Influence of coordination groups on the catalytic performances of organo-titanium compounds for disproportionation of methyl phenyl carbonate to synthesize diphenyl carbonate

Article abstract of DOI:10.1016/j.apcata.2017.04.011

The disproportionation of methyl phenyl carbonate (MPC) to synthesize diphenyl carbonate (DPC) catalyzed by organo-titanium compounds with different coordination groups was systematically investigated. The results of the molecular structure analysis, the catalytic performance evaluation and the chemical computational studies revealed that both the electron effect and steric hindrance of the coordination groups together affected the active Ti center and therefore influenced the catalytic performance of the catalysts. The influence of the electron effect was more important than the steric hindrance. The catalytic activity is in the order of Ti(O-iC₃H₇)₄?>?Ti(OC₆H₅)₄?>?Ti(OC₄H₉)₄?>?TiO(OOCCH₃)₂?>?TiO(acac)₂?>?Cp₂TiCl₂, and Ti(O-iC₃H₇)₄ exhibited the best catalytic performance, due to its appropriate steric hindrance and electron effect for the coordination group. Under the optimum condition (n(Cat.)/n(MPC)?=?0.04, reacted at 180?°C for 3?h), the MPC conversion of 90.4% and DPC selectivity of 99.6% were attained, respectively, which are comparable to those of other catalysts reported. Moreover, the catalyst is low cost, non-toxic and easily obtained by readily commercial-available purchase, and this catalytic system is also easy operation and convenient-controllable, indicating that it is conducive to future industrial application. In addition, a possible reaction mechanism catalyzed by the organo-titanium compounds with Lewis acid was also proposed for the process.

Full text of DOI:10.1016/j.apcata.2017.04.011

Accepted Manuscript
Title: Influence of coordination groups on the catalytic
performances of organo-titanium compounds for
disproportionation of methyl phenyl carbonate to synthesize
diphenyl carbonate
Authors: Songlin Wang, Tong Chen, Gongying Wang,
Chengxing Cui, Hongying Niu, Changgong Li
PII:
S0926-860X(17)30168-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.04.011
APCATA 16210
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
14-12-2016
15-3-2017
18-4-2017
Please cite this article as: Songlin Wang, Tong Chen, Gongying Wang, Chengxing
Cui, Hongying Niu, Changgong Li, Influence of coordination groups on the
catalytic performances of organo-titanium compounds for disproportionation of
methyl phenyl carbonate to synthesize diphenyl carbonate, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.04.011
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Influence of coordination groups on the catalytic performances of
organo-titanium compounds for disproportionation of methyl phenyl
carbonate to synthesize diphenyl carbonate
Songlin Wang a, b, Tong Chen b, , Gongying Wang , Chengxing Cui , Hongying Niu
b
a
a, Changgong Li a
a
School of Chemistry and Chemical Engineering, Henan Institute of Science and
Technology, Xinxiang 453003, China
b
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu
6
10041, China
Corresponding author. Tel.: +86 028 85250005; fax: +86 028 85220713.
E–mail: chentongw@sina.com.cn (Tong Chen)
Graphical abstract
1

