Studies on the carboxymethylation and methylation of bisphenol A with dimethyl carbonate over TiO2/SBA-15

Article abstract of DOI:10.1016/j.molcata.2009.08.027

Carboxymethylated species were selectively synthesized from dimethyl carbonate (DMC) and bisphenol A (BPA) over TiO₂/SBA-15. On the basis of catalyst characterization by means of XRD, FT-IR, HPLC and GC-MS, the relations between catalytic performance and catalyst properties were discussed. Si-O-Ti was active sites for reaction, and the interaction mode between Ti-O-Si and DMC was main factor to determine carboxymethylation and methylation. When DMC was attacked by Ti-O-Si on two oxygen atoms of CH₃-O moiety, BPA attacked carbonyl carbon to form carboxymethylated products. If the interaction occurs through the oxygen of C{double bond, long}O moiety, BPA attacked methyl carbon to form methylated products. Chemisorbed H₂O over TiO₂/SBA-15 made DMC to act as methylating agent. After chemisorbed H₂O was removed, carboxymethylated species of two-methylcarbonate-ended-BPA (DmC(1)) and one-methylcarbonate-ended-BPA (MmC(1)) were selectively synthesized.

Full text of DOI:10.1016/j.molcata.2009.08.027

Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Studies on the carboxymethylation and methylation of bisphenol A with
dimethyl carbonate over TiO2/SBA-15
Kunmei Sua, Zhenhuan Lia,∗, Bowen Chenga,∗, Kun liaoa, Dexin Shena, Yufei Wangb
a College of Materials and Chemical Engineering, and Tianjin Key Lab of Fiber Modification & Functional Fiber, Tianjin Polytechnic University, Tianjin 300160, China
b College of Chemical Engineering, Dalian University of Technology, Dalian 116012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2009
Received in revised form 30 August 2009
Accepted 31 August 2009
Available online 6 September 2009
Carboxymethylated species were selectively synthesized from dimethyl carbonate (DMC) and bisphenol
A (BPA) over TiO2/SBA-15. On the basis of catalyst characterization by means of XRD, FT-IR, HPLC and
GC–MS, the relations between catalytic performance and catalyst properties were discussed. Si–O–Ti
was active sites for reaction, and the interaction mode between Ti–O–Si and DMC was main factor
to determine carboxymethylation and methylation. When DMC was attacked by Ti–O–Si on two oxy-
gen atoms of CH3–O moiety, BPA attacked carbonyl carbon to form carboxymethylated products. If the
interaction occurs through the oxygen of C O moiety, BPA attacked methyl carbon to form methylated
products. Chemisorbed H2O over TiO2/SBA-15 made DMC to act as methylating agent. After chemisorbed
H2O was removed, carboxymethylated species of two-methylcarbonate-ended-BPA (DmC(1)) and one-
methylcarbonate-ended-BPA (MmC(1)) were selectively synthesized.
Keywords:
Carboxymethylation
Methylation
Dimethyl carbonate
Bisphenol A
© 2009 Elsevier B.V. All rights reserved.
TiO2/SBA-15
1. Introduction
Shaikh et al. [14] and Haba et al. [18] reported that the
methylcarbonate-ended oligomers were most reactive in produc-
Poly(bisphenol A carbonate) (PC) is an important thermoplastic
which is often produced by the interfacial polycondensation of BPA
and phosgene, however, phosgene process used highly toxic phos-
gene as the reagent and copious amounts of methylene chloride
as the solvent [1]. Thus it is increasingly important that an envi-
ronmentally friendly synthesis of PC is developed. In an attempt
to devise a phosgene-free synthesis of PC, diphenyl carbonate
(DPC) [1–6], CO [7–13] and DMC [14–18] were used as carbony-
lating agents to replace phosgene. The reaction between DPC and
BPA resulted in phenol as a condensation byproduct, but phenol
removing often took place under the conditions of rising tempera-
ture (higher than 300 ◦C) and lowering vacuum pressure (less than
13 Pa). However, these severe reaction conditions easily result in
fatal damages to the PC produced, such as the thermal instability of
BPA at high temperature can result in the discoloration of the final
product and complicated byproducts [19]. As a potential substitute
for DPC, DMC would react with BPA into polycarbonate precursors,
and it has a great advantage that there is no need to obtain DPC.
