Mixed alcohol dehydration over Bronsted and Lewis acidic catalysts

Mixed alcohols are attractive oxygenated products of biomass-derived syngas because they may be catalytically converted to a range of hydrocarbon products, including liquid hydrocarbon fuels. Catalytic dehydration to form olefins is a potential first step in the conversion of C-2-C-4 alcohols into longer-chain hydrocarbons. We describe here the physical and chemical characterization along with catalytic activity and selectivity of 4 Bronsted and Lewis acidic catalysts for the dehydration of two mixed alcohol feed streams that are representative of products from syngas conversion over K-CoMoS type catalysts (i.e., ethanol, 1-propanol, 1-butanol and 2-methyl-1-propanol). Specifically, a Lewis acidic Zr-incorporated mesoporous silicate (Zr-KIT-6), a commercial Al-containing mesoporous silicate (Al-MCM-41), a commercial microporous aluminosilicate (HZSM-5), and a commercial microporous silicoaluminophosphate (SAPO-34) were tested for mixed alcohol dehydration at 250, 300 and 350 degrees C. The zeolite materials exhibited high activity (>98% ethanol conversion) at all temperatures while the mesoporous materials only displayed significant activity (>10% ethanol conversion) at or above 300 degrees C. The turnover frequencies for ethanol dehydration at 300 degrees C decreased in the following order: HZSM-5 > SAPO-34 > AI-MCM-41 > Zr-KIT-6, suggesting that Bronsted acidic sites are more active than Lewis acidic sites for alcohol dehydration. At 300 degrees C, SAPO-34 produced the highest yield of olefin products from both a water-free ethanol rich feed stream and a C3+-alcohol rich feed stream containing water. Post-reaction characterization indicated changes in the Bronsted-to-Lewis acidic site ratios for Zr-KIT-6, Al-MCM-41 and HZSM-5. Ammonia temperature programmed desorption indicated that the acid sites of post-reaction samples could be regenerated following treatment in air. The post-reaction SAPO-34 catalyst contained more aromatic, methylated aromatic and polyaromatic compounds than its zeolite counterpart HZSM-5, while no aromatic compounds were observed on post-reaction Al-MCM-41 or Zr-KIT-6 catalysts. Olefin yield at 300 degrees C over SAPO-34 (>95%) was comparable to published values for the methanol-to-olefins process, indicating the potential industrial application of mixed alcohol dehydration. Furthermore, the olefin product distribution over SAPO-34 was tunable by the composition of the alcohol feed mixture. (C) 2015 Elsevier B.V. All rights reserved.