Shape-selective isopropylation of naphthalene over H-mordenite catalysts for environmentally friendly synthesis of 2,6-dialkylnaphthalene

Recent development of advanced polymer materials such as polyethylene naphthalate, polybutylene naphthalate and liquid crystalline polymers has created an increasing demand for 2,6-dialkylnaphthalene, which has spurred interest in shape-selective naphthalene alkylation. This work deals with mordenite-catalyzed shape-selective naphthalene isopropylation to produce 2,6-diisopropylnaphthalene (2,6-DIPN). Effects of dealumination of mordenite on the structural and acidic characteristics and on the shape selectivity and activity were examined by physicochemical analysis, TPD, solid-state ₂₇Al and ₂₉Si MAS NMR, XRD, as well as catalytic alkylation reactions. Dealumination removes octahedral Al species as well as tetrahedral Al species, decreases the unit cell dimensions and reduces the number of strong acid sites in mordenites. Proper dealumination can improve selectivity to 2,6-DIPN from 33 to 61 % and significantly increases 2,6/2,7 ratio. Improved selectivity to 2,6-DIPN upon proper dealumination was attributed to the decrease in mordenite acidity, reduction in unit cell dimension and removal of some strong acid sites. However, neither the change in selectivity nor that in activity is a simple function of dealumination degree or SiO₂/Al₂O₃ ratio. Some minor difference in the apparent framework SiO₂/Al₂O₃ ratio can result in a major difference in activity or selectivity. There exist optimum conditions of dealumination as well as optimum reaction conditions for achieving higher selectivity to 2,6-DIPN.