Surface area and porosity

Key point: Heterogeneous catalysts are high-surface-area materials formed as either finely divided substrates or crystallites with accessible internal pores. An ordinary dense solid is unsuitable as a catalyst because its surface area is quite low. Thusα-alumina, which is a dense material with a low specific surface area, is used much less as a catalyst support than the microcrystalline solid γ-alumina, which can be prepared with small particle size and therefore a high specific surface area (the surface area divided by the mass of the sample). The high surface area results from the many small but connect ed particles like those shown in Fig. 26.14, and a gram or so of a typical catalyst support has a surface area equal to that of a tennis court. Similarly, polycrystalline quartz is not used as a catalyst support but the high-surface-area versions of SiO₂ are widely used. In a typical heterogeneous catalyst this substrate surface is coated with active sites or particles, such as metals or metal oxides, producing a large number of active sites. Both γ-alumina and high-surface-area silica are metastable materials, but under ordinary conditions they do not convert to their more stable phases (α-alumina and polycrystalline quartz, respectively). The preparation of γ-alumina involves the dehydration of an aluminium oxide hydroxide:

Similarly, high-surface-area silica is prepared from the acidification of silicates to produce Si (OH)₄, which rapidly forms a hydrated silica gel from which much of the adsorbed water can be removed by heating. When viewed with an electron microscope, the texture of the silica or alumina appears to be that of a rough gravel bed with irregularly shaped voids between the interconnecting particles (as in Fig. 26.14). Other high-surface-area materials used as supports in heterogeneous catalysts include TiO₂, Cr₂O₃, ZnO, MgO, and carbon. Zeolites (Section 24.13) are examples of uniform catalysts. They are prepared as very fine crystals that contain large regular channels and cages defined by the crystal structure (Fig. 26.15). The openings in these channels vary from one crystalline form of the zeolite to the next, but are typically between 0.3 and 2 nm. The zeolite absorbs molecules small enough to enter the channels and excludes larger molecules. This selectivity, in com bination with catalytic sites inside the cages, provides a degree of control over catalytic reactions that is unattainable with silica gel or γ-alumina. The synthesis of new zeolites and similar shape-selective solids and the introduction of catalytic sites into them is a vigorous area of research (Section 24.12).

Figure 26.15 A view into the channels of zeolite theta-1 with an absorbed benzene molecule in the large central channel. (Based on A. Dyer, An introduction to molecular sieves. Wiley, Chichester (1988).)

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مواضيع ذات صلة

Surface acidic and basic sites

Methanol carbonylation: ethanoic acid synthesis

Palladium-catalysed C-C bond-forming reactions

Asymmetric oxidations

Wacker oxidation of alkenes

Hydroformylation

Hydrogenation of alkenes

Alkene metathesis

Homogeneous and heterogeneous catalysts

Selectivity

Catalytic efficiency and lifetime

Catalytic cycles