Theoretical Study On The Vibrational Spectrum And Reaction Mechanism Of Isomerization Of Chlorotoluene

Chlorotoluene are important chemical intermediates which are widely used for the production of pharmaceuticals and dyes.Among three isomers, p-chlorotoluene (PCT) has the highest value and the most market demand which usually can be produced by isomerization of m-chlorotoluene (MCT) catalyzed by acidic zeolite. Several reactions were investigated by using density functional theory (DFT) with cluster model in this paper, including the molecular structure and vibrational spectra of three isomers of chlorotoluene, the adsorption structure and proportions of o-chlorotoluene (OCT) in acid zeolite and zeolite by Ag+ exchanged,and the reaction mechanism of isomerization of OCT to MCT and PCT. The main and important conclusions are summarized as follows.The molecular structure and vibrational spectra of three isomers of chlorotoluene were studied at HF and B3LYP level with 6-311++G(d,p) basis set. According to the results, the optimized geometry by two methods had basically the same results, while the B3LYP method is superior to the HF method in calculated frequencies and good agreements between the experimentally recorded and the theoretically calculated vibration frequencies at B3LYP level via the application of scaling factors.The adsorption properties of OCT in 10-ring-pore structure, zeolite 3T and 5T cluster models have been investigate at ONIOM(B3LYP/6-31G(d,p):UFF) and B3LYP/6-31G(d) level respectively while Ag atom were presented by LANL2DZ effective core potential and its valence electrons were described by LANL2DZ basis set. It is found that the structure of HZSM-5 adsorbed OCT is mainly that benzene ring is interaction with H+ and forms aπ-H-complex. While the structure of AgZSM-5 adsorbed OCT is mainly that Cl atom is interaction with Ag atom and forms a Cl-Ag-complex. The three geometry structures of AgZSM-5 and HZSM-5 adsorbed OCT are similar respectively. We also found that the adsorption energy of OCT to the zeolite calculated by the ONIOM method is lower than other methods, and the adsorption structure of AgZSM-5 adsorbed OCT is more stable than its of HZSM-5 adsorbed OCT.At the B3LYP/6-3G(d) level, using 5T cluster, the methyl 1,2-shift reaction mechanism of isomerization of OCT to MCT and PCT by acid zeolite was studied. It was found that the process include three steps. The first step is acidic site of zeolite adsorbs the benzene ring of OCT and forms aπ-complex, then the proton of zeolite acid site attack the C atom of benzene ring neighboring methyl. The second step is that methyl occurs 1,2-shift and formed transition states. The last step is the proton on the benzene dissociated and return to zeolite then form HZSM-5 and isomers. The key rate process is methyl 1,2-shift and the activation barriers are 237.9KJ/mol and 235.9 KJ/mol which show a good agreement with experimental values.