmed by the previous literature as
been used as a catalyst to prepare TMBvO with 86% selectivity shown in Scheme 1. The order of the catalytic activities of the
by an eco-friendly oxidation of TMP-OH with molecular oxygen, catalysts on the basis of TMBvO selectivity is as follows:
8
as an oxidant, and n-butanol, as a cosolvent. In the modern CSASBA-15(0.2)
>
CSAMCM-41(0.2)
>
CSASBA-15(0.1).
industry, TMBvO is also produced in a catalytic process using CSASBA-15(0.2) exhibits the best performance with 98%
8
molecular oxygen in the presence of copper chloride.
TMP-OH conversion and 95% TMBvO selectivity and has
Kholdeeva and Zalomaeva explained clearly in a review that higher catalytic activity than other CSA functionalized meso-
2+
transition-metal supported catalysts are recently used in selec- porous silica catalysts because huge amounts of Cu -ions can
tive oxidation of phenols and methoxyarenes using environmen- be actively anchored on the inner pore walls of salicylaldimine
8
tally benign oxidants. The most efficient heterogeneous catalyst functionalized SBA-15. It means that the numerous accessible
(
a
divanadium-substituted polyoxometalate supported on active sites in CSASBA-15(0.2) can be effectively enhanced to
2
+
N-doped carbon nanotubes) for the synthesis of TMBQ has the TMP-OH oxidation reaction while the Cu -ions co-
been recently reported by Evtushok et al. Even though the ordinated with a square planar structure in salicylaldimine
8
above catalytic systems produce a better TMBvO selectivity, the functionalized SBA-15 produces high numbers of Lewis acid
best catalytic system with a highly active and an eco-friendly sites on the silica pore walls, as confirmed by acidity studies
mesostructured copper(II) catalyst is greatly needed to achieve using Fourier-transform infrared (FTIR) pyridine spectroscopy
12
TMBvO selectivity. Therefore, to the best of our knowledge, for for copper containing mesoporous silica catalysts. The cata-
the selective synthesis of TMBvO by liquid-phase oxidation of lytic results obtained by CSASBA-15 catalysts show that the
2+
TMP-OH and for the selective synthesis of DSBQs by oxidation most of the Cu -ions could be anchored to increase the active
of di/tri-substituted phenols, the mesoporous CSASBA-15 cata- sites on the silica surface of inner pores as shown in Scheme 2
lysts could not be successfully used so far.
and can also be further confirmed by the results of inductively
Since 21st century, in both academia and industry, green coupled-plasma atomic-emission spectroscopy (ICP-AES), FTIR,
catalytic technology has been significantly applied for the cata- electron spin resonance (ESR) spectroscopy and ultraviolet-visible
9
lytic oxidation of phenols to their corresponding quinones in a diffuse reflectance (UV–vis DRS) spectroscopy. According to the
13
halogen-free reaction system. In this study, for the selective literature, we have explained in detail that the donor atoms of
synthesis of TMBvO, the green catalytic oxidation of TMP-OH N and O on CSASBA-15(0.2) produce the steric and electronic
2+
using HP as a green oxidizing agent over CSA-SBA-15 catalysts effects to stabilise Cu ions and increase the hydrophobicity,
under a variety of reaction conditions was conducted. The which plays an efficient catalytic role in increasing TMBvO
CSA-SBA-15 catalysts were also used for selective synthesis of selectivity and also achieving a square planar coordination with
2+
DSBQs by the oxidation of di/tri-substituted phenols using HP huge numbers of Cu -ions in salicylaldimine functionalized
9
as the green oxidising agent. The catalytic results obtained for SBA-15. Moreover, in our study, the catalytic characterization
the selective synthesis of TMBvO and DSBQs using all the results obtained by ICP-AES, X-ray diffraction (XRD), N
2
-sorp-
CSA functionalized mesoporous silica catalysts were tion isotherm analysis, FTIR, ESR and UV-vis DRS, as shown in
thoroughly correlated and compared.
Fig. S1–S5,† strongly support that CSASBA-15(0.2) has a higher
catalytic activity than the other CSA functionalized meso-
porous silica catalysts whereas CSAMCM-41(0.2) gives much
lower TMBvO selectivity than CSASBA-15(0.2) due to the less
2
. Results and discussion
2
+
number of Cu -ions coordinated with a square planar struc-
Mesoporous CSASBA-15 catalysts, i.e. CSASBA-15(0.1) and ture. It is noteworthy that, compared to CSASBA-15(0.2),
CSASBA-15(0.2), which have the copper content of 0.097 wt% CSAMCM-41(0.2) has thinner mesopore walls, which could
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
a
Table 1 Oxidations of TMP-OH over different CSASBA-15 catalysts
b
TOF (min−1)
g
Catalysts
Cu (wt%)
Amounts of Cu in 50 mg (mmol)
TMP-OH conversion (%)
TMBvO selectivity (%)
CSASBA-15(0.1)
CSASBA-15(0.2)
CSASBA-15 (0.1)
CSASBA-15 (0.2)
CSAMCM-41(0.2)
SiSBA-15
0.097
0.198
0.096
0.197
0.192
—
0.00076
0.00155
0.00075
0.00155
0.00151
—
45
100
46
100
90
10
100
100
39
95
40
96
85
3
147
162
152
161
149
—
c
c
d
e
f
W-CSASBA-15(0.2)
W-CSASBA-15(0.2)
0.197
0.197
0.00155
0.00155
98
98
161
107
a
Reaction conditions: 50 mg of catalyst; 1 : 3 ratio of TMP-OH to HP (10 mmol TMP-OH; 30 mmol of HP); 15 ml of MeCN; reaction temperature,
b
c
rd
d
3
53 K; reaction time, 40 min. The results are determined by ICP-AES. The results were obtained in the 3 run. The catalyst was synthesized
9
,10
e
f
using the published procedure.
