| الوصف: |
Silicoaluminophosphate (SAPO) molecular sieves, SAPO-5, SAPO-11, SAPO-20 and SAPO-34, were successfully synthesized by a novel dry-gel conversion method. The isopropylation of biphenyl (BP) with propylene to selectively produce 4,4′-diisopropylbiphenyl (4,4′-DIPB) was investigated over these molecular sieves, with more emphasis on SAPO-5. SAPO-5 exhibited high selectivity for 4,4′-DIPB (65%) with high BP conversion (89%). SAPO-11 showed high selectivity for 4,4′-DIPB with low conversion, whereas SAPO-20 and SAPO-34 had only low activities for the isopropylation of BP. The activity was influenced by the acidity of the catalysts and the selectivity by their structures. The selectivity for 4,4′-DIPB was influenced by reaction temperature and by reaction period in the isopropylation of BP over SAPO-5. With the increase in temperature, the selectivity decreased; such a decrease was compensated by the increase of thermodynamically more stable 3,3′- and 3,4′-DIPBs, whereas the isomerization of 4,4′-DIPB was observed when the period was prolonged. However, the composition of 4,4′-DIPB in the encapsulated products was always higher than that in the bulk products. Propylene pressure had less effect on activity and selectivity. The isomerization of 4,4′-DIPB to 3,3′- and 3,4′-DIPBs was also studied over SAPO-5, and considerable isomerization was observed at high temperature (300 °C). Ceria modification of SAPO-5 was effective for enhancement of the selectivity for 4,4′-DIPB because of the decrease in the undesirable non-regioselective reactions on external acid sites. |