ACS MEETING NEWS
New ways to nab nitrogen approaches work efficiently on a large enough scale to be practical. For that, we have the Haber-Bosch chemical synthesis process. This brute-force industrial method employs a metal catalyst to couple H2 with N2 at high temperature STEPHEN K. RITTER, C&EN WASHINGTON and pressure to prepare NH3. Much of the NH3 is then converted to nitric acid by the ritish chemist Humphry Davy usher in a new revolution in agricultural Ostwald process to make the fertilizer amis best known for being the fertilizer production, much in the way that monium nitrate. first person to isolate several the Haber-Bosch industrial process curThe Haber-Bosch-Ostwald pathway elements, including sodium and rently used to make ammonia ushered in a requires a substantial industrial infrastruccalcium. Davy carried out his work electrofertilizer revolution when it was developed ture that consumes massive amounts of lytically more than 200 years ago as an early 100 years ago. energy and creates great volumes of carbon experimenter with batteries. Less known In one example, postdoctoral researcher dioxide and other pollutants. Researchers about Davy is that during experiments on Ross D. Milton and chemistry professor have therefore sought out environmentally water electrolysis, in which water is split Shelley D. Minteer of the University of friendlier and more sustainable approaches into hydrogen and oxygen, he found that Utah and coworkers have developed a to producing ammonia, ranging from therammonia formed on the cathode in his bioelectrochemical process in which an enmal solar reactors to engineered plants that electrolysis cell. Davy had unexpectedly zyme-based fuel cell produces NH3 from N2 make their own ammonia. The new stratecoupled N2 dissolved in the water from and H2 at room temperature and pressure. gies from Minteer and Nocera could be part the air with the H2 being formed to make In another example, chemistry professor of the solution everyone is looking for. NH3 (Phil. Trans. R. Soc. Lond. 1807, DOI: Daniel G. Nocera at Harvard University, “If electrolysis should become a viable 10.1098/rstl.1807.0001). biochemistry professor Pamela A. Silver at strategy for nitrogen fixation, it could be a Scientists and engineers have been Harvard Medical School, and coworkers are means of circumventing the Haber-Bosch fixated on this so-called nitrogen-fixing developing engineered bacteria that incorand Ostwald processes,” says chemistry process ever since, primarily as a means porate H2 from water electrolysis with N2 professor Robert H. Crabtree of Yale University, who specializes in cate– e– alytic strategies for alternative Potentiostat energy generation. Last year, Crabtree and ATP chemistry intellectual propH+ erty specialist Michael Jewess published a perspectives paper H2 N2 MV+• MV+• recounting Davy’s discovery and proposing that scientists ramp up efforts to develop sunlight-driven bioelectrochemical + 2+ 2+ systems for making NH3 (ACS 2H 2NH3 MV MV Sustainable Chem. Eng. 2016, DOI: 10.1021/acssuschemeng Proton-exchange Anode Cathode .6b01473). In a big picture way, ADP membrane electrocatalytic nitrogen fixIn this enzymatic fuel cell designed by Utah’s Milton and Minteer, a hydrogenase enzyme oxidizes ation for distributed fertilizer H2 and a nitrogenase reduces N2 in an overall process that produces NH3 and a small surplus of production would be a more electricity (MV = methyl viologen, ATP = adenosine triphosphate, ADP = adenosine diphosphate). sustainable method for individual farms to harvest N2 from the to make NH3 to prepare fertilizer. We now from the air to produce NH3. Minteer and air and make their own fertilizer, bypassing know that bacteria in the soil produce niNocera presented details of their research the current industrial production and distrogenase enzymes to pull N2 from the air at the recent American Chemical Society tribution systems, Crabtree suggests. to make NH3. The ammonia is subsequently national meeting in San Francisco. “This is leading the chemical side of converted to nitrate by other bacteria in the Following Davy’s discovery, chemists the nitrogen-fixation problem to progress soil so that it can be used by plants. began trying to develop electrosynthesis beyond prior mechanistic and biomimetic A new set of NH3 production strateprocedures to produce ammonia on a large concerns and take on real practical signifgies now combines the best of both these scale. And with an understanding of the biicance, as the recent work of the Minteer worlds: Davy’s electrochemical observation ological nitrogen-fixing process, chemists and Nocera groups shows,” Crabtree says. and nature’s enzymatic approach. If these have been trying to develop metal catalysts Although many researchers have exexperimental technologies prove successthat mimic enzymes. But researchers plored enzymatic approaches to NH3 ful on a larger scale, they could one day haven’t quite figured out how to make these production, Minteer’s group at Utah has
More sustainable approaches to synthesizing ammonia could re-revolutionize agricultural fertilizer production
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C&EN | CEN.ACS.ORG | MAY 1, 2017
CREDIT: ROSS MILTON
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CREDIT: NOCERA LAB/HARVARD UNIVERSITY
