Andrew D. Schmitz

Shape-selective isopropylation of naphthalene over mordenite catalysts: Computational analysis using MOPAC

Abstract In our experimental work on shape-selective isopropylation of naphthalene, the selectivity for 2,6-diisopropylnaphthalene (2,6-DIPN) and the ratio of 2,6-DIPN to 2,7-DIPN were increased by mordenite catalyst dealumination. However, it was not clear whether the differentiation between the two isomers was caused by their differences in molecular dimensions or in electronic properties. In this work we performed a computational analysis of the molecular dimensions and frontier electron density f r ( E ) using MOPAC program for naphthalene, isopropyl- and diisopropylnaphthalene. The f r ( E ) value for electrophilic substitution reaction represents the density of electrons in the highest occupied molecular orbital (HOMO). According to the frontier molecular orbital theory, the most reactive position (the carbon atom on which electrophilic attack occurs most likely) has the highest frontier electron density. The calculation shows that 2,6-DIPN has a slightly smaller critical diameter. More importantly, position 6 in 2-IPN has higher f r ( E ) value than that of position 7 in 2-IPN. This suggests that during 2-IPN isopropylation inside a mordenite channel, the formation of 2,6-DIPN is electronically more favored than that of 2,7-DIPN.

Shape-selective isopropylation of naphthalene. Reactivity of 2,6-diisopropylnaphthalene on dealuminated mordenites

Shape-selective isopropylation of naphthalene over a hydrogen-mordenite (HM) catalyst is an effective route for the production of 2,6-diisopropylnaphthalene (2,6-DIPN). Our previous work on naphthalene isopropylation has shown that 2,6-DIPN selectivity is substantially increased by HM dealumination. An assessment of 2,6-DIPN reactivity on the catalyst would be useful in understanding factors that control product selectivity. We measured 2,6-DIPN reactivity on three HM catalysts (SiO2/AI2O3 ratio 14, 38 and 74), under propylene pressure, in batch reactor tests at 200°C. The propylene pressure was varied to give propylene to 2,6-DIPN mole ratios of 4.0, 1.0 and 0.5. On HM14 and HM38 catalysts, 2,6-DIPN conversions were 12–27%, mostly to higher alkylates. Some isomerization of 2,6-DIPN occurred at the lowest propylene pressure, but was suppressed by increasing the propylene pressure. 2,6-DIPN is nearly unreactive on dealuminated mordenite (HM74). These results reveal that the high selectivity toward 2,6-DIPN in the isopropylation of naphthalene is partly attributable to the low reactivity of 2,6-DIPN on dealuminated mordenites.

Shape-selective isopropylation of naphthalene over dealuminated mordenites. Increasingβ-substitution selectivity by adding water

Isopropylation of naphthalene was examined over two dealuminated H-mordenites (HM) having SiO2/Al2O3 molar ratio of 38 (HM38) and 74 (HM74). Experiments were done in batch reactors at 200°C. With propylene as the alkylating agent, selectivity forβ-substitution of naphthalene increased when water was added, and reached a maximum with a water-to-catalyst mass ratio of 0.80 (ca. 45 mmol water/g-cat). Of particular importance was the increase inββ-selectivity to 2,6- and 2,7-diisopropylnaphthalene (2,6-DIPN and 2,7-DIPN) with added water. Without added water,ββ-selectivities were 56 and 77% for HM38 and HM74, respectively. Selectivities to the highly desired 2,6-DIPN isomer were 37 and 54%, respectively. With added water,ββ-selectivities increased to 90 and 95%, and 2,6-DIPN selectivities increased to 60 and 70% for HM38 and HM74, respectively. A large amount of added water always decreased the activity; however, mixed effects were observed for intermediate amounts of added water. Sorption of water on the catalyst surface seems to explain changes in isomer selectivity and catalytic activity. Coke deposits on the catalysts also decreased when water was added. Furthermore, it was shown that higherββ-selectivity could be obtained with isopropanol (i-PrOH) as the alkylating agent, rather than propylene, under the same conditions. Comparison experiments showed that this was due to water formed fromi-PrOH.