Basicity-Induced Trans-1,3,3,3-tetrafluoropropene Isomerization on Alkaline Earth Metal Fluoride Catalysts

The effective trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)) isomerization to cis-1,3,3,3-tetrafluoropropene (HFO-1234ze(Z)) is of great significance due to the wide applications of the product, for example, as heat exchanger, solvent, propellant etc. However, the reaction is limited by the low target product selectivity. In this work, group II metal fluorides (MF2, M = Mg, Ca, Sr, Ba) were prepared as promising catalysts for HFO-1234ze(E) isomerization. Considerable reactivity and selectivity for HFO-1234ze(E) conversion to HFO-1234ze(Z) was obtained, with the conversion rate and selectivity measured to be 16.9-18.7 and 88.7-97.9%, respectively. Analysis of byproduct components further revealed that CF3C equivalent to CH was the dominant main ingredient. Linear relationships between the exposed Br & ouml;nsted basic site density (F-) and CF3C equivalent to CH byproduct selectivity were obtained, indicating that the increase of Br & ouml;nsted basic sites could effectively enhance the dehydrofluorination of HFO-1234ze(E), thus promoting the yield of CF3C equivalent to CH. DFT calculations in combination with experimental characterizations were used to systematically investigate the reaction mechanisms for HFO-1234ze(E) isomerization. Multiple reaction pathways were proposed for the HFO-1234ze(Z) target product and CF3C equivalent to CH byproduct formation, with the one-step isomerization and C-1-F defluorination mechanism involved in the reaction pathways. The stronger CF3CH = CH* intermediate chemisorption on the BaF2 surface prohibited the reaction pathway for HFO-1234ze(Z) formation and desorption, in which the CF3CH = CH* intermediate encountered a subsequent dehydrogenation process and resulted in the formation of the CF3C equivalent to CH byproduct. This work provided promising catalysts for HFO-1234ze(E) isomerization with considerable reactivity, selectivity, and stability. More importantly, new insights of Lewis/Br & ouml;nsted acid-base pair-related catalytic mechanisms for HFO-1234ze(E) isomerization were proposed.