Non-hydrolytic sol–gel route to a family of hybrid mesoporous aluminosilicate ethanol dehydration catalysts

Abstract

Hybrid materials are intensely studied for potential applications in heterogeneous catalysis. Organic groups at the catalyst surface can modify not only its hydrophilicity, but also acidity, hydrothermal stability, porosity, etc. In some cases, tuning such properties leads to improved catalytic performance. Often, however, the organic moieties are limited to methyl groups introduced via post-grafting. Here, a series of mesoporous hybrid aluminosilicate materials was prepared in one pot by non-hydrolytic sol–gel (NHSG). Aromatic, aliphatic, pendant, and bridging organic groups were incorporated. The presence of the organic groups in the bulk and at the outermost surface of the materials was verified by solid-state NMR, IR, ToF–SIMS, and XPS. The hybrid aluminosilicates were tested as catalysts in the gas phase ethanol dehydration to ethylene, and most of them outperformed the inorganic catalyst benchmark. While a direct influence of surface hydrophobicity (as probed by water sorption and water contact angle measurements) appeared unlikely, characterization of acidity (IR-pyridine) revealed that the improved performance for hybrid catalysts could be correlated with a modification of the acidic properties. The latter are determined by the quality of the dispersion of Al centers in the form of isolated sites in the hybrid silica matrix, which itself appears to be influenced by the presence of organic groups in the non-aqueous synthesis. All in all, this study establishes a "ranking" for a variety of organic groups in terms of their influence on the catalyst activity.