Platinum clusters supported in zeolite LTL: Influence of catalyst morphology on performance in n-hexane reforming

KLTL zeolite-supported platinum catalysts were synthesized from aqueous tetraammineplatinum(II) nitrate solutions and nonacidic KLTL zeolite crystallites, including some with dimensions as little as 300 x 500 Å. The zeolite crystallites had various morphologies, some being predominantly disk-shaped particles and some predominantly mosaics of rod-like domains with a range of c-dimension lengths. The activity and selectivity of each catalyst were evaluated for dehydrocyclization of n-hexane in the presence of H₂ to form predominantly benzene at conversions of typically 45-90%. The data presented here provide a detailed characterization of the deactivation of such catalysts in the absence of sulfur. EXAFS data show that the platinum in each catalyst was present in clusters of about 20 atoms each, on average. Electron micrographs show that the platinum clusters were nearly evenly dispersed on the surfaces of the zeolite crystallites, including the intracrystalline and extracrystalline surfaces. The catalytic performance was virtually independent of the zeolite channel length, but activity, selectivity, and resistance to deactivation were found to be correlated with the ratio of the surface area external to the crystallite domains to that within the intracrystalline pores. The catalyst performance is dependent on this ratio (which is related to the zeolite morphology) as follows: in comparison with the others, the catalysts with the relatively low fractions of platinum outside the intracrystalline pores are more active, more selective for benzene formation, and more resistant to deactivation. One well-prepared catalyst, for example, gave greater than 90% selectivity for benzene and no measurable deactivation over 140 h of operation in a flow reactor at 420°C and atmospheric pressure. These data match those characterizing the most selective catalysts reported. Consistent with the interpretation of E. Iglesia and J. E. Baumgartner (in "New Frontiers in Catalysis" (L. Guczi, F. Solymosi, and P. Tetenyi, Eds.), p. 993, Studies in Surface Science and Catalysis, Vol. 75. Elsevier, Amsterdam, 1993), catalyst deactivation is associated with platinum outside the zeolite pores; it is hypothesized that coke formation outside the pores is relatively rapid and that a distinguishing characteristic of the best catalysts is the presence of most of the platinum in the intracrystalline pores, where coke formation may be inhibited by the constraints of the pores.