Novel synthesis of aluminum hydroxyfluorides AlFx(OH)3-x and application in E-1-chloro-3,3,3-trifluoropropene isomerization

Abstract

A facile, safe, and environmentally friendly preparation method is described for the synthesis of AlFx(OH)3-x by the activation of γ-Al2O3 using HCFO-1233zd(E) under various temperatures. Comprehensive characterization techniques including XRD, XPS, N2 sorption, SEM, TEM, 27Al MAS NMR, FTIR, and py-IR were employed, revealing that at lower temperatures such as 275 °C and 300 °C, the synthesis yielded AlF(OH)2, while at higher temperatures like 350 °C and 375 °C, pyrochlore AlF1.5(OH)1.5 and metastable hexagonal tungsten bronze-type (HTB) β-AlF3 were formed, with α-AlF3 detected at 325 °C. Notably, a significant quantity of AlF(OH)2 can be obtained at 325 °C within a specific time frame. This amorphous phase exhibits weak Lewis acid sites and a lower acid density, rendering it significantly more effective in the isomerization of HCFO-1233zd(E) compared to strong Lewis acid catalysts such as AlF1.5(OH)1.5 and β-AlF3, which could promote coke formation, leading to catalyst deactivation. Moreover, due to its limited further fluorination at 290 °C, the optimal catalyst exhibits prolonged lifespan and high efficiency in HCFO-1233zd(E) isomerization.