Two bunches of radishes allow comparison of a control crop grown naturally (left) and an experimental crop with enhanced growth through ammonia supplied by Nocera’s bionic leaf (right). provided the first evidence for bioelectrochemical NH3 production by a complete nitrogenase, rather than just one subunit of the enzyme (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201612500). The Utah team’s enzymatic fuel cell consists of two compartments. On one side, a hydrogenase enzyme oxidizes H2 supplied to the cell to form hydrogen ions and electrons. Because the enzyme is not efficient at directly interacting with the electrode, the researchers need to provide a redox-active compound as a go-between to pick up and drop off the electrons. They chose methyl viologen, a versatile compound often used for this electrochemical role. In the other compartment of the fuel cell, they use a nitrogenase enzyme to reduce N2 from the air to make NH3, using electrons supplied from the hydrogenase side of the cell through an external circuit. Methyl viologen again acts as a redox mediator to shuttle the electrons between the electrode and the enzyme. The hydrogen ions, needed to form NH3, migrate through a membrane separating the two compartments. As a bonus, the overall reaction generates a small excess of electricity. There’s a few catches to the bioelectrochemical system, Minteer explains. Nitrogenases are not commercially available, and when isolated from cultured bacteria they must be handled with care because the enzymes can be irreparably damaged by oxygen. In addition, nitrogenases require the coenzyme adenosine triphosphate (ATP) to operate; ATP undergoes hydrolysis to mediate energy transfer for nitrogen reduction. Minteer’s group had to devise a way to continuously supply ATP to the enzyme, which the researchers did by adding creatine phosphate to recycle adenosine diphosphate (ADP) into more ATP. The team has been able to produce small amounts of NH3 so far, Minteer says, and several challenges remain before scaling up. But she thinks those will be mostly related to materials design and enzyme engineering. One challenge is to address the oxygen
sensitivity of nitrogenase and the lifetime of radishes, showing that plants treated with the enzymes. Another is to develop a workthe bacteria weigh 150% more than untreataround to avoid the need for ATP. ed plants. “We can grow big radishes, really Looking to the future, Minteer is thinkbig radishes,” Nocera exclaims. ing about small-scale systems in which The technology is still at an early stage every farmer could use a solar cell to run an and nowhere near being put into practical enzymatic bioelectrosynthesis cell or set of use, Nocera stresses. “I just wanted to find cells to make ammonia, rather than buying out if we could actually do it,” he says. “The it delivered in trailer-mounted tanks as answer is yes.” many currently do. Farmers could use the Nocera’s team is now exploring ways excess electricity to help power their opera- to speed up NH3 production. The proof of tions, or they could sell it to the power grid. concept also points to the possibility of “We generally think of Haber-Bosch as modifying the microbes to synthesize other an intensive process that consumes energy. compounds. “One day we might be able to But with the right catalytic system design, make everything we need by tailoring these we can actually generate energy,” she says. bugs,” Nocera says. “You would have a so“Our technology would definitely enable lar-based manufacturing lab.” us to decentralize fertilizer production and Nocera notes the primary beneficiaries avoid building and running large industrial of the Haber-Bosch process have been peoplants.” ple living in developed Nocera’s group is countries with established already known for develinfrastructure. He views oping an “artificial leaf,” a the bionic leaf instead as a wireless solar-cell device means of boosting agriculthat mimics a natural leaf ture and food production by splitting water into in developing regions. In H2 and O2. The H2 can be fact, the strategy assumes stored and used as needed developing an infrastructo run fuel cells to generture won’t be needed at all. ate electricity. The team “The Haber-Bosch prohas recently been taking cess is one of the greatest the concept a step further scientific achievements to develop systems for in the 20th century,” says making liquid fuels, and John J. Watkins, chief exnow a hybrid artificial —John J. Watkins, chief ecutive officer of Fulcrum leaf-microbial system to executive officer, Fulcrum Bioscience. “Industriproduce NH3. Bioscience al-scale nitrogen fixation The new approach, a allowed agriculture proconstruct called a “bionic leaf,” is actually duction to increase enormously and feed an an engineered bacterium that effectively ever-increasing world population. Howevcarries out Haber-Bosch in a single mier, there has been a growing interest in alcrobial cell. The researchers designed a ternative, greener methods that move away Xanthobacter species to take H2 from the from centralized ammonia production.” artificial leaf and use a carbohydrogenase Watkins’ company is working toward inenzyme to couple it with CO2 from the air creasing the nitrogen-fixation rate of algae to make the bioplastic polyhydroxybubiofilms using genetic modification and tyrate. A number of bacteria are known to electrochemical methods. Part of his evalproduce such bioplastics that they store uation includes looking at the dynamics of as a fuel source, like people store fat. But the fertilizer market. the team also integrated the ability for the “The current system of purchasing microbe to absorb N2 from the air and use fertilizer is straightforward for farmers: its nitrogenase to couple it with H2 from Fertilizer is purchased, delivered, and then the polyhydroxybutyrate to make NH3. This applied,” Watkins says. Because agriculture trick replaces the need for ATP to power operates on narrow margins, farmers must the enzyme. balance increased yields against increased The researchers spray a solution contain- cost to remain profitable, he adds. That ing the polyhydroxybutyrate-storing bactemeans any new production process, like ria onto the soil like a nutrient, where NH3 the technologies being developed by the is produced and expelled into the ground, Minteer and Nocera groups, must be cost reminiscent of the way farmers apply liquid competitive, Watkins says. And they must ammonia to fields. Natural bacteria in the be simple to operate with low maintenance soil do the rest, converting the NH3 into demands. “Growing up in Iowa, I never nitrate that plants can absorb through their knew any farmers with an abundance of roots. Nocera’s group tested the strategy on free time or money.” ◾
“There has been a growing interest in alternative, greener methods that move away from centralized ammonia production.”
MAY 1, 2017 | CEN.ACS.ORG | C&EN
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