Furthermore, polycondensation steps of the prepolymers removing
methanol often took place under lower temperature.
ing high molecular weight polymers in the postpolycondensation
step compared with hydroxyl-ended oligomers. Therefore, the
selective synthesis of methylcarbonate-ended oligomers became
most important. However, the presence of two nonequivalent elec-
trophilic centers in DMC, the carbonyl and methyl moieties, make
it possible for BPA nucleophile to undergo methylation and car-
boxylation [20]. Methylation products of BPA are unreactive in the
postpolymerization step because they have no carbonate functional
end groups to react further. Therefore, minimization of methylation
byproducts is indispensably required. The central carbon of DMC,
due to sp2 hybridization, results in a greater positive charge of the
carbon atom, making it a harder center. The two methyl groups on
the other hand are the softer electrophilic centers, owing to the
acceptor atom having an sp3 orbital and a saturated carbon atom,
which has a weaker positive charge [21]. Therefore, in an exten-
sion of Pearson’s HSAB theory [22], it is possible to say that harder
nucleophiles in a reaction with DMC will undergo carboxymethyla-
tion (hard reaction, Scheme 1, reaction 1), while softer nucleophiles
will undergo methylation (soft reaction, Scheme 1, reaction 2). As
for phenols with an electron withdrawing group (EWG) and phe-
nols with electron donating groups (EDG), an EWG would make the
phenolic oxygen more polarizable, making it a softer donor atom.
Contrarily, an EDG would decrease the polarizability of the donor
atom, making the phenolic oxygen a harder donor atom [21]. It
could be speculated that harder BPA nucleophile in a reaction with
DMC will mainly underwent carboxymethylation.
∗
Corresponding authors. Tel.: +86 222452 8358; fax: +86 222452 8055.
E-mail addresses: zhenhuanli1975@yahoo.com.cn (Z. Li), Bowen15@tjpu.edu.cn
(B. Cheng).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.08.027

K. Su et al. / Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
61
(EO)20(PO)70(EO)20:HCl:TEOS:H2O = 2:60:4.5:15
(mass
ratio)
was prepared, stirred for 24 h at 40 ◦C, and then heated under
autogenous pressure in a sealed autoclave at 100 ◦C for 24 h. The
solid product was collected by filtration and calcined at 550 ◦C for
6 h. To prepare TiO2/SBA-15 catalysts, SBA-15 was impregnated
in ethanol solution of TBOT at room temperature for 24 h, and
then dried in an oven at 80 ◦C for 8 h to remove ethanol. The
TBOT-impregnated silica was calcined with an air stream at 550 ◦C
for 3 h. Other metal oxides were loaded on SBA-15 supports by the
wet impregnation of corresponding metal nitrate. After solvent
was removed by evaporation at 80 ◦C for 8 h, the catalysts were
calcined at 550 ◦C for 3 h.
Scheme 1. (1) Carboxymethylation and (2) methylation reactions.
Carboxymethylation of BPA with DMC was often catalyzed
by Lewis acid catalysts, for example, TiO2/SiO2 [23], (Bu2SnCl)2O
[18], Bu2SnO and titanium alkoxides. Haba et al. [18] reported
the synthesis of methylcarbonate-ended oligomers from the car-
boxymethylation of BPA with a large excess of DMC (68 times
the mole of initial BPA), methylcarbonate-ended oligomers could
be produced with 22% yield with respect to BPA converted after
48 h under a catalyst system of dibutyltinchlorooxide and dimethy-
laminopyridine. Also, they used a substantial amount of a 4 Å
molecular sieve (3.5 times the weight of initial BPA) to remove
the methanol coproduced to alleviate the equilibrium limitation.
Because the separation of homogeneous catalyst from products was
complicated, the development of heterogeneous catalysts with bet-
ter activity and higher selectivity was highly desirable in view of
regeneration and separation. However, there are few reports on the
development of active heterogeneous catalysts for this reaction.
