Silica microporosities

As pointed out earlier (Section 3.5), certain shapes of hysteresis loops are associated with specific pore structures. Thus, type HI loops are often obtained with agglomerates or compacts of spheroidal particles of fairly uniform size and array. Some corpuscular systems (e.g. certain silica gels) tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined. Types H3 and H4 have been obtained with adsorbents having slit-shaped pores or plate-like particles (in the case of H3). The Type I isotherm character associated with H4 is, of course, indicative of microporosity. 

Galameau A., Cambon H., Di Renzo F., Fajula F. True microporosity and surface area of mesoporous SBA-15 silicas as a function of synthesis temperature. Langmuir 2001 ... 

From these studies with SynChropak SEC packings and controlled porosity glass, it is concluded that the silica packing contains a population of micropores which are differentially accessible to low molecular weight probes of total permeation volume. It is not known, however, if the microporosity in the 100 and 300A SynChropak SEC packings is the result of the rather wide pore-size distribution and whether all silicas contain micropores. 

Materials. Methyltrioxorhenium, NH4Re04 and Re207 were purchased from Aldrich and used as received. The silica-alumina was Davicat 3113 (7.6 wt.% Al, BET surface area 573 mVg, pore volume 0.76 cmVg), provided by Grace-Davison (Columbia, MD). For reactions involving MeReOs, silica-alumina was pretreated by calcination for 12 h under 350 Torr O2 at 450°C to remove adsorbed water, hydrocarbons, and carbonates, then allowed to cool to room temperature under dynamic vacuum. The silica was Aerosil 200 (BET surface area 180 mVg, with no significant microporosity) from Degussa (Piscataway, NY). 

StOber silica particles also show a low density of the powder as precipitated. All reported literature values are at or below a density of 2.0 g cm-3, and van Helden et al. (14,15) reported values of as low as 1.61 g cm-3. These results are in accordance with the previously discussed microporosity and TEM substructure in the particles. 

Additional studies by Menon (32) have indicated the p can occur at lower pressures then those predicted by Equation 1 depending on the pore structure associated with the adsorbent. Empirically, adsorbents possessing microporosity exhibit a p that is 0.6-0.8 of the value predicted by Equation 1. This observation is attributed to the overlapping of potential fields in the adsorbent pores, thereby enhancing sorption of the gas at lower pressures. Experimental studies by Ozawa (33) have verified this trend as shown in Figure 4 for the C02/activated carbon system. Here the adsorption maxima for the gas occurs at a lower pressure than the critical pressure of carbon dioxide. It should also be noted that the amount of gas adsorbed is decreased at higher reduced temperatures and that additional compression is required to reach a defined adsorption maxima (i.e., at very high values of T it is sometimes difficult to discern a well-defined adsorption maximum). The above trend has also been found for other adsorbent/adsorbate systems, such as silica gel/C02. 

For the preparation of silica with microporosity (0<2O A) a possible approach is the elimination of the organic component of a silsesquioxane RSiOi 5. In this case, the organic groups serve as templates and their elimination results in the formation of the 30... 

First results are, however, rather positive. It was possible to coat silica layers with a selectivity, which was somewhat above the Knudsen selectivity. This shows the presence of at least some degree of microporosity with, fortunately, still a relatively high N2 permeance compared to that of the centrifugal cast tubes described in chapter 4. Coating with just 1 silica layer from an undiluted silica coating solution showed to give best results. Coating with a second layer, did not improve selectivity, it did only reduce the N2 permeance of the membrane with... 

The IUPAC/SCI/NPL programme on surface area standards examined a number of carbon blacks, activated charcoals, and silicas, and in the resulting publication [33] the results obtained in a number of laboratories were compared. As a result, two carbon blacks and two silicas lacking microporosity were accepted as standards. A major conclusion of this work, namely that outgassing conditions determine results obtained with high-area solids, was reinforced by the unsuccessful attempts made by the European Union s Community Bureau of Reference to obtain reproducible results with silica gels intended as reference standards [8e]. 

Literature N2-sorption data of two categories of microporous materials were studied. The first category includes three microporous silicas (Fig. 1, a) possessing a different percentage of microporosity and exhibit a low-pressure adsorption step but zero hysteresis. 

This well accepted method [28] has been used extensively in the characterization of M41S materials [11-12,14]. From the application of this method to MCM-41, it has been concluded that this material contains no significant amounts of microporosity. This is the main evidence presented so far in order to conclude that MCM-41 is exclusively mesoporous. As it happens with any good method its limitations need to be considered in order to avoid misinformation. In the case of the a method the choice of the reference isotherm is crucial. All the reference silicas should be nonporous in order to allow a reliable analysis of MCM-41. Unfortunately, we observed that most of them have a steep rise in their N2 adsorption isotherms at 77 K at low relative pressures and BET surface areas varying from 40 [29] to 400 mVg [30], For this reason, our sample of MCM-41 was heat-treated so as to sinter the silica particles and thus obtain a nonporous silica (BET surface area 1.5 mVg) and as similar as possible to our MCM-41. The N2 adsorption isotherms for a reference silica [29] and our sintered MCM-41 are shown in Figure 7. 

Figure 2 contains the CO2 isotherms at 273 K for all the samples. It is known that CO2 adsorption at subatmospheric pressure is a useful complementary technique for the characterisation of the narrow microporosity, due to the higher adsorption temperature (273 K) and the smaller kinetic diameter of CO2 [5,6]. As it can be seen in Figure 2, all the silica gels adsorbed CO2, indicating that they all contain narrow micropores. 

From the comparative analysis between N2 and water isotherms, it can be deduced that only silica containing mesoporosity show water adsorption on the range of high relative pressures. These results seems to indicate that water adsorption at low relative pressures occurs mainly on the microporosity, while the adsorption at higher P/Po seems to be due to the presence of mesoporosity. These conclusions have been also obtained from water adsorption on AC [7,8]. 

Waksmundzki et al. extensively examined the surface areas and microporosities of imprinted silica surfaces [44]. It was found that although the template itself had little effect on the total surface area, the sizes of the micropores were positively correlated to the size of the template. Subsequent studies on the sorption of template to silicas imprinted with pyridine [45-50], quinoline and acridine [45-47], and 2-picoline, 2,4-lutidine and 2,4,6-collidine [50], combined with thermodynamic studies on the heat of wetting of template or methanol/water sorption [47,51-53], led to the conclusion that these templates were adsorbed as multilayers to the silica. This observation supported the association mechanism hypothesis. The possibility of a footprint mechanism and an association mechanism coexisting in a concentration dependent fashion does not appear to have been considered. 

Figure 25.17 illustrates some experimental results about a sol-gel-derived microporous silica membrane and related powder containing nanodispersed ZnO. An important result is that the microporosity is maintained after successive treatments of H2S chemisorption and regeneration under air. 

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