Shape selectivity, trends

For the monomeric stationary phase. Figure 9.19 shows the SRM 869a ofTBN/BaP decreasing with increasing proportions of aceonitrile in the mobile phase, which should indicate increased shape selectivity with added acetonitrile. This is the same observed trend when organic solvents are added to water in a reversed-phase separation. However, the selectivity measured from the triphenylene/o-terphenyl pair and the chrysene/benzo[fl]anthracene pair showed exactly opposite trends in that the planarity and LIB selectivity was increased with decreasing proportions of acetonitrile. 

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

In this paper, we review primary and secondary shape selective acid catalysis with zeolites. Next, we discuss shape selectivity with metal containing zeolites.We conclude with a section that deals with future trends in shape selective catalysis. 

Molecular sieve science is growing rapidly. The uses of molecular sieves as shape selective catalysts for various chemical reactions continue to increase. Below, we illustrate several new trends in the use of zeolites and other molecular sieves as shape selective catalysts. 

The relative distribution of para + meta aromatics in methanol conversion was increased over Mo exchanged ZSM-5 but not on the pyridine poisoned sample. The same increase trend was also observed in the disproportionation reaction over Mo exchanged zeolites (Fig. 3), thus a reasonable explanation is the presence of internal Mo. Both methanol and toluene conversions performed on zeolites are molecular shape selective processes. Internal Mo will create diffusional hindrances which will favour the formation of para aromatics (product selectivity). 

The discovery that the protonated forms of zeolites could be used as active, stable and shape selective catalysts in hydrocarbon transformations has been of immense benefit to the refining and petrochemicals industry. The need for optimised microporous acid catalysts will continue as fuel specifications change and the requirements of the chemicals market shift. The likely growth in demand for synthetic fuels, including diesels, is one expected trend that could involve zeolite catalysts. Diverse feedstock chemicals and fine chemicals synthesis involving zeolite catalysts are also being developed. 

The PFR of aromatic esters [121-123] and amides [124] has been studied within faujasite and pentasil zeolites. In the former, the predominating prodnct is again the orf/jo-isomer, while in the latter, the para-isomer is favored. The observed differences have been attributed to the size and shape of the cavities and channels of the zeolites, which is consistent with the selectivity trends found in faujasites containing alkaline cation of diverse sizes. The same orf/tz>-selectivity is found in Y zeolites [125]. 

The second factor to consider is the catalyst conditions and the catalyst-particles, environment. From the point of view of the effectiveness factor, the size, shape, porosity, metal-loading, and other internal characteristics, must be the same. Otherwise, the intrinsic reaction rate and selectivity will be compromised. In practice there will be a trend to reduce costs by using less metal. This will change the process markedly, and other adjustments will be required. 

Diversity patterns are an important feature in the analysis of tropical forest communities. However, a comparison of existing studies is difficult because size and shape of plots, diameter limit of the included individuals, and criteria of site selection may differ considerably (Table 13.5). A simple but helpful measure is the calculation of species per 100 stems (Klinge et al. 1995) (Table 13.5). Another severe problem is that all individuals can not be identified to species level. In all reports a varying percentage of unidentified taxa is present. This reason impedes a calculation of indices of similarity or diversity. The generally observed trends in changing species diversity of Amazonian floodplain forests are as discussed below. 

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