Dispersed-phase catalysis

UTZETAL. Dispersed-Phase Catalysis in Coal Uqu action... 

There are, of course, many catalysts that are only available either as a dissolved comj und or as a solid. Solid surfaces are often essential to make certain reactions possible, particularly stereospecific reactions. However, in recent times homogeneous catalysts have been developed that promote stereospecific reactions. Phase transfer catalysis is a form of heterogeneous catalysis where the catalytic phase is a dispersed liquid phase, containing a dissolved catalyst. The reactants penetrate the dispersed phase, react there, and the reaction product dissolves again in the continuous phase, where it is protected from consecutive reactions. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

XPS is among the most frequently used techniques in catalysis. It yields information on the elemental composition, the oxidation state of the elements and, in favorable cases, on the dispersion of one phase over another [ J.W. Niemantsverdriet, Spectroscopy in Catalysis, An Introduction (2000), Wiley-VCH, Weinheim G. Ertl and J. Kiippers, Low Energy Electrons and Surface Chemistry (1985), VCH, Weinheim L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin Film Analysis (1986), North-Holland, Amsterdam]. 

Support materials are commonly u.sed in heterogeneous catalysis. Their major function is to maximize the dispersion of the active phase by providing a large surface area over which the active phase can be distributed. In this way the cataly.st material is shaped into a form suitable for use in technical reactors. Supports are not always chemically inert they can also show certain catalytic activity and often they act as a stabilizer for the actual active phase. A number of materials are u.sed as catalyst supports. Table 3.2 gives an overview. 

In spite of the large success of XRD in routine structural analysis of solids, this technique does present some limitations when applied to catalysis [1,9]. First, it can only detect crystalline phases, and fails to provide useful information on the amorphous or highly dispersed solid phases so common in catalysts [22], Second, due to its low sensitivity, the concentration of the crystalline phase in the sample needs to be reasonably high in order to be detected. Third, XRD probes bulk phases,... 

Catalysts have been bonded to insoluble polymers to allow, in principle, an appreciable simplification of PTC the catalyst represents a third insoluble phase which can be easily recovered at the end of the reaction by filtration, thus avoiding tedious processes of distillation, chromatographic separation and so on. This is of potential interest mainly from the industrial point of view, due to the possibility of carrying on both discontinuous processes with a dispersed catalyst and continuous processes with the catalyst on a fixed bed. This technique was named "triphase catalysis" by Regen (13,33,34). 

Reactions carried in aqueous multiphase catalysis are accompanied by mass transport steps at the L/L- as well as at the G/L-interface followed by chemical reaction, presumably within the bulk of the catalyst phase. Therefore an evaluation of mass transport rates in relation to the reaction rate is an essential task in order to gain a realistic mathematic expression for the overall reaction rate. Since the volume hold-ups of the liquid phases are the same and water exhibits a higher surface tension, it is obvious that the organic and gas phases are dispersed in the aqueous phase. In terms of the film model there are laminar boundary layers on both sides of an interphase where transport of the substrates takes place due to concentration gradients by diffusion. The overall transport coefficient /cl can then be calculated based on the resistances on both sides of the interphase (Eq. 1) ... 

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