Benzene, adsorbed on zeolites

The value of ext is determined by the local crystal structure and it changes as the structure evolves. The INS spectrum of benzene adsorbed on zeolite is shown in Fig. 5.8 ( 7.5.4.1) [17]. Here the benzene sits on an uncongested surface and intermolecular forces restrain the molecule only weakly. The basic spectral features of the benzene molecule remain visible in Fig. 5.8. The benzene is thus physisorbed and has not reacted with the surface. 

Fig. 5.8 Comparison of the calculated, upper trace, INS spectrum of benzene (as shown in Fig. 5.6) compared with the observed spectrum, lower trace, of benzene adsorbed on zeolite. Reproduced from [17] with permission from The Royal Society of Chemistry.

This, Eq. (5.21), is the external displacement tensor of benzene adsorbed on zeolite, as measured by diffraction. It is about twice that measured by INS, Eq. (5.18), which is a difference too great to be explained by simple experimental errors. 

Changes in relative peak intensity and marginal line shifts have been observed for benzene adsorbed on porous glass (26). More significantly, infrared spectroscopic evidence had been found in the appearance of inactive fundamentals for the lowering of molecular symmetry of benzene on adsorption on zeolites (47). 

Diffusional behavior of sorbed species is studied by NMR using one of three approaches the van Vleck method of moments, relaxation measurements, and the pulsed-field-gradient method. An example of the use of the method of moments is the work of Stevenson (194) on H resonances in zeolite H-Y (see Section III,K). Another is the study by Lechert and Wittem (284) of C6H6 and C6H3D3 adsorbed on zeolite Na-X. Analysis of second moments of H resonances allowed the intra- and intermolecular contributions to the spectra to be extracted. Similarly, second moments of H and 19F spectra of cyclohexane, benzene, fluorobenzene, and dioxane on Na-X provided information about orientation of molecules within zeolitic cavities (284-287). 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

Following the pioneering works of Ito and Fraissard (57) and Ripme-ester (58), 129Xe NMR of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used 11 and 13C NMR measurements of the adsorbed benzene in conjunction with 129Xe NMR and adsorption isotherm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR linewidths and chemical shifts and are compared with results obtained from other studies. 

Figure 7. Room temperature (a) 11, and (b) 13C NMR spectra of benzene adsorbed on NaY (Si/Al = 2.49) zeolite at various benzene coverages (0).

A study was made of the ultraviolet spectra of benzene, alkyl-, amino-, and nitro-derivatives of benzene, diphenyl-amine, triphenylmethane, triphenylcarbinol, and anthra-quinone adsorbed on zeolites with alkali exchange cations, on Ca- and Cu-zeolites, and on decationized zeolites. The spectra of molecules adsorbed on zeolites totally cationized with alkali cations show only absorption bands caused by molecular adsorption. The spectra of aniline, pyridine, triphenylcarbinol, and anthraquinone adsorbed on decationized zeolite and Ca-zeolite are characterized by absorption of the corresponding compounds in the ionized state. The absorption bands of ionized benzene and cumene molecules appear only after uv-excitation of the adsorbed molecules. The concentration of carbonium ions produced during adsorption of triphenylcarbinol on Ca-zeolite and on the decationized zeolite depends on the degree of dehydroxyla-tion of the zeolite. 

The Role of Excitation in the Ionization of Molecules Adsorbed on Zeolites. Ultraviolet irradiation of benzene and cumene (10), adsorbed on Na-zeolite, did not lead to the appearance of absorption bands reflect-... 

The effect of the co-adsorption of organic molecules on the Xe NMR spectra of xenon atoms adsorbed on zeolite NaY was studied by De Menorval et al. [327]. They found that the NMR shift depends on the kind of guest molecules, their concentration, and the density of xenon atoms inside the zeolite cages. The differences in the interaction of xenon atoms with aromatic molecules (benzene and trimethylbenzene) in comparison with saturated hydrocarbon chains like n-hexane were explained by an adsorption of the aromatic hydrocarbons parallel to the cage walls while the hydrocarbon chains are adsorbed perpendicular to the walls. 

EPR spectra of the N2H3 radical were produced by fast-electron irradiation of liquid hydrazine adsorbed on zeolite [7], by y irradiation of hydrazinates of X(C104)2 (X = Mg, Ca) and YCI2 (Y = Ca, Zn, Ba, Mg) [8], and by photolysis of hydrazine in a benzene solution containing di-ferf-butyl peroxide [20]. 

Deuterium NMR has recently been used to study molecular motion of organic adsorbates on alumina (1.) and in framework aluminosilicates (2). The advantage of NMR is that the quadrupole interaction dominates the spectrum. This intramolecular interaction depends on the average ordering and dynamics of the individual molecules. In the present work we describe NMR measurements of deuterated benzene in (Na)X and (Cs,Na)X zeolite. 

Hirschler and co-workers (82) have reported the formation of a radical when benzene was adsorbed on a calcium exchanged Y-type zeolite. A... 

Methane dehydroaromatization on zeolites Mo/HZSM-5 was also investigated by solid-state MAS NMR spectroscopy 162. Both variation of the state of the transition metal component and products (such as ethane, benzene, and ethylene) adsorbed in zeolite were observed after reaction at high temperature (900-1000 K). Molybdenum carbide species, dispersed on the external surface or in the internal channels of the zeolite catalysts, had formed during the reaction 162. ... 

Potassium ions in the natural clinoptilolite (3.5-5.2%) affect adsorption only slightly with respect to water vapor. n-Hexane as well as benzene vapors are adsorbed on the secondary porous structure of the zeolite (26). 

In zeolites the mobility of hydrocarbon molecules with double bonds is specifically restricted because of a specific interaction between the 7r-elec-trons and the zeolite (2). As expected, proton spin relaxation of benzene, cyclohexadiene, cyclohexene, and cyclohexane adsorbed on NaY reveals an increasing restriction of mobility with increasing number of -electrons (8, 4, 8). This is shown in Figure 1, where the longitudinal (7 ) and transverse (T2) proton relaxation times are plotted. 

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