COMPARISON OF PT/KL CATALYSTS PREPARED BY ION-EXCHANGE OR INCIPIENT WETNESS IMPREGNATION

Pt/KL catalysts prepared by ion exchange (IE), incipient wetness impregnation (IWI), and coimpregnation with KCl (IWI + KCI) have been characterized by dynamic techniques, chemisorption of H2 and CO, Fourier transform-infrared spectroscopy (FT-IR) of adsorbed CO, and catalytic tests using n-hexane (n-C6) and methylcyclopentane (MCP) conversions as probe reactions. Temperature-programmed reduction (TPR) shows significant differences between the IE and IWI catalysts. After calcination up to 400°C, the IWI samples contain Pt4+ ions that are reduced at 250°C, but IE samples contain Pt2+ particles that are reduced at 11°C, besides two Pt2+ species with TPR peaks at 80 and 150°C. High-resolution electron microscopy, adsorption, FT-IR, and catalytic results consistently show that, after reduction, the IWI catalysts contain smaller Pt particles located inside the zeolite channels, while the IE sample has larger particles, some of which are on the external surface. At high temperature, excess KCI reacts with zeolite protons, forming HCl which escapes. In comparison to the IE samples, the IWI catalysts are less acidic, less active for n-C6 conversion, more selective for dehydrocyclization, but less selective for hydrogenolysis, and they deactivate less. Since hydrogenolysis requires large Pt ensembles, the small Pt particles in the IWI catalysts produce less C1C5 compounds and less coke. The product distribution of MCP ring opening shows higher than statistical selectivity toward 3-methylpentane, suggesting that the MCP molecule becomes oriented inside the zeolite channels. © 1992.