The catalyst was washed before using in catalytic oxidation. TMP-OH was added in three portions
g
(3.33 mmol per 20 min) to the reaction mixture. Turnover frequency (TOF) = (moles of substrate consumed)/(moles of active sites in the catalyst
used × reaction time).
size, pore size, surface area, pore volume, and wall thickness)
of the CSASBA-15(0.2) catalyst contribute to increase its cata-
9
lytic activity in this liquid-phase catalytic oxidation. The struc-
tural and textural properties of the CSA functionalized meso-
porous silica catalysts are listed in Table S1.† This reaction was
also performed using SiSBA-15 synthesized using the previous
1
5
Scheme 1 Oxidation of TMP-OH.
procedure published by Selvaraj et al. In this case, TMP-OH
conversion (10%) with a trace amount of TMBvO selectivity
(
Table 1) can only be obtained by SiSBA-15. This catalytic
result is highly recommended in that the high activity is poss-
ibly caused by CSA functionalized hexagonal mesoporous
silica catalysts prepared with thick pore walls, which, is one of
the main factors to increase catalytic activity and plays an
important role in the production of TMBvO with higher
selectivity.
Among the CSA functionalized mesoporous silica catalysts,
CSASBA-15(0.2) and CSASBA-15(0.1) give the highest and
lowest TMBvO selectivity, respectively. Therefore, we have
taken the above two catalysts for reusability tests under the
reaction conditions listed in Table 2. Initially, the CSASBA-15
catalysts used in the production of TMBvO usually cause
lower product selectivity because the unreacted organic wastes
Scheme 2 Structure of CSASBA-15.
2
+
9
not support to incorporate the high numbers of Cu -ions.
The ICP-AES studies show that the silica pore walls of screen the majority of active sites and create the sintering
2
+
CSASBA-15(0.2) contain more Cu -ions than the other effect on the catalyst surface. Prior to reuse, we have regener-
9
CSASBA-15(0.1) and CSAMCM-41(0.2). As reported in the lit- ated the used CSASBA-15 catalysts by washing treatments.
1
4
29
erature, the results of Si magic-angle spinning nuclear Initially, the used catalysts were reactivated by filtration from
magnetic resonance (MAS NMR) spectroscopy show that the the reaction mixture, and then the catalysts were thoroughly
signal intensity of CSA functionalised mesoporous silica washed with the corresponding solvents such as acetonitrile
catalysts is much lower than that of siliceous SBA-15. This and dichloromethane for the complete removal of organics
observation clearly supports the stabilization of the copper and unreacted TMP-OH molecules and dried using a vacuum
ions by the silanol groups (defect sites), and an additional oven at an ambient temperature. The recovered recyclable and
1
2
point noted is that T and T peak signals further confirm active catalysts, e.g., CSASBA-15(0.2) and CSASBA-15(0.1), were
that large numbers of CSA are functionalized within the then reused for the reaction (Table 1). The recyclability experi-
inner surface of mesoporous silicas. The ESR and UV-vis DRS ments were performed for each run before initiation of the
spectroscopy results (Fig. S4 and S5†) prove that the catalytic reaction. After two runs, the TMBvO selectivity of
2
+
CSASBA-15(0.2) catalyst contains more Cu -ions coordinated CSASBA-15(0.2) increases slightly from 98 to 100%; this is
with a square planar structure than the other CSA functiona- attributed to the complete removal of the extra-framework
9
lized mesoporous silica catalysts. Additionally, the XRD CuO nanoparticle species from the active surface of the cata-
(Fig. S1†) and N
2
-sorption isotherm (Fig. S2†) results strongly lyst resulting in the correct orientation of TMP-OH on the
indicate that the physicochemical parameters (i.e. unit cell CSASBA-15(0.2) catalyst during oxidation. In the third run, the
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
TMP-OH conversion and TMBvO selectivity remain constant,
2
+
as shown in Table 1, indicating that the active Cu species
could not be further leached from the mesoporous matrix; this
is in good agreement with the ICP-AES results for filtrate solu-
2
+
tions, which did not reveal any active Cu species due to the
steric and electronic effects increased by the donor atoms of N
and O on CSASBA-15(0.2) as strongly indicated in the litera-
13
ture. On the basis of our catalytic results, it is strongly con-
firmed that the TMBvO selectivity could not be highly
achieved by CuO nanoparticles because its surface area is six-
1
1
times lower compared to that of CSASBA-15(0.2).
