Density functional theory calculations of the transition states for hydrogen exchange and dehydrogenation of methane by a Brönsted zeolitic proton

D. functional and semiempirical (MNDO) theories are used to det. transition states and the corresponding activation barriers of hydrogen exchange and dehydrogenation of methane catalyzed by a protonated zeolite cluster model. The nonlocal d. functional activation barriers were found to be 125 and 343 kJ/mol for hydrogen exchange and dehydrogenation, resp. From the imaginary frequency of one of the transition state Eigen modes, the reaction coordinates were deduced. Addnl., from the activation barrier and vibration, rotation, and translation partition functions, reaction rate consts. have been evaluated using transition state reaction rate theory. [on SciFinder (R)

Density functional theory calculations of the activation of methanol by a Bronsted zeolitic proton

Density functional theory is used to determine transition states, adsorption, and dissociative complexes of Bronsted- 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 theor

Alkylation and transalkylation reactions of aromatics

A symposium. D. functional theory calcns. were carried out to analyze the reaction energy of solid-state acid-catalyzed Me-transfer reactions. Different mechanistic routes for the alkylation of C6H6 and PhMe by MeOH were compared. An associative reaction path via an intermediate complex of MeoH and the substrate is the preferred route. The activation energy is 123 and .apprx.120 kJ/mol for C6H6 and PhMe, resp. A MeO-mediated path involves very high activation barriers compared to the associative route. However, coadsorbed H2O gives a large redn. of the activation energy for this reaction. Different mechanisms for PhMe transalkylation, involving Ph2CH2 as an intermediate, directly via Me transfer, and MeO-mediated, were compared. For the 1st mechanism, the preferred route is that where the reaction chain of elementary reactions is propagated via H- transfer. The rate-detg. step is the initial dehydrogenation, with an activation energy of +277 kJ/mol, which is present only in the very 1st step of the reaction chain. In the following steps, the initial dehydrogenation is replaced by proton-assisted cracking of Ph2CH2 as the step with the highest activation barrier. The direct mechanisms via Me transfer or via intermediate MeO do present activation barriers that are lower than the dehydrogenation step but higher than via Ph2CH2/H- transfer-mediated reaction. For small-pore zeolites, where large mols. like Ph2CH2 cannot be formed, they should be considered as optional routes for the transalkylation reactio

Density functional theory calculations of the activation of methanol by a Bronsted zeolitic proton

Density functional theory is used to determine transition states, adsorption, and dissociative complexes of Bronsted- 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 theor

Theoretical study of C-C bond formation in the methanol to gasoline process

Density functional theory is used to study one of the most successful routes to the production of synthetic fuels, the conversion of methanol to gasoline (MTG process) with an acidic zeolite. With our calculations we have determined transition states and adsorption complexes of reactants, intermediates, and products as well as the corresponding activation barriers and adsorption energies of the numerous reactions involved in such a process. Bronsted acid catalyzed methanol dehydration to dimethyl ether is the first step of the MTG process. Two different mechanisms are possible. One proceeds via an associative interaction between two methanol molecules, generating directly dimethyl ether, while the other proceeds via a methoxy surface species intermediate. The presence of water lowers the activation barrier of the last mechanism by more than 50 kJ/mol. Our calculations suggest that ethanol and ethyl methyl ether are the first formed species with a C-C bond. Several different mechanisms for those reactions have been studied. The activation barriers involved in such reactions are of the order of 300 kJ/mol for both ethanol and ethyl methyl ether. Without coadsorbed water, the activation barriers are 60 kJ/mol higher. In a following step ethylene is formed from alcohol or ether. Those reactions are very fast due to a very low activation barrier. Trimethyloxonium, proposed to be an intermediate in the formation of ethyl methyl ether, can be excluded as an intermediate for the C-C bond formation. Although it can be formed, its further reaction to ethanol or ethyl methyl ether involves activation barriers that are over 80-150 kJ/mol higher than their formation directly from dimethyl ether and methanol. Reaction paths for the formation of methane and formaldehyde, which are observed in reactions for very low methanol coverages, have also been studie

Theoretical study of the mechanism of surface methoxy and dimethyl ether formation from methanol catalyzed by zeolitic protons

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 presente

Activation of C-H and C-C bonds by an acidic zeolite : a density functional study

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 (CH3- 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 theor