Study On Synthesis Of Hierarchical HZSM-5 And Its Performance In Pyridine Bases Preparation Of Aldehyde-ammonia Condensation

Pyridine bases and their derivatives,as one of the most important chemical materials,are widely used in pharmaceutical,pesticide,fuel,synthetic rubber.Nowadays,pyridine bases are mainly produced by chemical technology of Chichibabin reaction in industry,and the catalyst in this technology is HZSM-5 zeolite molecular sieve.However,the catalyst used in this technology can deactivate easily due to coke deposition,causing an in-negligible loss in industry.Therefore,it becomes more and more important to find some effective ways to modify the HZSM-5 for the purpose of improving its activity and stability.In this paper,a series of hierarchical HZSM-5 zeolites were synthesized using hydrothermal method with four different soft mesoporous templates(CTAB?PTEOS?DDAC and PDD-AM)respectively.The results have shown that the hierarchical HZSM-5 sample synthesized with DDAC not only possesses connected micro-mesopores,relatively larger surface area and mesoporous volume,which combine the good shape selectivity and nice mass transfer,but also possesses a relatively larger amount of total acidic sites as well as the largest amount of strong acidic sites,which could provide enough active sites for the reaction.These good physicochemical properties of this catalyst finally enable the highest activity in the condensation of aldehyde and ammonia.Then,another group of hierarchical HZSM-5 zeolites with different SiO2/Al2O3(50,100,150,200,250)were synthesized with DDAC.The results have shown that the HZSM-5 sample with SiO2/Al2O3=100 not only possesses relatively higher crystallinity but also the largest surface area,total pore volume and relatively larger amount of acidic sites,so that it has a high yield of pyridine bases in the reaction,which could reach 49%at maximum.At last,conventional microporous HZSM-5(SiO2/Al2O3=100,no mesoporous template)and hierarchical HZSM-5(SiO2/Al2O3=100,DDAC as the mesoporous template)were synthesized and followed by the deposition of ZnO via ALD(atomic layer deposition)technology.The results have shown that,the depositions can cause the change of pore distribution to the catalysts.And,with the same ALD cycle numbers,the Zn content in hierarchical HZSM-5 is larger than that in conventional HZSM-5,which could be attributed to the much more mesopores decreasing the diffusive resistance for precursor DEZ.What's more,the deposited ZnO in hierarchical HZSM-5 was presented in layer-like form while in conventional HZSM-5 was presented in spot-like form.After the reaction,it has shown that sample DDAC&ALD/HZSM-5 not only exhibits the highest activity but also best stability.This could be attributed to its well-preserved mesopores and better distribution of acidic sites after ALD modification.In that case,these two factors may have a cooperative effect in helping forming the target product and reducing the coke deposition.