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Poly(ethylene oxide) oligomers, commonly referred to as polyethylene glycols or PEGs, are polyethers that are liquid at room temperature and have high enough melting points to enable their use as ion exchange media at elevated temperatures.  We have found PEG oligomers to be effective as alternative solvents for the ion-exchange of porous and layered oxide materials.  Our initial work explored lithium ion exchange into sodalite, using PEGs of varying molecular weight and end-capping groups. Based on the successful results of these experiments, exchange of catalytically active ions Mn²⁺, Fe²⁺, and Co²⁺ into hydrated and dehydrated Zeolite X (Na₈₀Al₈₀Si₁₁₂O₃₈₄·nH₂O) in PEG solvents was explored.  When attempted in aqueous solutions, exchange of these cations quickly leads to destruction of the zeolite structure within 1 – 2 exchange cycles.  However, in PEG oligomer solvents, the structure can be maintained and exchanges of 48% (Co²⁺), 80% (Mn²⁺), and 91% (Fe²⁺) are observed after one cycle under hydrated conditions.  When rigorous steps are taken to remove all water from the zeolite before exchange, absorption of the oligomers into the zeolite pores is promoted which hinders ion exchange; a maximum of 6% exchange is seen under dehydrated conditions. 

While the complete dehydration of the zeolite does hinder ion exchange in the PEG solvents, the transition metal ions that are incorporated into the zeolite under dehydrated conditions are more catalytically active than those exchanged in the presence of trace water.  This is likely due to prevention of hydrolytic attack on the zeolite framework and metal hydroxide formation.  Catalytic efficiency toward NO decomposition was compared for Mn²⁺, Fe²⁺, and Co²⁺ exchanged Zeolite X samples prepared in aqueous solution and prepared in oligomer solvents.  Turnover frequencies for samples exchanged in PEG oligomers under dehydrated conditions (0.0237 s^-1 for Na/Mn-X, 0.0213 s^-1 for Na/Fe-X, and 0.0190 s^-1 for Na/Co-X) are an order of magnitude higher than those exchanged in the presence of water.  Characterization of the composition and catalysis of these samples involved a very useful and continuing collaboration of the PI and her students with researchers at the National High Magnetic Field Laboratory.   

Exchange of rare earth ions having potentially useful luminescence properties was also explored.  Rare earth ions Nd³⁺ and Er³⁺ have been exchanged into hydrated and dehydrated Zeolite X using poly(ethylene glycol) oligomers as exchange solvents.  Under hydrated conditions, a maximum of 40% ion exchange was achieved, with the framework structure maintained through at least the first exchange cycle.  Although a maximum of only 8% ion exchange was observed under dehydrated conditions, structures were successfully maintained through several exchange cycles.  Luminescence and Raman spectroscopy studies indicate that ion exchange of RE³⁺ cations in aqueous solution leads to incorporation of the rare earth ions into the degraded framework, replacing Al³⁺ leached by the acidic exchange solution.  Ion exchange in PEG solutions prevents this process; the rare earth cations instead occupy the cation sites in the zeolite cages, leading to more optimal luminescence and highlighting the advantage of this method.  The rare earth and transition metal ion exchange research was the PhD project of Gina Canfield, who successfully defended her dissertation and is now at a postdoctoral position at the Air Force Research Laboratory at Tyndall AFB.

Adsorption of oligomer molecules into the cages during ion exchange is a nuisance in some respects (adsorbed PEG is usually removed after ion exchange by calcining the sample).  On the other hand, presence of PEG molecules in the cages may serve to increase the affinity of the zeolite to CO₂ adsorption.  Separation and sequestration of carbon dioxide to curb greenhouse gas emissions is of great industrial importance, and zeolites are often incorporated into membranes for CO₂ separation.  CO₂ molecules are physisorbed into zeolite cages due to their high surface area, cage size, and presence of cations in the cages.  Improved adsorption may be possible with incorporation of PEG oligomers into the cages, due to the polar nature of these polyethers.  Short-chain PEG molecules have a particularly high affinity for CO₂ but are usually liquids; trapping these oligomers in zeolite cages allows for their use in solid membranes.  We are investigating a variety of PEG-loaded Zeolite X samples to observe which combinations of exchanged cation and PEG oligomers leads to higher CO₂ adsorption.

Attempts to exchange metal ions such as Ag⁺ and Cu²⁺ into Zeolite X using PEG solvents led instead to reduction of these ions.  Ethylene glycol is often used as a reducing agent for the formation of silver nanoparticles (“polyol process”) and the same chemistry occurs in the PEG oligomers.  We are now investigating this as a potential avenue to form ordered arrays of silver nanoparticles in the zeolite matrix, either by adsorbing PEG into the zeolite and then soaking in aqueous silver nitrate solution to exchange and then reduce the Ag⁺ ions as they go into the cage, or by placing an unexchanged zeolite in a PEG solution of AgNO₃ and slowly heating.  This work is being carried out by a second year graduate student.