Study of the relationship between the acidity of amorphous silica-alumina supports and diesel selectivity in Fischer-Tropsch wax hydrocracking

Three amorphous silica-alumina supports with similar textural properties and different amounts of acidity were successfully synthesized through ammonium exchange processing and their structures, acidity properties and the coordination of Si and Al in the as-synthesized supports were characterized using XRD, N₂ adsorption-desorption, NH₃-TPD, Py-FTIR and NMR. Amorphous silica-alumina was impregnated into a solution of H₁₂N₄PtCl₂·XH₂O to obtain hydrocracking catalysts. The relationship between acidic properties of supports and catalytic performance was studied by hydrocracking of Fischer-Tropsch (F-T) wax to diesel in a continuous-flow type fixed-bed reactor as a model reaction. The results illustrated that the diesel selectivity was mainly related to Brønsted acid properties of the supports while Lewis acid showed little correlation. The amount of strong Brønsted acid was reversely related to the selectivity of diesel:the smaller the amount, the higher the selectivity. The as-synthesized Pt/B-1 catalyst showed high selectivity of 87.12% to diesel at the conversion of F-T wax of about 62.52%, under the reaction conditions of 370℃, 7.0 MPa, LHSV of 1.0 h^-1 and a hydrogen-to-wax ratio of 1 000:1. Pt/B-1 had better catalytic performance than Pt/ASA prepared by commercial amorphous silica-alumina (ASA).