Weisz window

In Table 3 the three common reactor types are compared. Obviously, the monolithic reactor in the Taylor-flow regime leads to a high degree of process intensification. When these numbers are recalculated into production rates, values of 40 mol/m3reactor-s were found. Figure 17 illustrates the high value in relation to the Weisz window of reality. This demonstrates the attractiveness of using monoliths in fast catalyzed gas-liquid-solid reactions. 

A catalytic process is commercially viable if the catalyst transformation is achieved within definite, practical limits of space and time. To quantify this aspect, one can determine the so-called space-time yield. This measure of activity is simply the amount of product obtained per unit time and per unit reaction space (where reaction space is usually the reactor volume). Weisz (79) pointed out that in industry the useful space-time yield is rarely less than 10"6 g/mol of reactant per cubic centimeter of volume of reactor space per second. This has been called the Weisz window on reality. Figure 9 (79) shows the Weisz window and other windows of chemical activity that apply to biochemistry and petroleum geochemistry (79). 

Useful chemical transformations are thus confined to the Weisz window on reality. The Weisz window is an important concept from various standpoints. For scale up, it is helpful in determining the limiting reactor sizing to be considered for a given intrinsic catalyst activity, as determined by laboratory experiments. During catalyst development, it provides guidance in assessing whether further catalyst development is necessary. It also indicates the directions for further research e.g., in the case of certain zeolites that were found to be too active... 

Perhaps several catalyst preparation steps will be necessary to tame the catalyst activity until it performs within the Weisz window. This can include diluting the active catalyst in ah inert matrix to provide a reduction of 10 to 20 per-unit volume activity. 

Figure 9. The Weisz window and other windows of activity.

To proceed with the topic of this section. Refs. 250 and 251 provide oversights of the application of contemporary surface science and bonding theory to catalytic situations. The development of bimetallic catalysts is discussed in Ref. 252. Finally, Weisz [253] discusses windows on reality the acceptable range of rates for a given type of catalyzed reaction is relatively narrow. The reaction becomes impractical if it is too slow, and if it is too fast, mass and heat transport problems become limiting. 

Based on this approach, a useful estimation was provided recently [35]. With regard to the reaction aspect, achievable space-time yields (STY) of currently operated catalytic reactors were considered, whereupon Weisz defined the following window of reality [36] ... 

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

The most straightforward cause of shape selectivity is the discrimination between molecules on the basis of their diffusion rates through the channels or cage windows. Microporous solids act as true molecular sieves, because the well-defined pores are able to select molecules on the basis of differences in dimensions of 0.1 A or less. Examples of strong molecular sieving effects include the selection of normal alkanes over branched ones by small-pore solids and the selection of para-substituted over ortho- and meta-substituted aromatics over medium-pore zeolites. This type of selectivity according to molecular diffusion rate may act on both reactant and product molecules. The much faster dehydration of n-butanol compared to isobutanol over Ca-A demonstrated by Frilette and Weisz is the classic example of reactant diffusion... 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

Figure4.1.4 Windows of rates of biochemical and chemical processes (hatched areas overall range gray majority of catalytic processes). Adapted from Weisz (1982) and Moulijn, Makkee, and Van Diepen (2004).

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