Study of the influence of physical–chemical properties of steamed H-MOR zeolites in the mechanism of adsorption of fatty acids and their esterification

Abstract

The increasing use of fatty acid esters, on the industry and the substitution of traditional petroleum fuels, promotes the study of heterogeneous catalysts with a better environmental performance than their homogeneous counterparts. The mordenite, has attracted special attention, for its environmental friendliness, exceptional physical chemical properties and the availability of large natural deposits. In order to study the influence of its physical chemical properties over the mechanism of adsorption of fatty acids (FA) and their esterification, three new solids were obtained through hydrothermal treatment at 800–1000K of a mordenite with a Si/Al ratio of about six. The characterization by FTIR and XRD confirms the collapse of crystallinity with the increase of the treatment temperature. The partial exiting of tetrahedral aluminum out of the crystalline network, revealed by 29Si and 27Al MAS NMR leads to the diminution of the acidity and the increasing hydrophobicity, evidenced by FTIR spectroscopy and adsorption isotherms of stearic acid. The kinetic evaluation of FA esterification reveals a pseudo-second order model. These results suggest a change in the adsorption mechanism: In the original mordenite and treated at low temperatures, a higher density of acid sites favors the adsorption of the carboxyl group of FA molecules by hydrogen-bonding to the zeolite Al–OH–Si groups, while the tails adsorb on the less polar surroundings. At greater treatment temperatures, the taller hydrophobicity induces the adsorption of the FA molecules onto the lesser polar (and lesser active) sites of the zeolite with its carboxyl groups oriented away from the zeolitic surface.