Zn-promoted synthesis of diphenyl carbonate via transesterification over Ti–Zn double oxide catalyst

Article abstract of DOI:10.1007/s11164-016-2791-5

An environmentally friendly heterogeneous catalyst, Ti–Zn double oxide, was prepared using a sol–gel method and firstly used for the synthesis of diphenyl carbonate (DPC) via the transesterification of phenol and dimethyl carbonate. The catalyst exhibited excellent catalytic activity. Moreover, the effects of Ti to Zn molar ratio, calcination temperature and catalyst amount on the catalytic performances of Ti–Zn double oxide have been investigated. The characterization results of XRD, TGA–DSC, ICP-AES and NH₃-TPD showed that amorphous TiO₂ was the active sites, and amorphous ZnO was the promoter. Also the amount of Zn remarkably affected the acid amounts of the catalysts, and the calcination temperature not only influenced the acid amount, but also affected the acid strength. Besides, the weak surface acid sites were responsible for the synthesis of MPC and DPC, whereas the strong acid sites favored the formation of a by-product, anisole. The phenol conversion and the transesterification selectivity reached 41.2 and 98.2% over 0.3?g 5TiZn-250 for 8?h, respectively. Furthermore, the prepared catalyst could be reused for three runs without drastic decrease in activity. The slight decreased activity was attributed to the phase change of Ti–Zn double oxide and the leaching of Ti.

Full text of DOI:10.1007/s11164-016-2791-5

Res Chem Intermed
DOI 10.1007/s11164-016-2791-5
Zn-promoted synthesis of diphenyl carbonate
via transesterification over Ti–Zn double oxide
catalyst
Yingmin Qu1,2 Songlin Wang3 Tong Chen1
Gongying Wang1
•
•
•
Received: 23 June 2016 / Accepted: 19 October 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract An environmentally friendly heterogeneous catalyst, Ti–Zn double oxide,
was prepared using a sol–gel method and firstly used for the synthesis of diphenyl
carbonate (DPC) via the transesterification of phenol and dimethyl carbonate. The
catalyst exhibited excellent catalytic activity. Moreover, the effects of Ti to Zn
molar ratio, calcination temperature and catalyst amount on the catalytic perfor-
mances of Ti–Zn double oxide have been investigated. The characterization results
of XRD, TGA–DSC, ICP-AES and NH3-TPD showed that amorphous TiO2 was the
active sites, and amorphous ZnO was the promoter. Also the amount of Zn
remarkably affected the acid amounts of the catalysts, and the calcination temper-
ature not only influenced the acid amount, but also affected the acid strength.
Besides, the weak surface acid sites were responsible for the synthesis of MPC and
DPC, whereas the strong acid sites favored the formation of a by-product, anisole.
The phenol conversion and the transesterification selectivity reached 41.2 and
98.2% over 0.3 g 5TiZn-250 for 8 h, respectively. Furthermore, the prepared cat-
alyst could be reused for three runs without drastic decrease in activity. The slight
decreased activity was attributed to the phase change of Ti–Zn double oxide and the
leaching of Ti.
Keywords Ti–Zn double oxide Á Transesterification Á Diphenyl carbonate Á
Dimethyl carbonate
&
Tong Chen
chentongw@cioc.ac.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
123

