Density functional theory is used to determine transition states, adsorption, and dissociative complexes of Brpnsted-acid-activated methanol. The respective activation barriers and adsorption and desorption energies for the reactions of hydrogen exchange and dehydration of methanol are presented. The activation barriers were found to be 11 and 212 kJ/mol for hydrogen exchange and dehydration, respectively. The methoxonium ion intermediate of the hydrogen exchange reaction was found to be a transition state corresponding to a maximum in the potential energy surface, rather than a chemisorbed species. The dehydration reaction forms a methoxy group that is a methyl group surface-bonded to the basic oxygen lattice. An analysis of the equilibrium constants shows that for both reactions methanol will adsorb initially with the hydroxyl group directed to the basic oxygen of the zeolite cluster model, perpendicular to the zeolitic surface (end-on). The dehydration reaction proceeds via a fast equilibration between this first mode of adsorption (end-on) and an adsorption mode where now the methyl group is directed to the basic oxygen of the zeolite cluster, parallel to the zeolite surface (side-on). From the calculated activation barrier and vibrational, rotational, and translational partition functions, reaction rate constants have been evaluated using transition state reaction rate theory.

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Density functional theory is used to determine transition states and the corresponding energy barriers of the reactions related to C-H bond activation of hydrogen exchange and dehydrogenation of ethane catalyzed by a protonated zeolite as well as hydride transfer between methanol and a methoxide (CH 3 -zeolite) species. Additionally the C-C bond activation involved in the acid catalyzed cracking reaction of ethane was investigated. The computed activation barriers are 118 for hydrogen exchange, 202 for hydride transfer, 292 for cracking and finally 297 for dehydrogenation, all in kilojoules per mole. For the cracking reaction, two different transition states with the same activation barrier have been obtained, dependent on the approach of the ethane molecule to the zeolite cluster. A study of the relation between acidity and the structure of the zeolite shows that the transition state for the hydrogen exchange reaction is rather covalent and its geometry resembles the well-known carbonium ion, while the others are rather ionic carbenium ions. From the calculated activation barriers as well as vibrational, rotational, and translational partition functions, reaction rate constants have been evaluated by means of the transition state reaction rate theory.

Density functional theory is used to study the zeolite acid catalyzed methanol dehydration to dimethyl ether. Three different reaction pathways are proposed. In the first, methanol adsorption and surface methoxy species formation are the initial elementary steps for this reaction. Subsequent dimethyl ether formation by reaction of a new methanol molecule with the surface methoxy species takes place. The second path involves the simultaneous adsorption and activation of two methanol molecules with formation of dimethyl ether and water in one step. The third path involves also the simultaneous adsorption and activation of two methanol molecules. The difference is that, like in the first path, initially a methoxy surface species will be formed from dehydration of one of the methanol molecules, and this will be followed by dimethyl ether formation. The second path appears to be the preferred route for dimethyl ether formation, since its activation barrier is lower than the other two paths. The effect of making the zeolitic cluster slightly more acidic (by lengthening the Si−H bond distances) over the activation barriers of dimethyl ether formation has been studied. Changes on the order of 5 kJ/mol are observed. An analysis of the reaction rate constants for the three reaction paths of methanol dehydration is also presented.