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

Abstract

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 (C 2 H 6 /H 2 /H 2 O/inert ) 10/1.5/2/32 or C 2 H 6 /H 2 /CO 2 /inert ) 10/2/5/83, 600-630°C reaction temperature, atmospheric pressure). From kinetic measurements, a negative dependence of the reaction rate in H 2 and C 2 H 4 partial pressures was observed, while the dependence on steam partial pressure was positive. Isotopic labeling experiments showed that the negative effect of H 2 and C 2 H 4 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 C 2 H 6 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 order could be explained by partial occupation of the surface by adsorbed surface species. Methane and ethyne were the main byproducts of the dehydrogenation reaction. Cofeed experiments with 13 C 12 CH 6 / 12 C 2 H 4 indicated that both methane and ethyne are produced via ethene, and not directly from ethane. D 2 /C 2 H 6 cofeeding experiments revealed further that one H at the time is replaced with D in ethene, that addition of CO 2 does not affect the H-D distribution in ethene and ethyne and that practically no H-D exchange takes place over the support. The activation energy of the dehydrogenation of ethane over the present catalyst has in a previous study been determined to 116 kJ/mol [Virnovskaia, A.; Morandi, S.; Rytter, E.; Ghiotti, G.; Olsbye, U. Characterization of Pt,Sn/Mg(Al)O Catalysts for Light Alkane Dehydrogenation by FT-IR Spectroscopy and Catalytic Measurements. J. Phys. Chem. C 2007, 111, 14732]. The apparent activation energy for ethene hydrogenation over the same catalyst could be determined to -40 kJ/mol in the temperature range 497-596°C, indicating that in this temperature range the decrease in surface coverage with temperature overcompensates the increase of the rate constant of the rate determining step of the hydrogenation reaction.

The role of hydrogen during Pt–Ga nanocatalyst formation

“…Alternatively, active metals may be deposited onto the LDH-derived supports via conventional impregnation techniques, as in the welldescribed Pt-Sn/Mg(Al)O x system. [19][20][21][22] Since many applications for LDH-derived materials have already emerged, with more expected to come, deeper insight into their chemistry is highly desirable.…”

Effects of composition and metal particle size on ethane dehydrogenation over PtxSn100−x/Mg(Al)O (70⩽x⩽100)

“…The most active catalyst for ethane dehydrogenation is platinum; however, in its pure form, it exhibits low ethene selectivity and rapid coke deposition, which contributes to catalyst deactivation [6]. Isotopic studies have revealed that both methane formation and coke accumulation originate from ethene re-adsorption and dehydrogenation [6,7]. The addition of a second metal, such as tin, indium, or gallium, to platinum has been found to be effective in increasing ethene selectivity and suppressing coke formation [6,[8][9][10][11].…”

Non-oxidative coupling of methane catalysed by supported tungsten hydride onto alumina and silica–alumina in classical and H2 permeable membrane fixed-bed reactors

“…8,9 Moreover, ethane dehydrogenation into ethylene is central for polyethylene production. 10 Accounts reporting heavy hydrocarbon formation from CH 4 are predominantly based upon supported metal systems. [11][12][13] For instance Re and Mo supported on H-ZSM-5 convert CH 4 mainly to aromatics, with the selectivity to C 2 H 6 being less than 7%.…”