Abstract
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•Enhanced formation of unstable 1,4,6‐triisopropylnaphthane (1,4,6‐TriIPN) at low temperatures, and the increase of stable 1,3,6‐ and 1,3,7‐DIPN at higher temperatures.•The selectivities for TriIPN isomers keep constant during the reaction without their isomerization.•Enhanced selectivities for β,β‐diisopropylnaphthalene (β,β‐DIPN) in the initial stage of the catalysis.•New type of thermodynamic control favored the stable β,β‐DIPN over fresh catalyst.
The isopropylation of naphthalene (NP) was carried out over a BEA zeolite (BEA38; SiO2/Al2O3 = 38) focused on the selectivities for diisopropylnaphthalene (DIPN) and triisopropylnaphthalene (TriIPN) isomers. The isopropylation gave possible eight DIPN isomers including β,β- (2,6- and 2,7-), α,β- (1,3-, 1,6-, and 1,7-), and α,α- (1,4- and 1,5-). The catalysis over BEA works two types of controls: kinetic control operates to form predominantly bulky and unstable α,α-DIPN at low temperatures, and thermodynamic controls work for the predominant formation of the slim and stable β,β-DIPN at high temperatures, although the intermediately bulky and stable α,β-DIPN are the major products through both controls. The enhanced selectivities for β,β-DIPN were observed at the early stages of the catalysis in the range of 200−300 °C, which operate under new type of thermodynamic control over fresh catalyst through thermodynamically preferred transition states; however, they decreased with the increase in the selectivities for α,α- and α,β-DIPN, and converged after prolonged reaction period. The isopropylation of DIPN isomers gives TriIPN isomers: unstable and bulky 1,3,5- and 1,4,6-TriIPN with α,α,β-substitution, and stable and slim 1,3,7- and 1,3,6-TriIPN with α,β,β-substitution. The low temperatures favor the former isomers, whereas the selectivity for the latter isomers increases with increasing reaction temperature. These results indicate that TriIPN isomers principally form under kinetic control at low temperatures, and thermodynamic controls participate in the catalysis at high temperatures. The selectivities for TriIPN isomers kept constant during the reaction at all temperatures: 200, 250, and 300 °C. The catalysis occurs inside the BEA channels and allow even the formation of bulky 1,3,5- and 1,4,6-TriIPN; however, all isomers cannot be isomerized to the others in the channels and on the external surfaces. Severe coke-deposition occurred during the catalysis, particularly in the early stages; however, the catalyst is recovered by the calcination with a small change in catalytic activity.