Study On The Preparation Of Hierarchical HZSM-5 Zeolites And The Performance Of The Thiophene Alkylation

With the increasing strictness of environmental regulations and vehicle gasoline sulfur standards,a new technology for obtaining ultralow sulfur gasoline by olefin alkylation of thiopenic sulfur?OATS?was developed,which can achieve the purpose of controlling the contents of sulfur and olefin in FCC gasoline without additional reactants.It can avoid the loss of the octane number for gasoline on account of the separation of desulfurization reaction and hydrogenation process.For the restrictions of steric hindrance and reaction mechanism of OATS,catalysts with appropriate pore structure and acidity are urgently required.Therefore,the research and development of the novel acid catalysts for the alkylation desulfurization reaction are really of great significant for clean gasoline production.In this work,the HZSM-5 composite zeolites with both mesoporous and microporous structure were obtained by treating in different alkali solutions under different conditions.The catalysts were characterized by XRD,XRF,SEM,BET,TEM,N2-adsorption/desorption,NH3-TPD,Py-IR,XPS and so on.The effects of different alkali solutions and reaction conditions on pore structure and acidity of HZSM-5 zeolite were investigated.The performance of thiophene alkylation on HZSM-5 zeolites was studied with a small-scale fixed bed reactor by using model compounds as feedstock.The deactivation and regeneration behaviors were observed,and the technological conditions of the catalyst were investigated.The main contents and results are described as follow:1 Tetrapropylammonium hydroxides?TPAOH?with different concentrations were used to treat HZSM-5 zeolites with different SiO₂/Al2O₃ ratios to obtain meso-microporous composite molecular sieves.And the catalysts were used in the OAST.The results showed that the SiO₂/Al2O₃ ratios of HZSM-5 zeolites and the TPAOH concentration cause the zeolite desilication and mesopore production.The mesopore diameter increased with the increasing of TPAOH concentration and SiO₂/Al2O₃ ratio.The acidity of HZSM-5 was regulated by TPAOH solution.HZSM-5?SiO₂/Al2O₃=50?was obviously higher than the other prepared molecular sieves in the relative acid content and the strength of both weak and moderate acid and B acid content.HZSM-5?SiO₂/Al2O₃=50?treated by TPAOH in different concentration showed excellent performance on the thiophene alkylation.2 Different concentration of NaOH and Na₂CO₃ was adopted to treat HZSM-5 with different SiO₂/Al2O₃ ratios.The hierarchical porous HZSM-5?SiO₂/Al2O₃=50?zeolites were prepared and tested by OAST reaction.The results showed that the hierarchical zeolites with meso-micropore could be obtained through the treatment with low concentration NaOH solution.Because NaOH belongs to a strong alkaline,it can damage the micropores strongly.Moreover,the rate of silicon dissolution in NaOH is rapid,which can make the demetallation uncontrollable.After the desilicification,pore size,acid content and heat stability of the catalysts are higher,which is not favorable to thiophene alkylation.But Na₂CO₃ solution is a mild alkaline solution,and the speed of desilicication is moderate.It can develop the composites with meso-microporous structure.The HZSM-5 framework is not collapse and alkali treatment have little influence on microporous even at the highest concentration.The HZSM-5 zeolite treated by 4 mol · L-1 Na₂CO₃ for 2 hours has the advantages on thermal stability,moderate diameter and acidity.The thiophene alkylation activity is high,and the conversion of thiophene alkylation reaches to 97.2%.And the thiophene alkylation activity is stable at the same time.3 The hierarchical zeolites with meso-micropore were prepared from HZSM-5?n?SiO₂?/n?Al2O₃?=50?,by secondary crystallization with TPAOH after pretreated with Na₂CO₃,or being treated with Na₂CO₃ and TPAOH simultaneously.The OAST experiments were also performed on the above-mentioned catalysts.After preteated with 4 mol · L-1 Na₂CO₃,HZSM-5 zeolite was added into TPAOH solution for secondary crystallized.The optimum conditions for that treatment are as followed:the TPAOH concentration was 0.3 mol·L-1,the crystallization temperature was 170 ?and the crystallization time was 24 h.HZSM-5?CO₃2--TPA+,0.3-24-170?had moderate pore diameter and acidity,and the highest thiophene eonversion of 99.1%was achieved.But when the HZSM-5 was treated by Na₂CO₃ and TPAOH simultaneously,the dissolved SiO₂ blocked the pores and channels,and covered the catalyst surface,leading to lower acidity and smaller pore diameter,therefore it can't be applied in the OAST.4 Deactivation and regeneration were studied in this work.The results indicated that the catalyst deactivation is due to the deposition of organic macromolecules on the catalyst surface,which appear in the 1-hexene oligomerization and 1-hexane and hexane polymerization.The amounts of carbon deposition can be reduced by coke-burning regeneration of coked zeolite catalyst under 550 ?.The activity of regenerated catalyst is close to fresh catalyst.5 The effect of process conditions on OATS performance on typical catalyst was studied by the orthogonal experiments.The results indicated that the main factors influencing the thiophene alkylation reaction on Cat-f catalyst are the reaction temperature and space velocity.When the reaction temperature is 120 ?,the reaction pressure is 1.0 MPa,and the reaction space velocity is 1.0 h-1,the conversion rate of thiophene alkylation is the highest.