Y. Qu et al.
Introduction
Diphenyl carbonate (DPC) is extensively used to produce various organic compounds
and polymer materials, particularly as an important intermediate for the synthesis of
polycarbonate in non-phosgene processes [1–3]. The processes developed for the
synthesis of DPC with different carbonyl sources involve phosgenation, oxidative
carbonylation of phenol, esterification of CO2 with phenol, phenolysis of urea, and
transesterification of carbonates with phenol [4]. The phosgenation technique has the
disadvantages of using corrosive and severely toxic phosgene as a raw material and
the formation of corrosive hydrogen chloride as a by-product. Though the oxidative
carbonylation of phenol is considered to be a prospective process for synthesis of
DPC, the employment of a noble catalyst and the low yield of DPC limit its
industrialization [5]. Similarly, DPC produced via direct reaction of phenol with urea
or CO2 also suffer low yield [6]. Besides, the subsequent decarbonylation of
transesterification of dimethyl oxalate (DMO) and phenol to DPC hampers its
application [7]. In this context, the transesterification of dimethyl carbonate (DMC)
and phenol is thought to be the most suitable method for commercial production of
DPC with no hazard to the environment. Generally, this route involves the
transesterification of DMC and phenol to methyl phenyl carbonate (MPC; Eq. 1)
followed by the further transesterification (Eq. 2) and/or a disproportionation step to
DPC (Eq. 3) [8]. Moreover, side reactions also may occur in this process (Eq. 4) [9].
ð1Þ
ð2Þ
ð3Þ
ð4Þ
The transesterification reaction of DMC and phenol is thermodynamically
unfavorable, and the low yield and selectivity to DPC were obtained even at
elevated temperatures [10]. Therefore, an effective catalyst is highly desirable for
this process. The liquid-phase transesterification of DMC and phenol was usually
catalyzed by homogeneous catalysts such as conventional Lewis acids, organo-
titanium, and organotin compounds [11–13]. Because of the separation problems
123

Zn-promoted synthesis of diphenyl carbonate via…
between homogeneous catalysts and DPC as well as the recovery of catalysts, more
attention has been paid to heterogeneous catalysts like MoO3/SiO2, TiO2/SiO2, Ti-
HMS, TiO2/CNT, PbO/MgO, Pb–Zn composite oxide, 12-molybdophosphoric acid
supported on titania (HPMo/TiO2) and so on [14–23]. Although these catalysts had
good performances in the transesterification, their stability for this process was not
very satisfactory. Moreover, some catalysts containing heavy metals, supported
organotin, etc., are harmful to the environment. Therefore, the development of
active solid catalysts with high catalytic activity, desirable stability and nontoxicity
is necessary. In transesterification, TiO2 as catalysts have been widely studied in
various fields with advantages of ease of synthesis, low cost, innocuity and
environmental friendliness [24, 25]. As the activity of pure TiO2, especially after
heat treatment at high temperature, is low, some methods have been developed to
improve the catalytic activity of TiO2 including dispersion of TiO2 on supports,
complex of TiO2 with other oxides, etc. Furthermore, it was reported that
amorphous ZnO played a role as the promoter in the DPC synthesis via
transesterification over lead and zinc double oxide catalyst [26]. Besides, TiO2
shows strong interaction with transition metal oxides, which probable makes Ti–Zn
double oxide stable [27]. In addition, square-shaped Zn/Ti oxides were also reported
to be an excellent catalyst for the transesterification of dibutyl carbonate with
phenol to synthesize DPC [28]. These facts give some credence to Ti–Zn double
oxide catalyst providing preferable performance in the transesterification reaction.
Therefore, in the present work, Ti–Zn double oxide, a green solid, was prepared
via a sol–gel process, and first used to catalyze the transesterification of DMC and
phenol. Ti–Zn double oxide catalyst revealed superior catalytic activity compared
with other heterogeneous catalysts reported. The influences of molar ratio of Ti to
Zn, calcination temperature and catalyst amount on catalytic behavior were also
investigated.
Experimental
Catalyst preparation
Ti–Zn double oxide catalyst was prepared by a sol–gel method. An appropriate
amount of zinc acetate was added to a mixed solution of ethanol (200 mL) with
acetic acid (48 mL) containing 6.80 g tetra-n-butyl titanate [Ti(OBu)4] with stirring
at 80 °C for 1 h, and then the pH of the solution was adjusted to the range between
5.6 and 6 using a mixture of 2-aminoethanol and H2O (volume ratio of 1:1),
followed by evaporation of solvent at 120 °C until dry and being calcined in a
muffle furnace at the desired temperature for 4 h. The prepared catalysts are denoted
as xTiZn-T, in which x is the molar ratio of Ti to Zn, and T is the calcination
temperature. TiO2 and ZnO, as the controls, were also synthesized via the same
procedure.
123

