Enhancement of para-selectivity in the phenol oxidation with H2O2 over Ti-MCM-68 zeolite catalyst

Article abstract of DOI:10.1039/c5gc01237f

Although titanium silicalite-1 (TS-1) with an MFI framework (interconnected 10-ring micropores) is a useful catalyst for phenol oxidation, Ti-MCM-68, a novel MSE-type titanosilicate with 12-ring and 10-ring micropores, has been developed as a phenol oxidation catalyst with high activity and selectivity toward the p-isomer, hydroquinone. This report describes the improvement in catalytic activity and para-selectivity of Ti-MCM-68 for phenol oxidation. Thermal treatment after Ti insertion significantly enhanced the catalytic activity of Ti-MCM-68, mainly due to an improvement in the hydrophobicity in the micropores. Silylation of the external surface of Ti-MCM-68 with Ph₂SiCl₂, followed by Ti insertion and calcination, showed greater para-selectivity, indicating that the selective formation of p-isomers occurred within one-dimensional 12-ring micropores in the Ti-MCM-68. The addition of ethanol as a cosolvent to the reaction mixture resulted in the best para-selectivity (>94%) and activity among the modified Ti-MCM-68 catalysts.

Full text of DOI:10.1039/c5gc01237f

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Enhancement of para-selectivity in the phenol
oxidation with H2O2 over Ti-MCM-68 zeolite
catalyst†
Cite this: DOI: 10.1039/c5gc01237f
S. Inagaki, Y. Tsuboi, M. Sasaki, K. Mamiya, S. Park and Y. Kubota*
Although titanium silicalite-1 (TS-1) with an MFI framework (interconnected 10-ring micropores) is a
useful catalyst for phenol oxidation, Ti-MCM-68, a novel MSE-type titanosilicate with 12-ring and 10-ring
micropores, has been developed as a phenol oxidation catalyst with high activity and selectivity toward
the p-isomer, hydroquinone. This report describes the improvement in catalytic activity and para-selecti-
vity of Ti-MCM-68 for phenol oxidation. Thermal treatment after Ti insertion significantly enhanced the
catalytic activity of Ti-MCM-68, mainly due to an improvement in the hydrophobicity in the micropores.
Silylation of the external surface of Ti-MCM-68 with Ph2SiCl2, followed by Ti insertion and calcination,
showed greater para-selectivity, indicating that the selective formation of p-isomers occurred within one-
dimensional 12-ring micropores in the Ti-MCM-68. The addition of ethanol as a cosolvent to the reaction
mixture resulted in the best para-selectivity (>94%) and activity among the modified Ti-MCM-68 catalysts.
Received 7th June 2015,
Accepted 20th August 2015
DOI: 10.1039/c5gc01237f
www.rsc.org/greenchem
bisphenol-A into p-isopropenylphenol and phenol, and oxi-
dation of p-isopropenylphenol into HQ with the simultaneous
Introduction
Partial oxidation reactions are important for the production of formation of acetone (Fig. S1†). The phenol and acetone
practical and functional chemicals. In conventional oxidation (formed during the process as coproducts) can be used as the
processes, powerful oxidizing agents, such as KMnO4, NaClO3, starting materials. In addition, no significant quantity of salt
CrO3, m-chloroperoxybenzoic acid (MCPBA), peracetic acid and waste is formed. This is a typical “green” chemical process;
other peroxycarboxylic acids/alkyl hydroperoxides, are utilized. however, more straightforward routes to the desired product of
For example, hydroquinone (HQ), a key intermediate in the HQ warrant investigation.
