The mechanisms of ethene methylation with methanol and dimethyl ether have been investigated using a 128T cluster of ZSM-5 zeolite modeled by ONIOM(B3LYP/6-31G(d,p):UFF) and ONIOM(M06-2X/6-311+G(2df,2p):UFF) calculations. The effects of the infinite zeolitic framework on the model of the zeolite nanopocket, which consisted of a quantum cluster of 34 tetrahedral units and of 128 tetrahedral units modeled on the UFF level, were also included. The zeolitic Madelung potential was reproduced by a set of point charges generated by the SCREEP method. The energies for the adsorption of methanol and dimethyl ether on H-ZSM-5 from an ONIOM2(M06-2X/6-311+G(2df,2p):UFF)+SCREEP calculation are -26.3 and -29.4 kcal/mol, respectively, which are in good agreement with the experimental data. Dissociative and associative mechanisms of the ethene methylation by methanol and dimethyl ether have been considered. For the dissociative mechanism, the methylation reaction of ethene starts with the protonation of methanol or dimethyl ether by the acidic zeolite proton to form a surface methoxide intermediate, which subsequently reacts with an ethene molecule forming a propoxide intermediate. The propoxide intermediate is then deprotonated to form the propene product. The activation energies for the first step are computed to be 41.2 and 42.9 kcal/ mol for methanol and dimethyl ether, respectively. The activation energies for the subsequent second and third reaction steps are 21.4 and 26.5 kcal/mol, respectively. For the associative mechanism, protonation and methylation take place simultaneously without formation of a surface methoxide. The calculated activation barriers are 29.0 and 33.0 kcal/mol for methanol and dimethyl ether, respectively, suggesting that methanol should be slightly more reactive than dimethyl ether for the methylation of ethene. The final step in the associative mechanism, the deprotonation of the propoxide intermediate to give the adsorbed propene product, has an activation energy of 25.4 kcal/mol. The results indicate that the associative pathway is favored over the dissociative one and that the rate-determining step of this reaction is the ethene methylation step.

The initial stage of glycerol conversion over H-ZSM-5 zeolite has been investigated using density functional theory (DFT) calculations on an embedded cluster model consisting of 128 tetrahedrally coordinated atoms. It is found that glycerol dehydration to acrolein and acetol proceeds favourably via a stepwise mechanism. The formation of an alkoxide species upon the first dehydration requires the highest activation energy (42.5 kcal mol(-1)) and can be considered as the rate determining step of the reaction. The intrinsic activation energies for the first dehydration are virtually the same for both acrolein and acetol formation, respectively, suggesting the competitive removal of the primary and secondary OH groups. A high selectivity to acrolein at moderate temperatures can be attributed to the selective activation of the stronger adsorption mode of glycerol through the secondary OH group and the kinetically favoured subsequent consecutive steps. In addition, the less reactive nature of acrolein relative to acetol precludes it from being converted to other products upon conversion to glycerol. In accordance with typical endothermic reactions, the forward rate constant for glycerol dehydration significantly increases with increasing reaction temperature.

The mechanism of alkene oxidation with hydrogen peroxide over the titanium silicalite-1 (TS-1) defect is investigated using a 65T nanocluster, TiSi64O97H74, and calculated at the 9T/65T two-layered ONIOM(B3LYP/6-31G(d,p):UFF) level. The intermediate titanium hydroperoxo in the bidentate form, Ti(η 2 -OOH), occurring through the single-step double proton-transfer mechanism aided by a neighboring silanol group, is proffered as the active species in the oxidation process. It is noted that this species is influenced by the number of water molecules surrounding the active region. The formation of titanium peroxo species, Ti(η 2 -OO-), consistent with the role of water in hydroperoxo-peroxo interconversion in the TS-1/H2O/H2O2 system, results from the step in which an additional water molecule is introduced into the hydrated Ti(η 2 -OOH) complex. The step in which oxygen is abstracted during the epoxide formation is determined to be the reaction rate determining step, and is reactive to a number of methyl groups substituted to the active CC bond of alkene molecules. The evident activation energies for ethylene, propylene, and trans-2-butylene are estimated to be 15.5, 13.6, and 12.2 kcal/mol, respectively. These results agree with the reactivity series of the gas-phase calculations and compare favorably with the known apparent activation energy of 1-hexene of 15.5 ± 1.5 kcal/mol obtained from experiment.