Green Synthesis Of Molecular Sieve Membranes And Its Application In Adsorption And Separation

Molecular sieves are widely used in the fields of adsorption and separation because of their uniform molecular scale pores,unique adsorption properties and excellent stability.High selectivity can be reaized on the continuous molecular sieve membrane formed on the porous support by the differences in adsorption and diffusion properties among different molecules.Molecular sieve membrane has great application potential in many industries,such as petroleum,chemical,energy ect.However,the synthesis of molecular sieve membranes has many issues,like long synthesis time,use of organic template,low material efficiency etc,which is far from the requirements of green chemistry.The high membrane cost,mediocre separation performance?permeability and selectivity?,poor repeatability and scale-up restrict the application of molecular sieve membrane.In this study,we systematically studied the synthesis and characterizations of DDR and SAPO-34membranes prepared by microwave heating and oil-bath heating with purposes of reducing synthesis time without sacrificing membrane performance.Their performance for CO₂-CH₄ separation and methanol-DMC separation were investigated.The synthesis of high-performance DDR membranes was greatly limited by lengthy synthesis time and tough template removal.All-silica DDR zeolite membrane was prepared on porous ceramic tubes with microwave-aided hydrothermal synthesis.Compared with traditional hydrothermal synthesis,microwave heating significantly accelerated the crystallization kinetics of DDR membranes by reducing the synthesis time from 2 to 6 d to1?h.Membrane thickness was also reduced from?5??m to less than 1.5??m.These improvements can be attributed to the fast and homogeneous heating from microwave heating,which resulted into fast nucleation and crystallization as well as smaller crystals and thinner membrane.Template removal by rapid thermal processing was not successful for DDR membranes which could be the result of high thermal stress at high calcination temperature.Low temperature calcination under ozone environment can remove template molecules effectively without generating defects,which can be ascribed to small thermal stress at low calcination temperature.Outstanding CO₂-CH₄ separation performance was obtained,with CO₂ permeance of 4.7?×?10^-7 mol/?m²·s·Pa?and CO₂-CH₄ selectivity of 190at 0.14?MPa pressure drop.The high separation permeance is the synergy of optimized seeding,microwave-aided synthesis and mild template removal under ozone environment.However,the high cost of microwave equipment is not conducive to industrial application,it is necessary to further develop a cheaper and faster synthesis method.Dimethyl carbonate has wide applications in many industries.Its synthesis usually involves the separation of methanol and dimethyl carbonate,which is energy intensive for traditional thermal separation.In this study,small-pore SAPO-34 molecular sieve membrane was used to achieve efficient separation of methanol-dimethyl carbonate via pervaporation and vapor separation.Thin SAPO-34 zeolite membranes were prepared on tubular ceramic supports by tubular reactor+oil-bath heating.The crystallization time of SAPO-34 zeolite membrane synthesized by this method was greatly shortened from 5 h to1 h,and the membrane thickness was reduced from 5?m to 1?m.Vapor separation of methanol-DMC mixtures on SAPO-34 membranes was systematically studied,including pressure drop,feed composition,temperature,flow rate,membrane degradation etc.SAPO-34 membranes demonstrated outstanding vapor separation performance,with methanol flux?15?kg/?m²·h?and methanol/DMC separation factor?1000 at 403?K and0.6?MPa pressure drop for 90methanol:10DMCwt%feed.SAPO-34 membranes exhibited great stability in?20?d continuous test.Methanol flux decreased 50%in 20 d continuous test,but it can be recovered to 80%of original value by a simple methanol rinse.Methanol/DMC separation factor exhibited significant increase with time,which can be attributed to the defect blocking by the chemical bonding of DMC in the defects,as verified by XPS and FT-IR analysis.Too much DMC adsorption on crystal surface hindered the free permeation of methanol.The methanol adsorption induced expansion of SAPO-34 crystals also helped shrinking defects effectively,which contributed to high separation factor.The addition of 1%water in the feed had little impact on separation performance.Compared with polymer membranes and other inorganic membranes,SAPO-34 membrane has excellent separation performance and high stability,and has great application prospects.High performance DDR and SAPO-34 molecular sieve membranes were successfully prepared by microwave heating and oil-bath heating.Fast heating reduced synthesis time significantly,improved synthesis efficiency and reduced the process energy consumption.At the same time,the thickness of the film is greatly reduced.Permeance was significantly increased without sacrificing selectivity.The molecular sieve membranes prepared in this study exhibited excellent separation performance in natural gas purification and methanol-dimethyl carbonate separation,and have a broad application prospect.