PLS versus zeolites as sorbents and catalysts II. Terpene conversions in alumina-pillared clays and phosphates and medium pore zeolites Original Research Article

The reactions of α-pinene, limonene, and α-terpinene in several alumina-pillared clays (PILCs) and a layered α-tin phosphate analogue (Al-PILP) have been investigated under Lewis acid conditions and compared with the mid-pore zeolites USY, NH4+-ZSM-5 (with SiO2/Al2O3ratios = 35 and 235), and H+-mordenite. The bicyclic α-pinene gives the highest conversion, all catalysts giving > 50% camphene at 100°C. Total yields show that USY is the strongest acid, after which 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 fenchane carbocation-derived products are produced, indicating that all the solids promote formation of the norbornyl cation intermediate. Selectivities in the unsubstituted PILCs is comparable with those in the zeolites (e.g. both FAZA and USY show selectivity against limonene production in the α-pinene reaction). BP-PILC also shows appreciable activity for α-pinene at 25°C (as does USY) whereas Al-PILP is inactive. Specific carbocation precursors are deduced from the product distributions and a ‘carbocation cascade’ based on pore acidity provides a rationalisation of the results. However, site-selectivity effects 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 means of generating terpene derived carbocations in the solid state.