Synthesis, Characterization And Catalytic Performance Of Mesoporous MCM-41 Molecular Sieves With Highly Hydrothermal Stability

Since the discovery of the crystalline mesoporous M41 s materials,Mesoporous molecular sieves,with well-defined structure,uniform size distribution（2-50nm）and high surface areas,have been the most extensively studied.However,thinner pore wall,the amorphous state and large number of surface silanol of mesoporous materials led to its poor hydrothermal stability.The design and synthesis of high condensation degree of microstructural units can increase pore wall thickness and the degree of condensation of mesoporous molecular sieves,reduce the number of surface hydroxyl groups of silicon and improve hydrothermal stability of mesoporous molecular sieves.It will provide a theoretical basis for the design and synthesis of high condensation degree of microstructural units,and put forward new ideas and new strategies for preparing high hydrothermal stability of mesoporous molecular sieve,and promote mesoporous molecular sieve industrial applications.Therefore,design and synthesis of microstructure silica units with the high degree of silica condensation to prepare the mesoporous molecular sieves can strengthen its pore wall thickness and condensation degree,which are important in enhancement of hydrothermal stability.However,the approach of achieving controllable synthetic zeolite microstructural units and introducing into the pore walls of mesoporous molecular sieve is the key factor in the process.In this work,the industrial inorganic silicate Na₂O·3.3SiO₂ was selected as a cheaper source of silica instead of expensive organic precursors,using CH3（CH₂）15·（CH3）3NBr（CTAB）as surfactant,and a series of mesoporous materials with hydrothermal stability were prepared by synthesis of zeolite microstructural units under control.This dissertation mainly was consisted by the following several aspects.（1）TZM Mesoporous molecular sieves were synthesized.Routing of desiliconisation ZSM-5 was designed and optimized.The influence of the pH value of the assembly process,the crystallization temperature and crystallization time on the structure was investigated,and the structure,texture,surface hydroxyl sites were characterized.Optimal condition for the synthesis of ZSM-5 microstructure unit by selective desilication was: pH≈13,T=70oC,t=60min;Optimal condition for mesoporous assembly process was: Si:CTAB=1:0.2,pH=9.5,crystallization temperature T=120oC,and crystallization time t=48h.TZM material was single-phase mesoporous molecular sieve containing ZSM-5 a single 5-membered ring or a single 4-membered ring primary structural unit.Specific surface area of mesoporous TZM molecular sieve is 959 m2 · g^-1.（2）TBM Mesoporous molecular sieves were synthesized.Routing of desiliconisation β was designed and optimized,The influence of the pH value of the assembly process,the crystallization temperature and crystallization time on the structure was investigated,and the structure,texture,surface hydroxyl sites and ect.were characterized.Optimal condition for the synthesis of β microstructure unit by selective desilication was: pH≈11,T=50oC,t=70min;Optimal condition for mesoporous assembly process was: Si:CTAB=1:0.2,pH=9.5,crystallization temperature T=120oC,and crystallization time t=48h.TZM material was single-phase mesoporous molecular sieve containingβa double 5-membered ring primary structural unit.Specific surface area of mesoporous TBM molecular sieve is 857 m2 · g^-1.（3）TMM Mesoporous molecular sieves were synthesized.Routing of desiliconisation MOR was designed and optimized,The influence of the pH value of the assembly process,the crystallization temperature and crystallization time on the structure was investigated,and the structure,texture,surface hydroxyl sites and ect.were characterized.Optimal condition for the synthesis of MOR microstructure unit by selective desilication was: pH≈11.5,T=80oC,t=60min;Optimal condition for mesoporous assembly process was: Si:CTAB=1:0.2,pH=9.5,crystallization temperature T=120oC,and crystallization time t=48h.TMM material was single-phase mesoporous molecular sieve containingβa single 12-membered ring primary structural unit.Specific surface area of mesoporous TBM molecular sieve is 830.2 m2 ? g^-1.（4）Mesoporous molecular sives containing ZSM-5 microstructure unit was synthesized by micro-mesoporous double template method.Synthetic route of ZSM-5 microstructure unit was designed and optimized,the influence of the pH value of the assembly process,the crystallization temperature and crystallization time on the structure was investigated.Optimal condition for the synthesis of ZSM-5 microstructure unit was: n（Si）:n（Al）:n（TPAOH）:n（H₂O）=3.14:0.14:3.25:14.1,pH≈11,crystallization temperature T=150oC,crystallization time t=44h;Optimal condition for mesoporous assembly process was: 3.14SiO₂ :0.14 NaAl O₂:3.25TPAOH: 4.053 CTAB : 88.5H₂0,pH≈9.5,crystallization temperature T=120oC,and crystallization time t=48h.ZZM material was single-phase mesoporous molecular sieve containing ZSM-5 a single 5-membered ring or a single 4-membered ring primary structural unit.Specific surface area of mesoporous TZM molecular sieve is 915 m2 · g^-1.（5）The hydrothermal stability of above materials were investigated.The surface area,pore volume and pore size of those materials decreased with augmentation of hydrothermal treatment time.（a）The mesoporous structure of TZM materials was retained,the remaining ratio of surface area,pore volume and pore diameter were 89%,80% and 88.6%,respectively,for the sample under hydrothermal treatment for 8 d.Even the hydrothermal treatment time up to 10 days,it was 86.3%,,79% and 90.5%;（b）The mesoporous structure of TBM materials was retained,the remaining ratio of surface area,pore volume and pore diameter were 82%,76.3%,and 92.7% for the sample under hydrothermal treatment for 8 d.Even up to 10 days,it was 79%,70.3% and 88.8%;（c）The remaining ratio of surface area,pore volume and pore diameter were 86.9%,73.1% and 83.7% for TMM under hydrothermal treatment for 8 d.Even up to 10 days,it was 81%,66.7% and 82.9%.This indicates that the micro-structural units introduced into mesoporous molecular sieve pore wall framework can significantly improve the hydrothermal stability of mesoporous molecular sieves.（6）The catalytic performance of catalysts using above mesoporous materials after hydrothermal treatment for 8d was investigated.（a）the conversion of CH₄ and CO₂ were 77.7% and 68.2 for the sample of 5%Ni/TZM-0,it was 82.7% and 84.6% for 5%Ni/TZM-8;（b）the conversion of CH₄ and CO₂ were 48.8% and 70.5% for the sample of 5%Ni/TBM-0,it was 45.0% and 69.5% for 5%Ni/TBM-8;（c）the conversion of CH₄ and CO₂ were 46.8% and 66.5% for the sample of 5%Ni/TZM-0,it was 43.5% and 59.8% for 5%Ni/TZM-8;（d）the conversion of CH₄ and CO₂ were 47.2% and 51.9% for the sample of 5%Ni/TZM-0.