Effect of Propylene Pressure on Selectivity

Matsuda and his co-workers also showed that extensive isomerization of 4,4 -DIPB on a HM(20) catalyst was observed at high conversion of biphenyl in the presence of limited amounts of propylene (initial propylene/biphenyl ratio=1.0). However, the selectivity was almost constant during the reaction although the selectivity for 4,4 -DIPB was less than that of HM(IO) catalyst. They suggest that the low isomerization activity of HM(IO) is related to its weak acid strength. 

Another interesting feature of the reaction of 4,4 -DIPB is that no further isopropylation to polyisopropylbiphenyls was observed under high pressure of propylene. There is not enough space in the pores for the transition state of further isopropylation of 4,4 -DIPB. This is one of the reasons for the highly regioselective formation of 4,4 -DIPB. 

Takahata and his co-workers found an increase in the selectivity for 4,4 -DIPB with increasing propylene pressure over HM with a low Si02/Al203 ratio. Fellman suggested that the increase in selectivity was due to the accumulation of propylene at active sites. However, it is obvious from above discussion that the 

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Effect of Propylene Pressure on Selectivity - The partial pressure of propylene is also one of key factors for selective formation of 4,4 -DIPB.22"25 The high partial pressure of propylene effectively enhanced the isopropylation, but the selectivity for 4,4 -DIPB decreased at lower partial pressures over HM(220), as shown in Figure 6.24-25 However, the isomerization of 4-IPBP did not occur at any propylene pressures. Because 3,4 -DIPB is a more thermodynamically stable isomer than 4,4 -DIPB,43 this decrease of the selectivity was ascribed to the isomerization of 4,4 -DIPB to 3,4 -DIPB, not to the lower selectivity to 4,4 -DIPB. Figure 7 shows the effect of propylene pressure on the selectivities for 4,4 -DIPB in bulk and encapsulated products. The selectivity of 4,4 -DIPB inside the pores was almost constant at every pressure. These results indicate that the isomerization does not occur inside the pores but at the external acid sites. The effect of the pressure on the isomerization of 4,4 -DIPB was similar to that on the isopropylation of biphenyl. 4,4 -DIPB itself isomerizes significantly to 3,3 - and 3,4 -DIPB over the catalyst in the absence of propylene. However, no significant isomerization of 4,4 -DIPB occurred in the presence of sufficient propylene pressure. On the other hand, the selectivity of 4,4 -DIPB in encapsulated DIPB isomers was almost constant at any pressure. These differences support the hypothesis that the isomerization of 4,4 -DIPB to 3,4 -DIPB occurs on the external surfaces. The isomerization of 4,4 -DIPB under high pressures is considered to be retarded by the preferential adsorption of propylene on acid... 

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