Diffusion benzene-silicalite

TTie diffusion coefficients obtained for benzene diffusing a Silicalite-1 and HZSM-5 in the present study agree well with those reported in the literature.[7] Figure 10(a) shows that our... 

Possibly the earliest theoretical study of diffusion of aromatics in zeolites was published in 1987 by Nowak et al. (89), who considered diffusion of benzene and toluene in the pores of silicalite and theta-1. Theta-1 (90) has a unidimensional medium-sized pore opening bounded by 10-rings. In this study, only the straight channel of silicalite was considered, making the... 

Similar energy minimization calculations were reported for benzene and p-xylene in silicalite (92). Diffusion coefficients were estimated from minimum energy paths through the pore. The value for benzene, 27.6 kJ/mol, is in good agreement with that of Pickett et al. (91). For the bulkier p-xylene molecule, the activation barrier was predicted to be slightly lower (23 kJ/... 

The predicted diffusion behavior of benzene showed the familiar anisotropy observed for other molecules in silicalite, although evolving over a far longer time period. The calculated diffusion coefficients at temperatures between 200 and 500 K varied by 6 orders of magnitude. The orientationally averaged value at 300 K is 1.1 X 10-12 m2/s, approximately 1-2 orders of... 

The complexity of modeling the adsorption of benzene in silicalite has already been discussed in the section concerned with diffusion. A TST study by Snurr et al. (106) led to the identification of 27 unique sorption minima in the asymmetric unit. Given this result, it is unsurprising that there have been relatively few simulation studies of this system. However,... 

Ga-MFI zeolites have been reported to exhibit catalytic activity in conversion of light paraffins to aromatics [3] and therefore the diffusion data of aromatics are of interest to explain the mechanism of the catalytic reaction.There has been done much of both theoretical and experimental investigations on the motion of aromatics (benzene, toluene, xylene etc.) in ZSM-5 zeolite and silicalite crystals cf.e.g. 

Figure 3 compares the self-diffusivities determined on the basis of NMR tracer exchange measurements involving benzene in silicalite [61 with the results... 

Fig. 12 Comparison of corrected diffusivity of benzene in silicalite-1 and H-ZSM-5 at low sorbate concentrations. Van Den-Begin et al., square wave o Eic and Ruthven, ZLC A, V Zikanova et al., piezometric silicalite-1, + H-ZSM-5, SSFR NMR tracer exchange. From Shen and Rees [69]...

Abstract Infrared spectroscopic methodsfor the measurement of adsorption and adsorption kinetics of some aromatics (benzene, ethylbenzene, p-xylene), pyridine, and paraffins in solid microporous materials such as zeolites (MOR, ZSM-5, silicalite-1) are described as well as the evaluation of the spectroscopically obtained data. The adsorption isotherms are of the Langmuir-Freundlich type. Isosteric heats of adsorption, transport diffusivities, and activation energies of diffusion as deduced from the spectroscopic measurements are compared with literature data as far as available, and they are found to be in reasonable agreement with results provided by independent techniques. Special attention is paid to sorption and sorption kinetics of binary mixtures, especially the problems of co- and counter-diffusion. ... 

Table 4 I Diffusivities of benzene in H-ZSM-5 or silicalite-1 and comparison with literatme ...

Fig.lO Normalized corrected diffusivities obtained experimentally for several systems ( ) CF4 in silicalite, (Q) benzene in Na-Y, (A) CO2 in silicalite, (O) Ar in silicalite, (V) N2 in silicalite, ( ) D2 in Na-X. The solid line gives the (1-0) dependence, and the dotted line the Darken approximation ... 

The effect of subtle differences in certain properties of sorbate molecules on the diffusivities can be very intriguingly demonstrated in Fig. 16. With shape and size close to that of p-xylene, the long, rigid molecule, p-dichloro-benzene (p-DCB) displays an FR behaviour in silicalite-1 very similar to that of p-xylene. The flexible saturated cyclic hydrocarbons diffuse much more slowly within the channel framework of silicalite-1 than their rigid aromatic... 

ABSTRACT. The principle features of the frequency-response i paratus developed at Imperial College is described. The apparatus has been used in both its a) full and b) single-step frequency modes to determine the diffiisivities of various hydrocarbons in silicalite-1. Die effect of temperature and loading of sorbate in the silicalite-1 has been ascertained. The effect of the introduction of A1 atoms into the framework of silicalite-1 on the diffusivity of benzene has been detmnined. The diffusion of benzene in NaX has been studied and diffusion coefficients obtained which agree with NMR pulsed field gradient measurements, n-Butane and 2-butyne hydrocarbons were found to generate out-of-phase response curves by the full FR method which could only be fitted by introducing two diffusion coefficients into the solution of the appropriate diffusion equation. 

TABLE 2. Corrected diffusion coefficient, D, of benzene in Silicalite-1... 

In conclusion, therefore, these studies have shown that benzene diffuses 2-3 orders of magnitude faster in the large channels of NaX at 400K than in the smaller channels of ZSM-5/Silicalite-l. Secondly, the single-step method can follow sorption/desorption processes which only take -200 ms to attain equilibrium. 

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