The
CSASBA-15(0.2) catalyst, before applying to the liquid-phase
oxidation of TMP-OH, was washed more than four times using
the solvents, acetonitrile and dichloromethane, to remove the
non-framework CuO nanoparticles completely, and dried
using a vacuum oven at an ambient temperature. Finally, the
treated catalyst was used as a washed CSASBA-15(0.2) catalyst Fig. 1 Oxidations of TMP-OH at different temperatures over
in this catalytic reaction. The catalytic activity of the recyclable W-CSASBA-15(0.2). Reaction conditions: 50 mg of catalyst,
0
.00155 mmol of Cu; 1 : 3 ratio of TMP-OH : HP (10 mmol TMP-OH,
CSASBA-15(0.2) catalyst, in the third run, is almost similarly
obtained to CSASBA-15(0.2), as shown in Table 1. Based on
this result, it is seen that the majority of active copper species
on the surface of the CSASBA-15 catalyst could not be leached,
as an irreversible manner. In addition, as a proof of the recycl-
ability, the ICP-AES results also confirm the absence of copper
species. After obtaining the results for the three runs over
CSASBA-15(0.2), hot-catalyst filtration experiments were per-
formed twice at 353 K for this oxidation reaction. In this case,
the obtained catalytic results prove that the filtered solutions
produce ∼2–5% of TMBvO selectivity with 5–7% of TMP-OH
conversion, which may be formed by HP. The above catalytic
results confirm that the oxidation of TMP-OH occurs only on
the surface of the green CSASBA-15(0.2) catalyst. Therefore,
W-CSASBA-15(0.2) has an outstanding catalytic activity and
30 mmol of HP); 15 mL of MeCN; reaction time, 40 min.
1
6
leads to a green heterogeneous catalytic process.
It is
obviously noted that the washing experiments confirm the
complete removal of non-framework CuO nanoparticles on the
surface of the catalyst, as the ICP-AES results show in
Table S1,† and this catalytic result also closely matches that of
the hot-catalyst filtrate solution of CSASBA-15(0.2). When the
TMP-OH is added in three portions to the reaction, we have
undoubtedly observed that the TMP-OH oxidation is 100%
complete, as shown in Table 1. Moreover, the catalytic charac-
Fig. 2 Oxidations of TMP-OH at different times over W-CSASBA-
1
1
5(0.2). Reaction conditions: 50 mg of catalyst, 0.00155 mmol of Cu;
: 3 ratio of TMP-OH : HP (10 mmol TMP-OH, 30 mmol of HP); 15 mL of
MeCN; reaction temperature, 353 K.
2
terization results, such as XRD, N -sorption isotherm, and
UV-vis DRS, prove that the structural and textural parameters,
2
+
including the environment of the Cu species coordinated to increase on reaching 100% TMBvO selectivity when the reac-
the surface of CSASBA-15(0.2) are maintained (Table S1†). The tion temperatures and times are increased from 303 to 353 K
recycling results show that W-CSASBA-15(0.2) is unprecedented and 30 to 40 min, respectively. With a further increase in the
as a green catalyst among the metallosilicate mesoporous temperature and time from 353 to 383 K and 40 to 180 min,
1
6
catalysts.
respectively, the TMP-OH conversion and TMBvO selectivity
The optimal reaction parameters, i.e. temperature, time, could not be changed because TMBvO shows kinetic stability,
17
and reactant ratio, were investigated using W-CSASBA-15(0.2) as confirmed and reported in the literature. Additionally, the
because it is the most active catalyst of the CSA functionalized TMHQ selectivity increases with decreasing temperature from
mesoporous silica catalysts. Oxidation of TMP-OH was carried 353 to 318 K and time from 40 to 30 min. The catalytic results
out at different reaction temperatures and times under the obtained using different temperatures and times clearly show
reaction conditions listed correspondingly in Fig. 1 and 2. The that the TMHQ selectivity increases with either decreasing
1
6,17
rates of TMP-OH consumption and TMBvO formation temperature or time.
The TOF increases from 303 to 353 K.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
a
Table 2 Oxidations of TMP-OH by TBHP over W-CSASBA-15(0.2)
g
Temperature
K)
Time
(min)
TMP-OH
conversion (%)
TMBvO
selectivity (%)
TOF
−1
(
(min )
298
333
353
353
353
353
353
333
333
30
30
30
40
180
40
180
180
180
31
79
88
91
97
98
99
68f
53
42
50
57
78
69
53
45
37
58
67
170
189
98
35
105
35
b
c
d
e
24
19
a
Reaction conditions: 50 mg of W-CSASBA-15(0.2), 0.00155 mmol of
Cu; 1 : 3 ratio of TMP-OH to TBHP (10 mmol TMP-OH; 30 mmol of
b
TBHP); 15 ml of MeCN. 1 : 6 ratio of TMP-OH to TBHP (10 mmol
c
TMP-OH; 60 mmol of TBHP). 1 : 8 ratio of TMP-OH to TBHP
d
(
10 mmol TMP-OH; 80 mmol of TBHP).
O
2
(excess) was used as an
e
oxidizing agent. 1,2,4-Trimethylbenzene was used instead of TMP-OH
Fig. 3 Variations of TMP-OH to HP ratio over W-CSASBA-15(0.2). with urea hydrogen peroxide as the oxidizing agent and MeOH as the
f
g
Reaction conditions: 50 mg of catalyst, 0.00155 mmol of Cu; 15 mL of solvent. Conversion of 1,2,4-trimethylbenzene. Turnover frequency
(
TOF) = (moles of substrate consumed)/(moles of active sites in the
MeCN; reaction temperature 353 K; reaction time, 40 min.
catalyst used × reaction time).
