A series of alumina-supported Fe-based catalysts is prepared via a dry impregnation method in the presence of a phosphorus source (phosphate salt) and then used for the catalytic dehydrogenation of propane. Specifically, supported catalysts with Fe:P molar ratios of 1:1, 2:1, and 3:1 are prepared and their chemical composition, textural properties, and redox properties are characterized with an array of techniques. In the nonoxidative dehydrogenation (PDH) of propane at 600 °C and atmospheric pressure, the most active catalyst (Fe:P ratio of 3:1) exhibits 15% propane conversion and >80% C 3 H 6 selectivity. The calculated activity is 9.9 mmol/(h g Fe ) (mass basis) or 13 μmol/(h m 2 ) (surface area basis), with a corresponding TOF of 19 h −1 . During the initial stages of reaction under PDH conditions, the precatalyst is reduced and Fe(0) species are generated, eventually giving way to iron carbide species. During this induction period, significant carbon is incorporated into the catalyst and propylene selectivity is low. Only after the iron carbide phase appears do the reactivity and selectivity achieve steady-state conditions with high propylene selectivity and good activity. The addition of the phosphorus source in the precatalyst is found to be important in obtaining a catalyst with superior performance.
A series of ternary mixed metal oxides containing Group III A elements (In, Ga, Al) is prepared by means of an alcoholic co‐precipitation method. Specifically, oxide catalysts with a molar composition of In/Ga/Al=5:15:80, 10:10:80, and 15:5:80 are reported. The chemical composition, redox properties, and catalyst structures are fully characterized, with the results suggesting that the indium, gallium, and aluminum moieties are well‐dispersed in the catalysts. The catalysts are evaluated for propane dehydrogenation (PDH) at 570 and 600 °C under 1 atm total pressure. The most effective catalyst with a composition of In/Ga/Al=5:15:80 provides 17 % conversion and approximately 86 % C3H6 selectivity with an initial activity of 4.6 mmol h−1 gcat−1 and 24.1 μmol h−1 m−2. The intrinsic activity on an active metal (i.e. indium and gallium) basis is approximately 3 times that of the In2O3–Ga2O3 family and approximately 3–9 times that of the In2O3–Al2O3 family. The catalyst deactivates with time on stream, and regeneration tests show that removal of surface coke and recovery of an In2O3 state helps to regain the initial activity, whereas reducing In2O3 domains into In0 does not allow for recovery of the performance. Raman analysis of the carbonaceous species deposited on the catalyst indicates catalysts with higher gallium content give more graphitic carbon, which correlates with higher C3H6 selectivity, whereas catalysts with more disordered coke are associated with lower selectivity. However, higher gallium content causes more coke formation, which leads to faster deactivation. This initial study of this family of mixed oxides suggests that an ideal In/Ga ratio may exist whereby catalyst properties may be optimized.
We report the synthesis, characterization, and enhanced propane dehydrogenation properties of hierarchical Ga-MFI zeolite catalysts synthesized by two different methods: (i) repetitive branching and (ii) utilization of a long chain alkyl SDA. Structural, compositional, and morphological characterizations confirm that the Ga-MFI catalyst materials have hierarchical structures including micropores and mesopores in a single particle and show that Si/Ga ratios comparable to bulk Ga-MFI catalysts can be obtained. Acid site analysis using NH 3 -TPD and pyridine adsorption followed by in situ IR spectroscopy allowed quantification of the number and types of acid sites present and show that the hierarchical catalysts have considerably higher Lewis acid site concentrations than the bulk catalysts. The hierarchical Ga-MFI catalysts show superior PDH performance (at 600 °C) compared to bulk Ga-MFI catalysts. Propane conversion rates are increased 2−6-fold and propylene selectivities are 10−100% higher. We also examine the effects of synthesis variationssuch as addition of 3-mercaptopropyltrimethoxysilane during synthesis and H + ion-exchangeand find that both these steps have a beneficial effect on PDH properties. Calculations suggest that PDH in both the bulk and the hierarchical Ga-MFI is not diffusion-limited. Therefore, the superior performance of hierarchical Ga-MFI is due to intrinsically higher activity and selectivity. Potential reasons for this behavior are outlined. The present findings showing enhancement of PDH in hierarchical Ga-MFI catalysts suggests that the utility of the hierarchical zeolites is not limited to diffusion-limited reactions involving large, bulky molecules and that they may be useful in more diverse applications involving gas-phase reactions with small molecules.
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