A single-step process outperforms the current two-step route for making lactide, a key precursor of polylactic acid (PLA), a popular biodegradable polymer, say researchers at the Center for Surface Chemistry and Catalysis, KU Leuven, Heverlee, Belgium. The zeolite-catalyzed method not only eliminates the cost and complexity of a second step but also reduces byproduct formation, they add. “Products made of PLA now can become cheaper and greener,” enthuses Bert Sels, a professor at the Center and head of the team that developed the process.
In the route, aqueous lactic acid produced via fermentation goes to a stirred-tank reactor where a highly selective zeolite catalyst converts it into lactide at yields of up to 85% in a single pass (Figure 1). The process relies on a commercial zeolite, H-Beta, that has shown stability and reusability over several consecutive runs of the process. More details appear in a recent article in Science.
The researchers aim to scale up the process so they can gain a better understanding of its technical and economic feasibility. “However, since the ingredients of our process only contain commercial compounds, there is a very good chance that our one-step approach will defeat the current two-step approach. The zeolite, for instance, is available commercially and its performance is nearly ideal. Obviously, catalyst regeneration is a point of action under study, but the results are promising,” notes Sels.
“A cost-based comparison at this point — in the early stage of development of the new process — is hard due to lack of specific upscaling parameters. However, knowing that the excessive purification steps in place in the current process are superfluous in our case (absence of racemization and no irreversible side-products) and that the process is simplified from two stages to one, while running at lower temperatures and ambient pressures with a reusable catalyst (unlike non-recovered metal salts), we are convinced that it will be viable.”
The key challenges are developing a continuous version of the process and adapting it to a larger scale, he says.
“The KU Leuven patent on our discovery was recently sold to a chemical company that intends to apply the production process on an industrial scale,” notes Sels. The firm, a large petrochemical company, may start up a several-thousand-ton/yr pilot plant within the next two to three years, he believes.
The approach promises to have wider applicability. “We have been able to show that the technology also works for cyclization of other alpha-hydroxy acids, other than lactic acid,” he adds.