Improving Product Yield in the Direct Carboxylation of Glycerol with CO2 through the Tailored Selection of Dehydrating Agents

Nurul Razali,James Mcgregor

doi:10.3390/catal11010138

Nurul Razali, James Mcgregor

Open Access

https://doi.org/10.3390/catal11010138

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Abstract

Improved yields of, and selectivities to, value-added products synthesised from glycerol are shown to be achieved through the judicious selection of dehydrating agents and through the development of improved catalysts. The direct carboxylation of glycerol with CO2 over lanthanum-based catalysts can yield glycerol carbonate in the presence of basic species, or acetins in the presence of acidic molecules. The formation of glycerol carbonate is thermodynamically limited; removal of produced water shifts the chemical equilibrium to the product side. Acetonitrile, benzonitrile and adiponitrile have been investigated as basic dehydrating agents to promote glycerol carbonate synthesis. In parallel, acetic anhydride has been studied as an acidic dehydrating agent to promote acetin formation. Alongside this, the influence of the catalyst synthesis method has been investigated allowing links between the physicochemical properties of the catalyst and catalytic performance to be determined. The use of acetonitrile and La catalysts allows the results for the novel dehydrating agents to be benchmarked against literature data. Notably, adiponitrile exhibits significantly enhanced performance over other dehydrating agents, e.g., achieving a 5-fold increase in glycerol carbonate yield with respect to acetonitrile. This is in part ascribed to the fact that each molecule of adiponitrile has two nitrile functionalities to promote the reactive removal of water. In addition, mechanistic insights show that adiponitrile results in reduced by-product formation. Considering by-product formation, 4-hydroxymethyl(oxazolidin)-2-one (4-HMO) has, for the first time, been observed in all reaction systems using cyanated species. Studies investigating the influence of the catalyst synthesis route show a complex relationship between surface basicity, surface area, crystallite phase and reactivity. These results suggest alternative strategies to maximise the yield of desirable products from glycerol through tailoring the reaction chemistry and by-product formation via an appropriate choice of dehydrating agents and co-reagents.

Highlights

Glycerol is a by-product of biodiesel synthesis, 10 wt.% of glycerol is produced for every kilogram of biodiesel, and glycerol produced in this way contributes to ~67% of glycerol manufactured globally [1]
commercial La2O3 (C-La2O3), CP-La2O2CO3, SG-La2O2CO3 and HT-La2O2CO3 were characterised using scanning electron microscopy (SEM) and analysed using ImageJ software in order to determine their morphology at the macro-scale (Figure 1)
Acetins can be formed in the presence of basic species through secondary reactions

Summary

Introduction

Glycerol is a by-product of biodiesel synthesis, 10 wt.% of glycerol is produced for every kilogram of biodiesel, and glycerol produced in this way contributes to ~67% of glycerol manufactured globally [1]. Potential products which can be synthesised include glycerol carbonate and acetins. Glycerol carbonate has desirable properties such as low flammability, biodegradability, is non-toxic and has a high boiling point. These make it suitable for a wide range of applications including as an electrolyte in lithium-ion batteries, as a solvent and in surfactants [3]. Acetins are commercially valuable fuel additives, in particular for biodiesel (and their synthesis from a by-product of biodiesel production is attractive). Triacetin has additional applications in cosmetics, pharmaceuticals and food industries [4,5,6]; it has been suggested as a source of food energy on space missions [7]

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