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Chemical Tailoring of Porous Silica Xerogels: Local Structure by Vibrational Spectroscopy
Author(s) -
Fidalgo Alexandra,
Ilharco Laura M.
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305079
Subject(s) - silanol , siloxane , sol gel , hydrolysis , fourier transform infrared spectroscopy , infrared spectroscopy , materials science , porosity , silica gel , methyltrimethoxysilane , cyclohexane , condensation polymer , chemical engineering , analytical chemistry (journal) , chemistry , organic chemistry , catalysis , composite material , polymer , nanotechnology , coating , engineering
Abstract Monolithic porous silica xerogels were synthesized by the sol–gel process, and their local structure was analysed by vibrational spectroscopy. The silica alcogels were prepared by a two‐step hydrolytic polycondensation of tetraethoxysilane (TEOS) in isopropanol, with a water/TEOS molar ratio of 4. The hydrolysis step was catalysed by hydrochloric acid (HCl), with different HCl/TEOS molar ratios (ranging from 0.0005 to 0.009), and the condensation step was catalysed by ammonia (NH 3 ), with different NH 3 /HCl molar ratios (ranging from 0.7 to 1.7). After appropriate ageing, the alcogels were washed with isopropanol and subcritically dried at atmospheric pressure. The diffuse reflectance infrared Fourier transform (DRIFT) spectra were analysed in terms of the main siloxane rings that form the silica particles, taking into account the splitting of the ν as SiOSi mode into pairs of longitudinal and transverse optic components, by long‐range Coulomb interactions. It was proven that the proportion of residual silanol groups (which correlates with hydrophilicity), and the fraction of siloxane 6‐rings (which correlates with porosity) may be tailored by adequate catalytic conditions, mostly by the hydrolysis pH. This was explained in terms of the reactions' mechanisms taking place in the two‐step sol–gel process followed.