An international study led by the Chemistry Department at University College London (UCL) in collaboration with the Center for Molecular Modeling (CMM) at Ghent University and the Central Laser Facility (CLF) has made a new and unexpected discovery about industrial catalysts that could help to increase the efficiency of the production of chemical building blocks from non-fossil feedstocks such as methanol. Their findings could lead to a reduction of the cost and environmental impact of the process, making it a viable alternative to fossil fuels.
Methanol conversion towards chemicals of higher energy density is a sustainable alternative to fossil fuels. To spur the chemical reactions needed to turn methanol into complex chains of carbon like gasoline industrial catalysts are used. One of the most popular catalysts are zeolites, which are microporous, crystalline structures made up of silicon, aluminium and oxygen atoms that form a framework of cavities and channels, known as pores. Their structure means they are often referred to as molecular sieves. The stability and efficiency of zeolites is a major concern, as the build-up of carbon deposits tend to block the pores, eventually leading to deactivation of the process. At this point, the zeolite must be replaced or reactivated through an expensive, energy-intensive and environmentally unfriendly industrial process that effectively burns away whatever is hindering the catalytic process. 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.
Researchers at UCL, UGent, and CLF were able to provide answers to these questions by applying Kerr gated Raman spectroscopy in combination with molecular simulations. Raman spectroscopy is one of the most common techniques used to investigate catalysis as it can tell which molecules are present during the reaction. However, it suffers from background fluorescence that easily drowns out the Raman signal of interest. That is where the Kerr gate comes into play. In contrast to Raman fluorescence, which is an instantaneous process, the background fluorescence is delayed by a few picoseconds. The Kerr gate can filter out this unwanted fluorescence leaving scientists with a clear Raman signal to read.
Using Kerr gated Raman spectroscopy the team succeeded to resolve the molecular fingerprints at all stages of the catalytic reaction. To interpret the complex Raman spectra, containing contributions from a lot of different hydrocarbons that were produced during the process, molecular simulations were performed by CMM researchers. The combination of experimental observations and theoretical calculations enabled to pinpoint the presence of important hydrocarbon species during the process. In particular, 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 deactivate. This was a completely new and unexpected discovery. ‘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 teamleader Andy Beale of UCL.
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 University College London, Ghent University, the Central Laser Facility, and the Catalysis Hub – a consortium 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.
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