Catalysts For Dehydrogenation Of Alkanes Research Articles

Control of Intrazeolitic Gallium Cation Content and Its Effects on C 2 Dehydrogenation in Ga-MFI Catalysts

Ga-doped MFI zeolites are selective C 2 and higher alkane dehydrogenation catalysts. The balance between zeolite proton sites and exchanged Ga-cations, which is known to critically affect the formation of aromatics, is itself greatly affected by the extent of Ga reduction, steaming, and treatment with oxidizers. Also, although the reaction can be inhibited by H 2, and CH 4, at short times on stream the presence of CH 4 can in some cases enhance the formation of aromatics. The reaction of ethane at low alkane pressures has been used to separate these effects from site deactivation by coking. The kinetics studies were supplemented with site characterization experiments, including thermal analysis of adsorbed 1-propanamine, TPO of the carbonaceous residues, XRD, and FTIR of the catalyst and of adsorbed 1-propanamine and pyridine. These studies show that at reaction conditions small quantities of H 2O and/or O 2 can transform intrazeolite Ga-cation sites back to protons and gallium oxide. This transformation inhibits the formation of aromatics except in the case of low levels of protons, where the yield to aromatics can actually increase. The results are interpreted in terms of the dual-function mechanism for alkane dehydrogenation.

MFI-Type Ferrisilicate Catalysts for the Oxidative Dehydrogenation of Alkanes

The influence of extra-framework Al via AlCl3 deposition on the structure of the active Fe sites in Fe/ZSM-35 was investigated by 29Si MAS NMR, UV–visible diffuse reflectance spectroscopy, infrared spectroscopy, UV Raman spectroscopy, and 57Fe Mössbauer spectroscopy combined with catalytic decomposition of N2O. It is found that high-temperature treatment considerably modifies the iron speciation. UV–visible diffuse reflectance spectra and in situ infrared spectra of NO adsorption show that a significant amount of clustered and cationic Fe species is transformed into extra-framework Fe–O–Al mixed oxide species by high-temperature treatment. UV Raman spectra and 57Fe Mössbauer spectra suggest the generation of a binuclear Fe site stabilized by extra-framework Al species with a feature Raman band at 875 cm−1, which is associated with the high activity of N2O decomposition. A possible explanation is that the addition of extra-framework Al species is beneficial for the formation of binuclear active Fe sites bound to extra-framework Al.

Dynamic process of carbon deposition on Pt and Pt–Sn catalysts for alkane dehydrogenation

The process of carbon deposition on Pt/Al 2O 3 and Pt–Sn/Al 2O 3 catalysts under different conditions was investigated in a multifunctional apparatus which can be used for hydrogen chemisorption, TPO and TPSR determinations. The results showed that on both Pt and Pt–Sn catalysts, carbon deposition is a dynamic process. The carbonaceous materials which originate from the metal surface may be converted to carbon through successive dehydrogenation on metal sites, or they may continuously migrate to the support with irreversible coke formation through condensation or hydrogen transfer reactions. The results of chemisorption and thermal desorption of ethylene on platinum and platinum-tin catalysts showed that the incorporation of tin reduced the strength of chemisorption of the hydrocarbons on the metal surface, so that the carbon precursors can migrate towards the support more easily. The proportion of carbon deposited on the support of the platinum-tin catalysts was greater than that of the platinum catalysts. With the same level of carbon deposition, 30% of the metal surface on platinum-tin catalyst remained uncovered, whereas only 10% of the metal surface of the platinum catalysts was bare. The high stability of Pt-Sn/Al 2O 3 catalysts is closely related to the larger bare metal surface under high-temperature conditions.