Beds of zeolite

Introduction of PFG NMR to zeolite science and technology has revolutionized our understanding of intracrystalline diffusion [19]. In many cases, molecular uptake by beds of zeolites turned out to be limited by external processes such as resistances, surface barriers or the finite rate of sorbate supply, rather than by intracrystalline diffusion, as previously assumed [10, 20-24]. Thus, the magnitude of intracrystalline diffusivities had to be corrected by up to five orders of magnitude to higher values [25, 26],... 

MOG [Mobil olefins to gasoline] A process for converting dilute streams of C2- to C4-hydrocarbons to gasoline, using a fluidized bed of zeolite ZSM-5 catalyst. Developed by Mobil Research Development Corporation and piloted in 1990. 

Figure 3.1 Principal mass transfer resistances in a packed bed of zeolite beads [69].

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

Pulsed-field-gradient NMR (291-292), in which spin echoes are measured in the presence of a time-dependent magnetic field gradient, has been used to determine effective diffusion coefficients, Deff, in beds of zeolite powder. Barrer (35) quotes the expression for the spin-echo amplitude given by Karger (259) in the form ... 

In principle, the determination of molecular uptake may be based on any experimentally accessible quantity which is a function of the amount adsorbed. Being directly sensitive to a certain molecular species, in this respect the application of spectroscopic methods is particularly suitable. IR spectroscopy has been successfully applied to studying molecular uptake by beds of zeolite catalysts [26-28] as well as—in combination with IR microscopy [29, 30]—on individual crystallites. Similarly, NMR spectroscopy has also been used to monitor the time dependence of the sorbate concentration within porous media [31]. Moreover, recent progress in NMR imaging allows the observation of concentration profiles within porous media with spatial resolution below the mm region [32-34],... 

The process of adsorption of n-hexane in a bed of zeolite Na X was monitored by NMR imaging in combination with PFG NMR (99).The intracrystalline diffusivities were found to depend exclusively on the given sorbate concentration, independent of the time interval elapsed since the onset of the adsorption process. The authors may conclude that adscnhent accommodation during the process of adsorption is not of significant influence on the molecular mobility (99). 

It is primarily the span of absolnte pressnres, conpled with differences in mole fractions (caused by pressure shifts and/or by admitting different streams to the column), that drive PSA separations. Equilibrium selectivity can canse composition shifts to occur simply by changing the pressure. To illustrate this point, consider an adsorbent bed of zeolite 5A filled with air (assnmed to be only oxygen and nitrogen) at 3 atm. The mole fractions in the gas phase of oxygen and nitrogen are... 

Since the chemical shift of Xe also depends on the nature and the concentration of molecules adsorbed in addition to xenon within the zeolite structure [30,39], Xe NMR may be applied as well to map the sorbate concentration within adsorbent-adsorbate systems [40-43]. The principle of this method is illustrated in Fig. 5, which shows the Xe NMR spectra and the corresponding distribution of the adsorbate under study (hexamethylbenzene) within a bed of zeolite NaY [42]. The spectrum shown in Fig. 5a was measured after a finite amount of liquid hexamethylbenzene was introduced into the sample, which... 

In many cases the rate of molecular mass transfer through the bed of zeolite crystallites (see Sec. III.D.) is found to be so fast that the propagator determined in the PFG NMR experiment may be easily separated into its two constituents. 

As a noninvasive method, NMR spectroscopy provides ideal conditions for the in situ measurement of the dynamical and structural properties of systems undergoing internal changes. This is true in particular for the observation of catalytic reactions in beds of zeolite crystallites (204-208] and, very recently, of the space... 

H NMR imaging has been used to study the diffusion of pure hydrocarbons (benzene, n-hexane,) during their adsorption in or desorption from a fixed bed of zeolite crystallites. This technique is used to visualize the progression of the diffusing molecules in the zeolite bed and to determine their intracrystallite diffusion coefficients. In the case of competitive adsorption, it gives the time dependence of the distribution of the two coadsorbed gases. 

As an example. Fig. 1 shows the propagator representation of molecular self-diffusion of ethane in beds of zeolite Na,Ca-A with two different crystalhte sizes. Being symmetric in z, for simphcity the propagator is only represented... 

Fig. 2 Parameters of molecular transport in beds of zeolite crystallites as accessible by PPG NMR measurements...

The influence of confining boundaries on the effective diffusivity as reflected by Eqs. 15a,b has been repeatedly applied to determine the pore surface in rocks or beds of sand grains [106-115]. In [116], this concept has been for the first time successfully applied to beds of zeolites. Figure 9 shows the results of these studies, which have been performed with two different samples of Na-X, one loaded with n-hexane (two molecules per supercage), the other with n-hexane and hexafluoromethane (one molecule per supercage for either). In both samples, for the n-hexane measurements a temperature of 298 K was chosen, where the n-hexane molecules were found to be totally confined so that the data analysis could be based on Eq. 15a. The measure-... 

Fig. 18 Apparent coefficients of intracrystalline self-diffusion of n-hexane as observed by time- and space-resolved PFG NMR in a bed of zeolite Na-X with restricted ( ) and unrestricted ( ) sorbate supply in dependence on the sorbate concentration. The real diffusivities open symbols) were calculated from these values by using the correspondence presented by Fig. 3. The full line with the indicated error bars represents the range of intracrystalline diffusivities as observed in previous PFG NMR studies with closed sample tubes. From [163] with permission...

NMR of adsorbed xenon and NMR imaging new methods to stndy the diffusion of gaseous hydrocarbons in a fixed bed of zeolite... 

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