The Quantum Chemistry Study Of The Methanol Dehydration To Dimethyl Ether On The HZSM-5Zeolite

In recent years, looking for alternative energy resources is the universal attention topic all over theworld, and the awareness of people for protecting environment is becoming deeper and deeper, so dimethylether (DME) is considered as a very promising alternative energy resources thanks to its betterenvironmentally-friendly and combustion performances as well as properties similar to traditional fuels.Now the main industry technology process for the production of DME is the methanol dehydration ingaseous phase to DME over solid acid catalysts. ZSM-5zeolite has very good catalytic performance forthe dehydration of methanol in the gas phase to DME, because of its high catalytic activity at lowtemperature, resistance-water stability and ability of resistance to carbon deposition. However, the currentmethods are not enough for researching the structure and acid of ZSM-5zeolite in detail, and the reactionmechanism of the methanol molecular dehydration to DME is still in debated. So in this paper, thequantum chemistry calculation method is used to research the structure and acidity of ZSM-5zeolite indetail as well as the methanol adsorption complexes and dehydration reaction mechanism on ZSM-5zeolite, which has very important significance for the researches on the catalysts of methanol dehydrationto DME. The main research contents and results are showed as follows:1. Using density functional theory (DFT) and B3LYP functions to calculate the structure parametersand the acidity of the cluster models of ZSM-5zeolite, investigating the size of cluster models, the size ofbasis sets and different terminal atoms of cluster models which have influence on the structure parametersand the acidity of ZSM-5zeolite. The structure parameters and Mulliken charge distribution of clustermodels of ZSM-5zeolite can be gained by the structures optimization calculations for the cluster models of ZSM-5zeolite, and the deprotonation energies can be obtained by the optimization calculations for thecluster models with the except of the protons on the active sites, which can be used to investigate therelation between the structures of zeolite cluster models and their acidity. The calculation results show: thesize of cluster models of ZSM-5zeolite has little impact on the acidity of the active sites of ZSM-5zeolite,but has a little influence on the structure parameters of cluster models of ZSM-5zeolite; the size of basissets has a certain effect in the acidity for active sites of ZSM-5zeolite, and the greater basis sets are, theacidity of the active sites on cluster models is lower; different terminal atoms of cluster models have someinfluence on the acidity of active sites on cluster models, and the acidity of the zeolite cluster modelssaturated using hydrogen atom is stronger than that of the cluster models saturated using hydroxl atoms.2. Using DFT and MP2calculation methods to study the adsorption complexes and stability ofmethanol molecule on HZSM-5zeolite and their stability, investigating the influence of the size of clustermodels on the adsorption complexes of methanol molecule and their stability. According to the structureoptimization calculation for the adsorption complexes of the methanol molecule on the cluster models, thestructural parameters and the adsorption energies of the methanol adsorption complexes can be obtained.The calculation results show: there are two kinds of the methanol adsorption complexes on ZSM-5zeolite,end-on mode and side-on mode. The adsorption energies of the end-on and side-on adsorption complexesare-80kJ/mol and-59kJ/mol, respectively. According to the adsorption energies, the methanol end-onadsorption complexes is more stable than the methanol side-on adsorption complexes. The end-onadsorption complexes could be conversed into the side-on adsorption complexes through a transition state,and the side-on adsorption complexes is more advantageous to the methanol dehydration reaction. The sizeof cluster models of HZSM-5zeolite has no impact on the two adsorption modes of methanol molecules. The result obtained using MP2method to calculate the methanol adsorption complexes and their stabilityis consistent with that obtained using DFT method.3. On the basis of the theoretical calculations for the methanol molecule adsorption, use DFT methodat B3LYP/6-31G(d, p) level to study the microcosmic interaction mechanism of methanol dehydration toDME on the3T cluster model of ZSM-5zeolite, and there are two kinds of reaction mechanisms of themethanol dehydration to DME: Rideal-Eley (R-E) mechanism and Langmuir-Hinshelwood (L-H)mechanism. According to the whole optimization for the stagnation points in the reactions, the calculationsfor the activation energies in the reactions, and the intrinsic reaction coordinate (IRC) calculation in orderto confirm the transition states responsible, the reasonable reaction mechanism can be decided. Thecalculation results show: In the R-E mechanism, the reaction proceeds via two distinct steps. Firstly, onemethanol molecule in the side-on adsorption mode dehydrate, leaving a methyl group attached to the basicoxygen of the zeolite, the activation energy for this step is203kJ/mol. Then the second methanol moleculeadsorbs on the cluster model with the surface methoxy species, to generate DME by the reaction of thesecond methanol molecule with the surface methoxy species, and the activation energy for the second stepis162kJ/mol. According to the activation energies, the rate-limiting step for DME formation in the R-Emechanism is the dehydration of the first methanol molecule. In the L-H mechanism, the two methanolmolecules meanwhile adsorb on the cluster model of HZSM-5zeolite and are activated at the same time, togenerate DME and water molecule in one step, the activation energy for the L-H mechanism is121kJ/mol.According to the activation energies in the R-E mechanism and the L-H mechanism, we consider that theL-H mechanism is more advantageous to the methanol dehydration to DME reaction.