Influence of Ti substitution on the 29Si NMR spectra of silicalite.: A computational study

The effect of the substitution of Si by Ti in the framework of the zeolite silicalite on the Si-29 NMR chemical shift is studied with a combined ab initio/molecular mechanics method. Substitution causes a shift of the 29Si NMR signal of neighbouring Si nuclei of only similar to 1 ppm to lower fields, while the dependence of the chemical shift on the average T-O-T angle remains unaltered. Hydration of the framework Ti site is found to strongly influence the Si-29 NMR chemical shift via structural distortions of the lattice and by hydrolysis of Si-O-Ti bridges. The effect of hydrogen bonds and of the presence of Ti on the chemical shift of Q(3) species is also found to be negligible. Ti47-49 NMR Shielding constants strongly depend on the local structure around Ti and show variations of up to 150 ppm upon hydration.