Thereafter, the TOF is still sustained when temperature is
increased from 353 to 383 K. It is noticeably found on the HMBP-OH, as shown in Table 2. These catalytic studies
basis of catalytic results that the TMP-OH conversion as well as confirm that the water molecules of HP could not affect the
TMBvO selectivity could not be affected even when the catalytic activity of this system as reported in previous
1
8
reaction is conducted at a higher temperature (383 K). studies.
Comparing the catalytic results obtained over
When this reaction is performed using a 1 : 3 ratio of TMP-OH W-CSASBA-15(0.2) using different reaction parameters clearly
to HP under the reaction conditions listed in Fig. 3, confirms that the excellent TMP-OH oxidation is achieved via a
W-CSASBA-15(0.2) produces a higher TMBvO selectivity. The reaction at 353 K for 40 min using a 1 : 3 ratio of TMP-OH to
−
1
reaction rate (TOF = 161 min ) for the ratio 1 : 3 is higher as HP with a higher TMP-OH conversion as well as a higher
compared to other ratios, 1 : 1 and 2 : 5, because there is no de- TMBvO selectivity.
activation and no sintering effect caused, whereas the catalytic
activity decreases in TMP-OH to HP ratios ranging from 1 : 1 to (0.2) with the reaction parameters and with different solvents
: 5 because the byproducts, oxidative C–C coupling product listed in Table 3 was performed to achieve higher TMBvO
HMBP-OH) and dihydroxyarene product (TMHQ), could be selectivity. MeCN is a common polar aprotic solvent that can
The liquid-phase oxidation of TMP-OH over W-CSASBA-15
2
(
formed by increasing the amounts of HP (TMP-OH to HP form complexes on the catalyst surface at ambient temperature
ratios, 1 : 5, 1 : 7). The diffusion rate of W-CSASBA-15(0.2) as (353 K) and preferentially enable attack on the active sites to
well as TMP-OH conversion decreases for the ratios 1 : 1 and achieve higher TMP-OH conversion. The TMP-OH conversion
2
: 5. The TMBvO selectivity could not be improved by these (38%) and TMBvO selectivity (48%) decrease when this reac-
ratios because the active sites could be partially screened by tion is conducted, under similar reaction conditions, with
unreacted organics deposited on the inner pore walls while the 5 mL of MeCN, because large amounts of TMP-OH may be in-
reactants in these ratios may not reach an equilibrium state. soluble to react with HP. However, the conversion as well as
When this reaction is performed over W-CSASBA-15(0.2) using selectivity is not majorly affected when the reaction is carried
tert-butyl hydroperoxide (TBHP) as the oxidizing agent instead out using 30 mL of MeCN under the same reaction conditions,
of HP at optimal temperatures, times, and reaction ratios while other solvents were also used for this catalytic oxidation,
listed in Table 2, the TMBvO selectivity gradually decreases as noted in Table 3. The order of the dielectric constants of
compared with that using HP while the selectivity of by-pro- the solvents is MeCN > MeOH > ethanol > ACvO > MeCOOH.
ducts, HMBP-OH and TMHQ, can be increased. However, as All solvents could offer lower TMBvO selectivity than MeCN
shown in Table 2, the TMBvO selectivity decreases almost because of increasing by-product selectivity (TMHQ and
threefold when molecular oxygen is used as the oxidizing HMBP-OH) as shown in Table 3. This is experimentally con-
agent instead of HP under the same reaction conditions firmed when the same solvents are used at either higher or
1
6,17
because HMBP-OH selectivity reached 63%. When pseudocu- lower reaction temperatures, as reported in the literature.
mene is used instead of TMP-OH with urea hydrogen peroxide To confirm the effect of the solvent on the catalytic activity,
UHP) as the oxidizing agent and MeOH as the solvent, the the catalytic reaction is performed without a solvent at 353 K
(
TMP-OH conversion and TMBvO selectivity significantly for 40 min; the TMP-OH conversion and TMBvO selectivity
decrease with increasing selectivity of by-products, TMHQ and reach 15% and 19%, respectively. Additionally, the catalytic
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
a
Table 3 Oxidations of TMP-OH in different solvents over W-CSASBA-15(0.2)
TOF (min−1)
d
Temperature (K)
Solvent (15 ml)
Dielectric constant
TMP-OH conversion (%)
TMBvO selectivity (%)
353
333
330
330
353
348
353
353
353
MeCN
MeOH
ACvO
37.5
33
21
6.2
6.2
24.3
37.5
37.5
—
100
85
72
37
68
78
38
100
15
98
73
81
52
80
39
48
98
19
161
137
116
60
110
126
139
161
24
MeCOOH
MeCOOH
Ethanol
b
MeCNc
MeCN
Solvent free
a
Reaction conditions: 50 mg of W-CSASBA-15(0.2), 0.00155 mmol of Cu; 1 : 3 ratio of TMP-OH to HP (10 mmol TMP-OH; 30 mmol of HP); reac-
b
c
d
tion time, 40 min. 5 ml of solvent. 30 ml of solvent. Turnover frequency (TOF) = (moles of substrate consumed)/(moles of active sites in the
catalyst used × reaction time).
results show that the TOF of this reaction for MeCN, used as a CSASBA-15(0.2), recyclable CSASBA-15(0.2), and W-CSASBA-15
solvent, is higher than that of other solvents, because many (0.2), are used under the reaction conditions listed in Table 4.
solvents do not easily create complexes between the reactants When the calcined CSASBA-15(0.2) catalyst is used to catalyse
and the active surface of the catalyst while they may block or the oxidation of 2,6-di-tert-butylphenol (DTBP-OH) at 3 h
screen the numerous active sites on the surface of the cata- (Table 4), excellent catalytic activity is obtained with a 100%
1
9,20
lyst.
