Tuning The Framework Aluminum Distribution And Pore Structure Of ZSM-5zeolite

ZSM-5 zeolite with uniform and ordered micropores,adjustable acidity,and high thermal and hydrothermal stability has become an important shape-selective catalyst in the petrochemical industry.Acid property and pore structure are the two crucial factors affecting the catalytic performance of ZSM-5 zeolite-based catalysts.On the one hand,the distribution of Br（?）nsted（B）acid sites,which is dependent on the distribution of framework Al atoms,affects the reaction path and thereby changes the distribution of the reaction products;on the other hand,creating hierarchical pore structure in ZSM-5zeolites can alleviate the steric hindrance and diffusion limit in reactions involving bulky molecules.In this thesis,tunning of the framework aluminum distribution and hierarchical pore structure of ZSM-5 zeolite is achieved by utilizing special Si and Al sources and regulating the dosage and types of organotemplates.In the first part of this thesis,we present an organotemplate-free approach for the controllable synthesis of ZSM-5 zeolite with a high content of Al pairs selectively located at the channel intersections,by using a submolten salt（SMS）depolymerized rectorite as both the Al source and heteropical crystalline seeds.The characterizations reveal that the oligomeric aluminosilicate species like 5-and 6-membered ring structures originally existing in the SMS rectorite serve as crystalline seeds to promote zeolite nucleation;simultaneously,the Al-O-（Si-O）_1,2-Al sequences contained in the oligomeric aluminosilicate species are in-situ transformed into the final zeolite framework,leading to the generation of abundant Al pairs selectively located at the channel intersections.Such an unusual Al distribution endows the synthesized ZSM-5 zeolite with excellent selectivity to aromatics in the methanol to aromatics（MTA）reaction.In the second part of this thesis,we report the synthesis of hierarchical ZSM-5zeolites with secondary inter-crystalline pores from the SMS depolymerized rectorite and without using any mesoporous template.The influence of the synthetic parameters on the texture properties of the resulting products was comprehensively investigated and the results reveal that:the key to the successful synthesis of hierarchical ZSM-5 zeolites with secondary inter-crystalline pores is the suitable addition of tetrapropylammonium bromide（TPABr）and the adequate aging of the gel precursor.Both of them facilitate the generation of zeolitic nuclei together with the SMS depolymerized rectorite;and the secondary pores are formed due to the agglomeration of nano-rod crystallites grown from these nuclei.The results of the n-octane hydroisomerization reaction show that increasing the content of secondary pores and external specific surface area is beneficial to improving the selectivity to C8 isomers and reducing the selectivity to cracking products.With the aim of further increasing the content of secondary pores in hierarchical ZSM-5 zeolites,the third part of this thesis proposes a novel approach to synthesize the hierarchical ZSM-5 zeolites with secondary intra-crystalline pores.This approach is achieved by using a phenol-formaldehyde resin（PFR）as the mesoporous template that is featured by its remarkable water-solubility,thermoset property and no NOx and halogen emmisions when being removed from the resulting products by calcination.The synthesized hierarchical ZSM-5 zeolite possesses not only higher BET specific surface area,total pore volume,external specific surface area and mesopore volume,which are0.23,1,1.3 and 4.2 times higher than those of the commercial ZSM-5 zeolite,respectively,but also excellent hydrothermal stability and less strong B acid sites.Thanks to its rich secondary intra-crystalline pores and reduced strong B acid sites,the hierarchical ZSM-5supported catalyst exhibits outstanding selectivities to mono-and di-branched C8 isomers and lower selectivities to cracking products in n-octane hydroisomerization reaction.