Soft Template Assisted Synthesis Of Hierarchical ZSM-5 Zeolite With Different Morphology

ZSM-5 is crystalline aluminosilicate,which has excellent physicochemical properties such as good shape selectivity,adjustable acidity and high thermal/hydrothermal stability and,therefore,has wide application in ion-exchange,gas separation,adsorption and especially in heterogeneous catalysis.It should be noted that ZSM-5 is microporous zeolite,which is unfavorable to the diffusion of bulky-molecule product.Besides,it is easy to deactivate caused by coke deposition.Hierarchical ZSM-5 which possesses micropores and mesopores/macropores simultaneously can greatly solve the problem.Accordingly,the main purpose of this study is to fabricate hierarchical ZSM-5 with the assistance of soft template and also study their catalytic properties in methanol to gasoline?MTG?reaction.Firstly,hierarchical ZSM-5 has been synthesized using conventional tetrapropylammonium hydroxide as organic structure directing agent?OSDA?and anionic polyacrylamide?APAM?as mesoporogen.The effect of APAM amount on the properties of different samples was studied in this section.Results show that all samples demonstrate the morphology of aggregates consisted of nanocrystals.Sample synthesized without APAM only has inter-crystalline mesopores formed by stacked nanocrystals,and the presence of APAM make the samples possess inter-and intra-crystalline mesopores simultaneously.When applied in MTG reaction,the desired C₅₊gasoline yield is similar with the sample synthesized without APAM,while the catalytic lifetime gets greatly prolonged with increasing the amount of APAM.Secondly,we use silicalite-1?S-1?as seed and amphiphilic organosilane dimethyloctadecyl[3-?trimethoxysilyl?propyl]ammonium chloride as mesoporogen to synthesize hierarchical ZSM-5.The effects of key synthesis parameters?TPOAC/SiO₂molar ratio,S-1 seed content?on the final samples were systematically studied.Results show that when the S-1 amount is 2%?the SiO₂ in S-1 seed suspension is 2%relative to the total SiO₂?,the morphology of samples change gradually with increasing the amount of TPOAC.When TPOAC/SiO₂=0.065,the sample demonstrates the morphology of multilamellar ZSM-5 nanosheets.When applied in MTG reaction,the sample's catalytic lifetime prolongs to 36 h with the similar yield of C₅₊compared with sample synthesized without TPOAC.When changing the amount of S-1 seed?relative to the total silica content?:the sample demonstrates the irregular cuboid consisted of nanocrystals with relative less addition of S-1 seed?0.5%?;the morphology of the samples are multilamellar ZSM-5 nanosheets when S-1 seed amount varies between 2%⁵%.Finally,the crystallization mechanism of multilamellar ZSM-5 nanosheet is mainly studied.The characterization results of samples crystallized for different time have been specially discussed and the probable crystallization mechanism was revealed:firstly form MCM-41-like materials and then transform to ZSM-5 zeolite.