29Si NMR and Raman Glimpses into the Molecular Structures of Acid and Base Set Silica Gels Obtained from TEOS and Na-Silicate

The molecular structures of various silica gels are compared using 29Si NMR and laser Raman spectroscopy. When gels are deposited onto a porous organic resin from tetra ethyl orthosilicate, TEOS, at acidic or basic conditions, their appearance and physical properties show dramatic differences. It is demonstrated that these differences are associated with distinct Q3/Q4 Si–O–Si connectivity ratios in the two materials although both are mainly constructed from 3 and 5 member siloxane rings (3R and 5R). Comparative NMR and Raman spectroscopic experiments on these gels and related model calculations indicate that Raman is disproportionally more sensitive for Q1 and Q2 connectivities than for Q3 and Q4. This is an important new aspect to consider in the Raman identification of Qn connectivities in amorphous silicates. It is also shown that the molecular structures of TEOS based gels differ when made in the presence or absence of the porous organic substrate. Hence the substrate seems to exert structure directi...