Confined-space Synthesis Of Mesoporous Aluminosilicates:Tuning The Walls Of Mesophase And Its Hydrothermal Stability

Much attention was paid on mesoporous aluminosilicates（MAs）because of their large pore size and specific surface area.However,the bad hydrothermal stability due to the amorphous thin walls of mesophase hinder its application in severe chemical process.To solve this problem,our research group introduced the primary and secondary structural units of zeolite Y,improving significantly the hydrothermal stability.In spite of this,the hydrothermal stability of MAs required much more promotion to meet the needs of fluid catalytic cracking conditions.In addition to the crystallization of walls of mesophase,increasing the thickness of the mesophase wall was another procedure to solve this problem.Methylcellulose（MC）have the interesting thermal reversible properties.In this process,a three-dimensional space structure would be formed above 50°C due to its hydrophobic association structure.Moreover,it will return to a nematic liquid crystal structure and at room temperature.It can be reasonably deduced that this reversible thermal behavior of MC makes it a confined space to control the growth of MAs.At high temperature,the micelles would undergo crystallization in the three-dimensional network of MC.In this process,the agglomeration between the small MAs crystallites can be avoided and the growth of MAs crystal will be maximized.As a result of this,MAs with large unit cell and thick walls will be obtained.Just in this novel idea,synthesis of MAs with thick walls was conducted:（1）In the low F68 concentration system,F68(PEO₈₀PPO₃₀PEO₈₀)micelles interacted with silica-alumina gel in the network structure of MC.Since the unit cells were not aggregated,they could grow larger,and mesoporous molecular sieves with larger unit cell parameters and pore walls were obtained.Meanwhile,the mesh structure of MC can be removed by washing at room temperature.Characterization results showed that MAs had a specific surface area of 821.5 m²·g^-1,pore volume of 0.74 cm³·g^-1,wall thickness of 10.55 nm,respectively.Moreover,in this process the raw material consumption was 0.70 g F68/g MAs,and 19.9 g H₂O/g MAs.Compared with the conventional synthesis procedure（1.26 g P123/g MAs and 36.4 g H₂O/g MAs）the utilization of raw materials improved greatly.With a severe hydrothermal process at 800℃and 100%water vapor for10 h,the sample showed a specific surface area 257.6 m²·g^-1 and pore volume 0.21 cm³·g^-1,with the retention ratio were 31.4%and 28.4%,respectively.（2）In the low F68 concentration system,MC is added in the crystallization stage,anionic polymer Polyacrylic acid（PAA）was introduced in the obtaining of MAs,in which PAA could increase the wall thickness.This effect could be ascribed to the two effects:（1）the presence of PAA favor the the crystallization at high temperature,inhibiting the crystallization at room temperature;（2）Additional PAA domains will be present due to the weak interaction between PAA and PEO segments.As a result of this,space between micelles will be large,which favors the increase of wall thickness.Characterization results showed that MAs had a specific surface area of 865.9 m²·g^-1,pore volume of 0.60 cm³·g^-1,wall thickness of 12.67 nm,respectively.Moreover,in this process the raw material consumption was 0.68 g F68/g MAs,and 18.6 g H₂O/g MAs.After a severe hydrothermal process,the sample showed a specific surface area276.2 m²·g^-1 and pore volume 0.15 cm³·g^-1,with the retention ratio were31.9%and 25.0%,respectively.（3）In the low F68 concentration system,MC is added in the crystallization stage,Sodium Perfluorononenyloxybenzenesulfonate（OBS）was introduced in synthesis of MAs.In this strategy,interaction of OBS with PEO segments made volume of PEO larger,leading to the increase of wall thickness.In addition to this,hydrophilicity of the PEO section will be less due to the introduction of OBS.Less water content in MAs will favor the improvement of hydrothermal stability.Characterization results showed that MAs had a specific surface area of 932.6 m²·g^-1,pore volume of 0.59 cm³·g^-1,wall thickness of 17.07 nm,respectively.Moreover,in this process the raw material consumption was 0.68 g F68/g MAs,and 17.6 g H₂O/g MAs.After a severe hydrothermal process,the sample showed a specific surface area 332.9 m²·g^-1 and pore volume 0.17 cm³·g^-1,with the retention ratio were 35.7%and 28.9%,respectively.