Hydrogen transfer and activation of propane and methane on ZSM5-based catalysts

Hydrogen exchange between undeuterated and perdeuterated light alkanes (CD₄-C₃H₈, C₃D₈-C₃H₈) occurs on H-ZSM5 and on Ga- and Zn-exchanged H-ZSM5 at 773 K. Alkane conversion to aromatics occurs much more slowly because it is limited by rate of disposal of H-atoms formed in C-H scission steps and not by C-H bond activation. Kinetic coupling of these C-H activation steps with hydrogen transfer to acceptor sites (Ga ⁿ⁺, Zn ^m+) and ultimately to stoichiometric hydrogen acceptors (H⁺, CO₂,O₂, CO) often increases alkane activation rates and the selectivity to unsaturated products. Reactions of¹³ CH₄/C₃H₈ mixtures at 773 K lead only to unlabelled alkane, alkene, and aromatic products, even though exchange between CD₄ and C₃H₈ occurs at these reaction conditions. This suggests that the non-oxidative conversion of CH₄ to higher hydrocarbons on solid acids is limited by elementary steps that occur after the initial activation of C-H bonds.