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Neutron scattering uncovers the secrets of catalytic selectivity.
Central to making a chemical commercially is the design of the catalyst that speeds up the relevant chemical reaction. It must be highly selective, directing the chemical pathway to maximise the amount of the final product at the expense of unwanted side-products. Catalysts used to make heavy chemicals are typically solids with a large surface area, over which the reacting molecules flow. The reactants bind to the catalyst surface generating an intermediate structure which facilitates the formation of the product. Understanding how and what intermediates form allows chemical manufacturers to modify the catalyst surface in order to get the best possible outcome. Since profits depend on productivity, the details of the catalyst formulation are always a closelyguarded secret!
A standard way of identifying chemical structures such as catalytic intermediates is via their characteristic bond vibrations. This is typically done using infrared spectroscopy but neutron scattering can also provide crucial complementary information.
This was indeed the case in a recent investigation of the catalytic mechanism underlying the synthesis of methyl chloride (CH3Cl) from methanol (CH3OH) and hydrogen chloride over an alumina catalyst. Nearly a million tonnes of methyl chloride are manufactured globally every year; it is used to make a wide range of everyday materials – from plastics to pharmaceuticals. The collaboration involved ISIS researchers, the University of Glasgow and one of the UK’s main chemical manufacturers, Ineos ChlorVinyls, which produces methyl chloride at its Runcorn complex. The company wanted to develop an improved catalyst that would eliminate the byproduct, dimethyl ether.
To understand the reaction process required identifying all the intermediates on the catalyst surface. Infrared studies revealed that the main one was a methoxy species (CH3O) formed by the fragmentation of methanol. However, there was a largish feature in the spectrum that could not be identified, which could have been due to a competing intermediate.
This was where neutrons came into their own. The vibrational energies associated with the feature were identified with inelastic neutron spectroscopy. Using two spectrometers, TOSCA and MARI which measure the energies of the neutrons before and after passing through the sample, the small energy differences resulting from interactions with the telltale molecular vibrations could be detected and assigned to a particular molecular structure.
The results revealed that the suspect feature was actually part of the methoxy spectrum, indicating that only this intermediate formed. Based on this information, Ineos ChlorVinyls was able to optimise the catalyst to make its manufacturing process much more efficient.
Stewart Parker (ISIS), D Lenon (University of Glasgow)
Research date: December 2005
An infrared and inelastic neutron scattering spectroscopic investigation on the interaction of alumina and methanol, AR McInroyet al., Phys. Chem. Chem. Phys. 7 (2005) 3093.
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