External surface, catalytic role

It should be emphasized that the active sites located on the external surface, often in small amounts compared to the inner sites (<1% for crystallites of 1 //m), play a catalytic role. Generally, this leads to a selectivity decrease, the external surface lacking the shape selective properties of the inner pores.. However, recent results show that reactions which can occur only on the external surface of zeolites or just within the pore mouth are very selective, suggesting a shape selective influence of external surface depending on the nature of the substrate (Table 1.2). 

All these aspects were thoroughly discussed by lecturers and participants during the round table organized during the Poitiers School on The Future Trends in Zeolite Applications . Special emphasis was placed on the role played by the sites at the external surface (pockets, etc.) or at the pore mouth, by mesopores, extraframework aluminum species, as well as by the polarity of reactant and product molecules. Other important topics dealt with the remarkable catalytic properties of BEA zeolites for fine chemical synthesis, the potential of mesoporous molecular sieves, zeolitic membranes and the role of combinatorial catalysis in the development of zeolite catalysts. It is our hope that the fruits of these discussions will appear in the literature or even better as new and environmentally friendly products or processes. 

In montmorillonite, similar to other minerals, when the size of the exchanged cation is similar to the pore sizes in the crystal lattice, cations can build into the crystal lattice and, consequently, they reduce the negative layer charge (Chapter 1, Section 1.3.3.2). Other neutral molecules or cationic substances (Chapter 1, Sections 1.3.3.1 and 1.3.3.2) can also be sorbed in the interlayer space and on the external surfaces as well. They play an important role in defining the internal and total surface area and catalytic properties, and they may have an effect on the hydrophobicity of the mineral, as well as playing an important role in the production of pillared materials, etc. 

Non-contact atomic force microscope (AFM) and N2 absorption measurements on beta zeolites reveal the extreme irregularity of the external crystal surface which can make up a considerable proportion of the total surface area. A catalytic test, the acylation of 2-methoxynaphthalene, shows that active sites on the outer surface play an important role in the catalytic activity of the zeolite. Attempts to influence the external surface area and its catalytic activity through synthesis or post-synthesis modification such as dealumination show that the principle influence on the external surface comes from the synthesis procedure. 

ESAs except for some increase in the mesopore range (1-20 nm). A catalytic test showed that in all cases and even after extensive dealumination, the external surface area plays an important role in the catalytic activity of the zeolite. 

External Surfaces of Zeolites. All of the work reviewed above dealt with the internal pores of zeolites. In situations where zeolite particles are large and molecules of interest are easily accommodated in the zeolite pores, it is entirely appropriate to focus exclusively on the internal pores. There is growing interest, however, in situations where the external surfaces of zeolites can play a role in catalytic and separation processes. Molecules entering or exiting zeolite pores at an external surface can in some cases experience large resistances to transport that can contribute to the net resistance to diffusion experienced by mobile... 

The role of external or internal surface catalytic sites was also examined with the aim of understanding whether the reaction occurs in the whole volume of the catalyst or only on the external surface. To this end, two USY catalysts with different particle sizes are compared the first is composed of granules between 28 and 35 mesh, and the other of granules larger than 60 mesh. Both these catalysts exhibit the same activity, suggesting that the process does not take place only on the external surface. 

Physical transport processes can play an especially important role in heterogeneous catalysis. Besides film diffusion on the gas/liquid boundary there can also be diffison of the reactants (products) through a boundary layer to (from) the external surface of the solid material and additionally diffusion of them through the porous interior to from the active catalyst sites. Heat and mass transfer processes influence the observed catalytic rates. For instance, as discussed previously the intrinsic rates of catalytic processes follow the Arrhenius... 

