Transesterification of Jatropha curcas oil and soybean oil on Al-SBA-15 catalysts

Al-SBA-15 samples with molar Si/Al ratios of 3, 22 and 73 were synthesized and tested for the acid catalyzed transesterification of Jatropha curcas oil with methanol. Being non-edible, low-cost and acidic, Jatropha oil is an interesting feed for biodiesel production, as its use would not entrain competition between food and fuel markets. For fundamental reasons, soybean oil, a typical high quality feed, was also investigated, as its negligible free fatty acids content would not mask the extent of transesterification by the simultaneous occurrence esterification. All the catalysts were characterized as to their composition, structure and texture by ICP-AES, Al-27 MAS NMR, X-ray diffraction and nitrogen physisorption. Surface acidity was assessed by ammonia adsorption microcalorimetry. The nature of the organic material adsorbed/trapped on the catalyst during reaction was determined by GC-MS after solvent extraction. The possibility of recovering the activity of the catalayst after a reaction cycle was also checked. In the case of soybean oil, the best-performing catalyst was that with Si/Al = 3 (90 mol% fatty acids methyl esters yield at 180 degrees C, reaction time 24h, methanol/oil ratio 12). In the case of Jatropha oil, the three catalysts showed a virtually identical performance (ca. 100 mol% methyl esters yield at 180 degrees C, reaction time 24 h, methanol/oil ratio 12), in spite of their remarkable differences in terms of both concentration and strength distribution of the acid sites. Esterification of the free fatty acids present in the feed occurred in parallel to triglycerides transesterification, both reactions being catalysed by the acid sites of the catalyst, as well as by the acids present in the feed. Several secondary reactions occurred during the transformation of both soybean oil and Jatropha oil. The corresponding reaction pathways were outlined.