Hydrothermal And Ionothermal Synthesis Of HZSM-5/SAPO-5 Composite Molecular Sieve And Their Catalytic Behaviors In Isobutane Aromatization

With the increasing lack of oil resources,aromatization of inexpensive low carbon hydrocarbon which main products are benzene,toluene,ethylbenzene and xylene?BTEX?,has become a hot research topic.Traditional catalyst of aromatization is HZSM-5 with strong acidity which is not benefit for the life of catalysts due to the crack of hydrocarbon on the strong acidic sites,thus restrict its application in industry.At present,most modification methods were focused on metal modification,acid treatment,alkali treatment,steaming treatment and so on,by which the strong acidity of HZSM-5 were decreased,the diffusion capability of reactant and products were increased,both together could improve the selectivity of BTEX and stability of catalysts in aromatization of low carbon hydrocarbon.In this article,the synthesis conditions of nano SAPO-5 were firstly optimized by adjusting the mole ratio of SAPO sol and crystallization conditions,and the effect of silicon content on the size of SAPO-5 particles was investigated in hydrothermal and ionothermal system.Then a certain mass of modified HZSM-5 by PDDA?poly dimethel diallyl ammonium chloride?were added into the synthetic sol of SAPO-5 under hydrothermal and ionothermal,a series of different ratio of HZSM-5?solid?to SAPO-5?liquid?were prepared,XRD,TG,FT-IR,SEM,TEM,N₂ adsorption-desorption,NH₃-TPD and Py-IR were utilized to characterize the structures and acidic properties of molecular sieves,and the effects of solid-liquid ratio on the morphology,pore structure,acidity,BTEX selectivity of isobutane aromatization and catalytic stability of HZSM-5/SAPO-5 were investigated.the conclusion are as follows:1)SAPO-5 molecular sieve were hydrothermally synthesized in 130?for6h followed by 180?for 24h when the mole ratio of SAPO-5 sol is60H₂O:Al₂O₃:P₂O₅:0.4¹.5SiO₂:1.2TEA,and the effects of SiO₂/P₂O₅ ratio on the structure and morphology of SAPO-5 were investigated.The results showed that the crystal size of SAPO-5 was the smallest when the ratio of SiO₂/P₂O₅ is1.2.Based on the above results,HZSM-5?PDDA?was added into 20ml crystallized liquid of nano SAPO-5 in 130?for 6h and adsorbing SAPO-5seeds for 30 min,then crystallized in 180?for 24 h,thus a series of HZSM-5/SAPO-5 composite molecular sieves with the ratio of solid to liquid1/20,2/20,3/20 and 4/20?g/ml?were finally synthesized.The results show that HZSM-5/SAPO-5 composite molecular sieve is HZSM-5 covered by about40nm SAPO-5 round crystals,but the coating is thicker.The strong B acid of HZSM-5/SAPO-5 composites were significantly decreased by introducting SAPO-5,the cracks of isobutane was effectively restrained and the selectivity of BTEX was improved to 88%,far higher than HZSM-5;About 20nm mesopores of composites which was possibly due to the accumulation of SAPO-5 on the surface of HZSM-5 were favorable for the improvememt of the diffusion rate,which also accounts for the promotion of the stability of reaction;the liquid yield is a little bit lower The aromatization performance was better when the ratio of solid to liquid was 2/20?g/ml?.2)SAPO-5 was ionothermally synthesized in 180?for 72 h when the mole ratio of synthetic sol was 54EU:2.0P₂O₅:1.5Al₂O₃:0.5HF:0.4SiO₂,followed by the effect of the ratio of SiO₂/P₂O₅?0.2⁰.7?on crystal size of SAPO-5 was investigated.The results showed that the crystal size of SAPO-5was the smallest?about 40nm?when the ratio of SiO₂/P₂O₅ is 0.6.Then HZSM-5?PDDA?with SAPO-5 nano-crystals adsorbed on their surface electrostatically were introduced into the ionothermal synthetic sol of nano-SAPO-5 and crytallized in 180?for 72h,a series of HZSM-5/SAPO-5core-shell molecular sieves with the ratio of solid to liquid being 0.1/8.5,0.2/8.5,0.3/8.5 and 0.4/8.5?g/ml?were prepared.The results showed that HZSM-5/SAPO-5 composites were core-shell structures of HZSM-5 covered by40nm SAPO-5 round crystals,but the coating was incomplete;and the mesopores were about 50nm larger and distributed widely than that of the composites synthesized hydrothermally;the introduction of SAPO-5 decreased the strong Bronsted acid sites of the composites and supressed the cracking of isobutane,thus the selectivity of BTEX and the stability of the reaction were promoted slightly higher than that of the composites from hydrothermal system,but the liquid yield was still lower.The catalytic performance of aromatization was better when the ratio of solid to liquid of HZSM-5/SAPO-5 was0.2/8.5?g/ml?.3)HZSM-5/SAPO-5 composite molecular sieves were hydrothermal synthesized by the two stage crystallization method of 130?for 3h followed by 180?for 24h,before which nano SAPO-5 crystallites synthesized ionthermally were adsorbed to the surface of HZSM-5.The results showed that HZSM-5/SAPO-5 core-shell composites were also obtained base on the above optimum conditions,the SAPO-5 coating on the surface of HZSM-5 composed of about 40nm SAPO-5 round crystals was well completed and distributedly uniform,and the quantity of mesopores was more than that of composites prepared ionothermally and hydrothermally;the BTEX selectivity was more than 95%and the stability of the reaction and the liquid yelid were higher than ionothermal;the activity and stability of aromatization were the best when the ratio of solid to liquid was 4/20?g/ml?.