Kinetic Mechanism and Site Requirements of Cooperative Reactions on Metal Oxides PDF Download

Author: Houqian Li
Publisher:
ISBN:
Category : Chemical kinetics
Languages : en
Pages : 0

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Book Description
The reaction mechanism and site requirements of cooperative C-C coupling and self-deoxygenation catalyzed by Lewis acid-base pairs on mixed metal oxides was elucidated with the combination of diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy (DRIFTS-MS) measurements, kinetic study, isotopic study, and density functional theory (DFT) calculations. The complex reaction network as well as the mixed metal oxides were dissected and investigated. Over Zn1Zr10Oz, acetone enolate was detected by infrared spectroscopy and the C-C coupling was found following Eley-Rideal mechanism where a surface enolate reacts with a gas phase acetone and produces an acetone dimer. The acetone surface reaction was probed by DRIFTS-MS system on ZrO2 and Zn1Zr10Oz to examine the reaction pathway of acetone to isobutene as well as the roles of incorporated ZnÎþ+. An acetone trimer, 2,6-dimethyl-2,5-heptadien-4-one (phorone-A) which was formed via aldol condensation of mesityl oxide enolate and acetone, was hypothesized as direct precursor of isobutene. This key intermediate can only be detected on Zn1Zr10Oz but not on ZrO2 which is likely caused by weaker acidity and stronger basicity over former material. However, the DRFITS-MS measurements were performed in the absence of water and water can possibly mediate reaction pathway. Kinetic study combined with DFT calculation was performed to provide detailed insights. From in situ DRIFTS and solid state nuclear magnetic resonance results, water does not introduce Bronsted acidity onto the surface of Zn1Zr10Oz. Instead, presence of water hinders dehydration of diacetone alcohol but facilitates decomposition of this intermediate which enables stable production of isobutene from diacetone alcohol intramolecular rearrangement under appropriate conditions. The ZnxTiyOz mixed metal oxides with preferential exposure of TiO2 (101) or (001) facet were used to investigate the site requirements at molecular-level. Hydroxyls with faster hydrogen transfer rate on ZnxTiyOz (101) likely facilitate diacetone alcohol decomposition and hinder formation of mesityl oxide more efficiently. This work demonstrates the reaction mechanism and site requirements for producing value-added chemicals from abundant biomass-derived precursors. Such strategies to investigate complex reaction network and materials also provide an example of studying similar complicated systems and are potentially of broader impact in other catalytic applications.