Ultrathin catalysts on demand

Korean chemists have taken acidic zeolite catalysts to the limit in terms of thickness - creating ultrathin nano-sheets that are efficient and long-lived catalysts for hydrocarbon cracking and other petrochemical applications.

Zeolites are already used in the petrochemicals industry, but making the catalysts very thin means that reactant molecules can easily diffuse into the zeolite structure and product molecules can get out quickly, explains Ryong Ryoo of the Korean advanced institute of science and technology (KAIST) in Daejeon. This improves the efficiency of the catalyst and reduces unwanted side reactions that can produce polymeric hydrocarbon ’coke’ that clogs the zeolite pores and eventually kills the catalytic activity.

To make the thin sheets, Ryoo and his team used a surfactant as a template to direct the growth of the zeolite structure. The surfactant molecule has a polar ’head’ group - with two quaternary ammonium groups around which the aluminosilicate zeolite crystal grows - and a long hydrocarbon ’tail’, which prevents the sheets from aggregating together into larger, three dimensional crystals. When the surfactant is removed, these flakes pile up randomly with gaps in between which further aids diffusion to the catalyst sites.

’Di-quaternary ammonium cations are very effective structure-directing groups for MFI zeolites,’ says Ryoo, ’but so far people have used small cations that end up totally included in the zeolite structure.’ Using the protruding tails to prevent the crystals from growing in one dimension gives thin sheets which are only as thick as a single unit cell - the smallest repeating unit in the crystal lattice.

Ryoo admits that his original intention was for the surfactant to form micelles and to build the zeolite on the surface of those micelles to make a controlled three dimensional porous crystalline network. But Duncan Macquarrie from the University of York, UK, thinks that the nano-sheet structure could have advantages over such mesoporous zeolites: ’One of the benefits of microporous zeolites is shape selectivity - molecules react based on whether they can fit into the pore or not - and you tend to lose some of that in mesoporous structures.’ Macquarrie adds that the thinness of the sheets also improves accessibility of catalytic sites within the structure - ’it’s potentially the best of both worlds,’ he says.

The team has tested the nano-sheets as catalysts in some representative reactions, using bulky molecules to determine the effect of increased diffusion within the zeolite structure. The nano-sheet catalysts showed increased activity in all the reactions compared with the same weight of a standard zeolite catalyst. When the team used their catalyst to convert methanol to hydrocarbon fuel, they also saw that the nano-sheets had greatly extended catalyst lifetimes. Ryoo attributes this to the lack of formation of unwanted ’coke’ byproducts, which block the micropores of conventional zeolites because the product molecules cannot diffuse out fast enough. With the nano-sheets, the small amount of coke that does form is mostly on the catalyst surface, so doesn’t affect catalytic activity.

Zeolite nano-sheets could also have applications outside of catalysis, explains Mike Anderson from the University of Manchester, UK. ’A long term goal of zeolite chemists has been to make large single crystal films of zeolites, which could be perfect membranes for separation.’ He adds that Ryoo’s work suggests a possible general method for making such films: ’if you could pre-organise the surfactant on a flat sheet, then perhaps you could encourage the zeolite to grow into very thin but extensive film structures.’

Phillip Broadwith