Hydroisomerization of n-hexadecane on zeolite catalysts

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(NH3)(4)Cl-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/Hbeta 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/Hbeta, 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/Hbeta 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