AlPO4–Al2O3 catalysts with low alumina content. Part IV.—Effect of fluoride ion addition on texture, surface acidity and catalytic performance in cyclohexene and cumene conversions

Abstract

Fluoride-modified (2.5 wt.% F^–) AlPO₄–Al₂O₃(APAl–A; 5–25 wt.% Al₂O₃) catalysts have been prepared and compared for their catalytic activity/selectivity in cyclohexene conversion and cumene cracking. The characterization of catalysts modified with F^– was performed using X-ray diffraction (XRD), ²⁷Al and ³¹P NMR and diffuse reflectance FTIR (DRIFT) spectroscopies and N₂ adsorption methods, and by the measurement of the surface acid character by chemisorption of pyridine (PY) and 2,6-dimethylpyridine (DMPY). Fluoride addition shifted the transition of AlPO₄ from an amorphous to a crystalline (α-cristobalite) phase at a lower temperature. Fluoride-treated APAl-A catalysts contained both tetrahedral and octahedral aluminium (their ratio varied with anion type) while phosphorus always remained in P(OAl)₄ environments. Moreover, two types of Al surface hydroxy groups at 3786 and 3738 cm^–1, and only one type of P surface hydroxy group at 3670 cm^–1, are found in the DRIFT spectra of the APAl-A catalysts. The DRIFT measurements also revealed that fluoride ion substitutes hydroxy groups on tetrahedrally coordinated Al and P centres in the surface. The PY and DMPY adsorption (pulse chemisorption and DRIFT spectroscopy) indicates that the addition of fluoride ion appreciably modifies the surface acidity of the AlPO₄–Al₂O₃ catalyst by increasing the number and strength of Brønsted acid sites. Furthermore, the addition of fluoride increases remarkably the activity for both processes and, for cyclohexene conversion, in contrast to pure AlPO₄–Al₂O₃ catalysts, where skeletal isomerization is the only catalysed reaction, bimolecular hydrogen transfer also takes place. The hydrogen transfer/skeletal isomerization ratio decreases as the Al₂O₃ loading increases although with all APAI-A-FM (fluoride-modified) catalysts the skeletal isomerization always predominates. The catalytic activities were correlated with both the acidity and the acid strength of the catalyst. Fluoride addition transformed a low activity cracking/dehydrogenation catalyst to one selective for cracking and exhibiting much greater catalytic activity.