HEXANE CRACKING IN ZSM-5 - IN-SITU C-13 CROSS-POLARIZATION MAGIC-ANGLE-SPINNING NMR AND FLOW REACTOR/GC EXPERIMENTS

The conversion of n-hexane by ZSM-5 has been studied in closed samples by 13C cross-polarization (CP) magic-angle-spinning (MAS) NMR and by flaw reactor/GC analysis. The NMR experiments carried out after various heat treatments and subsequent quenching of the sealed samples to room temperature, result in product distributions which are comparable to those from flow reactor/GC experiments at full conversion, with propane (47 mol%) and butanes (25 mol%) as the main products. The differences in product distributions observed by NMR and the flow reactor/GC experiments, especially the absence of olefins in all NMR experiments, are ascribed to the difference in contact time in the two experimental setups. Based on the complementary data obtained from the two techniques, it is proposed that, at temperatures below 300°C, isomerization to the methylpentanes is dominant, while at higher temperatures cracking of the reactant and the methylpentanes takes over. Dimethylbutanes could not be detected by NMR, and it is concluded that the formation of these Isomers is inhibited in the medium-pore zeolite. © 1993 Academic Press, Inc.