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

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 Pt 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 Pt 2 O 2+, balancing negative charges on the zeolite framework. The results of temperature-programmed H 2 reduction (H 2 —TPR) suggested that Pt 2 O 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 H 2 dissociation, generating neutral platinum hydride and zeolite Brønsted acid sites, [Pt— n H] 0 and H + . A similar platinum species, x [Pt 0 — n H], was obtained from homolytic H 2 dissociation on Pt 0 x . When H 2 was removed from the system, electrons were transferred from Pt 0 atoms or Pt 0 x nanoparticles to the zeolite protons. When H 2 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 H 2 -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 n H 2 n +1 + (species 1) and [Pt 0 —H]/ZO − H + (species 2) sites, in dynamic equilibrium with gas-phase alkane and H 2 . 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.