Abstract In the foreseeable future, human beings will continue to live on this blue planet named Earth. Our planet has limited space, resources and environmental carrying capacity, yet the human population and quality of life will continue to rise. Economic development and environmental protection are often conflicting goals, and it is a great challenge for us to achieve both at the same time: to develop sustainably. Green chemistry—the aim of making the whole industrial chain of chemistry and chemical industry environmentally friendly—has contributed, and will further contribute, to the sustainable development of the chemical industry. Recently, NSR talked with Buxing Han—an academician of the Chinese Academy of Sciences and one of the leading scientists in this field. Han introduced the major scientific issues of green chemistry and talked about the challenges and future developments of this field. GREEN CHEMISTRY NSR: How was the green chemistry concept proposed? And what is its goal? Han: Most manufactured products involve one or more chemical processes. Chemical industry provides us with basic chemicals, materials and fuels, and has made irreplaceable contributions throughout the history of human civilization. However, it is undeniable that traditional chemical industry is accompanied by a serious waste of resources and heavy pollution of water, air and soil. It is essential, but hugely challenging, that we transform the chemical industry, making it capable of producing various products to meet our daily needs while avoiding any possible negative impact on the environment. Against this background, the concept of green chemistry was proposed by American researchers in the early 1990s. Later, 12 proposed major principles of green chemistry have been generally accepted by the chemistry community. The definition of green chemistry can be briefly given as the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. The goal of green chemistry is to eliminate pollution at the source, to enhance efficiency, to save resources and energy, and to achieve sustainable development of chemistry and the chemical industry. NSR: What are the key scientific ideas that will enable us to reach the goals of green chemistry? Han: Green chemistry aims to make the whole production chain of the chemical industry clean. So we should use non-toxic and renewable green reactants, employ green solvents, green catalysts and green additives to synthesize environmentally friendly products using synthetic routes that have high atom economy. It is also very important to reduce production costs and guarantee economic rationality. Generally speaking, there are two major ways to achieve these goals. The first one is to transform every atom in the reactants into products, which means that no by-product is produced during the manufacturing process. However, by-products cannot be totally avoided in most chemical processes. Once released to the environment, they may cause pollution of water, soil and air. So, the second protocol is to use the by-products of one chemical process as the reactants of another process. The design of the entire processing cycle to utilize every atom of the feedstocks is another effective way to avoid pollution, and generally speaking, the application of this second protocol is more common. View largeDownload slide Buxing Han, Academician of the Chinese Academy of Sciences, Fellow of The World Academy of Sciences (TWAS) for the advancement of science in developing countries, and one of the leading scientists in the field of green chemistry (Courtesy of Buxing Han). View largeDownload slide Buxing Han, Academician of the Chinese Academy of Sciences, Fellow of The World Academy of Sciences (TWAS) for the advancement of science in developing countries, and one of the leading scientists in the field of green chemistry (Courtesy of Buxing Han). NSR: How can green chemistry influence materials science? Han: Chemistry and materials science are tightly connected. It is an important component of green chemistry to synthesize environmentally friendly high-performance materials using green raw materials and green technologies. Many aspects of the materials industry may cause pollution. First, many materials production processes cause pollution. Second, some commonly produced and widely used materials themselves contain harmful substances and are toxic to human health. Third, some materials may turn into pollutants during their after-use disposal. Both the materials production processes and materials themselves should be green. Interdisciplinary studies of green chemistry and materials science are receiving increasing attention. Green technologies have been developed, and various green materials have been designed, produced and used, such as coatings, medical, decorative and polymer materials. Examples include: waterborne coatings developed by substituting organic solvents for water; degradable polymer materials with unique properties synthesized from CO2 and biomass; and green polymer materials produced by modification of natural materials such as cellulose. These new materials do not rely on fossil resources. They take advantage of natural resources and could contain special functional groups that are difficult to synthesize artificially. It is of crucial significance for materials science to make the whole cycle green, from design, production, usage to recovery. GREEN SOLVENTS NSR: You have contributed a lot in the field of green solvents. Why are solvents important for green chemistry? Han: Tens of millions of tons organic solvents are released to the atmosphere every year. This is a great waste, and moreover it greatly pollutes the air. Green solvents have many advantages: they are non-toxic, easily reusable and able to enhance the efficiency of chemical processes. One typical green solvent is supercritical fluids, which