Design of Chlorofluorocarbon (CFC) Alternatives
Over the last 75 years, we have developed many halogenated products that have become an essential part of our lives. For example, Teflon and the chlorofluorocarbons (CFCs) used for refrigerants. They are not without their drawbacks, however, and halogenated gases like CFCs are linked to the depletion of the ozone layer.
Dr. Masaaki Yamabe joins us to talk about his efforts to improve halogenated products. He previously held the position of research director and general manager at Asahi Glass Company‘s research center in Yokohama, Japan and was also the Vice-President of the Chemical Society of Japan. Today, Dr. Yamabe is the director of the Fluorine Center at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan. In 1997, he was awarded the Best of the Best Stratospheric Ozone Protection Award from the Environmental Protection Agency.
Frank Ling (FL) talks to Masaaki Yamabe (MY). Below is an edited transcript:
FL: Perhaps we can begin with a brief introduction on fluorine chemistry and how fluorinated compounds are used in real life.
MY: In 1928, in the United States, Dr. Thomas Midgley invented the first fluorinated compound useful, very useful for refrigeration in place of SO2 (sulfur dioxide) or ammonia, which are both very toxic. This new compound dichlorodifluoromethane (CCl2F2) has excellent properties for refrigeration and is non-toxic. Following this invention, a variety of chlorofluorocarbons were also invented in DuPont and commercialized. We usually use the code number instead of the chemical structure or nomenclature. For example, CCl3F (CFC-11) for blowing agent for polymers (e.g. polyurethane) and CFC-12 (dichlorodifluoromethane) for the refrigerants in automobiles and air-conditioning. And another is CFC-113, this is trichlorotrifluoroethane, an excellent cleaning agent for the micro-electronics industry. So, we have been enjoying those developed CFCs or fluorine chemicals in industry. In the middle of the 1980s, the world-wide production of these CFC compounds reached 8 million tons.
FL: When did people first realize that these halogenated compounds could have adverse effects on the environment?
MY: In 1974, Professor Sherwood Rowland (Nobel, Chemistry 1995) and postdoctoral fellow Mario Molina (Nobel, Chemistry 1995) at the University of California at Irvine presented a paper at the annual meeting of the American Chemical Society: those non-toxic excellent CFCs would destroy the ozone layer in the stratospheric atmosphere high above. This was quite a surprise for the chemical industry, for the fluorine industry. This was just a hypothesis and at that moment in 1974, nobody easily believed that. We had no evidence of the ozone layer had been destroyed by CFCs. But those two, Sherry Rowland and Mario Molina, gave us the theory of destroying ozone molecules by the CFC molecules, so the scientists, for example at NASA, started to investigate what was happening in the stratosphere: the concentration of CFCs and the concentration of ozone. In 1984, all the governments of the United Nations recognized the actual decrease of ozone concentration in the stratosphere, which had been triggered by the activation of CFCs by the very strong UV light in the stratosphere. The UV light triggered the scission of the bonding C-Cl bond, not C-F bond. Chlorofluorocarbons have both the C-Cl and C-F bond, but the C-F bond is very strong and will not be easily broken. C-Cl will be easily broken and the excited chlorine will trigger the decomposition of ozone molecules. This is a chain reaction. One excited chlorine molecule can destroy 10,000 or 100,000 ozone molecules. The worldwide consensus has been made. we have to protect the ozone layer so have to stop producing CFCs. In 1987, the Montreal Protocol, an international treaty, was ratified by the United Nations and worldwide, it was decided that CFC production would be stopped in developing countries by the end of 1995.
So, what has happened in industry since that time? Chemists and scientists in industry or universities had to develop alternatives to CFCs with the same properties but much more environmentally friendly, so this has been a big challenge. We fluorine chemists have succeeded in developing a variety of alternatives for CFCs. S ome of these possible candidates include HCFCs (hydrochlorofluorocarbons) or HFCs (hydrofluorocarbons). The principle of the development of the alternative is to introduce hydrogen atom into the molecule because the CFCs have a very long atmospheric lifetime. So, those CFCs compounds have a long lifetime and can diffuse gradually into the stratosphere. They are not destroyed in the troposphere, but if we can introduce hydrogen atoms into the molecule, the C-H bond will be attacked by the OH radical present in the troposphere. The atmospheric lifetimes of these molecules will become much shorter. The CFCs’ lifetime is more than 75 years, longer than 50 years, a very long time. But if we can introduce hydrogen, the atmospheric lifetime will be much more shorter, for example 5 to 10 years. Most compounds can not come up to the stratosphere because during their diffusion up the troposphere, they will be destroyed by OH radicals.
Fluorine chemical manufacturers have stopped to produce CFCs and have started to produce much more environmentally friendly HCFCs and HFCs. This is a very good success story as written in the book Industry Genius. But another problem was raised — we actually have two serious problems in the global environment — global warming. The degradation of the ozone layer will be solved by introducing new alternatives to CFCs, but among those alternative CFCs, for example hydrofluorocarbon, have a rather high global warming potential, in another words, a greenhouse gas.
We ratified the Montreal Protocol in 1987. In just ten years another protocol, the Kyoto Protocol, for reducing global warming and greenhouse gases was ratified. Six greenhouse gases were defined at that time. The major greenhouse gas is carbon dioxide. Another five are minor. Three of them are fluorine containing gases, HFC, PFC (perfluorocarbon), and SF6 (sulfur hexafluoride) which are very important to industry. PFCs do not contain hydrogen but have a very long lifetime. So, our research center is now doing scientific research, developing alternatives to these greenhouse gases HFCs, PFCs, and SF6. These are the next generation of chemicals for a sustainable society.
FL: Do you believe we will find a solution that will satisfy the requirements for the ozone layer as well as for the global warming?
MY: The most critical property of these chemical compounds is the atmospheric lifetime. So, we design those molecules from synthesis. evaluate these chemicals, and also use computational methods to predict what kind of molecule will be suitable for the sustainable society but this is a very difficult challenge. So now, HFCs (hydrofluorocarbons) are a very good substitute for CFCs to protect the ozone layer, but this is said to be a strong greenhouse gas. The properties are excellent for industry, so it is very difficulty to find another alternative. The best solution to prevent global warming is the responsible use of those HCFCs, meaning the best way is emission control so we have to recycle, recover, and reduce the emissions as much as possible. This is the best way to prevent global warming rather than to develop new alternatives.
FL: Dr. Yamabe, thank you for joining us today.
MY: My pleasure.