Base-catalyzed gels

Acid- and base-catalyzed gels yield micro- (pore width less than 2 nm) and meso-porous(2-50 nm) materials, respectively, upon heating. An add-catalyzed gel which is weakly branched and contains surface functionalities that promote further condensation collapses to give micropores. This example highlights a crucial point the imtial tnicrosirnctnre and surface functionality of a gel dictates the properties of the heat-treated product. 

According to Ro et al.17 gels made from low water content sols contain residual organic groups, caused by incomplete hydrolysis, which contributes to the formation of micropores during the thermal treatment. Acid-catalyzed gels show slit-shaped micropores and have a fibrous or plate-like structure. Base-catalyzed gels have cylindrical pores and spherical particles. 

Relative to base-catalyzed gels, those eatalyzed with acid have a larger number of small pores, and it is difficult to dry them without significant pore collapse. However, a two-step process in which the precursors are initially hydrolyzed with acid, followed by gelation under basic conditions, does lead to robust polymeric aerogels. In fact, this process yields the lowest density aerogels on record. 

Figure 5.27 Schematic representation of the structural evolution during drying for (a) acid-catalyzed gels, (b) base-catalyzed gels, (c) coUoidal gel aged under conditions of high silica solubility, and (d) colloidal gel composed of weakly bonded particles. (From Ref. 40.)...

Figure 5.29 Linear shrinkage versus temperature during constant heating rate sintering at 2°C/min for an acid-catalyzed sUica gel (A), a base-catalyzed silica gel (B) and a colloidal siUca gel. The acid- and base-catalyzed gels were prepared with a water concentration of 4 mol H2O per mole of TEOS. (From Ref. 43.)...

Silica gels obtained from acid hydrolysis of alkoxides have a mean pore size in the range of 3 nm. For base catalyzed gels, the mean pore size is shifted toward higher values. The behavior of silica gels varies with respect to these different textural properties. 

Associated to the pore size and the pore size distribution the gels have different perme-abihty values. Base catalyzed gels have a liquid permeability in the range of 10-20 nm, acid catalyzed gels in the range of a few nm. In both cases the silica network is covered with... 

In comparison, base catalyzed gels which consist of an arrangement of large particles and which exhibit a higher mean pore size are less sensitive than acid ones. Basic gels show a weak syneresis phenomenon and the associated shrinkage is quite of low extent. 

Figure 14-8. Evolution of the elastic and mechanical properties E, a and Kic as a function of the bulk density (a) for the neutral and base catalyzed gels, and (b) for sintered gels.

Similar studies have been performed [15] using a base-catalyzed gel made by the two-step process (identical to gel B2 from ref. 14). No change in dimensions was observed when the sample was soaked in ethanol-acetone solutions ranging from 0 to 100% ethanol. When soaked in ethanol-water solutions, contraction was relatively rapid, and the rate increased with water concentration. (The initial pore liquid for this gel was 9 ethanol/1 water by volume.) Gels soaked in solutions containing more than 80 vol% water turned white and opaque. This was not microsyneresis [7], which can cause the same appearance as shown in Fig. 16, the SEM reveals spherical regions... 

Based on decomposition of the MAS Si NMR results (Fig. 44) [170] and estimates of the extent of dehydroxylation according to Eq. 14, the maximum percentage of silicons incorporated in three-membered rings in the two-step acid-base-catalyzed gels studied by Drinker and coworkers exceeds 20%. The NMR results indicate that after heating at intermediate temperatures (e.g., 600°C), the silica gel surface is composed primarily of OH terminated ... 

Linear thermal shrinkage at constant heating rates for particulate gel made using flamegenerated particles (Colloidal Gel), base-catalyzed gel from TEOS (B2), and acid-catalyzed gel from TEOS (A2). (Reprinted by permission of publisher from Brinker el al. [S6], copyright 1984. Elsevier Science Publ. Co., Inc.)... 

Nogami and Moriya [58] showed that the chemical and microstructural differences between acid- and base-catalyzed gels were reflected in their densification behavior. The initial shrinkage of the base-catalyzed gel could be described using Frenkel s equation, Eq. 14. The acid-catalyzed gels sintered at lower temperatures, and the kinetics of densification did not... 

Isothermal plots of /K and viscosity for base-catalyzed gel (B2) after heating at 2 C/min ( ) to indicated temperature K = (Reprinted by permission of... 

The same sort of experiments were performed [15] using a two-step base-catalyzed gel called B2 [14] (made from TEOS with r = 3.7, pH 7.9). When soaked in the pore liquid (89 vol% ethanol and 11 % water) the shear modulus increased for as much as one year, as shown in Fig. 26 the rate of stiffening was greater at 35°C than at room temperature. Other samples were soaked in various ethanol-water solutions or allowed to dry partially before measurement. Samples aged in pure ethanol or in ethanol-acetone solutions showed neither shrinkage nor increase in modulus over a period of... 

Schematic representation of base-catalyzed gel (A), desiccated xerogel (B), and TEM micrograph of xerogel prepared by two-step acid-base-catalyzed hydrolysis of TEOS (r = 3.8) followed by drying at 50°C, bar = 100 nm (C) [5].

Raman spectra of the two-step acid-base-catalyzed gel (r = 3.8) after drying at 50°C or heating to 200, 600, or 1100°C compared to the spectrum of conventional u-SiOj. Xerbgel samples treated at 50 to 600°C possess high surface areas (>800 m /g), whereas the 1100°C sample and v-SiOi are fully dense (341. 

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