The ordered mesoporous silicas like SBA-15 [24] and MCM-41
[25,26], with adjustable pore sizes, very high specific surface areas
and large pore volume, provided a special platform for the develop-
ment of advanced materials with different chemical functionalities
[27–29]. Substitution of Si by Ti created more attention, and the
conventional way to introduce Ti in mesoporous silicas was usu-
ally achieved by direct synthesis. Postsynthetic incorporation of
Ti in mesoporous silicates is much less investigated; however, the
postsynthetic incorporation of Ti seems to have a great advantage,
that is, the opportunity to start with a highly ordered mesoporous
material with a very high inner surface and pore volume. Herein, the
carboxymethylation of BPA with DMC was promoted by TiO2/SBA-
15 which was prepared by postsynthetic process. Si–O–Ti was
active sites for reaction, and the interaction mode between Ti–O–Si
and DMC was main factor to determine carboxymethylation and
methylation. Chemisorbed water blocked the carboxymethylation,
and 10 wt% TiO2/SBA-15 gave the best results to produce MmC(1)
and DmC(1) in 25.3% and 3.6% yields at 10 h. In addition, the effects
of TiO2 loading, temperature, time, catalyst amount, phenol/DMC
mole ratio on reaction were investigated, and possible reaction
mechanism was provided.
2.3. Characterization
XRD patterns were measured on a Bruker AXS (Germany) using
Cu K␣ radiation. The date was recorded from 1◦ to 80◦ (2ꢀ). FT-
IR spectrum was obtained on a Nicolet Nexus 470 FT-IR analyzer
using the KBr method. Reaction products were analyzed by HPLC
and GC–MS (HP5972) (capillary column: 30 m SE-30, 0.25 mm ID,
and 0.25 ␮m film thickness) to identify products and polycarbonate
precursors. Quantitative analysis was carried on a gas chromato-
graph (Agilent 6890N GC with a FID detector, HP-5/DB-5 capillary
column) with cetane as interior standard. The HPLC analyses were
carried out by a reverse-phase method on a Waters 2690 separa-
tion module (Waters Associates, Milford, MA). As an UV detector, a
Waters 2487 Dual Absorbance Detector was used at the wavelength
of UV 240 nm. The reverse-phase LC column was Waters Lichrosorb
RP18 analytical column. A mobile phase flow rate of 1 mL/min was
adjusted with a gradient mode of acetonitrile and water from 65/35
to isocratic of acetonitrile. The reaction samples were diluted with
THF and 5 ␮L was injected. The Millenium 32 software was used for
data reduction and integration. It gave a good separation and UV
sensitivity to polycarbonate precursors and BPA.
2.4. Reaction procedure
Reactions were carried out in a four neck flask, equipped with
a thermometer, nitrogen inlet, dropping funnel and fractionating
column connected to a liquid dividing head. BPA and catalyst were
charged into flask under nitrogen atmosphere. When mixture was
heated to 160 ◦C, DMC was added drop-wise, and reaction was con-
tinued for a certain period of time. During the reaction, a distillate
of DMC and methanol was collected slowly in a receiver flask. After
reaction finished, mixture was cooled and analyzed. DmC(1) and
DmC(1) yields were based on initial BPA.
2. Experiments
3. Results and discussion
2.1. Chemical reagents
The carboxymethylation of BPA with DMC mainly produced
MmC(1) and DmC(1), and heavier products from further reaction
of MmC(1) and DmC(1) were not detected by HPLC analyses, unlike
in the case of BPA and DPC. In addition, there were little byproducts
formed from O-methylation of BPA (A), ortho-methylation of BPA
with DMC (B and C) (see Scheme 2).
(NH4)6Mo7O24·4H2O was purchased from Tianjin Chemical
Reagent Co. (IV) and used as received. DMC (Tianjin Chemical
Reagent Research Institute) was fractionally distilled and stored
over molecular sieve (4 Å). Tetrabutoxy titanium (TBOT) was
purchased from Tainjin Kermel Chemical Reagents Development
Center. BPA (Tianjin Chemical Reagent Co. (III)) was of analytic
reagent (AR) grade and used without further purification. Other
chemical reagents were AR grade and used as received from local
manufactures without further purification.
