Role of Water on Zeolite-Catalyzed Dehydration of Polyalcohols and EVOH Polymer
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Chemistry↗
Engineering topics
Crossley, Steven P.
Publications and source records attributed to Crossley, Steven P..
A Perspective on Catalysis in Solid Acids
Solid acids are among the most important heterogeneous catalysts, finding many applications ranging from hydrocarbon cracking and alkane isomerization [1] in petroleum refining to valorization of biomass [2]. Even though the subject is quite old, new materials and applications are still being developed. Some of the new applications involve solid Lewis acids, such as metal-substituted, siliceous zeolites (e.g. Sn-BEA [3-5]); but new Brønsted acids [6] and applications for Brønsted acids [7-9] are appearing as well. Although there have been many excellent reviews of solid acidity and methods for characterizing solid acids [10-12], there is still much that we do not know about the nature of acid sites in solids and the factors which are most important for influencing catalytic activity. In this perspective, we will discuss some concepts that we feel are important for understanding these materials and the reactions they catalyze. Because our understanding of Lewis acids is much less advanced, we will restrict our comments to solids with Brønsted acidity and refer the reader to other recent publications on Lewis acids [13-15].
36 MATERIALS SCIENCE↗
Stabilization of furanics to cyclic ketone building blocks in the vapor phase
Furanics are produced in high abundance from the decomposition of biomass. The thermal and chemical instability of these species leads to the formation of humins upon condensation. The ring rearrangement of furfural to form 2-cyclopentenone and cyclopentanone is known to occur in the condensed aqueous phase, but this requires operation in condensed acidic media where humin formation readily occurs. High hydrogen pressures are typically used to offset rapid polymerization reactions, limiting the yields of stable unsaturated products that result. We report that furfural can be selectively converted to 2-cyclopentenone and cyclopentanone in a single step over supported TiO 2 catalysts with both model compounds and real biomass-derived streams in the vapor phase. Selectivity for ring rearrangement vs. C—O cleavage over TiO 2 supported Ru and Pd catalysts can be tuned by manipulating the water partial pressure. The formation of these products in the absence of a condensed acidic stream also enables the tuning of reaction environments to favor the selective formation of unsaturated ketones, which could be valuable diolefin precursors. The incorporation of a TiO 2 support in the catalysts tested leads to the suppression of C—C hydrogenolysis/decarbonylation and enhancement of ring rearrangement reactions. The nature of the active sites for selective Csingle bondO cleavage as well as vapor phase ring rearrangement are discussed.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Solvent-mediated charge separation drives alternative hydrogenation path of furanics in liquid water
Compared to the vapour phase, liquid-phase heterogeneous catalysis provides additional degrees of freedom for reaction engineering, but the multifaceted solvent effects complicate analysis of the reaction mechanism. Here, using furfural as an example, we reveal the important role of water-mediated protonation in a typical hydrogenation reaction over a supported Pd catalyst. Depending on the solvent, we have observed different reaction orders with respect to the partial pressure of H 2 , as well as distinct selectivity towards hydrogenation of the conjugated C=O and C=C double bonds. Free energy calculations show that H 2 O participates directly in the kinetically relevant reaction step and provides an additional channel for hydrogenation of the aldehyde group, in which hydrogen bypasses the direct surface reaction via a hydrogen-bonded water network. In conclusion, this solution-mediated reaction pathway shows the potential role of the solvent for tuning the selectivity of metal-catalysed hydrogenation when charge separation on the metal surface is feasible.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Hydrogenation of o -Cresol at the Water/Pt(111) Interface
Catalytic experiments were performed to investigate the hydrogenation of o-cresol to 2-methylcyclohexanol on a platinum catalyst in water. It has been observed experimentally both here and in prior studies that intermediate ketones desorb from the catalyst surface in the vapor phase or in hydrocarbon solvents, but they are not observed when reactions are carried out in the aqueous phase. Density functional theory was employed to explore the atomic-scale mechanism of o-cresol hydrogenation at the water/Pt(111) interface. Here, we show that water plays a dual role of accelerating the rate of hydrogenation of these ketones by shuttling hydrogen from the surface while also decreasing the rate of desorption of the less polar ketone intermediate from the catalyst surface. These two effects, when combined, explain the lack of observation of these ketone intermediates when this hydrogenation reaction is carried out in the aqueous phase.
Li, Yaping↗