Effect of acid modification of kaolin and metakaolin on Bronsted acidity and catalytic properties in the synthesis of octahydro-2H-chromen-4-ol from vanillin and isopulegol

Natural kaolin and the metakaolin obtained by calcination of kaolin at 650 degrees C were chemically activated using 0.25-3.0 mol/dm(3) HCl solutions. Structural and textural properties of samples were characterized by X-ray diffraction, elemental analysis and N-2-adsorption/desorption analyses. The amount of Bronsted acid sites (BAS) was determined by infrared spectroscopy using pyridine as probe molecule. The amount of BAS for kaolin rose with increasing HCl concentration up to 1.0 M, and decreased for higher concentrations, that is related with the leaching of Al from the solid. Effect of HCl concentration on amount of BAS was negligible for metakaolin. The catalytic performance of these materials was investigated in the Prins cyclization of (-)-isopulegol with vanillin to form octahydro-2H-chromen-4-ol, carried out in toluene at 50 degrees C. The reaction rate and the selectivity toward octahydro-2H-chromen-4-ol depended on the Bronsted acidity, structural and textural properties of the catalysts. The reaction rate in the presence of kaolin modified by HCl correlated with the amount of BAS. HCl-kaolin samples were more active compared with HCl-metakaolin, while selectivity reaction toward octahydro-2H-chromen-4-ol was larger in the presence of HCl-metakaolin. (C) 2016 Elsevier B.V. All rights reserved.