Active Sites For Dehydrogenation Research Articles

Enhanced Catalytic Performance of Cr/MOR for Ethane Dehydrogenation Through Dealumination

Dehydrogenation of ethane to ethylene assisted by CO2 was investigated over CrOx catalysts supported on MOR zeolites. The catalysts were characterized by XRD, N2 adsorption, SEM, 27Al and 29Si MAS NMR, XPS, DRIFTS, H2-TPR and laser Raman spectroscopy. The catalysts exhibit enhanced activity and stability after MOR dealumination, with an optimal ethane conversion of ~ 38% and ethylene yield of ~ 33%. Characterization results indicate that the improved catalytic performance derives from the increased silanol nests of MOR support created by acidic dealumination, which are in favor of dispersion and stabilization of CrOx species, resulting in more reducible Cr(VI) considered as the active site for dehydrogenation.

Mo oxide supported on sulfated hafnia: Novel solid acid catalyst for direct activation of ethane & propane

Shale gas revolution has opened opportunities for conversion of light alkanes to value-added chemicals like ethylene and propylene. Literature suggests the use of bifunctional catalyst, consisting of a solid acid such as HZSM-5 and a metal oxide for direct conversion [7]. Here, we demonstrate a catalyst consisting of a novel Mo oxide-doped sulfated hafnia (SH) solid acid to convert ethane and propane.Mo-SH was synthesized via impregnation and characterized using BET, XPS, SEM-EDS, DRIFTS, NH3-TPD, XANES and TPO. Catalyst acidity was measured with NH3-TPD and DRIFTS. SEM-EDS and XPS were used to ensure the target loading of Mo on SH. XANES showed that Mo oxides are converted to Mo carbides with time, which are known as the active sites for dehydrogenation, the initial reaction step. Selectivity for ethane is 40–50 % to ethylene and for propane is 70–90 % to propylene at 550−600°C.No significant deactivation was observed for ∼16 h on stream.

Kinetic and Isotopic Study of Ethane Dehydrogenation over a Semicommercial Pt,Sn/Mg(Al)O Catalyst

Mechanistic and kinetic information on the ethane dehydrogenation reaction over a semicommercial Pt,Sn/Mg(Al)O catalyst has been elucidated from catalytic testing and isotopic labeling experiments under reaction conditions close to those used in the commercial dehydrogenation process (C2H6/H2/H2O/inert = 10/1.5/2/32 or C2H6/H2/CO2/inert = 10/2/5/83, 600−630 °C reaction temperature, atmospheric pressure). From kinetic measurements, a negative dependence of the reaction rate in H2 and C2H4 partial pressures was observed, while the dependence on steam partial pressure was positive. Isotopic labeling experiments showed that the negative effect of H2 and C2H4 could not be attributed to the reverse reaction, but rather to competitive adsorption at the active sites for dehydrogenation. The observed reaction rate with respect to C2H6 was close to first order. By fitting the experimental data to the rate equation derived from the elementary steps of ethane dehydrogenation, the observed deviation from the first ord...

The roles of gallium hydride and Brønsted acidity in light alkane dehydrogenation mechanisms using Ga-exchanged HZSM-5 catalysts: A DFT pathway analysis

We have used density functional theory (DFT) to study light alkane dehydrogenation by Ga-exchanged HZSM-5 by considering two types of catalytic sites: a mono-Al site of the form Z−[HGaX]+ (X=H, CH3, OH, Cl) and a di-Al site of the form Z2−[GaH]2+. For the mono-Al site, we report a new, direct one-step dehydrogenation mechanism; however, we conclude in general that mono-Al sites in ZSM-5 are not likely responsible for alkane dehydrogentation, as calculated activation energies are too high compared to experimental values (∼60kcal/mol versus 39kcal/mol). Instead, we propose [GaH]2+ residing near di-Al sites (Z2−[GaH]2+) are more active sites for dehydrogenation. We report a three-step mechanism for di-Al sites consisting of (1) C–H activation, followed by (2) alkene desorption and (3) H2 removal. We find that as Al–Al distance increases, the activation barrier for C–H activation decreases (ranging from 85.72 to 38.38 to 19.69kcal/mol), while the barrier for H2 removal increases (ranging from 15.49 to 36.71 to 47.38kcal/mol)—resulting in an optimal Al–Al separation distance of 4.53Å arising from these competing trends. As a result, we propose a simple ‘structure-to-activity’ correlation based on the Sabatier principle, which could be used to model and design the catalyst with required dehydrogenation activity.

