Heterogeneous Catalytic Dehydrative Reactions for the Conversion of Biomass-derived C4 Chemicals

Abstract

The utilization of biomass for the production of fine chemicals has been attracted as an alternative way to conventional petrochemical processes. As biomass-derived feedstocks contain abundant oxygen atoms, selective removal of oxygen is required to produce fine chemicals previously obtained from petroleum. Dehydrative reactions such as dehydration and esterification are effective to reduce oxygen content of biomass-derived feedstock as well as to produce desired functional groups. In this thesis, heterogeneous dehydrative reactions for the conversion of biomass-derived C4 chemicals including 2,3-butanediol and 1-butanol are studied. At first, a novel type of dehydration reaction that produces 2,3-epoxybutane from 2,3-butanediol (dehydrative epoxidation) is discovered and explored. Among a number of tested basic catalysts, the CsOx/SiO2 catalyst showed outstanding performance for the dehydrative epoxidation of 2,3-butanediol and is considered to be the most promising catalyst for this type of reaction. In order to identify the superiority of the CsOx/SiO2 catalyst and a mechanism of the reaction, structure-activity relationships were studied along with density functional theory (DFT) calculations. The following features are found to be responsible for the excellent activity of the CsOx/SiO2 catalyst: i) strong basic sites formed by Cs+, ii) low penetration of Cs+ into SiO2 which permits basic sites to be accessible to the reactant, iii) stable basic sites due to the strong interactions between Cs+ and SiO2 surface, and iv) mildly acidic surface of SiO2 which is advantageous for the elimination to H2O. In addition, the dehydrative epoxidation involves an inversion of chirality (e.g. meso-2,3-butanediol (R,S) to trans-2,3-epoxybutane (R,R or S,S)), which is in agreement with DFT results that the reaction follows a stereospecific SN2-like mechanism. The 2,3-epoxybutane produced from the dehydrative epoxidation of 2,3-butanediol can be further utilized to produce fine chemicals. When 2,3-epoxybutane was reacted over basic lithium phosphate catalyst, isomerized product, 3-buten-2-ol, was obtained with high selectivity. Furthermore, it was found that 3-buten-2-ol is an ideal precursor for the production of 1,3-butadiene. Reaction of 3-buten-2-ol over acidic mesoporous aluminosilicate (Al-MCM-41) led to dominant formation of 1,3-butadiene via acid-catalyzed dehydration. On the basis of the results, heterogeneous catalytic process for the production of 1,3-butadiene from 2,3-butanediol is proposed. Esterification of 1-butanol with carboxylic acid produces various esters which can be utilized to environmentally friendly solvents and precursors for fragrances. Esterification reactions are industrially conducted with homogeneous mineral acid catalysts, which causes process and environmental problems. Heterogeneous acid catalyst for the esterficiation reactions is essential to overcome the current problems. Zr-WOx clusters on WOx/ZrO2 catalyst are known to be active sites for the acid catalyzed reactions, such as dehydration of alcohols and alkane isomerization reactions. However synthetic methods for producing high density of Zr-WOx clusters with high surface areas are not currently available. A facile method for preparing mesoporous Zr-WOx/SiO2 is proposed and the effect of Zr/W ratio on its structure and acidity was examined. Results showed that the sequential hydrolysis of zirconium and tungsten via soft-templating resulted in the formation of Zr-WOx clusters with uniform mesopore structures and a high acidity. The prepared Zr-WOx/SiO2 was characterized by N2 physisorption, XRD, TEM, XPS, UV-Vis spectroscopy, NH3-TPD and in-situ FTIR. Catalytic performance for the esterification of 1-butanol with acetic acid was evaluated. The materials had a high surface area of over 500 m2/g and ordered cylindrical pores with a uniform size of ca. 5 nm. Below a Zr/W ratio of ≈0.5, the zirconium was primarily associated with tungstate rather than SiO2, which indicates the formation of Zr-WOx clusters. The highest density of Zr-WOx clusters was obtained at a Zr/W ratio of 0.3 with a strong Brønsted acidity. Consequently, Zr-WOx/SiO2, as a Zr/W ratio of 0.3 exhibited the highest activity with a significantly improved performance compared to HZSM-5 and WOx/ZrO2 catalysts.