Granados, M. L.; Alba-Rubio, A. C.; Vila, F.; Alonso, D. M.; Mariscal, R.
Recently, we have found that forming a slurry by mixing Ca oxide with biodiesel (approximately 10–15 g of biodiesel per gram of Ca oxide) results both in protection against poisoning by atmospheric CO2 and H2O and, when the slurry is used as a catalyst, in a remarkable increase in reaction rate [M. Lopez Granados et al., Energy Fuel 23 (2009) 2259–2263]. By conducting catalytic tests and DRIFT studies with either sunflower oil or model compounds revealed that the presence of minute amounts of monoglycerides (MG) and/or diglycerides (DG) in the biodiesel were the reason for the reaction rate promotion. To observe this effect, the Ca oxide–biodiesel slurry must be pretreated with methanol for a few minutes before proceeding with the transesterification reaction. The DRIFT studies demonstrated that the transesterification of MG and DG during the pretreatment with methanol results in the releasing of glycerol, which then reacts with the catalyst surface, resulting in the formation of very active surface Ca glyceroxide species. In view of this information, it was also demonstrated that the two-step procedure of mixing biodiesel with the catalyst and then carrying out methanol pretreatment could be substituted with pretreatment of the Ca oxide with methanol containing a few milligrams of glycerol per gram of Ca oxide before proceeding with the reaction. In this latter case, the slurry was not required. The DRIFT studies demonstrated that this simpler pretreatment also resulted in the formation of very active surface Ca glyceroxide species.