Adsorbates in zeolites

The NMR spectra of adsorbed xenon were obtained on a Bruker MSL-300 spectrometer operating at 83.0 MHZ and 295K. Typically, 2000-40000 signal acquisitions were accumulated for each spectrum with a recycle delay of 0.3s between 90 pulses. The Xe NMR chemical shifts were referenced to that of external xenon gas extrapolated to zero pressure using Jameson s equation [11]. All resonance signals of xenon adsorbed in zeolites were shifted downfield from the reference but were taken to be positive in this report. 

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. ... 

In Zeolite A. The location and distribution of methane within the a-cages of zeolite A were found to be very similar to those found in zeolite Y, consistent with the structural similarities of these two materials. Cohen de Lara et al. (50) complemented their extensive experimental work with MD calculations of a single methane molecule adsorbed in zeolite A. At... 

Fig. 58. Proton relaxation times T, and T2 for water adsorbed in zeolite Na-X at 12 MHz...

Fraissard and co-workers were the first to take advantage of these properties of l29Xe for the study of xenon adsorbed in zeolites (336-344). They have demonstrated that the 129Xe chemical shift is then a sum of several... 

The delocalized state can be considered to be a transition state, and transition state theory [105], a well-known methodology for the calculation of the kinetics of events, [12,88,106-108] can be applied. In the present model description of diffusion in a zeolite, the transition state methodology for the calculation of the self-diffusion coefficient of molecules in zeolites with linear channels and different dimensionalities of the channel system is applied [88], The transition state, defined by the delocalized state of movement of molecules adsorbed in zeolites, is established during the solution of the equation of motion of molecules whose adsorption is described by a model Hamiltonian, which describes the zeolite as a three-dimensional array of N identical cells, each containing N0 identical sites [104], This result is very interesting, since adsorption and diffusion states in zeolites have been noticed [88],... 

Typically, the UV Raman spectra of various hydrocarbons adsorbed in zeolites have been found to be similar to their spectra in solution, as a pure liquid, or as a pure solid (25). This is an important finding because the UV Raman spectra of free molecules (which are relatively quick and easy to measure) can be used for fingerprint identification of adsorbed species. One minor exception to this rule is the Raman spectrum of naphthalene, which shows some changes in the pattern of peak intensities between solid naphthalene and naphthalene adsorbed in ultrastable Y-zeolite. In this case, the adsorbed naphthalene spectrum more closely resembles that of the molecule in solution with benzene or CCI4, which suggests that interaction with the pore walls of the zeolite was similar to solvent interactions. The smaller pore diameters and pore intersections in zeolite MFI compared to Y-zeolite might be expected to produce more pronounced changes in molecular vibrational spectra as a consequence of steric interactions of the molecules with the pore walls. 

Type I isotherms [3] are char8u terized by an as)onptotic approach to a saturation cap8u ity with increasing pressure. This class of isotherms is most commonly observed for gases or vapors (water is an exception) adsorbed in zeolites or activated carbon. A t3rpical set of Type 1 adsorption isotherms is shown in Fig. 1. Several questions may be asked about sets of isotherms like these. How is the saturation capacity measured Is the saturation capau i1y a constant or does it decrease with temperature as suggested by Fig. 1 ... 

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

This review is concerned primarily with optical spectroscopic methods for characterising zeolites and molecules adsorbed in zeolites. The electromagnetic spectrum spans the range from radiofrequencies to X-radiation. Spectroscopic techniques included in this range are, in order of increasing frequency, NMR, EPR, infrared, UV-VIS and Raman, XPS, XAS and Mossbauer spectroscopies. 

The coefficients of intraciystalline self-diffusion of the n-alkanes from propane to n-hexane adsorbed in zeolite ZSM-5 are studied by means of the PFG NMR technique over a temperature range from -20 to 380 °C (96). The diffusivities are found to decrease monotonically with increasing chain lengths. Over the considered temperature range, the diffusivities in ZSM-5 are found to be intermediate between those for Na X and NaCa A zeolite, and the diffusivities of n-alkanes are independent of the Si/Al ratio of the zeolite lattice (96). 

