Pillared layered structuresvs. zeolites as sorbents and catalysts

Abstract

The pore structures of several alumina-pillared clays (PILCs) have been investigatedvia conversions of ethylbenzene (EB) to diethylbenzenes (DEB) and to styrene (Styr) under Lewis acid conditions. PILCs (including those containing Fe in the pillar) give higher EB→DEB conversion, with a comparable selectivity towards p-DEB to that of ZSM-5s (SiO₂ /AlO₂=35, 235) and other mid-pore zeolites. Detailed comparisons of catalytic properties suggest the PILCs contain specific pore exit channels, or dimensions similar to those found in ZSM-5 zeolites, rather than a random supermesh structure. They differ from mid-pore zeolites in acting more as acidic reaction vessels (conversely the shape-selectivity characteristic of ZSM-5s depends on pore access–exit modes). This implies that the alumina pillar controls sorbate molecular orientation during this bimolecular reaction. Zeolites do not give EB→Styr conversion, and the PILCs themselves are active only at >450 °C, and then non-selectively. They become much more active and highly selective (over a wide temperature range: 100–600 °C) only after cation exchange. Beidellitic-PILCs are more active and selective than montmorillonite-PILCs and those containing mixed Fe²⁺Fe³⁺ /Al³⁺ pillars (e.g. FAZA, derived from a Greek bentonite) more than those with unsubstituted alumina pillars. Conversion yields for FAZA lie in the order: Ni²⁺>Co²⁺>Cu²⁺ , with Ni²⁺-FAZA the most unfavoured as regards disproportionation (2.5% at 450 °C). Although Cu²⁺-FAZA gives low conversions of styrene (<12% at 500 °C) it shows the highest selectivity (100%). The high activity and selectivity for K⁺-FAZA suggest that KFeO₂ is formed at the alumina pillar, as suggested for commercial α-Fe₂O₃ catalysts. Possible reaction mechanisms and active sites are proposed.

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