Fabrication And Properties Of NaA Zeolite Membranes On Porous Si₃N₄ Supports

NaA zeolite membranes have been widely used in the dehydration of organic solvents and the gas separation due to the specific pore structure and the strong hydrophilicity.Due to the poor mechanical properties,small pore sizes and porosity of supports,the industrial NaA zeolite membranes had low permeation flux and easily broken in usage,which would affect the separation efficiency of membranes.In regard to the issue above,the porous silicon nitride ceramics with excellent mechanical behavior were chosen as a new support of NaA zeolite membranes in this paper.The preparation and the properties of NaA zeolite membranes on Si₃N₄supports were investigated in detail.The target was to investigate the preparation condition optimization of NaA seeds,the analysis of pore structures and properties of Si₃N₄ supports and the fabrication of the NaA zeolite membranes.The optimal preparation condition of NaA seeds was obtained by the orthogonal design.The aging time,synthesis temperature and synthesis time were chosen to design an L₉(3³)orthogonal experiment.The crystallinity and particle size distribution were chosen as the evaluation indexes.The optimal preparation condition obtained of NaA seeds was aging 5 h,crystallizing 3 h at 100?.The pore structures and gas permeability of porous silicon nitride ceramics via self-propagating high temperature synthesis(SHS)were investigated.The porous Si₃N₄ ceramics with tailored pore structures were fabricated by altering the particle sizes and amount of Polymethylmethacrylate(PMMA).With the increasing content of PMMA,the open porosity of samples increased and the flexural strength of samples decreased.The tortuosity which showed irregular variation affected gas permeability directly.The samples with 20 wt.%content of PMMA exhibited the maximum Darcian and non-Darcian constants with the smallest tortuosity.There wasn't much influence on the open porosity of samples as the particle size of PMMA increased.Affected by the curvature of pore structures,the tortuosity of samples decreased and the permeation constants increased.Moreover,the comparison of permeability coefficients with other ceramics via different pore forming methods in literature was presented.The specimens exhibited great permeability due to the large pore sizes created by the elongated and coarsened?-Si₃N₄ grains during the SHS process,providing a low-cost and environmentally friendly method for preparing high permeability porous Si₃N₄ supports.The hot dip-coating method and the vertical rotating spraying method were used to cover the Si₃N₄supports with nucleation seeds.The gas permselectivity of NaA zeolite membranes obtained by the hot dip-coating method should be improved further.Increasing the synthesis temperature,extending the synthesis time and improving the concentration of nucleation seeds made the NaA zeolite membranes become denser.However,further lengthening the synthesis time would lead to the transforming of NaA zeolite to Na X zeolite with much more thermodynamic stability.The home-made vertical rotating spraying device made up for the poor process stability of the hot dip-coating method.The O₂/N₂ permselectivity of NaA zeolite membranes via vertical rotating spraying was 4.54.Comparing with the structure and performance of NaA zeolite membranes reported in the literature,it was confirmed that the porous silicon nitride supports had a promoting effect on the permeation efficiency of NaA zeolite membranes.The interlayer of Y₂O₃-SiO₂-Al₂O₃(YSA)on the porous Si₃N₄ supports was fabricated by pressureless sintering.The phase composition and the microstructures of YSA interlayer were investigated by altering content of raw materials and sintering temperatures.The sintering process of YSA interlayers was incomplete under the low temperature,and the liquid phase would fall into the porous Si₃N₄support during the sintering process under the higher temperature,which would affect the formation of interlayer.However,the formation of interlayer made a positive effect on densifying the NaA zeolite membranes and flattening the surface of porous Si₃N₄ supports.The optimal sintering temperature of YSA inlayer was1400°C.The YSA-Si₃N₄ supports with different microstructures were selected to prepare NaA zeolite membranes under the same experiment condition.It had been found that the YSA interlayers were effective to promote the quality of NaA zeolite membranes.