The structures of six different extraframework aluminum (EFAL) species, possibly present in zeolites, were studied by density functional theory methods. A T 6 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 (Al 3+ , Al(OH) 2+ , AlO + , Al(OH) 2 + , AlO(OH), and Al(OH) 3 ). The monovalent cations prefer to attain bicoordination with the framework AlO 4moiety, 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 Al 3+ ‚T 6 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.
The effect of extra‐framework aluminum (EFAL) species on the acidity of a HT6 cluster (T=Si, Al) was studied by density functional calculations. A higher acid strength, compared to an isolated site, was only found with Al(OH)2+. The increase in acid strength was not due to Brønsted/Lewis acid synergism, as previously reported, but to the stabilization of the conjugated base through hydrogen bonding with the EFAL hydroxy groups (see picture).
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