Globally, manufacturers are trying to make vehicles lighter because this not only reduces fuel consumption but also reduces the environmental footprint.
Over the last several decades, airplanes, cars, bicycles and other modes of transportation have become lighter with the use of carbon fiber composites. Carbon fiber is the ideal manufacturing material for many parts because it is considerably lighter, two-times stiffer and five-times stronger than steel. However, with the industry’s growing dependence on petroleum products to produce carbon fiber, could renewable sources provide an alternative solution?
Gregg Beckham, a group leader at the National Renewable Energy Laboratory (NREL), together with an interdisciplinary team, reported the outcomes of computational and experimental investigations on the conversion of lignocellulosic biomass into acrylonitrile – a bio-based chemical which is used as a key precursor for making carbon fiber. The study was published in the December 2017 issue of Science.
Acrlyonitrile is considered to be major commodity chemical, and it is today manufactured through an intricate petroleum-based process at the industrial scale. Propylene, which is obtained from natural gas or oil, is combined with oxygen, ammonia and a complex catalyst. The resulting reaction produces high amounts of heat and a toxic by-product – hydrogen cyanide. At present, acrylonitrile is made using the catalyst, which is both expensive and relatively complex, and researchers are yet to fully understand its mechanism.
"That's where our study comes in," Beckham said. "Acrylonitrile prices have witnessed large fluctuations in the past, which has in turn led to lower adoption rates for carbon fibers for making cars and planes lighter weight. If you can stabilize the acrylonitrile price by providing a new feedstock from which to make acrylonitrile, in this case renewably-sourced sugars from lignocellulosic biomass, we might be able to make carbon fiber cheaper and more widely adopted for everyday transportation applications."
Several years ago, the Department of Energy (DOE) pitched a proposal that asked: Is it possible to make acrylonitrile from plant waste material? It was conceived to come up with innovative ideas to make acrylonitrile from renewable feedstocks. The materials included wood chips, rice straw, wheat straw and corn stover.. These are the inedible parts of the plant that can be easily broken down into sugars and subsequently converted to a wide range of bio-based products for daily use, for example fuels like ethanol and other chemicals.
"If we could do this in an economically viable way, it could potentially decouple the acrylonitrile price from petroleum and offer a green carbon fiber alternative to using fossil fuels," Beckham said.
Beckham and the group went on to develop an entirely different process. In the NREL process, sugars derived from waste plant materials are used and converted to an intermediate known as 3-hydroxypropionic acid (3-HP). Then, using a simple catalyst and new chemistry, named nitrilation, the team converted 3-HP to acrylonitrile at high yields. The catalyst, which was utilized for the nitrilation chemistry, is roughly three times less costly than the catalyst utilized in the petroleum-based process and it is a simpler process. Since the chemistry is endothermic, it does not generate excess amounts of heat. Also, it does not produce the harmful byproduct hydrogen cyanide, unlike the petroleum-based process. The bio-based process instead produces alcohol and water as its by-products.
From a green chemistry standpoint, the bio-based acrylonitrile manufacturing process provides a number of advantages when compared to the petroleum-based process being used today. "That's the crux of the study," Beckham said.
XSEDE's Role in the Chemistry
Beckham is not new to the eXtreme Science and Engineering Discovery Environment (XSEDE) that is supported by the National Science Foundation. As a principal investigator, Beckham has been using XSEDE resources, such as Stampede1, Comet, Bridges and now Stampede2, for approximately nine years. The Texas Advanced Computing Center deploys and maintains Stampede1 and Stampede2 (currently #12 on the Top500 list).
While most of the chemistry and biological research performed for this project was purely experimental, the team initially hypothesized the mechanism of the nitrilation chemistry. Vassili Vorotnikov of NREL is also a postdoctoral researcher in the team. He was recruited to run periodic density functional theory calculations on Stampede1 and also the machines at NREL so as to explain the mechanism of this novel chemistry.
Over a couple of months and more than several millions of CPU-hours used on Stampede1, the team was able to explain the chemistry of this innovative catalytic process. "The experiments and computations lined up nicely," Vorotnikov said.
Since the researchers had an allocation on Stampede1, they were able to quickly turn around a complete mechanistic picture of the way this chemistry works."This will help us and other Top500 institutions to develop this chemistry further and design catalysts and processes more rationally," Vorotnikov said. "XSEDE and the predictions of Stampede1 are pointing the way forward on how to improve nitrilation chemistry, how we can apply it to other molecules, and how we can make other renewable products for industry."
"After the initial experimental discovery, we wanted to get this work out quickly," Beckham continued. "Stampede1 afforded a great deal of bandwidth for doing these expensive, computationally intensive density functional theory calculations. It was fast and readily available and just a great machine to do these kind of calculations on, allowing us to turn around the mechanistic work in only a matter of months."
There is a huge community of chemical engineers, biologists, and chemists, who are developing ways for using plant waste materials to make everyday materials and chemicals instead of using petroleum. While researchers have attempted to do this before with acrylonitrile, no one has been as successful in the context of devising high yielding processes with commercial potential for this specific product. With their latest finding, the researchers hope that their work would be quickly applied to the industry.
Scaling the process up to make 50 kilograms of acrylonitril is the next immediate step. The researchers are collaborating with a number of companies, which include a catalyst company to develop the required catalyst for pilot-scale operation; a research institute to scale the catalytic and separations process; an agriculture firm to help scale up the biology to make 3-HP from sugars; a car manufacturer to test the mechanical properties of the ensuing composites; and a carbon fiber company to manufacture carbon fibers from the bio-based acrylonitrile.
"We'll be doing more fundamental research as well," Beckham said. "Beyond scaling acrylonitrile production, we are also excited about is using this powerful, robust chemistry to make other everyday materials that people can use from bio-based resources. There are lots of applications for nitriles out there — applications we've not yet discovered."