In-situ High Pressure Study On The Structures Of Several Zeolites

In 1756,people firstly discovered the natural aluminosilicate,and since then,scientists opened up a new world,gradually began to realize the characters of zeolites,and started to transform and utilize these characters.Zeolite is a kind of porous functional material with uniform pore size.Its unique limited pore structure,easily accessing acidic sites,good hydrothermal stability and environmental friendly characteristics,lead to its good performance in adsorption,ion exchange and catalysis.Therefore,zeolite is widely used in petrochemical industry,catalysis,control of heavy metal water pollution,gas adsorption and separation and other fields.The research on zeolites has experienced nearly a hundred years of development.People have explored its structure,properties and applications in detail,and many new zeolites with different structures have been synthesized artificially.In order to improve the function of zeolites,it is necessary to have a clearer understanding of its structure.The function of zeolites is determined by the shape and size of its pore and the position of acid sites.The pore size and dimension determine whether the molecules can pass through,and the stability is also a major factor testing the function of zeolites.In the past few decades,many experiments have studied on how zeolites response to temperature,but the influence of pressure has not been very clear yet.On the other hand,the decomposition reaction which is difficult to carry out under high pressure has always been a curious but difficult direction to navigate.For a decomposition reaction,the reaction will increase the total number of moles in the system,and pressure is not conducive to the positive shift of its chemical equilibrium.But if we can "stash" the products of the reaction into the pores of the zeolites,it is equivalent to constantly removing the products from the system,causing the chemical equilibrium to shift to the right.In order to achieve this purpose,we must first master the ability of zeolites to withstand pressure.In this paper,high pressure in-situ Raman and X-ray diffraction experiments were carried out on several common and commercialized zeolites.Their structural evolution under high pressure were explored through nuclear magnetic resonance and other characterization methods,combined with theoretical calculations.The main progress and conclusions are as follows:1.This paper focuses on the most common and time-honored Zeolite A to study its structural stability under high pressure.Zeolite 3A is taken as example for in-situ high pressure experiments.Through the analysis of the results of X-ray diffraction,Raman and NMR,it is known that the skeleton structure(LTA structure)of Zeolite 3A will gradually shrink under pressure and begin to collapse at about 4.3 GPa.The double quaternary rings begin to break,and the sod cages separate from each other and become independent.When the pressure is below 8 GPa,the skeleton structure can restore original structure to a certain extent after recovered,but when the pressure reaches more than 8 GPa,the LTA structure will be permanently damaged and cannot restore.If we fill the cage with water molecules,when the pressure is low,the presence of water molecules will make the zeolites shrink faster because of the exist of the van der waals force between water and the skeleton.However,when the pressure is high,the presence of water molecules will play a supporting role and improve the ability of the LTA skeleton to withstand the pressure.2.On the basis of the above studies,this paper also explores the response of several other common zeolites to pressure: Y-type molecular sieves(FAU framework)have similar bearing capacity as A-type molecular sieves,which also start to collapse at around 4.5 GPa,and are completely damaged at around 7-8 GPa,which cannot be recovered.However,filling the Y molecular sieve channels with water molecules will aggravate the collapse of FAU structure.We suppose that it is because of the “spilling out” of water molecular since Y molecular sieve have bigger pore size.ZSM-5 with MFI skeleton will be completely amorphous at around 8 GPa.SAPO-34 with CHA structure has the largest bulk modulus among common zeolites,which is also verified by our experiments under high pressure.The pure SAPO-34 can withstand a pressure of 14 GPa,while the pore filled with water molecules can retain a certain structural periodicity under a pressure of more than 30 GPa.