Green chemistry – challenges and opportunities

Green chemistry – challenges and opportunities
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Dr Audrey Moores, of the Department of Chemistry at McGill University, discusses some of the challenges in green chemistry today and how her team is working to overcome them.

Dr Audrey Moores’ research group at McGill University, Canada, works at the interfaces between the fields of material chemistry, co-ordination chemistry, and organic synthesis. It conducts research in the domains of catalysis using both heterogeneous and homogeneous approaches and more sustainable nanoparticle synthesis. It has a special interest in magnetic particles and nanocrystallites of cellulose in catalysis, novel synthesis of nanoparticles in solid phase, and nanoparticles in ionic liquid media. Speaking to SciTech Europa, Moores discussed some of the challenges in green chemistry today and how her team is working to overcome them.

What do you feel are some of the biggest challenges in green chemistry today (i.e. the inefficiency of processes for synthesising chemical products and subsequent issues in resource conservation and environmental and health concerns related to the chemical wastes)?

Green chemistry as a field has recently celebrated its 25th anniversary; it is no longer a new or emerging discipline. In the earlier days, there was a distinct focus on areas related to the development of a more sustainable chemistry enterprise, including the evolution of more efficient processes designed to diminish waste and exposure to toxicity, and they are themes which have persisted over time. As such, they have become areas that have been very well researched, and while they remain challenges in themselves, over the years the emphasis has shifted a little towards the concept of systems: systems thinking, life cycle analysis, being able to properly measure the impact of a process and, moreover, of changing a particular process to something more sustainable. Indeed, this paradigm is now central to green chemistry, and there is a sense that this will continue.

I am the associate editor of the journal ACS Sustainable Chemistry and Engineering, and my colleagues and I are placing increasing weight on metrics in the papers we publish as the way we approach our discipline evolves. We are progressing from studying a specific reaction to trying to understand its integration within a bigger picture. This is technically different to the more traditional approaches as we are now increasingly utilising software and databases of information. We have entered into a very different kind of territory, and it is through the integration of a multidisciplinary approach that we are going to be able to do this more inclusively.

Multidisciplinarity, of course, has its own inherent challenges, not least working to overcome technical language barriers, for instance. How do you approach this?

At McGill, over the years we have held a number of green chemistry initiatives in an effort to bring more interdisciplinarity into both our research and teaching practice. Here, using funding for several centres, we have always tried to include people from engineering and management as well as from scientific disciplines, and, more recently, we have started the McGill Sustainable Systems Initiative-Materials theme, which I co-lead with two colleagues.

This sees teams of people from pharmacology, management, law, engineering, natural resources, and scientific disciplines such as physics and chemistry come together to solve problems. From this incredible richness of diversity comes a richness of new ideas and approaches.

For the last five years now with a colleague from management, we have also held a workshop on sustainable innovation, where we bring together students from an MBA programme with students from graduate studies in engineering and graduate studies in science. They form teams and are asked to design an innovative product, competing against each other. Within this, each team needs to be able to articulate the financial aspect and the strategy aspect (which comes from the MBA side), as well as to overcome technical barriers and to propose technical changes and improvements (from the science and engineering side) in order to hit new markets and to have more niche applications.

From this experience we have learned that in order to work together people need to be together in the same room to solve problems, and that it is through the hardship of trying to figure out what the other person is trying to say that you actually start to initiate true collaborations. Indeed, this initiative has seen the creation of a number of start-ups by the students who have taken part, and, essentially, this is a result of them being able to call upon the people they met and worked with there to help them to fill the gaps in their own areas of expertise when they are attempting to raise funds, set up companies, and develop and market new products.

Traditional solvents have led to various environmental and health concerns, meaning that various cleaner solvents have been evaluated as replacements. What are your thoughts on this area?

There has been a lot of progress recently in replacing solvents, although there is still a lot of work to be done. The pharmaceutical industry has similarly taken great strides here, working in a number of areas to solve this problem, with many others now taking up the work they began so as to implement a real reduction in solvent use and a reduction in their toxicity.

Toxicity has become a topic for research in itself, of course, and there has certainly been a lot of work happening in this area. Being able to evaluate toxicity to living organisms is a topic that is being developed massively at the moment, and much of the work here is now placing an increased emphasis on high-throughput techniques in an effort to expedite the discovery of whether something is toxic or not and, if it is, then what it is toxic for.

A major obstacle to using renewable biomass as feedstock is the need for novel chemistry to transform the large amounts of biomass selectively and efficiently, in its natural state, without extensive functionalisation, defunctionalisation, or protection. How can this be tackled?

Our group is certainly working in this area, as are many others elsewhere.

