Transalkylation of Biphenyl and Naphthalene

The ethylation of naphthalene was not shape-selective even over HM. However, the transethylation of naphthalene with polyethylbenzenes over HY gave high selectivity for 2,6-DEN with high yield. Tetraethylbenzene was the best transalkylating agent among di-, tri-, and tetraethylbenzenes to form 

6- DEN. The highest selectivity and yield of 2,6-DEN were 62.4 and 46.6%, respectively, at 110 °C for 8 h. Because the silica-alumina catalyst yielded 

6- DEN in high selectivity, the transethylation was not explained by the shape-selective catalysis, either. The key factor for the high selectivity is the steric hindrance and high electron density at the substitution site of naphthalene. 

The product distribution in the zeolite-catalysed alkylation of polynuclear aromatics depends on the structure of zeolite pores. High regioselectivities were observed in the HM catalysed isopropylation of polynuclear aromatics, such as biphenyl, naphthalene, p-terphenyl, and dibenzofuran, to yield predominantly the least bulky products e.g., 4,4 -DIPB for biphenyl, and 2,6-DIPN for naphthalene. These reactions are controlled by steric restriction at the transition state inside the pores and by the entrance of intermediate products molecules into the pores. On the other hand, the catalysts with large-pore HY and HL zeolite are controlled at low temperature by the electron density of the reactant molecule and at higher temperature by the stability of the product molecules because their pores have enough space for a transition state, which allow the formation of all corresponding isomers. 

Dealumination of HM resulted in common features for both the isopropylation of biphenyl and naphthalene. The dealumination of HM decreases the blocking of the pores by coke deposition in the both reactions because of the decrease in acid sites at intracrystalline and external surfaces. It also reduced the non-regioselective isopropylation at the external surface, and enhanced the reaction to yield the least bulky product molecules. The pressure of propylene is also an important key factor for high yield of 4,4 -DIPB in the isopropylation of biphenyl, because the isomerization of 4,4 -DIPB occurred significantly under low pressure of propylene at the external acid sites. 

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