"Making soap from plastic has never been possible before. When our group first got their hands on soap made from plastic, they were super excited. At that time, Xu Zhen took a picture for me. The moment I received the photo, I understood that this meant that we had created a completely new technological path and proved that it was possible. Associate Professor Liu Guoliang of Virginia Tech recalled.
In other words, in the future, some of the ingredients in soap or detergents we buy from the market may come from recycled plastic waste.
Photo丨Liu Guoliang (Source: Liu Guoliang)
Recently, based on factors such as the economic value of recycling plastic products, the Virginia Tech team reported in Science a universal and effective method to convert plastics into soap or other cleaning agents [1].
It is worth noting that this method can simultaneously convert polyethylene (PE) and polypropylene (PP) into fatty acids (the main component of soap), with a conversion rate of about 80%. Moreover, the average molar mass of the two plastics is high, with PE achieving 700 daltons and PP 670 daltons.
The study addresses two issues. On the one hand, this method avoids the emission of large amounts of carbon dioxide or other greenhouse gases when plastics are recycled. This relatively environmentally friendly recycling method greatly reduces carbon emissions; On the other hand, the team developed a new way to transform low-value substances into high-value products. Compared to extracting fatty acids from animals, this method is more economically competitive.
From the perspective of market demand, the demand for soap and plastic is almost the same. In terms of economic value, the market price of soap is 2 to 3 times that of plastic under the same weight conditions. The current average price in the market is about $3550/ton for soap and detergent, and about $1150/ton for polyethylene.
Economic value is one of the key considerations in plastic recycling. As labor costs increase, so does the cost of recycling plastics. So, how can recycling plastics generate more economic value and profit? Government subsidies or tax rebates are one way to make a profit, but it is not sustainable.
The industrial-scale techno-economic analysis of the study proves that the process is economically viable without any subsidies, pointing a new direction for the sustainable recycling of plastics.
Realize the simultaneous recycling of both PE and PP plastics
Liu Guoliang's research interests are diverse, mainly focusing on the application of energy and environmental science. Specific directions include polymers, polymer film composites, functional carbon fibers, polymer recycling and upcycling, etc. In particular, there is a strong focus on the application and recycling of plastics and the development of appropriate strategies for related research. Previously, the team studied producing polymers as prestigents, such as converting plastics into carbon materials.
When studying plastic waste recycling, they will choose the prospects of its final recycling application according to the specific situation of the polymer. Taking polystyrene (Polystyrene, PS) recycling as an example, PS contains a large number of aromatic ring groups, and when recycling PS, the research team focused on how to rationally use and recover aromatic ring groups.
In the new study, plastic and soap appear to be two unrelated substances. So, how did the research team come up with the idea of transforming the former into the latter? Is the transformation process simple and has the possibility of industrial transformation?
In fact, back in around 2018, Liu Guoliang had the idea of turning plastic into soap. Then, from the initial study in 2020 to the final result, it took about two years. From the characteristics of molecular structure, PE and fatty acids are very similar. Both are composed of unique, simple molecular structures of carbon chains, while fatty acids have an additional functional group at the end of the chain. This suggests that it is possible to convert PE to fatty acids.
Fatty acids are basically derived from animal fats or vegetable oils. A common use of animal fat is in surfactants, and soap is representative of its downstream products. Liu Guoliang pointed out, "In fact, our converted fatty acids are more similar to animal fats. So we thought we could try to make plastic into surfactants such as soap and detergent. ”
(Source: Virginia Tech)
The molecular chain of PE is about 3000 carbon atoms long, so what happens if you make its molecular chain smaller, and how to efficiently shorten its atomic chain?
From a thermal point of view, the team broke the atomic chain structure of PE from a long chain to a short chain, not only degrading it into wax (short-chain PE), but also reducing the production of small gas molecules. Then, the researchers used methods such as manganese stearate catalytic oxidation to eventually convert the wax into fatty acids. When the researchers first got soap made from PE and PP, they got better product performance, and the conversion rate of fatty acids achieved about 80%.
Figure 丨Cycling PE and PP to fatty acids in a temperature gradient reactor (Source: Science)
After the breakthrough in PE, the team found that a similar structure exists in PP. "The fatty acids we commonly use have an unsaturated structure. In some cases, PP is actually a molecule that can be made into a fatty acid structure. Liu Guoliang said.
