Mesoporous silica-anchored organotin as heterogeneous catalyst for the transesterification of dimethyl carbonate with phenol

Article abstract of DOI:10.1007/s11164-016-2530-y

A simple scheme for a mesoporous silica-anchored organotin catalyst was developed for the transesterification of dimethyl carbonate with phenol to diphenyl carbonate. N₂-sorption, TEM, UV–Vis, and elemental analysis combined with ²⁹Si and ¹³C NMR measurements evidenced the formation of mesoporous organic–inorganic hybrid silica with a highly dispersed tetrahedral Sn species. The catalyst exhibited excellent activity and reusability in the transesterification. With a catalyst of 1.0?g, a reaction temperature of 150–180?°C, and a reaction time of 9?h, the phenol conversion and transesterification selectivity reached 51.1 and 99.9?%, respectively. The phenol conversion just decreased from 41.2 to 35.0?% after five runs with 0.5?g of catalyst. The improved stability was attributed to the strong covalent bonding between the organotin and mesoporous silica.

Full text of DOI:10.1007/s11164-016-2530-y

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Res Chem Intermed
DOI 10.1007/s11164-016-2530-y
Mesoporous silica-anchored organotin as heterogeneous
catalyst for the transesterification of dimethyl
carbonate with phenol
Yuanzhuo Zhang1,2 Songlin Wang3 Zhongliang Xiao1,2
•
•
•
Tong Chen1 Gongying Wang1
•
Received: 16 January 2016 / Accepted: 23 March 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract A simple scheme for a mesoporous silica-anchored organotin catalyst
was developed for the transesterification of dimethyl carbonate with phenol to
diphenyl carbonate. N2-sorption, TEM, UV–Vis, and elemental analysis combined
with 29Si and 13C NMR measurements evidenced the formation of mesoporous
organic–inorganic hybrid silica with a highly dispersed tetrahedral Sn species. The
catalyst exhibited excellent activity and reusability in the transesterification. With a
catalyst of 1.0 g, a reaction temperature of 150–180 °C, and a reaction time of 9 h,
the phenol conversion and transesterification selectivity reached 51.1 and 99.9 %,
respectively. The phenol conversion just decreased from 41.2 to 35.0 % after five
runs with 0.5 g of catalyst. The improved stability was attributed to the strong
covalent bonding between the organotin and mesoporous silica.
Keywords Organotin Á Silica-anchored Á Transesterification Á Diphenyl carbonate Á
Phenol
Introduction
Diphenyl carbonate (DPC) is an important green chemical intermediate for the
production of many organic chemicals, especially for the synthesis of polycarbonate
by the melt polymerization process [1–3]. The transesterification of dimethyl
&
Tong Chen
chentongw@sina.com.cn
1
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041,
People’s Republic of China
2
3
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology,
Xinxiang 453003, People’s Republic of China
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Y. Zhang et al.
carbonate (DMC) with phenol is considered to be the most suitable phosgene-free
route to synthesize DPC [4–6]. This reaction is usually carried out in the liquid
phase, and the organotin catalysts display excellent catalytic performance [7].
However, the organotin compounds are usually highly toxic, and difficult to be
separated from the reaction system and recovered for reuse. A scientifically
rewarding strategy to overcome the problems is to anchor the organotins onto an
insoluble support, which is also a general trend nowadays in catalysis [8, 9].
In this context, anchored organotins on insoluble support have been successfully
developed, especially in the use of polymers as carriers. With regard to
transesterification reactions, excellent catalytic properties have been reported for
cross-linked polystyrene-anchored dichlorobutyltins, distannoxanes, and trichlor-
otins [10–12]. For example, the polystyrene-anchored (CH2)11–SnCl3 could display
an average conversion degree of 76 % after 2 h in the transesterification between
ethyl acetate and n-octanol. The catalyst was recycled at least nine times without
loss of activity, and the residual tin content averaged just 5 3 ppm [11]. Although
the use of polymer supports may give advantages of facile functionalization and
high loading, there are the disadvantages of poor mechanical stability, solvent
swelling, and even decomposing at the relatively high temperature. Compared to
polymer carriers, silica exhibits better thermal and chemical stabilities, so the silica-
anchored organotin catalyst may provide better stability. The covalent anchoring of
molecule catalysts on the silica surface is generally achieved by the reaction
between surface silanols and a Si(OEt)3 group of molecule-substituted triethoxysi-
lane [13]. However, the organotin-silane is not a ready-made reagent, and often
requires complex organic synthesis. For this reason, only a few silica-supported
organotins have been reported so far, including dialkyl dimethoxyltin [14],
dibutylpropyltin hydride [15], undecyltrichlorohexyltin [16], and propyl methox-
yldichlorotin [17]. The use of silica-supported organotins to catalyze transesteri-
fication reactions of DMC and phenol has not yet been reported.
In this paper, one kind of organotin-substituted triethoxysilane was synthesized
by a simple method, and then anchored on the mesoporous silica to yield a silica-