Y. Qu et al.
Characterization
Powder X-ray diffraction (XRD) was performed on a Philip X’Pert PRO MPD
˚
diffractometer using a radiation source of Cu Ka (k = 1.54056 A) at 45 kV and
40 mA. The samples were scanned in the 2h ranges from 5° to 80° with a step time
count of 1 s and step size of 0.02°.
The surface acid properties of the catalysts were studied via temperature
programmed desorption (TPD) of NH3 and conducted on a TPD flow system
equipped with a TCD detector. Generally, the catalyst was pretreated at 250 °C for
1 h under N2 (20 mL min-1) and then cooled to 40 °C at which NH3 was adsorbed
for 2 h and sample was flushed with N2 for 2 h to remove any physically adsorbed
NH3 followed by heating of the sample to 600 °C at 10 °C min-1 to desorb NH3.
TGA–DSC experiments were carried out in a TGA/DSC 2 LF/1100/155
apparatus. About 15 mg of sample was placed in an alumina crucible. Heating
was started at 15 °C min-1 from room temperature to 900 °C with 50 mL min-1
air.
The element contents were determined using an inductively coupled plasma-
atomic emission spectrometry (ICP-AES) method on an IRIS 1000 instrument
(Thermo Electron, USA).
Reaction procedure
The transesterification of DMC with phenol was conducted in a 100-mL three-
necked round-bottom flask, equipped with a magnetic stirring bar, nitrogen inlet,
dropping funnel and fractionating column connected to a liquid dividing head under
refluxing condition at atmosphere pressure. Typically, under nitrogen atmosphere,
phenol and the desired catalyst were charged into the flask. When the mixture was
heated to 175 °C, DMC was added dropwise under continuous stirring, and the
reaction temperature was kept at 150–180 °C under a refluxing condition. During
the reaction procedure, a distillate composed of DMC and methanol was slowly
collected in a receiver flask to break the reaction equilibrium limitation to promote
the reaction towards the formation of MPC and DPC. Upon completion of the
reaction, the mixture was cooled to room temperature and the catalyst was separated
by centrifugation, then washed with DMC and dried at 200 °C. Qualitative and
quantitative analysis of the reaction products and the distillates were carried out on a
HP-6890/5973 system and a gas chromatograph (Agilent Technologies 7820A)
equipped with a DB-35 capillary column (30 m 9 320 lm 9 0.25 lm) and a flame
ionization detector (FID). The conversion of phenol and the selectivity of MPC and
DPC were calculated by GC results using a correction factor normalization method.
123

Zn-promoted synthesis of diphenyl carbonate via…
Results and discussion
Characterization
XRD patterns of the 5TiZn-T with varied calcination temperature were shown in
Fig. 1. Below 300 °C, there was no peak in the XRD of catalysts, indicating that the
Ti specie and Zn specie may exist in a microcrystalline structure or amorphous state.
The characteristic peaks of anatase were observed in 5TiZn-400 and 5TiZn-500, and
became sharper with calcination temperature increasing due to the improved
crystallinity of TiO2 and the particle growth. It can be deduced that increasing
calcination temperature improves the crystallinity of TiO2. In addition, the
characteristic peaks of ZnO existed after reuse of 5TiZn-250 for three times. The
results suggested that the additive Zn didn’t change the phase of TiO2 and was
highly dispersed in the catalysts.
Figure 2 represented the NH3-TPD characterization of TiO2-250 and xTiZn-
T. The peaks in the high and low temperature regions can be assigned to desorption
of NH3 from the strong and weak acid sites, respectively. Generally, the peaks in the
high temperature (T [ 400 °C) regions are ascribed to the desorption of NH3 from
strong acid sites, while the peaks in the low temperature (T \ 400 °C) regions are
attributed to the desorption of NH3 from weak acid sites [29]. As shown in Fig. 2,
the peaks appeared both in the low and high temperature region, confirming that
there existed simultaneously weak and strong acid sites on the surface of xTiZn-T. It
can also be observed clearly that the amount of weak acid sites decreased
significantly with the calcination temperature of 5TiZn-T increasing or after reuse
for three times, or with the decreased Ti content. The decreased amount of weak
acid sites with the increased calcination temperature of 5TiZn-T might be ascribed
Fig. 1 XRD patterns of (a) 5TiZn-500; (b) 5TiZn-400; (c) 5TiZn-300; (d) 5TiZn-250; (e) 5TiZn-250
reused
123