manufacture of polymeric materials, e.g., PEEK (poly[ether
Hydrogen peroxide (H2O2) is a preferable oxidant for envir-
ether ketone]) resin, is produced via the oxidation of aniline onmentally benign liquid-phase reactions because it can
(with stoichiometric quantities of MnO2 and H2SO4) to oxidize organic compounds with an atom efficiency of 47%
p-benzoquinone, followed by reduction through stoichiometric and with the generation of water as the only coproduct, theo-
consumption of Fe and H2O (see the ESI, Fig. S1†).1 This retically. Since titanium-substituted molecular sieves with iso-
process is not environmentally friendly because the production lated tetrahedral Ti species in the zeolite framework are
of HQ is accompanied by the formation of one equivalent of required for selective oxidation reactions with H2O2,2–6
a
(NH4)2SO4 and FeO as well as four equivalents of MnSO4. Alter- variety of titanosilicate catalysts, such as TS-1,7 TS-2,8 Ti-beta
natively, a novel environmentally benign route has been deve- (Ti-BEA),9–11 Ti-ZSM-12 (Ti-MTW),12 Ti-ZSM-48,13 Ti-mordenite
loped by Upjohn.1 This newer route consists of bisphenol-A (Ti-MOR),14 Ti-SSZ-33,15 Ti-ITQ-7 (Ti-ISV),16 Ti-SSZ-23 (Ti-
synthesis starting with two equivalents of phenol and one STT)17 and Ti-MCM-22 (Ti-MWW),18–21 have been developed
equivalent of acetone, followed by heterolytic cracking of the for “green” and sustainable chemical processes.
Titanium silicalite-1 (TS-1) with MFI topology (10 × 10-
membered rings) is an industrially useful titanosilicate, which
is effective for phenol oxidation with H2O2 to HQ and catechol
Division of Materials Science and Chemical Engineering, Yokohama National
(CL) (Fig. S1†). Up to 85% para-selectivity has been achieved
through post-synthetic modification of TS-1 or with the aid of
a solvent in the reaction.22–24 Recently, Ti-MCM-68 25 and Ti-
YNU-2 26 have been developed as phenol oxidation catalysts
with high activity and selectivity toward hydroquinone. These
are novel MSE-type titanosilicates with a 12 × 10 × 10R pore
University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.
E-mail: kubota@ynu.ac.jp
†Electronic supplementary information (ESI) available: Detailed preparation pro-
cedures of catalysts, textural properties, reaction pathways, schematic diagram of
the preparation of Ti-MCM-68, graphics for size-comparison, schemes for plaus-
ible mechanism of phenol oxidation, and results of FT-IR, 13C CP MAS NMR,
TG-DTA, and N2-adsorption. See DOI: 10.1039/c5gc01237f
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Fig. 1 Projections of the (a) 10R window (0.51 × 0.55 nm) in TS-1 (MFI viewed along [010]) and (b) 12R window (0.64 × 0.68 nm) in Ti-MCM-68
(MSE viewed along [001]). The ionic radii of oxygen (red) and silicon (orange) in the center of each figure are 0.135 and 0.04 nm, respectively.
system in which a 12R straight channel intersects with two framework was isomorphous substitution of conventional
twisted 10R channels.27 These materials also contain a large Al-MCM-68 to Ti-MCM-68, which was accomplished using
cavity (18 × 12R) accessible only through the 10R pores. As post-synthetic modification of Al-MCM-68 (dealumination by
shown in Fig. 1a, the crystallographic dimensions of the 10R acid treatment followed by gas-phase Ti insertion using TiCl4
window of the straight channel in TS-1 (MFI) were 0.51 × as the Ti source, see Fig. S2†).25 The present report describes
0.55 nm when the ionic radii of O2− and Si4+ were defined as the improvement in catalytic activity and para-selectivity of
0.135 and 0.04 nm, respectively.28 Although breathing of the Ti-MCM-68 for phenol oxidation, and the effects of post-syn-
pore window could make the aperture expand momentarily, thetic silylation of external site defects and the addition of a
diffusion of a phenol molecule into the 10R straight channel cosolvent on catalytic performance.
of TS-1 was limited. In contrast, a 12R aperture in Ti-MCM-68
(MSE), 0.64 × 0.68 nm (Fig. 1b),28 is wide enough to allow a
phenol molecule to enter. Therefore, Ti-MCM-68 is a promis-
ing catalyst for selective p-hydroquinone production with high
efficiency for phenol oxidation.
Results and discussion
Catalyst preparation
The MSE-type molecular sieves can be obtained hydrother-
mally only as an aluminosilicate, and the Si/Al molar ratio of
the product is limited to the range of 9–12.29–32 While
Al-MCM-68 can be synthesized using N,N,N′,N′-tetraethyl-
Fig. 2 shows powder XRD patterns of the as-synthesized
sample and the post-synthetically treated samples. The as-syn-
thesized product possessed an MSE framework without any
impure phases, and after subsequent calcination and acid
treatment, the framework structures remained unchanged.