Due to the high similarity of PE and PP, it has been challenging to recycle both plastics and their mixtures simultaneously in previous studies. The use of catalysts to convert plastics into other high-value products is one of the means of conversion, but often catalysts are only effective for one material and may be completely ineffective against another.
The study provides a new way to convert low-value substances into high-value products without the need for catalysts by breaking the form of PE and PP molecular chain structures. In addition, the researchers also investigated the reaction mechanism during the interruption of the molecular chain, as well as the mechanism of oxidation and the next reaction.
PE and PP have huge differences in chemical reactions, and the problem of selectivity between the two is bypassed by this method. "And our approach is universal and will most likely be applicable to the recycling of other materials." Liu Guoliang said.
Fig. 丨PE is degraded into wax and further converted into fatty acids (Source: Science)
After confirming the scientific principles, they found Bobby M. Smith of Oak Ridge National Laboratory in the United States. Bobby G. Sumpter used density functional–based tight-binding (DFTB) simulations to gain molecular understanding of temperature gradient degradation of PE and PP and to evaluate subsequent oxidative upcycle reactions.
The tightly combined approach of DFTB and extension can simulate relatively large systems and reasonable time scales with good accuracy, but is much faster than the traditional ab initio computational density functional theory method.
In addition, in terms of economy, Professor Zhang Qikun of Shandong Normal University carried out a series of calculations to prove the feasibility of economic profitability of this technology.
Related papers (Source: Science)
Recently, a related paper was published in Science [1].
Zhen Xu and Nuwayo Eric Munyaneza, Ph.D. students at Virginia Tech, co-first authors of the paper, were led by Associate Professor Guoliang Liu and Bobby M. Smith of Oak Ridge National Laboratory. Bobby G. Sumpter serves as co-corresponding author of the paper.
New sustainable ideas for plastic recycling
Liu Guoliang has an interdisciplinary background in chemistry, chemical engineering, physics, materials, electronic appliances, computer science, etc., which invisibly gives more innovation and curiosity to his scientific research and exploration. He received his bachelor's degree in chemical engineering from the School of Materials Chemistry and Chemical Engineering of Zhejiang University, the Advanced Class of Engineering Education and the Intensive Course of Innovation and Entrepreneurship Management of Zhu Kezhen College. He received his Ph.D. in chemical engineering from the University of Wisconsin-Madison, under the tutelage of Paul M. Professor Paul F. Nealey.
Subsequently, he engaged in postdoctoral research at Northwestern University, co-supervisor of Chad M. Professor Chad A. Mirkin and was named a Distinguished Fellow of the university's International Institute of Nanotechnology. During this time, he collaborated with Nobel laureate James Fraser Stoddart in the field of supramolecules. In the fall of 2014, he joined Virginia Tech as a faculty member and founded a research group.
"In terms of topic selection, I made a detailed analysis of the research results and directions during my doctoral and postdoctoral studies, decided to leave the original research direction that was relatively comfortable, innovated independently, and found a 'new world' driven by my own interests - energy and environment. Plastic waste, for example, is an obvious and important environmental issue. Liu Guoliang said.
(Source: Virginia Tech)
The publication of a paper is a reflection of the phased results, but it does not mean the end of the research. Liu Guoliang said: "Although we have successfully verified the feasibility of the technology of converting plastic into soap, we are not in a hurry to submit papers. Even after submission, we still have a lot of things that we are not particularly satisfied with the work. Therefore, until now, relevant research has also been ongoing. ”
As a next step, the team hopes to test the performance of the plastic converted products and then further determine the possibility of helping the relevant fields based on the relevant properties. Their plan will focus on two areas. On the one hand, the principle of this conversion reaction process is explored from the perspective of basic sciences such as chemistry, chemical engineering, and machinery; On the other hand, from the perspective of practicality, continue to explore ways to improve product conversion rate and adjust the degree of regulation.
For the industrialization and development of this technology, Liu Guoliang said frankly that based on the research results, he hopes to provide a new idea for the sustainability of plastic recycling for the field. "While we remain optimistic, we must also maintain a clear understanding that the transformation of technology from the laboratory to the industrial can be a long process. We look forward to cooperating with the industry in the future to realize the application of technology in production and life as soon as possible. ”
Resources:
1.Xu，Z., et al. Chemical upcycling of polyethylene, polypropylene, and mixtures to high-value surfactants. Science 381,6658,666 - 671(2023). https://www.science.org/doi/10.1126/science.adh0993