Silica-Based Materials - Amorphous

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

Catalysis by sol gel doped silica-based materials has become in the last 20 years a prominent tool to synthesize a vast number of useful molecules both in the laboratory and in industrial plants.12 The underlying basic concept of all sol-gel applications is unique one or more host molecules are entrapped by a sol-gel process within the cages of an amorphous metal oxide where they are accessible to diffusible reactants through the inner pore network, which leads to chemical interactions and reactions (Figure 5.3). 

Methods for preparation of silica samples exhibiting or not porosity are known since a long time [10]. Before the discovery of mesoporous silica [11] zeohtes were the most important materials with micropores forming a regular array of channels with uniform size [12-14]. Mesoporous materials were mainly represented by amorphous silica [1], pillared clays, sihcates [15,16] and certain forms of alumina. A common characteristic of those materials is the irregular spatial distribution of pores and a wide spectrum of pore sizes [17]. In 1992 Beck et al. announced the synthesis and characterization of new mesoporous silica-based materials [11,18]. These mesoporous molecular sieves received the general... 

Gervais, C., Baboimeau, R, and Smith, M.E. 2001. Detection, quantification, and magnetic field dependence of solid-state NMR of X-O-Y (XY = Si, Ti) hnkages Implications for characterising amorphous titania-silica based materials. J. Phys. Chem. B 105 1971-1977. 

It is worth noting that both Pd-aUoy and sUica-based membranes present some problem about material instability in the WGS environment. The Pd-aUoy membranes can be negatively affected by surface carbonization, sulfur poisoning, and hydrothermal embrittlement, whereas the amorphous silica-based membranes can show some degradation caused by the condensation reaction of sUanol in hydrothermal conditions (Tang et al., 2010). In particular, the siliceous MFI-type zeolite membranes, constituted by a crystalline microporous zeolite membrane, in recent years have been seen as attractive candidates for the WGS reaction because of the high-temperature hydrogen separation and for their intrinsic sulfur tolerance and hydrothermal stability. 

The Temkin isotherm assumes that the adsorption enthalpy Aa// (per site) decreases linearly upon increasing coverage. Examples of heats of adsorption decreasing linearly with coverage are reported in the literature, as for instance in the case of NH3 adsorbed on hydroxylated silica, either crystalline, [48] or amorphous, [49] as well as in the case of CH3OH adsorption on silica-based materials [26]. 

Chemical methods to determine the crystalline content in silica have been reviewed (6). These are based on the solubility of amorphous silica in a variety of solvents, acids or bases, with respect to relatively inert crystalline silica, and include differences in reactivity in high temperature fusions with strong bases. These methods ate qualitative, however, and fail to satisfy regulatory requirements to determine crystallinity at 0.1% concentration in bulk materials. 

However, with respect to ee, the same catalyst immobilized on amorphous silica performed even better (conversion 72%, ee 92%) than the one immobilized on MCM-41. This example illustrates an important issue, i.e., OMS-based catalysts have to be compared with those based on amorphous silica or silica-alumina. If the amorphous materials perform as well or even better than the OMS materials, then there is no advantage in using the significantly more expensive OMSs. However, in those cases where the catalytic reaction benefits from the regular and well-defined pore systems of the OMS materials, such materials can be very attractive, e.g., for the conversion of bulkier molecules or to overcome transport limitations in more narrow pores. 

Materials based on amorphous silica fibres are of special interest these are manufactured in a variety of textile forms (cloth, tape, rope, etc.) which can be used for several applications (thermal, electric insulation) for service temperatures generally up to about 1000°C. Typically they contain 93-96% Si02 about 4% Al203 and small quantities of Ca or alkali oxides. 

Although micro- and not mesoporous but worth mentioning in any case is a new class of materials in this evolution series of porous silica-based hybrid materials, the so-called ZOLs (zeolites with organic group as lattice). As we will see, many studies in the past focused on the organic functionalization of amorphous silica porous... 

Bigi, F., Carloni, S., Maggi, R., Mazzacani, A. and Sartori, G. Nitroaldol condensation promoted by organic bases tethered to amorphous silica and MCM-41-type materials. Stud. Surf. Sci. Catal., 2000, 130D, 3501-3506. 

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