Acid treatment successfully produced highly dealuminated
MCM-68 (Si/Al > 1000, determined by ICP-AES analysis), likely
including some site defects (vacancy defects) in the MSE
framework. During TiCl4 treatment at 600 °C, the Ti species
was inserted into the site defects of dealuminated MCM-68,
which was confirmed by the sole presence of the UV-Vis peak
at ca. 210 nm (Fig. 3), corresponding to 4-coordinated Ti
species in the silicate framework. Other possible peaks corres-
ponding to 5- or 6-coordinated Ti species would appear at
around 250–290 nm, which are not observed in Fig. 3 but dis-
cussed elsewhere.5,26 It should be noted that the dealuminated
MCM-68 free of Ti atoms gave no obvious UV-vis peaks
(Fig. 3a). Another sign of Ti-incorporation into the framework
is the presence of a characteristic IR band at around 960 cm−1
(Fig. S3†), which is consistent with earlier examples.40,41 The
bicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium
diiodide
(TEBOP2+(I−)2) or another quaternary ammonium cation as a
structure-directing agent (SDA),33 researchers34 have reported
the synthesis of an UZM-35 zeolite, which has topology identi-
cal to MSE, but can be synthesized using dimethyldipropyl-
ammonium hydroxide as the SDA. Recently, a wide variety of
MSE-type aluminosilicates were developed during hydro-
thermal conversion of zeolite Y (FAU)35 or via seed-assisted
crystallization under OSDA-free synthetic conditions.36,37
A
precursor of the pure silica version of the MSE topology
(YNU-2P) and its stabilized microporous version (YNU-2)26,38
have been synthesized by utilizing the steam-assisted crystalli-
zation (SAC) method.39
Conversely, for the titanosilicate version, direct crystalliza-
tion of Ti-MSE remained difficult using any of the methods
effective for aluminosilicates or pure-silica materials. One
option for introducing a sufficient amount of Ti into the
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Fig. 3 DR UV-vis spectra of (a) deAl-MCM-68, (b) Ti-MCM-68, (c) Ti-
MCM-68_cal, (d) Ph2Si-Ti-MCM-68, (e) Si/Ti-MCM-68_cal, and (f) TS-1.
a UV-Vis peak at ca. 210 nm (Fig. 3). After the combustion of
the phenyl groups by calcination of Ph2Si-Ti-MCM-68 at
650 °C, the Ti content of the calcined sample (Si/Ti-
MCM-68_cal) was 0.3 mmol g−1, very similar to Ti-MCM-68_cal
(0.24 mmol g−1), indicating that TiCl4 molecules penetrated
into the micropores within Ph2Si-MCM-68, independent of the
phenyl groups situated on the external surface.
Fig. 2 XRD patterns of (a) calcined Al-MCM-68, (b) dealuminated
MCM-68, (c) Ti-MCM-68, (d) Ti-MCM-68_cal, (e) Ph2Si-MCM-68, (f)
Ph2Si/Ti-MCM-68, and (g) Si/Ti-MCM-68_cal.
FE-SEM observations (Fig. 4) indicated that all of the
MCM-68 samples were tiny particles (50–100 nm) and that
TS-1 consisted of agglomerates (ca. 300 nm) of tiny particles
(50–100 nm). The nitrogen adsorption–desorption isotherms
(Fig. S7†) indicated that Ti-MCM-68, Ti-MCM-68_cal and Si/Ti-
Ti contents of both Ti-MCM-68 and its calcined version (Ti-
MCM-68_cal) were ca. 0.24 mmol g−1 (Table 1).
The Ph2SiCl2 treatment of dealuminated MCM-68 gave
Ph2Si-modified MCM-68 with ca. 0.1 mmol g−1 Ph2Si on the
surface (ca. 1.1 molecules per nm2), as confirmed by 13C MAS
NMR and TG-DTA analyses (Fig. S4 and S5,† respectively).
In the 13C CP MAS NMR, two peaks at 133.9 and 137.9 ppm
were assigned to the phenyl group in Ph2Si on the MCM-68
surface; the peaks at 128.4, 124.8 and 20.2 ppm were assigned
to toluene, which was a residue after silylation. In the TG-DTA,
a decrease in weight (8.1 wt%) at the lower temperature (from
room temperature to ca. 100 °C) was caused by desorption of
water, toluene and adsorbed molecules, and a typical decrease
(1.5 wt%) at around 300 °C occurred due to combustion of the
phenyl group immobilized on the surface of dealuminated
MCM-68. The Ph2SiCl2 molecules that were larger than the
12R micropores (see Fig. S6†) could react selectively with the
silanols at the external site defects of dealuminated MCM-68.
