Synthesis And Characterization Of ZSM-35 Zeolite

ZSM-35 is an important zeolite catalyst which is widely used in isomerization,polymerization,aromatization and cracking reactions.The crystal size and acidity of ZSM-35zeolite have significant effects on the reactivity,selectivity and stability during the reactions.ZSM-35 synthesized through hydrothermal method,however,shows low reactivity and short lifespan during the reactions because of its large crystals and improper distribution of acid sites,especially for cracking and isomerization of large molecules.Small crystals have more outer active sites exposed to the reactants,which increases the conversion of large molecules during the reactions.On the other hand,the short channels within the small crystals are unfavorable for the reactants conversion and could alter the products distribution?selectivity?.Thus,zeolites of various crystal size and acidity should be synthesized and used in different reactions.Effects of different factors during the synthesis process on crystal size and acidity of ZSM-35 zeolite were explored in this study.Synthesis conditions were explored at first and it was found that relatively moderate temperature?170 ^oC?,high alkalinity?pH>14?as well as short crystallizing time?36 h?were favorable for the formation of ZSM-35 with high cystallinity and relatively small crystals.The source of aluminum,compared with other components,has a larger effect on the formation of ZSM-35,with the performance of Na?AlO₂?>C₉H₂₁AlO₃>Al₂?SO₄?₃.Crystallization temperature determins the speed of crystal nucleus formation and the following crystal growth process.Relatively low crystallization temperature can increase the amount of crystal nucleus,which is favorable for the formation of small crystals.High temperature,however,is beneficial for the crystal growth and the formation of large crystals.Alkalinity of the solution can affect the dissociation speed of Si and Al sources,which can ultimately influence the crystalline phase of product.Al ion is always prior to embed into the skeleton of zeolite during the synthesis process in solutions with high alkalinity,which could suppress the insertion of Si into the zeolite skeleton.As a result,the skeleton Si/Al decreases with increasing solution alkalinity,leading to the formation of products with more acid sites.The high alkalinity could also increase the crystal grow and thus the formation of large crystals.The use of organic molecules as templates or structure directing agents?SDA?in zeolite synthesis was introduced.Ethanediamine?EDA?,cyclohexane?CHA?and Pyrrolidine?THP?were used as single SDA separately during the experiments,and the pure ZSM-35 can be obtained in the order of THP>EDA>CHA?easy to difficult?.EDA is used to fill the inner channel and provide NH₄⁺to keep the charge balance of skeleton,while CHA act as the template for the formation of the skeleton and THP go into the channel to support the framework.The combination of these two templates were introduced in this study as multi-template.The use of multi-template can increase the amount of crystal nucleus and thus make it possible to adjust the crystal size and acidity of the products.The seed-assisted synthesis of ZSM-35 has also been studied.The TG-DTA and hydrothermal aging results show that the as-synthesized ZSM-35zeolite has good thermal and hydrothermal stability and the crystal phase is stable under rigorous conditions.The as-synthesized ZSM-35 zeolite can be modified by NH₄⁺ion-exchange,hydrothermal aging and?NH₄?₂SiF₆ deposition which can tailor its acidity and improve the number of available active sites inside the channels of zeolites.