3.1. The effects of various catalysts on carboxymethylation and
methylation
As shown in Table 1, TiO2/SBA-15 showed highest activi-
ties among TiO2 nanoparticle, MoO3/SBA-15, ZnO/SBA-15 and
Pb2O3/SBA-15. MmC(1) and DmC(1) could be produced with 25.3%
and 3.6% yields with respect to BPA converted over TiO2/SBA-15 at
10 h. Reutilization catalystic test indicated that RTiO2/SBA-15 could
be reused and showed even higher activity for DmC(1) synthesis.
When TiO2 nanoparticle acted as catalyst, MmC(1) and DmC(1)
2.2. Sample preparation
SBA-15 was synthesized in accordance with the published pro-
cedure [30] using tri-block copolymer poly(ethylene oxide)–poly
(propylene oxide)–poly(ethylene oxide) [(EO)20(PO)70(EO)20
]
as template in acidic conditions. In brief, solution of
a

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K. Su et al. / Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
Scheme 2. The reaction of BPA and DMC over TiO2/SBA-15.
yields declined to 7.4% and 1.2%. Although MoO3 and Pb3O4 sup-
ported catalyst displayed the high activity in reaction between
DMC and phenol [26,31], no DmC(1) was detected, and only 9.1%
and 3.4% MmC(1) yields were obtained over MoO3/SBA-15 and
Pb3O4/SBA-15. When reaction was carried out over ZnO/SBA-
15, the selectivity of carboxymethylated byproducts amounted
to 41.7%. The main difference in the two catalytic reactions of
ZnO/SBA-15 and TiO2/SBA-15 was the relative percentages of car-
boxymethylation and methylation at the completion of reaction,
signifying that ZnO/SBA-15 has great effect on the hard–soft behav-
ior of this class of nucleophile. Although (C4H9)2SnO showed the
high activity in carboxymethylation, it belonged to homogeneous
catalysts which resulted in difficult separation of catalysts from
products. Considering the stability and facile recovery and regen-
eration after reaction, silica-supported titania was chosen as better
catalyst.
The investigation of heterogeneous catalyst framework was
conducted via FT-IR spectroscopy, and results were shown in
Fig. 1. TiO2/SBA-15 showed a strong absorption peak at 960 cm−1
that could be ascribed to the vibration of O3–Si–O–Ti in sup-
ported molecular sieve [32]. The bands at 800 cm−1 and 1090 cm−1
were for symmetric Si–O–Si stretching vibration and asymmetric
Si–O–Si stretching vibration, respectively. IR bands at 1630 cm−1
and 3450 cm−1 were attributed to bending vibration and stretching
vibration of the O–H bond of chemisorbed H2O on Lewis acid sites
[33]. The characterized results indicated that the Si–O–Ti species
over TiO2/SBA-15 had high activity for carboxymethylation of BPA
with DMC.
Ti–O–Si has tetrahedral symmetry of Ti with four surrounding
O anions [34], so DMC adsorbs preferentially the surface Ti–O–Si
bonds to form cis–cis and cis–trans species together with tetrahe-
dral Ti changing into octahedral Ti with six surrounding O anions
(I) or pentahedral Ti with five surrounding O anions (II and III) (see
Scheme 3) [35]. When the interaction between Ti–O–Si and DMC
occurs through the two oxygen atoms of the CH3–O moiety, the
electrostatic perturbation in adducts Ti–O–Si and DMC with cis–cis
species (I) induces an elongation (weakening) of the C–OCH3 bonds,
a fact which can explain the high catalytic activity of TiO2/SBA-15 in
carboxymethylation reactions. If the interaction between Ti–O–Si
and DMC occurs through the oxygen of the C O moiety, in adducts
Fig. 1. FT-IR of supported catalyst (a) TiO2/SBA-15, (b) ZnO/SBA-15, (c) MoO3/SBA-
15, and (d) Pb2O3/SBA-15.
Ti–O–Si and DMC with cis–cis and cis–trans species (II and III)
an elongation of the O–CH3 bonds occurs, which justifies the cat-
alytic activity of TiO2/SBA-15 in methylation reactions. When DMC
adsorbed the surface ZnO/SBA-15, much the interaction occurs
through the oxygen of the C O moiety, which could explain the
selectivity of methylation byproducts amounted to 41.7%.
