Investigating the active phase of Ca-based glycerol polymerization catalysts: On the importance of calcium glycerolate.

[Display omitted] • The active phase of calcium-based catalysts for glycerol polymerization was investigated. • Ca-glycerolate was identified as being the in situ formed solid active phase. • A mechanism is proposed to explain the in situ formation of calcium glycerolate. • A mechanism is proposed for the polymerization of glycerol catalyzed by Ca-glycerolate. Ca-based catalysts are of strong interest for glycerol polymerization reaction due to their high catalytic performances, their wide availability and the absence of any toxicity. In the present study, we investigated the active phase of CaO, Ca(OH) 2 , CaCO 3 and calcium diglyceroxide used as catalysts for this reaction. The solids were analyzed by XRD, solid-state 13C-NMR and TGA-DSC. XRD evidenced the presence of calcium glycerolate in the spent catalysts when CaO, Ca(OH) 2 and calcium diglyceroxide were used as starting materials. This was corroborated by the TGA-DSC results. Further, the SS NMR results showed that the as-formed calcium glycerolate is a mixture of linear Ca(C 3 H 7 O 3) and cyclic-branched Ca(C 3 H 6 O 3). Moreover, the recycled catalyst issued, e.g. , from CaO was more efficient that the initial catalyst with a glycerol conversion of 47 % (vs. 22 % initially for CaO) and a 70 % (vs. 55 %) selectivity to polyglycerols higher than tetraglycerol at 245 °C after 4 h in the presence of 3.5 mol.% of catalyst. As the main outcome, we have shown that Ca-glycerolate is the actual solid active phase, which is formed in situ from CaO and Ca(OH) 2 playing the role of catalyst precursors. A mechanism involving the dissolution of CaO and Ca(OH) 2 , the formation of Ca-glycerolate and its precipitation followed by crystallization, supported by characterization studies, is proposed. [ABSTRACT FROM AUTHOR]