Increased sulfur tolerance of Pt/KL catalysts prepared by vapor-phase impregnation and containing a Tm promoter

Tm-containing Pt/KL catalysts were prepared by a variety of techniques, including incipient wetness impregnation (IWI), ion exchange (IE)? and vapor-phase impregnation (VPI) methods. The Pt morphology resulting from the addition of Tm and Pt sequentially, using the VPI method, was found to yield the greatest enhancement to the aromatization performance of the Pt/KL cataIvsts studied. The presence of Tm in the sequential VPI Pt/Tm/KL catalyst resulted reproducibly in a catalyst with higher Pt dispersion than that in an unpromoted VPI catalyst, as determined by ESAFS analysis and DRIFTS of adsorbed CO. VPI catalysts give more finely dispersed Pt clusters than either conventional IWI or IE methods. From TPO of poisoned catalysts, Tm was also found to act as a getter for sulfur, so it delays the poisoning of Pt under sulfur-containing feeds, as further evidenced by reaction studies. In addition, the initial activity of the Tm-promoted VPI catalysts was found to be higher than that of the unpromoted Pt/KL VPI catalysts, suggesting that Tm may directly modify Pt or even participate in accelerating the aromatization reaction. The amount and method of incorporation of Tm were found to be critical to the morphology of the Pt clusters and, subsequently, to catalyst performance under sulfur-free and sulfur-poisoned reaction conditions. While the sequential vapor-phase impregnation method with a small amount of Tm (0.15%) yielded a catalyst with improved catalytic properties, some of the other methods such as coimpregnation of Pt and Tm were found to hinder the dispersion of Pt. This may cause the blocking of the L-zeeoite channels, as demonstrated by DRIFTS of adsorbed CO, and a higher deactivation rate in the reaction. (C) 2000 Academic Press.