Synthesis And Properties Of Mordenite And Mordenite Membranes

Mordenite(MOR) is a kind of high-silica zeolite,which has elliptical pore channels(0.695nm×0.581nm) parallel to each other with the cell composition of Na₈[(AlO₂)₈(SiO₂)₄₀]24H₂O.Due to its high thermal and acid stabilities, hydrophilicity and appropriate pore size,MOR zeolite has been widely used for catalytic reactions and separation processes in acidic media.In addition,MOR zeolite has also been extensively studied as an inorganic membrane material.This paper is consisted of two parts:(â… ) Preparation and catalytic properties of powdery mordenite;and(â…¡) Preparation,modification and pervaporation performance of mordenite membranes.The details are discussed below:Firstly,the influence of silica source(silica sol and silica gel) on the crystallization process of mordenite zeolite was investigated using a synthesis mixture with n(Na₂O)/n(SiO₂)=0.2,n(SiO₂)/n(Al₂O₃)=30,n(H₂O)/n(SiO₂)=26.It was found that the products of the crystallization were totally different using silica sol or silica gel as silica source even if the other conditions were the same.A series of experiments were designed to give highlights on these results.It was proved that the n(SiO₂)/n(Al₂O₃) ratio of the gel rather than the apparent n(SiO₂)/n(Al₂O₃) ratio of the synthesis mixture was the determining factor for the products of crystallization.Secondly,the effect of heating methods(microwave heating versus conventional heating) on the crystallization process of mordenite was also studied using a synthesis mixture with n(Na₂O)/n(SiO₂)=0.2,n(SiO₂)/n(Al₂O₃)=30, n(H₂O)/n(SiO₂)=26.It was shown that microwave heating can not only accelerate the crystallization rate but also inhibit the formation of impurities and increase the specific surface area of the products when using silica gel as silica source.However, the products obtained were all in the form of hard solid using either microwave heating or conventional heating when using silica sol as silica source.Thirdly,the conversion and yields of the gaseous and liquid products for JP-10 cracking were studied at 400℃and atmospheric pressure over three H-mordenite catalysts(HMOR,HMOR-OH and HMOR-H),which were prepared by different methods using Na-mordnite powder as the starting materials.The results show that the synthesized mordenite samples have satisfactory stability against strong acid and alkali solution.Among these three H-mordenite catalysts,the acid quantities, initial catalytic activity and the deactivation rate all follow the same order as HMOR-OH＞HMOR＞HMOR-H.Based on the results obtained during the first 60 min reaction period,the average conversions of JP-10 cracking are 69.7%,60.9% and 54.1%for HMOR,HMOR-OH and HMOR-H catalysts,respectively.The yields of the gaseous cracking products follow the same order as the conversion, whereas the yields of the liquid products follow the opposite order.Finally,factors influencing pervaporation performance of mordenite membranes prepared using seeded hydrothermal synthesis(secondary growth) were discussed.The results demonstrate that the factors such as concentration of seed solution,silica source and Si/Al ratio in the synthesis mixture have influences with different degree on the pervaporation performance of mordenite membranes.It was found that the too high or too low concentration of seed solution is not favorable for the formation of high quality mordenite membranes.The use of silica sol as silica source is better than silica gel for the formation of high quality mordenite membranes.The higher content of Al in the synthesis mixture also favors the formation of high quality mordenite membranes.In addition,effect of post-treatment by EDTA and NH₄F aqueous solution has also been explored on pervaporation performance of mordenite membranes.The results show that post-treatment either using EDTA or using NH₄F aqueous solution can improve both the total fluxes and the water/ethanol separation factors of the mordenite membrane under appropriate treatment conditions.