PLS vs. zeolites as sorbents and catalysts. 5. Evidence for Brønsted/Lewis acid crossover and high acidity in conversions of C1–3 alcohols in some alumina-pillared smectite clays Original Research Article

Conversions of methanol, ethanol, and iso-propanol have been investigated in the activated alumina-pillared montmorillonite (BP-PILC), beidellite/montmorillonite (FAZA), and saponite (ATOS)-based PILCs obtained from standard [Al13O4(H2O)12(OH)24]7+ Keggin-ion. Product outcomes for MeOH differ from those on smectite clays themselves and on alumina (or transition metal ion oxides): all give rise to dimethylether, rather than methylformate or formic acid. Systematic study of conversion, yields and selectivity for cation-exchanged FAZA show that the large changes observed must be ascribed to both steric effects and selective blocking of proton-containing active sites. The latter are most important in Ni2+-exchanged FAZA-containing catalysts and are attributed to generation of highly acidic Lewis sites, e.g. hydrocarbons alone (in a distribution similar to the MTG process) are obtained on Ni2+-FAZA. Apart from ethene and acetaldehyde, EtOH conversion also gives diethylether, not obtained on smectites themselves, and produced via a bimolecular reaction. The total dehydration/dehydrogenation ratio varies in the order BP-PILC>FAZA>>AZA=ATOS, BP-PILC being the most acidic and most selective. Dehydration and dehydrogenation activity with temperature go through a crossover point. This is ascribed to “iso-acidity”, i.e. the iso-acidic point is where Lewis and Brønsted acids are equally strong. Trends in iso-acidity with metal-ion exchange in FAZA materials suggest that the Lewis acid sites are at the alumina pillar. Detailed study of reaction kinetics and contact times leads to the conclusion that saponite surfaces have higher dehydration activity than those in montmorillonite.