The subsequent TiCl4 treatment of Ph2Si-modified MCM-68
successfully allowed Ti insertion in site defects located on the
micropore surface, which was supported by the appearance of
MCM-68_cal had similar BET surface areas (ca. 500 m2 g−1
)
and micropore volumes (0.19–0.20 cm3 g−1). Calcination of Ti-
MCM-68 at 650 °C improved hydrophobicity, as confirmed by
the water adsorption isotherm (Fig. 5). The hydrophobicity of
Si/Ti-MCM-68_cal was moderate, and was between that of Ti-
MCM-68 and Ti-MCM-68_cal. 29Si MAS NMR and FT-IR indi-
cated the decrease in (SiO)3SiOH (Q3) peaks and silanols (iso-
lated and hydrogen-bonded species), respectively (not shown).
Improvement in para-selectivity
Table 1 lists the results of phenol oxidation with H2O2 on tita-
nosilicate catalysts prepared for this study. First, the catalytic
performance of titanosilicates was investigated under con-
ditions in which water from the H2O2 solution and an excess
of phenol were the reaction media. No meta isomer (resorci-
nol) was detected and no interconversion was observed
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Table 1 Oxidation of phenol with H2O2 catalyzed by various titanosilicates
Yieldc (%)
Total
Catalyst
Ti contenta (mmol g−1
)
Cosolvent
TONb
HQd
CLd
p-BQd
para-Sel.e (%)
Ti-MCM-68
0.235
None
MeOH
EtOH
MeCN
None
MeOH
EtOH
MeCN
None
MeOH
EtOH
MeCN
Neat
90
75
10.0
8.1
6.5
5.9
14.0
9.4
26.3
33.1
45.8
39.0
28.3
42.9
77.6
56.3
8.6
3.5
0.4
1.2
2.9
7.6
2.8
3.4
5.5
6.5
3.4
4.6
6.3
6.6
1.4
1.4
4.2
0.0
1.8
0.0
0.9
1.2
0.0
0.0
2.9
1.0
0.0
0.0
2.7
0.0
0.0
0.0
0.0
65.0
95.1
92.0
78.3
78.3
92.2
93.1
88.4
81.9
92.7
94.4
90.3
56.8
75.3
71.9
48.7
134
118
314
324
446
419
259
335
651
460
90
15.2
13.1
35.0
35.9
49.2
47.4
35.7
46.7
82.3
65.3
15.2
5.7
Ti-MCM-68_cal
Si/Ti-MCM-68_cal
TS-1
0.241
0.300
0.373
MeOH
EtOH
MeCN
32
27
47
4.3
3.5
4.0
4.8
8.1
Reaction conditions: catalyst, 20 mg; phenol, 21.25 mmol; H2O2 (31 wt%), 4.25 mmol; H2O, 17.8 mmol; cosolvent, 4.0 g; temperature, 70 °C;
time, 60 min. a Determined by ICP analysis. b Turnover number (moles of [HQ + CL + p-BQ] per mole of Ti site). c Product yields based on added
H2O2 after exhaustive acetylation of the products with excess Ac2O–K2CO3, the derivatized products were analyzed by GC (0.25 mm × 30 m ×
1.00 μm DB-1 column, internal standard: anisole, detector: FID). d HQ = hydroquinone, CL = catechol, p-BQ = p-benzoquinone. e Selectivity to
para-isomers of dihydroxybenzenes and quinones (moles of [HQ + p-BQ] per mole of [HQ + CL + p-BQ]).
Fig. 5 H2O adsorption isotherms at 298 K of (a) TS-1, (b) Ti-MCM-68,
(c) Ti-MCM-68_cal, and (d) Si/Ti-MCM-68_cal.
between HQ and CL in independent experiments. The total
yield in Ti-MCM-68_cal (35.0%) was significantly greater than
that in Ti-MCM-68 (10.0%), which agreed well with a previous
report.25 Water adsorption isotherm measurements (Fig. 5)
suggested that this improvement in catalytic activity was due
to an increase in hydrophobicity. The Si/Ti-MCM-68_cal also
showed a high total yield (35.7%) comparable to Ti-MCM-68_
cal (35.0%), strongly suggesting that tetrahedral Ti species
Fig. 4 Typical FE-SEM images of (a) Ti-MCM-68 and (b) TS-1.
