Bulk structures of silicas

The bulk structures of silicas are classified as crystalline and amorphous polymorphs. More than 35 well-defined crystalline silicas are known, which are well-characterized by the Si-O length, the Si-O-Si bond angle, and the Si-O bond topology and coordination (10). Some of the crystalline polymorphs are collected in Table I. Because of the lack of sufficiently precise methods to assess the long-range structural order, amorphous silicas remain poorly characterized. They can be loosely discriminated according to their dispersity, bulk density, and type of pore structure. 

This chapter does not cover the most common aspects of the solid-state NMR techniques employed in the study of heterogeneous catalysts such techniques are described in Chapter 4. Since this chapter emphasizes the surface characterization of silica and alumina systems and silica aluminas by NMR methods, only those technical aspects highly relevant to surface characterization and not otherwise emphasized in this volume are explicitly discussed here. NMR studies of zeolites and clays are treated in separate chapters, and the bulk structures of silica and alumina systems are covered by Eckert. Unavoidably this chapter is also concerned with dynamics at the surface, although the amount of detailed work on that subject to date is limited. With the increasing availability of variable-temperature solid-state NMR equipment, however, one can expect that attention devoted to dynamics at surfaces will increase markedly during the next few years. 

Even though the structure of active sites has been specified, it seems essential that the bulk structure of silica-alumina composites be described in a manner which is consistent with the data. The high surface area (500-600 ... 

In a bulk silica matrix that differs from the silica nanomatrix regarding only the matrix size but has a similar network structure of silica, several kinetic parameters have been studied and the results demonstrated a diffusion controlled mechanism for penetration of other species into the silica matrix [89-93]. When the silica is used as a catalyst matrix in the liquid phase, slow diffusion of reactants to the catalytic sites within the silica rendered the reaction diffusion controlled [90]. It was also reported that the reduction rate of encapsulated ferricytochrome by sodium dithionite decreased in a bulk silica matrix by an order of magnitude compared to its original reaction rate in a homogeneous solution [89], In gas-phase reactions in the silica matrix, diffusion limitations were observed occasionally [93],... 

Several recent studies7,9,10,23,26 have reported attempts to create silicon nitride by direct ammoniation of silica, usually as a spin-off of the integrated circuit technology research. Most of these studies agree that at temperatures about 1473 K up to 20 -25 % (w/w) nitrogen can be incorporated, but silicon nitride is seldom formed. The final product of this direct nitridation method is silicon-oxynitride (Si2N20) with residual silica. The nitridation is not restricted to the surface, but the N diffuses also into the bulk structure of the silica. No adequate mechanisms were presented to explain the observed reactions. 

Prior to introducing the defect, the bulk structure of pure silica chabazite was relaxed to the... 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

These are methods for the simulation of a flat amorphous surface. To simulate the atomic structure of a porous oxide adsorbent like silica gel, one may first simulate the bulk amorphous silica. Then cut out of it globules and arrange them in space to model the pore structure of silica gel. Other applications of this idea include the creation of pores such as those found in porous glass by deleting atoms from a simulated block of solid in such a way as to leave a cylindrical pore. 

Figure 4. SEM micrographs of the bulk structure of the (a) granular and (b) spray-dried silica gel. The water glass serves as cement for the granulate fragments to form a spherical particle during the spray-drying process.

The macroscopically different bulk structures of granulate and spray-dried silica gels are composed of small, almost spherically shaped particles, which can be seen in an enlarged transmission electron microscopic (TEM) micrograph (Figure 5). The latter... 

Figure 1. A schematic representation of the structure of silica. The Si" ion is fourfold coordinated with oxygens. The oxygens in the bulk are bound by two metal centres. In the interface singly and doubly coordinated oxygens can be found. The charge of the broken SiO bond can be compensated by the uptake of a proton SiO + <=> SiOH .

The data given in Table 1.21 show that the similar valnes of AG and thickness of the SBW layer have been obtained for all the adsorbents with the exception of A, For WBW, the thickness of the layer of structurally ordered liquid is minimal for Ap and maximal for A . For A , and Ab samples, the AG, and Js parameters have not been determined taking into account superposition of the narrow and wide components of the signals. The wide component of the bonded water signal disappears as a function of rise in the bulk deusity of silica. However, the maximal thickness of the water layer structurally ordered by the surface is observed on slight compaction of the material. Thus, for practically equal values for A aud Abe, the thickness of the bound water layer varies more than... 

Porous silicas employed as packings in HPLC are amorphous i.e., they do not possess a long-range order of their bulk structure. Most information on the bulk and surface structure of silica and its bonded derivatives can be drawn from Si and C solid-state NMR spectroscopy [7],... 

Figure 9.7 SEM micrographs of lluorinated silica aerogel (a, b) before abrasion, (c, d) after abrasion with sandpaper (100 cycles), and (e, f) after scratching with a knife. Images b, d, and f show that after mechanical damage the exposed aerogel surface has a similar nanoscale topography as before abrasion. This is due to the bulk structure of the silica aerogel consisting of a nanoporous framework of sihca nanoparticles. Reproduced with permission from [50]. Copyright 2014 American Chemical Society.

To design a gradient-type membrane, the model structure of ZSM-5 with different Si/Al ratio such as 1, 10, 30, 60, 90, and 300 and only silica were generated. An allsilica model is shown in Figure 2.1. We have optimized the individual bulk structure of one unit cell with CASTEP and the Si/Al ratio of 300 was chosen to mimic a situation of very low A1 content. It has to be mentioned that for a gradient-type membrane, the compatibility of its component, which means the combination of two MFI layers with varied Si/Al ratio, is an integral part to achieving a stable membrane. 

The application of infrared photoacoustic spectroscopy to characterize silica and alumina samples is reported. High quality infrared photoacoustic spectra illuminate structural changes between different forms of silica and alumina, as well as permit adsorbate structure to be probed. Adsorption studies on aerosil suggest adsorbed species shield the electric fields due to particle-particle interactions and induce changes in the vibrational spectra of the adsorbates as well as in the bulk phonon band. It is shown that different forms of aluminum oxides and hydroxides could be distinguished by the infrared spectra. 

The use of small polyhedral silsesquioxanes as molecular models of silica surfaces has been studied increasingly in recent years, because catalysts, etc., attached to such species are relatively easy to handle and characterize when compared to bulk silica. The preparation and structures of silsesquioxanes used in this type of work are described later in Sections II,C, IV,E, and IV,F. Further recent examples may be found in Ref. 5. 

Small-angle X-ray scattering (SAXS) data have made it possible to deduce the localisation of organic additives (pigments) in the bulk of isotactic polypropylene (iPP) [344]. This work has confirmed that the additives are located in the amorphous phase, in spite of their crucial influence on the formation of the crystalline phase of iPP. SAXS has also been used to study the 3D structure of different carbon-black aggregates, and silica-filled SBR rubber compounds [345]. 

---