Aldol condensations of carbonyl compounds for C–C bond formation are a very important class of reactions in organic synthesis and upgrading of biomass-derived feedstocks. However, the atomic level understanding of reaction mechanisms and structure–activity correlation on widely used transition metal oxide catalysts are limited due to the high degree of structural heterogeneity of catalysts such as commercial TiO 2 powders. Here, we provide a deep understanding of the reaction mechanisms, kinetics, and structure–function relationships for vapor phase acetone aldol condensation through the controlled synthesis of two catalysts with high surface areas and clean, dominant facets, coupled with detailed characterization and kinetic studies that are further assisted by density functional theory (DFT) calculations. Temperature-dependent diffuse reflectance infrared Fourier transform spectroscopy showed the existence of abundant acetone bonded to surface hydroxyl groups (acetone-O s H) and acetone bonded to Lewis acid sites (acetone-Ti 5c ) on the surface of both {101} and {001} facet dominant TiO 2 . Intermolecular C–C coupling of theenolate intermediate from acetone-Ti 5c and a vicinal acetone-O s H is a kinetically relevant step, which is consistent with kinetic and isotopic studies as well as DFT calculations. The {001} facet showed a lower apparent activation energy (or higher activity) than the {101} facet. This is likely caused by the weaker Lewis acid and Brønsted base strengths of the {001} facet which favors the reprotonation–desorption of the coupled intermediate, making the C–C coupling step more exothermic on the {001} facet and resulting in an earlier transition state with a lower activation barrier. It is also possible that the {001} facet has a smoother surface configuration and less steric hindrance during intermolecular C–C bond formation than the {101} facet.

Bimolecular and monomolecular cracking mechanisms of alkanes simultaneously occur and have a competitive relationship, which strongly influences product distribution. In this work, the density functional theory (DFT) is firstly carried out to elucidate two cracking mechanisms in HZSM-5 and HY zeolites. It is found that the overall apparent reaction barrier for the monomolecular cracking reaction at 750 K in the HZSM-5 zeolite is 5.30 kcal/mol, much lower than that (23.12 kcal/mol) for bimolecular cracking reaction, indicating that monomolecular mechanism is predominant in the HZSM-5 zeolite. In contrast, the bimolecular mechanism is predominant in the HY zeolite due to lower apparent reaction barrier energy barrier (6.95 kcal/mol) for bimolecular cracking reaction than that (24.34 kcal/mol) for the monomolecular cracking reaction. Moreover, the intrinsic reason for the different mechanisms is further elucidated. The confinement effect can effectively decrease energy barrier when the size of transition state is comparable to pore-size of zeolite.The insights in this work will be of great significance to the understanding of confinement on catalytic cracking mechanism and to the design of highly efficient cracking catalysts.

Hydrosilanes are useful precursors for the preparation of many different silicon-containing molecules, and they are transformed into value-added molecules generally through Si–H bond activation. Herein, we report the synthesis of a series of porphyrinic metal–organic framework (MOF) composites that were encapsulated with bimetallic PtCu nanowires (PtCu@Ir-PCN-222) and their catalytic performances toward Si–H bond functionalization. Catalytic results showed that PtCu@Ir-PCN-222 was efficient for the hydrolytic oxidation of hydrosilane and CO2 hydrosilylation. Especially, the turnover number in PtCu@Ir-PCN-222-catalyzed hydrosilane oxidation was up to 27,857 based on Pt. Theoretical studies disclosed that Ir­(III) porphyrin might interact with hydrosilane via a η2-(H–Si) coordination model, and meanwhile, PtCu alloys displayed higher Si–H activation ability than the sole Pt nanoparticles because of the well-tuned Pt and Cu electronic structures in the bimetallic PtCu nanowires.

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