A simple Google search of “green chemistry” and “hydrogen peroxide” will quickly display many links to the reported environmental benefits of using hydrogen peroxide in chemical reactions. From a winner of the Presidential Green Chemistry Challenge award in 1999 to more recent “green” chemistry lab experiments, a cursory glance will lead you to believe that hydrogen peroxide is a model reagent. This view has scarcely shifted in the more than twenty years of growth in the field of sustainable chemistry. But are the commonly touted benefits enough for this chemical to be considered “green”?
Hydrogen peroxide is known to produce water as an innocuous reaction byproduct, and it’s used as a safer alternative to chlorinated oxidants. But green chemistry is, in a large part, also about looking at the entire life cycle of a product, material or chemical. It’s evaluating even useful chemicals and asking if they’re ideal or if they can be improved upon. It’s therefore surprising that hydrogen peroxide is commonly cited as a “green” reagent – surprising for reasons like its associated safety concerns and unsustainable production process. Discussion of what makes something “green” – a reaction, product, feedstock, material or process – is at its core a discussion of metrics. How do we measure environmental, health and safety improvements? The challenges and perceptions of hydrogen peroxide beg the question: who determines what trade-offs are worthwhile, and which aren’t?
In 2005, a team of scientists from NASA and Honeywell assembled a historical review of “Hydrogen Peroxide Accidents and Incidents.” The report illustrates clearly that challenges arise when the concentration of hydrogen peroxide is raised for lab or industrial applications to 30% or more - ten times higher than what you’d use to disinfect a cut.
In addition to being a strong oxidant and corrosive, upon interaction with some organic compounds hydrogen peroxide can transform into an explosive ingredient. This is exactly what happened in Helena, Montana in 1989 when a runaway train collided with another train and derailed. Over 26,000 gallons of 70% hydrogen peroxide spilled and reacted with spilled isopropyl alcohol and ground contaminants. Windows at nearby Carroll College were blown out by the fire and explosions that ultimately resulted in the evacuation of 3,500 residents and six million dollars of clean up and repairs. The very qualities that make hydrogen peroxide a highly effective bleaching agent for the textile and paper industries are the same that pose a risk to safety.
Of course, there’s no need to panic about hydrogen peroxide in your medicine cabinet. The IARC and Public Health England are just two of many organizations that don’t consider hydrogen peroxide to be a carcinogen in humans, and most hydrogen peroxide that you buy at the store is likely only three to six percent concentrated. Just as hydrochloric acid in your stomach is different from the 6 molar HCl under the hood, you can imagine that these differences in concentration command very different considerations.
That said, hydrogen peroxide at these higher concentrations is hazardous to human health and the environment. It’s corrosive to metal, skin and eyes and is acutely toxic. In a 2014 press release about sun screen the American Chemical Society noted that, “high amounts of hydrogen peroxide can harm phytoplankton, the microscopic algae that feed everything from small fish to shrimp to whales.” It’s in the second-highest health hazard category for both humans and for long-term harm to aquatic life. It’s not hard to imagine that its production, transportation, storage and application on a large, industrial scale would pose problems.
Hydrogen peroxide expands, sometimes dangerously, when heated or boiled, meaning that even stationary storage or processes like distillation can become dangerous. There are numerous accounts of leaking drums or slightly contaminated hydrogen peroxide causing explosions as a result of its high reactivity. This can be dangerous even on small, laboratory scales. In 1957 at Rocky Flats, for example, radioactive plutonium was released into a lab because hydrogen peroxide was exposed to minor impurities. Traces of iron, copper and nickel initiated a catalytic reaction in a glove box, pressurizing the box until the radioactive materials were ejected outwards. More recently in 2010, Chemical and Engineering News reported that a mixture of hydrogen peroxide (35%) and acetic anhydride resulted in an explosion at Northwestern University, seriously injuring a chemist.
There are many more accounts of lab incidents – sometimes fatal ones - resulting from hydrogen peroxide use that could illustrate this point: it’s not that hydrogen peroxide should never be used, but that its reputation among green chemists as being harmless deserves more scrutiny. Hydrogen peroxide has, in many cases, served society well; it’s used to treat drinking water, remove stains, produce pharmaceuticals, and as a disinfectant in homes and hospitals.
This brings us to the actual production processes used to make hydrogen peroxide. The challenges with this chemical go beyond storage, use, and transportation. Because green chemists must use systems thinking – looking at the big picture, from the time a material is extracted from the earth through its manufacture and ultimate disposal – it’s key to look at how chemicals are made.
Hydrogen peroxide is produced almost exclusively using the anthraquinone process. The very first step in its manufacture, as described by the New Zealand Institute of Chemistry, poses problems for the chemist trying to be “green.” To begin with, a palladium catalyst is called for to carry out the initial hydrogenation. Palladium is well-known to be a critical material: difficult to extract and expensive to obtain while being an inherently finite resource. The social and environmental impacts of extracting materials like platinum group metals (PGMs) can be devastating, and market stability is unreliable with an estimated 88% of the world’s PGM supply being located in South Africa. There is huge water and energy consumption associated with PGM mining, and it produces greenhouse gases and difficult to handle solid waste streams. On top of that, we’ve previously explored the challenges around most industrially produced hydrogen, particularly the fact that it’s almost exclusively sourced from non-renewables.
And this is just the first of four main steps. Additional steps require a non-polar solvent like benzene – a human carcinogen - to dissolve the anthraquinone that’s been produced, followed by additional solvents to dissolve even more quinone byproducts that result from that reaction.
With hydrogen peroxide being one of the world’s top 100 most important chemicals, it’s extremely unlikely we’ll see a reduction in its use anytime soon. What if we could at least produce hydrogen peroxide in a green and sustainable way? This is an opportunity for innovation that would serve nearly every facet of industrial chemistry. Researchers are developing routes to produce hydrogen peroxide directly to be more step-economic. These include via noble-metal catalysis, fuel cells and plasma methods. While none of these methods are perfect – for example, continued use of palladium – it’s a step in the right direction in achieving more efficient, greener production. Headwaters Technology Innovation Group, for example, earned a 2007 Presidential Green Chemistry Challenge Award based on their less energy-intensive, direct synthesis of hydrogen peroxide, but it relied on the use of palladium-platinum nanocatalysts. This method has the added benefits of producing higher yields at a lower cost although the process has not been taken to large scale. In China, researchers have likewise worked to develop a palladium-catalyzed but more efficient, less hazardous synthesis of hydrogen peroxide. Even in this team’s conclusion they state that there is still much work to do, and that “the most promising technology in the future will be direct synthesis of hydrogen peroxide from hydrogen and oxygen without using anthraquinone as the reaction carrier.”
So where does this leave chemists? It’s our duty as scientists to keep asking how we can improve – for the environment, for our health - the processes and materials around us. Time and time again, serendipity and perseverance have proven that anything can be achieved through chemistry. Surely, there is a sustainable, safe method of producing hydrogen peroxide or an entirely new reagent that will allow the chemical transformations that are considered dependent on it to occur.
This is yet another opportunity for chemists and chemical engineers to design greener chemicals, chemistries, and processes that enable a more sustainable future. While hydrogen peroxide has its advantages, we can and should do better. Are you up to this challenge of pursuing greener options at each stage of production and use not just for hydrogen peroxide, but across all of chemistry?
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