Staged O2 Introduction and Selective H2 Combustion during Catalytic Reactions of Alkanes on Cation-Exchanged H-ZSM5

Stoichiometric reactions can be used to remove H2 and the associated kinetic and thermodynamic barriers that lead to low dehydrocyclodimerization selectivity during alkane reactions on cation-modified H-ZSM5. O2 co-reactants can form H2O in exothermic reactions that balance the enthalpy of endothermic dehydrogenation steps. O2 reacts preferentially with H2 via homogeneous and heterogeneous pathways, but also with hydrocarbons as H2 is depleted; thus, it must be gradually introduced as H2 forms in dehydrogenation reactions. Staged O2 protocols significantly increased aromatics yields during C3H8 reactions on unexchanged and on Ga- and Zn-exchanged H-ZSM5. On Ga/H-ZSM5, the maximum aromatic yields increased from 54% to 68% and aromatization/cracking selectivity ratios increased from 2.1 to 3.9 when O2 was introduced gradually into a gradientless batch reactor as H2 formed. O2 introduced was converted to H2O with >95% selectivity; an equivalent amount of O2 initially added with C3H8 led to low H2O selectivities (<60%). Similar effects of O2 addition and of staging protocols were observed for alkane reactions on H-ZSM5 and Zn/H-ZSM5. Staging strategies led to selective use of O2 to remove thermodynamic and kinetic bottlenecks and to unprecedented aromatics yields during alkane reactions on cation-exchanged H-ZSM5.