Highlights





Electron effect and steric hindrance of the coordination group together affected
active Ti center and thereby influenced the catalytic performance.
Ti(O-iC displayed excellent catalytic performance compared to other
3
H )
7 4
catalysts reported.
Suitable steric hindrance and electron effect for the coordination group played
important role for outstanding catalytic performance.
Influence of the electron effect was more important than that of the steric
hindrance.
This work offered an attractive catalyst candidate towards efficient and green
processes for MPC disproportionation.
Abstract: The disproportionation of methyl phenyl carbonate (MPC) to synthesize
diphenyl carbonate (DPC) catalyzed by organo-titanium compounds with different
coordination groups was systematically investigated. The results of the molecular
structure analysis, the catalytic performance evaluation and the chemical
computational studies revealed that both the electron effect and steric hindrance of the
coordination groups together affected the active Ti center and therefore influenced the
catalytic performance of the catalysts. The influence of the electron effect was more
important than the steric hindrance. The catalytic activity is in the order of
Ti(O-iC
3
H
7
)
4
> Ti(OC
6
H
5
)
4
> Ti(OC H
4 9
)
4
> TiO(OOCCH
3
)
2
> TiO(acac)
2
> Cp
2
TiCl ,
2
and Ti(O-iC
3
H )
7 4
exhibited the best catalytic performance, due to its appropriate steric
hindrance and electron effect for the coordination group. Under the optimum
condition (n(Cat.)/n(MPC)=0.04, reacted at 180 ºC for 3 h), the MPC conversion of
9
0.4% and DPC selectivity of 99.6% were attained, respectively, which are
comparable to those of other catalysts reported. Moreover, the catalyst is low cost,
non-toxic and easily obtained by readily commercial-available purchase, and this
catalytic system is also easy operation and convenient-controllable, indicating that it
is conducive to future industrial application. In addition, a possible reaction
2

mechanism catalyzed by the organo-titanium compounds with Lewis acid was also
proposed for the process.
Keywords: methyl phenyl carbonate; diphenyl carbonate; disproportionaton;
organo-titanium compounds; steric hindrance and electron effect
1
. Introduction
Polycarbonates (PC), as one of the most useful engineering thermoplastics with
high transparency, good thermal resistance and mechanical property, was widely used
for the synthesis of automobiles, office equipments, medical devices and construction
glazing, ect [1, 2]. Diphenyl carbonate (DPC), an important and green chemical
intermediate, was frequently employed as the key raw material for the non-phosgene
production of PC by a melt polymerization process, and was also utilized in the
synthesis of many organic compounds and polymer materials [3-7]. Nowadays, the
transesterification of dimethyl carbonate (DMC) and phenol was viewed as one of the
most promising routes to synthesize DPC without using phosgene. This route was a
two-step process consisting of the transesterification of phenol and DMC into methyl
phenyl carbonate (MPC) (Scheme 1, Eq. (1)), followed by the further
transesterification reaction of phenol and MPC (Scheme 1, Eq. (2)) or the
disproportionation of MPC to DPC (Scheme 1, Eq. (3)) [8, 9]. The equilibrium
–
5
o
o
constants K
2
(1.2×10 , 25 C) and K (0.19, 25 C) in the second step showed that
3
DPC was mainly generated by the disproportionation of MPC rather than the further
transesterification between MPC and phenol [10, 11]. Clearly, the first step was the
–
5
o
rate-controlling step, since the equilibrium constant K
1
(6.3×10 , 25 C) was much
smaller than K , even at elevated temperatures. Thus, previous studies have been
3
mostly focused on the first-step transesterification reaction of phenol and DMC. And
the reactivities of the catalysts were also based on the calculation of the total yield of
MPC and DPC [12-17]. However, little attention has been paid to the
3

