Characterization And Performance Of Cr-based Catalysts For The Synthesis Of CFCs Alternatives

Chlorofluorocarbons(CFCs) are considered as stratosphere ozone depletion substances(ODS),which were classified as controlled substances in the Montreal Protocol and its amendments.Hence,trans-utilization of CFCs or developing CFCs substitutes is a very important subject in fluorination circle.CCl₂F₂(CFC-12),once widely used as refrigerants and propellants,has been found to be the main culprit for the destruction of the stratospheric ozone layer.One of the main research objectives of this study is how to convert CCl₂F₂ to valuable chemical compounds of tetrafluoromethane(CF₄).There is intermediate product CClF₃ during the vapor-phase fluorination of CCl₂F₂ to synthesize CF₄,therefore,a comparative study of CClF₃ as reactant for this vapor-phase fluorination was also performed.In this study,we describe the use of CrO_X-Y₂O₃ catalysts to achieve the conversion of CCl₂F₂ to CF₄. The CrO_X-Al₂O₃ catalysts prepared by a deposition-precipitation method had also been employed to test for the fluorination of chlorotrifluoro-methane(CClF₃) to synthesize tetrafluoromethane(CF₄).1,1,1,2-tetrafluoethane(HFC-134a) has been considered as one of the promising alternative of chlorofluorocarbons(CFCs).In this paper,the CrOx-Y₂O₃ catalysts prepared by a precipitation method had been employed to test for the fluorination of 2-chloro-1,11l-trifluoroethane(CF₃CH₂Cl) to synthesize 1,1,1,2-tetrafluoroethane (CF₃CH₂F).The catalysts were characterized by BET,XRD,Raman spectroscopy, UV-Vis and EPR techniques.The effects of yttrium precursor and promotion of In on Cr based catalysts in this fluorination catalytic performance were investigated.1.A series of CrO_X-Y₂O₃ catalysts calcined at different temperatures were prepared by a deposition-precipitation method and tested for vapor-phase fluorination of CCl₂F₂ and CClF₃ to synthesize CF₄.The effect of the calcination temperature on the CrO_X species was investigated.Also,a comparative study of CCl₂F₂ and CClF₃ for Cl/F exchange reactions over CrYF catalysts was conducted.The CrYF-6 catalyst possesses the highest catalytic performance.The highest activity could be attributed to that the chromia species such as YCrO₄ and CrO₃ formed on the catalyst surface are easier to be activated and in turn transformed into the catalytically active species.The CrYO-8 catalyst predominantly existed in form of YCrO₃,which is difficult to be activated,thus the catalytic activity decreased remarkably.The reaction convert CCl₂F₂ to CClF₃ is very fast,while the convert CClF₃ to CF₄ appears to be a rate-control step.2.The fluorination of CClF₃ to synthesize CF₄ was performed on various CrO_x-Al₂O₃ catalysts calcined at different temperatures.The highest activity was obtained on a pre-fluorinated catalyst calcined at 600℃,about 92%of CF₄ yield was achieved at 400℃over the CrAlF-6 catalyst for Cl/F exchange reactions in CClF₃.The effect of calcination temperature on the catalyst composition and structure was investigated.XRD,UV-vis and Raman results indicated that the relative content of the crystalline Cr₂O₃ on the surface of the catalyst increased with increasing calcination temperature,It was found that the catalytic activity was related to the content of Cr₂O₃ phase on the surface of the CrAlO catalysts,which was difficult to be fluorinated to the active sites.The highest activity obtained for the CrYF-6 catalyst could be attributed to the lowest Cr₂O₃ content on its surface.The presence of small crystalline size of AlF₃,which was originated from the Al₂O₃ support, resulted in increased activity.The deactivation of catalyst could be attributed to increasing content and growing crystalline size of Cr₂O₃ on the catalyst surface.3.The synthesis of 1,1,1,2-tetrafluoroethane from 2-chloro-1,1,1-trifluoroethane was performed over CrO_X-Y₂O₃ catalysts prepared by a precipitation method.The effect of yttrium precursor on the CrO_X species was investigated.The catalysts prepared using hydroxide,chlorideand nitrate yttrium precursors were denoted as CrYO-H, CrYO-Cl and CrYO-N,respectively.Catalytic activity decreased in the following order:CrYF-H＞CrYF-Cl＞CrYF-N.The highest activity obtained on the CrYF-H catalyst could be attributed to the highest content of Cr⁶⁺ on the surface.This species then transformed into an active species such as CrO_XF_Y or Cr(OH)_XF_Y during the activation process.The presence of CrF₃,which was originated from the Cr⁶⁺,resulted in decreased activity.4.The fluorination of 2-chloro-1,1,1-trifluoroethane(CF₃CH₂Cl) to synthesize 1,1,1,2-tetrafluoroethane(CF₃CH₂F) was performed on CrO_X-Y₂O₃ catalysts doped with In promoter.The catalytic activity increased with increasing In content.For example,about 22%of CF₃CH₂Cl conversion was obtained on the CrYF catalyst at 340℃,while about 28%of conversion was obtained on the 10%In-CrYF catalyst. The effect of In content on the catalyst composition and structure was investigated. XRD,UV-vis,Raman and EPR results indicated that the CrO_X species with high oxidation state(Cr⁶⁺ and Cr⁵⁺) were well dispersed on the catalyst surface when the In loading increased.It was found that the catalytic activity was related to the high oxidation state of Cr species,which were liable to be fluorinated to the active sites. Also,it was assumed that InF₃ enchanced the resistance to carbon deposition of the catalyst.