Catalysis porous solids

It would be difficult to over-estimate the extent to which the BET method has contributed to the development of those branches of physical chemistry such as heterogeneous catalysis, adsorption or particle size estimation, which involve finely divided or porous solids in all of these fields the BET surface area is a household phrase. But it is perhaps the very breadth of its scope which has led to a somewhat uncritical application of the method as a kind of infallible yardstick, and to a lack of appreciation of the nature of its basic assumptions or of the circumstances under which it may, or may not, be expected to yield a reliable result. This is particularly true of those solids which contain very fine pores and give rise to Langmuir-type isotherms, for the BET procedure may then give quite erroneous values for the surface area. If the pores are rather larger—tens to hundreds of Angstroms in width—the pore size distribution may be calculated from the adsorption isotherm of a vapour with the aid of the Kelvin equation, and within recent years a number of detailed procedures for carrying out the calculation have been put forward but all too often the limitations on the validity of the results, and the difficulty of interpretation in terms of the actual solid, tend to be insufficiently stressed or even entirely overlooked. And in the time-honoured method for the estimation of surface area from measurements of adsorption from solution, the complications introduced by... 

Diffusion in porous solids is usually the most important factor con-troUing mass transfer in adsorption, ion exchange, drying, heterogeneous catalysis, leaching, and many other applications. Some of the... 

The discipline of materials science, for example, has a major impact on innovation in catalysis. Developments in the field of porous solids have led to some... 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

In this chapter, we demonstrate the potential of such agents as catalysts/promoters in key steps for the derivatization of sugars. The most significant catalytic approaches in carbohydrate chemistry that use aluminosilicate porous materials, namely zeolites and montmorillonite clays, are reviewed and discussed. Silica gel is a porous solid silicate that has also been used for heterogeneous catalysis of organic reactions in general. We include here its usefulness as promoter and reagent support for the reactions under consideration. 

Another silicon-based porous solid which has been used in heterogeneous catalysis of organic reactions is silica gel. However, unlike zeolites and clays, silica gel is... 

Catalysis by solids depends on the amount of surface exposed to the fluid. Large specific surface is obtained with small particles, but primarily with highly porous structures. For instance, to achieve 1 m2/cc the diameter of a sphere must be reduced to 6(10-4) cm, but porous catalysts may have several hundred m2/cc. Practical limitations exist to the smallness of particles that can be used, such as pressure drop and entrainment. In fixed or moving beds, particle diameters are several millimeters, in fluidized beds they may be less than 0.1 mm. 

A good deal of information presented in this chapter has been adapted from textbooks and monographs. For further reading, all of them can be recommended. They contain enormous amounts of additional information and give excellent reviews not only on all topics related to catalysis on micro- and mesoporous solids but also on catalysis and on porous solids in general. 

The understanding of bio- and chemo-catalytic functionalities, their integration in recognizing materials (doped materials, membranes, tubes, conductive materials, biomarker detection, etc.) and the development of smart composite materials (e.g., bio-polymer-metal) are all necessary elements to reach above objectives. It is thus necessary to create the conditions to realize a cross-fertilization between scientific areas such as catalysis, membrane technology, biotech materials, porous solids, nanocomposites, etc., which so far have had limited interaction. Synergic interactions are the key factor to realizing the advanced nanoengineered devices cited above. 

Cross-fertilization Advanced nanoengineered devices by integration of catalysis, membranes, biotech materials, porous solids, and nanocomposites concepts... 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

N. Floquet, J. P. Coulomb, G. Andre and R. Kahn, Studies in Surface Science and Catalysis, 2007, 160(Characterization of Porous Solids VII), 375. 

Catalysis is of crucial importance for the chemical industry, the number of catalysts applied in industry is very large and catalysts come in many different forms, from heterogeneous catalysts in the form of porous solids over homogeneous catalysts dissolved in the liquid reaction mixture to biological catalysts in the form of enzymes. 

These porous solids obviously have very high surface areas, and they lend themselves naturally to service in catalysis. Palladium catalysts supported on alumina aerogels have been used successfully to remove CO and NO from automobile exhausts,13 and a V205/Ti02 aerogel is itself a catalyst for the selective reduction of NO to N2 and water by gaseous ammonia.14... 

Adsorption in porous solids, an important topic in catalysis and other areas, is presented in Section 9.7, in which the adsorption hysteresis and capillary condensation are introduced. 

FIGURE 4.14 SAXS plots for ACFs prepared by (a) C02 and (b) steam activation. (From Lozano-Castello, D., et al., Studies in Surface Science and Catalysis, Characterisation of Porous Solids V, vol. 128, Elsevier Science, the Netherlands, 523-532, 2000. With permission.)... 

Kaneko K, Setoyama N, and Suzuki T. Ultramicropore characterization by He adsorption. In Rouquerol J, et al., eds. Characterization of Porous Solids III. Studies in Surface Science and Catalysis, vol. 87, the Netherlands Elsevier Science. 1994 pp. 593-602. 

In heterogeneous catalysis reactants have to be transported to the catalyst and (if the catalyst is a porous, solid particle) also through the pores of the particle to the active material. In this case all kinds of transport resistance s may play a role, which prevent the catalyst from being fully effective in its industrial application. Furthermore, because appreciable heat effects accompany most reactions, heat has to be removed from the particle or supplied to it in order to keep it in the appropriate temperature range (where the catalyst is really fully effective). Furthermore, heterogeneous catalysis is one of the most complex branches of chemical kinetics. Rarely do we know the compositions, properties or concentrations of the reaction intermediates that exist on the surfaces covered with the catalytically effective material. TTie chemical factors that govern reaction rates under these conditions are less well known than in homogeneous catalysis. Yet solid catalysts display specificities for particular reactions, and selectivity s for desired products, that in most practical cases cannot be equaled in other ways. Thus use of solid catalysts and the proper (mathematical) tools to describe their performance are essential. 

In the last decade, a variety of microporous and mesoporous materials have been developed for applications in catalysis, chromatography and adsorption. Great attention has been paid to the control of (i) pore surface chemistry and (ii) textural properties such as pore size distribution, pore size and shape. Recently, a new field of applications for these materials has been highlighted [1-3] by forcing a non-wetting liquid to invade a porous solid by means of an external pressure, mechanical energy can be converted to interfacial energy. The capillary pressure, Pc p, required for pore intrusion can be written from the Laplace-Washbum relation,... 

---