Ethanol Dehydration Over Hydrophobic Aluminum and Niobium Silicates: Influence of Homogeneity of Metal Mixing on Catalytic Activity and Stability of Si−C Bonds

Herein we report on the stability and catalytic activity of aluminum and niobium silicates hybridized with phenylene bridges in the gas phase dehydration of (bio)ethanol to (bio)ethylene. Highly porous hybrid metallosilicate catalysts were prepared by non-hydrolytic sol-gel technique. The introduction of organic groups took place during the polycondensation reactions, as verified by IR and MAS NMR. The homogeneity of active metal sites distribution was influenced by condensation route chosen, as studied by XPS and ToF-SIMS. Catalytic performance was evaluated in a fully-automated gas-phase fixed-bed micro-reactor. The (hydro)thermal stability was assessed by measuring weight loss (TGA), loss of surface area (see Figure), and extent of hydrolysis of Si−C bonds (IR, MS, GC). More homogeneous metal distribution provided samples with higher catalytic activity, however, the hydrothermal stability decreased abruptly. Hydrolysis of Si−C bonds was observed already at 200 °C. The hydrothermal stability was improved by applying precursors with Si−C(sp3) bonds. Based on the data acquired we conclude that the observed stability drop is due to the introduction of partial charges into the silica network by metals with low electronegativity thus making the metallosilicates prone to hydrolysis