Preparation And Adsorption Properties Of All-silicon Multi-stage Molecular Sieve Silicalite-1

With the development of industry,the level of industrialization in developing countries is constantly improving,and the environmental pollution problems brought about by industrial development are becoming more and more serious.Therefore,many experts have tried to conduct in-depth research on prevention and control of environmental pollution.It has been found that molecular sieves have achieved great results in this respect and have great development and utilization value.Silicalite-1 molecular sieve is widely used in the prevention and treatment of environmental pollution due to its special structural structure and physical and chemical properties.Silicalite-1 molecular sieve belongs to all-silicon molecular sieve and is the all-silicon form of ZSM-5 molecular sieve.It has the same framework structure as ZSM-5molecular sieve,but it does not contain aluminum element in the framework.Its unique pore structure has high hydrothermal stability,hydrophobicity,shape-selective catalysis,and has been developed continuously in the field of chemical industry.The main work of this paper is as follows:1.The Silicalite-1 molecular sieve was prepared by hydrothermal synthesis using tetrapropylammonium bromide as template.The factors affecting the synthesis of Silicalite-1molecular sieve were studied by adjusting the molar ratio of template to silica,crystallization time and temperature.Firstly,from the XRD analysis,it is difficult to synthesize Silicalite-1molecular sieve at a low temperature of 100°C.Secondly,from the nitrogen adsorption desorption test data of the sample,the specific surface area of the sample gradually increases with the increase of the crystallization temperature when the crystallization time is 24 h and the molar ratio of the template to the silica is the same.For example,when the molar ratio of template agent（TPABr）to silicon is 0.2 and crystallized at 100°C for 24 h,the specific surface area of the sample was 289.26 m²/g,and the specific surface area was 448.16 m²/g and 454.92m²/g at 120°C and 140°C,respectively.Therefore,the sample prepared at 140°C has better nitrogen adsorption performance under the molar ratio of the same template agent（TPABr）to silicon.Similarly,with the increase of the molar ratio of template agent（TPABr）to silica at140°C for 24 h,the specific surface area of the sample first increased and then decreased,the internal structure of the zeolite was destroyed.For example,when the molar ratio of template agent（TPABr）to silica was 0.2,0.35,0.5,the specific surface areas of the samples were 454.92m²/g,474.35 m²/g,and 367.97 m²/g,respectively.When crystallized at 140°C for 30 h,the specific surface area of the sample gradually decreased with the increase of the molar ratio of templatie agent（TPABr）to silica.The crystallization time was 24 h relative to the crystallization time of 30 h,and the nitrogen adsorption performance of the sample was better.In conclusion,the nitrogen adsorption performance of the samples synthesized under the conditions of the molar ratio of template agent（TPABr）to silica 0.35 and the crystallization time of 24h at 140°C was better.2.Silicalite-1 zeolite was synthesized by changing the amount of water,crystallization temperature and time with tetrapropyl ammonium hydroxide as template.The results of N₂adsorption-desorption test show that the specific surface area of the samples increases with the increase of crystallization temperature under the condition of 24 h crystallization time.When the molar ratio of water to tetraethyl orthosilicate was 90,100 and 110,the phase difference of specific surface area was about 125 m²/g,35 m²/g and 30 m²/g with the change of crystallization temperature,respectively.Although the specific surface area of the sample increases with the increase of temperature under the three different molar ratios of water to ethyl orthosilicate,the difference shown that the structure of the sample was partially destroyed to some extent.At the same time,compared with the crystallization time of 24 h and 30 h,it was found that the crystallization time of 24 h was relatively better.3.Silicalite-1 molecular sieves were synthesized using a double templating agent.The two templating agents were tetrapropylammonium bromide and tetrapropylammonium hydroxide.Firstly,in the XRD characterization,by comparison with the standard card,there were diffraction peaks at the positions of the diffraction peaks of the five standard features,and no obvious peaks were found at other positions.Secondly,the samples were characterized by N2adsorption and desorption.When the molar ratio of template（TPABr and TPAOH）to silica was0.3,the specific surface area of the samples synthesized by fumed silica as silicon source changed with the crystallization temperature.Compared with the samples synthesized by silica and ethyl orthosilicate as silicon source,the specific surface area of the samples synthesized by fumed silica,silica and ethyl orthosilicate varied greatly,among which fumed silica,silica and ethyl orthosilicate crystallized with each other.The difference of specific surface area of fumed silica,silica and tetraethyl orthosilicate with the change of crystallization temperature is 114.25m2/g,29.9 m²/g and 6.09 m²/g,respectively.It is shown that the specific surface area of the samples synthesized from fumed silica as a source of silicon was easily changed by changing the crystallization temperature.The same conclusion could also be reached when the molar ratio of template（TPABr and TPAOH）to silica was 0.25.