ABSTRACT Hydrogen separation and combustion subsequent to coal gasification is highly attractive ... more ABSTRACT Hydrogen separation and combustion subsequent to coal gasification is highly attractive as an environmentally benign method of energy generation. Siliceous zeolites are thermally and chemically stable microporous materials that can satisfy the function of a gas separation membrane for such high temperature (>473 K) processes. Ensuing steam generation via hydrogen combustion can consequently occur without significant energy loss. Silicalite-1 is attractive for the separation of smaller H2 (2.89 Å) from larger CO2, CH4, N2 and O2 molecules with kinetic diameters of 3.30, 3.80, 3.64 and 3.46 Å, respectively. The current study employs molecular dynamics and grand canonical Monte Carlo approaches to predict single-component gas diffusivities and adsorption isotherms for H2, CO2, CH4, N2 and O2 in silicalite-1 at 273–1,073 K. The respective gas diffusivities and adsorption loadings determined in this study enable prediction of separation characteristics of silicalite-1 at relevant process conditions. Adsorption of all gases, excluding H2, is relatively high at ambient temperature and significantly affects overall mass transport and separation selectivity. Hydrogen adsorption is relatively low even at ambient temperature, and at elevated temperatures (>473 K), adsorption of all gases is low, resulting in mass transport and separation selectivity that is dependent upon molecular diffusivity.
Carbonic anhydrase is an enzyme that catalyzes the hydration and dissociation of carbon dioxide, ... more Carbonic anhydrase is an enzyme that catalyzes the hydration and dissociation of carbon dioxide, a greenhouse gas, to bicarbonate under mild conditions; this reaction can be used for CO2 sequestration. Due to its reaction specificity, carbonic anhydrase can be an energy-effective biocatalyst for CO2 mitigation. In this study, bovine carbonic anhydrase is immobilized on native and functionalized mesostructured cellular foam (MCF) silica, which has an ordered three-dimensional pore structure with high porosity and high surface area, to investigate the effect of surface modification on catalytic activity (kcat, kcat/Km). The heat of immobilization (ΔH) is measured by flow microcalorimetry (FMC) to elucidate the fundamental interactions between bovine carbonic anhydrase and the mesoporous host. The FMC data provide invaluable insight allowing optimization of surface properties of mesoporous silica host to produce highly efficient immobilized carbonic anhydrase systems.
The selective ammoxidation of propane into acrylonitrile catalyzed by the Mo-V-Te-Nb-O mixed meta... more The selective ammoxidation of propane into acrylonitrile catalyzed by the Mo-V-Te-Nb-O mixed metal oxide received considerable attention because of its environmentally friendly nature. This process consists of a series of steps involving propane oxidative dehydrogenation (ODH), ammonia and O2 activation, and N-insertion into C3 surface intermediates. However, the limited fundamental understanding of the catalyst has hindered the progress in improving their activity and selectivity required for commercial applications. In this study, density functional theory (DFT) calculations were performed to investigate the mechanism of propane ammoxidation over cluster models of the proposed selective and active sites present in the surface ab plane of the M1 phase, in order to understand the roles of the different metal cation species in the reaction. We have calculated the activation energy barriers for the oxidative dehydrogenation (ODH) of propane on vanadyl, molybdyl, and telluryl oxo group...
ABSTRACT The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-N... more ABSTRACT The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-Nb-O system has received significant attention because it is more environmentally benign than the current process of propene ammoxidation and relies on more abundant propane feedstock. The reaction mechanism is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation, ammonia and O-2 activation, and NHx insertion into C-3 intermediates. In this study density functional theory calculations have been performed to investigate the energetics of ammonia adsorption and activation in the proposed active center in the ab plane of the M1 phase. The formation of NHx (x = 0, 1, 2, 3) species is found to be highly favored on reduced, oxo-depleted metal sites. The reduced Mo site is determined to be the most favorable site for ammonia activation by comparing the reaction energy profiles for the sequential dehydrogenation of ammonia on the various metal sites. The activation barrier for the initial H abstraction from ammonia was found to depend strongly on the surface sites that stabilize H and NH2, and is as low as 0.28 eV when NH2 is stabilized by the reduced Mo site and H is abstracted by the telluryl oxo group. The subsequent step of surface NH insertion into a pi-allyl gas intermediate was also found to have a low activation energy barrier of 0.03 eV on the reduced Mo site.
