Dr Zoe Schnepp
In the past, chemists were free to play with any element or molecule they wanted. Hazards such as bioaccumulation were unknown and, importantly, unexpected. Chemists busied themselves making devices, materials and medicines for the 20th century world with no idea of the problems these products might cause. In the process, chemistry (and chemicals) got a pretty dreadful reputation! Now we have to keep up with the demands and needs of a 21st century population, as well as find solutions to problems like the energy crisis.
So what are the next challenges for chemistry? Energy is certainly the biggest in my opinion. There are numerous options, with solar being perhaps the most attractive. The energy will also need to be stored, which is another big area of research. Another area that is becoming really interesting is where we source our feedstocks. Most school-age children will learn about fractional distillation of crude oil to produce molecules for the chemical industry (as well as the major fraction going to fuels). If oil becomes scarce then we will need alternative feedstocks and again nature may provide the answer. There is a lot of attention in the media about biofuels but similar chemistry is also being used to make useful molecules for the chemical industry. Plant matter (biomass) can be broken down in a biorefinery to make a whole range of molecules that can then be used to produce the drugs, plastics and other materials we use in our everyday lives.
A large challenge that I’ve mentioned briefly this week is resources. Elements that we use in devices and materials have to be sourced from the Earth. Many of these are mined from the Earth’s crust and some are present only in very small quantities. These scarce elements are expensive and several of them are becoming very important in modern technologies. Most importantly, some elements such as platinum or indium will become increasingly important in future technologies such as solar capture or fuel cells. Finding alternative ways to make these technologies work without rare elements is one possibility. In the meantime, the careful use of resources is essential.
There are so many other challenges I could discuss here. If you are interested in reading further, there is some great information (and a white paper) on the webpage of the Royal Society of Chemistry.[i] Scientists have always been good at solving problems, that’s the main reason that most of us do research! I’d like to think that the big challenges of the future represent some great opportunities.
Dr Zoe Schnepp
As I mentioned yesterday, a big area of research in chemistry is controlling the size and shape of different materials. I talked about materials for water purification but that’s just one possible application. By controlling the size and shape of particles of a material you can do some really amazing things. You might have come across the example of gold already. It’s a really unreactive metal in the bulk state – that’s why people have used it for millennia in jewellery after all! But reduce gold down to nanoparticles and it can do amazing things like purify car exhaust.[i]
Size and shape is most important for a class of materials called catalysts. These speed up chemical reactions and they are fundamental to many aspects of our lives. They will also be crucial in many future applications such as hydrogen-powered cars and capturing solar energy. There are many ways that scientists can control how catalysts are formed, but maybe the most exciting way is to copy nature!
Living organisms have been controlling size and shape of materials for millions of years. Mammals generate bones out of a hard mineral called calcium phosphate. Bones have a microscopic honeycomb structure that allows incorporation of cells and blood vessels and also keeps the bones from being too heavy. Sea creatures create a spectacular range of shells that become even more amazing when you view them under the microscope![ii] The best thing about this ‘biomineralization’ is that living organisms create these structures under ambient conditions and from water. In this sense, they have designed the ultimate green materials chemistry.
There are many ways that we can copy nature and control the microscopic architecture of materials. It’s a huge field of research and there are a lot of books on the subject. One way is to use the natural materials themselves as a template.
For example, by coating a leaf skeleton in a solution of iron and heating, we were able to replicate the microscopic vessels of the leaf in a magnetic material called iron carbide (an important catalyst for a range of processes).[iii] Another possibility is to use some of the remarkable polymers (long molecules) that nature produces. Seaweed is a particularly nice example. Brown seaweeds produce a polymer called alginate and this can be used to make nanowires of superconductors. The polymer is able to control how the crystals of the superconductor grow.[iv]
- Magnetic Leaf
This method of using natural materials to create useful materials is the main area of research in my group.[v] It’s maybe not the conventional idea of a chemistry lab! We have boxes of sawdust (that we’re using to make water filtration materials) alongside tubs of gelatin (to make materials for fuel cells).[vi] As well as being interesting, this type of science is becoming increasingly attractive to industry. Waste materials from industry and agriculture often have very low value. In fact with increasing taxes on landfill and burning, waste materials now often have negative value – the represent a cost to the producer. So if we can take a waste material such as sawdust and create a useful material like a water filter it is not only attractive in terms of sustainability, but may generate valuable income.
Dr Zoe Schnepp
Enough energy from sunlight strikes our planet in one hour to provide all the energy needed for human activity in one year.
Given this astonishing fact, it is not surprising that governments all over the world now consider the harvesting of solar energy to be a priority. Several approaches exist, the most well known being the direct conversion of sunlight into electricity. However, sunlight is not constant and so to ensure a reliable national power supply an energy storage system is required. This cannot just be a daily charge-recharge cycle. For energy security most countries require a storage buffer. At the moment this often takes the form of an oil stockpile. Batteries could provide part of the solution, but current technology does not have the energy capacity or stability for large-scale long-term storage.
