ROLES OF CARRIER GASES ON DEACTIVATION AND COKING IN ZEOLITE-BETA DURING CUMENE DISPROPORTIONATION

The influence of carrier gases (N-2, H-2, He, and CO2) on the catalyti c activity, stability, and coke formation in zeolite beta during cumen e disproportionation reaction is discussed, The reaction intermediates as well as the carbonaceous residues were characterized by C-13 NMR s pectroscopy under proton cross-polarization and magic-angle spinning a nd by thermogravimetric method. The effects of carrier gas dilution on coke formation, catalyst deactivation, and product shape selectivity have also been examined by varying carrier gas (N-2) to reactant molar ratios (0.2-20), The amount of total coke decreases linearly with inc reasing N-2/cumene ratios above a value near 2, It is also found that the coke induced shape selectivity is notable only at extreme dilution . In the presence of various carrier gases, a notable decrease in cata lytic activity has been found to obey the order N-2 > H-2 > He > CO2, whereas a reverse order was observed for the catalytic stability, More over, the amount of coke deposit is found to decrease linearly with th e kinetic diameter of the carrier gases, Hence, the incorporation of c arrier gases resulted in a decrease in the amount of coke deposition w hich is mainly due to the transport of coke precursors and less bulky carbonaceous compounds (soft coke), Similarly, as proposed by the tran sition complex solvation model, the carrier gas molecules stabilize th e biphenyl alkane reaction intermediates by van der Walls interactions and prevent them from further dissociation into product molecules. Wi th the exception of H-2, the combination of the carrier gas transport effect and transition complex solvation model is used to describe the observed trends in the activity and stability of the catalyst. (C) 199 6 Academic Press, Inc.