Abstract
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.
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