In situ C-13 MAS NMR study of n-hexane conversion on Pt and Pd supported on basic materials .1. Identification of the main reaction pathways and comparison with microcatalytic reactor tests

n-Hexane conversion was studied in situ on Pt and Pd supported on aluminum-stabilized magnesium oxide and Pt on Zeoite KL catalysts (Pt/Mg(Al)O, Pd/Mg(Al)O and Pt/KL) by means of C-13 MAS NMR spectroscopy. n-Hexane 1-C-13 was used as a labelled reactant. Forty NMR lines corresponding to 14 different products were resolved and identified. The NMR line assignments were confirmed by adsorption of model compounds. The NMR results were further quantified and compared with continuous flow microreactor tests. Four parallel reaction pathways were identified under flow conditions: isomerization, cracking, dehydrocyclization, and dehydrogenation. Aromatization occurs via two reaction routes: (1) n-hexane dehydrogenation towards hexadienes and hexatrienes, followed by dehydrocyclization; (2) dehydrocyclization of n-hexane followed by dehydrogenation of a cyclic intermediate. The former reaction pathway is prevented under NMR batch conditions. High pressures induced in the NMR cells at high reaction temperatures (573, 653 K) shift the reaction equilibrium towards hydrogenation. NMR experiments showed that on Pt catalysts aromatization occurs via a cyclohexane intermediate, whereas on Pd it takes place via methylcyclopentane ring enlargement. (C) 1996 Academic Press, Inc.