Activation of Hydrogen and Hexane over Pt,H-Mordenite Hydroisomerization Catalysts Ferenc Lo ´nyi,* Anita Kova ´cs, Ágnes Szegedi, and Jo ´zsef Valyon Institute of Nanochemistry and Catalysis, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u. 59-67, 1025 Budapest, Hungary ReceiVed: December 5, 2008; ReVised Manuscript ReceiVed: April 1, 2009 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 sTPR) 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, [PtsnH] 0 and H + . A similar platinum species, x[Pt 0 snH], 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 sH]/ZO - C n H 2n+1 + (species 1) and [Pt 0 sH]/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. 1. Introduction The isomerization of alkanes to isoalkanes is a fuel-upgrading technology of outstanding importance. 1-4 In thermodynamic equilibrium, branched alkanes are favored components at lower temperatures. Strong Lewis acids, such as Friedel-Crafts catalysts, initiate the reaction in the liquid phase at relatively low temperature. However, mainly as a result of the extremely corrosive nature and poor stability of the catalyst, the Friedel-Crafts process has been replaced by heterogeneous catalytic technology. 3,4 Over Pt/alumina, alkane hydroisomer- ization proceeds at temperatures as high as 723-773 K. In 1957, Weisz and Swegler 5 suggested a mechanism for this reaction, which is often referred to as the “classical” mechanism of bifunctional hydroisomerization. Accordingly, (i) the reactant alkane is first dehydrogenated on Pt sites, giving alkene and molecular hydrogen; (ii) then, the alkene is protonated on an acid site; (iii) the obtained alkyl carbenium ion transforms into an isoalkyl carbenium ion; and (iv) the catalyst releases isoalkene, which is then (v) hydrogenated to product isoalkane on Pt site. In steps iv and v, the active acid and metal sites are regenerated. The kinetic equation of the reaction was derived by assuming that the alkane/alkene/H 2 system rapidly equili- brates in the presence of Pt, H 2 , and acid sites and that isomerization of the alkyl carbenium ion governs the reaction rate. For most reaction systems, the obtained equation properly describes the observed kinetics. 6,7 As a significant development of the past decade, it was found that the reaction temperature can be decreased to about 473-523 K by using a catalyst wherein the active metal component, platinum, is supported on a strong solid Brønsted acid, such as chlorinated alumina, sulfated metal oxide, or zeolite. 1-3,8 A high isomerization activity and selectivity at reasonably low reaction temperature, tolerance to sulfur and water in the alkane feed, and a long lifetime have made the zeolite-supported Pt catalysts very attractive for the petrochemical industry. Although attempts have been made to interpret results of alkane hydroconversion over Pt/(strong solid acid) catalyst by the classical mechanism, 9-12 some observations do not comply with that mechanistic picture. For instance, H-zeolites and sulfated zirconia were found to have isomerization activity even without Pt present. 2,13-15 Moreover, the possible role of alkenes as reaction intermediates was strongly questioned, considering that the equilibrium alkene concentration was very low at the relatively low temperatures where these catalysts were still active. 2,6,7,9 The reaction order in H 2 was found to depend on the nature of the support. In accordance with the classical mechanism, the H 2 reaction order was negative in the reaction over Pt/zeolite, but unexpectedly, it was positive over Pt/sulfated zirconia. 16,17 Although different mechanisms can satisfy a single kinetics, the kinetic variance of the reaction suggests that the classical mechanism cannot be the only operative mechanism. * To whom correspondence should be addressed. E-mail: lonyi@ chemres.hu. Phone: +361-438-1162. J. Phys. Chem. C 2009, 113, 10527–10540 10527 10.1021/jp810716f CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009