Study On The Preparation And Performance Of Water Treatment Membrane Based On MWW-Type 2D Zeolites

In recent years,great efforts have been applied to the 2D materials since the discovery of graphene,which expanded the choice of building blocks for membranes.A high-performance laminar membranes with unique nanopores and/or nanochannels prepared by 2D layered materials,has been proved to be superior in controlling mass transport within nano-and sub-nanoscales.The single crystal nanoflake with high aspect ratio,special pore size and excellent stability make 2D zeolites a advanced membrane material.However,there is almost no relevant research involved in the field of water treatment.In order to explore the possibilities of 2D zeolite in water treatment membranes and alleviate the increasingly serious water pollution problem,2D MWW-type zeolite sheets are exfolitated and used as the building blocks for lamellar membrane.The research contents and conclusions are as follows:A 2D ITQ-2 zeolite membrane was prepared by vacuum filtration using the stripped 2D ITQ-2 zeolite as the membrane building block.First of all,MCM-22 precursor,MCM-22(P),was synthesized by hydrothermal method,followed by high-temperature swelling to obtain the MCM-22(S).A molecular sieve 2D ITQ-2 sheet with MWW monolayer structure was produced via acidification ultrasound,and high-temperature roasting of MCM-22(S).The optimal raw material ratio and reaction time for MCM-22(P)synthesis were explored,and series of characterizations were conduct to analyze the materials.Despite a flake-like material with a specific surface area was obtained,the exfoliation environment of high temperature and strong acid destroys the layered structure of the ITQ-2 sheets and causes the edges to curl,which makes the ITQ-2 sheets poorly dispersible in the solvent.This property prevents it from being assembled into a 2D lamellar membrane.Therefore,the 2D ITQ-2 zeolite membrane exhibits poor separation performance,which is not suitable for application in the field of water treatment.Based on the above conclusions,the exfoliation method of ITQ-2 sheets was improved,and a stable dispersion of MWW sheet suspension was obtained.The 2D MWW sheet membrane was assembled by the vacuum filtration method using the 2D MWW sheet as the membrane building block.Simultaneously,the performance of the membrane was characterized in detail.A uniformly dispersed MWW sheet suspension was prepared via sonicating of MCM-22(RT-S)obtained by swelling at room temperature in the viscous liquid hydroxyl-terminated polybutadiene(HTPB).The 2D MWW sheet possesses a MWW single-layer structure,high aspect ratio and the thickness of 2.5 nm,while only silanol exits on the surface.It is not easy to swell when exposed to water.Additionally,the pores on the molecular sieve are not easily deformed,and the exfoliated layer in HTPB is easier to spread,which is beneficial to prepare a 2D lamellar membrane with precise screening performance because of milder exfoliation conditions.Through the vacuum filtration of the 2D MWW sheet/toluene dispersion system,the lamellar(MH)membrane based on the 2D MWW sheet was successfully prepared on the nylon support membrane.The separation performance of the membrane was tested with a cross-flow device.The flux of pure water reached 11.3 L m-2 h-1 bar-1,the rejection rate of the negatively charged dye such as indigo carmine(1.5 × 0.5 nm)exceeded 99.5%,and the rejection rate of the positively charged dye Rhodamine B with a molecule larger than 1.5nm reaches 90%.The rejection of positively charged dyes with smaller molecules is reduced to a certain extent.Dye rejection experiments of different sizes and electrical properties reveals that the mass transfer of the 2D MWW membrane is mainly affected by the Donan effect during sieving negatively charged dyes,which is caused by the negative charge on the surface of the membrane;When sieving positively charged dyes,it is affected by both size sieving and Donan effect.In the stability evaluation,the results of the immersion and the ultrasonic experiments certify that the membrane quality did not change.The results of cross-flow test show that the membrane can maintain stable flux and retention under different pressures,exhibiting excellent compression and shear resistance;The long-term stable operation at different pHs proves its excellent chemical stability.In summary,with 2D MWW-type zeolite sheet as the membrane structure unit,the preparation methods of 2D MWW zeolites are explored,and a novel and simple strategy to prepare the 2D zeolite lamellar water treatment membrane is proposed.A simple vacuum filtration method was carried out to prepare a 2D MWW nanosheet lamellar membrane with good flux,accurate screening,and high stability,which exhibits great generalizability.The simple preparation method with excellent performance provides a novel approach for the development of 2D zeolite material membranes and membrane water treatment technology.