anchored organotin catalyst. This catalyst was used as a heterogeneous catalyst to
catalyze the liquid transesterification reaction between DMC and phenol, and the
catalytic and recyclability properties was presented and discussed.
Experimental
Reagents
Pluronic P123 triblock polymer was purchased from Aldrich Chemical Co.
3-chloropropyl triethoxysilane was obtained from Sinopharm: [CH(31)CH2(2)O]3
1
SiCH2(3)CH2(4)CH(25)Cl, H NMR (300 MHz, CDCl3, 293 K): d 3.67–0.77 (m, 2H,
H3), 1.20 (t, 9H, H1), 1.81–1.92 (m, 2H, H4), 3.47–3.55 (m, 2H, H5), 3.79 (q, 6H,
H2); 13C NMR (75 MHz, CDCl3, 293 K): d 8.1 (C3), 18.2 (C1), 26.5 (C4), 47.4 (C5),
58.4 (C2). The other chemicals were obtained from local manufacturers. Diethyl
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Mesoporous silica-anchored organotin as heterogeneous…
ether was dried and distilled over Na prior to use, and the other chemicals were used
as received.
Preparation of mesoporous silica-anchored organotin catalyst
The preparation of mesoporous silica-anchored organotin primarily involved the
synthesis of organotin-silane and the anchoring of organotin on the mesoporous
silica. Mesoporous silica was prepared using a reported procedure at 100 °C [18].
The template was removed by extraction using Soxhlet apparatus in ethanol, and
dried at 120 °C overnight under vacuum.
Synthesis of organotin-silane
A suspension of magnesium powder (4.0 g), 3-chloropropyl triethoxysilane (8.0 g),
and a small amount of iodine in 60 mL diethyl ether was heated to reflux with
stirring under an inert atmosphere for 2 h. After cooling, the supernatant liquid was
decanted into another container. Then, SnCl4 (17.0 g) was added dropwise to the
obtained liquid. The unreacted SnCl4 was reduced by addition of excess
magnesium, and reflux for 5 h. Some non-volatile solvents, such as toluene, could
be added during this manipulation. The resulting mixture was filtered, and the
filtrate was obtained as a colorless or faint yellow liquid. The atomic ratio of
Cl:Sn:Si was 2.83:0.83:1 as determined by ICP-AES and Mohr titration.
Anchoring of organotin on mesoporous silica
The activated silica powder was suspended in the above solution, and refluxed under
stirring for 12 h. A white solid was recovered by filtration, then washed with
ethanol, and dried at 80 °C for 12 h. The sample was named as SiO2–Sn, with a
chemical formula of C12.7H47.0O20.0Cl2.5Si8.3Sn1.3. After each catalytic run, the
catalyst was filtrated, washed with DMC, and dried at 80 °C overnight. The
recovered samples were designated as SiO2–Sn-x, where x indicated the recycle
number.
Characterizations
The textural properties were measured from nitrogen sorption isotherms at -196 °C
with a Builder SSA-4200 system. Prior to measurement, the samples were degassed
at 200 °C for 6 h. The surface areas were calculated by the BET method, and the
pore size distributions were calculated from the adsorption branch of the isotherms
using the BJH method. A JEM-2010 high-resolution transmission electron
microscope with an electron kinetic energy of 300 kV was used to scan the silica
and catalysts on the Cu grid. The Sn and Si content were determined with an
inductively coupled plasma-atomic emission spectrometer (IRIS 1000). Elemental
analyses (C and H) were carried out with a Euro EA 3000 elemental analyzer. The
chlorine was detected by the Mohr titration method [19]. The solid-state NMR
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Y. Zhang et al.
spectra were recorded on a Bruker Avance 300 M solid-state spectrometer at
resonance frequencies of 125 MHz for 13C and 99 MHz for 29Si.
Reaction procedure
The reaction was carried out in a 100 mL glass flask equipped with a thermometer, a
nitrogen inlet, a dropping funnel and a fractionating column connected with a liquid
dividing head. Phenol and catalyst were charged into the flask under a nitrogen
atmosphere. DMC was added drop-wise into the flask after the mixture was heated
to 175 °C. The reaction temperature was kept at 150–180 °C during the period of
reaction. The products were analyzed by a gas chromatograph equipped with a FID
detector.
Results and discussion
Catalyst synthesis and characterization
The organotin-alkoxysilanes have been prepared through the hydrosilylation of
alkenyl organotin by triethoxysilane [16], or the hydrostannylation of alkenyl
trialkoxysilane by organotin hydride [15]. The former method used noble metal
catalysts, and the latter method needed expensive active organotin hydride
precursors. In this work, the propyltin-silane (C) was synthesized by the reaction
between SnCl4 and the Grignard reagent (B) of 3-chloropropyl triethoxysilane (A),
as shown in Scheme 1. The organotin had connected with a Si(OEt)3 group after
organotin synthesis. This was simpler compared to that reported previously. The
anchored organotin catalyst SiO2–Sn was then prepared by the silylation of
Compound C and mesoporous silica. The atomic ratio of Cl:Sn in Compound C was
3.41:1, but it was decreased to 1.97:1 after organotin immobilization on the silica,
O
O Si
O
O
O Si
O
Mg
SnCl4
Mg/Ether
Reflux
MgCl
Cl
Toluene
Reflux
A
B
OH
O
O Si
O
SiO2
OH
OH
O
O
O
SnCl3
SnCl3
Si
Reflux
SiO2-Sn
C
Scheme 1 Synthetic pathway to the mesoporous silica-anchored organotin catalyst
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