To achieve the highly selective synthesis of TMBvO, di-tert-butyl-p-benzoquinone (DTBBQvO) selectivity. A 1,4-di-
MeCN is found to be a beneficial solvent under these reaction ketone can be formed on the aromatic rings of para-substi-
conditions, as clearly shown by the above-mentioned catalytic tuted 2,6-di-tert-butylphenols (DTBP-OHs) during these oxi-
results.
dations, and we have clearly observed that the elimination
In organic synthetic chemistry, quinone derivatives play an groups can be easily eliminated from the para positions of
2
1
important role in biosystems and are being used in the syn- DTBP-OHs, as experimentally confirmed elsewhere. The alkyl
1
thesis of vitamins. To explore the advantages of a liquid groups, which are electron-donating, lead to form DSBQs. The
phase catalytic system for the synthesis of quinone derivatives, higher DTBBQvO selectivity is obtained at longer reaction
the CSA functionalized mesoporous silica catalysts, i.e. times; however, at low reaction times (entries 2–4), the higher
a
Table 4 Oxidations of di/tri-substituted phenols over W-CSASBA-15(0.2)
Entry
Time (h)
Substrate
Major product
Selectivity (%)
Conversion (%)
e
b
1
2
2
2
91
96
96
98
100
100
c
d
b
2
3
4
3
3
3
94
98
98
97
100
100
c
d
b
2
2
2
96
99
99
95
99
99
c
d
b
1
1
1
93
99
99
95
98
98
c
d
f
b
5
1
1
1
94
99
99
96
100
100
c
d
f
b
6
1
1
1
93
97
97
94
98
98
c
d
a
b
Reaction conditions: 50 mg of catalyst; 10 mmol of substrate; 30 mmol of HP; 10 ml of MeCN; reaction temperature, 330 K. CSASBA-15(0.2),
c
d
rd
e
0
.00155 mmol of Cu. W-CSASBA-15(0.2), 0.00155 mmol of Cu. The results were obtained in the 3 run. 10 ml of hexane was used as a
f
2
solvent. The mixture of solvents (6 ml of dioxane : 4 ml of H O).
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
selectivities for 2,6-diisopropyl-p-benzoquinone (DIPBQ) and (97%), 2,6-dimethylphenol (99%), 2,6-dimethylhydroquinone
,6-dimethyl-p-benzoquinone (DMBQ) are obtained because of (>97%), 2,6-dimethoxyhydroquinone (>97%), 2,6-di-tert-butyl-
steric effects, which decrease in the order of t-butyl > isopropyl hydroquinone, HP (30% in H O), TBHP (70% in H O), UHP
methyl; a similar trend is previously reported. The oxi- (97%), MeCN (99.5%), ACvO (99.5%,), MeOH (99.8%),
2
2
2
2
1
>
dations of 2,6-disubstituted hydroquinones at short reaction MeCOOH (99.7%), ethanol (≥99.5%), hexane (99%), and
times show the excellent selectivities for DSBQs (entries 1, 5 dioxane (>99%) were purchased from Sigma-Aldrich Inc.
and 6); however, 2,6-disubstituted hydroquinones cost (USA), TCI (Japan), and BHZ (China) Chemical Inc. (China)
approximately four times more than that of 2,6-disubstituted and used as received without further purification.
phenols. Moreover, the coupling products, i.e. 3,5,3′,5′-tetra-
tert-butyldiphenoquinone (TTBDPQ), 3,5,3′,5′-tetraisopropyl-
diphenoquinone (TIPDPQ), 3,5,3′,5′-tetramethyldipheno-
quinone (TMDPQ), and 3,5,3′,5′-tetramethoxydiphenoquinone
TMoDPQ), can be formed with lower selectivities (1–9%).
3
.2. Synthesis of SBA-15 and MCM-41
The well-ordered mesoporous SBA-15 molecular sieve material
was synthesized using the pH-adjusting direct hydrothermal
method according to the previous procedure published by
Selvaraj and Kawi. In a typical synthesis, 4 g of Pluronic P123
with 25 ml of water was stirred to obtain a clear solution.
Thereafter, a required amount of dilute HCl solution was
added, and the mixture solution was again stirred for another
(
W-CSASBA-15(0.2) produces larger amounts of DSBQs than
CSASBA-15(0.2), as shown in Table 4, because the trace
amounts of CuO nanoparticles, from the active surface of the
catalyst, can be completely removed using washing treatments.
