Activity and selectivity of different base catalysts in synthesis of guaifenesin from guaiacol and glycidol of biomass origin

Abstract

Guaiacol and glycidol can be obtained from biomass valorization. Guaiacol (2-methoxyphenol) and glycidol (2,3-epoxy-1-propanol) have been used for the efficient synthesis of guaifenesin ((RS)-3-(2-methoxyphenoxy) propane-1,2-diol). Different catalysts such as hydrotalcite (HT), calcined hydrotalcite (CHT), calcinated hydrotalcite supported on hexagonal mesoporous silica, magnesium oxide, alumina and, potassium promoted zirconium oxide were synthesized, out of which CHT was found to be the most active, selective and reusable catalyst. The catalyst characterization was done by different techniques. Both Oxide and hydroxide phases were observed on calcination of HT in air at 450 °C for 6 h. CHT possess both acidic and basic sites and basicity of CHT was the highest. Crystallite size, surface area and pore size of CHT play important role in catalytic activity and selectivity. Reaction was carried out in a batch reactor and influence of different parameters was systematically studied. The reaction mechanism involving two sites, acidic and basic, was proposed. A suitable kinetic model was developed and fitted against experimental data. A second order rate equation was derived on the basis of Langmuir–Hinshelwood–Hougen–Watson mechanism with weak adsorption of reactants, intermediates and products. Kinetics was used to predict reaction conditions to obtain guaifenesin selectively. Guaifenesin was efficiently obtained with 94.8% selectivity at guaiacol conversion of 38.2% over CHT at 80 °C after 4 h.