Acid-catalyzed production of biodiesel over arenesulfonic SBA-15: Insights into the role of water in the reaction network

This work presents a systematic approach to understand the effect of the presence of water in highly acidic crude palm oil–typical conditions of low-grade oleaginous feedstock on the performance of arene-SO3H-SBA-15 catalyst in the batch-production of biodiesel. The addition of small amounts of water (1 wt%) to the reaction medium led to a clear reduction of the observed yield to fatty acid methyl esters (FAME), being this decay usually attributed to the highly hydrophilic nature of arenesulfonic acid groups, and the associated difficulties of hydrophobic substrates to access these catalytic acid sites. However, the addition of larger amounts of water –up to 10 wt%– did not cause a proportional decay in the yield to FAME, but a higher production of free fatty acids (FFA). This is attributed to the promotion of acid-catalyzed hydrolysis of both starting triglycerides and formed FAME. The net result is not only a significant reduction of the final FAME yield, but also the appearance of high acid values, i.e. FFA contents, in the final biodiesel. Consequently, the overall process is simultaneously affected by transesterification, esterification and hydrolysis reactions, all of them catalyzed by Brønsted acid sites and dependent on the reaction conditions –temperature and water concentration– to different extents. Several strategies devoted to manage such behavior of sulfonic acid-modified SBA-15 catalysts in presence of water, aiming to maximize FAME yield while minimizing FFA content, have been explored: (1) minimization of the water content in the reacting media by pre-drying of feedstock and catalyst; (2) addition of molecular sieves to the reacting media as water scavengers, (3) hydrophobization of the catalyst surface to minimize the water uptake by the catalyst; and (4) use of a decreasing reaction temperature profile in order to first promote transesterification at high temperature and then reduce the temperature to keep at a minimum the hydrolysis of formed FAME. All these strategies resulted in an improvement of the catalytic performance, especially the use of a decreasing temperature profile. The results showed by the latter strategy open new possibilities and reaction pathways in which readily available, low-grade, cheap oleaginous feedstock with high water and FFA contents can be efficiently converted into biodiesel.