Recyclable CSASBA-15(0.2) has a similar catalytic activity for
these catalytic oxidations, as shown in Table 4. Hot-catalyst fil-
tration experiments were also performed twice at specific
temperatures and times (not shown in Table 4) during these
oxidation reactions over CSASBA-15(0.2). The filtrate solutions
contain very small amounts of the conversion product (2–7%),
which may form via HP; this again confirms that excellent
selective synthesis of DSBQs has been achieved on the surface
of the W-CSASBA-15(0.2) catalyst resulting in a higher catalytic
activity and providing further proof of its validity as a green
1
5
1
h for the hydronium ions to be associated with the alkylene
oxide units. Next, 9 g of TEOS was added, and the resulting
mixture was stirred for 24 h at 313 K. The product was recov-
ered by filtration, washed several times with water and dried
overnight at 373 K. Finally, the obtained SBA-15 was calcined
in air at 813 K for 6 h for the complete removal of the surfac-
tant template. The mesoporous MCM-41(40) was alkaline-
hydrothermally synthesized using cetyltrimethylammonium
bromide (CTMABr) as the structuring agent according to a
2
2
well-known synthetic procedure published by Selvaraj et al.
1
6,17,20
heterogeneous process.
Overall, the results show that
W-CSASBA-15(0.2) is a very active, recyclable, and unpre-
cedented heterogeneous catalyst for the highly selective syn-
3.3. Synthesis of CSASBA-15 and CSAMCM-41(0.2)
thesis of DSBQs. Comparison of the results obtained here with The mesoporous CSASBA-15 catalysts, CSASBA-15(0.1) and
those reported in the literature shows that the W-CSASBA-15 CSASBA-15(0.2), were synthesized by the post-grafting method,
(0.2) catalyst has exceptional catalytic activity for the oxidation as shown in Scheme 1, using amino-functionalized SBA-15
of DSBQs compared with that of other solid catalysts because (APTES-SBA-15) with appropriate amounts of salicylaldehyde
it has thicker pore walls than CuMCM-41 and other meso- and Cu(NO ) ·3H O according to the previous procedure pub-
3
2
2
1
6,17,20
9
porous metallosilicate MCM-41 catalysts.
lished by Selvaraj et al. In a typical synthesis of CSASBA-15
0.2), one gram of SBA-15 was stirred with 1.8 g of amino-
propyltriethoxysilane (3-APTES) in chloroform at room tem-
perature for 15 h under an N atmosphere. Thereafter, the
stirred mixture was filtered and washed using the solvents
such as chloroform and dichloromethane, and a white solid of
(
2
3
. Experimental
3
.1. Materials
For the preparation of the CSA functionalized mesoporous APTES-SBA-15 was collected. APTES-SBA-15 was then refluxed
3
silica catalysts, all chemicals, i.e. triblock copolymer poly(ethyl- with 5 g of salicylaldehyde in methanol (150 cm ) for 4 h at
ene glycol)-block-poly(propylene glycol)-block-poly(ethylene 333 K. The yellowish solid was then collected by the filtration
glycol) (Pluronic P123; molecular weight
=
5800, of the refluxed mixture. The obtained solid material was dried
EO20PO70EO20), cetyltrimethylammonium bromide (CTMABr, in a desiccator. Thereafter, 1.22 g of yellowish solid was stirred
C H N(CH ) Br), sodium metasilicate (44–47% SiO ), tetra- with 0.5 g of Cu(NO ) ·3H O solution (it was dissolved by the
1
6
33
3 3
2
3 2
2
ethylorthosilicate (TEOS; 98%), sulphuric acid (99.99%), hydro- appropriate amount of methanol) at room temperature for
chloric acid (HCl; 37%), aminopropyltriethoxysilane, salicylal- 15 h. The formation of a green solid product was filtered and
dehyde, chloroform, dichloromethane, methanol and copper washed with methanol using Soxhlet. The obtained product
(
II) nitrate trihydrate (Cu(NO
3 2 2
) ·3H O), were purchased from was dried under vacuum and is designated as CSASBA-15 (0.2).
Aldrich Chemical Inc. and used as received without further This typical procedure was adopted for the preparation of thin-
9
,10
purification. Deionized water was used throughout the synth- walled mesoporous CSAMCM-41(0.2).
Bulk amounts of cat-
eses. For the selective synthesis of p-benzoquinones, all chemi- alysts, CSASBA-15(0.2), CSASBA-15(0.1) and CSAMCM-41(0.2),
cals, i.e. TMP-OH (≥97%), 2,3,5-trimethylphenol (99%), pseu- were synthesized using the above synthetic procedures for the
documene (98%), DTBP-OH (99%), 2,6-diisopropylphenol selective synthesis of benzoquinones.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
3
.3. Investigation of catalytic stability
Conflicts of interest
The mesoporous CSASBA-15 catalysts, e.g., CSASBA-15(0.2) and
CSASBA-15(0.1), used in oxidation reactions were regenerated
by washing and drying at ambient temperature, as reported
There are no conflicts to declare.
9
previously, and the treated catalysts are denoted as recyclable
CSASBA-15 catalysts, i.e. CSASBA-15(0.2) and CSASBA-15(0.1). Acknowledgements
Before application to the catalytic oxidation of TMP-OH,
The authors extend their appreciation to the Deanship of
Scientific Research at King Khalid University for funding this
work through General Research Project under grant number
G.R.P.146/1440.
CSASBA-15(0.2) was simply washed several times using sol-
vents, such as acetonitrile and dichloromethane, to improve
the catalytic activity through the removal of any non-frame-
work CuO nanoparticle species deposited on the active sur-
9
faces of the catalysts.
References
3
.4. Characterization
1
2
3
(a) I. V. Kozhevnikov, Catal. Rev., 1995, 37, 311;
b) S. Fujibayashi, K. Nakayama, Y. Nishiyama and Y. Ishii,
Chem. Lett., 1994, 7, 1345.
