Sulfate ion induced formation of unsaturated coordination of AlF3 nanocluster catalyst confined in carbon framework for catalytic dehydrofluorination

The gas-phase catalytic dehydrofluorination of hydrofluorocarbons (HFCs) to produce hydrofluoroolefins is an important strategy for the resource utilization of HFCs. AlF3 is the most suitable and efficient catalyst for dehydrofluorination of HFCs. However, sintering and preservation of coordinatively unsaturated aluminum centers pose significant challenges for AlF3 catalysts. Herein, we prepared carbon-confined AlF3 nanocluster catalysts with aluminum fumarate metal organic frameworks (AlFu-MOF) as the precursors, followed by carbonization and fluorination treatments. During catalyst preparation, anion ligand in the aluminum salt plays a major role in the formation of coordinatively unsaturated Al centers. Compared with Al(NO3)3·9H2O and AlCl3·6H2O, AlFu-MOF (named AlFu-S) prepared with Al2(SO4)3·18H2O exhibits a higher concentration of Al-OH species. Additionally, the residual SO42- ions can indeed retard the crystallization of AlF3 at elevated temperatures. Hence, the decomposition of Al-OH groups and the presence of residual SO42- ions in AlFu-S contribute together to the abundance of unsaturated coordinated Al centers in the finally carbon-confined AlF3 nanocluster catalyst (named AlFu-S-550-F). Also, the confinement effect of the carbon material formed by the carbonization of AlFu-S effectively prevent AlF3 nanocluster from sintering and phase transition, hence, in the dehydrofluorination of HFC-134a, AlFu-S-550-F demonstrates excellent stability compared to AlF3 catalysts prepared by precipitation methods. In summary, this study provides a strategy for the preservation of coordinatively unsaturated aluminum centers in AlF3 catalysts due to sintering and crystal structure transformation. Furthermore, the methodology proposed in this work is also applicable to the preparation of other coordinatively unsaturated metal catalysts.