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were successfully formed in the MSE framework during TiCl4
treatment, regardless of Ph2SiCl2 pre-treatment.
Table 1 lists typical results for phenol oxidation over titano-
silicate catalysts upon the addition of MeOH, ethanol (EtOH)
All three Ti-MCM-68 catalysts possessed greater para-selecti- or acetonitrile (MeCN) as a cosolvent. No oxidation products of
vity compared to TS-1 (56.8%). The greater para-selectivity in EtOH, such as acetaldehyde (MeCHO) and/or acetic acid
Ti-MCM-68_cal (78.3%) compared to that in Ti-MCM-68 (AcOH), were detected in this reaction system. When MeOH or
(65.0%) was probably caused by the predominant shape-selec- EtOH was added to the TS-1 system, the total yield of the pro-
tive reaction in the hydrophobic micropores to give p-isomers. ducts greatly decreased even though para-selectivity was
Wilkenhöner et al.22 suggested two possible reaction mecha- slightly improved. This improvement may have been caused by
nisms (A and B) for the formation of CL (o-isomer) during lower opportunity of o-hydroxylation on the external surface
phenol oxidation over TS-1; mechanism A involves a phenol due to the covering of external tetrahedral Ti sites with alco-
molecule coordinated to Ti-peroxo species that was trans- hols. As an additional factor, the presence of alcohols inside
formed into CL through a penta-coordinate trigonal bipyrami- 12-ring micropores might cause a little more steric restriction
dal Ti site; mechanism B involves the formation of CL without forcing para-substitution to occur. Meanwhile, the addition of
coordination of phenol to the active titanium site, which MeCN in the TS-1 system decreased the total yield as well as
occurred via a six-membered ring transition state involving para-selectivity.
phenol. Both mechanisms indicate that, for less restricted geo-
In contrast, in all three Ti-MCM-68 systems, a dramatic
metric Ti sites (as on the external surface of TS-1), CL is the improvement of catalytic activity and para-selectivity occurred
preferred product. Conversely, the formation of p-isomer, HQ, upon the addition of a cosolvent. For example, the addition of
can be assisted neither by coordination nor by a six-membered EtOH in the Si/Ti-MCM-68_cal system resulted in 82.3% total
ring transition state. It should be formed in a shape-selective yield and 94.4% para-selectivity. The relative hydrophilicity of
way under limiting geometrical conditions, as in the suitable Ti-MCM-68 (Fig. 5) makes the hydrogen bonding of MeOH or
micropores. The 10R pores of TS-1 (0.51 × 0.55 nm) are a little EtOH with Si–OH in the external surface of catalysts more
too small to show sufficient shape-selectivity. Thus, in the likely, narrowing the pore mouth and covering external active
liquid-phase reaction, the 12R micropores (0.64 × 0.68 nm) in Ti sites. Therefore, Ti-MCM-68 in MeOH or EtOH exhibited
Ti-MCM-68 may efficiently promote a restricted geometry, greater para-selectivity at relatively lower catalytic activity, com-
leading to selective formation of hydroquinone. The above- pared to neat (cosolvent-free) conditions. The more hydro-
mentioned mechanism B could be partially applicable to the phobic Ti-MCM-68_cal containing only a trace amount of
present case by removing the assistance of a six-membered external Si–OH groups could interact with a lower amount of
ring transition state but with the aid of steric restriction by a MeOH or EtOH, resulting in a much more hydrophobic exter-
suitable 12R micropore, as shown in Fig. S8.† The slightly nal surface, without pore narrowing, that helped guide phenol
wider pores in Ti-MCM-68 dramatically enhanced the diffusiv- molecules into the 12R micropores and to allow shape-selec-
ity of phenol molecules while maintaining the restricted geo- tive reactions to occur. These factors may have improved the
metry inside the pore, causing the para-selective reaction catalytic activity and greater para-selectivity for phenol oxi-
predominantly compared to the relatively narrow micropores dation over Ti-MCM-68_cal in the presence of a cosolvent.
