29 June 2005: Novel ion-exchange technique for zeolites

UK chemists have demonstrated a novel electrochemical method for exchanging ions in zeolites. 

The technique can efficiently insert protons and other ions into a range of zeolites under ambient conditions and with minimal impact on the zeolites’ structural integrity. It can also be used as a potentially important analytical tool to measure the rate at which ions diffuse through zeolites, something that is significant for the tailoring of zeolites for specific applications.

Zeolites - porous aluminosilicates - are used in a wide range of applications, from simple ion-exchange processes to gas separations and catalysis. Nearly all of these applications depend on the presence of specific ions within the pores of the zeolite framework.

Most zeolites are synthesised with sodium ions in the pores; if another ion is required the sodium must be exchanged. This is done by making a slurry of the zeolite in a solution of the ion to be exchanged and applying heat. A certain amount of exchange will occur after which the solution is then refreshed and the process repeated a number of times.

’This is a relatively crude and inefficient process and it is difficult to monitor how the exchange is progressing,’ said Robert Dryfe, who has been developing the new technique with colleagues at the University of Manchester.

’Many applications, notably the cracking of hydrocarbons, require the zeolite to contain a proton,’ said Dryfe. ’Simply trying to exchange the sodium with hydrogen ions by washing the zeolite in an acid is difficult because the acid attacks the aluminium in the zeolite framework, effectively dissolving the structure.’

In this case the exchange must be done with extremely weak acid solutions or by exchanging the sodium initially with ammonium, before driving off ammonia by heating, leaving a proton behind. Proton exchange is currently not feasible for many zeolites.

The Manchester team has adopted a different approach, using so-called liquid???-liquid electrochemistry, with an organic liquid phase in contact with an aqueous phase. The zeolite is suspended in the aqueous solution, together with the ion that is being exchanged. If this is hydrogen, protons are present in sufficiently small concentrations to form only an extremely weak acid that will not attack the zeolite.

The organic phase contains an organic electrolyte to permit conduction, and a sequestering agent that is specific to the ion being removed from the zeolite. 

Dryfe and colleagues demonstrated the concept by exchanging sodium ions in zeolite Y with protons. The organic solvent in the system was 1,2-dichloroethane, and the sodium-sequestering agent a crown ether.

When a small potential difference (tuned to the transfer energy of the ion being removed from the zeolite) is applied across the organic-aqueous interface, sodium ions are preferentially pulled across the interface (because of the lower hydrophobicity of sodium compared to hydrogen, and sodium’s higher affinity for the crown ether), where they are captured by the crown ether and effectively removed from the system.

This drives the equilibrium in favour of the hydrogen ions. The tendency will be for the sodium to vacate the zeolite, to be replaced by protons.

’We have demonstrated a proton exchange rate of 77 per cent with zeolite Y, which is high,’ said Dryfe.

The researchers have unpublished x-ray diffraction data to show that the structure of the zeolite remains intact following the electrochemical process. Solid-state NMR has demonstrated that the pores are indeed occupied by protons.

’We have also conducted preliminary studies on more aluminous zeolites which are especially susceptible to damage by acids during proton exchange,’ said Dryfe. ’The early results look extremely promising and it appears that this could be a viable means of achieving proton exchange in some of these more delicate zeolites.’

It is possible to monitor the rate and extent of the ion exchange by measuring the currents produced during the process, something that cannot readily be done with the traditional wet chemistry approach.

The team has also positioned a membrane of a silicalite zeolite across an organic-aqueous interface and used an electric field to drive ions through the membrane. ’The membrane acts like an electrified sieve, allowing only sufficiently small ions to pass through,’ Dryfe said. ’By measuring the current we can monitor the rate of the movement of the ions through the structure. This could be a useful analytical tool for investigating how different ions diffuse through different zeolite structures.’ Simon Hadlington