Preparation Of Dichlorobenzene By Isomerization Of Dichlorobenzene Catalyzed By Solid Acid

In this paper, the catalytic performance of supported phosphotungstic acid and various zeolites in p-dichlorobenzene isomerization reaction were evaluated. The results showed that nanoscale ZSM-5 and ZSM-12 zeolites exhibited higher activity and selectivity for m-dichlorobenzene. Therefore, modified ZSM-12 zeolite and ZSM-5 zeolite were prepared by acid treatment, alkali treatment, ion exchange and isomorphous replacement, respectively. The structural and acidic properties of the modified zeolites were characterized by XRD, SEM, XRF, N2 adsorption, NH3-TPD and Py-IR. The catalytic performance was evaluated in the p-dichlorobenzene isomerization reaction on the fixed bed micro-reaction apparatus over the modified zeolites. It has been seen that the ZSM-12 zeolite modified by sodium hydroxide exhibited higher conversion of p-chlorobenzene and selectivity for m-dichlorobenzene due to the increased acid amount and mesorpore volume. The modified ZSM-12 zeolite treated with 0.8 mol/L NaOH solution at 45℃ for 0.5 h exhibited best catalytic performance, the p-chlorobenzene conversion of 20.1% and selectivity of 95.5% for m-dichlorobenzene were obtained at the reaction time of 5 h, respectively.The ZSM-5 zeolite was modified by citric acid and sodium hydroxide, respectively. The results showed that samples after modification possessed more defects and the strength of strong acid sites reduced, while the surface acidic density increased, which lead to the conversion of p-chlorobenzene and the selectivity for m-dichlorobenzene increased. The strength of strong acid sites of nanosized ZSM-5 zeolites modified by ion exchange and isomorphous replacement reduced and the amount of acid sites decreased in different degrees. Therefore, the conversion of p-chlorobenzene decreased and the selectivity for m-dichlorobenzene increased. For the nanoscale ZSM-5 modified by mixed alkali treatment, the strong acid sites had no obvious change, but the total acid content increased, the second mesoporous was generated. As a result, the conversion of p-dichlorobenzene and the selectivity for m-dichlorobenzene were increased, and the deactivation rate of the modified nanosized ZSM-5 zeolites reduced. NZ5-0.4HAT sample showed the best catalytic performance. The p-chlorobenzene conversion of 53.1% and the selectivity of 90.8% for m-dichlorobenzene over the NZ5-0.4HAT sample was obtained at reaction time of 5 h. The lifetime of catalysts before and after modification were investigated. The results showed that the deactivation rate of nanosized ZSM-5 zeolite modified by mixed alkali decreased compared with the unmodified sample. After reaction of 30 h, the modified nanosized ZSM-5 zeolite still kept the high conversion of p-dichlorobenzene and the selectivity for m-dichlorobenzene. Moreover, the modified sample had less carbon deposition and activation energy of burning charcoal, therefore, the carbon deposition of modified sample was easier to remove.