Plastics from Plants
Can plants replace petroleum in the production of plastics? Absolutely. Biomass is an organic plant-based substance that is used as a renewable source of fuel, but it can also be used to produce p-xylene, a chemical that can take the place of conventional petroleum in the production of plastic consumer goods. If biomass can be used to produce sufficiently high yields of p-xylene, it could reduce the country’s dependency on fossil fuels. Researchers at the Catalysis Center for Energy Innovation (CCEI) have discovered a process that boosts biomass-derived p-xylene yields to unprecedented levels. These findings have the potential to help researchers discover other chemicals deriving from biomass that can be used to produce plastics.
Experimentalists at CCEI fed two chemicals produced from biomass into a small batch reactor along with a catalyst, which is a material that speeds up a chemical reaction. The biomass-derived chemicals were 2,5-dimethylfuran (DMF) and ethylene, and the catalyst was a zeolite, which is a material naturally found in clay. When zeolite was added, p-xylene was produced as a result of a chemical reaction. The researchers expected this to occur, but they did not anticipate what happened when they increased the amount of zeolite disproportionately to the amount of chemicals. To their surprise, p-xylene was produced at significantly higher rates than ever before.
So keep adding more zeolites, right? Not so fast.
By adding more, they ended up with a peculiar set of results. At a certain point, the rate hit a plateau and no longer increased as greater amounts of zeolite were added to the reactor pot. The experimentalists needed to know why the production rate stopped increasing at a certain zeolite threshold, so they turned to their computational colleagues for insight.
The computational researchers in CCEI leveraged their expertise to study the problem at two different length scales, similar to the way an entomologist might study a bee using the naked eye as well as under a microscope. At the atomic length scale, researchers used a complex theory that provided information about how the individual molecules interacted with each other. This information was then used to build a computer model of the reactor. Now, the researchers had a way to expose what happens at the reactor length scale for direct comparison with the experiment.
The key finding in the computational research was that two chemical reactions occurred. During the first step of the biomass conversion process, the chemicals (DMF and ethylene) combine to form an intermediate molecule. This is referred to as a cycloaddition reaction. In the second step, the intermediate molecule is broken down into two molecules -- the desired p-xylene and water. This is known as a dehydration reaction. The calculations also provided information about how quickly each of the reactions took place.
When researchers put this information into the computer model, it revealed the mystery of the plateau. They realized that without the zeolite catalyst present for the second reaction, the intermediate molecule was broken down too slowly to produce p-xylene at high rates. Interestingly, adding the catalyst did not affect the speed of the first reaction, yet it sped up the second reaction so much that it was now faster than the first reaction! Unfortunately, once enough of the zeolite had been added to fully speed up the second reaction, the overall rate of p-xylene production plateaued, because it was now limited by the first reaction.
These findings were recently highlighted as an American Chemical Society’s Editors' Choice article in the journal ACS Catalysis and illustrate the important interplay between experimental and computational research within CCEI. The experiment's peculiar results produced a set of questions that could not be explained, leading to computational investigations at both the atomic and reactor length scales. With this two-front approach, CCEI researchers mimicked the lab experiment computationally, discovered an unknown reaction at the molecular level, explained the peculiar results, validated their findings, and gained a deeper understanding of the limited, but critical role of the catalyst during the second reaction. These findings could prove useful in the investigation of other chemicals and materials used to produce plastics from biomass.
Using renewable sources such as biomass to produce p-xylene used in plastics and other products holds the promise of protecting the planet while bringing the United States one step closer to energy independence.
This work was supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
RE Patet, N Nikbin, CL Williams, S Green, CC Chang, W Fan, S Caratzoulas, PJ Dauenhauer, and DG Vlachos. 2015. "Kinetic Regime Change in the Tandem Dehydrative Aromatization of Furan Diels–Alder Products." ACS Catalysis 5:2367-2375. DOI: 10.1021/cs5020783