Farming, namely plant cultivation and livestock breeding, has been the primary means for human food production for thousands of years. Beginning with inorganic inputs such as carbon dioxide (CO2), water (H2O), nitrogen gas (N2) and minerals, complex organic nutrition is formed in plants and livestock through various biological routes. Numerous chemical transformations are involved, such as photosynthetic conversion of CO2 and H2O into glucose via chlorophyll, nitrogen fixation to ammonia by a certain group of prokaryotes, amino acid and lipid formation, and protein synthesis from amino acids in living cells with the know-hows encoded in DNA transferred over the generations.

Chemistry and chemical engineering have contributed tremendously to modern farming. Most famously, the Haber–Bosch process — a chemical reaction between N2 and H2 to form ammonia — provides nitrogen for the food production chain. The extensive use of chemically synthesized ammonia magnifies farming productivity through enhanced mass transfer, as illustrated by roughly half the nitrogen in our body coming from a high-temperature, high-pressure chemical reaction1. To put this into perspective, rice paddies with cyanobacteria covering 1 km2 fix only 0.16 tons of nitrogen daily2, whereas typical ammonia plants of a few square kilometres fix 2,000–3,000 tons daily3, a boost of three to four orders of magnitude.

In meeting the Sustainable Development Goals set by the United Nations, traditional farming is facing challenges such as land scarcity, freshwater shortages, wild swings in temperature and weather conditions, intense greenhouse gas emissions, and excessive use of pesticides and fertilizers. Furthermore, the unequal distribution of land, water and other resources threatens food security for vulnerable populations in geographically unfavourable locations. Consequently, new approaches enhancing efficiency, reduced reliance on natural resources, greater utilization of renewable energy and substitution of environmentally harmful chemicals are paramount. Can we once again implement chemistry and chemical engineering approaches to come up with innovative solutions for supplementing the next generation of farming?

To confront these prevailing challenges, an integrated concept of green chemical farming is proposed. In contrast to conventional farming, where most chemical transformations occur via biological routes, green chemical farming involves the transformation of one or more farming components using chemical processes at any point throughout the production operation, either through enhancement or replacement, with a particular focus on reducing the reliance on land, water, weather and non-renewable resources, increasing efficiency and/or lowering carbon footprint.

## Key principles

Conventionally, forage grasses are planted in pastures, which are consumed by cows either directly or after harvesting as hay; beef products are then produced after cows are slaughtered and processed. By way of comparison, the green chemical farming approach replaces, at the outset, plant-based biomass with chemically produced functional molecules. Key chemical transformations, ideally driven by renewable energy, include H2O to hydrogen (H2), CO2 to organics, N2 conversion to ammonia and amino acid synthesis. Chemical feedstock will then be leveraged as building blocks by a range of microbes, such as edible microalgae and yeast, for more complex food ingredients and flavouring. These nutrient-laden microorganisms are subsequently harvested and processed as agricultural products and can be integrated with the rest of our food processing flow. Figure 1 illustrates how green chemical farming may deliver new pathways to meat analogues that can complement protein (for example, beef) production. Herein, adaptability of chemically farmed nutrients into familiar food products can address consumer apprehension. In fact, the same nutrients can be upstream substrates for other emerging food technologies, including three-dimensional printing, non-animal-derived meat or nutrient substrates for laboratory-grown meat.

