PLS VERSUS ZEOLITES AS SORBENTS AND CATALYSTS .2. TERPENE CONVERSIONSIN ALUMINA-PILLARED CLAYS AND PHOSPHATES AND MEDIUM PORE ZEOLITES

The reactions of alpha-pinene, limonene, and alpha-terpinene in severa l alumina-pillared clays (PILCs) and a layered alpha-tin phosphate ana logue (Al-PILP) have been investigated under Lewis acid conditions and compared with the mid-pore zeolites USY, NH4+-ZSM-5 (with SiO2/Al2O3 ratios = 35 and 235), and H+-mordenite. The bicyclic alpha-pinene give s the highest conversion, all catalysts giving > 50% camphene at 100 d egrees C. Total yields show that USY is the strongest acid, after whic h the acidity order is: BP-PILC = ZSM-5 (35) > FAZA. H+-mordenite, and the layered phosphate appears to be less acid than the PILCs. No fenc hane carbocation-derived products are produced, indicating that all th e solids promote formation of the norbornyl cation intermediate. Selec tivities in the unsubstituted PILCs is comparable with those in the ze olites (e.g. both FAZA and USY show selectivity against limonene produ ction in the alpha-pinene reaction). BP-PILC also shows appreciable ac tivity for alpha-pinene at 25 degrees C (as does USY) whereas Al-PILP is inactive. Specific carbocation precursors are deduced from the prod uct distributions and a 'carbocation cascade' based on pore acidity pr ovides a rationalisation of the results. However, site-selectivity eff ects do come into play in K+- and Ca2+-PILCs and it also appears that limonene occupies a specific site in USY. The reactions provide a mean s of generating terpene derived carbocations in the solid state.