Polystyrene-divinylbenzene resins bearing linear acyl groups with up to eight carbon atoms in the chain have been prepared and sulfonated to obtain oleophilic, strongly acidic ion-exchange catalysts. They were tested in the esterification of stearic acid with methanol at 85 °C using edible oil as the reaction medium to mimic raw materials for the production of biodiesel with a high content of free fatty acids. The acylation of the catalysts made them more oleophilic hence less active than a conventional strongly acidic ion-exchange catalyst when the reaction mixture was saturated with methanol (20 %, w/w), because of their lowered swelling ability. However at relatively low methanol concentration (5 %, w/w) not only the acylated catalysts were generally more active than the conventional catalyst, but their activity showed an excellent correlation with the degree of acylation and the number of carbon atoms of the acyl groups. The results clearly show that this does not depend on their swelling behaviour under duty rules, but rather to enhanced adsorption of stearic acid into the catalyst facilitated by the intermixing of the acylic and sulfonic groups at the molecular level.
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