Characterization of the morphology of Pt clusters incorporated in a KL zeolite by vapor phase and incipient wetness impregnation. Influence of Pt particle morphology on aromatization activity and deactivation

Two series of Pt/KL catalysts with varying metal loading were synthesized by the methods of incipient wetness impregnation (IWI) and vapor phase impregnation (VPI) to compare the effects of the different morphologies that result when the metal loading and, in particular, the preparation method are varied. Catalysts were characterized by a Variety of techniques. TEM and DRIFTS studies indicated that on the low-loading samples the majority of particles were located inside the channels of the L-zeolite. In agreement with recent studies, the DRIFTS results evidenced the formation of Pt carbonyls, which further support the presence of very small particles. EXAFS and TEM showed that the VPI catalysts resulted in smaller particles than the catalysts prepared by the nn method. In addition, EXAFS demonstrated for this series a higher degree of interaction with the L-zeolite framework oxygen atoms. Pulse testing of the methylcyclopentane ring opening showed that the very small clusters produced by the VPI preparation did not result in collimation of the MCP molecule, implying that the reactants and products can easily diffuse over the Pt cluster. This is in contrast with the particles produced by the IWI method, which clearly displayed a collimation effect. The characteristic morphology produced by the VPI method was found to improve the performance of the catalyst under clean and sulfur-poisoned conditions, enhancing the catalyst's resistance to the formation of coke and decreasing the particle agglomeration rate. (C) 1999 Elsevier Science B.V. All rights reserved.