Zeolites — minerals formed from silicon and aluminum — have intricate networks of acidic micropores that can adsorb molecules and induce changes in reactivity. Consequently, zeolites are widely used as catalysts, most notably in the petrochemical industry. The micropores, however, have one drawback: their confining environment also traps unwanted chemical by-products that eventually deactivate the catalyst.

Now, a team led by Ryong Ryoo from the Korea Advanced Institute of Science and Technology1 has discovered a way to produce sheets of zeolite crystals only two nanometers across — a single crystal lattice unit wide. The exposed structure and narrow pores of these nanosheets dramatically improve the material's performance as a catalyst.

Fig. 1: (a) A surfactant directs silicate anions to form into (b) thin zeolite sheets, stacked alternately with the surfactant. (c) Reducing the sodium concentration produces agglomerates of individual zeolite nanosheets.

The key to making the zeolite nanosheets lies in a surfactant developed by Ryoo and his colleagues. The surfactant has a C22 hydrocarbon ‘tail’, and a pair of positively charged ammonium ions separated by a C6 hydrocarbon chain as its ‘head’. When this surfactant was mixed into an aqueous solution containing silicate and aluminum ions, the long hydrophobic tails aggregated together into phase-separated layers known as micelles. Inside each micelle, the ammonium head groups directed the formation of thin sheets of a zeolite called MFI (Fig. 1).

Heating the resulting gel produced thin plates composed of alternating stacks of surfactant micelles and MFI zeolite. Single nanosheets of the zeolite were isolated by reducing the initial sodium concentration, a discovery Ryoo credits to lead author Minkee Choi, who recognized that sodium can alter the polymerization rates of silicate within the zeolite.

The nanosheets had more than triple the catalytic activities and lifetimes of typical zeolites in reactions such as the ‘cracking’ of polyethylene and the conversion of methanol into gasoline. Analyses revealed that reaction by-products formed only on the surface of the nanosheets, not inside the micropores — meaning the zeolites retained their catalytic capabilities much longer.

“The most valuable information we obtained in this study is that such thin zeolite nanosheets are very good as a catalyst with a long catalytic lifetime,” says Ryoo. The next challenge for the team is extending the zeolite sheets to large-area membranes that can separate gases efficiently.