Progressive isomorphous incorporation of TiIV (or BIII) heteroatoms into the MFI structure of as-synthesized silicalite-1 caused a decrease in the amount of siloxy groups (anions), requisite for counter-balancing the structural directing agent (cation), as determined using 1H MAS NMR to quantify the silanol protons H-bonded to the siloxy oxygen. This revealed the negative charge on the incorporated heteroatoms, identifying them as TiO5 (or BO4) sites.
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Parker et al. (2006) studied this question.
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