Shape-selective polymerization

The present paper describes the phenomena of shape-selective polymerization involved in converting C--C olefins over HZSM-5 to higher boiling olefins and the similarities and differences compared to amorphous silica-alumina. The channel systems of ZSM-5 (8), Figure 1, impose shape-selective constraints on the shape of the large molecules accounting for the differences with amorphous catalysts. 

The applications of the ZSM-5 family of zeolites for shape-selective cracking of paraffins in the gasoline (2, 10), distillate (11) and lube oil range (12) have all been reported. In this paper, we have established evidence of the converse reaction, shape-selective polymerization, to produce hydrocarbons in the same product range. 

Separations of PAH isomers by shape-selective phases are useful if the sample contains compounds within a limited number of aromatic rings, otherwise the sample becomes too complex to analyze because molecules with different ring sizes coelute. This makes identification of the components difficult even when selective fluorescence detectors are used. Multidimensional LC should be applied in cases where it is necessary to characterize completely all the components of a complex polyaromatic compound mixture. Normal-phase LC on polar bonded silica gives eluent fractions consisting of molecules with similar numbers of pi-electrons. These can then be further separated on a monomeric-type reversed-phase column into different isomer types. Finally, a shape-selective polymeric Cig reversed-phase column (or liquid crystal coliunn in capillary GC) may be used to separate the specific PAH isomers that differ in planarity and Ub) ratios. Two-dimensional LC is a powerful tool for the separation of PAHs in complex environmental samples because of the higher peak capacity than single coliunn LC. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

FIGURE 5.11 Variations in column shape selectivity (axBN/BaP) as afunctionof aUcyl-phase length for monomeric and polymeric phases. (Adapted from Sander, L.C. and Wise, S.A., Anal. Chem., 59, 2309, 1987. With permission.)... 

Analytical shape computation techniques were applied for the detection of cavities and the calculation of molecular surface properties of isolated cavity features and other ordered formations within these resultant alkyl stationary-phase simulation models [227]. Deep cavities (8-10 A wide) within the alkyl chains were identified for Cig polymeric models representing shape selective stationary phases (Figure 5.23). Similar-structure cavities with significant alkyl-chain ordered regions (>11 A) were isolated from two independent Cig models (differing in temperature,... 

FIGURE 9.20 Effect of mobile phase composition on shape selectivity with a polymeric octadecyl-polysiloxane stationary phase, column using (a) SRM 869a (b) triphenylene/o-terphenyl (c) chrysene/benzo[a]anthracene with column outlet pressure 20.0 MPa and flow rate 1 mL/min at pump head. (Reprinted from J. W. Coym, 1. G Dorsey,... 

Stationary phases with a high density of bonded alkyl groups can differentiate between two molecules of identical size where one is planar and the other twisted out of plane. This shape selectivity has been described by Sander and Wise [53] for polymeric stationary phases, where in the preparation, water has been added on purpose and trichloro alkyl silanes have been used. The selectivity for the retention of tetrabenzonaphthalene (TEN) and benzo[a]pyrene (BaP) was taken as a measure to differentiate between polymeric and standard RP columns. With standard ( monomeric ) RP columns, the twisted TBN elutes after the planar BaP, which on the other hand is more strongly retarded as TBN on polymeric stationary phases. In these cases the relative retention of TBN/ BaP is smaller than 1, whereas with monomeric phases the value is >1.5. The separation of the standards on three different phases is shown in Figure 2.9. These stationary phases have superior selectivity for the separation of polyaromatic hydrocarbons in environmental analysis. Tanaka et al. [54] introduced the relative retention of triphenylene (planar) and o-terphenyl (twisted), which are more easily available, as tracers for shape selectivity. However, shape selectivity is not restricted to polymeric phases, monomeric ones can also exhibit shape selectivity when a high carbon content is achieved (e.g., with RP30) and silica with a pore diameter >15 nm is used [55]. Also, stationary phases with bonded cholestane moieties can exhibit shape selectivity. 

FIGURE 2.9 Demonstration of shape selectivity according to Sander and Wise. Differentiation between monomeric and polymeric RP columns. TBN tetrabenzonaphthalene (twisted) BaP benzoMpyrene (planar). 

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