| Aromatic hydrocarbons as one of most important basic chemical raw materials are mainly derived from petroleum and coal tar.But the research and development of new aromatization technology is extremely urgent because of the exhaustion of the mineral resources and the adjustment of national energy policy.In recent years,due to the impact of the national west east gas transmission project,adverse consequence is excess production of low carbon hydrocarbons.How to transform the low carbon hydrocarbons into the downstream products with high added value has become the focus of the researchers.The technology of aromatization of low carbon hydrocarbons has come into being,and HZSM-5 zeolite as a aromatization catalyst has greatly restricted the industrial application because of its many defects.In this paper,HZSM-5/SAPO-5 composite molecular sieves were synthesized by using two-stage crystallization method under hydrothermal condition.First,the surface of HZSM-5 was midified with poly cationic reagent or cationic reagent,and the SAPO-5 microcrystal in aging products were adsorbed by pretreated HZSM-5,then HZSM-5/SAPO-5 composite molecular sieve obtained by crystallization process of the mixture in a high temperature.And the optimal synthesis solution is determined by adjusting the concentration of the composite system,crystallization temperature,crystallization time,composite ratio of solid and liquid and silica content of SAPO-5 zeolite.The finished producct is HZSM-5 / SAPO-5 composite molecular sieves with high acidity,different pore size distribution and strong shell,continuous and high coverage.As the SAPO-5 zeolite covers the outer surface of the HZSM-5 molecular sieves,the acidity of HZSM-5 decreases and the pore size increases,thus inhibiting the occurrence of aromatization side reactions and the rate of coke inactivation.In this paper,the crystal structure and solid acidity of all prepared samples were represented by XRD,SEM,TEM,FT-IR,NH3-TPD and Py-IR.The effect of isobutane aromatization on the catalytic performance of isobutane aromatization reaction were investigated.And the following conclusions can be drawn:1)The synthesis ratio of SAPO-5: Al2O3 : P2O5 : 1.0SiO2 : template : 60H2 O.Preparation conditions: stirring temperature is room temperature(25?~30?);the first growth crystallization temperature 130 ?,time 6 h;the second growth crystallization temperature 200 ?,time 24 h.The pretreated nuclear phase HZSM-5 was added before the start of the second growth crystallization and then stirred for 0.5 ~ 1h to prepare HZSM-5 / SAPO-5 composite composite molecular sieve.2)The synthetic solid-liquid ratio has a great influence on the appearance and catalytic performance of the composite molecular sieve.With the increase of the proportion of HZSM-5 molecular sieves,becomes smaller and the agglomeration phenomenon of the composite molecular sieve is weakened,B acid and total acid increased gradually,but L acid almost does not change.The aromatization reaction of isobutane showed that the catalytic performance of aromatization was the best when the solid-liquid ratio was 3:20(g: ml).3)Silicon content affects the appearance of SAPO-5 molecular sieve.The results show that the growth rate of SAPO-5 molecular sieve along the a-axis will be greater than that of the c-axis,and the particles will gradually decrease,and the acid and acid strength of the SAPO-5 molecular sieve will show the law that increase first and then lower to stability.By adjusting the silicon content of the composite molecular sieve,the L acid,B acid and B / L of the molecular sieve can be optimized to make the molecular sieve have suitable acid and acid strength.And the synergetic catalytic effect of the two-phase molecular sieve is enhanced,which improves the selectivity of BTEX and the conversion of raw materials.When the silicon content of the composite molecular sieve is 1.0,the solid acid properties of the molecular sieve are more favorable for the aromatization reaction.4)HZSM-5 moleclar sieve was pretreated with two different cationic reagents: polydiallyldimethylammonium chloride solution(PDDA)and cetyltrimethylammonium bromide(CTAB)respectively.Then,two kinds of composite molecular sieves of CSZP and CSZC were synthesized respectively.Then,the microstructure and acidity of CSZP composite molecular sieve,CSZC composite molecular sieve,MZS mechanical mixed molecular sieve and HZSM-5 molecular sieve were analyzed: The two-phase structure of composite molecular sieves are much closer,the SAPO-5 molecular sieves cover the surface activitic sites of HZSM-5 raw powder effectively,and CSZP composite molecular sieve with suitable acid and the acid strength in isobutane aromatization reaction showed higher BTEX selectivity and catalytic stability;... |
PDF Full Text Request