of TS-1. Intracrystalline diffusivity of phenol in Ti-beta (12R, For Si/Ti-MCM-68_cal, the addition of EtOH improved both
0.66 × 0.67 nm) was much greater (D = 3.9 × 10−16 m2 s−1) than the catalytic activity and para-selectivity compared to MeOH
that in TS-1 (D = 5.2 × 10−17 m2 s−1) at 30 °C in water.23 This addition, probably due to the greater hydrophobicity of the
can explain the much greater para-selectivity over Ti-MCM-68 ethyl group compared to the methyl group on the external
catalysts. Note that Si/Ti-MCM-68_cal gave excellent para- surface of titanosilicate particles. When isopropyl alcohol
selectivity (81.9%), suggesting that Ph2SiCl2 molecules reacted (i-PrOH) was added as a cosolvent to the reaction system,
with the site defects on the external surface of dealuminated Si/Ti-MCM-68_cal produced a total yield of 65% with greater
MCM-68, while TiCl4 molecules selectively reacted with the para-selectivity (93%), indicating that isopropyl groups on
site defects on the micropore surface.
the external surface were hydrophobic enough to guide
phenol into the micropores, but were too bulky to allow
easy access of a phenol molecule into the 12R-pore. This
tendency was also observed for Ti-MCM-68 and Ti-
Effect of cosolvent
In the TS-1 catalyst system, para-selectivity improved from 56% MCM-68_cal. Consequently, EtOH addition could improve
to 68% when methanol (MeOH) was added as a cosolvent to both the catalytic activity and para-selectivity of the Ti-
the reaction mixture.22 One possible reason proposed was that MCM-68 catalyst for phenol oxidation, probably due to
the coordination of the protic solvent increases the size of the enhancement of hydrophobicity via covering of external Ti
active Ti site. In the narrow TS-1 micropores, this coordination sites and Si–OH with EtOH. Although MeCN also appears to
provides geometric constraints for an approaching phenol be a good cosolvent for improving the hydrophobicity of the
molecule. Therefore, phenol will approach the bulky Ti site external surface of Ti-MCM-68, the MeCN molecules may
with the OH group pointing away from the Ti site, producing interact more strongly with active Ti sites located on the
HQ.22 The effect of a cosolvent in the Ti-MCM-68 system for micropore surface and deactivate the catalytic active sites
phenol oxidation was also investigated.
during phenol oxidation.
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aqueous solution of H2O2 (466 mg, 4.25 mmol) were loaded
into a 35 mL glass pressure tube. When the cosolvent was
used, 4.0 g of MeOH, EtOH, i-PrOH, or MeCN was added to
Experimental
Preparation of titanosilicates
Ti-MCM-68 was prepared from Al-MCM-68 through dealumina- the reaction mixture. The whole mixture was immediately
tion by nitric acid treatment, TiCl4 treatment and immersed in a pre-heated oil bath to start the reaction. The
calcination.25,30–33 The detailed synthetic procedure of Ti- reaction mixture was stirred at 70 °C for 60 min, and then
MCM-68 is available in the ESI.† However, for this study, an immediately cooled in an ice bath to stop the reaction. Sulfo-
alternative synthetic route to Ti-MCM-68 was developed to lane (2.0 g as a suitable unreactive solvent for derivatization),
insert the Ti species into internal site defects selectively. In a anisole (225 mg, 2.0 mmol, as an internal standard), and the
typical procedure, the silylation of external site defects of deal- catalyst was removed from the reaction mixture by centrifu-
uminated MCM-68 (1.0 g) with Ph2SiCl2 (0.2 g) was conducted gation. The supernatant (100 mg) was treated with an excess
under reflux conditions for 24 h in toluene (50 mL), followed amount of acetic anhydride (ca. 200 mg, 2 mmol) and K2CO3
by recovery of the silylated solid through filtration and drying (ca. 300 mg) at 35 °C for 10 min to achieve exhaustive acetyl-
at room temperature. Next, the Ph2Si-modified sample was ation of the phenolic OH groups. After removing the residual
exposed to a flow of argon bubbled through a vessel containing inorganic solids by filtration through a membrane filter and
TiCl4. The TiCl4 flow was continued for 60 min at 600 °C. The adjusting the concentration to the one suitable for gas chrom-
resulting Ti-Ph2Si-MCM-68 was treated thermally at 650 °C for atography (GC) with chloroform, the GC analysis was per-
4 h in a furnace to give Si/Ti-MCM-68_cal.
formed on a Shimadzu GC-2014 instrument equipped with a
TS-1 was obtained from the Asia Catalyst Project in collabor- flame ionization detector using a capillary column of DB-1
ation with the Catalysis Society of Japan as an Asia-reference (30 m length, 0.25 mm diameter, and 1.00 μm film thickness).
catalyst TS-1 (catalogue specification: Si/Ti = 35).
