| Core-shell composite material is one of the hotspot in the field of material research,the researchers can choose the appropriate material as core material and shell material to prepare satisfied core-shell materials according to the materials properties and performance The core-shell composites can be synthesized by composite two or more than two kinds of materials with a variety of different methods The molecular sieve of SAPO-34 used in this paper is a kind of CHA(0.38x0.38 nm)topological structure.The aperture size of this kind of molecular sieve is similar to CH4(0.38 nm)and CO2(0.33 nm),and SAPO-34 molecular sieve can separate CH4 and CO2.Moreover,?-Al2O3 is a good absorbent for CO2,so ?-Al2O3/SAPO-34 is mainly used for the separation of CH4 and CO2.It is reutilization of energy,and it is also a kind of environmental protection.For ?-Al2O3 it has a phase transition from ? to ?,which leads to the loss of thermal stability.We use gel method and impregnation method with lanthanum nitrate hexahydrate to modify ?-Al2O3,in a certain extent,which improves its thermal stability at high temperatures.Hydrothermal synthesis method is used for the preparation of core-shell materials,including the in-situ growth method and secondary growth method.The morphologies and properties of the core-shell materials prepared by different synthetic methods are investigated.The addition of La improves the thermal stability in a certain extent,because lanthanum elements can enter the aluminum oxide crystals' holes.It plays an important role in the ion diffusion of Al3+ and O2-,and hinders the nucleation process of alpha phase as well.Within the scope of a certain degree,with the increment of lanthanum content,the specific surface area,pore volume and thermal stability of ?-Al2O3 will gradually increase at high temperature.The effects of different templates,molar ratios and adding sequence on the preparation of core-shell materials were investigated by in-situ growth method.When triethylamine is as the template agent,we failed to prepare of core-shell materials.The density of the molecular sieve membrane has a big difference between the core shell materials prepared with four ethyl ammonium hydroxide as the template and different ratios and different adding sequences.A pure phase SAPO-34 molecular sieve can be obtained in the precursor solution by using the ratio one(1Al2O3:2P2O5:0.6SiO2:4TEAOH:140H2O),and only a small number of molecular sieve crystals grow on the surface of Al2O3.The synthesis method with the ratio of two(1Al2O3:1.448P2O5:0.13SiO2:1.96TEAOH:219.7H2O)not only makes SAPO-34 in the precursor solution,but on the surface of Al2O3,but it can not form a dense molecular sieve membrane.When four ethyl hydroxide and di-n-propylamine are used as the template,the addition of di-n-propylamine causes the precursor solution to dissolve more thoroughly and makes the of the mixture more evenly.Therefore the SAPO-34 molecular sieve grows more evenly on the surface of the carrier to relatively dense molecular sieve membrane.We discuss the influences of aging time and crystallization time on the formation of the molecular sieve membrane.The experimental results show that the aging time of 48 h,and the crystallization time of 24 h is relatively suitable for the preparation of zeolite membranes.With the secondary growth method,the seed layer was prepared by drop coating method.The crystal seed can promote the formation of crystal nucleus,it can provide all the growth surfaces for the crystal nucleus at the same time.It is advantageous to obtain zeolite membrane with smaller crystal particle size.The molecular sieve membrane is arranged closely and a uniform thickness can be obtained by secondary growth method.In the study of the influence of the synthesis conditions,we obtained the best effect of the molecular sieve membrane under the condition of aging time with 18 h and crystallization time with 24 h. |
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