Adsorbates alkanes

For propane, n-pentane and n-hexane the differential heats of adsorption over FER dropped more rapidly, right after 1 molecule was adsorbed per Bronsted acid site. Similar results were obtained with TON. In contrast, with MOR and FAU the drop in the differential heats of adsorption for n-alkanes occurred at lower coverages, indicating that only a certain fraction of the Bronsted acid sites were accessible to the adsorbing alkane probe molecules. With MFI the drop did not occur until 2 molecules of n-alkane were adsorbed per Bronsted acid site, suggesting perhaps a higher stoichiometry of about two n-alkanes per Bronsted acid site. In the cases of i-butane and i-pentane the drop occurred around one alkane per Bronsted acid site. Finally, n-butane adsorption isotherms measured over TON framework type catalysts having three different A1 contents (Si/Al2 = 90, 104, 128) showed Henry coefficients to increase with increase in the A1 content [5], Based... 

In a similar way, Schultz and coworkers [87,125] proposed a relation for the calculation of y, but based on a different variable, that is [a (y )x,2 - l is the section of the adsorbed alkane and y is the surface free energy of the injected n-alkanes (usually, C5Hi2-Ci2H26), obtained at room temperature [66,93]. [—AGa] may be calculated using the following equations... 

The measured enthalpies of adsorption of alkanes into zeolites are very large, and can be calculated from this formula [13]. The dielectric constants for the open networks formed by the aluminium silicate frameworks of zeolites can be related to the known and measured dielectric and optical properties of bulk quartz. The calculation assiunes that the zeolite framework tessellates a hyperbolic surface as described in Chapter 2. The polarisabilities of different adsorbate alkanes are also known. When the calculations are carried out for a whole range of alkanes as adsorbate molecules and for different zeolites (with differing pore structure and size) the agreement between measured and predicted heats of adsorption is excellent (cf. Fig. 3.2). The results depend... 

The sextet-doublet model was adapted for 1-5 dehydrocyclization, the reverse of cyclopentane hydrogenolysis, and it was proposed that physically adsorbed alkane reacts with chemisorbed hydrogen according to a push-pull mechanism (772) (Scheme 55). 

Table 9-1 provides retention volume data for several sample compounds in a model GAC system (benzene plus helium carrier gas) and the corresponding GSC system (helium carrier gas data of Fig. 9-1). For every sample compound, the use of the adsorbing carrier gas decreases its retention volume, and this is fairly typical of GAC separation. Increasing the concentration of benzene in the carrier gas further decreases C/ for all the compounds of Table 9-1, implying that partition effects [case (4)] are relatively unimportant. With the exception of the weakly adsorbing alkanes and those samples with retention volumes smaller than that of benzene (the carrier gas component), the GAC system of Table 9-1 gives two elution bands when a single sample compound is injected into the column. One band corresponds to the injected compound and the other is a benzene band. For these latter compounds it is apparent that adsorption proceeds (at least in part) by case (3) adsorption of the sample... 

Dimensions of alkane molecules sorbed by zeolites are comparable to the dimensions of zeolite pores. So the diffusion and intramolecular dynamics of alkanes confined in the narrow pores of zeolite is defined by the size and geometry of the zeolite pores as well as by dimensions and the structure of the adsorbed alkane. This means that alkanes with different structures and dimensions can exhibit a unique NMR line shape in the zeolite structure of the same type. 

Fig. 17 shows NMR line shapes for some of C -Cjg alkanes adsorbed in zeolite ZSM-5. Due to the difference in the intramolecular motion of these alkanes, the line shapes are different. This property can be used as a fingerprint test for identification of adsorbed alkanes in zeolites of similar and different structures. 

Due to the expected noticeable difference in the H chemical shifts of the methyl and methylene groups (eg, 1.0 and 1.5 ppm for the CHj and CH groups of propane), H MAS NMR allows identification of these separate fragments in adsorbed alkanes [136]). This gives us a chance to get into details of the mechanism of H/D exchange for CyC alkanes and therefore to clarify the pathways of these alkanes activation on zeolite catalysts. 

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