Preparation Of Hierarchical Porous Silicalite-1 And Study On Gas Adsorption And Separation

Zeolite was the cage-type inorganic material,which had a regular pore structure and consisted of the tetrahedron primary structural units.It favored numerous applications in catalysis,adsorption,separation,ion exchange,host and object assembly and so on because of its ordered pore structure,acidity and alkalinity,higher hydrothermal stability,cheap production and other characteristics.MFI-type zeolite was the most widely used zeolite.Recent studies have shown that pure silica MFI type zeolite Silicalite-1,which did not contain aluminum,had a good adsorption effect on small molecules gas such as CH₄,N₂,CO₂,C₂H₆ and O2,especially for the CH₄/N₂ mixed gas,the adsorption separation effect reached a high adsorption selectivity compared with ordinary adsorbents.With the improvement of industrial production requirements and the development of science,zeolite modification was more and more,of which hierarchical porous zeolite was a very important one.When hierarchical porous zeolite appeared,the problems of stacking carbon,stacking macromolecules,low catalyst efficiency and diffusion resistance troubled catalysis for many yearswere resolved.However,there had been no application in the field of adsorption and separation of hierarchical porous zeolite.The main reason was that,compared to the micropores structure,the surface area and the amount of micropores of hierarchical porous zeolite decreased,which lead directly to a decrease of gas adsorption capacity and adsorption selectivity of hierarchical porous zeolite.This was contrary to the characteristics of an adsorbent,so the role of hierarchical porous structure on the adsorption separation basically left to rot and no researchers have attempted to find or synthesize a hierarchical porous zeolite that did not affect the adsorption capacity and adsorption selectivity but also reflect the role of hierarchical porous structure.Based on the above two points,in this article,a method of a crystal seeds-assisted rapid synthesis microporous and hierarchical porous Silicalite-1was explored.The phase,morphology and pore structure of Silicalite-1 were characterized by XRD,SEM,TEM and liquid nitrogen adsorption,and then the CH₄,N₂,CO₂ and C₂H₆ adsorption separation properties of hierarchical porous Silicalite-1 and microporous Silicalite-1 were also investigated studying the role of hierarchical porous structure in adsorption separation and verifying whether hierarchical porous Silicalite-1 in this article not only had a higher adsorption capacity and adsorption selectivity but also could reflect hierarchical porous structure performance.The main contents and results are listed as follows:Firstly,under hydrothermal synthesis,we explored a method of a crystal seeds-assisted rapid synthesis Silicalite-1.Change the addition of the key rawmaterial,and the optimum raw material ratio for microporous Silicalite-1 was 1:0.1: 0.1: 0.15: 50 of SiO2?silica sol?: ethylamine : TPABr : NaOH : H2 O and crystal seeds?10% weight of SiO2?.Compare the optimum raw material ratio for microporous Silicalite-1,mesoporous Silicalite-1 was the addition of potassium fluoride?0.3 of SiO2?.The reaction temperature and time of the two pore types Silicalite-1 were 453 K and 20 hours,and compared with the conventional synthesis method,this method greatly shortened the reaction time.The CH₄,N₂,CO₂ and C₂H₆ adsorption properties were investigated for the two pore types Silicalite-1 at 298 K at 1 bar pressure and the results shown that microporous Silicalite-1 displayed a higher adsorption capacity of CH₄,N₂,CO₂ and C₂H₆ and adsorption selectivity of CH₄/N₂?CO₂/N₂?CO₂/CH₄?C₂H₆/CH₄,however,although the adsorption capacity of the mesoporous Silicalite-1 was lower,the adsorption selectivity of CH₄/N₂ was the same as microporous Silicalite-1;on the contrary,the CO₂/CH₄,CO₂/N₂ and C₂H₆/CH₄ adsorption selectivity on mesoporous Silicalite-1 was improved.Secondly,based on the method of the above rapid synthesis Silicalite-1,the silicon source was turned into fumed silica to obtain microporous and hierarchical porous Silicalite-1 zeolite.Differented from microporous and mesoporous Silicalite-1 obtained the silica sol as the silicon source,the surface area microporous Silicalite-1 and hierarchical porous Silicalite-1 obtained the fumed silica as the silicon source was basically the same,and the adsorption capacity of CH₄,N₂,CO₂ and C₂H₆ and the adsorption selectivity of CH₄/N₂,CO₂/N₂,CO₂/CH₄ and C₂H₆/CH₄ of both were also the same.Based on the above analysis,the gas breakthrough test of CH₄/N₂,CO₂/N₂,CO₂/CH₄ and C₂H₆/CH₄ mixed gas of microporous Silicalite-1 and hierarchical porous Silicalite-1 were carried out respectively,and the results showed that the breakthrough time of four mixture of two components was shorter on hierarchical porous Silicalite-1than microporous Silicalite-1,and the hold time of CH₄/N₂ and C₂H₆/CH₄ was the same on hierarchical porous Silicalite-1 and microporous Silicalite-1.Further draw conclusion,when the gas molecule with smaller molecular dynamics diameter in the mixture was weak adsorbate and the gas molecule with larger molecular dynamics diameter was strong adsorbent?weak adsorbent first penetrated out and strong adsorbent second penetrated out in gas breakthrough test?,compared microporous Silicalite-1,the breakthrough time became shorter and the hold time unchanged on hierarchical porous Silicalite-1,which shortened cycle time,improved the efficiency and achieved a better effect of pressure swing adsorption.