Another possibility is using solar energy to generate a fuel, in much the same way as plants use sunlight to convert carbon dioxide and water into energy-rich carbohydrates. Chemical fuels offer a much higher energy density (amount of energy per unit of mass) than batteries and can be stored for use either in stationary power plants or in vehicles. However, ‘copying nature’ is not straightforward. Photosynthesis is actually quite inefficient and so to make artificial photosynthesis a viable industry we can’t just settle with copying nature. We need to go one better.
Photosynthesis in plants involves two main steps, both of which are driven by sunlight. One step splits water into hydrogen and oxygen. This hydrogen is not released as a gas but is transported as a positively charged hydrogen ion to another enzyme. Here, the hydrogen ion is combined with carbon dioxide to generate sugars. For a chemist, copying this exquisite multistep process is extremely difficult! One alternative is just to focus on part of the photosynthesis reaction: the water splitting. If we can generate materials to split water into hydrogen and oxygen, we could generate hydrogen gas, which is an energy-rich and clean fuel. The materials use energy from sunlight to drive the water splitting and are called photocatalysts.
This system has a lot of potential but many challenges need to be overcome. Current photocatalysts have quite low efficiency and many only work using UV light. When water is split, the hydrogen and oxygen gases need to be kept separate to avoid creating an explosive mixture! Furthermore, many existing photocatalysts for water splitting use toxic elements such as cadmium or extremely rare and expensive elements such as platinum. Viable, large-scale hydrogen production from sunlight will require efficient photocatalysts based on cheap materials and simple preparation methods.
It’s at this stage that you can envisage some of the enormous challenges facing scientists. We’ve already mentioned cadmium being toxic – it’s banned from many applications under EU RoHS (Restriction of Hazardous Substances) legislation.[i] But in artificial photosynthesis, there are materials containing cadmium that work really well! Should we continue to use cadmium, arguing that it may end up being the only material that works? Or perhaps we can learn a lot about the science of artificial photosynthesis by studying cadmium? It’s a very difficult problem and certainly not one that is confined to cadmium, or indeed to artificial photosynthesis. There are countless cases of toxic or expensive elements that perform their jobs extremely well. This is why some toxic elements are exempted from EU chemical hazard regulation for certain devices. I would argue that we have a unique opportunity. In terms of implementing the technology, we are in the very early stages with artificial photosynthesis. There is a lot more work to be done to make this very promising chemistry work and it could genuinely revolutionize our world. If we consider sustainability now, then we won’t be faced with a big clean-up operation in 50 or 100 years.
Dr Zoe Schnepp
I tried an experiment today. I typed the word ‘chemical’ into a Google image search. Alongside images of glassware filled with colourful fluid and The Chemical Brothers in concert I got lots of hazard warning signs (TOXIC, WARNING, HARMFUL, RADIOACTIVE) and people in protective suits. Sadly, the search also returned many images from the recent conflict in Syria. Chemicals seem to be synonymous with danger, harm and even death, so can chemistry ever really be called ‘green’?
Many of the chemicals responsible for this negative image were the result of a lack of foresight. With the advent of world-changing technologies in the 20th Century, it was inconceivable to scientists and industries at the time that many of the products they were making might harm people or the earth on which we live. CFC refrigerants were lauded at the time of discovery for being a non-toxic and ‘inert’ alternative to the much more dangerous and commonly-used ammonia. It was decades later that the complex atmospheric interaction of CFCs with ozone was discovered. The insecticide DDT was also once a success story, being used for example to combat malaria. Likewise, Thalidomide was initially used effectively to control morning sickness in pregnant women. Obviously, the image of the chemical industry has not been helped by some cases of appalling cover-ups. But the point is that these chemicals, and many others, were never designed to do the harm that they did. They were created with the goal of improving our lives. The terrible effects on human health and the environment were unforeseen.
With cases like DDT in mind, chemists in the US in the 90s coined the term ‘Green Chemistry’ and wrote a set of twelve principles. This was not a new field of chemistry, but rather a philosophy, a set of values to be used by all chemists when designing a new molecule or process. The twelve principles include minimization of energy usage and waste but also the design of new molecules to be non-toxic. The idea is that sustainability should be considered from the very first stages of a new research process, rather than after a new molecule or material has already been created. Of course the same principles can be applied to existing processes and in fact there are many examples of industrial processes that have been made much cleaner and more energy efficient through the application of Green Chemistry. But the long term goal is that sustainability should be considered at the design stage.
So can chemistry ever really be green? Will we ever have a world where all industrial processes produce harmless waste or even no waste at all? Can we generate all of the chemicals that we use in our everyday lives (medicines, detergents, electronic materials, food ingredients to name just a few!) from entirely renewable resources? It’s certainly going to be a challenge and there are many sceptics. But there are also some remarkable and exciting Green Chemistry success stories, some of which I hope to talk about in this blog over the next week!
Dr Zoe Schnepp is a Birmingham Fellow in the School of Chemistry at the University of Birmingham.