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

摘要

Abstract 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.