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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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Mota et al. (2004) studied this question.
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