Leaching and homogeneous contribution in liquid phase reaction catalysed by solids: The case of triglycerides methanolysis using CaO

This work investigates two fundamental aspects faced in the heterogeneous catalytic reaction in liquid phase: leaching of species from the solid and the participation of these species in the catalytic reaction. Particularly we have focused on the transesterification of triglycerides with methanol catalysed by solid CaO to produce biodiesel. The first aspect affects the industrial application as extensive leaching may threaten the reusability and the environmental sustainability of catalyst. A method based on in situ conductivity measurements has been used to quantify the amount of solid that is solubilised. This in situ methodology is very helpful in the case of solids sensitive to CO2 and H2O present in ambient atmosphere, as it is the case of CaO. The amount of CaO that can be solubilised in methanol, and for the first time, in glycerol–methanol and in biodiesel–glycerol–methanol mixtures has been investigated. Larger amount of leached species was observed when glycerol is present because Ca diglyceroxide is formed due to the reaction between CaO and glycerol and this is a more soluble compound than CaO. Notwithstanding this, the solid can be reutilised for a reasonably large number of runs under the catalyst loading frequently used (larger than 1 wt% referred to oil). The second investigated aspect refers whether the solid can be considered just a source of leached active homogeneous species. It was found that the homogeneous contribution arising from the leached species can be considered negligible provided that the catalyst loading is that often used (close to or larger than 1 wt% referred to oil). The methodology and principles used in this work may be also relevant to other heterogeneous reaction carried out in liquid phase.