CaO as Drop‐In Colloidal Catalysts for the Synthesis of Higher Polyglycerols

Abstract

Glycerol is an attractive renewable building block for the synthesis of polyglycerols, which find application in the cosmetic and pharmaceutical industries. The selective etherification of glycerol to higher oligomers was studied in the presence of CaO colloids and the data are compared with those obtained from NaOH and CaO. The materials were prepared by dispersing CaO, CaCO3, or Ca(OH)2 onto a carbon nanofiber (CNF) support. Colloidal nanoparticles were subsequently dispensed from the CNF into the reaction mixture to give CaO colloids that have a higher activity than equimolar amounts of bulk CaO and NaOH. Optimization of the reaction conditions allowed us to obtain a product with Gardner color number <2, containing no acrolein and minimal cyclic byproducts. The differences in the CaO colloids originating from CNF and bulk CaO were probed using light scattering and conductivity measurements. The results confirmed that the higher activity of the colloids originating from CaO/CNF was due to their more rapid formation and smaller size compared with colloids from bulk CaO. We thus have developed a practical method for the synthesis of polyglycerols containing low amounts of Ca.

Homogeneously-acid catalyzed oligomerization of glycerol

“…Purification of the oligoglycerol fractions has been then undertaken. As previously pointed out by B. M. Weckhuysen and co-workers, 12 we observed that at a low conversion the reaction media is nearly colourless but turned to dark brown at a conversion close to 80%. This change of colour was attributed to the formation of side products notably stemming from the dehydration of glycerol and oligoglycerols to readily polymerizable chemicals (exact elucidation of the chemical composition of the black materials was not trivial).…”

Alkaline-Based Catalysts for Glycerol Polymerization Reaction: A Review

“…This could be linked to the fact that the polymerization reaction at higher temperatures released more water, hence promoting the formation of colloidal Ca(OH) 2 according to Equation (1). Further, in the same group, Kirby et al [44] reported that the active phase derived from CaO/CNF catalyst might be the colloidal Ca(OH) 2 particles and Ca 2+ .The amounts of Ca and Mg leaching from dolomite were also determined by ICP-OES during the glycerol polymerization reaction by Barros et al [7]. According to these authors, 49% Ca and 4% Mg leached into the medium at 245 • C after 24 h of reaction.…”

“…Several types of catalysts have been reported in the literature to polymerize glycerol to PGs, including strong acids [39][40][41], alkali-modified zeolites [42,43], bulk or supported alkaline oxides [3,5,28,44], alkaline carbonate and hydroxide [21,28,45,46] and impregnated basic mesoporous solids [47]. Among all these catalysts, alkaline homogeneous and heterogeneous catalysts gave better performances than acid catalysts in terms of selectivity to PGs [9,41].…”

“…They used carbon nanofibers (CNF) as a support for CaO to yield larger specific surface areas while the active species (CaO) was mostly unaffected by the presence of the CNF as the interactions between CNF as a support and active phase were low. The highest conversion was obtained in the presence of 14% CaO/CNF with a glycerol conversion was 76% with selectivities to di-, tri-, tetra-and higher oligomers of 38%, 20%, 10% and 5%, respectively, at 220 • C after 24 h of reaction [6,44,61]. Table 3 summarizes the results of the studies on glycerol polymerization over heterogeneous catalysts.…”

“…Kirby et al [44] also studied the effect of temperature on the polymerization of glycerol and the coloration of products in the presence of CaO-CNF. They performed tests at the temperatures of 180, 200, 220, 240 and 260 °C and observed glycerol conversions of ~10, ~35, ~55, 80 and 100%, respectively.…”