disproportionation of MPC relative to the first-step transesterification. In fact, MPC
must undergo the further disproportionation to generate DPC [18, 19].
To avoid the unfavorable thermodynamic balance limitation on the
transesterification reaction, as a result of the K
1
smaller than K , it could be divided
3
into two reaction processes, namely including the first-step transesterification reaction
and the second independent disproportionation reaction, which was more conducive to
improve the yield of the target product DPC, the conversion of raw materials and the
intermediate MPC [18]. Furthermore, it was also found that the catalysts were suitable
for the first-step transesterification reaction, whereas they did not exhibit good
catalytic performance for this disproportionation reaction [19]. Therefore, the
exhaustive and intensive study concentrated on the disproportionation of MPC was
also very important.
Nevertheless, up to now, the research on MPC disproportionation is rarely reported,
and only the catalytic potentials of organo-tin compounds [20], metal based catalysts
like PbO [21], MoO
3
/SiO
2
[22] and PbO-ZrO
2
[23] have been preliminarily
investigated by our research group. The results show that although organo-tin
compounds are highly active, it is well known that these compounds are inconvenient
for industrial application, due to their high cost, toxicity and environmental
contamination. Moreover, metal based catalysts also seem to be unfavorable on the
economic feasibility of future large-scale production, because of their lower catalytic
activities. Thus, developing highly efficient, economical and environmentally benign
sustainable catalytic system and getting insight into the mechanism for this process is
still in a great necessity. On the other hand, titanium esters (Ti(OC
Ti(O-iC , ect.) [24], titanocene complexes (Cp TiCl ) [25], titanium oxide
acetylacetonate (TiO(acac) ) [26] and titanyl acetate (TiO(OOCCH ) [27] are typical
6
H
5
)
4
, Ti(OC
4
H
9
)
4
,
3
H
7
)
4
2
2
2
)
3 2
samples of Lewis acids, and they are frequently employed as catalysts for the
esterification/transesterification reaction, owning to their much lower price and facile
commercial-available purchase. These catalysts are active in the first-step
transesterification, however, they have not been used as catalysts for MPC
4

disproportionation so far. The MPC disproportionation is a nucleophilic substitution
reaction, and it is in essence the Lewis acid catalysis process because Brönsted acid
easily results in the decarboxylation of MPC into by-product anisole [18-23]. As our
continuous work for exploiting broader kinds of attractive catalysts on the catalytic
reaction, it would be quite intriguing to compare the catalytic behaviors of these
titanium compounds in terms of their Lewis acid property regarding the Ti center
connected to the different coordination groups.
In the present work, organo-titanium compounds with different coordination groups
are conducted for MPC disproportionation, and the relationship between the steric
hindrance and the electron effect of the coordination groups on their catalytic
properties of the organo-titanium compounds are systematically investigated. Also the
interactions of the respective Lewis acid-base complex intermediates for
organo-titanium compounds are theoretically explored by chemical computational
studies. Meanwhile, a possible reaction mechanism catalyzed by organo-titanium
compounds is also discussed for the disproportionation.
2
. Experimental
2
.1. Materials
Titanium oxide acetylacetonate (TiO(acac)
2
, 99.6%), tetraphenyl titanate
, ≥98%), titanium isopropoxide
, ≥98%) were purchased from Alfa Aesar Chemical Reagent Company.
TiCl , 99.7%) was provided from Hubei Huabang
Chemical Reagent Company of China. Titanyl acetate (TiO(OOCCH ) was
synthesized according to the literature [27]. Cp TiCl , TiO(acac) , Ti(OC and
TiO(OOCCH were solid powder. Ti(OC and Ti(O-iC were colorless
transparent liquid. Before the reaction, Cp TiCl , TiO(acac)
TiO(OOCCH were dried in a vacuum oven at 70 ºC, while Ti(OC
Ti(O-iC were dried at 20 ºC in the presence of 4A molecular sieves.
(
Ti(OC
6
H
5
)
4
, 99.6%), tetrabutyl titanate (Ti(OC
4 9 4
H )
(
Ti(O-iC
3
H )
7 4
Titanocene dichloride (Cp
2
2
)
3 2
2
2
2
6
H )
5 4
3
)
2
H
4 9
)
4
3
H )
7 4
2
2
2
, Ti(OC
6
H
5
)
4
and
and
3
)
2
4
H
)
9 4
3
H )
7 4
2
.2. Catalytic performance tests
5

