Zeolites mobility measurement

Molecular mobility measurement, hydrocarbons in zeolites, 39 351-410 benzene... 

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

Molecular Mobility Measurement of Hydrocarbons in Zeolites by NMR Techniques... 

SIMS (61,64,86), microscopy (65), XPS (56), electron microprobe techniques (14,66), electron paramagnetic resonance (EPR) (67) and luminescence experiments (68) have been successfully employed to probe and study V mobility and reactivity on a catalyst surface. TEM, STEM and energy dispersive X-ray emission (EDX) measurements have indicated that V interaction with REY-crystals induced vanadate clusters formation (65). Vanadium was also found capable of reacting with rare-earths outside the zeolite cavities to form LaVQ4... 

To understand the effects of the different ligands on V-migration during thermal treatments, vanadyl naphthenate precursor has also been used as dopants. The results for two calcined samples (EuYV(n)AAAC and AAAV(n)EuYC), are presented in Figs. 7 and 8, respectively. In contrast to the results shown in Fig. 1-6, no measurable V migration occurs from the zeolite to the matrix (Fig. 7) and only a trace of V moves from matrix to the zeolite (Fig. 8). Thus vanadium precursors can alter V mobility during calcination... 

The adsorption behavior of benzene on dehydrated NaX and NaY zeolites has been investigated directly by H and 13C NMR measurements of the adsorbed benzene and indirectly by the combination of 129Xe NMR and isotherm measurements of the co-adsorbed xenon. Powdered zeolite samples of various Si/Al ratios and with varied adsorbate concentrations were investigated. Detailed macroscopic and microscopic adsorption phenomena of benzene in NaX and NaY zeolites, including the loading capacity, mobility, and sites of adsorption are presented in terms of measurements of NMR linewidths and chemical shifts. 

The agreement between the experimental data and the equation achieved using the assumption of cation-hopping conduction [114] testify that this hypothesis is in concordance with the experimental data. It is interesting to recognize the difference between the measured apparent activation energies for sodium and calcium zeolites (see Table 4.10 [114]). As was hitherto observed for the polarization phenomena, Na+ cations are more mobile than Ca2+ cations this effect also explains the observed differences in the measured apparent activation energies for cationic conduction for Na+ and Ca2+. 

At this time, the locations of cations in zeolites have been determined primarily by X-ray diffraction (XRD) techniques. Unfortunately, this method has the drawback of being able to locate only the most stationary cations in zeolites. In some studies of hydrated zeolites, less than 50% of the total cation population can be accounted for. A higher percentage of the cations can be located in dehydrated samples, but the effect of the dehydration step on the location of the cations is generally not well known. NMR measurements, on the other hand, are most sensitive to mobile cations and cations in high symmetry sites. 

Ail interpretations based on the assumptions with respect to use of the Nernst-Planck relationships are, however, subject to sizable uncertainty because the constancy attributed to the diffusion coefficients used in these relationships is susceptible to sizable variability. Bead volume variations are ignored. This leads to variations in ion mobilities which dictate changes in the diffusion coefficients of the Nemst-Planck relationships. Sizable divergence of measurements firom prediction can be expected on this basis. Even in rigid ion exchangers such as zeolites the difference in size of the counterions exchanged usually affects their mobility, and so leads to variations of the diffusivities [3]. 

In general, catalyst type does not appear to effect VMI, since units using identical catalysts may have very different VMIs. However, in certain cases, the catalyst may have an affect. Two examples are shown in Table 1, refinery 19 and 20. Unit 19 switched from a base catalyst to a catalyst with a vanadium trap, and the VMI dropped dramatically. This result is not unexpected, since a vanadium trap, when operating properly, is expected to immobilize vanadium so it cannot destroy zeolite and produce coke and gas. Unit 20 was a unit starting up. Initially the VMI was low, but the inventory was largely start-up Beat and the VMI being measured was likely a combination of the Unit 20 and the unit where the Beat was from. However, the VMI increased as the Beat left the unit and the higher VMI is likely more representative of the actually unit V mobility. 

The variable temperature measurements of MAS NMR offer very variable information on the dynamic character of the protons of zeolites as solid-acid catalysts. The temperature-dependent lineshape of MAS NMR spectra of acidic protons (acidic OH groups) in zeolites shows the first and unambiguous evidence for proton migration in zeolite structure. There is a possibility that the mobility of protons influences a catalytic activity of solid-acid catalysts. 

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