Oxygen Assisted Dehydrogenation of Ethane over Alumina Supported Pt-Sn Catalysts
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The oxygen-assisted dehydrogenation of ethane (ODE) over supported Pt-Sn catalysts seems to be a promising alternative for the conventional steam cracking of ethane for the production of ethylene. However, despite the fact that much research has been performed on this catalytic system during the last years, little is known about the precise role of the Pt-Sn catalyst on the overall product distribution. The focus of this research work lay therefore on gaining a better understanding of the role of tin in alumina-supported Pt-Sn catalysts under ODE conditions. To this extent a series of Pt-Sn catalysts with increasing tin loading (1.0 wt% Pt; 0.16 – 2.5 wt% Sn) were synthesized by means of incipient wetness impregnation. Additionally, a 1.0 wt% Pt/Al2O3 and a 2.5 wt% Sn/Al2O3 catalyst were prepared for comparison. All experiments were performed at 923 K with an ethane-to-oxygen ratio of 2 and a total gas flow of 1000 mL.min-1. The catalysts were tested either non-diluted in the presence of hydrogen or 10-fold diluted in either the absence or presence of hydrogen in the feed gas. It was found that the effects of tin addition to the Pt catalyst depended strongly on other reaction parameters such as the presence of hydrogen and the total metal loading in the reactor. In the absence of hydrogen the addition of tin increased the conversion of both ethane and oxygen and improved the catalytic stability of the diluted catalysts. Furthermore, the ethylene selectivity increased dramatically at the expense of CO and CO2. Addition of hydrogen limited the promotional effects of tin on the ethane conversion significantly, but increased the ethylene selectivity by approximately 3-fold for all catalysts, with a selectivity as high as 80% for the PtSn4.00 catalyst (Pt/Sn = 0.25 at/at). The hydrogen also improved the catalytic stability, and based on a visual comparison of the spent catalysts after 11 hours time on stream, this was ascribed to a significantly lower degree of coke formation. In the case of ODE over non-diluted Pt-Sn catalysts the trends in conversion and selectivity became more complex. Tin had a negative effect on the ethane conversion, but the selectivity towards ethylene, acetylene and water increased dramatically. Even the catalyst with the smallest amount of tin used in this work, i.e. with only 0.16 wt% Sn (Pt/Sn = 4 at/at), showed significant differences with the monometallic Pt/Al2O3 catalyst. Due to the increased metal loading in the reactor the oxygen conversion was initially over 99.8% for all catalysts. This remained the case for the Pt catalyst during the entire experiment. For the Sn-promoted catalysts on the other hand, the oxygen conversion dropped suddenly after 3 hours time on stream, only to stabilize at around 99% conversion after 6 hours time on stream. The same trends were observed for all selectivities, with the exception of acetylene, suggesting that the Pt-Sn catalysts possessed certain highly active sites that deactivated completely within these first 6 hours time on stream. Contrary to the diluted catalysts in the presence of hydrogen, the non-diluted catalysts showed severe coking. Combined with the complete conversion of oxygen within the catalyst bed and an increased production of molecular hydrogen, it was concluded that the catalyst bed consisted of an oxygen-rich zone on one side and an oxygen-lean zone on the other side. A physical mixture of the two monometallic catalysts was compared with a Pt catalyst diluted with only alumina and one of the coimpregnated Pt-Sn catalysts. It was shown that the physical mixture possessed nearly the same catalytic behaviour as the coimpregnated catalyst after several hours time on stream, differing significantly from the diluted Pt catalyst. Combined with the fact that the monometallic Sn/Al2O3 catalyst had approximately the same low activity as the blank alumina, it was concluded that tin had to be mobile under the ODE conditions used in this work, leading to the formation of bimetallic Pt-Sn particles similar to those in the coimpregnated catalysts.