Theoretical Studies On The Mechanisms Of Aromatics Methylation Over H-ZSM-5

Methylation of aromatics over zeolites can be employed to produce important petrochemical products. With the rapid development of computational chemical softwares and computer harewares, theoretical calculation has become a significant technique for investigating the reaction mechanisms of aromatics methylation over zeolites. Theoretical calculation on the reaction mechanisms can provide a theoretical basis for optimizing zeolite catalyst design. In this study, the density functional theory (DFT) and "our own N-layered integrated molecular orbital+molecular mechanics"(ONIOM) methods were employed to investigate the methylation reactions of benzene, toluene and4-methylbiphenyl (4-MBP) over H-ZSM-5. The reaction mechanisms were clarified. The effects of Bransted (B) acidic strength and temperatures on the reaction mechanisms were discussed. The relationship between catalytic activity of H-ZSM-5for methylation reaction and the electronic properties of aromatics as well as the interaction between aromatics and the zeolite framework were illustrated. The results were summarized as follows:(1) The stepwise and concerted mechanisms of benzene methylation with methanol over H-ZSM-5were studied. The calculated intrinsic activition energies and rate constants illustrate that the methylation of benzene with methanol occurring along the stepwise mechanism is more kinetically preferred in the pores of H-ZSM-5(128T model). As temperature (673-773K) rises, the difference between the stepwise and concerted mechanisms in rate constant becomes smaller. On the external zeolite surface, the increasing B acidic strength (5Tâ†'12T) or the raise of temperature (673-773K) is more beneficial to the stepwise mechanism.(2) The reactions of toluene methylation with dimethyl carbonate (DMC) and methanol in the pores of H-ZSM-5(128T model) were studied. By the analysis of the changes of bond length parameters after DMC adsorbing on the zeolite, the paths of toluene methylation with DMC are proposed. The dominant path is DMC full decomposition to methoxide, methanol and carbon dioxide prior to toluene methylation with methoxide forming xylene isomers. The stepwise path is more kinetically favored than the concerted path in toluene methylation with methanol. Methanol is more active than DMC as the methylation reagent in toluene methylation. Through the structural optimization of methylation transition states, it is found that the formation of a meta-transition state structure is restricted by the H-ZSM-5pores. Meta-xylene (MX) is the main product in xylene isomerization, however, the generated MX is restrained in the pores attributting to the highest desorption energy and further isomerizates to para-xylene (PX) prior to ortho-xylene (OX). With the increasing temperature (573-733K), the extent of increase of rate constant from isomerization of MX to OX is larger comparing to isomerization of MX to PX, the selectivity to PX is therefore decreased.On the external H-ZSM-5surface (12T model), the rate constants for the formation of three xylene isomers in toluene methylation with methanol are similar. The isomerization reactions of xylene occur during the formation of MX. As temperature (573-733K) rises, the relative rate constant of PX isomerization to MX and OX isomerization to MX increases, the selectivity to PX is therefore decreased. Furthermore, with the B acidic strength becoming stronger (5Tâ†'12T), compared to methylation of toluene, the extent of increase of catalytic activity from the zeolite for xylene isomerization is much larger.(3) The stepwise and concerted mechanisms of4-MBP methylation with methanol over H-ZSM-5were studied. The calculated intrinsic activition energies and rate constants illustrate that the stepwise path is more kinetically preferred and can obtain a larger selectivity difference between the formation of3,4’-dimethylbiphenyl (3,4’-DMBP) and4,4’-DMBP in the pores of H-ZSM-5(128T model). Through the structural optimization of methylation transition states, it is found that the3-methylation transition state structure is unstable due to the intramolecular torsional stress of4-MBP. At573-773K, the rate constant of4-MBP stepwise methylation is higher than that of4-MBP isomerization. In conclusion, in the pores of H-ZSM-5,4-MBP and methanol undergos methylation along the stepwise path to form4,4’-DMBP. In the straight channel of H-ZSM-5, the difference between3,4’-DMBP and4,4’-DMBP in diffusion barrier is beneficial to obtain4,4’-DMBP.On the external zeolite surface (12T model), at573-773K, the rate constants of4-MBP isomerization is higher than that of4-MBP methylation. In4-MBP methylation, the intrinsic activation energies and rate constants for the formation of3,4’-DMBP and4,4’-DMBP are similar. With the increasing temperature, the rate constant for the formation of3,4’-DMBP increases faster, therefore decreases4,4’-DMBP selectivity.(4) Through comparing the intrinsic activation energies and rate constants of aromatics methylation reactions, it is proved that H-ZSM-5shows the highest catalytic activity for toluene methylation, followed by4-MBP methylation, the catalytic activity for benzene methylation is the lowest. The catalytic activity of H-ZSM-5for methylation reaction depends on the electronic density of specific position of aromatic carbon as well as the interaction between aromatics and the zeolite framework atoms. The former is the dominant factor.