3.2. Effect of TiO2 loading on carboxymethylation and
methylation
The X-ray powder diffraction analysis was undertaken to deter-
mine the composition and crystallinity of Ti species. The low-angle
XRD patterns of TiO2/SBA-15 were displayed in Fig. 2(a). The well-
defined diffraction peak indexable as 100 reflection, which was
associated with SBA-15 type mesostructures, was observed for
all these samples. The peak intensity was almost kept with an
increase in TiO2 loading up to 30 wt%, suggesting that the long-
Table 1
Effects of catalyst on carboxymethylation and methylation.
Catalyst
Product distribution (mol%)
YMmC(1) (mol%)
YDPC(1) (mol%)
MmC(1)
DmC(1)
Methylation products
TiO2
80.5
83.4
81.2
99.0
58.3
91.8
80.5
12.6
11.7
13.5
–
–
–
6.9
4.9
5.3
1.0
41.7
8.2
7.4
25.3
23.4
9.1
4.2
3.4
1.2
3.6
3.9
–
–
–
TiO2/SBA-15
RTiO2/SBA-15
MoO3/SBA-15
ZnO/SBA-15
Pb3O4/SBA-15
(C4H9)2SnO
18.1
1.4
44.3
10.0
Reaction conditions: BPA 11.4 g, DMC 18 g, catalyst 0.5 g (with 10 wt% metal oxide loading), 160 ◦C, 10 h. RTiO2/SiMCM-41 is the secondly used catalyst which is activated at
550 ◦C for 3 h.

K. Su et al. / Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
63
Scheme 3. DMC adsorbs the surface Ti–O–Si bonds to form cis–cis and cis–trans species. Tetrahedral Ti with four surrounding O anions transfer into octahedral Ti with six
surrounding O anions (I) and pentahedral Ti with five surrounding O anions (II and III).
range regularity of hexagonal arrays of mesopores was sustained
after the introduction of TiO2 by wet impregnation technique. How-
ever 110 and 200 reflections decreased as TiO2 loading increasing,
which suggested that TiO2 species deposited inside the mesopore
channels. The extensive-angle XRD showed that there were no
characteristic diffraction peaks of TiO2 crystal when the loading
amount of TiO2 was less than 10 wt%. When TiO2 loading increased
to 20 wt%, the XRD peaks of anatase TiO2 crystal appeared, which
showed the presence of bulk titanium oxide crystal on catalyst
surface.
The FT-IR spectra of catalysts were listed in Figs. 3 and 4. The
broad absorption band for OH stretching around 3400 cm−1 and
the corresponding HOH bending at 1630 cm−1 occurred [33]. As
TiO2 loading increasing, the absorption band at 960 cm−1 became
stronger. The stronger absorption peaks at 960 cm−1 indicated the
more titanium was incorporated into SBA-15 framework, i.e., much
more Si–O–Ti species existed over TiO2/SBA-15 [32]. 639 cm−1 sug-
gested that Ti–O–Ti species appear on TiO2/SBA-15. It was worthy
of noting that there were two weak absorption peaks at 2336 cm−1
and 2360 cm−1 in 10 wt% TiO2/SBA-15. Those two weak bands were
Fig. 3. FT-IR of TiO2/SBA-15 with different TiO2 loadings: (a) 2.5 wt%, (b) 5 wt%, (c)
10 wt%, (d) 20 wt%, and (e) 30 wt%.
attributed to vibration of chemisorbed CO2 over defect sites which
were mainly caused by O vacancies [36] and tetrahedral envi-
ronment [37,38]. It was well known that Ti–O–Si species have
tetrahedral symmetry of Ti with four surrounding O anions [34]
and tetrahedral TiO4 has been thought to be important to cat-
Fig. 2. XRD patterns of TiO2/SBA-15 with different TiO2 loadings at small-angle and extent-angle: (a) 2.5 wt%, (b) 5 wt%, (c) 10 wt%, (d) 20 wt%, and (e) 30 wt%.

