Dialkylation of Naphthalene with Isopropanol Over H₃PO₄/MCM-41 Catalysts for the Environmentally Friendly Synthesis of 2,6-Dialkylnaphthalene

AlMCM-41 materials with SiO₂/Al₂O₃ molar ratios 20, 70, 110, 150, 200, and Si-MCM-41 were synthesized following standard procedures, and loaded with different amounts of H₃PO₄. The catalysts were well characterized by powder X-ray diffraction (XRD), nitrogen adsorption studies, and solid state ²⁹Si, ²⁷Al, ³¹P, ²³Na and ¹H MAS NMR spectroscopy. Acidity measurement by FT-IR spectroscopy monitoring of pyridine adsorption reveals that the incorporation of Al in the framework generates Brønsted and Lewis acidity, which increases with the increase in metal content. By loading H₃PO₄ on the AlMCM-41, Brønsted sites increases substantially, but the Lewis sites decrease to a large extent. Liquid phase isopropylation of naphthalene with isopropanol in n-hexane under autogeneous pressure was performed in the temperature range 473-623 K. The optimum feed molar ratio was found to be 1:2:10 (naphthalene:isopropanol:n-hexane), where the naphthalene conversion reaches to 85.5%. The β-and β,β-selectivities over 30 wt% H₃PO₄/AlMCM-41(200) were 97.0 and 84.8, respectively, at 85.5% conversion. The main products of the reaction, mono and diisopropylnaphthalenes, were analyzed and identified by gas chromatography and confirmed by GC-MS. The conversion and selectivity of the products are discussed from the point of view of catalyst characteristics and reaction conditions. Graphical Abstract [graphic removed]