Abstract The last few decades have seen growing interest in developing highly active and selectiv... more Abstract The last few decades have seen growing interest in developing highly active and selective catalysts for selective oxidation of lower alkanes to alkenes. Many catalytic systems have been suggested for these reactions and very recently, it has been proposed that MoVTeNbO catalysts show good activity and selectivity for propane ammoxidation reaction due to presence of two major crystalline phases, called M1 and M2. Researchers have paid special attention to the M1 phase which contributes to the high activity and selectivity to produce acrylonitrile via propane ammoxidation. The surface ab (basal) planes of M1 which are oriented perpendicular to the length of the rod like crystals have been suggested to contain the active and selective sites for this reaction, however this conclusion is still being debated. Therefore, it is very important to establish the role of ab planes of M1 in this catalytic reaction. In this study, phase pure M1 MoVTeNbO catalyst was prepared by hydrother...
The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and microwave-assisted HT ... more The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and microwave-assisted HT synthesis methods (MW) employing two different Ta precursors, Ta ethoxide and a custom-made Ta oxalate complex. The profile intensity analysis of the HAADF-STEM image of M1 phases oriented along [hk0] directions from the surface to bulk region of HAADF-STEM images indicated that the chemical composition of surface ab planes is very similar to their composition in the bulk. The HAADF-STEM image analysis showed that synthesis methods have a significant impact on the Mo/V distribution in the MoVTeTaO M1 phases and their reactivity in propane ammoxidation. Enhanced acrylonitrile yield and 1st order irreversible reaction rate constants for propane consumption, normalized to the estimated surface ab plane areas, correlated with increased V content in the proposed catalytic center (S2-S4-S4-S7-S7). These observations lend further support to the idea that multiple VOx sites present in the surfac...
The vapor-phase hydrogenation of furan, crotonaldehyde and selective hydrogenation of acetylene w... more The vapor-phase hydrogenation of furan, crotonaldehyde and selective hydrogenation of acetylene were carried out over carbon-supported Pd and Pt nano-particle catalysts. Carbon-supported Pt and Pd nano-particles exhibited unique catalytic properties due to size-dependent electronic and geometric properties of the noble metal nano-particles for hydrogenation reactions. Platinum and palladium nano-particles smaller than 10 nm were synthesized by different methods, such as reduction by alcohols and microemulsion methods, followed by the nano-particle deposition on Vulcan XC-72 carbon support by adsorption and wet impregnation, respectively. The catalysts were characterized by the N2 porosimetry (surface area), X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM studies demonstrated well-defined distributions of the platinum and palladium particles and the particle size calculation showed that the metal particles were in the range of 2 to 7 nm. The characterization r...
Recently, the water-gas shift (WGS) reaction has become very important because of production of h... more Recently, the water-gas shift (WGS) reaction has become very important because of production of high purity H2 in conjunction with fuel cell power generation.. It has been predicted that Cu and Ni are promising active transition metals for the water-gas-shift reaction based on theoretical (DFT) calculations. However, Ni-based catalysts also catalyze methanation side-reaction under water-gas-shift reaction conditions. It is expected that the Cu addition to Ni can modify the Ni ensemble properties and electronic structure by the Cu-Ni alloy formation to improve the WGS reaction and suppress the methanation reaction. The goal of this work is to explore supported bimetallic Cu-Ni nanoparticles for low temperature water-gas-shift reaction in order to suppress the undesirable methanation activity. The activity and selectivity in many metal-catalyzed reactions were shown to be dependent on the size and structure of metal particles on the nanoscale. For these reasons, we investigated the ef...
