| The interest in investigating the conversion of 2,3-butanediol (2,3-BDL) to methyl ethyl ketone (MEK) over zeolites originates from the idea of synthesizing value-added chemicals by utilizing biomass-generated 2,3-BDL as the feed. Our specific objective here is to understand the catalytic and reaction variables that control the 2,3-BDL to MEK dehydration over a selection of zeolites, since previous findings suggested that biomass to MEK is a viable process. In the present study, a three-pronged approach was implemented to study the effects of temperature, zeolite structure, acid site density, and site acidity on the conversion of 2,3-BDL. First, vapor phase kinetic studies performed utilizing a differential reactor apparatus under specific temperature-programmed or isothermal flow conditions revealed the concurrent formation of pinacol rearrangement and intermolecular dehydration, and that the product distribution from these two pathways depended on pore size. Second, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis at RT elucidated the dehydration of 2,3-BDL to MEK, the dissociation of the cyclic ketal of 2,3-BDL and MEK, and the absence of aldol dimerization of MEK over zeolites. When combined with temperature step desorption/reaction (TSD/TSR), in-situ DRIFTS revealed aldol condensation of MEK, the formation of coke species with aromatic and/or polyenic nature, and the removal of adsorbed MEK species in larger pore materials by cyclic ketal formation. Characterization of the zeolites with atomic absorption spectroscopy (AAS), 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and pyridine titration coupled with in-situ DRIFTS revealed that zeolites having similar structures and Si/Al ratios could have very different Al site distributions. These subtle differences may account for the lack of product formation observed in the isothermal flow experiments on some catalysts. |
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