Hollownano-MgF2 supported catalysts: Highly active and stable in gas-phase dehydrofluorination of 1,1,1,3,3-pentafluoropropane

Abstract

In this work, different metal ions (Al3+, Cr3+, Fe3+, Co3+, Zn2+, Ni2+) were supported on a hollow nano-MgF2 to catalyze gas-phase dehydrofluorination of 1,1,1,3,3-pentafluoropropane (HFC-245fa) to 1,3,3,3-tetrafluoropropene (HFO-1234ze). The support, hollow nano-MgF2 prepared by a polyol mediated sol-gel method possesses large surface area (127 m2 g−1) and mesoporous structure (12 nm) with weak Lewis acidity after calcination at 350 °C. The catalysts were characterized by TEM, XRD, BET, NH3-TPD, EDX, pyridine-IR and XPS. The physicochemical characterization showed via simple impregnation with different metal ions, the textural and surface acidic properties of hollow nano-MgF2 were tuned, while the hollow nanosphere structure were maintained. The introduction of Al on hollow nano-MgF2 has resulted in higher catalyst activity and selectivity to desired product, trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)) than the other catalysts due to its largest surface area and numerous Lewis acid sites. The combination of the results of acidic properties and activity testing showed that the number of medium acid sites appeared to have a linear influence on the activity of hollow nano-MgF2 supported catalysts. The highest activity with HFC-245fa conversion of 54% and 82% selectivity to HFO-1234ze(E) was obtained at 280 °C on hollow nano-MgF2 supported 9 mol.% Al catalyst (9%Al/nano-MgF2), which also exhibited almost constant catalytic performances over 200 h. Surface Al species of Al/nano-MgF2 catalysts were identified, including Al(OH)xFy/AlOxFy, β-AlF3 and α-AlF3. The relative abundance of Al(OH)xFy/AlOxFy species was found to be highly correlated with the catalyst activity. The catalytic performances of 9%Al/nano-MgF2 are significantly higher than those of traditional AlF3 and fluorinated Cr2O3 for gas-phase dehydrofluorination reactions.