Synthesis, Characterization,\nand Application of Amorphous\nSilica–Aluminas Support for Fischer–Tropsch Wax Hydrocracking\nCatalysts

High-performance amorphous silica–aluminas (ASAs)\nwere prepared\nprior to the formation of the 10-membered ring (10-MR) ZSM-5 zeolite\nby regulating the hydrothermal processing time. Their structures,\nmorphologies, acidity properties, and Si–Al coordination were\nwell studied. Particularly, a facile FTIR method of in-situ adsorbing\nbulky 2,6-dimethlypyridine followed by pyridine adsorption was innovatively\nutilized to quantify the Brønsted acid sites in micropores. All\nthe ASAs samples were transformed into catalysts by loading with 0.5\nwt % Pt. The structure–activity relationship, especially from\nthe strength, density, and location of Brønsted acid sites, was\ninvestigated by Fischer–Tropsch synthesis (FTS) wax hydrocracking.\nThe evaluation results showed that the medium strong Brønsted\nacid sites located on the external surface played a crucial role in\nthe activity. Contrary to the general belief that larger pores favor\nthe production of heavy cracking fractions, the ASAs with a 10-MR\nmicroporous structure proved to be more effective for diesel production\nthan those with a 12-membered ring (12-MR). Strong Brønsted acid\nsites in micropores were less conducive to diesel production mainly\ndue to stronger adsorption at these sites and steric hindrance from\nthe microporous system. Furthermore, the Pt/AS-20 catalyst with few\nintramicropore Brønsted acid sites exhibited high diesel selectivity\n(83.3%) at 50.5% conversion under industrially relevant reaction conditions,\nwhich provides a significant opportunity to develop FTS wax hydrocracking\ncatalysts more rationally.