A two-step process for making lactic acid (LA) promises environmental and economic advantages over anaerobic sugar fermentation, the method currently used for commercial production of the precursor to polylactic acid (PLA), an increasingly popular biodegradable polymer (Figure 1). The technique uses as feedstock glycerol, a low-value byproduct of biodiesel manufacturing now in growing oversupply as biodiesel output expands, note its developers at the ETH Zurich, Zurich.
Not only is a glut of glycerol developing but also impurities in the byproduct rule out its use in current chemical and pharmaceutical processes, explain the researchers. The material doesn’t burn well and so isn’t a good energy source, they add.
The new process involves the biocatalytic oxidation of the glycerol to dihydroxyacetone (DHA) followed by isomerization of the DHA using a heterogeneous catalyst, report the team led by Konrad Hungerbühler and Javier Pérez-Ramirez of the university’s Institute for Chemical and Bioengineering. The catalyst consists of tin-containing MFI zeolites and is highly active, selective and recyclable, they add.
More details about the cascade process appear in a recent article in the journal Energy & Environmental Science.
The process cuts overall carbon dioxide emissions by 20% compared to the conventional method, according to the team’s analysis. Considering an LA price of $1,800/ton, making LA via DHA isomerization in methanol can provide a marginal profit 15 times higher than that of the conventional approach, the researchers estimate. “Our calculations are even rather conservative,” notes team member Merten Morales. “We assumed a glycerol feedstock of relatively good quality. But it also works with low-quality glycerol, which is even cheaper.”
According to Pérez-Ramirez, the next steps in the development are scaling up the catalyst for converting DHA into LA; developing a chemocatalytic route with selectivities higher than 90% for making DHA from glycerol; and using DHA as a building block for polymers other than PLA.
Key issues remaining, he says, are assessing the scalability and stability of the isomerization catalyst, determining the impact of glycerol purity on process stability, and piloting the process. One biodiesel producer already has expressed interest in helping to pilot the technology, notes Pérez-Ramirez.
The team now is working to develop a continuous version of the isomerization process. A continuous chemocatalytic process for converting glycerol to DHA would make the two-stage process even more appealing, the researchers add.