Catalytic hydrolysis of Dichlorodiuoromethane over MoO3/ZrO2 -TiO2 solid acid

The catalytic hydrolysis of Diuorodichloromethane(CFC-12) by solid acid of MoO 3 /ZrO 2 -TiO 2 calcined at different temperature had been studied. The effects of catalytic hydrolysis temperature and water vapor concentration on catalytic hydrolysis of CFC-12were also studied. The results showed that catalytic hydrolysis rate of CFC-12 reached to 98.65 % at 400 (cid:0) C when the MoO 3 /ZrO 2 -TiO 2 catalyst was calcined at 500 ℃ with a concentration of water vapor of 83.18% ,and the main hydrolysis products were CO, CO 2 , HF and HCl. After 30 hours’ continuous reaction, the hydrolysis rate of CFC-12 was 65.34%. The XRD result reveals that the main phase of solid MoO 3 /ZrO 2 -TiO 2 catalyst is the tetragonal Zr(MoO 4 ) 2 with dopedTiO 2 of anatase.


Introduction
Chloro uorocarbons (CFCs)have been broadly utilized in chemical industry due to their superior physical and chemical properties. However, more and more research showed that CFCs is harmful for ozone layer. Molina and Rowland proved that CFCs are a killer of ozone layer that protects humans against harmful ultraviolet radiation from the sun in 1974 [1,2]. Moreover, CFCs as green house gases have a great negative impact on human health and ecological environment. The research of CFCs has recently attracted many experts and scholars all over the world [3]. For example, in 1985, the Vienna Convention was established to restrict the use of CFCs for protecting the ozone layer, facilitated by United Nations Environment Program. With the enhancement of environmental awareness, 46 countries signed the Montreal Protocol (hereinafter referred to as protocol) in Montreal, Canada in 1987. Over time, the United Nation organized several meetings again, strengthen efforts that limited use of CFCs, modi ed protocol clearly stipulates that developing countries to stop using CFCs, CFCB, CCl 4, CH 3 CCl 3 .
So far, the technology of harmless treatment of chloro uorocarbons mainly includes chemical method, incineration, cement kiln, induction plasma, supercritical water and photocatalysis, etc [4][5][6][7].However this methods have some limitations, therefore, it is urgent to pursue safe and e cient method of hydrolysis of CFCs. Solid super acid [8][9][10][11]is one of the new catalytic materials that has developed rapidly in recent years. The solid super acid catalytic has advantage of excellent catalytic activity, pollution-free, stability and reusability, thus this type of materials also become very popular in catalytic hydrolysis of CFC-12 [12][13][14]. The catalytic performance of MoO 3 /ZrO 2 -TiO 2 solid acid catalysts calcined at different temperatures for the hydrolysis of CFC-12 was studied, including the catalyst life. The effects of hydrolysis temperature and steam concentration on the reaction were also investigated [15][16][17].The conditions of catalytic hydrolysis of CFC-12 [18][19][20] were optimized, which provided a theoretical basis for a large number of harmless treatment of CFC-12.

Catalyst preparation
The MoO 3 /TiO 2 -ZrO 2 catalyst were prepared by a wetness impregnation. Firstly, 5 ml TiCl 4 solution was added into 30 ml absolute ethyl alcohol solution in ice bath conditions. Then 6.2968 g ZrOCl 2 8 H 2 O was dissolved in mother solution. Secondly, the pH was adjusted to 8 with 10% ammonia, then took out the solid , washed out the Clwith deionized water and dried it at 110 0 C. Next, the precursor was impregnated in 0.25 mol/L (NH 4 ) 6 MoO 24 ·4H 2 O at 60 0 C for 6 hours, Finally, the impregnation liquid was ltered , dried at 110 0 C and calcined at a certain temperature for 3 hours.

