HIGH-TEMPERATURE INSITU MAGIC ANGLE SPINNING NMR-STUDIES OF CHEMICAL-REACTIONS ON CATALYSTS

We report in situ magic angle spinning (MAS) studies of chemical reactions on zeolite catalysts in the high-temperature range (573-673 K) used in typical catalytic processes. These studies were carried out using a novel high-temperature MAS probe which is also described in this contribution. The chemical studies reported here were selected based on either earlier studies at lower temperatures or the failure to observe any reactions using probes with lower temperature limits. All reactions were carried out in zeolite HZSM-5. Cracking of ethylene oligomers was studied at 623 K. A product distribution consistent with the formation of a pentacoordinated carbonium ion intermediate was observed which was not seen in a previous investigation at a lower temperature. Furthermore, direct observation of three-coordinate carbenium ion formation during cracking at 523-623 K was achieved. The conversion of methanol to gasoline (MTG) on HZSM-5 was studied with a temperature jump to 623 K. Important differences observed in this study relative to previous investigations at lower temperatures include well-resolved signals for adsorbed vs exogenous (gas phase) methanol and dimethyl ether and a higher yield of aromatics. Two less-reactive methane derivatives were also studied to look for analogies to MTG chemistry. Methyl iodide began reacting on HZSM-5 at ca. 583 K to form light aliphatics and at 623 K was completely consumed to form methane, ethane, and benzene. As was previously observed at lower temperatures, dimethyl sulfide formed trimethylsulfonium ion which proved to be stable even at 673 K in the present contribution.