Preparation and characterization of Pt/ZnO-Cr 2O 3 catalyst for low-temperature dehydrogenation of isobutane

Although Pt/ZnO catalyst is effective for low-temperature dehydrogenation of isobutane to isobutene, it is deactivated by the steam added to the reaction system to inhibit coke deposition. Adding Cr, Al, and Ga to ZnO as a third component to the Pt/ZnO catalyst can prevent this deactivation. In this study, the preparation method of ZnO-Cr 2O 3 and the Cr content in ZnO-Cr 2O 3 were examined in this context. Pt/ZnO-Cr 2O 3 catalyst prepared by coprecipitation from homogeneous aqueous nitrates of Zn and Cr with urea exhibited the highest catalytic activity of the catalysts prepared. Isobutane conversion over this catalyst increased with Cr content up to a constant rate at 10 mol%. The characterization of catalysts revealed that Pt/ZnO-Cr 2O 3 catalyst with large BET surface area and small ZnO crystallite size has high catalytic activity in general. However, it appears that the catalytic activity is affected by the number of free electrons rather than by the BET surface area. In order to generate many free electrons, extensive replacement of Zn with Cr in the ZnO lattice is required, necessitating mixing of Zn and Cr on the atomic scale. This mixing itself results in higher BET surface area and smaller ZnO crystallites. It was also found that most of the Pt surface was covered by carbon, and a small exposed Pt area, which exists in the interface between Pt and ZnO, in where coke was removed by the catalysis of ZnO is the active site for dehydrogenation.

Adsorption data extracted from kinetic equations of some hydrocarbon conversions on nickel

Adsorption equilibrium constants and adsorption enthalpies have been calculated from kinetic data of cyclohexane dehydrogenation and ethane hydrogenolysis on metallic nickel. From these data the bond strength of hydrogen and carbon with the active sites of nickel and the activation barrier of C−H and H−H bond dissociation were calculated by the recently developed method of Shustorovich. The calculated data indicate that the active sites for dehydrogenation differ (or in part differ) from those of hydrogenolysis.

Electrocatalytic Chemistry of the Transition Metal Complexes. II. Dehydrogenation of Cyclohexene Catalyzed by the Electroreduced Cobalt Complex of α,β,γ,δ-Tetraphenylporphine

Abstract The metal complexes of α,β,γ,δ-tetraphenylporphine (M(TPP), M=Co(II), Ni(II), FeCl(III), Pd(II), Pt(II), Cu(II), MnCl(III), and V(IV)O) were electrolyzed at −2.0 V vs. Ag wire in the presence of cyclohexene under nitrogen atmosphere. The cobalt complex only had the catalytic activity of dehydrogenating cyclohexene to 1,3- and 1,4-cyclohexadiene and benzene. With the use of dimethylformamide, dioxane, and hexamethylphosphoric triamide as solvent, cyclohexadienes were produced, and with that of benzonitrile, and dimethyl sulfoxide, benzene was produced and cyclohexadienes were scarcely produced at all. In all cases, the current efficiency of the products was much higher than 100%, indicating that the dehydrogenation reaction is catalytic. The composition of the products also varied with the reaction temperature. When ethanol was added to the reaction solution, the amount of cyclohexadienes increased with benzonitrile used as solvent. The ESR spectrum observed at g=2.003 under electrolysis at the second wave potential is due to the anion radical of cobalt(I) tetraphenylporphine. From polarographic data and other experiments, the species is considered to be an intermediate for dehydrogenation of cyclohexene. The active sites for dehydrogenation are considered to be both the central metal atom and the conjugated ring.