Laser heating of the sample combined with MAS yields an enhanced resolution of the H NMR spectra of molecules adsorbed in zeolites. The fast increase of the temperature fi om a value at which the reaction is too slow to be measured to the reaction temperature allows the determination of reaction rates and hydrogen transfer rates in order of magnitude of more than one per minute. The time evolution of the system can be monitored by H NMR in steps, which are limited by the longitudinal relaxation time. 

We now report how theoretical methods can be used to provide information on the adsorption, diffusion, and reactivity of hydrocarbons within acidic zeolite catalysts. In Section A, dealing with adsorption, the physical chemistry of molecules adsorbed in zeolites is reviewed. Furthermore, in this section the results of hydrocarbon diffusion as these data are obtained from the use of the same theoretical methods are described. In Section B we summarize the capability of the quantum-chemical approaches. In this section, the contribution of the theoretical approaches to the understanding of physical chemistry of zeolite catalysis is reported. Finally, in Section C, using this information, we study the kinetics of a reaction catalyzed by acidic zeolite. This last section also illustrates the gaps that persist in the theoretical approaches to allow the investigation of a full catalytic cycle. 

A.N. Fitch, H. Jobic A. Renouprez (1986). J. Phys. Chem., 90, 1311-1318. localisation of benzene in sodium Y zeolite by powder neutron diffraction. (Somewhat less detailed is R. Goyal, A.N. Fitch, H. Jobic (2000). J. Phys. Chem., B104, 2878-2884. Powder neutron and X-ray diffraction studies of benzene adsorbed in zeolite ZSM-5.)... 

Hindered rotations of the hydrogen molecule adsorbed in zeolites represent a sensitive probe of their adsorption sites. Systematic studies of the INS spectra of hydrogen in different zeolites can lead to an... 

Aromatic molecules such as naphthalene will emit phosphorescence when adsorbed in zeolites exchanged with heavy ions such as thallium. This was exploited in a convenient, zeolite TlY-coated optical fiber format in order to detect naphthalene.[131]... 

Figure 20 H PFG NMR Fourier transform NMR spectra of an ethane-ethene mixture (1.5 molecules ethane and 1 molecule ethene per supercage) adsorbed in zeolite NaX at 293 K, with g = 2.8 Tm and A = 4 ms. The chemical shift 8 refers to TMS. (From Ref. 172.)...

The C isotropic chemical shifts of adsorbates in zeolites are typically within 3 ppm of their corresponding values in solution. Small downfield shifts attributable to hydrogen bonding are often seen, mirroring the familiar solvent effects ob.served with changes in concentration or solvent. For example. Fig. 7 shows that gas-phase methanol has a chemical shift of 48.0 ppm vs 53.1 ppm in zeolite HZSM-5 at the same temperature 31. Thus, in many cases, one can use C shifts from solution data to help assign MAS spectra of products formed during in situ experiments. Because of the small shifts mentioned above, however, it is... 

Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z" ) created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. 

Species formed from acetylene (Ay) adsorbed in zeolite Y, mordenite, beta and ZSM-5 have been studied by IR spectroscopy. The dynamics of Ay physisorption has been characterized by the frequency response method (FR). The rate of micropore diffusion governed the transport in Na-mordenite, while sorption was the rate limiting process step for all the H-zeolites. The equilibrium constants (Ka) of Ay sorption have been determined applying the Langmuir rate equation to describe the pressure dependence of the sorption time constants. The -octane hydroconversion activity of Pt/H-zeolites was found to increase linearly with the Ka of Ay sorption on the H-zeoIites. 

NMR Shifts of Xenon Atoms Adsorbed in Zeolite Pores... 

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