Silica gels and xerogels prepared by sol-gel process: acid catalysts and microstructure

NOEL JAVIER MORALES MENDOZA; CRISTIÁN HUCK IRIART; MARIA LIDIA HERRERA; SILVIA N. GOYANES; ROBERTO J. CANDAL

Congreso; 23ª Reunión Anual de Usuarios Del Laboratorion Nacional de Luz Sincrotrón; 2013

This work is about the e®ect of the type of acid used as catalyst on the microstruc-ture of silica hydrogels and xerogels prepared by sol-gel process. Small Angle X ray Scattering (SAXS), BET sorptometry and low resolution nuclear magnetic res-onance 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 xero-gels by drying and ¯ring (450 C). SAXS measurements on gels were performed at DO2A-SAXS2, wavelength= 1.7968 º A, q range: 0.0055 - 0.10 A ¡1 . Fired xerogels were analyzed at DO2A-SAXS1, wavelength= 1.5400 º A, q range: 0.0177 - 0.4747 A ¡1 . T2 relaxation times were determined with a Minispec, mq60, Brucker. N2 ad-sorption isotherms were determined with a Micromeritics ASAP 2020. Geli¯cation 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 ag-gregates. HCl and HNO3 produced polymeric type gels with Rg1 and Rg2 smaller than for HF. Gel structure collapsed after ¯ring 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 ¯rst lead to mesoporous and the 2nd to microporous xerogels.