784
Chemistry Letters Vol.35, No.7 (2006)
Synthesis of Diphenyl Carbonate from CO2, Phenoxide, and CCl4 with ZnCl2 as Catalyst
Zhenhuan Li,ꢀ1;2 Zhangfeng Qin,1 Huaqing Zhu,1 and Jianguo Wang1
1State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,
P. O. Box 165, Taiyuan, Shanxi 030001, P. R. China
2Graduate School of the Chinese Academy of Sciences, Beijing 100080, P. R. China
(Received March 16, 2006; CL-060318; E-mail: zhenhuanli@sohu.com)
OK
OH
Direct synthesis of diphenyl carbonate (DPC) from phen-
OK
oxide, CO2 and CCl4 in one pot was realized with ZnCl2 as cat-
alyst. Trichloromethyl cation may act on potassium phenyl car-
bonate, which brings the carbonyl more easily attached on by the
phenoxide. Onium salts promote the reaction greatly, especially
with phenol and potassium carbonate instead of phenoxide as
reactants. Moreover, the substituent on the aromatic ring has
significant effects on the selectivity and yield of carbonate.
O
O
O
CO2
C
O
(1) CCl4
ZnCl2
C
O
CCl3
(PPC)
PhOK
(TPC)
O
(2)
(Rearrangement)
O
C
O
CCl4
PhOK
C
O
PS
OK
ZnCl2
(DPC)
Scheme 1. The possible reaction mechanism.
Diphenyl carbonate (DPC) is a key raw material for the pro-
duction of aromatic polycarbonates. As yet, it can be obtained
through the phosgenation of phenol,1 oxidative carbonylation
of phenol,2 transesterification of dimethyl carbonate (DMC),3
or dimethyl oxalate (DMO) with phenol.4 All those methods
have certain disadvantages, for example, the phosgenation proc-
ess uses severely toxic phosgene as raw material,5 oxidative
carbonylation method needs noble metal catalyst and costly
procedures, and transesterification demands intricate multistage
procedures with a critical thermodynamic limitation,6 which
results in very low DPC yield and selectivity. It will be un-
doubtedly a much more economical option if DPC can be
synthesized directly from phenol and CO2.
In this work, the direct synthesis of DPC from phenoxide,
CO2, and CCl4 in one pot was realized with ZnCl2 as catalyst.
The effects of reaction temperature, promoter, and substituent
attached to the aromatic ring on the carbonate yield were inves-
tigated.
The reaction results are summarized in Table 1. In the
presence of an inert atmosphere N2, no reaction was observed
at 100 ꢁC (Entry 1). With the introduction of CO2, DPC is detect-
ed even at 80 ꢁC (Entry 2). As the temperature is elevated to
100 ꢁC, phenyl salicylate (PS) appears as the main by-product
(Entry 3). It seems that with the increase of reaction temperature,
the yields of both DPC and PS increase, but the selectivity to
DPC decreases slightly. At 120 ꢁC, the yields of DPC and PS
come up to 3.6% and 4.6%, respectively (Entry 4).
According to the above described results, it is found that the
presence of CO2 is an important factor for synthesis DPC. One
possible reaction process is started with the nucleophilic attack
of the potassium phenoxide to CO2 and it produces potassium
phenyl carbonate (PPC).7 During the reaction, trichloromethyl
cation CCl3ꢀþ is formed on the catalyst surface, which is similar
to the destruction of carbon tetrachloride over lanthanide oxide-
based catalyst.8 Thus, potassium phenyl carbonate may either
form trichloromethyl phenyl carbonate (TPC) through binding
with CCl3ꢀþ (Scheme 1, pathway 1) or be rearranged into salicy-
late7 (Scheme 1, pathway 2). After transesterification with phen-
oxide, TPC changes into DPC, KCl, and phosgene. The phos-
gene, produces in situ during the reaction, reacts with phenoxide
to produce another molecular DPC. In one word, one molecular
CO2 couples with one molecular CCl4 to produce two molecular
DPC.
ZnCl2
4PhOK þ CCl4 þ CO2 ꢂꢂꢂꢂꢂ! 2DPC þ 4KCl:
ð1Þ
Further tests proved that the yield and selectivity of DPC can
be improved significantly by using the reactants of phenol
+ K2CO3 as a substitute for potassium phenoxide with certain
co-catalysts like onium salts. As listed in Table 2, with potassi-
um phenoxide as reactant, the main product is PS (Entries 1–5).
With tetramethylammonium bromide (Me4NBr) as co-catalyst,
the formation of DPC and especially PS is suppressed in compar-
ison with the reaction without co-catalyst (Entry 2). Using tetra-
butylammonium bromide (Bu4NBr) as co-catalyst, the yield of
PS increases moderately and a small amount of phenyl p-hy-
droxybenzoate (PPB) is detected even at such a low temperature
(Entry 3). With cetyltrimethylammonium bromide (CeMe3NBr)
as co-catalyst, DPC yield was also suppressed (Entry 4). These
results suggested that onium salt with large organic group
(Bu4NBr) can enhance the conversion of phenoxide obviously,
while the onium salt with small organic group (Me4NBr) is fac-
ile to enhance the selectivity to DPC. This may be related to their
difference in the steric hindrance. The reaction at 120 ꢁC exhibits
high selectivity to PS (Entry 5), which proved further that higher
Table 1. Synthesis of DPC form potassium phenoxide in liquid
CCl4 at different temperaturesa
Yield/%b
Temperature
/ꢁC
Entry
Catalyst
ZnCl2
DPC
PS
In the presence of N2 (1 MPa)
100
In the presence of CO2 (1 MPa)
1
—
—
2
3
4
80
100
120
ZnCl2
ZnCl2
ZnCl2
0.1
1.4
3.6
—
1.7
4.6
aReaction conditions: PhOK 30 mmol, catalyst 2.0 g, CCl4
40 mL, reaction duration 6 h. bYield based on phenoxide
was determined by GC analysis.
Copyright Ó 2006 The Chemical Society of Japan