ABSTRACT Hydrogen separation and combustion subsequent to coal gasification is highly attractive ... more ABSTRACT Hydrogen separation and combustion subsequent to coal gasification is highly attractive as an environmentally benign method of energy generation. Siliceous zeolites are thermally and chemically stable microporous materials that can satisfy the function of a gas separation membrane for such high temperature (>473 K) processes. Ensuing steam generation via hydrogen combustion can consequently occur without significant energy loss. Silicalite-1 is attractive for the separation of smaller H2 (2.89 Å) from larger CO2, CH4, N2 and O2 molecules with kinetic diameters of 3.30, 3.80, 3.64 and 3.46 Å, respectively. The current study employs molecular dynamics and grand canonical Monte Carlo approaches to predict single-component gas diffusivities and adsorption isotherms for H2, CO2, CH4, N2 and O2 in silicalite-1 at 273–1,073 K. The respective gas diffusivities and adsorption loadings determined in this study enable prediction of separation characteristics of silicalite-1 at relevant process conditions. Adsorption of all gases, excluding H2, is relatively high at ambient temperature and significantly affects overall mass transport and separation selectivity. Hydrogen adsorption is relatively low even at ambient temperature, and at elevated temperatures (>473 K), adsorption of all gases is low, resulting in mass transport and separation selectivity that is dependent upon molecular diffusivity.
Carbonic anhydrase is an enzyme that catalyzes the hydration and dissociation of carbon dioxide, ... more Carbonic anhydrase is an enzyme that catalyzes the hydration and dissociation of carbon dioxide, a greenhouse gas, to bicarbonate under mild conditions; this reaction can be used for CO2 sequestration. Due to its reaction specificity, carbonic anhydrase can be an energy-effective biocatalyst for CO2 mitigation. In this study, bovine carbonic anhydrase is immobilized on native and functionalized mesostructured cellular foam (MCF) silica, which has an ordered three-dimensional pore structure with high porosity and high surface area, to investigate the effect of surface modification on catalytic activity (kcat, kcat/Km). The heat of immobilization (ΔH) is measured by flow microcalorimetry (FMC) to elucidate the fundamental interactions between bovine carbonic anhydrase and the mesoporous host. The FMC data provide invaluable insight allowing optimization of surface properties of mesoporous silica host to produce highly efficient immobilized carbonic anhydrase systems.
The selective ammoxidation of propane into acrylonitrile catalyzed by the Mo-V-Te-Nb-O mixed meta... more The selective ammoxidation of propane into acrylonitrile catalyzed by the Mo-V-Te-Nb-O mixed metal oxide received considerable attention because of its environmentally friendly nature. This process consists of a series of steps involving propane oxidative dehydrogenation (ODH), ammonia and O2 activation, and N-insertion into C3 surface intermediates. However, the limited fundamental understanding of the catalyst has hindered the progress in improving their activity and selectivity required for commercial applications. In this study, density functional theory (DFT) calculations were performed to investigate the mechanism of propane ammoxidation over cluster models of the proposed selective and active sites present in the surface ab plane of the M1 phase, in order to understand the roles of the different metal cation species in the reaction. We have calculated the activation energy barriers for the oxidative dehydrogenation (ODH) of propane on vanadyl, molybdyl, and telluryl oxo group...
ABSTRACT The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-N... more ABSTRACT The selective ammoxidation of propane into acrylonitrile catalyzed by the bulk Mo-V-Te-Nb-O system has received significant attention because it is more environmentally benign than the current process of propene ammoxidation and relies on more abundant propane feedstock. The reaction mechanism is proposed to consist of a series of elementary steps including propane oxidative dehydrogenation, ammonia and O-2 activation, and NHx insertion into C-3 intermediates. In this study density functional theory calculations have been performed to investigate the energetics of ammonia adsorption and activation in the proposed active center in the ab plane of the M1 phase. The formation of NHx (x = 0, 1, 2, 3) species is found to be highly favored on reduced, oxo-depleted metal sites. The reduced Mo site is determined to be the most favorable site for ammonia activation by comparing the reaction energy profiles for the sequential dehydrogenation of ammonia on the various metal sites. The activation barrier for the initial H abstraction from ammonia was found to depend strongly on the surface sites that stabilize H and NH2, and is as low as 0.28 eV when NH2 is stabilized by the reduced Mo site and H is abstracted by the telluryl oxo group. The subsequent step of surface NH insertion into a pi-allyl gas intermediate was also found to have a low activation energy barrier of 0.03 eV on the reduced Mo site.
