Microcalorimetric, reaction kinetics and DFT studies of Pt-Zn/X-zeolite for isobutane dehydrogenation

Microcalorimetric measurements of the adsorption of H(2)and C2H4 were carri ed out at 300 K on a Pt-Zn/X-zeolite catalyst (Pt: Zn atomic ratio equal to 1 : 1). The initial heats of H-2 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 kin etics measurements for isobutane dehydrogenation over the Pt-Zn/X-zeolite c atalyst were carried out at temperatures from 673 to 773 K, at isobutane pr essures from 0.01 to 0.04 atm, and at hydrogen pressures from 0.1 to 0.7 at m. The catalyst shows high activity and selectivity for dehydrogenation of isobutane to isobutylene. The reaction kinetics can be described with a Hor iuti-Polanyi reaction scheme. DFT calculations were carried out for the ads orption 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 bind ing energies of pi -bonded ethylene, di-a-bonded ethylene, and ethylidyne s pecies on atop, bridged, and three-fold Pt sites, respectively. These effec ts are most significant for the bonding of ethylidyne species, and they are least significant for pi -bonded ethylene species. Results from DFT calcul ations for the adsorption of formaldehyde show that addition of Zn to Pt we akens the di-or-bonding at Pt-Pt sites; however, this weakening effect of Z n on formaldehyde adsorption is less significant than the effect on ethylen e adsorption. Moreover, the preferred location for adsorption of formaldehy de on PtZn(011) is a Pt-Zn site, whereas the preferred location for adsorpt ion of ethylene is a Pt-Pt site. Thus, formaldehyde is adsorbed more strong ly by 53 kJ/mol on PtZn(011) compared to the di-sigma -adsorption of ethyle ne, whereas formaldehyde and ethylene adsorb in the di-rr-forms with compar able energies on Pt(111). This preferred adsorption of formaldehyde compare d to ethylene on PtZn(011) may be at least partially responsible for the en hanced selectivity of Pt-Zn-based catalysts for hydrogenation of C=O groups compared to C=C bonds in alpha,beta -unsaturated aldehydes.