分子筛催化剂上正十六烷的临氢异构化反应

Hydroisomerization of n-Hexadecane on Zeolite Catalysts

对分子筛催化剂上正十六烷的临氢异构化反应进行了研究 ,考察了不同分子筛催化剂的活性和选择性 ,并对临氢异构化反应的产物分布进行了分析 .在此基础上 ,对正十六烷的临氢异构化反应机理进行了初步探讨 .结果表明 ,正十六烷在中孔分子筛催化剂上的临氢异构化反应主要在分子筛外表面和孔口进行 ,分子筛的择形作用对异构选择性的影响有限 .这与一般情况下低分子量正构烷烃在择形分子筛上的临氢异构化反应有所不同 .临氢异构化反应的异构选择性主要取决于催化剂酸性组分的酸性质 ,弱酸和中等强度的酸对异构化反应有利 ,而催化剂的活性则由酸性组分中B酸中心的数量和加氢组分的活性共同决定 .

Hydroisomerization of n hexadecane on zeolite catalysts was studied and the reaction mechanism was discussed, which may provide new clues to developing hydroisomerization catalysts with high selectivity. The catalysts were prepared as follows: zeolite and alumina were mixed and then extruded to make a support, after drying and calcination, platinum was loaded on by wet impregnation with Pt(NH 3) 4Cl 2 solution. The hydroisomerization of n hexadecane was carried out in a continuous fixed bed reactor with down flow. The catalysts were loaded in the reactor and reduced for 4 h. Then n hexadecane was pumped into the reactor at different temperatures. The reaction results indicated that Pt/MCM 22 and Pt/ZSM 5 catalysts show higher activity but lower selectivity than Pt/Hβ and Pt/SAPO 11 catalysts. The selectivity of Pt/SAPO 11 is the highest, more than 90% at the conversion of 80%. From the cracked product distribution, it could be seen that, for Pt/ZSM 5 and Pt/Hβ, the fractions of small molecule products, such as C 3, C 4 and C 5, are more than those of long chain products, but for Pt/SAPO 11, the cracked product distribution is nearly symmetrical, which means that there is secondary cracking occurred for Pt/ZSM 5 and Pt/Hβ but no for Pt/SAPO 11. The symmetrical distribution of cracked products is the characteristics of large pore zeolites or amorphous catalysts, so it can be concluded that the hydroisomerization takes place on the external surface or at the pore mouth of SAPO 11, but not in the pore channel. The different zeolites showed different behavior for the isomerization, which can be attributed to their different acidity. The strong acidity of MCM 22 and ZSM 5 leads to poor isomerization selectivity. SAPO 11 has high selectivity, indicating that the weak and mild acids are favorable to the isomerization.