Aluminum Distribution in Low Si/Al Zeolites:  Dehydrated Na−Clinoptilolite

We have investigated the siting and distribution of Al atoms in the zeolite clinoptilolite using periodic lattice simulation techniques. A novel procedure is presented for the study of the unresolved problem concerning the Al atoms siting in heulandite-structured materials. The resulting structural models are in excellent agreement with experimental studies and show preferential aluminum siting at T2 and minimal aluminum occupancy of T5. We show how Al−Al and Al−Na interactions are important in the siting of Al atoms in low Si/Al regimes. We also show the importance of lattice relaxation in finding the lowest energy aluminum distribution and the failure of standard Monte Carlo techniques in this context.