FLUORIDE TREATMENT OF ALPO4-AL2O3 CATALYSTS .2. POISONING EXPERIMENTSBY BASES FOR CYCLOHEXENE CONVERSION AND CUMENE CRACKING

Bronsted acid sites on fluoride-modified AlPO4-Al2O3 (2.5 wt% F; APAl- P-2.5F) catalyst are poisoned by the presence of 2,6-dimethylpyridine (DMPY) and hexamethyldisilazane (HMDS), thus decreasing the catalytic activity for cyclohexene and cumene reaction processes, while the effe ct of pyridine (PY) was scarce. Besides, the drop in activity for cycl ohexene conversion was accompanied by a change in reaction selectivity so that hydrogen transfer sites are much more sensitive to base poiso ning (getting greater as the poisoning effect increased) than isomeriz ation sites. Moreover, surface trimethylsilyl (TMS) complexes (formed by covalent reaction of HMDS with surface hydroxyls) decomposed and th us, the activity progressively increased at increasing time intervals, thus reaching greater values (at ca. 4 h) than the unpoisoned APAl-P- 2.5F catalyst. So, DMPY was more suitable than PY and HMDS for the poi soning of Bronsted acid sites on APAl-P-F catalyst.