Deactivation of organosulfonic acid functionalized silica catalysts during biodiesel synthesis

The reusability of silica functionalized with 4-ethyl-benzene sulfonic acid groups used as catalyst in biodiesel production from sunflower oil/methanol mixtures has been investigated. This material was used for four runs under batch mode operation, at different reaction temperatures (373, 423 and 473 K), with a catalyst loading of 1.5 wt.% referred to oil and with a methanol/oil molar ratio equal to 6. The catalyst is significantly deactivated during the first run, while the activity for the second and successive runs are very similar. Fresh and used catalysts were characterized by chemical analysis, N-2 adsorption-desorption isotherms, infrared spectroscopy and evolved gas analysis by mass spectrometry. Leaching of the organosulfonic groups and adsorption of organic compounds onto the acid sites was detected in the used catalysts. Reactants and products are involved in the leaching process, although glycerine has the highest leaching capacity. The organic deposits are formed by side reactions involving reactants and/or products. The solid porous structure remained unchanged after catalyst use, indicating that sintering or other alterations of the porous network can be discarded as source of deactivation. Leaching and deposition effects occur predominantly during the first run, slowing down notably in subsequent cycles. Both leaching and organic deposits participate of the deactivation; the latter increases its impact at reaction temperatures higher than 423 K. Partial catalyst regeneration by removal of the organic adsorbates could not be achieved by treatment at high temperature because the deposits and the organosulfonic acid sites were combusted simultaneously. (C) 2010 Elsevier B.V. All rights reserved.