I have studied. Supercritical CO2 is one example. Besides their non-toxic character, the most significant feature of supercritical fluid solvents is that they are easy to recycle. Simply by reducing pressure, we can separate solvent and product, and the solvent can be reused. This procedure can guarantee that no solvent remains in the products. Using the special properties of supercritical fluids, many green technologies that are difficult or impossible to achieve using conventional solvents can be developed. Today, supercritical technologies have been applied in many fields, such as extraction and fractionation, pharmaceuticals, chemical reactions, materials production and dyeing. More applications of this kind of solvent will appear in the future. Ionic liquids are another kind of widely studied green solvent in the past 20 years. Ionic liquids are non-volatile and can be designed to be non-toxic and to possess special Tens of millions of tons organic solvents are released to the atmosphere every year. This is a great waste, and moreover it greatly pollutes the air. —Buxing Han properties and functions. Ionic liquids can dissolve both organic and inorganic compounds and can be widely applied in chemical reactions, separation, materials production and other fields. Traditional solvents interact with solute molecules mainly through the van der Waals force and hydrogen bonding. But for ionic liquids, electrostatic interaction exists too. This property can be exploited in useful ways. Many chemical reactions are sensitive to electrostatic forces. Thus, ionic liquids are able to tune or enhance reaction efficiency, or even to accomplish reactions that are impossible in other solvents. For example, some metal-catalysed reactions can be conducted in ionic liquids under metal-free conditions; the ionic liquids act as both catalyst and solvent. And in many cases the non-volatile nature of ionic liquids makes it easy for products to be separated, and the solvent reused. Another important green solvent is water. We can substitute water for organic solvents in some reactions. However, the solubility of most organics in water is very low, and how to further effectively utilize this solvent is an interesting topic. In addition, there is work on other green solvents such as liquid polymers, biomass-based solvents and switchable solvents. NSR: Are solvents essential for modern chemical industry? Can we construct reaction systems without them? Han: It would be pretty good to avoid solvent usage if possible. Solvents are not directly involved in chemical reactions and we have to separate them after use in most cases. We have conducted many solvent-free reactions, including liquid-liquid, liquid-solid, gas-liquid and solid-solid reaction systems, which show some unique advantages. However, solvent-free synthesis has certain limitations, and thus more than 60% of chemical processes need a solvent. During reaction, separation and materials production, the solvent can enhance mass transfer and heat transfer, thus promoting effective interactions between reactant molecules, increasing production efficiency and preventing local overheating. And in many cases, interactions between solvent, reactants and products can effectively control reaction pathways and tune the morphology and size of the materials produced. GREEN CHEMISTRY IN MANY BRANCHES OF CHEMISTRY NSR: Your original research was in physical chemistry. How did you step into the field of green chemistry? Han: Strictly speaking, my original research field was chemical thermodynamics, and I am now focusing on the interdisciplinary area of chemical thermodynamics and green chemistry. Chemical thermodynamics is a classical and significant branch of chemistry. And as a relatively new branch of chemistry, green chemistry needs to refresh some of the traditional modes of thinking. Many of the significant problems that need to be reconsidered and resolved lie within the category of chemical thermodynamics. So, the overlap of these two branches of chemistry became an area of cutting-edge research, and it is able to widen the scope of chemical thermodynamics, to solve key problems of green chemistry, and to promote the development of both. NSR: Besides chemical thermodynamics, are other branches of chemistry also connected with green chemistry? Han: Yes, of course. Green chemistry is a rather multidisciplinary field, which is tightly related with chemistry, chemical industry and materials science. For example, in the field of catalysis we need to design high-performance, non-toxic, stable and recyclable catalysts. In the field of fine chemicals and pharmaceuticals we usually need massive amounts of raw materials to produce only a very small amount of products. This results in enormous pollution and waste. Thus, it would be rather important for these fields to optimize the synthesis methods and to create more atom-economic reactions. Theoretical chemistry and computational chemistry also play important roles. For example, if we can develop better computational methods, capable of predicting the efficiency of catalytic reactions, experimental trials for catalysts optimization and screening would become much easier and the development of green chemistry would speed up. CHALLENGES AND PROSPECTS NSR: Are there difficulties in transferring green chemistry technologies into industry? Han: There are many conundrums in the transfer process. In addition to guaranteeing that the technologies are green, the most significant concern is economical viability. Many company operators come to me to talk about their factories, wondering whether we could provide or recommend some green technologies to them. They are eager to make their production processes green and to meet the government's requirements. But once we came to the stage of industrialization, there exists the problem of cost