Cartography of catalysts

UK researchers are creating a definitive road map of the internal structure of porous supports for catalysts. The microscopic cartography will provide chemists with important information for formulating the most efficient catalytic system for a given process.

Many catalysts are carried in porous supports and the nature of the pores is crucial to the operation of the catalytic system. Generally there is a trade-off between small pore size - to increase the internal surface area of the support - and large pores - to enable the reactants and products to diffuse rapidly through the system. Chemists must choose the most appropriate support at the lowest cost.

’During the reaction, some of the pores can become clogged up and this can have important consequences on the efficiency of the process,’ said team leader Sean Rigby, of the department of chemical engineering at the University of Bath. ’In some hydrocarbon cracking reactions, for example, coke deposits can block the maze of pores and interfere with the diffusion of reactants and products in and out of the system. We need information about the inside of the structure so that we can predict what will happen during the reactions and how this will affect the efficiency of the process.’

To get an idea of the internal pore structure of various supports, which are typically made from silica, alumina or carbon, various techniques can be used, such as measuring the adsorption of gas, which can be related to the size and number of pores, or forcing mercury into the structure, a process called mercury porosimetry. ’These are relatively crude techniques that usually assume that the pores are parallel bundles of hollow cylinders, which we know is not the case,’ said Rigby.

His team has been working with Sandy Chudek at the University of Dundee, using magnetic resonance imaging to obtain accurate three-dimensional data on the structure of pores within mesoporous silica pellets, with pore sizes between 10 and 50 nm. ’From this we produce a map of the number of pores, their different sizes, and where they occur throughout the structure,’ Rigby told Chemistry World. ’We can then take experimental measurements and relate these to the image data.’

They have successfully carried out new modelling studies to predict how mercury will behave in a particular sample of mesoporous silica. ’The physics of entrapment of mercury in these materials is more complex than existing models allow for, and we believe we have developed the most predictive model to date,’ said Rigby.

The team has also made predictions for how hydrocarbon gases condense in mesoporous supports, where the condensation occurs and how this affects the mass transport of the reactants and products in the system. ’We have obtained good agreement between our models and our experimental observations,’ said Rigby. The data await publication. Simon Hadlington