Y. Qu et al.
Fig. 2 The NH3-TPD profiles of (a) 1TiZn-250, (b) 2TiZn-250, (c) 5TiZn-250 used, (d) 5TiZn-250,
(e) 5TiZn-300, (f) 5TiZn-400 and (g) 5TiZn-500
to the decreased surface area of catalyst, the decreased amount of Ti–OH and the
phase transformation from the amorphous phase to the anatase, which was
demonstrated using the TGA/DSC curves of the 5TiZn-120 (Fig. 3). When catalyst
was calcined at low temperature, with the calcination temperature increasing, the
amount of Ti–OH decreased which led to the decreased amount of acidic sites. In
addition, the peaks at high temperature for the catalysts calcined at low temperature
(below 300 °C) might partly be due to the decomposition of acetate and the relative
amount of strong acid sites to weak acid sites in a catalyst becoming stronger
evidently with the increase of calcination temperature from 400 to 500 °C.
Furthermore, in comparison with the 5TiZn-250, the peaks at low temperatures
shifted toward higher temperatures with increased calcination temperature, illus-
trating that the strength of weak acid of 5TiZn-T became stronger with increasing
calcination temperature. Nevertheless, the addition of Zn to TiO2 didn’t change the
strength of acid sites of the xTiZn-250. The NH3-TPD results indicated that the
incorporation of the Zn component into Ti affected the number of acidic sites,
especially the amount of weak acid sites. Also, the calcination temperature
influenced the strength and the amount of acid sites.
Figure 3 showed the TGA/DSC curves of the 5TiZn-120. The TGA curve could
be divided into two main weight-loss stages. The first weight loss below 250 °C
could be attributed to the evaporation of solvents, such as water and alcohol, and
organic acid such as acetic acid, corresponding to an endothermic peak at
120–200 °C. The second step that occurred in the temperature range of 250–500 °C
resulted from the dehydroxylation of Ti–OH into TiO2 and the decomposition of
Zn(OAc)2 to ZnO as corroborated by a endothermic peak at 250–350 °C and the
combustion of the organic residues and the phase transformation revealed by the
XRD patterns corresponding to a sharp exothermic peak at 350–500 °C [30, 31].
123

Zn-promoted synthesis of diphenyl carbonate via…
Fig. 3 TGA/DSC analysis of the 5TiZn-120
Catalytic activity
Effect of molar ratio of Ti to Zn
Table 1 showed the results of catalytic activities of xTiZn-250. It was notable that the
phenol conversions increased steadily with increasing the molar ratio of Ti to Zn up to
3:1, followed by slowly decreasing at higher Ti content. However, the selectivity for
MPC and DPC monotonously increased with the increase of Ti content. 36.4% of
phenol conversion and 99.2% of selectivity for MPC and DPC over 5TiZn-250 were
obtained, respectively. In the case of the absence of any catalysts, no yield of DPC
was gained and only a trace amount of phenol was converted, suggesting that the
transesterification was difficult to incur without catalyst.
For comparison, the transesterification over TiO2-250 and ZnO-250 were
performed and the phenol conversions and the transesterification selectivity were
15.7, 0.4% and approximatively 100, 91.4%, respectively. These results demon-
strated that ZnO was the promoter, which could improve markedly the activity of
the catalysts. It is well known that in this transesterification, the weak acid sites
promote the synthesis of MPC and DPC, and strong acid sites are responsible for the
anisole (AN) formation [32]. The decrease in phenol conversion with decreased Ti
content was attributed to the decrease of weak acid sites and the decreased active
sites of Ti. The decreased selectivity of MPC and DPC over catalysts with decreased
Ti content was due to the decreased active sites of Ti.
Effect of calcination temperature
The effect of calcination temperature on the catalytic activity of 5TiZn-T was
displayed in Table 2. It was observed that the conversion of phenol and the
selectivity of MPC and DPC increased with the calcination temperature of 5TiZn-
123

