IN-SITU C-13 MAS NMR-STUDY OF N-HEXANE CONVERSION ON PT AND PD SUPPORTED ON BASIC MATERIALS .2. ON THE MECHANISM OF ISOMERIZATION AND HYDROCRACKING

C-13 MAS NMR spectroscopy was performed in situ to investigate the mec hanisms of n-hexane isomerization and hydrocracking on Pt and Pd suppo rted on Al-stabilized magnesia (Pt/Mg(Al)O and Pd/Mg(Al)O), and Pt on KL zeolite (Pt/KL). All the catalysts had high metal dispersion, the m etal particle sizes being 13, 11, and 18 Angstrom, respectively. n-Hex ane 1-C-13 was used for in situ label tracer experiments. C-13 MAS NMR spectra were obtained during the time course of the reaction at 573 a nd 653 K. The NMR results were then quantified, and the reaction kinet ics were studied. Identification of the primary and secondary labeled reaction products led to the conclusion that both cyclic and bond-shif t isomerization mechanisms operate on the three catalysts. In the case of Pt/Mg(Al)O, the cyclic mechanism accounts for 80% of the isomeriza tion products. In the case of Pt/KL and Pd/Mg(Al)O, the contribution o f bond-shift reactions increases due to restricted formation of the me thylcyclopentane intermediate on the former and to suppressed hydrogen olysis of methylcyclopentane on the latter, A nonselective cyclic isom erization mechanism operates on magnesia catalysts, while on Pt/KL sel ective bisecondary bond rupturing occurs. Mechanistic pathways of bond -shift and hydrocracking reactions involve both 1,3- and 2,4-metallocy clobutane intermediates in the case of magnesia-supported catalysts, w hile in the case of the Pt/KL catalyst a 1,3-metallocyclobutane interm ediate is preferentially formed. Only terminal scission occurs on Pt/K L. The Pd catalyst demonstrates enhanced activity in demethylation. Th e observed differences in the mechanistic pathways are explained on th e basis of the specific properties of the metal and support. (C) 1996 Academic Press, Inc.