Synthesis, characterization and activity of AL-MCM-41 catalyst for hydroxyalkylation of epoxides

Abstract

Perfumery chemicals and intermediates are produced on a large scale by Friedel Crafts alkylation or acylation of aromatic compounds in the presence of Lewis acid catalyst. However, problem in the industrial process of perfumery chemical and intermediate manufacture like toxity, corrosivity and production of pollutants, make convenient to change the conventional Lewis acid AlCl3 or FeCl3 catalysts by acid solid catalyst. Thus, Al-MCM-41 catalysts were prepared with various SiO2:Al2O3 ratios via direct and secondary syntheses using sodium aluminate as the aluminium source. Al-MCM-41 was characterized by X-ray Diffraction (XRD), Surface Area Analyzer Instrument and Fourier Transform Infrared Spectroscopy (FTIR). The results indicate that Al-MCM-41 sample with a uniform hexagonal pore structure and high surface area was synthesized. Structural studies by 27Al and 29Si MAS NMR spectroscopy indicated that Al are in the tetrahedral form and located in the framework. The presence of distorted framework aluminium was also observed, more significantly in the secondary aluminated samples. Maximum amount of Al was incorporated by direct synthesis with SiO2:Al2O3 ratio of 10 and a calculated Si/Al ratio of 15.2. Acidity studies using Pyridine Desorption Measurement and Temperature Programmed Desorption of Ammonia (TPD-NH3) show that the acidity of Al-MCM-41 increases with increase in Al incorporation into the MCM-41 framework. The potential of H-Al-MCM-41; as a heterogeneous catalyst was studied in the hydroxyalkylation of benzene with propylene oxide as a model reaction. Favourable reaction conditions such as SiO2:Al2O3 ratios, temperature, time on stream, the reactant mole ratio and solvent have significant influence on the distribution of products. Gas chromatography analysis indicates that H-Al-MCM-41 with SiO2:Al2O3 ratio of 10 demonstrates the highest catalytic activity with a conversion of benzene and selectivity of 92.3% and 87.5% respectively. The formation of 2-phenyl-1-propanol was favourable occurred at a temperature of 393 K after 24 hours with propylene oxide to benzene mole ratio of 0.5 using nitrobenzene as the solvent. The activity enhancement for catalyst is associated with the presence of distorted tricoordinated aluminium as Lewis acid sites. The strength of Lewis acid sites was correlated to appropriate aluminium content, temperature, B/L ratio, crystallinity and surface area of sample which played a role in order to improve catalytic activity of Al-MCM-41. Aprotic dipolar solvent such as nitrobenzene stabilized the unstable intermediate of propoxy cations to prevent propylene oxides oligomerisation. The results indicate that instead of aluminium content, solvent and reactant mole ratio also play a role to give high conversion and selectivity of 2-phenyl-1-propanol.