MPC was synthesized and purified in the laboratory, and its purity was 99.8%
determined by high performance liquid chromatography. The synthesis procedure was
as follows: due to the small thermodynamic equilibrium constant of MPC
disproportionation reaction, MPC could be synthesized beneficially by the reversible
disproportionation of DMC and DPC. Subsequently, the separation of high purity
MPC was carried out by the pressure reduction distillation and rectification. Finally,
the quantitatively analysis of MPC was performed by the high performance liquid
chromatography.
The liquid-phase disproportionation reaction of MPC was carried out in a 100 mL
three necked round bottom flask fitted with a fractionation column connected to a
liquid dividing head. Typically, the reactor was gently purged with nitrogen to
evacuate the air, and then 150 mmol MPC and an appropriate amount of catalyst were
added into the reactor under nitrogen atmosphere. The reactor was gradually heated to
the desired temperature in an oil bath, and it was subsequently maintained at the
desired temperature with a constant stirring rate. During the reaction, DMC was
continuously distilled out by the liquid dividing head attached to a receiver flask.
After the reaction, the reactor was slowly cooled to room temperature (20 ºC).
After the reaction, the reaction mixture and the distilled liquid are respectively
detected by GC-MS instrument with a HP-5 capillary chromatography packed column
(30 m × 250 μm × 0.25 um) on a HP-6890/5973 system. The reaction mixture is
mainly composed of the raw material MPC and the product DPC, and there is also a
tiny amount of by-product anisole. The distilled liquid is DMC without any
by-product. The products were quantitatively analyzed by GC system (Agilent
Technologies 7820A) with a flame ionization detector (HP/DB, capillary
chromatography packed column) and a DB–35 capillary column (30 m × 320 μm ×
0
.25 μm) using the correction factor normalization method.
The conversion of MPC, the yield and selectivity of DPC were calculated as
follows:
6

no  n
MPC
Conv/% 
100%
no
nDPC
Y /% 
100%
no
Y %
Conv%
Sel / % 
100%
As to the calculation, Conv was denoted as the conversion of MPC, %; n and nMPC
0
were the mole substance amounts of MPC before and after the reaction, respectively;
Y was denoted as the yield of DPC, %; nDPC was the mole substance amount of DPC;
Sel was denoted as the selectivity of DPC, %.
2
.3. Theoretical studies
To understand the catalyzing behavior of these titanium compounds, their
geometrical structures were fully optimized without any restriction. The
three-parameter fit to the exchange-correlation potential suggested by Beck in
conjunction with the correlation functional suggested by Lee, Yang and Parr (B3LYP)
was adopted with the basis set of 6-31++G (d, p) level [28]. Frequency calculations
were carried out at the optimized structures to confirm a real local minimum (no
imaginary frequency). The gaps between the frontier orbitals, i.e., the highest
occupied molecular orbital (HOMO) and the lowest unoccupied orbital (LUMO),
were correlated to the catalyzing ability. All calculations were performed with
Gaussian 09 programs package [29].
3
. Results and discussion
3
.1. Effect of the coordination groups on the disproportionate performances of the
organo-titanium compounds
Organo-titanium compounds with different coordination groups are used to catalyze
the disproportionation of MPC, and the effect of the coordination groups on their
catalytic performances are investigated and the results are summarized in Table 1. The
catalyst dosage is on the basis of the active Ti center, and the mole ratio of Ti to MPC
is 0.04.
7

