Organosulfonic acid functionalized montmorillonites as solid catalysts for (trans) esterification of free fatty acids and (waste) oils

Abstract

Organosulfonic acid functionalized montemorillonite-MMT (K10 and cloisite Na+) were prepared by different methodologies: (a) one-pot organosilylation or sulfonation, and (b) two-step organosilylation followed by sulfonation or oxidation. The physicochemical characterization ensured the success of the selected methodologies to promote HSO3-functionalization including high S contents (2.84 mmol S/g) and acidities up to 6.03 mmol H+/g (K10_PhSO3H).All the organosulfonic acid functionalized MMT were tested as acid catalyst in the esterification of free fatty acids in a hybrid feedstock model. Conversions of 100% were obtained after 60 min for K10_PhTESO3H, K10_SO3H, Cloi_PhTESO3H and Cloi_H2SO4, in accordance with the acid character of these catalysts, prepared by one-pot and two-step methodologies involving a sulfonation step. The MMT catalysts obtained by direct sulfonation exhibited the highest TOF whereas those obtained by two-step organosilylation-sulfonation showed the highest stability and reusability. The increase of FFA carbon chain length led to higher conversions, showing the prevalence of inductive effect and absence of steric effects on the esterification mechanism using these organosulfonic acid functionalized catalysts. Moreover, K10-PhSO3H was applied as solid acid catalyst in a simultaneous (trans) esterification of different feedstock types for biodiesel production giving up to 100% of FAMEs under moderate conditions.