Shape selectivity spectroscopy

NMR spectroscopy is essential for the structure determination of carotenoid isomers because the TI-NMR signals of the olefinic range are characteristic for the arrangement of the isomers. The stereoisomers of astaxanthin, as shown in Figure 4.16, can be separated on a shape-selective C30 capillary column with methanol under isocratic conditions. 

In summary, NMR spectroscopy is an extremely versatile tool useful that enables researchers to understand the structure of natural products such as carotenoids. For a full structural assignment, the compound of interest has to be separated from coeluents. Thus, it is a prerequisite to employ tailored stationary phases with high shape selectivity for the separation in the closed-loop on-line LC-NMR system. For the NMR detection, microcoils prove to be advantageous for small quantities of sample. Overall, the closed-loop system of HPLC and NMR detection is very advantageous for the structural elucidation of air- and UV-sensitive carotenoids. 

Shape selectivity can be induced by differences in the diffusivities of the reactants and/or the products or by steric constraints of the transition state. A schematic representation of the three types of shape selectivity, i.e., the limitations of the access of some of the reactants to the pore system (reactant selectivity), the limitation of the diffusion of some of the products out of the pores (product selectivity) and constraints in forming certain transition states (transition state selectivity) are given in Fig. 8. Differentiation between the latter two is difficult as the kinetic results may be disguised when the overall rate is influenced by the rates of diffusion. In situ IR and NMR spectroscopy have contributed much to our understanding of these complex phenomena. The aspects of shape selectivity have been extensively discussed and excellent reviews exist [242,243,244]. The examples given here should only illustrate what can be achieved by employing a zeolite and why the pathway of a particular reaction is influenced. 

Similar behavior was discovered in subsequent studies for ZSM-5 (772,174) and ZSM-11 (173) zeolites synthesized with aluminum and boron in the zeolite lattice and for boron-synthesized ZSM-11 zeolites (173). The modification of the ZSM-5 and ZSM-11 samples produced a minor improvement in shape selectivity and a large decrease in acidity and hence activity. The initial heat for the B-ZSM-11 sample decreased from 160 kJ mol" for Al-ZSM-11 to 65 kJ mor , and the acidity decreased to 10% of the original value. The q-d curve also showed a maximum at high coverages, which was attributed to the formation of a NH NHa) complex on reacting B—OH—NH3 with NH3. Dehydroxylation at 1073 K increased the initial heat to 170 kJ mol", a value comparable to the initial heat of 185 kJ mol" on Al-ZSM-11, and it sharpened the maximum in the q-9 curve. This behavior is apparently due to the formation of a few strong Lewis acid sites. The sample synthesized with both boron and aluminum behaved differently than those with only aluminum or boron. The q-6 curve for this sample showed maxima at about 145-175 kJ mol" and at about 60-70 kJ mol for 673 and 1073 K dehydroxylation temperatures, respectively. The acidity of this sample was 30% lower than an Al-ZSM-11 sample with similar Si/Al ratio. The initial heat for the aluminum zeolite was 170 to 190 kJ mol". It was shown, with IR spectroscopy of adsorbed ammonia, that the boron-modified samples showed little or no Brpnsted acidity. 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

The results presented in this paper show that diffuse reflectance infrared Fourier transform spectroscopy, used in conjunction with controlled environment techniques and gas chromatography/mass spectrometry, can be a powerful tool for the study of catalysis by product (shape) selective molecular sieve materials. By utilizing spectral differencing techniques it is possible to track the variations in protonated site occupancy and the formation of stable organic species that occur during exposure of molecular sieve catalysts to organic reactants. 

Metal clusters in zeolites are catalysts for a number of reactions, including alkene hydrogenation and alkane hydrocracking. The former is an example of shape selective catalysis, whereby straight diain alkenes can enter the zeolite pores and react but branched alkenes cannot enter and so do not substantially react. The latter have been apidied commerdally. Pt dusters in the zeolites KL and BaKL are remarkably selective catalysts for the dehydroi dization of n-hexane to give benzene, and they are now applied commerdally. The origin of the selectivity is still not fully understood, but it may be primarily a consequence of the smallness of the Pt clusters, which consist of only about S or 6 atoms on average, as determined by EXAFS spectroscopy, H2 chemisorption, and electron microscopy. 

The main task of selective spectroscopy is to measure a homogeneous optical band because the shape of this band indudes important information cxriKeming the molecular dynamics and the electron-phonon interaction. By now, there are three main experimental methods for selective spectroscopy, fluorescence line narrowing (FLN), spectral hole-burning (HB) and various types of photon echoes (PE). 

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