As can be seen in Table 1, evidently, the disproportionation reaction could not
proceed without catalyst. Organo-titanium compounds with different coordination
groups have notable catalytic activities for the disproportionation, and the selectivities
of DPC are more than 96.0%. Among them, titanium esters exhibit higher catalytic
activities, giving the MPC conversion of more than 86.2%. For these titanium
compounds tested, the sequence of the catalytic activities is observed as follows:
Ti(O-iC
3
H
7
)
4
> Ti(OC
6
H
5
)
4
> Ti(OC H
4 9
)
4
> TiO(OOCCH
3
)
2
> TiO(acac)
2
> Cp
2
TiCl ,
2
which may be resulted from the influence of both the steric hindrance and the electron
effect of the coordinating groups associated with the active Ti center [30].
Organo-titanium is as Lewis acid and the Ti is active center. The electron effect of
the coordination group influences the electron cloud density of the Ti, while the steric
hindrance of the coordination group influences the difficulty level of Ti contacting
with the reactant molecules [31-34]. Specifically, they together affect the catalytic
activity of the Ti. Cp
2
TiCl displays sandwich molecular structure [25], and its steric
2
hindrance is much larger than other organo-titanium compounds, increasing the
difficulty of Ti contacting with the oxygen of the carbonyl group of MPC. Therefore,
its reactivity is the lowest, and only 31.4% MPC conversion is obtained. Likewise, the
steric hindrance of Ti(O-iC
and thus the reactivity of Ti(O-iC
Ti(OC
As to Ti(OC ) , the corresponding coordination groups of the Ti
3
H
7
)
4
is smaller than those of Ti(OC
6
H
5
)
4
and Ti(OC
4
H ) ,
9
4
3
H
7
)
4
is higher in comparison with Ti(OC
6
H
5
4
) and
4
H
9
4
) .
6
H
5
)
4
and Ti(OC
4
H
9
4
are phenoxyl and butoxyl. As a consequence of the conjugated effect of benzene ring,
the electron withdrawing ability of phenoxyl group is stronger than that of butoxyl
group, and therefore the Lewis acidity of the Ti in Ti(OC
Ti(OC . Although the steric hindered space of the phenoxyl group is larger than
that of butoxyl group, the reactivity of Ti(OC is still higher than Ti(OC
suggesting that the influence of the electron withdrawing effect is larger than that of
the steric hindrance. Furthermore, the high Lewis acidity of Ti(OC makes the
conversion of MPC close to that of Ti(O-iC . Nevertheless, the high Lewis acidity
6
H
5
) is higher than that in
4
4
H )
9 4
6
H
5
)
4
4
H
9
4
) ,
6
H
)
5 4
3
H )
7 4
of Ti(OC
6
H
5
)
4
also results in the decarboxylation of MPC to produce anisole [20, 35],
8

thereby causing the selectivity of DPC lower than those over Ti(O-iC
Ti(OC
TiO(OOCCH
octahedral structure [27]. Its steric hindrance is significantly greater than that of
Ti(OC . Accordingly, the reactivity of TiO(OOCCH is lower than Ti(OC
The steric hindrance of TiO(OOCCH is remarkably bigger compared to that of
TiO(acac) , but from the point of view of the electron effect, the electron withdrawing
ability of acetoxy group is stronger than that of acetylacetonate group, making the
Lewis acidity of TiO(OOCCH higher than that of TiO(acac) , and hence the
catalytic activity of TiO(OOCCH is higher than that of TiO(acac) . The results also
3
H
7
)
4
and
4
H
9
4
) .
3
2
) usually appears as the dimmers and displays six coordination
4
H
9
)
4
3
)
2
4
H
9
4
) .
)
3 2
2
3
)
2
2
3
)
2
2
show that the influence of the electron effect is greater than that of the steric
hindrance.
3
.2. Theoretical studies
The disproportionation of MPC is Lewis acid catalysis process. As shown in Fig. 1,
according to the literatures [36, 37], the Lewis acid-base complex intermediate is
formed by the interaction/coordination of MPC with the metal Ti center via the
oxygen of the carbonyl group, and then this interaction activates MPC to undergo
subsequent nucleophilic attack. In view of the orbital interaction of the Lewis
acid-base complex intermediate, it is thought as a result of an overlap between the
lower unoccupied molecular orbital (LUMO) of the Lewis acid Ti center and the
higher occupied molecular orbital (HOMO) of the oxygen Lewis base of the carbonyl
group, as shown in Fig. 2. If the energy gaps between HOMOcarbonyl–LUMOcomplex of
respective species are quite close, the Lewis acid-base interactions will be more
effective for readily activating MPC to undergo subsequent nucleophilic attack.
Therefore, the chemical computational studies are carried out, and the results are
listed in Table 2.
According to the energies calculated in Table 2, TiO(acac)
2
shows the lowest orbital
energy, meaning that the strongest Lewis acid-base interaction occurs between
TiO(acac)
2
and the carbonyl group of MPC. And the interaction intensity of the
9

