Ethanol dehydration over hybrid aluminosilicate catalysts prepared by non-hydrolytic sol-gel

Styskalik, Ales [UCL] Leonova, Lucie [UCL] Pinkas, Jiri Debecker, Damien P. [UCL]

The dehydration of (bio)ethanol to ethylene is an essential catalytic reaction in the perspective of the development of bio-based industry.[1] Traditional catalysts employed in this reaction are fully inorganic: alumina, silica-alumina, and HZSM-5.[2] Each of these systems come with their limitations: only moderate activity in the case of Al2O3 and silica-alumina, and rapid deactivation by coking in the case of zeolite catalysts. Recently, we have shown, that non-hydrolytic sol-gel (NHSG) provides highly homogeneous and porous aluminosilicate materials exhibiting superior activity and long-term stability in ethanol dehydration.[3] The ethylene selectivity was improved by one-pot incorporation of organic groups.[4] In this follow-up study, the best performing NHSG-prepared aluminosilicate catalysts (fully inorganic) were post-synthetically modified by grafting with trimethysilyl groups. In such a way, the Brønsted acid sites (≡Si−O(H)∙∙∙Al≡) were transformed into Lewis acid sites (≡Si−OSiMe3 and Al≡). The number of reacted ≡Si−OH moieties and thus the trimethylsilyl groups loading and Brønsted/Lewis ratio was controlled via a temperature-vacuum pretreatment of aluminosilicate samples. Structure, porosity, acidity, and hydrophobicity of NHSG-prepared catalysts were closely followed by MAS NMR studies, N2 physisorption, IR-pyridine analyses, and water adsorption. Moreover, aluminosilicates were tested in a gas-phase fixed-bed catalytic reactor in ethanol dehydration. Lewis acid Al sites were found to exhibit a high ethylene selectivity even at relatively low temperatures (200−300 °C). These tailored NHSG-prepared aluminosilicate catalysts exhibited high ethylene productivities, markedly outperforming commercial silica-alumina.