Catalyst characterization
In order to understand the morphology of catalyst, this paper has taken the following characterization tools. Firstly, the surface composition of the catalysts is analyzed by X-ray, the instruments are produced in Germany, the model is BRUKER D8ADVANCE. Secondly, the surface morphology of catalysts is analyzed by scanning electron microscopy with spectrum analyzer, the equipment is produced by FEI company of United States, its model is NOVA NANOSEM-450.The crystal structure of the catalyst was analyzed by transmission electron microscope, the equipment was produced by Japan Electronics Company, model JEM2106.BET N 2 isotherm adsorption desorption was used to determine the speci c surface area and pore distribution of the catalyst, the equipment was produced by Macchik Bayer company, its model is BELSORP -max .

Catalyst experiment
The catalyst hydrolysistest was performed in a xed bed reactor at one atmospheric pressure. 1.00 g of catalyst (MoO 3 /TiO 2 -ZrO 2 ) and 50 g SiO 2 were added into quartz tubes (Φ30×700 mm). A mixed gas stream of CFC-12, H 2 O(g), O 2 and He was introduced into the reactor with a total ow rate of 15 ml/min. The catalytic performance of MoO 3 /ZrO 2 -TiO 2 solid acid catalyst calcined at different temperature for the catalytic hydrolysis of CFC-12 was investigated. The effects of hydrolysis temperature and steam concentration on the reaction were also investigated. After 1.5 hours, the hydrolysis rate of CFC-12 was tested. The catalyst stability was analyzed after 30 hours. indicate that the components are mainly amorphous with calcination temperature of 500 ℃. When calcination time above 3 hours, the better crystal structures of MoO 3 /ZrO 2 -TiO 2 are observed in the XRD patterns.
TEM patterns of MoO 3 / ZrO 2 -TiO 2 calcined at different temperature It can be seen from the TEM diagram of MoO 3 /TiO 2 -ZrO 2 .When the calcination temperature is 400 ℃, the catalyst mainly exists in amorphous state. With the increase of calcination temperature, the catalyst calcined at 500 ℃ mainly exists in crystalline phase. The selected area diffraction pattern in the lower right corner is concentric ring, which shows that the sample is polycrystalline, which is consistent with the XRD results. When the temperature continued to rise to 600 ℃, the speci c surface area and catalytic activity of the catalyst decreased, the main reason is the sintering of catalyst.
BET N 2 adsorption and desorption patterns of MoO 3 /ZrO 2 -TiO 2 calcined at different temperature As can be seen from table 1, with the increase of calcination temperature, the speci c surface area of the catalyst decreases more sharply, which indicates that the catalyst has a certain degree of sintering.
Combined with XRD, with the increase of calcination temperature, the intensity of diffraction peak increased to a certain extent, and the speci c surface area of catalyst calcined at 600 ℃ decreased 224.795 compared with that at 400 ℃ It can be seen from the gure that the isotherm characteristics of all samples are concave to the relative pressure axis. The adsorption curve is not consistent with the desorption curve, that is, there is a hysteresis loop. The P/P 0 values of the catalysts calcined at 400 ℃ and 500 ℃ are 0.45-0.99, which is a typical IV isotherm accompanied by H2 hysteresis loop. The characteristics of these isotherms also re ect the formation of inter granular mesopores. The catalyst calcined at 600 ℃ is characterized by type IV isotherm and H1 hysteresis loop, which is the result of homogeneous pore condensation. When the relative pressure is 0.75 and 0.9, the change slope of the isotherm is higher, which indicates that the mesoporous materials have better uniformity.

