Structural and surface study of calcium glyceroxide, an active phase for biodiesel production under heterogeneous catalysis

Calcium diglyceroxide has been reported as a very active phase in the transesterification of triglycerides with methanol for biodiesel production. This work reports on the determination of the crystal structure of Ca diglyceroxide by Patterson-based direct methods from synchrotron X-ray powder diffraction (XRD) data in combination with characterisation by Differential Thermal Analysis-Thermogravimetry (DTA-TG), Scanning Electron Microscopy (SEM) and X-ray Photoelectron and Fourier Transform-Infrared spectroscopies (XPS and FT-IR). Its crystal structure is formed by molecular tetramers held together by a complex H-bond network. Both the XRD structural determination and the O1s XPS core level indicate the presence of a basic non-protonated O− anion at the surface of Ca diglyceroxide. Along with the presence of surface lipophilic CHx units, this O− anion may be the origin of this molecule’s high activity relative to CaO.

Two relevant surface features of Calcium diglyceroxide have been disclosed by surface characterisation and determination of its crystal structure: the presence of basic unsaturated O anion and the presence of lipophilic CHx units. Both characteristics can favour two key steps of the reaction mechanism: methoxide formation and nucleophilic attack of the latter to the carbonyl groups of triglyceride molecule.Figure optionsDownload high-quality image (123 K)Download as PowerPoint slideHighlights
► Surface characterisation and solving of the crystal structure are reported.
► Presence of surface basic O anions and of lipophilic CHx units has been disclosed.
► Both properties can favour two key steps of the reaction mechanism.
► First property can favour the methanol activation to methoxide species.
► The second one allows the approaching of hydrophobic triglyceride to the surface.