Green Synthesis And Hydrothermal Stability Of SAPO-11 Molecular Sieve

With the rapid development of economy,it attracts the public’s attention of the shortage of petrochemical resources and environmental pollution.As a substitute for petrochemical diesel,bio-diesel has attracted wide attention worldwide for its environmental friendliness and renewable advantages.However,its wide application is limited by its poor thermal and chemical stability,high viscosity and high turbidity point temperature.Thus,hydrodeoxygenation（HDO）is required to convert fatty acids into paraffins.In HDO reaction,as a main by-product,generation of H₂O results in a hydrothermal environment.Therefore,hydrothermal stable catalysts are required for HDO.SAPO-11 is a common support of HDO catalysts,which has an unsatisfied hydrothermal stability.In order to solve the above problems,the defect structure affecting the hydrothermal stability of SAPO-11 was repaired and improved in this paper by increasing the Si/Al and adding rare earth.Ni/Ln-SAPO-11 catalyst with excellent hydrothermal stability was synthesized using rice husk ash（RHA）as silicon source and mesoporous filler for stearic acid hydrodeoxygenation.Firstly,the effect of silicon content on hydrothermal stability of SAPO-11 was investigated.The affecting factor of hydrothermal stability was determined by analyzing phase structure,pore structure,morphology and skeleton atomic coordination environment.After hydrothermal treatment,the micropores of SAPO-11 collapse,the surface area loss is serious,the crystal surface is etched,and the aluminum atoms are removed from the zeolite skeleton,which results in the decrease of hydrothermal stability.The study showed that SAPO-11 molecular sieve with rich silicon did have better resistance to hydrothermal damage,and the micropore retention increased from 41.5%to70%.The hydrothermal stability of molecular sieve was optimized by rare earth ion modification.After selection,SAPO-0.02Y showed the best relative crystallinity（RC=93.8%）,pore retention（66.3%）and Br?nsted acid retention,and Al had more stable coordination environment and better hydrothermal stability.Secondly,a fast,economical and environmentally friendly method for preparing carbon silicon composites from rice husk ash（RHA）was proposed.Carbon-silicon composites were obtained by anaerobic calcination of RHA,which act as both silicon source and mesoporous filler.The phase structure,skeleton vibration,element composition and morphology of RHA at different calcination temperatures（400℃,500℃,600℃）were characterized and analyzed.The SAPO-11 molecular sieve was synthesized by microwave irradiation at a lower crystallization temperature（200℃）and a shorter duration（4 h）.The SAPO-11 molecular sieve had a high mesoporous volume（0.08-0.14cm³/g）due to the mesoporous packing of carbon in RHA.The Ni/SAPO-11 catalyst showed a high conversion rate of stearic acid to iso-alkanes（>95%）.When Ni/SAPO-400was used as catalyst,iso-C_15-18had the highest yield（7.9%）,and C₁₆and C₁₈also had the highest yield.Finally,rare earth modification approach was applied to SAPO-11 molecular sieve synthesized with RHA.The results showed that Ni/SAPO-Y catalyst had less impurity,better crystallinity,larger mesoporous volume and better retention of Br?nsted acid.After the addition of rare earth Y,the interaction and adsorption between Ni substance and SAPO-11 carrier were stronger,and dispersion of nickel particles was improved.Hydrodeoxygenation of stearic acid showed that the conversion rate of Ni/SAPO-Y and the yield of C_15-18were significantly increased,and the percentage contents of C₁₅and C₁₇were significantly increased.After reused for three times,micropores volume decreased and Ni particles agglomerated,leading to a decrease of conversion and the yield of C_15-18.It is worth noting that Br?nsted acids of Ni/SAPO-Y were preserved and the yield of iso-C_15-18was 10%.The method proposed in this thesis is important for optimizing the hydrothermal stability of SAPO-11,synthesizing SAPO-11 using RHA,and developing efficient catalysts for bio-oil upgrading.At the same time,this synthesis method also provides a reference for the synthesis of other types of SAPO molecular sieves.