Y. Qu et al.
Table 1 Catalytic performance of xTiZn-250 on the transesterification
Catalyst
Phenol conversion (%)
Yield (%)
MPC
Selectivity (%)
DPC
AN
None
0.86
15.7
35.8
36.4
38.4
41.2
33.2
24.6
0.4
0.86
10.1
19.6
20.4
20.7
20.4
18.0
7.1
0
0
0
0
100
TiO2-250
10TiZn-250
5TiZn-250
4TiZn-250
3TiZn-250
2TiZn-250
1TiZn-250
ZnO-250
5.6
100
16.2
15.8
17.3
17.2
9.1
100
0.2
99.2
98.8
91.1
81.6
66.4
90.0
0.4
3.6
6.1
9.3
8.2
0.3
\0.1
\0.1
Reaction conditions: phenol 10 g, n(phenol) = n(DMC), catalyst 0.15 g, reaction time 8 h, reaction
temperature 160–180 °C
Table 2 Effect of calcination temperature of 5TiZn-T on the transesterification
T (°C)
Phenol conversion (%)
Yield (%)
MPC
Selectivity (%)
DPC
AN
MPC ? DPC
DPC
200
250
300
400
500
34.7
36.4
28.4
24.6
6.6
21.5
20.4
14.9
13.0
2.7
12.4
15.8
12.6
4.5
0.8
0.2
0.9
7.1
3.6
97.6
99.2
96.9
81.7
45.2
35.7
43.3
44.4
18.3
4.5
0.3
Reaction conditions: phenol 10 g, n(phenol) = n(DMC), catalyst 0.15 g, reaction time 8 h, reaction
temperature 160–180 °C
T increasing from 200 to 250 °C, then decreased afterwards. The maximum phenol
conversion of 36.4% and transesterification selectivity of 99.2% were obtained with
5TiZn-250, which was higher than that over other catalysts [4]. The decreased
catalytic performance of 5TiZn-T with increasing calcination temperature was
ascribed to the decreased amount of weak acid sites, which was the active site in the
transesterification of phenol and DMC [28], and was also due to the formation of
anatase possessing low catalytic activity [33]. The low selectivity at higher
calcination temperature was attributed to the acid strength becoming stronger and
the increased relative amount of strong acid sites to weak acid sites, which favor the
formation of AN [28]. The results elucidated that the composition of production was
closely related to the calcination temperature of 5TiZn-T.
Effect of catalyst amount
The effect of 5TiZn-250 amount on the transesterification was determined in Fig. 4.
The phenol conversion increased significantly with the catalyst amount increasing
123