The unreacted H2O2 was quantified using the iodometry
method involving titration of a well-stirred solution of the reac-
tion mixture (1.0 g) and KI (0.2 g) in 2.0 mol L−1 hydrochloric
Characterization
All titanosilicates used in this study were characterized by acid (50 mL) with 0.1 mol L−1 aqueous Na2S2O3 solution.
powder X-ray diffraction (XRD, Ultima-IV, Rigaku) using CuKα
radiation at 40 kV and 20 mA. The textural properties, specific
surface area and micropore volume of titanosilicates were eval-
uated using the nitrogen adsorption–desorption isotherm
Conclusions
(BELSORP-max-1-N, Bel Japan) at 77 K. The morphologies of The Ti-MCM-68 catalyst with an MSE structure demonstrated
zeolite catalysts were observed by field emission scanning elec- greater catalytic activity and para-selectivity for phenol oxi-
tron microscopy (FE-SEM, SU9000, Hitachi). The contents of Ti dation with H2O2 compared to the conventional catalyst TS-1
in titanosilicates were measured using inductively coupled with an MFI structure. The high para-selectivity of Ti-MCM-68
plasma atomic emission spectrometry (ICP-AES, ICPE-9000, was due to the predominant shape-selective reaction in hydro-
Shimadzu). The coordination states of Ti species in the titano- phobic 12R micropores to give p-isomers, unlike the behaviour
silicates were evaluated using diffuse reflectance UV-vis spec- of the 10R micropores in TS-1. The high catalytic activity of
troscopy (DR/UV-Vis, V-550 spectrometer, JASCO). The organic Ti-MCM-68 was caused by the high diffusivity of phenol within
contents in Ph2Si-modified materials were measured using the 12R channels. Thermal treatment after Ti-insertion signifi-
thermogravimetry (Thermo-Plus, Rigaku).
cantly enhanced the catalytic activity of Ti-MCM-68, due
The FT-IR spectra of titanosilicates were recorded on a mainly to an increase in the hydrophobic nature inside the
JASCO FT/IR-6100 spectrometer equipped with a TGS detector. pores. Note that Si/Ti-MCM-68 showed greater para-selectivity
A titanosilicate sample (ca. 1 mg) diluted with KBr powder (ca. for phenol oxidation in the presence of EtOH as a cosolvent.
100 mg) was used for the measurement in the region of The high para-selectivity (>94%) of Si/Ti-MCM-68_cal while
700–1000 cm−1
.
maintaining a high turnover number (TON) is considered to
The solid-state magic angle spinning nuclear magnetic be due to enhancement of hydrophobicity by covering external
resonance (MAS NMR) spectra were obtained using an Ti sites through Si–OH and EtOH interactions.
AVANCEIII 600 (Bruker) instrument operating at 600 MHz for
1H and 150 MHz for 13C. All of the MAS NMR spectra were
obtained at room temperature in a 4 mm diameter ZrO2 tube.
Acknowledgements
The 13C chemical shifts were determined using glycine. The
cross-polarization (CP) MAS NMR measurements were This work was financially supported in part by Grants-in-Aid for
obtained using 2000 pulses with a recycle time of 4.0 s at a Scientific Research (no. 13199071 and 23760741) from the
spinning rate of 10 kHz.
Japan Society for the Promotion of Science (JSPS). We are grate-
ful to Prof. T. Tatsumi of Tokyo Institute of Technology for
helpful discussion. We thank Dr A. Endo of National Institute
Reaction procedures
The oxidation of phenol with H2O2 was performed as follows. of Advanced Industrial Science and Technology and Mr Y. Ohno
Catalyst (20 mg), phenol (2.00 g, 21.25 mmol) and a 31 wt% of Yokohama National University for technical assistance.
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