Abstract The last few decades have seen growing interest in developing highly active and selectiv... more Abstract The last few decades have seen growing interest in developing highly active and selective catalysts for selective oxidation of lower alkanes to alkenes. Many catalytic systems have been suggested for these reactions and very recently, it has been proposed that MoVTeNbO catalysts show good activity and selectivity for propane ammoxidation reaction due to presence of two major crystalline phases, called M1 and M2. Researchers have paid special attention to the M1 phase which contributes to the high activity and selectivity to produce acrylonitrile via propane ammoxidation. The surface ab (basal) planes of M1 which are oriented perpendicular to the length of the rod like crystals have been suggested to contain the active and selective sites for this reaction, however this conclusion is still being debated. Therefore, it is very important to establish the role of ab planes of M1 in this catalytic reaction. In this study, phase pure M1 MoVTeNbO catalyst was prepared by hydrother...
The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and microwave-assisted HT ... more The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and microwave-assisted HT synthesis methods (MW) employing two different Ta precursors, Ta ethoxide and a custom-made Ta oxalate complex. The profile intensity analysis of the HAADF-STEM image of M1 phases oriented along [hk0] directions from the surface to bulk region of HAADF-STEM images indicated that the chemical composition of surface ab planes is very similar to their composition in the bulk. The HAADF-STEM image analysis showed that synthesis methods have a significant impact on the Mo/V distribution in the MoVTeTaO M1 phases and their reactivity in propane ammoxidation. Enhanced acrylonitrile yield and 1st order irreversible reaction rate constants for propane consumption, normalized to the estimated surface ab plane areas, correlated with increased V content in the proposed catalytic center (S2-S4-S4-S7-S7). These observations lend further support to the idea that multiple VOx sites present in the surfac...
The vapor-phase hydrogenation of furan, crotonaldehyde and selective hydrogenation of acetylene w... more The vapor-phase hydrogenation of furan, crotonaldehyde and selective hydrogenation of acetylene were carried out over carbon-supported Pd and Pt nano-particle catalysts. Carbon-supported Pt and Pd nano-particles exhibited unique catalytic properties due to size-dependent electronic and geometric properties of the noble metal nano-particles for hydrogenation reactions. Platinum and palladium nano-particles smaller than 10 nm were synthesized by different methods, such as reduction by alcohols and microemulsion methods, followed by the nano-particle deposition on Vulcan XC-72 carbon support by adsorption and wet impregnation, respectively. The catalysts were characterized by the N2 porosimetry (surface area), X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM studies demonstrated well-defined distributions of the platinum and palladium particles and the particle size calculation showed that the metal particles were in the range of 2 to 7 nm. The characterization r...
Recently, the water-gas shift (WGS) reaction has become very important because of production of h... more Recently, the water-gas shift (WGS) reaction has become very important because of production of high purity H2 in conjunction with fuel cell power generation.. It has been predicted that Cu and Ni are promising active transition metals for the water-gas-shift reaction based on theoretical (DFT) calculations. However, Ni-based catalysts also catalyze methanation side-reaction under water-gas-shift reaction conditions. It is expected that the Cu addition to Ni can modify the Ni ensemble properties and electronic structure by the Cu-Ni alloy formation to improve the WGS reaction and suppress the methanation reaction. The goal of this work is to explore supported bimetallic Cu-Ni nanoparticles for low temperature water-gas-shift reaction in order to suppress the undesirable methanation activity. The activity and selectivity in many metal-catalyzed reactions were shown to be dependent on the size and structure of metal particles on the nanoscale. For these reasons, we investigated the ef...
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Papers by Vadim Guliants