A Density Functional Theory Based Approach to Extraframework Aluminum Species in Zeolites

The structures of six different extraframework aluminum (EFAL) species, possibly present in zeolites, were studied by density functional theory methods. A T6 cluster (T = Si, Al), with different Si/Al ratios, was used to simulate the real zeolite Y structure and the coordination of the chosen EFAL species (Al3+, Al(OH)2+, AlO+, Al(OH)2+, AlO(OH), and Al(OH)3). The monovalent cations prefer to attain bicoordination with the framework AlO4- moiety, while di- and trivalent cations usually achieve tetracoordination. One important result is that, in all cases, coordination occurs with the oxygen atoms nearest to the framework aluminum ones. A single water molecule addition to the optimized Al3+·T6 cluster produces a strongly exothermic reaction, leading to formation of a hydroxyaluminum cation and an acidic site on the zeolite. The addition of a second water molecule produces only minor energetic and structural changes.