Zn-promoted synthesis of diphenyl carbonate via…
Fig. 4 Effect of catalyst amount on the catalytic performance of 5TiZn-250. Reaction conditions: phenol
10 g, n(phenol) = n(DMC), reaction time 8 h, reaction temperature 160–180 °C
from 0.05 to 0.30 g, with the selectivity decreasing slightly, and remained almost
constant with the further increase of catalyst amount. The conversion of phenol and
the selectivity of the transesterification reached 41.2 and 98.2%, respectively, with
0.30 g of catalysts, which was significantly higher than that with V–Cu composite
oxide (37.0% of phenol conversion and 96.8% of selectivity, respectively) [34].
Nevertheless, with the increase of catalyst amount, the decarboxylation of MPC to
AN increased, which decreased the selectivity of the transesterification [20].
Therefore, the optimal amount of catalyst was 0.3 g.
Reusability test of catalyst
The reusability of 5TiZn-250 was investigated and the results are listed in Table 3.
The used catalyst was filtered and washed with DMC, and dried at 200 °C. As
shown in Table 3, the phenol conversion slowly decreased from 41.2 to 37.5%, with
the transesterification selectivity slightly improving, indicating that the catalyst
could be reused. XRD results suggested that the characteristic peaks of ZnO
appeared in the used catalyst, implying that the structural change occurred during
the reaction, which may be the reason for the decreased phenol conversion for the
spent catalyst. In addition, the total acidic amount of reused catalyst reduced,
leading to the catalytic activity decrease. Furthermore, the molar ratio of Ti to Zn
dissolved in solution was 15:1.35 in the first run, which also caused the decreased
activity of the catalyst. On the whole, 5TiZn-250 was more stabilized than other
TiO2 -based catalysts and other metal-based catalysts [14, 26]. The excellent
stability of 5TiZn-250 maybe a result from the strong interaction between TiO2 and
ZnO [27].
123

Y. Qu et al.
Table 3 The reusability of 5TiZn-250
Run time
Phenol conversion (%)
Yield (%)
MPC
Selectivity (%)
DPC
MPC ? DPC
DPC
1
2
3
41.2
38.7
37.5
22.9
21.5
21.0
17.6
17.1
16.5
98.2
99.7
42.7
44.2
44.0
[99.9
Reaction conditions: phenol 10 g, n(phenol) = n(DMC), catalyst 0.15 g, reaction time 8 h, reaction
temperature 160–180 °C
Conclusions
A green and economic catalyst of Ti–Zn double oxide was prepared employing a
sol–gel method and firstly used as a heterogeneous catalyst for the transesterification
of DMC and phenol with excellent catalytic activity and high stability. The molar
ratio of Ti to Zn and calcination temperature significantly affected the catalytic
performance of the catalysts. 41.2% of phenol conversion and 98.2% of selectivity
were achieved over 0.3 g of 5TiZn-250. Moreover, the 5TiZn-250 catalysts could be
used for three times with the phenol conversion slowly decreasing from 41.2 to
37.5%, the stabilization of which was ascribed to the strong interaction of TiO2 with
ZnO. The decreased catalytic activity was caused by the phase change and the
leaching of Ti.
Acknowledgements Financial support for this work from 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. 2013TD0010) and the Key Scientific Research Project
of Colleges and Universities in Henan Province (17A150026) are greatly acknowledged.
References
1. W.B. Kim, U.A. Joshi, J.S. Lee, Ind. Eng. Chem. Res. 43, 1897 (2004)
2. J. Haubrock, M. Raspe, G.F. Versteeg, H.A. Kooijman, R. Taylor, J.A. Hogendoorn, Ind. Eng. Chem.
Res. 47, 9854 (2008)
3. K.M. Deshmukh, Z.S. Qureshi, K.P. Dhake, B.M. Bhanage, Catal. Commun. 12, 207 (2010)
4. Q. Wang, C.H. Li, M. Guo, S.J. Luo, C.W. Hu, Inorg. Chem. Front. 2, 47 (2015)
5. H.Y. Niu, J. Yao, Y. Wang, G.Y. Wang, Catal. Commun. 8, 355 (2007)
6. W. Xue, J. Zhang, Y. Wang, X. Zhao, Q. Zhao, Catal. Commun. 6, 431 (2005)
7. X. Ma, J. Gong, S. Wang, F. He, H. Guo, X. Yang, G. Xu, J. Mol. Catal. A Chem. 237, 1 (2005)
8. H. Lee, S.J. Kim, B.S. Ahn, W.K. Lee, H.S. Kim, Catal. Today 87, 139 (2003)
9. Y. Ono, Appl. Catal. A Gen. 155, 133 (1997)
10. X. Yin, Y. Zeng, J. Yao, H. Zhang, Z.Y. Deng, G.Y. Wang, Ind. Eng. Chem. Res. 53, 19087 (2014)
11. H.Y. Niu, J. Yao, Y. Wang, G.Y. Wang, J. Mol. Catal. A Chem. 235, 240 (2005)
12. H.J. Lee, J.Y. Bae, O.S. Kwon, S.J. Kim, S.D. Lee, H.S. Kim, J. Organomet. Chem. 689, 1816 (2004)
13. Z.P. Du, W.K. Kang, T. Chen, J. Yao, G.Y. Wang, J. Mol. Catal. A Chem. 246, 200 (2006)
14. Y.T. Kim, E.D. Park, Appl. Catal. A Gen. 356, 211 (2009)
15. U.A. Joshi, S.H. Choi, J.S. Jang, J.S. Lee, Catal. Lett. 123, 115 (2008)
16. S.W. Luo, T. Chen, D.S. Tong, Y. Zeng, Y.C. Lei, G.Y. Wang, Chin. J. Catal. 28, 937 (2007)
17. X. Zhou, X. Ge, R.Z. Tang, T. Chen, G.Y. Wang, Chin. J. Catal. 35, 481 (2014)
123

