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 of (CH4)-C-13/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.