title: Dehydration of Methanol and Ethanol over Silica-Supported Heteropoly Acids in the Gas Phase
Creators: name: Alfaze, Rawan
Publisher: name: University of Liverpool Repository
Year: 2022

description: Dehydration of methanol (MeOH) to dimethyl ether (DME) and of ethanol (EtOH) to diethyl ether (DEE) and ethene is of significant interest in relation to sustainable development. DME is a multimarket product that has attracted attention as a supplement to liquefied petroleum gas (LPG) and as a clean alternative to diesel. Ethene is the raw material for approximately 30% of all petrochemical products, and DEE is considered an eco-friendly transportation fuel as well as a valuable chemical. This thesis aims to investigate MeOH and EtOH dehydration over Brønsted solid-acid catalysts based on tungsten Keggin heteropoly acids (HPAs). Dehydration of MeOH to DME and of EtOH to DEE and ethene was investigated at the gas-solid interface in the presence of bulk and SiO2-supported HPAs (H3PW12O40, (HPW) and H4SiW12O40, (HSiW)) as catalysts. The acid strength, texture and structural integrity of these catalysts were characterised by use of ammonia adsorption calorimetry (NH3-MC), Brunauer-Emmett-Teller method (BET), X-ray diffraction (XRD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The strength of the acid sites in the HPA/SiO2 catalysts was demonstrated to increase monotonically with the HPA loading up to 100% loading, and HPW catalysts were stronger than HSiW catalysts at any loading. In the dehydration of MeOH and EtOH, the turnover reaction rate for HPW catalysts was higher than for HSiW catalysts, which agrees with their acid strength. Upon increasing HPA loading, alcohol conversion passed a maximum and scaled with the number of HPA surface proton sites rather than with the HPA loading. This indicated that alcohol dehydration occurred via a mechanism of surface-type HPA catalysis at the gas-solid interface rather than a bulk-type (pseudo-homogeneous) mechanism. In addition, the conversion of DEE to ethene and EtOH, which is a step in the dehydration of EtOH to ethene, was studied at the gas-solid interface in the presence of bulk and supported HPA catalysts at 130–250 °C and ambient pressure. The catalysts involved HSiW and HPW supported on SiO2, TiO2 and ZrO2, as well as the bulk acidic heteropoly salt CsPW (Cs2.5H0.5PW12O40). The DEE elimination process was demonstrated to be of zero order in the DEE partial pressure within the range of 6–24 kPa. The ethene yield increased as the temperature of reaction was increased, reaching 98% at 220–250 °C and weight hourly space velocity (WHSV) of 2.2 h-1. The most active HPA catalysts were silica-supported HPW and HSiW and bulk CsPW salt. The HPA catalysts outperformed zeolites HZSM-5 and ultra stable Y (USY), which have been reported elsewhere. A correlation between catalyst acid strength and catalyst activity was established. This correlation indicates that Brønsted acid sites played a vital role in the elimination of DEE over HPA catalysts. The results suggest that the reaction occurred through consecutive reaction pathways: DEE → C2H4 + EtOH followed by EtOH → C2H4 + H2O. In this scheme, ethene is both a primary product of DEE elimination and a secondary product via dehydration of the primary product EtOH. The work provided evidence that DEE elimination over bulk HPA and high-loaded HPA/SiO2 catalysts proceeded via a surface-type mechanism.