Acetylene Adsoroption And Reaction On The Aluminum Fluoride （AlF₃） Surface

Vinyl fluoride (VF) is an important organic fluoride, which has been widely used as refrigerant, aerosol spray, and organic intermediates. Fluoride can be synthesized by acetylene fluorination reaction on AlF3catalyst. To give a deep understanding of the reaction mechanism, as well as the adsorption behaviors of C2H2and HF on the AIF3catalyst is the important basis for the development of flurination catalysts with high performance at relatively low temperature. However, the related theoretical studies have been seldom reported. Using the density functional theory method, the adsorption and reaction behaviors of HF and C2H2on the α-AlF3(0001) surface were systematically investigated. On the basis of theoretical results, the interactions between HF and C2H2on α-AlF3(0001) surface were interpreted in electronic nature. By the calculation of the coadsorption behaviors of HF and C2H2on α-AlF3(0001) surface, the formation of CHFCH2and CHF2CH3on a-AlF3-(0001) surface were systematically studied. The present theoretical work is valuable for the development of fluorination catalyst with high performance at low temperature.(1) Firstly using density functional theory, the adsorption behaviors of HF at the α-AIF3(0001) surfaces with3F,2F,1Fand Al terminations at different coverages were systematically studied. Our results indicate that the physisorption occurs when HF adsorbs at the3F termination surface; Al-F and FHF structures form when HF adsorbs at the surface with2F and1F terminations, then HF molecule is activated and possibly takes part into the successive fluorination reactions; and dissociated adsorption occurs when HF adsorbs on the Al termination surface. The unsaturated coordination number of surface Al for the3F,2F,1Fand Al termination surfaces are0,1,2, and3. The coordination number of AIF2surface is saturated aftera HF molecule adsorption; while only physical adsorption would occur for the successive HF molecule. However, it can successively occur chemisorb at the1F and Al termination surfaces. Therefore, it can be reasonably deduced that the higher unsaturated coordination number of surface would have the higher activity for HF adsorption. Charge density difference(CDD) indicates that weak interaction occurs between HF and3F termination surface; while strong interactions occurs between HF and2F,1F and Al termination surfaces.(2) Using the3F terminal α-AlF3(0001) surfaces with and without F vacancy as modeling catalyst, the adsorption behaviors of C2H2on the α-AlF3(0001) surfaces at different coverages were systematically studied. Our results indicate that acetylene strongly interacts with aluminum fluoride when acetylene adsorbs on the3F terminal AlF3-p(1×1) supercell. Bridge and far bridge configurations were obtained at1.0ML coverage on the stoichiometric AlF3surface; while only bridge configuration were observed at0.5ML coverage. Similar structure was obtained at the α-AlF3(0001) surfaces with F vacancy. However, its adsorption energy increases by1.20eV compared to that on the stoichiometric surface. According to the analysis of Charge density difference and Density of States, strong electronic interactions occurs between C2H2and α-AlF3(0001) surface.(3) The coadsorption behaviors of HF and C2H2were systematically studied in the a-AlF3(0001)-p(2×1) supercell with a fluorine vacancy. The calculation results show that HF fills into the F surface vacancy and the H-F bond is activated. Correspondingly, the active hydrogen species forms. Then the H species transfers to the adsorbed C2H2molecule to form a new C-H bond. The C orbit of acetylene translates from sp into sp2hybridization, and the C2H3---. intermediate forms. After its desorption, CH2CHF forms and a new F vacancy is generated at the same time, which might make the reaction to occur continuously. The further fluorination reaction of CH2CHF is much similar to that of C2H2. The activated H species obtained form HF adsorbed in the F surface vacancy takes part into the hydrogenation of CH2CHF to form CH3CHF2sepceis. The reaction energy barrier is very high, indicating that the formation of CH3CHF2is thermodynamically unfavorable which is well consistent with experimental results. According to the present theoretical results, AIF3and HF both contribute to the fluoridation reaction, in which HF molecule provides the fluorine atoms, AIF3provides reactive sites with F surface vacancies. This work revealed the fluorination reaction mechanism of acetylene on the AIF3surface.