Catalysts microporous

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

Microporous catalysts such as MAPO-36 (43,44), which are excellent for selective oxidation of hydrocarbons (45), are highly beam-sensitive. Yet HRTEM... 

Bisphenol A-derived epoxy resins, 10 356 Bisphenol A epoxy novolacs, 10 370 Bisphenol A manufacture, microporous catalysts and, 14 420 Bisphenol A moiety, 10 355-356 Bisphenol A polycarbonate (BPA-PC),... 

Micropore diffusion, 1 596, 597-599 Microporous catalysts, in bisphenol A manufacture, 14 420 Microporous metal membranes, 15 813t Microporous particles, apparent effective diffusivity and, 15 729-730 Microporous range, pore diameters within, 16 812... 

Microporosity is a feature observed in many different materials (e g., activated carbons, aerogels, and xerogels). However, with regard to heterogeneous catalysis, zeolites are practically the only microporous catalysts used at present. The following chapter thus only addresses zeolites and their use in catalysis. 

Membranes can also be used as a reactor where catalysts are used frequently. The membrane may physically segregate the catalyst in the reactor, or have the catalyst immobilized in the porous/microporous structure or on the membrane surface. The membrane having the catalyst immobilized in/on it acts almost in the same way as a catalyst particle in a reactor does, except that separation of the product(s) takes place, in addition, through the membrane to the permeate side. All such configurations involve the bulk flow of the reaction mixture along the reactor length while diffusion of the reactants/products takes place generally in a perpendicular direction to/from the porous/microporous catalyst. 

Fig. 48. Experimental vanadium deposit distributions in a microporous catalyst (100 A micropore diameter/204 m2/g SA) macroporous catalyst (1300 A pore diameter/14.5 m2/g SA) and bimodal catalyst (120 A micropore, 25,000 A macropore diameters/200 m2/g SA) (Plumail e al., 1983).

Instead of introducing a degree of mesoporosity into a microporous catalyst, the problem can also be approached from the opposite direction. Kloetstra et al. reported the introduction of crystalline microporous domains inside mesoporous MCM-41 by the partial recrystalhzation of the pore walls.[81] The mesoporous host can be regarded as the aluminium and silicon source for the zeolite crystallization. 

Shape-selective effects may occur whenever the pore size of a microporous catalyst is in the same range as the diameter of the molecules or transition states involved in the reacting system. Common microporous materials are zeolites and related materials (aluminophospates, pillared clays, etc.) which possess a regular crystal lattice together with a well defined pore size. According to Weisz [111] and Csicsery [27], shape-selective effects may be classified into three types (Fig. 25). 

In the light of the previous discussion it is quite apparent that a detailed mathematical simulation of the combined chemical reaction and transport processes, which occur in microporous catalysts, would be highly desirable to support the exploration of the crucial parameters determining conversion and selectivity. Moreover, from the treatment of the basic types of catalyst selectivity in multiple reactions given in Section 6.2.6, it is clear that an analytical solution to this problem, if at all possible, will presumably not favor a convenient and efficient treatment of real world problems. This is because of the various assumptions and restrictions which usually have to be introduced in order to achive a complete or even an approximate solution. Hence, numerical methods are required. Concerning these, one basically has to distinguish between three fundamentally different types, namely molecular-dynamic models, stochastic models, and continuous models. 

When Z)eA is large and kv is small, ip will be small enough for tanh ip -> p and rf 1. The catalysis is then surface-controlled, as frequently happens with gas reactions proceeding in macropores. On the other hand, with DeA small and kv large, > 1 so that tanh ip l and t] 1/tp. This latter condition is more likely to be met with in catalysed solution reactions, particularly with mesoporous and microporous catalysts. To find out which situation applies to a given first-order reaction, one can combine eqns. (32) and (34) and rearrange... 

III.C. Elucidation of the Structures of Meso- and Microporous Catalysts by HRTEM... 

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

A. Garforth, S. Fiddy, Y. H. Lin, A. G. Siakhali, P. N. Sharratt, and J. Dwyer, Catalytic Degradation of High Density Polyethylene an Evaluation of Mesoporous and Microporous Catalysts using Thermal Analysis, Thermochim. Acta, 294, 65-69 (1997). 

K. Gobin and G. Manos, Polymer degradation to fuels over microporous catalysts as a novel tertiary plastic recycling method, Polym. Deg. Stab. 83, 267 (2004). 

More specifically, over microporous catalysts such as zeolites, cracking catalysts and clays, the lower the catalyst acidity ... 

Y. H. Lin, M. H. Yang, T. F. Yeh and M. D. Ger Catalytic Degradation of High Density PolyethyleneOver Mesoporous and Microporous Catalysts in a Fluidized-Bed Reactor Polym. Degrad. Stabil., 86, 121 (2004). 

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