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 Si-29, Al-27, P-31, Na-23 and H-1 MAS NMR spectroscopy. Acidity measurement by FTIR spectroscopy monitoring of pyridine adsorption reveals that the incorporation of Al in the framework generates Bronsted and Lewis acidity, which increases with the increase in metal content. By loading H3PO4 on the AlMCM-41, Bronsted 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 beta-and beta, beta-selectivities over 30 wt% H3PO4/AlMCM41(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.