Synthesis Of Ultrathin Zeolite Membranes And Their Applications In Gas Separations

Porous membrane materials have been widely studied by scientists for the excellent performance in separation technology,such as energy-saving,high-efficiency and non-pollution,etc.,which will help achieve the goals of“peak carbon dioxide emissions”and“carbon neutrality”.Among various types of membranes,zeolite membranes have higher potential for gas separations because of uniform molecular level pores,tunable surface properties,and excellent thermal and chemical stabilities.However,at present,zeolite membrane is not enough for practical application in separation industry,duo to the low gas permeance.Reducing the thickness of zeolite membrane is one of the effective ways to improve gas permeability.This paper focuses on the synthesis of ultra-thin zeolite membranes and the rapid gas separations,mainly divided into the following aspects:1.A surface gel conversion method was developed to synthesize ultra-thin MFI zeolite films:under the assist of steam and MFI seeds,the gel was gradually conversion into MFI crystals and these crystals intergrowth to form continuous membrane.The Si-MFI zeolite membrane with a thickness of 500 nm be prepared on porous alumina discs by reducing the thickness or weight of gel.During synthesis,the wettability of the gel on the disc plays an important role in the formation of ultrathin membranes.The gas separation of n-hexane over isomers show that the permeability of 500 nm thick Si-MFI zeolite membrane to n-hexane is2.4×10^-6mol s^-1m^-2Pa^-1,which exceeds other zeolite membranes.And,the separation factors more than 13.3 for n-hexane over isomers.In addition,surface gel conversion is a maneuverable and full utilization of materials method in the preparation of high-quality zeolite membranes.The rapid selective separation of hexane isomers provides an option for the application of zeolite membrane in the production of high-octane gasoline in petrochemical industry.2.The tightness and thickness of zeolite films are related to the selectivity and permeability of gas separation.The growth characteristics of polycrystalline zeolite membranes still challengeable for the synthesis of dense ultra-thin membranes.we developed a zeolite conversion synthesis method for growing Na A zeolite membrane on Na X layer.Under a low basicity hydrothermal solution,Na A zeolite was transformed on the surface of Na X zeolite,and continuous Na A polycrystalline layer was formed.The thickness of Na A zeolite membrane was successfully prepared in the submicron scale（about 500 nm）by using this method,and the selective separation test of C₂H₂and C₂H₄mixture was conducted.It showed high permeability and excellent selectivity to C₂H₂.Compared with the Na A membrane prepared by the traditional in-situ growth method,the separation coefficient is increased by 2.5-times,and the transmittance of C₂H₂is increased by about 3-times.Zeolite conversion method provides a new reference for the synthesis of high-performance ultra-thin zeolite membranes.3.SSZ-13 as a small-pore zeolite,has gradually become a point in membrane researcher,due to its potential application in some major separation processes in the chemical and energy industries,such as CO₂capture,H₂purification,small molecule hydrocarbon separation,organic dehydration,etc.In this work,we developed a fast,efficient and green method for the synthesis of ultra-thin and high-performance SSZ-13 zeolite membranes.TMAda OH and TEAOH were used as templates to prepare the growth gel.Under the action of a small amount of water vapor,the gel could be rapidly crystallized into a continuous dense zeolite film with uniform thickness.At the same time,the crystallization process of SSZ-13 zeolite membrane and the influence of template agent,crystallization temperature and gel layer thickness on its growth were systematically studied.The SSZ-13 zeolite membrane with a thickness of about 500 nm was successfully synthesized by optimizing the conditions.In the selective permeability test of H₂,C₂H₆and C₃H₈gases,the high H₂permeability and selective permeability were demonstrated.This method provides a new way for the synthesis of high performance SSZ-13 zeolite membrane.