One of the central tenets of green chemistry is that Nature has delivered a number of beautiful molecules, and chemists traditionally seek to deconstruct them and try to remake new ones. When it comes to protection, functionalisation, and defunctionalisation, these were significant problems when green chemistry started, but the last 25 years have seen a huge development in terms of the methodologies used.

Indeed, much progress has been made in the carbon-hydrogen bond (CH) activation area in chemistry, and we are increasingly able to use protecting agents, and indeed activating agents, to target one specific position, which is of huge benefit in terms of fewer sets and less wait.

The next step is to explore how we can do the same thing but with less chemistry, or even do more with less chemistry; perhaps a different molecule that is an easier target from a certain resource could do the same job? Put simply, the challenge for chemists is to now try to achieve function with less chemistry.

Doing more with less is, of course, something that is not particular to chemistry, and perhaps the approach we now need to take is to look at the service a molecule provides rather than whether we need a specific molecule, and, from that, we may be able to find that a different molecule can provide the very same service, despite the fact that it is almost philosophically interesting for a chemist to say maybe what we need is actually less chemistry or, in fact, smarter chemistry.

Plasmonic nanoparticles are exciting and promising candidates for light-activated catalysis. Where has your work focused here, perhaps with regard to the activation of molecular hydrogen and the hydrogenation of ketones and aldehydes via visible light irradiation?

In this particular example we tried to tackle the limitation of silver chemistry. Silver is a wonderful metal for hydrogenation, and C=O double bonds are important functionalities that you may want to hydrogenate or reduce, and when you do so you want to do it selectively without touching C=C double bonds. The problem there is often catalysts will do them both because they have similar mechanisms.

Silver is good because it is a terrible hydrogenation catalyst: it is very bad at activating H2, it makes the activation of H2 very complicated, and it has to go through a slightly different mechanism than the other metals, meaning that you can be more selective in efforts to hydrogenate or reduce the functionalities.

The problem concerns the energy of the electrons in the system, and in this particular example we wanted to use the fact that light can be absorbed by silver through a mechanism known as ‘plasmon resonance’, where electrons inside the nanoparticle oscillate in resonance with the incoming light, and this creates the strong absorption. We were able to harvest that energy and convert it into chemical energy with electrons that need higher energy and are able to activate H2.

Now, we are moving from having to use 20 bars of H2 to having to use a single atmosphere of H2, meaning that we can do the same chemistry with the use of much lower pressures.

Cellulose nanocrystals (CNCs), derived from cellulose, provide us with an opportunity to devise more sustainable solutions to current technological challenges. What are the biggest barriers to the development of these solutions?

CNCs are currently used in a number of areas, and there are people working for environmental remediation, people making electronics, making sensors, developing food applications, and working in packaging who are all using this incredible material.

CNCs are constructed from cellulose, and this is any form of cellulose – from plant waste, from paper pulp, from bamboo, and even from bacteria (although they will have slightly different properties if they are taken from bacteria) – and this cellulose has amazing photonic properties. They can also self-assemble and can be used to create iridescent films, meaning that they also find applications in areas such as sensing and in numerous exciting photonics applications, too.

In our work, we wanted to see how we could apply CNCs to catalysis – that is, as a chiral inducer in enantioselective catalysis – and we wanted to harvest the chirality and use it in the context of energy-selective catalyses.

The concept of using a surface to transfer chirality is very difficult, and a lot of effort has been made to achieve this in the past by other teams, who were met with very limited results. While the enantiomeric excess is not that good compared to what an organic chemist would like to see from this selective catalysis, our group, however, was able to demonstrate that we could achieve higher enantiomeric excess because the cellulose nanocrystals are able to convey chirality much more effectively than other forms of biopolymers.

Moving forwards, we want to try to find applications whereby we can take advantage of the chirality of this material.

Where will your future research interests lie in a more general sense?

Currently, we are doing a lot of work on plasmonic nanomaterials, and while I can’t go into too much detail yet, I can say that we are trying to work towards turning them into even more sustainable catalysts.

We are working very hard on biomass conversion at the moment, and we have some exciting discoveries evolving from this. Similarly, we are also continuing to work on the synthesis of nanoparticles, where we are trying to make nanomaterials, which have potential in a lot of application areas, such as energy, remediation, depollution, and so on.

Nanoparticles are very exciting for a more sustainable society, but their synthesis is difficult, and so we are working on making them in ways which use less solvents or often no solvent at all.

We also have a programme to try to launch even more nanomaterials with these kinds of methods.

 

Dr Audrey Moores

Department of Chemistry

McGill University

+1 438 884 2135

audrey.moores@mcgill.ca

http://www.mcgill.ca/moores-group/

This is a commercial article that will appear in SciTech Europa Quarterly issue 27, which will be published in June, 2018.

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