Intrinsic kinetics of lower alcohols: C2, C3 dehydration over Lewis acidic ordered mesoporous silicate: Zr-KIT-6

Abstract

KIT-6 materials are large pore cubic Ia3d mesoporous silicate, with tunable pore size (4-12 nm) and pore wall thickness (4-6 nm). The three-dimensional structure of KIT-6 provides more mass transfer channels within the pore structure and also reduces the propensity for pore blockage. With the incorporation of zirconium into KIT-6 structure, the materials displayed mild Lewis acidity exclusively. These characteristics allow Zr-KIT-6 to be a promising catalyst for alcohol dehydration to olefins. Therefore, the emerging biomass-based renewable chemicals industry will particularly benefit from the availability of such catalysts for dehydration of long-chain alcohols from biomass based feedstock. In this study, the dehydration of short-chain alcohols, including isopropanol (IPA) and ethanol (EtOH), were carried out over three Zr-MIT-6 samples with different Si/Zr ratios ranging from 20 to 100. In the temperature range of 180-300 °C, the Zr-KIT-6 materials were shown to be highly active for of IPA dehydration to propylene (selectivity 98.5%). While, ethylene formed with the selectivity of 70%-80% when dehydrating EtOH at 300-380 °C range. 30 h continuous run revealed slight catalyst deactivation for IPA dehydration; and the catalyst started to deactivate after 60 h for EtOH dehydration. Kinetic models were established for both of these two reactions. The activation energy for IPA and EtOH dehydration, estimated from intrinsic rate constants normalized with respect to the Lewis acid sites, were approximately 48.9 ± 0.5 kJ/mol and 79.5 ± 0.7 kJ/mol, respectively, which are found to be lower than or comparative with most other Brønsted or Lewis acidic heterogeneous catalysts reported in the literature for such reactions. This clearly shows that the Zr-KIT-6 materials are a superior and promising class of highly active, selective and durable alcohol dehydration catalysts. Although, IPA and EtOH are short-chain alcohols, establishing such activity is key to their potential use as solid acid catalysts for even bulkier substrates.