Shape and control — it’s crystal clear

An international collaboration led by Michael Anderson at ManchesterUniversity is embarking on a project to control the growth of zeolite crystals — non-porous materials used commercially in catalysts for cracking oil.

Zeolites make up around 30 per cent of laundry detergent powder, where their principal function is water softening, but most that are used industrially are synthetic. They are also used for separating gases — particularly air — and storing others, such as hydrogen.

Zeolite crystals are about a micron in size and their shape is crucial, but controlling the shape of the growing crystals is difficult. ‘The pores are about the size of, say, a benzene ring,’ Anderson explained. ‘The pores can run through the crystals in a number of different ways — in one direction, like tunnels running through from one side to the other, in a 2D network, or in 3D, running all through the crystals.’

Many of the most important zeolites are one-dimensional but, said Anderson, they tend to form very long needle-shaped crystals.

‘This gives very long tunnels running though the crystal, which easily become blocked,’ he said. ‘Ideally, what you want to is engineer crystals that are short and squat, with the pores still in one dimension, but running through the crystal’s short axis.’

By looking at how the crystals grow, the team hopes to be able to modify the way they are formed.

‘We will be using techniques such as atomic force microscopy and high-resolution electron microscopy to watch them grow in situ on an atomic scale,’ said Anderson. ‘We will combine that with theoretical calculations of what is going on as they grow, and try and couple the two together.’

He believes another potential application could be to make zeolites containing spiral pores. ‘In principle, these would be very good for chiral catalysis, making molecules of one-handedness,’ he said. ‘This would be very important for the pharmaceutical industry — if we could lock the chirality into the inorganic zeolite, we could use it as a template for stamping out chiral organic molecules.’