Ethane dehydrogenation on Pt/Mg(Al)O and PtSn/Mg(Al)O catalysts

The dehydrogenation of ethane to ethene on Sn-promoted Pt supported on calcined hydrotalcite, PtSn/Mg(Al)O, was investigated with the aim of understanding the effects of Sn on the local environment of the dispersed Pt, the catalyst activity and selectivity for dehydrogenation, and the formation of coke. The origins of methane, the primary byproduct of ethane dehydrogenation were also investigated as a part of this study. Pt/Mg(Al)O was reacted with tetra-n-butyl tin in order to introduce Sn selectively to the dispersed Pt particles. High-resolution TEM revealed the formation of a PtSn bimetallic phase upon introduction of Sn. The activity of PtSn/Mg(Al)O for ethane dehydrogenation at 873 K was highest for a Sn/Pt ratio of 0.3, whereas the ethene selectivity increased monotonically with increasing Sn/Pt ratio, reaching 100% for Sn/Pt = 0.4. Sn promotion also significantly decreased the deposition of coke. Addition of H2 to the feed enhanced the formation of ethene for H2/C2H6 ratios up to 0.58 for Pt/Mg(Al)O and 0.25 for PtSn/Mg(Al)O, but for higher ratios the product concentration of ethene decreased and approached that determined thermodynamically. The ethene selectivity decreased dramatically with increasing H2/C2H6 ratio for Pt/Mg(Al)O but only slightly for PtSn/Mg(Al)O. Coke formation was suppressed considerably by H2 addition to the feed, particularly for PtSn/Mg(Al)O. Isotopic tracer studies revealed that methane formation resulted primarily from the readsorption of ethene. A mechanism is proposed for ethane dehydrogenation and is used to interpret the effects of Sn promotion and the addition of H2 to the feed.