Solid acid catalysed acylation of 2-methoxy-naphthalene: role of intraparticle diffusional resistance

Abstract

Synthesis of fine chemicals and intermediates by using Friedel–Crafts acylations is important in organic chemical technology. In most of the cases very good yields and selectivity can be obtained with aluminium chloride as catalyst in conjunction with nitrobenzene as solvent. However, with the current environmental restrictions, replacement of aluminium chloride–nitrobenzene pair with solid acid catalysts has great industrial relevance. Acylation of 2-methoxynaphthalene (2-MON), also known as yarayara, with acetic anhydride, was carried out using different solid acid catalysts such as zeolites, acid activated clays, ion exchange resins and sulphated zirconia. The products of the reaction are precursors for many organic and pharmaceutical products. The reaction products were isolated and confirmed by their melting points, 1H-NMR, gas chromatography, etc. A mathematical model is built to describe the kinetics of the reaction. At high molar ratios of acetic anhydride to 2-MON, the reaction is limited by intraparticle diffusion of the substrate 2-MON. However, this barrier is overcome for equimolar qualities. The reaction proceeds through the Eley–Rideal type of mechanism, wherein the chemisorbed acetic anhydride generates a carbocation and acetic acid, and the carbocation reacts with 2-MON from the liquid phase within the pore space to form 1-acetyl-2-methoxy-naphthalene.