Simple Catalyst Pair Transforms Excess Glycerol Into Useful Compounds

Biodiesel may be a green fuel, but its production has a waste problem. Every year, producers of the alternative fuel create over a million tons of glycerol worldwide, much of which goes to waste. Turning this side product into something useful and salable would help transform biodiesel into a more profitable commodity. Some scientists have suggested creating acetylglycerols from the unwanted glycerol, since the compounds are used in many consumer products. In a new study, chemists report a set of reactions that produces acetylglycerols in high yields using low-cost and abundant materials (ACS Sustainable Chem. Eng. 2014, DOI: 10.1021/sc500006b).

Current strategies for turning glycerol into acetylglycerols require harsh conditions and produce low yields. For the new reactions, Kiyotomi Kaneda, a catalysis chemist from Osaka University, in Japan, and colleagues used a one-pot method to transform glycerol into 1,3-diacetylglycerol, which is used in pharmaceuticals, cosmetics, polymers, and food additives. The transformations required two catalysts: a silicate clay called montmorillonite that they embedded with lanthanum ions, and aluminum oxide embedded with copper nanoparticles. The lanthanum montmorillonite catalyst acetylated the glycerol, and the copper catalyst oxidized, isomerized, and hydrogenated the acetylated product.

The chemists churned out diacetylglycerol at yields as high as 98%. Previous methods produced mixtures of diacetylglycerol isomers, but this one was selective, with the target compound 1,3-diacetylglycerol accounting for 99% of the product.

The reactions run for 24 hours at 120 °C, a mild temperature relative to those used in previous methods. The catalysts are relatively easy to obtain as well. Montmorillonite clays are often used in industry, and copper is much cheaper than commonly used precious-metal catalysts such as palladium or platinum. Besides the catalysts and the starting material, the reactions require only air, acetic acid, hydrogen gas, and toluene as the solvent. Also, the chemists can easily filter out the catalysts so they don’t contaminate the products.

One of the problems with the glut of glycerol from biodiesel is that it is impure, says Adam F. Lee, a catalysis chemist from the University of Warwick, in England. Most biodiesel production uses a liquid catalyst that’s difficult to remove from the glycerol by-product, so it can’t be used directly for food or cosmetics. “So there’s a desperate search for something to do with all this glycerol, and there have been very few low-temperature, efficient processes to transform it into useful chemicals,” he says.

This new process is important both because of the mild conditions and the inexpensive, plentiful catalytic materials they use, Lee says. Most chemists would design a process that uses one catalyst and one reaction at a time, so putting together two catalysts that have different roles is unusual, he says, but “nice and straightforward.” However, for these reactions to be truly useful in an industrial setting, they would have to be engineered into a solventless, continuous-flow type of process, he says.