Microcalorimetric, reaction kinetics and DFT studies of Pt-Zn/X-zeolite for isobutane dehydrogenation
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abstract
Microcalorimetric measurements of the adsorption of H2 and C2H4 were carried out at 300 K on a Pt-Zn/X-zeolite catalyst (Pt : Zn atomic ratio equal to 1 : 1). The initial heats of H2 and C2H4 adsorption were equal to 75 and 122 kJ/mol. respectively, and these values are weaker than the values of 90 and 155 kJ/mol typically observed for supported Pt catalysts. Reaction kinetics measurements for isobutane dehydrogenation over the Pt-Zn/X-zeolite catalyst were carried out at temperatures from 673 to 773 K, at isobutane pressures from 0.01 to 0.04 atm, and at hydrogen pressures from 0.1 to 0.7 atm. The catalyst shows high activity and selectivity for dehydrogenation of isobutane to isobutylene. The reaction kinetics can be described with a Horiuti-Polanyi reaction scheme. DFT calculations were carried out for the adsorption of ethylene on slabs of Pt(111), Pt3Zn(111) and PtZn(011). Results from these calculations indicate that addition of Zn to Pt weakens the binding energies of π-bonded ethylene, di-σ-bonded ethylene, and ethylidyne species on atop, bridged, and three-fold Pt sites, respectively. These effects are most significant for the bonding of ethylidyne species, and they are least significant for π-bonded ethylene species. Results from DFT calculations for the adsorption of formaldehyde show that addition of Zn to Pt weakens the di-σ-bonding at Pt-Pt sites; however, this weakening effect of Zn on formaldehyde adsorption is less significant than the effect on ethylene adsorption. Moreover, the preferred location for adsorption of formaldehyde on PtZn(011) is a Pt-Zn site, whereas the preferred location for adsorption of ethylene is a Pt-Pt site. Thus, formaldehyde is adsorbed more strongly by 53 kJ/mol on PtZn(011) compared to the di-σ-adsorption of ethylene, whereas formaldehyde and ethylene adsorb in the di-σ-forms with comparable energies on Pt(111). This preferred adsorption of formaldehyde compared to ethylene on PtZn(011) may be at least partially responsible for the enhanced selectivity of Pt-Zn-based catalysts for hydrogenation of C=O groups compared to C=C bonds in α,β-unsaturated aldehydes.
