Gas-phase hydrolysis of tetraethyl orthosilicate (TEOS)

摘要

Gas-phase hydrolysis of TEOS was investigated at 600 to 900 deg C , at ordinary pressure, using a tubular reactor where TEOS was always in the presence of a large excess of water. Thee reaction stages could be d istinguished: (a) A short induction period when nucleation takes place; (b) TEOS polymerization; during this stage further hydrolysis on the surface of the soild 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 method: XRPD showed the characteristic pattern of amorphous silica; MAS-NMR of the ~(29)Si nuclide s howed that the majority of the si atoms wer bound to four Si atoms through O bridges (Q4 centers), hhowever, a substantial fraction of the Si atoms carried on OH group (Q3 centers), and a very miinor fraction of the Si atoms carried two OH groups (Q_2 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.