Zeolite-Catalyzed Dehydration of Biomass-Derived Alcohols for Light Olefins Production

Abstract

Ever-intensifying climate change by greenhouse gas emission resulting from the perpetual use of fossil fuels has imparted the prime importance to the sustainable development. Much attention, in this regard, has been devoted to the environmentally friendly production of various chemical products to sustain human life. Biomass-derived chemicals pave the environmentally benign ways of producing a variety of chemical building blocks owing to their carbon neutrality. The so-called shale gas revolution leading the lightening of cracking feed, on the other hand, has triggered a deficit of C3 – C4 olefins, which were mainly produced by naphtha cracking. This has urged researchers to efficient alternatives for the production of various light olefins (i.e., C2 – C4), indispensable chemical building blocks for petrochemical industry, using renewable and sustainable feedstocks, which is of particular significance in both academic and industrial perspectives. In this thesis, we present the environmentally friendly production of light olefins through zeolite-catalyzed dehydration of biomass-derived alcohols. Besides its intrinsic acidic properties, the uniformly defined pore structure of zeolite catalyst renders shape-selective catalysis over this class of catalysts where the pore architecture strongly affects the selectivity of reaction products. Both unique pore structure and distinctive acidic properties governing catalytic behavior for each dehydration reaction are thoroughly examined. Given the concomitant water in most fermentation alcohol products, the influence of introduced H2O in feed stream on the catalytic activity, in addition, has also been investigated. The low-temperature dehydration of ethanol to ethene in the presence of water vapor (H2O/ethanol = 0.2) is investigated over a series of cage-based, small-pore zeolites with different framework structures (i.e., LEV, CHA, AEI, RTH, and LTA) and/or crystallite sizes, and the catalytic results obtained are compared with those observed for the small-pore H-EU-12, the medium-pore H-ZSM-5, and the large-pore H-mordenite zeolites, all of which are channel-based and known to be active for this reaction. At 200 °C, H-SSZ-13, H-SSZ-39, and H-RTH were found to show superior ethene selectivity and yield compared to H-mordenite, the best low-temperature ethanol dehydration catalyst. H-SAPO-34 with the same framework topology as that of H-SSZ-13 (CHA), but with fairly weaker acidity, is characterized by a much lower initial ethene yield, suggesting an important role of strong acid sites in ethanol dehydration. This accounts for the poor catalytic performance of the other two cage-based, small-pore zeolites (i.e., H-levyne and H-LTA). Given their structural features with small pore apertures, the selective ethene formation over H-SSZ-39, H-SSZ-13, and H-RTH can be rationalized by product shape selectivity. It is noteworthy that the gradual decrease in ethene yield with time on stream observed only over H-levyne indicates that the cage volume of cage-based small-pore zeolites would also play a crucial role in this reaction. Zeolite crystal size was found to be a crucial factor affecting the catalyst durability in low-temperature ethanol dehydration, making nanocrystalline H-RTH best among the zeolites studied here. Dehydration of biomass-derived 1,3-butanediol provides an environmentally friendly alternative to current butadiene production methods. We compare the catalytic properties of the proton form of 11 medium-pore zeolites, with different framework topologies (i.e., MFI, FER, STI, STW, TON, MTT, *MRE, and AEL) and/or different framework Si/Al ratios for this reaction at 300 ºC under excess water conditions (H2O/1,3-butanediol = 45). It was found that high-silica (Si/Al = 130) H-ferrierite with intersecting 10- and 8-ring channels exhibits the highest butadiene yield, together with the best durability, among the zeolites tested. Due to its highly inactive nature, however, 3-buten-1-ol, one of the butenol reaction intermediates in 1,3-butanediol dehydration, also remained as a major by-product. IR spectroscopy with adsorbed 3-buten-1-ol shows that intramolecularly hydrogen-bonded 3-buten-1-ol exists predominantly in H-ZSM-5 with two intersecting 10-ring channels, while the one with no intramolecular hydrogen bonding is another species found not only in H-ferrierite but also in the one-dimensional 10-ring zeolite H-ZSM-22. If such is the case, the conformation of 3-buten-1-ol resulting from the first dehydration reaction of 1,3-butanediol is found to be of particularly relevant to butadiene formation. By combining both experimental and theoretical results, we may conclude that 1,3-butanediol dehydration over medium-pore zeolites may constitute another example of reaction intermediate shape selectivity. A diversity of green alternative production of light olefins from renewable biomass-derived alcohols can also be extended beyond the topics studied, a few of which has been considered at the last part of this thesis. Bio-based manufacture of isobutene and butadiene could also be attained through dehydration of isobutanol and 1,4-butanediol, respectively, the production of which is implemented commercially. Biomass-derived 1-propanol, in addition, can be dehydrated into butadiene. It is an undeniable truth that steady efforts devoted to discovering more efficient zeolite catalysts has developed various chemical process in petrochemical industry. Therefore, we believe that the investigation of zeolite-catalyzed dehydration of diverse biomass-derived alcohols based on the intrinsic pore topologies and the corresponding shape-selective properties of zeolites would provide the meaningful step forth to the enhancement in efficiency and feasibility of bio-based light olefins production.