Exploration of Poly(Ethylene Glycol) Oligomers as Ion Exchange Media for Porous and Layered Oxides

Zeolite structures consist of a negatively charged, porous, aluminosilicate framework with the ability to ion exchange its charge balancing cations. However, problems associated with traditional methods of aqueous ion exchange can adversely affect the behavior and the industrial quality of these zeolites. In this work, an alternative to traditional ion exchange media was explored. Poly ethylene glycol (PEG) oligomers are known to dissolve and transport cations; therefore it was theorized that these solvents could be used to mobilize and exchange ions into zeolite structures. The theory was first tested with the ion exchange of Li+ for Na+ into hydrated and dehydrated sodalite using a series of PEG oligomers with different chain lengths and end groups. The outstanding results from this phase of work, with over 90% Li ion exchange, prompted its continuation with the exchange of catalytically active transition metal ions (Mn2+, Fe2+, and Co2+) into hydrated and dehydrated Zeolite X. Use of these oligomer solvents helped maintain the zeolite structure and allowed for a maximum of 91% ion exchange under hydrated conditions. Although more extensive oligomer incorporation under dehydrated conditions allowed for only 6% exchange, the catalytic activity of these samples was vastly improved over traditionally exchanged samples. The maximum turnover frequency of dehydrated Mn oligomer exchanged samples toward decomposition of NO was 2.37 x 10-2 s-1, whereas that of hydrated Mn aqueous exchanged samples was 9.67 x 10-4 s-1. Parallel ion exchange experiments involving luminescent rare earth metals (Nd3+ and Er3+) gave similar positive results. In addition, data from Raman spectroscopy indicated that while the aqueous exchange method promoted rare earth framework substitution, the oligomer solvents maintained exchange into the zeolite cages. The optimal conditions provided by the oligomer exchange method allowed for improved luminescence of the RE ions. Given its success, this method has been extended to the ion exchange of layered oxides, such as perovskites and cobaltates. Although this work has just begun, promising initial data indicate the possibility of continuing this work far into the future.