INQUIMAE   12526
INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Silica gels and xerogels prepared by sol gel process: acid catalysts and microstructure
Autor/es:
NOE, J. MORALES M.; HUCK-IRIART, C.; M. L. HERRERA; GOYANES, S. N.; CANDAL, R.
Lugar:
Sao Paulo
Reunión:
Encuentro; Synchrotron Light Laboratory (LNLS) 23rd Annual Users Meeting (RAU); 2013
Resumen:
This work is about the effect of the type of acid used as catalyst on the microstructure of silica hydrogels and xerogels prepared by sol-gel process. Small Angle X ray Scattering (SAXS), BET sorptometry and low resolution nuclear magnetic resonance spectroscopy (H-RMN) were used to characterize the materials. Ethanol, water and tetraethoxysilane in a volumetric ratio 5:7.5:5 were used as precursors. HF, HCl or HNO3 acid were used as catalyst. Sols were turned to gels and xerogels by drying and firing (450 °C). SAXS measurements on gels were performed at DO2A-SAXS2, wavelength= 1.7968 , q range: 0.0055 - 0.10 A-1. Fired xerogels were analyzed at DO2A-SAXS1, wavelength= 1.5400 , q range: 0.0177 - 0.4747 A-1. T2 relaxation times were determined with a Minispec, mq60, Brucker. N2 adsorption isotherms were determined with a Micromeritics ASAP 2020. Gelification was 2 orders faster with HF. H-RMN analysis indicated that gels made with HCl or HNO3 have a more open structure containing trapped water. SAXS analysis was performed by 2 approaches. The 1st was based in a fractal arrange of poly-disperse spherical pores, considering sub units poly-dispersity. The 2nd was an empirical model that does not require previous knowledge of the system. Similar trends were obtained with both models. Gels have a fractal structure that depends on the type of acid. Rg2 is larger for HF; these structures may be associated with porous aggregates. HCl and HNO3 produced polymeric type gels with Rg1 and Rg2 smaller than for HF. Gel structure collapsed after firing with an increment in Rg1. Porod slope was 4.0 in the HF-xerogels and 2.7-2.6 for the others, indicating smooth and rough separation surfaces respectively. BET analysis showed that HF-xerogels were mesoporous while the other microporous. The size of microporous determined by BET and SAXS was similar. These results agree with a polymeric structure with rough separation surfaces. Due to the high nucleophilicity of F, HF leads to fast hydrolysis and condensation with formation of particle like units, while HNO3 and HCl produce polymeric type units. The first lead to mesoporous and the 2nd to microporous xerogels.