64
K. Su et al. / Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
3.3. Effect of reaction temperature on carboxymethylation and
methylation
As shown in Fig. 6, no DmC(1) was detected at 140 ◦C even after
10 h, and only 1.2% MmC(1) yield was obtained, however, methy-
latedproductselectivity achieved 88.3%. When reaction was carried
out at 160 ◦C, MmC(1) and DmC(1) yields achieved 0.8% and 13.6%,
and methylation product selectivity declined to 7.7%. Furthermore,
if reaction was carried out at 200 ◦C, MmC(1) and DmC(1) yield
increased to 4.6% and 21.1%, together with methylated product
selectivity further decline to 4.5%.
The yield of DmC(1) and MmC(1) increases as temperature
increasing, while the methylated product selectivity decreased.
Those behaviors could be explained by two effects: (1) When reac-
tion was carried at high temperature, the coproduced methanol
was separated easily by distillation, which alleviated the equilib-
rium limitation of the reaction between BPA and DMC. (2) Another
problem is the effect of water adsorption from ambient humid-
ity. Hydration could break Ti–O–Si bridging bonds exposed to the
surface to form Ti–OH hydroxyls which are a five-coordinated Ti
species [40]. The interaction between five-coordinated Ti species
and DMC occurred through the oxygen of the C O moiety, which
lead to high selectivity to methylation. If reaction was carried out
at 140 ◦C, heat activation may not ensure the complete absence
of chemisorbed water on catalyst surface [41]. When reaction
was carried out at 160 ◦C, water molecules were removed from
TiO2/SBA-15. Only when the chemisorbed water is completely
removed, the high selectivity to carboxymethylation reactions
might occur.
Fig. 4. FT-IR of TiO2/SBA-15 with different TiO2 loadings: (a) 2.5 wt%, (b) 5 wt%, (c)
10 wt%, (d) 20 wt%, and (e) 30 wt%.
alytic applications because it is attributed to the active center of
many reactions [39]. The structure defect in 10 wt% TiO2/SBA-15
was mainly caused by the appearance of more Si–O–Ti species in
Ti–Si binary oxides [34].
The effects of TiO2 loading on reaction were shown in Fig. 5.
When 2.5 wt% TiO2/SBA-15 was used as catalyst, no DmC(1) was
detected, and MmC(1) yield was 6.4%, however, methylation prod-
uct selectivity achieved the maximum 15.8%. When TiO2 loading
amounted to 5 wt%, DmC(1) yield achieved 0.32%, and MmC(1) yield
increased to 8.8%. The maximum DmC(1) yield of 0.54% and min-
imum methylation product selectivity of 2.3% were obtained over
catalyst with 10 wt% TiO2 loading. When reaction was catalyzed
by 30 wt% TiO2/SBA-15, DmC(1) yield decreased to 0.34%, while
methylation product selectivity increased to 8.0%, respectively. The
reason for the high activity of 10 wt% TiO2/SBA-15 for DmC(1) syn-
thesis might be that much more Si–O–Ti species existed on surface
of 10 wt%TiO2/SBA-15, and the electrophilic interaction of DMC was
augmented by Ti cation in Si–O–Ti species.
3.4. Effect of reaction time on carboxymethylation and
methylation
A further study was carried out to investigate the equilibrium
conversion of BPA over 10 wt% TiO2/SBA-15 with an extended reac-
tion time of 12 h. As shown in Fig. 7, DmC(1) yield increased from 0%
to 2.74% as reaction time increasing from 2 h to 12 h, and DmC(1)
selectivity showed an almost linear increase with time prolong-
ing, and it amounted to 16.4% at 12 h. MmC(1) yield achieved 13.2%
and its selectivity obtained 79.1% at 12 h. Carboxymethylation com-
monly proceeded by a condensation reaction (see Scheme 4), in
which methanol was eliminated in each step. As reaction time
Fig. 5. Effect of TiO2 loading on carboxymethylation and methylation (reaction condition: BPA 5.7 g, DMC 9 g, catalyst 0.5 g, 180 ◦C, 6 h).

K. Su et al. / Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
65
Fig. 6. Effect of reaction temperature on carboxymethylation and methylation (reaction conditions: BPA 11.4 g, DMC 18 g, 10 wt% TiO2/SBA-15 0.5 g, 10 h).