respective Lewis acid-base complex intermediates for organo-titanium compounds is
in the order of TiO(acac) > TiO(OOCCH > Cp TiCl > Ti(OC > Ti(OC
Ti(O-iC , which is different from the order of their catalytic activities, further
2
3
)
2
2
2
6
H
5
)
4
4
H
9
) >
4
3
H )
7 4
suggesting that the influence of the electron effect is not the only factor, and other
features like the steric hindrance also play very important role on this activation
process. Considering the structural similarity of Ti(OC
6
H
5
)
4
, Ti(OC
4
H
9
)
4
and
Ti(O-iC , Ti(OC with the larger steric hindrance and the lower orbital energy
3
H
7
)
4
6
H )
5 4
presents higher catalytic activity, indicating that the influence of the electron effect is
greater than that of the steric hindrance, which is in accordance with the above
analysis.
In general terms, the steric hindrance and the electron effect of the coordination
group together affect the active Ti center and therefore influence the catalytic activity.
The smaller the steric hindrance of the coordination group is, the higher the reactivity
of organo-titanium compound is. The stronger the electron withdrawing group is, the
stronger the Lewis acidity of the Ti is, and thereby higher catalytic activity is obtained.
Nevertheless, higher Lewis acidity can also results in MPC decomposition to
decarboxylate into by-product anisole [38]. Among these titanium compounds
explored, Ti(O-iC
3
H
7
4
) exhibits the best catalytic performance, the conversion of
MPC reaches up to 90.4%, and the yield and selectivity of DPC are 90.0% and 99.6%,
respectively, which are comparable to those of organo-tin compounds, metal oxides,
MoO
performances with the MPC conversion of 89.7% and DPC selectivity of 99.3% [20].
Moreover, Ti(O-iC is cheap, innocuous and commercially available. Even though
the TOF of 10 h for Ti(O-iC in the reaction system is lower than those catalyzed
3
/SiO
2
, and PbO-ZrO
2
, since BuSnO(OH) showed the best catalytic
3
H )
7 4
-1
3
H )
7 4
-1
-1
by organo-tin compounds (TOF: 18 h ) and mixed metal oxides (TOF: 54 h ), it is
noteworthy that the catalyst is not only appropriate for the first-step transesterification
reaction, but also can efficiently catalyze the second-step disproportionation reaction.
As a result, when the first-step transesterification reaction is finished, the catalyst does
not be separated or purified from the reaction mixture and may be directly performed
for the subsequent second-step disproportionation reaction by adjusting the reaction
1
0

conditions, which can simplify the operation process and improve the reaction
efficiency in practical production. These advantages suggest its good potential for
industrial application in the future. On the whole, as has been already stated,
Ti(O-iC
3
H
7
4
) is a suitable Lewis acid catalyst for the disproportionation, and in order
to better understand the catalytic reaction as well as realize the industrial application
preliminary explored, the effect of reaction time, reaction temperature as well as the
catalyst amount is further investigated for Ti(O-iC
.3. Effect of the reaction time
Fig. 3 shows the effect of reaction time on the disproportionation of MPC. The
3
H
7
)
4
.
3
conversion of MPC increases sharply with reaction time increase before 3 h, and the
selectivity of DPC slightly changes. After 3 h, MPC conversion of 90.4% is attained,
and the selectivity of DPC is 99.6%. Continuing to increase the reaction time, the
conversion of MPC and the selectivity of DPC keep almost unchanged thereafter. The
result indicates that the disproportionation nearly achieves equilibrium at 3 h. Hence,
the optimum reaction time is 3 h. Besides, the reaction operation is simple, mild and
controllable, which is conducive to the industrial application in future.
3
.4. Effect of the reaction temperature
Thermodynamic calculation shows that the disproportionation reaction temperature
controlled in the range of 160 to 200 ºC is favorable [18], thus the influence of
temperature on the disproportionation is studied and the results are presented in Fig. 4.
The conversion of MPC increases quickly from 76.5% to 89.3% when the reaction
o
temperature is raised from 150 to 180 C, and the selectivity of DPC slightly changes.
Nevertheless, the selectivity of DPC begins to decrease as the temperature further
o
increases to 200 C, which can be attributed to the decarboxylation decomposition of
MPC into anisole at the higher temperatures [39]. The results suggest that a higher
reaction temperature not only favors the production of DPC and in the mean time, but
also raises the risk of MPC decomposition. Therefore, the optimum reaction
temperature is 180 °C, and the conversion of MPC and the selectivity of DPC is
11

