23 Nov 2017
Toyin Omojola was awarded the Richardson Travel Bursary in December 2017 to visit a leading UK research laboratory for 2 months. Here, he tells us how his visit was an excellent opportunity for discussions on experimental and modelling techniques.
‘SCI’s Richardson Travel Bursary gave me the opportunity to visit a leading UK research laboratory for 2 months. This visiting research stay allowed me to extend my knowledge on the steady state conversion of methanol to hydrocarbons (MTH) over ZSM-5 zeolite catalysts by using a spatially resolved technique available at the School of Chemistry and Chemical Engineering at Queen’s University Belfast, Northern Ireland.
‘Until now, the main challenge involved in MTH conversion is the identification of the primary C-C bond formed over ZSM-5 catalysts and the governing mechanisms leading to its formation. Despite MTH technology being already implemented at industrial scale, coking of the catalyst also remains a major challenge. Control of the C-C coupling reaction such that formation of coke or undesirable aromatics is minimised would greatly enhance the competitiveness of this technology.
‘Identification of the primary C-C bond is complicated by the fact that, although olefins are identified in the gas phase as the primary C-C bond formed at very early stages of the reaction (low contact times), several researchers have identified aromatics as primary products particularly in the pores of the zeolite. The proponents of the latter suggest that these aromatics have a higher binding (adsorption) capacity than olefins to the active sites of the zeolite leading to its non-detection during the early stages of MTH conversion. Nonetheless, as contact time increases, these aromatics eventually desorb.
‘Ultimately, the steady state product distribution during the early stages of MTH conversion can only be controlled by determining the adsorption, desorption, surface reaction, pore chemistry as well as diffusion of species formed. Construction of a microkinetic model is inevitable as this allows the identification of mechanisms governing the formation of the primary C-C bond.
‘The spatially resolved technique developed by Prof. Goguet and co-workers is beneficial to solving the C-C bond conundrum. Here, axial gas phase concentration and temperature profiles can be obtained by moving a sampling probe along the length of the fixed bed reactor. When combined with surface techniques such as Raman spectroscopy or Infrared spectroscopy, this technique is powerful in determining the gas and surface concentrations of species during MTH conversion. This would then feed into a microkinetic model which helps determine the mechanism of C-C bond formation.
‘During the first month of my research visit, we identified key problems concerning the installation of the experimental rig particularly to the adaptation of ZSM-5 catalysts used in MTH conversion. Specifically, we focused on the detection of leaks as a vacuum system needs to be established at the start of the experiments. Also, inserting the thermocouples (outer diameter of 0.08 mm) into capillaries of 0.15 mm required special care and adapting the experimental set-up to match conditions from a transient technique used for MTH conversion required special considerations. Due to time constraints, we were only able to establish the right conditions for steady state MTH conversion using the spatially resolved system. Experiments were postponed to a 2-week period in January 2018.
‘In the future 2-week research visit, actual experiments would be conducted on zeolite catalysts during MTH conversion using the spatially resolved technique. It is expected that axial concentration and temperature profiles of gas species would be obtained and used for steady state kinetic modelling. This would ultimately lead to the identification of the first C-C bond and the understanding of the mechanisms governing its formation.
‘I benefitted immensely from detailed discussions with the REACT group at Queen’s University Belfast. I am especially grateful to Ciaran Coney and Prof. Alexandre Goguet with whom I had daily discussions on experimental and modelling techniques and the SCI for affording me this opportunity to visit the School of Chemistry and Chemical Engineering to conduct my experiments.’
University of Bath