Fig. 7. Effect of reaction time on carboxymethylation and methylation (reaction conditions: BPA 11.4 g, DMC 18 g, 10 wt% TiO2/SBA-15 0.5 g, 160 ◦C, 10 h).
extended to remove methanol, the equilibrium limitation of reac-
tion was alleviated.
catalyst usage increased to 0.75 g, MmC(1) and DmC(1) yields
increased to 10.4% and 0.64%, while methylated product selec-
tivity achieved the minimum 3.0%. When 1.0 g catalyst was used,
no DmC(1) was obtained, and MmC(1) yield was only 1.9%, while
methylated product selectivity achieved 11.8%, which might be
caused by TiO2/SBA-15 powder deposition at flask bottom to influ-
ence thermal conductivity. However, when 0.75 g catalyst was
used, the presence of suspended TiO2/SBA-15 nanosized solid
particles in solution exhibited the effective thermal conductivity
[42,43].
3.5. Effect of catalyst usage amount on carboxymethylation and
methylation
In order to improve DmC(1) yield, the influence of catalyst
usage amount was investigated, and results were shown in Fig. 8.
When the amount of catalyst usage was 0.25 g, the yields of
MmC(1) and DmC(1) were 5.6% and 0.44%. As the amount of
Scheme 4. The carboxymethylation of BPA with DMC over TiO2/SBA-15.

66
K. Su et al. / Journal of Molecular Catalysis A: Chemical 315 (2010) 60–68
Fig. 8. Effect of 10 wt% TiO2/SBA-15 amount on carboxymethylation and methylation (reaction conditions: BPA 11.4 g, DMC 18 g, 180 ◦C, 10 h).
3.6. Effect of DMC/BPA mole ratio on carboxymethylation and
methylation
3.7. Suggested mechanism
In the reaction between DMC and BPA over TiO2/SBA-15, DMC
was attacked by Ti–O–Si on two oxygen atoms of the CH3–O moi-
ety, which increased the positive character of its carbonyl group,
and the electrostatic perturbation in adducts Ti–O–Si and DMC
with cis–cis species (I) induces an elongation (weakening) of the
C–OCH3 bonds. Therefore, the oxygen of BPA mainly attacked the
carbonyl carbon to form the tetrahedral intermediate (Scheme 5).
H transferred from OH to the tetrahedral intermediate on one of
its methoxyl oxygens, and then DmC(1) or MmC(1) formed. This
process can explain the high catalytic activity of TiO2/SBA-15 in
carboxymethylation reactions. If the interaction occurs through the
oxygen of the C O moiety, in adducts Ti–O–Si and DMC with cis–cis
and cis–trans species (II and III) an elongation of the O–CH3 bonds
occurs, and then the oxygen of BPA mainly attacked the methyl
carbon to form methylated products.
Effect of DMC/BPA mole ratio on carboxymethylation and
methylation was investigated over 10 wt% TiO2/SAB-15, as shown
in Fig. 9. When reactions were carried out at 180 ◦C, MmC(1)
yield increased from 4.8% to 23.8% and DmC(1) yield increased
from 0.3% to 2.9% as the mole ratio of DMC to BPA increas-
ing from 2 to 6. However, when the molar ratio of DMC to
BPA increased to 8, DmC(1) and MmC(1) yields declined to
0.12% and 8.68%. The maximum DmC(1) selectivity of 10.8%
was obtained when the mole ratio of DMC to BPA reached 6.
The methylated product selectivity showed linear decrease from
4.5% to 0.5% as the mole ratio of DMC to BPA increasing from
2 to 8. It was worthy of noting that when the mole ratio of
DMC to BPA was 2 and 4, DmC(1) yields were only 0.3% and
0.58%.
Fig. 9. Effect of DMC/BPA mole ratio on carboxymethylation and methylation (reaction conditions: BPA 11.4 g, 10 wt% TiO2/SBA-15 0.5 g, 180 ◦C, 10 h).

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67
Scheme 5. The process of the carboxymethylation and methylation of BPA with DMC over TiO2/SBA-15.
4. Conclusions
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