8
3
9.3% and 99.4%, respectively.
.5. Effect of the catalyst amount
Fig. 5 shows the effect of catalyst amount on the disproportionation of MPC, and
the catalyst amount is based on the calculation of Ti(O-iC
expected, the conversion of MPC gradually increases as the mole ratio of
Ti(O-iC to MPC increases, and the selectivity of DPC remains almost unchanged.
H
3 7
4
) to MPC mole ratio. As
3
H )
7 4
When the mole ratio is increased to 0.04, the conversion of MPC reaches to 89.5%,
and the selectivity of DPC is 99.6%. With further increase the mole ratio to 0.06, the
conversion of MPC slightly increases, whereas the selectivity of DPC distinctly
decreases. The reason may be that Ti(O-iC
3
H
7
4
) is Lewis acid, and the acid amount of
the reaction system increases with the increase catalyst dosage. As a result, anisole is
easily produced, since higher acid amount can result in MPC decarboxylation [40].
Therefore, the appropriate mole ratio for the present system is 0.04.
3
.6. Possible mechanism for the catalytic reaction
In general, Ti(O-iC as Lewis acid is a typical kind of soluble transition metal
3
H )
7 4
coordination complexes catalyst in organic reaction, and the Ti is the active site. It is
worth noting that the disproportionation of MPC is a nucleophilic substitution
reaction process, and MPC is the only reaction reactant. Hence, the carbonyl group
must be activated firstly, and then the subsequent reaction takes place [20-22, 24, 25,
3
0, 41]. Moreover, FTIR-online detection technique is used to monitor the reaction
process (see Supplementary material). In view of these, a possible reaction
mechanism for the disproportionation catalyzed by Ti(O-iC is proposed in
3
H )
7 4
Scheme 2. The Ti firstly activates the oxygen of the carbonyl group to produce the
chelate, making the carbonyl carbon even electropositive. Subsequently, it attacks
another molecular MPC to form four atoms transition state Ti complexes, and then
undergoes the nucleophilic substitution to generate DPC and DMC, completing the
catalytic cycle. If the final product DMC continuously is distilled off, the reaction can
be continuously proceeded towards DPC.
1
2