In most cases, catalysis on zeolites occurs inside the intracrystalline voids. Nevertheless, a catalytic role is sometimes attributed to the external surface of the crystals, which for many crystallographic directions consists of a collection of pore mouths. The possibility of catalysis at pore mouths was first discussed by Venuto [41]. Catalytic sites at the pore mouths can have a strength and a structural environment different from those within the intracrystalline cavities. In principle, situations may occur where the concentration of reactants is totally different at the pore mouths compared to the crystal interior. When intracrystalline diffusion is slow compared to the rate of the chemical reaction, only active sites near the external surface of crystallites may be responsible for catalysis. The absence of intracrystalline diffusion restrictions doesnot necessarily imply that the pore mouths are a-selective catalytic enviromnents. They may provide a local geometry which is different from that available inside the crystals. 

Moreover, there is a great variety of adsorbents which, depending on the chemical structure of their internal or external surfaces, play the role of solid catalysts or their supports. The basic function of catalyst supports is to keep the catalytically active phase in a highly dispersed state. For this reason, catalyst supports are usually porous adsorbents characterized by a well developed inner structure and possessing a large surface area. A large surface area is, however, not always desirable. 

The catalysts for ammonia synthesis are porous particles with weenie and interlaced micro-pores. The active sites playing the role of surface catalysis are distributed on the internal surfaces formed by these micro-pores. The internal surface area of ammonia synthesis after reduction is about 10m -g -15m -g , and the external surface area is only 0.1 m g F So, the surface area playing the role of surface catalysis mainly is internal surface. The equivalent diameter of catalyst particles used in industrial ammonia reactor is between 1.5 mm and 13 mm, and the inhibition effect of diflfusion should be considered in real ammonia synthesis rates. When designing industrial reactor, the resistance of external diffusion can be neglected by increasing contact between gas flow and external sm-face of catalysts. The catalytic reaction processes for ammonia synthesis pertain to considerable internal diffusion process in most cases. 

An issue of debate is the relative roles of internal and external sites in the catalytic process. The effects of shape selectivity, clearly present in product distribution, seem to indicate a predominance of intra-porous hydroxylation. However, the different catechol/hydroquinone ratio in methanol (0.5) and acetone (1.3), could indicate a significant contribution of sites located on the outer surface of the crystals, particularly for crystallite sizes <0.3 xm. Tuel and others, studying the time course of the reaction and the solubility of tarry deposits, went further and concluded that catechol and hydroquinone were produced on different sites, external and internal respectively [49]. The effect of acetone and methanol simply reflected their ability to maintain external sites clean from tar deposits, which are soluble in the former and insoluble in the latter. On the other hand, Wilkenhoner and others concluded, with the support of kinetic constants estimated independently for internal and external sites, that catechol was also produced in the pores over the entire reaction profile, albeit at a lower rate [47]. The contribution of the outer surface for crystal sizes close to 0.1 (xm ranged from 46% in methanol to 69% in acetone. 

Overview In many industrial reactions, the overall rate of reaction is limited by the rate of mass transfer of reactants between the bulk fluid and the catalytic surface. By mas,s transfer, we mean any proces.s in which diffusion plays a role. In the rate laws and catalytic reaction steps described in Chapter 10 (diffusion, adsorption, surface reaction, desorption, and diffusion), we neglected the diffusion steps by saying we were operating under conditions where these steps are fast when compared to the other steps and thus could be neglected. We now examine the assumption that diffusion can be neglected. In this chapter we consider the external resistance to diffusion, and in the next chapter we consider internal resistance to diffusion. 

What is more interesting, at least from the theoretical point of view is that oscillatory behaviour might emerge as a result of the interaction between the system and the external noise applied. This phenomenon was earlier described in a radio-engineering context (Kuznetsov et ai, 1965). Studying the role of multiplicative coloured noise for the catalytic oxidation of CO on a platinum surface, de la Rubia et ai (1982) demonstrated that a limit cycle is induced by external noise. Similarly, Treutlein Schulten (1985) found noise induced limit cycles in the Bonhoffer-van der Pol model of neural pulses (see further Lefever Turner (1984)). 

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