NH 3 -TPD of MoO 3 /TiO 2 -ZrO 2 calcined at different temperatures
Temperature programmed desorption method uses pyridine, ammonia, 2,6-dimethylpyridine and tert Butyl Ammonium as adsorbents. Because of NH 3 have strong basicity (P Ka = 9.2), high stability and small molecular volume, its can be adsorbed on acid sites with different acid strength and easily enter into the pore channels of porous materials, it is an ideal acid probe molecule. The peak temperature is often related to the acid strength, peak area and acid amount .The desorption temperatures of NH 3 were 100 ℃ 150 ℃, 200 230 ℃ and 500 700 ℃, respectively, corresponding to the weak acid site desorption peak, medium strong acid site desorption peak and strong acid site desorption peak [21]. It can be seen from Fig. 6 that NH 3 has two strong acid sites (α and β) on MoO 3 /TiO 2 -ZrO 2 , the peak area is proportional to the amount of NH 3 adsorbed on the sample. That is, it is directly proportional to the amount of acid in the sample, the weak desorption peak of β indicates that the content of strong acid site is less, the αdesorption peak of MoO 3 /TiO 2 -ZrO 2 catalyst calcined at 500 ℃ was the strongest, indicating the highest content of weak acid. The results showed that the best hydrolysis effect of CFC-12 was obtained by Effect of calcination temperature on the hydrolysis rate of CFC-12 It can be seen in Fig.7 that the hydrolysis rate increases with the increase of temperature from 300 ℃ to 500 ℃, and decreases after 500 ℃. The highly hydrolysis rate reaches 98.65 % at 500 0 C. It is studied and analyzed that the occurrence of the catalyst is due to the temperature being too high, the catalyst is sintered, and the activity is lowered.
Effect of calcination time on hydrolysis rate of CFC-12 Fig.8 is based on a previous study of the optimal calcination temperature. The effect of calcination time on the hydrolysis rate of CFC-12 at the optimum calcination temperature was investigated. The experimental results show that the catalytic hydrolysis rate of CFC-12 is up to 98.65% when the calcination time is 3 hours,. When the calcination time is less than 3 hours, the calcination is incomplete and the structure is incomplete, so the hydrolysis rate is not too high. After calcination for 3 hours, the main structure of the solid acid catalyst MoO 3 /ZrO 2 -TiO 2 is tetragonal Zr(MoO 4 ) 2 doped anatase TiO 2 structure. The hydrolysis rate decreased with the increase of calcination time, which is mainly due to the corrosion of the catalyst by HCl and HF produced by the hydrolysis of CFC-12 and calcination time is too long, the catalyst will be sintered, the dispersion of TiO 2 in ZrO 2 will be reduced, resulting in the speci c surface area and activity of the catalyst will be reduced.
Effect of catalytic hydrolysis temperature on the hydrolysis rate of CFC-12 Fig. 9 shows that the hydrolysis rate of CFC-12 increases with the increase of catalytic hydrolysis temperature. The hydrolysis rate of CFC-12 reaches 98.65% at 400 ℃, it remains unchanged after 400 ℃.The rate of hydrolysis gradually increases due to the endothermic process of the reaction of CFC-12: The effect of water vapor concentration on the hydrolysis rate of CFC-12 Fig.10 shows that water vapor concentration has signi cant effect on the hydrolysis rate of CFC-12. In the absence of water vapor, the hydrolysis of CFC-12 was 14.31%.The hydrolysis rate of CFC-12 increased with the increase of steam concentration, and decreases after 83.18%.When the steam concentration was 83.18%, the hydrolysis rate reached the maximum value of 98.65%.The hydrolysis rate decreased with the further increase of steam concentration. The main reason is that the reaction rate is accelerated and the gas-solid contact time is shortened with the increase of water vapor concentration, as well as the lower reaction temperature as more water vapor involved in the system. It is concluded that the optimum volume fraction of water vapor is 83.18%.
The effect of reaction time on the hydrolysis rate of CFC-12 As shown in Figure 11, when the reaction time is less than 10 hours, the hydrolysis rate of CFC-12 is more than 98.00%, indicating that the solid acid of   Effect of Calcination Temperature on CFC-12 hydrolysis rate  Effect of catalytic hydrolysis temperature on hydrolysis rate of CFC-12 Figure 10 The effect of water vapor concentration on the CFC-12 hydrolysis Figure 11 The effect of reaction time on the CFC-12 hydrolysis Figure 12 MoO3/TiO2-ZrO2 SEM diagram of the reaction before and after Figure 13 Characterization of MoO3/TiO2-ZrO2 EDS after reaction