ALPO4-AL2O3 CATALYSTS WITH LOW ALUMINA-CONTENT .4. EFFECT OF FLUORIDE-ION ADDITION ON TEXTURE, SURFACE-ACIDITY AND CATALYTIC PERFORMANCE INCYCLOHEXENE AND CUMENE CONVERSIONS

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 charac terization of catalysts modified with F- was performed using X-ray dif fraction (XRD), Al-27 and P-31 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-d imethylpyridine (DMPY). Fluoride addition shifted the transition of Al PO4 from an amorphous to a crystalline (alpha-cristobalite) phase at a lower temperature. Fluoride-treated APAl-A catalysts contained both t etrahedral and octahedral aluminium (their ratio varied with anion typ e) while phosphorus always remained in P(OAl)4 environments. Moreover, two types of Al surface hydroxy groups at 3786 and 3738 cm-1, and onl y one type of P surface hydroxy group at 3670 cm-1, are found in the D RIFT spectra of the APAl-A catalysts. The DRIFT measurements also reve aled that fluoride ion substitutes hydroxy groups on tetrahedrally coo rdinated Al and P centres in the surface. The PY and DMPY adsorption ( pulse chemisorption and DRIFT spectroscopy) indicates that the additio n of fluoride ion appreciably modifies the surface acidity of the AlPO 4-Al2O3 catalyst by increasing the number and strength of Bronsted aci d sites. Furthermore, the addition of fluoride increases remarkably th e activity for both processes and, for cyclohexene conversion, in cont rast to pure AlPO4-Al2O3 catalysts, where skeletal isomerization is th e only catalysed reaction, bimolecular hydrogen transfer also takes pl ace. The hydrogen transfer/skeletal isomerization ratio decreases as t he Al2O3 loading increases although with all APAl-A-FM (fluoride-modif ied) catalysts the skeletal isomerization always predominates. The cat alytic activities were correlated with both the acidity and the acid s trength of the catalyst. Fluoride addition transformed a low activity cracking/dehydrogenation catalyst to one selective for cracking and ex hibiting much greater catalytic activity.