Designating Oxygen Anions in Al–ITQ-21 as Brønsted Acid Sites Using DFT Calculations

The relative energies of Al-containing
ITQ-21 (Al–ITQ-21),
which generates Brønsted acid sites, and the deprotonation energies
of the acid sites were calculated by ONIOM (ωB97X-D/6-311G­(d,p):PM6)
method. Model clusters composed of four unit cells of Al–ITQ-21,
which contained one Al atom and one acidic proton, were used for the
calculations. In addition, n-butane cracking and
ethanol conversion over Al–ITQ-21 with different Al concentrations
were evaluated by varying the temperatures for measuring the activation
energies for these reactions. From the theoretical calculations and
NMR measurement, oxygen anions in the ITQ-21 lattice could be designated
as the site where two types of acidic protons having different acid
strengths are bonded. The presence of the two types of acidic protons
was also confirmed by the activation energies corresponding to n-butane cracking and the conversion of ethanol into ethene
and diethyl ether.