4
. Conclusions
The catalytic performances of organo-titanium compounds with various
coordination groups for MPC disproportionation were investigated. The results
showed that the steric hindrance and electron effect of the coordination groups
together influenced their catalytic performances. The influence of the electron effect
was greater than that of the steric hindrance. The smaller the steric hindrance of the
coordination group was, the higher the reactivity of organo-titanium was. Besides, the
stronger the electron withdrawing effect of the coordination groups was, the stronger
the Lewis acidity of the Ti was, and the higher the catalytic activity was. Nevertheless,
excessive strong Lewis acidity easily resulted in the decarboxylation of MPC to
anisole. Among the titanium compounds screened, Ti(O-iC
3
H
7
4
) showed the best
catalytic performance for its suitable steric hindrance and electron effect for the
coordination groups. Under the optimal condition, with 0.04 of n(Cat.)/n(MPC), 180
oC of reaction temperature and 3 h of reaction time, MPC conversion of 90.4% and
DPC selectivity of 99.6% were achieved. Furthermore, the catalytic system was easily
handled and convenient, and especially the catalyst with relatively low price and
commercially available purchase was beneficial to industrial purposes in the near
future. Thus, this work offered an attractive alternative candidate of catalysts towards
efficient and green processes for transforming MPC disproportionation into DPC. In
addition, the possible reaction mechanism over Lewis acid titanium compounds was
proposed for the disproportionation. The active Ti firstly coordinated the oxygen of
the carbonyl group to form transition state Ti complexes, and then undergo the
nucleophilic substitution attack to generate DPC, completing the catalytic reaction.
Acknowledgments
We gratefully acknowledge the National High Technology Research and
Development Program of China (863 program, No. 2013AA031703), the Science and
Technology Support Program of Sichuan Province (No. 2013GZX0135), the Science
and Technology Innovation Program for Youth Team of Sichuan Province (No.
2
013TD0010), the Key Scientific Research Project of Colleges and Universities in
1
3

Henan Province (17A150026) and the Science Research Start-up Fund of Henan
Institute of Science and Technology (210010616003) for their financial support to this
research.
1
4

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7

Fig. 1. Schematic illustration of the oxygen activation of the carbonyl group via the Lewis
acid-base interactions.
Fig. 2. Two different orbital energies of the Lewis acid-base interactions, and (A) is more efficient
due to the lower orbital energies.
Fig. 3. Effect of reaction time on the disproportionation of MPC.
3 7 4
Reaction conditions: 180 ºC, n(Cat.)/n(MPC) 0.04, catalyst Ti(O-iC H ) , MPC 150 mmol.
1
8

Fig. 4. Effect of reaction temperature on the disproportionation of MPC.
Reaction conditions: 3 h, n(Cat.)/n(MPC) 0.04, catalyst Ti(O-iC , MPC 150 mmol.
3
7 4
H )
Fig. 5. Effect of catalyst amount on the disproportionation of MPC.
Reaction conditions: 180 ºC, 3 h, catalyst Ti(O-iC , MPC 150 mmol.
3
7 4
H )
Scheme 1. Transesterification reaction of DMC and phenol to produce DPC.
1
9

Scheme 2. Possible reaction mechanism for MPC disproportionation.
2
0

Table 1 Catalytic performance of organo-titanium compounds for MPC disproportionation
MPC DPC
Yields (%)
conversion (%) DPC Anisole selectivity (%)
Catalysts
—
trace
31.4
69.2
71.1
86.2
89.1
90.4
trace
0
trace
96.5
98.7
97.7
99.3
98.5
99.6
Cp
TiO(acac)
3 2
TiO(OOCCH )
2
TiCl
2
30.3 1.1
68.3 0.9
69.5 1.6
85.6 0.6
87.8 1.3
90.0 0.4
2
Ti(OC
Ti(OC
4
6
H
9
5
)
4
4
H
)
3 7 4
Ti(O-iC H )
Reaction condition: 180 ºC, 3 h, MPC 150 mmol, n(Cat.)/n(MPC) 0.04.
Table 2 Frontiers molecular orbitals of the catalysts and MPC
a
Catalysts
TiO(acac)
LUMOcomplex (eV) ΔE of HOMOcarbonyl–LUMOcomplex (eV)
2
-3.83
-3.22
-3.04
-2.02
-1.01
-0.94
3.21
3.82
4.00
5.02
6.03
6.10
3 2
TiO(OOCCH )
Cp
2
TiCl
2
H
H
Ti(OC
Ti(OC
6
4
5
9
)
4
4
)
3 7 4
Ti(O-iC H )
a
Modular measure of HOMO–LUMO involves the HOMO of MPC (HOMOcarbonyl) and the
LUMO of the related catalyst (LUMOcomplex), and the orbital energy of MPC is −7.04 eV.
2
1