and profit. No matter how green the process of a technology is, it could not be considered as a real ‘green technology’ as long as it is too expensive and cannot be industrialized. So, we have to create more highly effective and economically viable technologies to authentically promote the development of green industries. NSR: What is the state of international cooperation in this field? Han: International communication and cooperation has been widely conducted in recent years. Many international associations have been founded, such as the IUPAC (International Union of Pure and Applied Chemistry) Subcommittee on Green Chemistry. There are some regular conferences, such as the International IUPAC Conference on Green Chemistry and the International Conference on Green and Sustainable Chemistry. Some professional journals such as Green Chemistry have been established. All these are effective at promoting the development of this field and international cooperation. Although in some cases problems such as intellectual property rights can limit the process, there has been fruitful cooperation between universities, research institutions and companies in and out of China. NSR: What are your expectations for the development of green chemistry? Han: Green chemistry is an inevitable trend in the development of chemistry. It is a higher level of chemistry. Viewed from the scientific angle, it is a development of the traditional chemical mode of thinking and is refreshing the basic content of chemistry and the chemical industry. Viewed from the environmental angle, it could eliminate pollution at the source and support development that is coordinated with ecosystems and the environment. Viewed from the economical angle, it attempts to reduce production costs and to rationally use energy and resources, especially renewable resources. Viewed from the social angle, it coincides with the sustainable development of society. With the efforts of academia, industry, and governments, green chemistry has been developing continuously, and many chemical processes are now much cleaner than before. However, there is still a long way to go to achieve the final goals of green chemistry. I am confident that green chemistry will develop rapidly in the future and the chemical industry will embark on the road of sustainable development. CHINA’S SITUATION NSR: How is the development of green chemistry in China? Han: China started early in green chemistry research, not long after the concept was proposed. The Chinese government, academic communities and companies all paid great attention to this field. The National Natural Science Foundation, Chinese Academy of Sciences, and Ministry of Science and Technology of China have funded many related research projects. Chinese scientists and enterprisers have made a lot of significant contributions in basic research and technology development, which have been internationally recognized. A number of green new industries are also emerging in China. NSR: What are the unique problems facing China? Han: Compared with other countries, green chemistry is especially important for China. As the biggest developing country with a population of over 1.3 billion, we have to produce As the biggest developing country with a population of over 1.3 billion, we have to produce most chemical products we need at home. So the contradiction between economy, resources and environment is particularly apparent in China. —Buxing Han most chemical products we need at home. So the contradiction between economy, resources and environment is particularly apparent in China. On the other hand, our technology is still relatively backward compared with developed countries, and the waste of resources and pollution of the environment are more serious. Thus, it is particularly important for China to develop green chemistry and green technology and to realize high-efficiency clean production. Given the reality of limited resources and environmental capacity, it is a great challenge for us to properly resolve the contradictions between economy, resources and environment. In recent years, many highly polluting Chinese factories have been shut down. However, this is not a long-term solution. To achieve economic development and to solve the problem of pollution simultaneously, we have to rely on the further development of green chemistry and the utilization of green technologies. NSR: What is your advice for young scientists? Han: Green chemistry is still young and its pace of development will accelerate in the future. It is a long-term and arduous task to promote the development of green chemistry and to achieve sustainable development of the chemical industry. We need more young researchers to enter this field in order to realize these goals. In my mind, the future chemistry will definitely be environmentally benign, and able to benefit both the environment and human life. For young people in the field of green chemistry, I believe that the core quality they should possess is creativity. Green chemistry means breaking the traditional methods and creating new green techniques. Innovation is the soul of green chemistry. Finally I would like to mention that green chemistry is in its infancy. Its connotation, principle, content and goal will be constantly enriched and improved. This short interview cannot discuss this field systemically. The views mentioned above may be one-sided or inappropriate, and thus I welcome criticisms. Let us work together to promote the development of green chemistry. ACKNOWLEDGEMENTS The author thanks Professor Zhigang Shuai (Tsinghua University) for his contributions to this interview. Weijie Zhao is a news editor of NSR. © The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
National Science Review – Oxford University Press
Published: Nov 1, 2018
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