ABSTRACT
Lateral (adsorbate-adsorbate) interactions between adsorbed molecules affect various physical and chemical properties of microporous adsorbents and catalysts, influencing their functional properties. In this work, we studied the hydration of microporous AlPO4-11 aluminophosphate, which has an unusually ordered structure upon adsorption of water vapor, and according to 27Al NMR data, only tetrahedrally or octahedrally coordinated Al sites are present in the AlPO4-11. These 27Al NMR data are consistent with the results of density functional theory (DFT) calculations of hydrated AlPO4-11, which revealed the presence of a strong repulsive lateral interaction at the initial stage of adsorption, suppressing the adsorption of water on neighboring (separated by one -O-P-O- bridge) Al crystallographic sites. As a result, of all the different aluminum sites, only half of the Al1 sites adsorb two water molecules and acquire octahedral coordination.
ABSTRACT
In this work, isolated surface sites of vanadium oxide on the alumina surface were modeled and compared to experimental data obtained with 51V Solid-State Nuclear Magnetic Resonance (SSNMR) spectroscopy. The geometry of the centers on the (100), (110), and (111) planes of the spinel structure and (010) monoclinic alumina was modeled using density functional theory (DFT); their 51V NMR parameters were calculated using the Gauge-Including Projector Augmented Wave (GIPAW) method. The comparison of the simulated theoretical spectra with the experimental ones made it possible to find the sites that are likely present on the surface of real catalysts. The minimum energy pathways of propane oxidative dehydrogenation to propene were calculated for the dioxovanadium site in order to estimate its activity.
ABSTRACT
Biomass pyrolysis by solid acid catalysts is one of many promising technologies for sustainable production of hydrocarbon liquid fuels and value-added chemicals, but these complex chemical transformations are still poorly understood. A series of well-defined model SiO2 -supported alumina catalysts were synthesized and molecularly characterized, under dehydrated conditions and during biomass pyrolysis, with the aim of establishing fundamental catalyst structure-activity/selectivity relationships. The nature and corresponding acidity of the supported AlOx nanostructures on SiO2 were determined with 27 Al/1 Hâ NMR and IR spectroscopy of chemisorbed CO, and DFT calculations. Operando time-resolved IR-Raman-MS spectroscopy studies revealed the molecular transformations taking place during biomass pyrolysis. The molecular transformations during biomass pyrolysis depended on both the domain size of the AlOx cluster and molecular nature of the biomass feedstock. These new insights allowed the establishment of fundamental structure-activity/selectivity relationships during biomass pyrolysis.
ABSTRACT
Ab initio DFT calculations of (93)Nb NMR parameters using the NMR-CASTEP code were performed for a series of over fifty individual niobates, and a good agreement has been found with experimental NMR parameters. New experimental and calculated (93)Nb NMR data were obtained for several compounds, AlNbO4, VNb9O25, K8Nb6O19 and Cs3NbO8, which are of particular interest for catalysis. Several interesting trends have been identified between (93)Nb NMR parameters and the specifics of niobium site environments in niobates. These trends may serve as useful guidelines in interpreting (93)Nb NMR spectra of complex niobium oxide systems, including amorphous samples and niobium-based multicomponent heterogeneous catalysts. Potential applications of (93)Nb NMR to study solid polyoxoniobates are discussed.
