An electron microscope image of the sort of catalyst crystals under study at CLF.
Credit: Professor Andrew Beale et al. University College London
21 September 2020
Researchers working at STFC’s Central Laser Facility (CLF) investigating catalyst materials used in the production of gasoline from simple hydrocarbons, such as methanol, have made a new and unexpected discovery that could help to reduce the cost, and environmental impact, of the process.
Methanol is a clean energy option that can be produced from natural gas, coal and a number of renewable resources including biomass, landfill gas and industrial emissions – in fact, methanol was originally only produced from biomass and was called ‘wood alcohol’. While methanol is an easily handled combustible liquid, its energy density is only half that of gasoline, so, in the 1970s, a technique was developed that allows methanol to be converted into gasoline.
Because methanol is a simple hydrocarbon made up of just a single carbon molecule, catalysts are used to spur the chemical reactions needed to turn those single molecules into the complex chains of carbon that make up high-energy hydrocarbon fuels like gasoline.
One of the most commonly used catalysts are zeolites, which are microporous, crystalline structure made up of silicon, aluminium and oxygen molecules that form a framework of cavities, known as pores. Their structure means they are often referred to as molecular sieves. The size of the pores dictates the hydrocarbon produced at the end of the reaction – small pores result in light hydrocarbons, medium pores result in gasoline components.
Although zeolites are effective catalysts, their pores have a nasty habit of getting ‘gummed up’ and blocked with heavy hydrocarbons that get trapped within them. This causes the catalyst to become less efficient and eventually to deactivate altogether. At this point, the zeolite must be replaced or reactivated through an expensive, energy-intensive, and relatively dirty industrial process that effectively burns away whatever is gumming up the works. These zeolites are of high value to industry and discovering why zeolites deactivate and precisely when the process starts is of great interest to scientists and to industry.
Image of the catalyst in the micro-reactor where it is heated to reaction temperature. It sits beneath a quartz window through which a laser is focused to measure the catalyst as the reaction occurs.
Credit: Emma Campbell, University College London
One of the most common techniques used to investigate catalysis is Raman spectroscopy, which is a simple and non-destructive way to analyse a sample’s molecular and chemical structure. The technique works by exposing the object under investigation to intense radiation in the form of light photons from a laser. The photons interact with molecules in the sample and scatter, losing energy as they do so. By detecting the scattered photon and measuring the energy the photon has lost, scientists can identify the properties of the molecules from which it scattered.
Unfortunately, Raman spectroscopy creates a lot of background fluorescence – the light equivalent of white noise – that easily drowns out the emissions the researchers are interested in. This is where CLF’s expertise in a technique called Kerr gated Raman spectroscopy comes in.
Kerr gated Raman spectroscopy works like normal Raman spectroscopy but it has optical ‘gates’ that use the polarisation of light to filter out the unwanted fluorescence. The technique takes advantage of the fact that Raman fluorescence is an instantaneous process – light bounces off the molecules instantly, whereas the unwanted background fluorescence is delayed and occurs a few picoseconds to nanoseconds later.
Unfortunately, because a picosecond is just a thousandth of a billionth of a second, you have to be able to filter out the noise very quickly indeed and the faster you can do the gating the better. The CLF is one of the fastest systems for doing this in the world and is able to gate out unwanted light in just three to four picoseconds.
Using Kerr gated Raman spectroscopy the team were able to track all the stages of the reaction and watch as progressively longer chains of hydrocarbons were formed. In doing so, they were able to identify what sort of hydrocarbon was being formed at the moment the catalyst started to deactivate – like catching a criminal holding a knife at a murder scene, they were able to catch the culprit red handed.
In the past, it had been thought that a species of hydrocarbon called polycyclic aromatics (polyaromatics) were the main deactivating agents, but the new research has shown that this is not the case. In fact, shortly before the polyaromatics are formed, another species called polyenes are formed and, in fact, it is they that are present at the moment the catalyst started to activate. This was a completely new and unexpected discovery. To continue the crime scene analogy, it was as if the polyenes stabbed the victim and the polyaromatics were just the next hapless hydrocarbon to come along and pick up the knife – it turns out it was a case of mistaken identity all along.
“I think that this will change the way people think about how these materials deactivate because, as soon as you know who the culprits are, you are able to design a strategy,” said team-leader Professor Andy Beale of University College London.
There is a huge energy cost, as well as time cost, associated with reactivating catalysts frequently, so anything that can keep catalysts doing their job for longer will save money, energy and reduce the environmental impact – resulting in greener fuels. The study could be significant for industry especially as methanol can be produced from biomass stocks – meaning that gasoline may not have to come from fossil fuel stocks in the future.
The project is an ongoing collaboration between the Central Laser Facility, University College London, Ghent University, and the Catalysis Hub – a consortia created with the aim of establishing a world-leading programme of catalytic science in the UK. The findings have been published in the journal Nature Materials.
CLF’s Professor Mike Towrie:
“Kerr Gate Raman is remarkable because it makes it possible to detect tiny Raman signals from zeolite reactions that would otherwise be lost in a sea of fluorescence noise. The Ultra Laser Facility at the CLF allowed us to achieve a signal quality approximately 100 times better than if we’d used standard Raman. For me, this work has been exciting because it demonstrates the potential of Kerr Gate Raman to help answer many more questions in catalysis and in other areas such as battery science.”
Last updated: 05 October 2020