Catalytic dehydrogenation of alkanes on Pt/Na-[Fe]ZSM5 and staged O2 introduction for selective H2 removal

Small Pt clusters within Na-[Fe]ZSM5-protected channels catalyze C3H8 and C2H6 dehydrogenation with unprecedented turnover rates and catalyst stability. Alkene selectivities are greater than 97% at near-equilibrium alkane conversions. Mild oxidative treatments fully restored initial catalytic rates and selectivities. Exchange sites in Na-[Fe]ZSM5 lead to well-dispersed Pt precursors and catalytic Pt clusters, which reside within ZSM5 channels that inhibit the formation of large unreactive organic residues. The weak acidity of residual OH groups in [Fe]ZSM5 minimizes β-scission and oligomerization reactions, which lead to loss of alkene selectivity and to unreactive organic residues. Thermodynamic constraints were removed by selective combustion of H2 using O2 coreactants. More than 90% of the O2 introduced was used to form H2O from H2, even when hydrocarbons were the predominant available reactants. Equivalent O2 amounts cofed with C3H8 reactants led instead to ∼5% selectivity for H2 combustion. Hydrocarbon combustion was the predominant reaction and the cofed O2 was depleted before H2 could be formed and dehydrogenation approached equilibrium. Alkene yield enhancements of ∼1.6 above equilibrium were achieved by selective H2 removal using O2 staging. These yield enhancements exceed those achieved with previously reported three-stage reactor systems. The O2 staging approach reported here requires only one reactor and one catalytic composition; thus, it decreases significantly process complexity and cost. H2 removal by selective combustion using O2 requires precise control of O2 introduction and availability in order to avoid H2 depletion and high CO selectivities, which can lead to unreactive deposits and to catalyst deactivation during alkane dehydrogenation.