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

Abstract

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 directing effect. Characteristic structural differences have been identified also between the TEOS based silica gels and gels prepared from a commercial 3.22 SiO2/Na2O ratio aqueous sodium silicate solution. Moreover the structure of an “as synthesized” base set Na-silicate hydrogel is also different from that of its acid set counterpart: the former gel contains mainly 4 and 5 member and larger siloxane rings while the latter one virtually does not show any ring-associated Raman vibration. This result provides direct experimental support for the decades-old theoretical predictions that physical differences of acid and base set gels might be related to their ring and branched linear chain structures, respectively. We also demonstrate that the acid set gel gradually develops siloxane rings during its washing and aging procedures.