Effects of Structure on the Activity, Selectivity, and Stability of Pt-Sn-DeAlBEA for Propane Dehydrogenation

Abstract

Recent research has found that dealuminated zeolite BEA (DeAlBEA) is an attractive support for the dispersion of Pt and PtSn species that serve as catalysts for propane dehydrogenation (PDH). In this study, we report the preparation, structural characterization, and PDH activities of Pt-Sn-DeAlBEA catalysts as a function of the Pt/Al ratio (here Al represents the amount of Al present in the parent zeolite H-BEA). The support Sn-DeAlBEA was prepared by introduction of Sn to DeAlBEA. Characterization of this material by X-ray absorption spectroscopy (XAS) and UV−vis spectroscopy revealed that the Sn incorporated into the BEA framework as Sn(IV) cations. Pt-Sn-DeAlBEA catalysts were prepared with Pt/Al ratios (0.001−0.026) and were characterized with infrared (IR) spectroscopy of adsorbed probe molecules and XAS to understand the effect of changing Pt loading on the structure of Pt in Pt-Sn-DeAlBEA. Pt dispersion on DeAlBEA (i.e., Pt-DeAlBEA) produced Pt nanoparticles with an average Pt−Pt coordination number of 9 (∼25 Å) for Pt/Al ratios of 0.001 and above. By contrast, dispersion of Pt on Sn-DeAlBEA (Sn/Al = 0.15) produced Pt oligomers with an average Pt−Pt coordination number of 3 for Pt/Al = 0.001, but for Pt/Al ratios >0.013, Pt nanoparticles formed with a Pt−Pt coordination number of 9. Pt-Sn-DeAlBEA exhibited high selectivity to propene (>97%) and high dehydrogenation rates. Forward rate constants were calculated and compared with values determined for various Pt and PtSn catalysts reported in the literature. The Pt-Sn-DeAlBEA catalysts prepared in this study exhibited significantly higher forward rate constants than those previously reported for Pt and PtSn catalysts. The kinetics of PDH were measured for Pt-Sn-DeAlBEA catalysts with different Pt/Al ratios but identical Sn/Al ratios. In all cases, the kinetics are described by a Langmuir−Hinshelwood rate expression, which is first order in propane and is inhibited by propane adsorption. The similarity of the apparent activation energies and enthalpies of propane adsorption for all three catalysts suggests that the active species are very small Pt 3 Sn clusters strongly bound to the framework of DeAlBEA.