²⁹Si NMR Studies of Zeolite Precursor Solutions

Solution ²⁹Si NMR spectroscopy results of zeolite precursor solutions of composition 1 SiO₂:4 C₂H₅OH:0.36/n R⁺ⁿ[OH^-]_n:20 H₂O are reported. This work employs isotopically enriched ²⁹Si materials to aid in spectral interpretation. Using both 1D and 2D methods, spectra of solutions containing tetrapropylammonium hydroxide are wholly consistent with the existing silicate chemistry literature and indicate that the majority of the species are high-symmetry silicate clusters previously observed in aqueous solutions. The results are inconsistent with the nanoblock or nanoslab model proposed by Kirschhock and co-workers. Mixtures containing the 4,4‘-trimethylene-bis(1,1‘-dimethylpiperidinium) dihydroxide cation were also studied. These mixtures have similar speciation to the TPA solutions, although the relative populations of the species are different. Preliminary variable temperature ²⁹Si NMR of these mixtures shows that the exchange properties of the high-symmetry silicate species, most notably the tetrahedral tetramer, depend on the organocation identity.