P. Schudel, H. Mayer and O. Isler, in The Vitamins, ed.
W. H. Sebrell and R. S. Harris, Academic Press, New York,
1972, vol. 5, p. 165.
All the CSA functionalized mesoporous silica catalysts, includ-
ing washed, and recyclable catalysts, were characterized using
ICP-AES, a CHNS/O analyser, XRD, N -sorption isotherms, Si
2
(
2
9
MAS NMR spectroscopy, UV-vis DRS, field-emission scanning
electron microscopy, and transmission electron microscopy,
9
according to the published procedures.
W. Brenner, (Hoffman-La Roche). German Patent, 2221624,
1
972.
4 D. Turk, European Patent, 0127888B1, 1987.
(a) W. Brenner, German Patent, 2221624, 1972;
b) T. Isshiki, T. Yui, H. Uno and M. Abe, European Patent,
127888, 1984; (c) K. Takehira, M. Shimizu, Y. Watanabe,
H. Orita and T. Hayakawa, J. Chem. Soc., Chem. Commun.,
989, 1705; (d) K. Takehira, M. Shimizu, Y. Watanabe,
H. Orita and T. Hayakawa, Tetrahedron Lett., 1989, 30, 6691;
e) M. Shimizu, Y. Watanabe, H. Orita, T. Hayakawa and
K. Takehira, Bull. Chem. Soc. Jpn., 1992, 65, 1522;
f) A. Baiker, Z. Bodnar and T. Mallat, J. Mol. Catal. A:
3
.5. Oxidation of di-/tri-substituted phenols
5
For selective synthesis of benzoquinones, the liquid-phase oxi-
dation of di-/tri-substituted phenols was carried out using
various mesostructured CSA functionalized silica catalysts,
recyclable catalysts and washed catalysts under vigorous stir-
ring in a thermostated quartz-vessel reactor, as reported by
(
0
1
1
6k
Selvaraj.
shown in the ESI.†
The details of the experimental procedure are
(
(
Chem., 1996, 110, 55; (g) K. Takaki, Y. Shimasaki,
T. Shishido and K. Takehira, Bull. Chem. Soc. Jpn., 2002, 75,
4
. Conclusions
311.
Two-dimensional uniformly mesostructured CSASBA-15 cata-
lysts have been successfully used in the preparation of benzo-
quinones by liquid-phase oxidation of di-/tri-substituted
phenols. Washing studies confirm that the washed catalyst,
W-CSASBA-15(0.2), recovered completely from the non-frame-
work CuO nanoparticle species deposited on its surface has
been successfully used in these oxidations to determine its
catalytic activity. The recyclability and hot-catalytic filtration
experiments for CSASBA-15(0.2) have been successfully per-
formed. Recyclability studies of the oxidation reactions show
that huge amounts of square planner Cu species can be suc-
cessfully incorporated to increase Lewis acid active sites on the
silica surface of the CSASBA-15(0.2) catalyst. The results of the
hot-catalyst filtrations in these oxidation reactions clearly
confirm that the recyclable and W-CSASBA-15(0.2) catalysts are
true heterogeneous catalysts. We clearly conclude on the basis
of the catalytic results that the green mesoporous CSASBA-15
6 (a) V. A. Bushmilyev, T. A. Kondratyeva, M. A. Lipkin and
A. V. Kondratyev, Khim.-Farm. Zh., 1991, 5, 65;
(b) M. Constantini, L. Krumenaker and F. Igersheim, French
Patent, 2552425, 1983.
7 (a) H. Chao-Yang and J. E. Lyons, European Patent, 93540,
1983; (b) D. L. Tomaja, L. H. Vogt and J. G. Wirth, J. Org.
Chem., 1970, 35, 2029; (c) S. Ito, K. Aihara and
M. Matsumoto, Tetrahedron Lett., 1983, 24, 5249;
(d) M. Shimizu, T. Hayakawa and K. Takehira, Tetrahedron
Lett., 1989, 30, 4711; (e) V. I. Korenskii, V. D. Skobeleva,
V. G. Kharchuk, I. P. Kolenko, V. L. Volkov, G. S. Zakharova
and V. N. Vinogradova, J. Gen. Chem. USSR, 1985, 55, 1750;
(f) O. A. Kholdeeva, A. V. Golovin, R. I. Maksimovskaya and
I. V. Kozhevnikov, J. Mol. Catal., 1992, 75, 235.
8 (a) H. Sun, K. Harms and J. Sundermeyer, J. Am. Chem.
Soc., 2004, 126, 9550; (b) H. Sun, X. Li and J. Sundermeyer,
J. Mol. Catal. A: Chem., 2005, 240, 119; (c) W. Bonrath and
T. Netscher, Appl. Catal., A, 2005, 280, 55–73;
(d) O. A. Kholdeeva and O. V. Zalomaeva, Coord. Chem.
