Study of preparation parameters of powder and pelletized Pt/KL catalysts for n-hexane aromatization

Abstract

Pt/KL powder and pelletized catalysts were synthesized by different methods including ion exchange (IE), incipient wetness impregnation (IWI), and vapor phase impregnation (VPI) methods. A detailed characterization was conducted on the various samples in order to determine the distributions of Pt particle size and location resulting from each preparation, as well as the relationships between these distributions and the catalytic performance. The catalysts were pretreated at two different reduction temperatures — 400 and 500°C — to investigate the sensitivity of each catalyst to thermal treatment. All catalysts showed high dispersion, with H/Pt ratios greater than unity. However, FTIR of adsorbed CO showed that more important than dispersion, it is the distribution of Pt cluster size and location, which influences the resulting catalytic stability. Standard IE catalysts were found to have a high fraction of Pt particles external to the L-zeolite and were the most sensitive to thermal treatment, displaying Pt migration out of the L-zeolite. These catalysts deactivated rapidly by coke formation. The addition of excess K + ions in the exchange solution and a longer aging period, as suggested in the patent literature, improves the Pt dispersion. IWI and VPI catalysts showed a majority of Pt clusters inside the L-zeolite channels. However, after high temperature reduction treatment, the IWI catalysts displayed particle growth inside the channels, in contrast with the VPI. The IWI catalysts were found to deactivate to about half their initial activity, while the VPI maintained the highest activity and stability. Different VPI methods including a moderate vacuum and a helium flow technique were examined and they showed similar Pt cluster distributions and catalytic performance to the catalysts synthesized using high vacuum. Preparation directly on the pellet for the methods investigated in this work (standard IE, IWI, and VPI) resulted in large fractions of Pt clusters external to the pores and likely on the binder. Thus, rapid deactivation by coke formation was evident. These results indicate that pelletized catalysts prepared by any of these techniques would require preparation on the powder prior to pellet extrusion.