Gas-phase hydrolysis of tetraethyl orthosilicate (TEOS)

Gas-phase hydrolysis of TEOS was investigated at 600 to 900°C, at ordinary pressure, using a tubular reactor where TEOS was always in the presence of a large excess of water. Three reaction stages could be distinguished: (a) A short induction period when nucleation takes place; (b) TEOS polymerization; during this stage further hydrolysis on the surface of the solid particles and cross-linking by condensation reactions also occur; (c) after quantitative conversion of TEOS severe structural rearrangements of the polymeric product take place. The nanophase products were analysed by various methods: XRPD showed the characteristic pattern of amorphous silica; MAS-NMR of the ²⁹Si nuclide showed that the majority of the Si atoms were bound to four Si atoms through O bridges (Q₄ centers), however, a substantial fraction of the Si atoms carried one OH group (Q₃ centers), and a very minor fraction of the Si atoms carried two OH groups (Q₂ centers); BET surface area measurements showed an area reduction as TEOS polymerization advanced to completion, while the products of longer residence times showed a large increase of surface area. The area decrease was probably due to a compaction of structure by polymerization and cross-linking, whereas the area increase was probably caused by hydrolysis and bond rearrangement reactions of the polymer.