Fluoridation Of Al-Based Catalyst:a Density Functional Theory Study

Aluminum fluoride has no toxicity, high temperature resistant, and no pollution to the environment. It has been widely applied into heterogeneous catalytic reactions, due to its high Lewis acid, and can replace AlCl3catalyst. In the present theis, the adsorption and reaction behaviors of HF on the α-Al2O3(0001) surface were systematically investigated using density functional theory method. The fluorination process of α-Al2O3(0001) surface was interpretated in theory. Then, the fluorination mechanism of CCl2F2on AIF3was discussed.The adsorption and reaction behaviors of HF on the α-Al2O3(0001) surface are systematically investigated using density functional theory method. By increasing the number of HF molecules in a p(2×1) α-Al2O3(0001) slab, we find that HF is chemically dissociated at low coverage; while both physical and dissociative adsorption occurs at a3/2ML coverage. At the same coverage (1.0ML), diverse configurations of the dissociated HF are obtained in the p(2×1) model; while only one is observed in the p(1×1) slab due to its smaller surface area compared with the former one. Preliminary fluorination reaction study suggests that the total energy of two dissociated HF in the p(2×1) slab increases by1.00and0.72eV for the formation and desorption of water intermediate, respectively. The coadsorption behaviors of HF and H2O indicate that the pre-adsorbed water is unfavorable for the fluorination of Al2O3, which is well consistent with the experimental results. The calculated density of states show that the peak of σH-F disappears, while the peaks of σH-O and σAl-F are observed at-8.4and-5～3eV for the dissociated HF. Charge density difference analysis indicates that the dissociated F atom attracts electrons, while no obvious changes on electrons are observed for the surface Al atoms.By substituting the oxygen atoms of α-Al2O3(0001) supercell for fluorine atoms, the fluorination process of α-Al2O3was systematically studied by density functional theory method. Based on the partially fluorination results, the XRD pattern of AIF3/Al2O3was obtained in theory. According to the adsorption behaviors of HF at the surface, interspace, and the interface of AlF3/Al2O3model, HF chemically adsorbs to form F-H-F species when it adsorbs at the surface and interspace of AlF3; while it reacts with Al2O3and dissociates to form O-H and Al-F bonds when it adsorbs at the interface of AlF3/Al2O3model. We also calculated the water desorption from the interface of AlF3/Al2O3. The relative energy comparison of water at surface, interspace, and the interface of AlF3/Al2O3model revealed that this fluorination process is thermodynamically favorable.Based on the theoretical results above, the adsorption behaviors of CCl2F2and CH2Cl2on the α-AlF3(0001) surface with different terminations were systematically calculated using density functional theory method. It was indicated that the SIF surface shows high activity for CCl2F2and CH2Cl2. However, only weak physical adsorption occurs on the S2F surface for CCl2F2and CH2Cl2due to the high stability of S2F surface. Similar results were obtained on the6-fold coordinated Al ions of the S3F surface, which has no Lewis acid sites and reactive activity for CCl2F2reactions.