In the 21st century, the chemical industry is shifting toward green chemistry, or chemistry that minimizes environmental impact. It is one of the key forces for achieving carbon neutrality by 2050.
Basic Knowledge
The 12 principles of green chemistry (Anastas & Warner, 1998):
① Prevention of waste (prevent generation rather than treat waste)
② Maximization of atom economy (incorporate as many atoms of the raw materials as possible into the products)
③ Avoidance of hazardous substances
④ Design of safer chemicals (reduce toxicity while maintaining function)
⑤ Reduction of solvents and auxiliaries (or use safe alternatives such as water or supercritical CO2)
⑥ Energy efficiency (prioritize reactions at room temperature and pressure)
⑦ Renewable feedstocks (biological resources instead of fossil resources)
⑧ Reduction of derivatives (minimize protecting and deprotecting groups)
⑨ Preference for catalysis (catalytic reactions instead of stoichiometric reactions)
⑩ Design for degradation (decompose after use without environmental burden)
⑪ Real-time analysis (monitor before pollution occurs)
⑫ Inherently safer chemistry for accident prevention (design processes that do not handle hazardous substances)
Environmental problems and chemistry’s responsibility:
・Global warming: CO2 (fossil fuels), CH4 (livestock and rice paddies), N2O (fertilizers), and fluorocarbons are greenhouse gases
・Ozone-layer depletion: CFCs → completely phased out under the Montreal Protocol → replacement HFCs → further replacements HFOs
・Acid rain: sulfuric and nitric acids from SOx and NOx
・Eutrophication: abnormal algal growth and red tides caused by N and P emissions into water bodies
・Marine plastics: microplastic pollution
・POPs (persistent organic pollutants): DDT, PCB, and dioxins; Stockholm Convention
・Mercury pollution: reduced worldwide under the Minamata Convention
Carbon-neutrality strategies:
・Conversion to renewable energy (solar, wind, and hydropower)
・Green hydrogen (hydrogen produced by electrolysis using renewable energy) to decarbonize the chemical industry
・CO2 capture, utilization, and storage (CCUS): capture using MOFs and amine absorbents, and use CO2 as a feedstock (methanol, urea, and others)
・Circular economy: circular design aiming for zero waste
Green chemistry (12 principles; Anastas & Warner, 1998)
Atom economy (Atom Economy) (= mass of product / total mass of reactants × 100)
E-factor (kg of waste / kg of product; lower is better)
Carbon neutrality (zero net CO2 emissions; 2050 target)
CCUS (CO2 capture, utilization, and storage; Carbon Capture Utilization & Storage)
Stockholm Convention/Montreal Protocol (regulation of POPs and fluorocarbons)
Further Study (Principles and Applications)
Example of atom economy: benzene+CH2=CHCH3→cumene→phenol+acetone (the cumene process). Not only phenol but also acetone is a useful by-product, making this an ideal process close to 100% atom economy.
In contrast, the Friedel–Crafts reaction uses AlCl3 stoichiometrically and discards large amounts of aluminum salts as by-products. Replacement with new-generation solid-acid catalysts such as zeolites is advancing.
E-factor: 1–5 for bulk chemicals, 5–50 for fine chemicals, and 25–100 or more for pharmaceuticals. The pharmaceutical industry has the highest waste ratio and has great potential for improvement.
Using CO2 as a feedstock: Urea synthesis (CO2+2NH3→CO(NH2)2+H2O) is a long-established CO2-utilization reaction. New research is accelerating internationally on CO2→methanol (CO2 + 3H2 → CH3OH + H2O), CO2→polycarbonates (polypropylene carbonate), CO2→ethylene, and other synthetic fuels and plastics.
SDG goals: 7 (energy), 9 (industrial infrastructure), 12 (responsible production), 13 (climate action), and 14 (oceans) are directly related to chemistry. It is an era in which the chemical industry’s responsibilities and opportunities coexist.
- The 12 principles of green chemistry guide 21st-century chemistry.
- Atom economy = mass of products / total mass of reactants.
- E-factor = waste / product; it is high for pharmaceuticals.
- Carbon neutrality by 2050 is a global goal.
- Technologies that use CO2 as a feedstock are expanding.
- POPs, fluorocarbons, and mercury are regulated by international treaties.
- A circular economy aims for zero waste.
Points to Note (Common Confusions and Frequent Mistakes)
① Yield and atom economy are different concepts: yield is “amount of desired product produced ÷ theoretical maximum amount,” while atom economy is “mass of desired product ÷ mass of all reactants.” Even with a 100% yield, atom economy is low if many by-products are formed. ② Carbon neutrality ≠ zero CO2 emissions (net zero is achieved by balancing emissions and absorption). Afforestation and CCS are also combined. ③ Not all bioplastics are biodegradable: bio-based (plant-derived) and biodegradable are independent concepts. Bio-PET is plant-derived but does not biodegrade. ④ Regulation of fluorocarbons was decided under the 1987 Montreal Protocol, and HFCs (replacement fluorocarbons) are also being reduced under the 2016 Kigali Amendment.
Practice
- Name five of the 12 principles of green chemistry and explain them briefly.
- Explain why phenol production by the cumene process is advantageous in terms of atom economy.
- State three roles that chemistry should play in achieving carbon neutrality.