Effect of calcination temperature and fluorination treatment on NiF2-AlF3 catalysts for dehydrofluorination of 1, 1, 1, 2-tetrafluoroethane to synthesize trifluoroethylene
Graphical abstract
Introduction
1, 1, 1, 2-tetrafluoroethane (CF3CH2F, HFC-134a) was considered as a promising alternative to dichlorodifluoromethane (CF2Cl2) due to its zero ozone depletion potential (ODP), and has been widely used in the refrigeration industry [1]. However, according to the Kyoto Protocol, CF3CFH2 with high Global Warming Potential (GWP, > 1300) contributes greatly to the greenhouse effect and should be largely phased out [2]. CF3CFH2 has been replaced eventually by the 2, 3, 3, 3-tetrafluoro-propene (HFO-1234yf). Consequently, the transformation of CF3CFH2 are attracting increasing attention [3].
Numerous methods for the dispose of CF3CFH2 have been reported, such as combustion, photodissociation, water plasmas and catalytic dehydrofluorination [[4], [5], [6], [7]]. Among these methods, the dehydrofluorination of CF3CFH2 to trifluoroethylene (CF2=CHF) is a promising process due to the high additional value of CF2=CHF for fluoropolymer. The routes to catalytic dehydrofluorination of hydrofluorocarbons are normally used Mg2P2O7 and AlF3, Fe2O3-CdO-Al2O3 and Pd/AlF3 as catalysts [[8], [9], [10], [11], [12]]. Teinz et al found that AlF3 was exclusively selective toward the dehydrohalogenation of 3-chloro-1, 1, 1, 3-tetrafluorobutane, owing to the presence of strong Lewis acid sites [13]. Okazaki et al disclosed that the product distribution was dependent on the acid strength in the CF3CH3 dehydrofluorination over AlF3 catalyst [8]. In addition, Li et al reported that the strength effect of Lewis acid sites of phosphate catalysts, and found that the weak Lewis acid sites were the only active sites for the dehydrofluorination of CF3CH3 to CF2=CH2, while the strong Lewis acid sties resulting in the formation of undesirable coke [9,10]. A previous work reported by our group found that the NiO doped Al2O3 catalyst was a good candidate for the dehydrofluorination of CF3CFH2 to CF2=CHF [14]. These investigations reveal that the catalytic dehydrofluorination performance significantly depend on the acidity of the applied catalysts. To date, the effect of Lewis acid sites distribution and activity of NiF2-AlF3 catalysts for dehydrofluorination of CF3CFH2 are not clearly investigated.
Herein, NiF2-AlF3 catalysts were prepared and characterized to investigate the relationship between acid sites and catalytic activity for the dehydrofluorination of CF3CFH2 to CF2CHF. BET, SEM, XRD, UV-DRS, Raman and XPS characterization techniques were employed to disclose the effect of calcination temperature and fluorination on the structure and acidity of the resultant NiF2-AlF3 catalysts. The surface acid sites of the obtained catalysts were characterized by Py-IR. The Raman and TG techniques were used to analyse the carbon deposition during the catalytic process. The evolution of acid active sites of NiF2-AlF3 catalyst correlated to the catalytic performance for dehydrofluorination reaction was also discussed.
Section snippets
Catalyst preparation
The Al2O3 supports were impregnated with aqueous solution of Ni(NO3)2·6H2O and dried at 100 °C overnight. The impregnated supports were calcined at 400, 500, 600 and 700 °C for 3 h in air, respectively. The resultant samples were denoted as NiAlO-4, NiAlO-5, NiAlO-6 and NiAlO-7. The molar ratio of Al/Ni was 9. Prior to use, the prepared NiAlO-x catalysts were subject to the fluorination treatment. The fluorination was carried out in a stainless-steel tubular reactor with a diameter of 3 cm and
Structural characterization
Table 1 lists the analytical results of the surface areas and composition of the prepared NiAlO and NiAlF catalysts. The surface area of these catalysts decreased as increasing the calcination temperature. For NiAlF catalyst, its surface area was much smaller than corresponding NiAlO catalyst. For example, the NiAlO-5 catalyst calcined at 500 °C had a surface area of 171 m2 g−1, while only 72 m2 g−1 was obtained on the NiAlF-5 catalyst. The surface element composition of the prepared catalysts
Conclusion
A series of NiAlF catalysts derived from the fluorination of NiO-Al2O3 at different calcination temperatures, were used for the dehydrofluorination of CF3CH2F to synthesize CF2CHF. The highest activity was obtained on a fluorinated catalyst calcined at 500 ◦C, with a reaction rate of 2.13 mmol min−1 gcat.−1 at 450 ◦C. Calcination temperature of the NiAlO catalysts and fluorination treatment have great influence on the structure and acidity of the obtained catalysts. With the increasing
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (Nos. 21603069 and 91534115), Education Foundation of Hubei Province (No. Q20184502) and the Science Foundation from Hubei Polytechnic University (No. 16xjz05R). Min Liu acknowledges the support from China Scholarship Council - University of Melbourne Research Scholarship (No. 201606260063).
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These authors contributed equally to this work.