Research On Coking Behavior Over Catalysts For Toluene Disproportionation And Modification Of Surface

PX is widely used as raw materials of synthesis of PTA and polyester. With the demandof PTA and polyester, p-xylene is demanded eagerly. Toluene disproportionation becomes aneffective way to increase the yield of benzene and xylene. In process of toluenedisproportionation reaction, carbon deposition over catalysts is an important factor resulting incatalyst deactivation. Therefore research on coking over catalysts and its applications havebecome one of the most important topics.Behavior of carbon deposition over catalysts surface in toluene disproportionation isstudied by using the method of TG. In the toluene disproportionation, amount of coking overcatalysts increases with both pressure of toluene and reaction temperature, and the initialcoking temperature decreases with partial pressure of toluene. Coking over catalysts resultsmainly from the relatively stronger acid sites on the surface of the catalysts. Compared to HMcatalyst, HZSM-5catalyst has higher catalytic activity and using stability in toluenedisproportionation.Tetrabutyl titanate, ethyl orthosilicate and dietyl phosphite are used for depositingmodification of chemical vapor-phase(CVD) and chemical liquid-phase(CLD) on HZSM-5catalyst. The catalyst samples are characterized by XRD, TPD, IR and SEM, respectively.Properties and coking resistance of modified catalysts are investigated on details.Densities of weaker acid sites over HZSM-5deposited with oxides containing Ti, Si andP in chemical vapor coating change little and the density of stronger acid sites decreases, inwhich acid intensity on HZSM-5zeolite deposited with oxides containing P enhancesobviously. HZSM-5catalyst deposited with oxides containing Si and P can greatly improveboth catalytic activity and coking resistance on catalyst. Toluene conversion increases from8.9%to11.8%, and coking rate decreases from4.7%to2.4%on HZSM-5coating P oxide inCVD.Densities of weaker acid sites over HZSM-5deposited with oxides containing Ti, Si andP in CLD change obviously. Both aicd strength and acid desity on HZSM-5coating Timodified in CLD decrease resulting in low catalytic acitivity. Both strength of acid andcatalytic acitivity on HZSM-5coating P oxide of CLD increase and coking reisistancedecreases greatly. Acid desity of HZSM-5coating Si oxide of CLD decreases obviously, however the acid strength changes little. So catalytic acitity and coking resistance are relatedto acid denstiy of catalysts, but mostly depend on acid strength of catalysts. Over HZSM-5catalyst coating P oxide of CLD, toluene conversion increases from8.9%to24.9%and cokingrate decreases from4.7%to4.1%.Effects of content of γ-Al2O3supporter in HZSM-5catalyst on toluene disproportionationare evaluated. Dilution effect of supporter in HZSM-5/γ-Al2O3always excess dispersion effectdue to its larger specific surface on active component HZSM-5. So HZSM-5specific surfacedecreases with the amount of supporter resultimg in catalyst activity decline linearly fortoluene disproportionation. Supporter also affects coking resistance on catalyst for toluenedisproportionation to some degree. In consideration of catalytic activity and coking resistance,the best loading content of γ-Al2O3is50%.CVD and CLD technique above can be effectively applied to surface modification ofindustrial catalyst HAT-097for toluene disproportionation, but catalytic activity and cokingresistance of HAT-097for toluene disproportionation have small promotion due to its maturemodification and catalytic activity are close to the optimum.