Activation of Hydrogen and Hexane over Pt,H-Mordenite Hydroisomerization Catalysts

摘要

Various Pt species were shown to be present in Pt/H-mordenite catalyst preparations containing 1.5 and 2.6 wt % Pt. Air calcination of the catalyst precursor generated large Pr~0_X metal particles (average size of > 10 nm) by autoreduction, whereas a fraction of the platinum was converted to cations and oxycations, such as Pt~(2+) and Pt2O~(2+), balancing negative charges on the zeolite framework. The results of temperature-programmed H2 reduction (H2-TPR) suggested that Pt2O~(2+) was reduced to Pt~+ below 550 K. The Pt~(n+) cations (n = 1 or 2) became reduced to Pt~0 and Pt~0_x above 550 K. Reduction was introduced by heterolytic H2 dissociation, generating neutral platinum hydride and zeolite Br0nsted acid sites, [Pt-nH]~0 and H~+. A similar platinum species, x[Pt~0-nH], was obtained from homolytic H2 dissociation on Pt~0_x. When H2 was removed from the system, electrons were transferred from Pt~0 atoms or Pt~0_x nanoparticles to the zeolite protons. When H2 was released, acid sites were annihilated, and the highly dispersed metal again became the zeolite cation Pt~(n+). The oxidation state and the chemical environment of the platinum were characterized by the vibrational spectra of chemisorbed CO. The spectral feature in the 2090-2100 cm~(-1) range, present in the spectrum of each H2-reduced catalyst, was shown to stem from two overlapping component bands. These bands were assigned to CO bound to Pt~+ and Pt~0. The results confirm that the active surface intermediates of alkane hydroisomerization are platinum hydride/carbenium ion and platinum hydride/zeolite proton pair sites, such as [Pt~0-H]/ZO~-C_nH_(2n+1)~+ (species 1) and [Pr~0-H]/ZO~-H~+ (species 2) sites, in dynamic equilibrium with gas-phase alkane and H2. Hydrogen promotes release of the alkane from species 1 by generating species 2 (hydride transfer). If the rate of isomer formation is governed by the transformation rate of the carbenium ion, this suggested mechanism corresponds to kinetics that is first-order in hexane and negative-order in hydrogen. The large Pt~0_x clusters were shown to catalyze the saturation of the eventually formed alkenes and, thereby, to suppress coke formation and catalyst deactivation.