Catalytic Pathways and Kinetic Requirements for Alkanal Deoxygenation on Solid Tungstosilicic Acid Clusters

Kinetic measurements and acid site titrations were carried out to interrogate the reaction network, probe the mechanism of several concomitant catalytic cycles, and explain their connection during deoxygenation of light alkanals (C_<i>n</i>H_2<i>n</i>O, <i>n</i> = 3–6) on tungstosilicic acid clusters (H₄SiW₁₂O₄₀) that leads to hydrocarbons (e.g., light alkenes, dienes, and larger aromatics) and larger oxygenates (e.g., alkenals). The three primary pathways are (1) intermolecular CC bond formation, which couples two alkanal molecules in aldol-condensation reactions followed by rapid dehydration, forming a larger alkenal (C_2<i>n</i>H_{4<i>n</i>–2}O), (2) intramolecular CC bond formation, which converts an alkanal directly to an <i>n</i>-alkene (C_<i>n</i>H_2<i>n</i>) by accepting a hydride ion from H donor and ejecting a H₂O molecule, and (3) isomerization–dehydration, which involves self-isomerization of an alkanal to form an allylic alcohol and then rapid dehydration to produce an <i>n</i>-diene (C_<i>n</i>H_{2<i>n</i>–2}). The initial intermolecular CC bond formation is followed by a series of sequential intermolecular CC bond formation steps; during each of these steps an additional alkanal unit is added onto the carbon chain to evolve a larger alkenal (C_3<i>n</i>H_{6<i>n</i>–4}O and C_4<i>n</i>H_{8<i>n</i>–6}O), which upon its cyclization–dehydration reaction forms hydrocarbons (C_<i>tn</i>H_{2<i>tn</i>–2<i>t</i>}, <i>t</i> = 2–4, including cycloalkadienes or aromatics). The intermolecular and intramolecular CC bond formation cycles are catalytically coupled through intermolecular H-transfer events, whereas the intermolecular CC bond formation and isomerization–dehydration pathways share a coadsorbed alkanal–alkenol pair as the common reaction intermediate. The carbon number of alkanals determines their hydride ion affinities, the stabilities of their enol tautomers, and the extent of van der Waals interactions with the tungstosilicic clusters; these factors influence the stabilities of the transition states or the abundances of reaction intermediates in the kinetically relevant steps and in turn the reactivities and selectivities of the various cycles

Gas-Dependent Active Sites on Cu/ZnO Clusters for CH₃OH Synthesis

This study describes an instantaneously gas-induced dynamic transition of an industrial Cu/ZnO/Al2O3 catalyst. Cu/ZnO clusters become “alive” and lead to a promotion in reaction rate by almost one magnitude, in response to the variation of the reactant components. The promotional changes are functions of either CO2-to-CO or H2O-to-H2 ratio which determines the oxygen chemical potential thus drives Cu/ZnO clusters to undergo reconstruction and allows the maximum formation of Cu–Zn2+ sites for CH3OH synthesis

Gas-Dependent Active Sites on Cu/ZnO Clusters for CH₃OH Synthesis

This study describes an instantaneously gas-induced dynamic transition of an industrial Cu/ZnO/Al2O3 catalyst. Cu/ZnO clusters become “alive” and lead to a promotion in reaction rate by almost one magnitude, in response to the variation of the reactant components. The promotional changes are functions of either CO2-to-CO or H2O-to-H2 ratio which determines the oxygen chemical potential thus drives Cu/ZnO clusters to undergo reconstruction and allows the maximum formation of Cu–Zn2+ sites for CH3OH synthesis

Gas-Dependent Active Sites on Cu/ZnO Clusters for CH₃OH Synthesis

This study describes an instantaneously gas-induced dynamic transition of an industrial Cu/ZnO/Al2O3 catalyst. Cu/ZnO clusters become “alive” and lead to a promotion in reaction rate by almost one magnitude, in response to the variation of the reactant components. The promotional changes are functions of either CO2-to-CO or H2O-to-H2 ratio which determines the oxygen chemical potential thus drives Cu/ZnO clusters to undergo reconstruction and allows the maximum formation of Cu–Zn2+ sites for CH3OH synthesis

Gas-Dependent Active Sites on Cu/ZnO Clusters for CH₃OH Synthesis

This study describes an instantaneously gas-induced dynamic transition of an industrial Cu/ZnO/Al2O3 catalyst. Cu/ZnO clusters become “alive” and lead to a promotion in reaction rate by almost one magnitude, in response to the variation of the reactant components. The promotional changes are functions of either CO2-to-CO or H2O-to-H2 ratio which determines the oxygen chemical potential thus drives Cu/ZnO clusters to undergo reconstruction and allows the maximum formation of Cu–Zn2+ sites for CH3OH synthesis