Synthesis, Characterization, and Application of Amorphous Silica–Aluminas Support for Fischer–Tropsch Wax Hydrocracking Catalysts

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