SIMS/XPS study on the deactivation and reactivation of B-MFI catalysts used in the vapour-phase Beckmann rearrangement

A weakly acidic borosilicate of MFI-type (B-MFI) was utilized as a catalyst for the Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam in a vapour phase process. XPS and SIMS depth profiling techniques were applied to study the fresh catalyst as well as the deactivation and the reactivation phenomena on and below the catalyst surface. The catalyst deactivation could not be attributed to excessive amounts of carbon deposited on the surface or to a pronounced and irreversible deboration of the zeolite. A pore blocking effect caused by an enrichment of nitrogen-containing species inside the pore system was observed. These species were presumably formed by ring-opening reactions of unsaturated nitriles preferentially formed inside of the zeolite, Furthermore, distinct changes of the SiO+/SiOH+ fragment ion profiles in the surface near regions of the pore system were observed but the SiO+/SiOH+ fragment ion ratios in the topmost atomic layers of the fresh and the reactivated catalyst were nearly identical. This indicates that the level of SiOH-groups at the catalyst surface, which are active sites for the given process, was retained or reestablished during the whole time of catalyst service and the regeneration, whereas the silanol group density in the pore system below the surface was tremendously affected. Indications were obtained that Na is located at lattice sites, whereas Ca is able to diffuse inside the pore system of the B-MFI catalyst. (C) 1998 Academic Press.