Rev., 2016, 306, 302–330; (e) V. Y. Evtushok, A. N. Suboch,
2
+
(0.2) is a highly active, recyclable, and environmentally friendly
solid catalyst with an outstanding catalytic activity, among
mesoporous metallosilicate catalysts.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
O. Y. Podyacheva, O. A. Stonkus, V. I. Zaikovskii,
Y. A. Chesalov, L. S. Kibis and O. A. Kholdeeva, ACS Catal.,
N. Ravasio, Green Chem., 2007, 9, 731; (h) J. S. Reddy, P. Liu
and A. Sayari, Appl. Catal., A, 1996, 148, 7; (i) M. Selvaraj,
D.-W. Park, I. Kim, S. Kawi and C. S. Ha, Dalton Trans.,
2012, 41, 14204; ( j) M. Selvaraj, D.-W. Park, I. Kim, S. Kawi
and C. S. Ha, Dalton Trans., 2012, 41, 9633; (k) M. Selvaraj,
Catal. Sci. Technol., 2014, 4, 2674.
2
018, 8, 1297–1307.
9
M. Selvaraj, V. Narayanan and S. Kawi, Microporous
Mesoporous Mater., 2010, 132, 494.
1
0 S. Jana, B. Dutta, R. Bera and S. Koner, Langmuir, 2007, 23,
492.
2
17 (a) O. A. Kholdeeva, V. N. Romannikov, N. N. Trukhan and
V. N. Parmon, RU Patent, 2164510, 2000; (b) N. N. Trukhan,
V. N. Romannikov, E. A. Paukshtis, A. N. Shmakov and
O. A. Kholdeeva, J. Catal., 2001, 202, 110;
(c) O. A. Kholdeeva, N. N. Trukhan, M. P. Vanina,
V. N. Romannikov, V. N. Parmon, J. Mrowiec-Bialon and
A. B. Jarzebski, Catal. Today, 2002, 75, 203; (d) J. Mrowiec-
Białoń, A. B. Jarzębski, O. A. Kholdeeva, N. N. Trukhan,
V. I. Zaikovskii, V. V. Kriventsov and Z. Olejniczak, Appl.
Catal., A, 2004, 273, 47; (e) O. A. Kholdeeva and
N. N. Trukhan, Russ. Chem. Rev., 2006, 75, 411.
1
1 (a) X. Meng, Z. Sun, S. Lin, M. Yang, X. Yang, J. Sun,
D. Jiang, F.-S. Xiao and S. Chen, Appl. Catal., A, 2002, 236,
1
7; (b) H. Tang, Y. Ren, B. Yue, S. Yan and H. He, J. Mol.
Catal. A: Chem., 2006, 260, 121.
2 J. Andas, F. Adam and I. A. Rahman, Appl. Surf. Sci., 2013,
1
1
2
84, 503–513.
3 E. F. Murphy, L. Schmid, T. Burgi, M. Maciejewski,
A. Baiker, D. Gunther and M. Schneider, Chem. Mater.,
2
001, 13, 1296.
1
4 U. G. Singh, R. T. Williams, K. R. Hallam and G. C. Allen,
J. Solid State Chem., 2005, 178, 3405.
5 M. Selvaraj and S. Kawi, Chem. Mater., 2007, 19, 509.
18 M. Carril, P. Altmann, M. Drees, W. Bonrath, T. Netscher,
J. Schütz and F. E. Kühn, J. Catal., 2011, 283, 55.
1
1
6 (a) I. D. Ivanchikova, M. K. Kovalev, M. S. Mel’gunov, 19 Y. Li, W. Liu, M. Wu, Z. Yi and J. Zhang, J. Mol. Catal.,
A. N. Shmakov and O. A. Kholdeeva, Catal. Sci. Technol., 2007, 261, 73.
014, 4, 200; (b) O. A. Kholdeeva, I. D. Ivanchikova, 20 M. Selvaraj, M. Kandaswamy, D. W. Park and C. S. Ha,
2
M. Guidotti, C. Pirovano, N. Ravasio, M. V. Barmatova and
Catal. Today, 2010, 158, 377
Y. A. Chesalov, Adv. Synth. Catal., 2009, 351, 1877; (a) M. Selvaraj and S. Kawi, Microporous Mesoporous Mater.,
c) O. A. Kholdeeva, I. D. Ivanchikova, M. Guidotti, 2007, 101, 240; (b) M. Selvaraj, M. Kandaswamy, D. W. Park
N. Ravasio, M. Sgobba and M. V. Barmatova, Catal. Today, and C. S. Ha, Catal. Today, 2010, 158, 286; (c) M. Selvaraj,
009, 141, 330; (d) O. A. Kholdeeva, M. S. Melgunov, D. W. Park, S. Kawi and I. Kim, Appl. Catal., A, 2012, 17, 415;
(
2
A. N. Shmakov, N. N. Trukhan, V. V. Kriventsov, (d) S. E. Dapurkar, A. Sakthivel and P. Selvam, J. Mol. Catal. A:
V. I. Zaikovskii, M. E. Malyshev and V. N. Romannikov, Chem., 2004, 223, 241.
Catal. Today, 2004, 91–92, 205; (e) A. Tuel and L. G. Hubert- 21 M. Shimizu, H. Orita, T. Hayakawa, Y. Watanabe and
Pfalzgraf, J. Catal., 2003, 217, 343; (f) T. Maschmeyer,
F. Rey, G. Sankar and J. M. Thomas, Nature, 1995, 378, 159; 22 M. Selvaraj, S. W. Song and S. Kawi, Microporous
g) O. A. Kholdeeva, I. D. Ivanchikova, M. Guidotti and Mesoporous Mater., 2008, 110, 472.
M. Takehira, Bull. Chem. Soc. Jpn., 1991, 64, 2583.
(
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.