Mechanisms of Hydrogen Exchange of Methane with H-Zeolite Y: An ab Initio Embedded Cluster Study James M. Vollmer and Thanh N. Truong* Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, UniVersity of Utah, 315 S 1400 E Rm. 2020, Salt Lake City, Utah 84112 ReceiVed: March 3, 2000; In Final Form: April 21, 2000 We present ab initio embedded cluster studies on the mechanism of hydrogen exchange of methane with H-Zeolite Y. We found that inclusion of the Madelung field stabilizes the formation of a carbonium-like transition state, and consequently reduces the reaction barrier by 17-23 kJ/mol, relative to the corresponding bare cluster predictions. Using the CCSD(T)/6-31G(d,p) level of theory, including zero-point energy (∼10 kJ/mol) and tunneling (1.6 kJ/mol) corrections, the activation energy is predicted to be 124 ( 5 and 137 ( 5 kJ/mol for hydrogen exchange from two different binding sites. These predictions agree well with the experimental estimate of 122-130 kJ/mol. We also found that it is necessary to include the Madelung potential to find preferrential proton siting at site O1 versus site O4, in agreement with experimental observation. Introduction Zeolites are important technological materials due to their many applications. Some important catalytic applications include catalysts for petroleum refining, synfuel production, and petro- chemical production. 1-4 The Brønsted acid site has been established as the primary active site for zeolite catalysis; however, little is still known of the mechanistic details for reactions occurring at these sites. For instance, in zeolite catalyzed cracking, isomerization, and alkylation of hydrocar- bons, most proposed mechanisms involve proton transfers from the Brønsted site to the hydrocarbon adsorbate to form car- bonium (pentacoordinated carbocations) or carbenium ions (tri- coordinated carbocations) as reactive intermediates; 5-9 however, there is still little empirical confirmation of the nature of these intermediates. Hydrogen/deuterium exchange of methane with the Brønsted proton of zeolites has been used as a prototypical reaction of hydrocarbons with zeolites. From a technological point of view, this is an important step in the catalytic conversion of methane to desired products or liquid fuels which is currently one of the greatest challenges in catalysis science. From a fundamental point of view, this reaction is among the simplest elementary processes that can be studied experimentally and theoretically to provide understanding for interactions of hydrocarbons with zeolites at the molecular level. Even for this simple reaction, its mechanism is not yet fully understood, despite numerous studies. The original experimental and theoretical work for this reaction were reported by Kramer et al. in Nature in 1993. 10 By measuring the evolution of the infrared absorption spectrum, the authors were able to extract the isotope exchange rates between CD 4 and the Brønsted proton of H-Zeolite Y and H-ZSM-5 in the temperature range between 620 and 750 K. Their theoretical results from ab initio quantum cluster calcula- tions at the Hartree-Fock (HF) level using a tri-tetrahedral (3T) H 3 Si-OH-Al(OH) 2 -O-SiH 3 cluster did not support the existence of the carbonium-like structure either as the transition state or the intermediate. In fact, the authors reported a transition state structure resembling two free H atoms lying between a CH 3 radical and the zeolite framework. Later theoretical studies done by Evleth et al., 11 using a 1T (H 2 OAlH 2 OH) cluster at the HF/6-31G* level of theory, predicted a transition state, whose structure and charge distribution were indicative of CH 3 - sH 2 Z + species (where Z represents the zeolite cluster). Theoretical studies done by Blaskowski et al., 12 using a 3T (H 3 Si-OH- Al(OH) 2 -O-SiH 3 ) cluster and various local and nonlocal density functional methods (DFT), with the double  plus polarization (DZPV) basis set, and by Truong, 13 using a 3T cluster and a nonlocal hybrid DFT method, both predict transition states similar to Kramer et al. Consequently, to date the nature of the transition state for this reaction is still unclear. All models agree that the transition state geometry resembles a CH 3 fragment, loosely bound to the transferring protons, which are bound to the zeolite framework to some degree. There is, however, still some dispute on the net charge of the CH 3 fragment, as well as the proximity of the exchanging protons to the zeolite framework. It is not surprising that stable carbonium ion intermediates were not found in these studies, since it is common knowledge that carbocation stability is directly related to the number of alkyl substituents. With no substituents to stabilize it, the CH 5 + ion is very unstable, and therefore not readily observable. What is surprising is that none of these predictions support a “carbonium ion-like” structure or charge distribution for the transition state, as one would expect based on the typical proton- transfer mechanisms applied to zeolite catalysis. It has been noted that the degree of ionicity of the transition state-lattice interaction is an important parameter, since the more ionic the transition state, the smaller the correlation between activation energy and the differences in proton affinity between the oxygen atom that is deprotonated and the oxygen atom that becomes protonated after the reaction. 4 The implication of this is that the activity differences Kramer et al. simulated for different zeolites, based solely on proton affinity differences between the various active sites, could be perturbed significantly if a more ionic transition state was found. * Corresponding author. E-mail: Truong@chemistry.utah.edu. 6308 J. Phys. Chem. B 2000, 104, 6308-6312 10.1021/jp0008445 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000