Zn-promoted synthesis of diphenyl carbonate via…
18. M. Cao, Y.Z. Meng, Y.X. Lu, Catal. Commun. 6, 802 (2005)
19. Z.H. Li, Y.J. Wang, X.S. Ding, X.Q. Zhao, J. Nat. Gas Chem. 18, 104 (2009)
20. S.L. Wang, R.Z. Tang, Y.Z. Zhang, T. Chen, G.Y. Wang, Chem. Eng. Sci. 138, 93 (2015)
21. D.S. Tong, J. Yao, Y. Wang, H.Y. Niu, G.Y. Wang, J. Mol. Catal. A Chem. 268, 120 (2007)
22. R.Z. Tang, T. Chen, Y. Chen, Y.Z. Zhang, G.Y. Wang, Chin. J. Catal. 35, 457 (2014)
23. Y. Zhang, S. Wang, Z. Xiao, T. Chen, G. Wang, Res. Chem. Intermed. (2016). doi:10.1007/s11164-
016-2530-y
24. X. Liu, M.M. Yu, Z.Q. Zhang, X. Zhao, J. Li, Res. Chem. Intermed. 42, 5197 (2013)
25. N. Ganji, A. Allahverdi, F. Naeimpoor, M. Mahinroosta, Res. Chem. Intermed. 42, 5395 (2016)
26. W. Zhou, X. Zhao, Y. Wang, J. Zhang, Appl. Catal. A Gen. 260, 19 (2004)
27. S. Wang, Y. Liu, Y. Shi, X. Ma, J. Gong, AIChE J. 54, 741 (2008)
28. P. Wang, S. Liu, F. Zhou, B. Yang, A.S. Alshammaric, Y. Deng, RSC Adv. 5, 84621 (2015)
29. Y. Liu, X. Ma, S. Wang, J. Gong, Appl. Catal. B Environ. 77, 125 (2007)
30. S.F. Wang, F. Gu, M.K. Lu, C.F. Song, S.W. Liu, D. Xu, D.R. Yuan, Mater. Res. Bull. 38, 1283
(2003)
31. Z.Q. Wang, X.G. Yang, S.Y. Liu, J. Hu, H. Zhang, G.Y. Wang, RSC Adv. 5, 87311 (2015)
32. S. Wang, X. Ma, J. Gong, N. Gao, H. Guo, X. Yang, G. Xu, Catal. Today 93–95, 377 (2004)
33. X. Ge, B. Li, J. Hu, T. Chen, G. Wang, X. Hu, Chem. Eng. (China) 39, 61 (2011)
34. D. Tong, T. Chen, J. Yao, Y. Wang, G. Wang, D. Shi, Z. Li, Z. Chen, Chin. J. Catal. 28(3), 190
(2007)
123