Although green chemical farming has not been demonstrated at the system level in full capacity, its individual components are feasible. Innovation in CO2 adsorption materials is making carbon capture practical in power plants4, offering CO2 as an abundant feedstock at less than US$100 per ton5. The Hydrogen Shot initiative, launched this year by the US Department of Energy, seeks to reduce the cost of clean hydrogen by 80% to US$1 per 1 kg in 1 decade (www.energy.gov/eere/fuelcells/hydrogen-shot) so that H2 becomes a cheap commodity for converting CO2 and N2 to green chemicals. Additionally, the development of multilayer anodes for seawater splitting with resistance to chloride corrosion is providing new pathways for hydrogen fuel generation via seawater electrolysis6. Electro- and photo-catalytic activation of CO2 and N2 are under extensive development. In fact, breakthroughs have recently been made in CO2 recycling with an electrified copper and carbon catalyst that can produce ethanol with over 90% efficiency7. The energy source remains solar but the means of harnessing this power are totally different from photosynthesis. More so, chemical synthetic routes from renewable carbon sources to amino acids have recently been established8,9. Chemical feedstock derived from CO2, N2 and other renewable carbon feedstocks present opportunity as nutrients for single-cell protein production; for instance, ethanol can be used as the sole carbon source to grow model bacterial species and produce structurally diverse chemicals10,11, while acetate has been proved to promote microalgae growth rate by a factor of two12. Single-cell protein productivity can be as high as 4 g l−1 h−1, with an estimated cost of US\$1.5–2.6 kg−1 (refs. 13,14). Furthermore, fermentation companies such as Calysta, LanzaTech and Perfect Day are exploring ways to use CO2 as a renewable carbon source for biological products, while the development of a chemical–biochemical hybrid pathway has recently proved the feasibility of starch synthesis from CO2 and H2 in a cell-free system15.

Green chemical farming presents advantages for productivity and sustainability. In conventional land-based farming, plant cultivation is inefficient in harvesting solar energy with a typical growth cycle of months. Livestock breeding normally takes weeks to months and even years while incurring pronounced negative environmental impacts. Both processes have substantial demands on land and fresh water. On the other hand, green chemical farming leverages the higher space–time productivity of chemical processes and accelerated growth rate of microbes to produce food at much greater efficiency. The production cycle for digestible chemicals is in the range of hours, while microbe cultivation requires only days to accomplish. It is important to further note that green chemical farming does not suffer from the slow mass transfer between gas phase (feedstock) and liquid phase (microbe growth media) that otherwise hinders production rate in direct biological conversion of gaseous CO2 and H2 into food components via fermentation.

Needless to say, green chemical farming is particularly applicable in regions where the resources required for traditional farming are unavailable (Fig. 1, bottom section). For instance, Singapore, with nearly 6 million people and 730 km2 territory, imports about 90% of its food, causing supply risks and national security issues that have been further worsened by the onset of the COVID-19 pandemic. Green chemical farming would alleviate these difficulties as both adequate CO2 supply and the infrastructure required for upgrading CO2 into digestible organic chemicals are in place, given the strong chemical industry in the country. Singapore is certainly not a stand-alone example among dozens of megacities worldwide, particularly in a post-pandemic era where emphasis will be placed on food-chain-supply resilience. Furthermore, regions with little freshwater supply such as the sunny Sahara would also benefit from this approach. Cheap and abundant renewable electricity from solar farming delivers affordable, abundant, localized and uninterruptable energy supply, while surrounding seawater provides a H2 source for CO2 and N2 upgrading, as well as media for microalgae growth.

Nevertheless, green chemical farming requires substantial research and development contributions from experts in various sectors to overcome many technical barriers. First, the feasibility of the approach must be proved using systems-level demonstrations. Concurrently, chemists and chemical engineers must design improved catalytic systems to enhance the efficiency of key chemical reactions involved, while biochemists and bioengineers must work to identify and overcome the rate-limiting steps in biomass and product formation from non-conventional chemical substrates (for example, ethanol). Following successful integration of each individual component, cradle-to-grave life cycle and technoeconomic analyses must be conducted to thoroughly understand the carbon footprint, comparative advantages and weaknesses, and issues for improvement; likewise, rigorous studies on food safety for the downstream processing of microbes into fine products are required. Consumer perception then becomes key for social acceptance of green-chemical-farmed products, for which governmental regulators, academia and food industry partners must work hand-in-hand to ensure these safe and nutritious food products can go from farm to fork sustainably. Lastly, environmentally extended input–output analyses must be adopted to elucidate how these processes will affect tertiary components of the economy, including business-as-usual food production, to thoroughly understand how green chemical farming ultimately affects the business landscape. Once feasibility is proved at the system level, government regulatory and industry leaders must establish benchmarks for integrating the new technology into the current food production system in a way that is both economically and politically viable, while remaining safe and appealing to the public.