Intracrystalline dynamics

The increase in time resolution of advanced sorption uptake methods and the joint use of sorption and radio-spectroscopic techniques allow for a more detailed analysis of the so-called "non-Fickian" behaviour of sorbing species in the intracrystalline bulk phase [18,28,29,76]. Correspondingly, information on molecular dynamics has been obtained for n-butane and 2-but ne in NFI zeolites by means of the single step frequency response method and C n.m.r. line-shape analysis [29]. As can be seen from Figures 4 and 5, the ad- / desorption for both sorbates proceeds very quickly, but with a... 

We used desorption of deactivated catalysts in vacuo at reaction temperatures into the ion source of a mass spectrometer as a method of examining desorbable intracrystalline fouling products. The method of dissolution of deactivated catalyst, followed by adsorbate analysis, that was reported by Venuto et al. (3, 4) was also used. The latter method gives composition and quantity of total adsorbate. The vacuum desorption technique provides information on the mobility—i.e., desorption dynamics, of desorbable (rather than total) adsorbed fouling products. 

Deformation by intracrystalline slip and dynamic reaystallization. These are important mechanisms for the development of crystallographic preferred orientation (CPO) and property anisotropy. Slip bands associated with intracrystalline flow may easily be seen in optical reflection microscopy of previously flattened and polished surfaces of specimens that are subsequently deformed as described above. CPO developments are less easily demonstrated, because it required the making of thin sections of deformed ice and the use of a simple universal stage to determine the orientation of the crystallographic c axis. 

Abstract As a non-invasive technique, NMR spectroscopy allows the observation of molecular transport in porous media without any disturbance of their intrinsic molecular dynamics. The space scale of the diffusion phenomena accessible by NMR ranges from the elementary steps (as studied, e.g., by line-shape analysis or relaxometry) up to macroscopic dimensions. Being able to follow molecular diffusion paths from ca. 100 nm up to ca. 100 xm, PPG NMR has proven to be a particularly versatile tool for diffusion studies in heterogeneous systems. With respect to zeolites, PFG NMR is able to provide direct information about the rate of molecular migration in the intracrystalline space and through assemblages of zeolite crystallites as well as about possible transport resistances on the outer surface of the crystallites (surface barriers). 

Fig. 25 Dependencies of the diffusion coefficients of n-butane in silicalite-1 on the root of mean square displacements at different temperatures and comparison with the results of dynamic MC simulations for a barrier separation of 3 jxm with the assumption that jumps across the barriers occur with an activation energy exceeding that of intracrystalline diffusion by 21.5 kj mol h Filled and open symbols correspond to measurements performed with two different samples of silicalite-1. From [216,217], with permission...

On the other hand, if the time scale of diffusion in the crystal is very long compared to that in the particle the system dynamics will be controlled by the intracrystalline diffusion with the characteristic length being the crystal dimension. The initial stage of the dynamics is the filling of pore space of macropore with adsorbate, which is very fast and usually of the order of one second. Furthermore, this capacity is usually very small compared to the capacity of the micropore hence the initial stage is usually not measurable. 

For many zeolitic systems the equilibrium isotherm can jbe represented in an approximate way by the Langmuir model while the intracrystalline diffu-sivity varies with concentration according to Eq. (6.12). Model 2c is intended to describe the dynamic behavior of such systems under conditions of intracrystalline diffusion control. 

It is well known that zeolites enhance selectivity based on the size of their intracrystalline pores. Zeolite crystals exclude or capture molecules based on the ratio of molecule size to pore size. Measurement of pore size by crystal size (e.g.. X-ray diffraction) fails to account for the influence of the dynamics of the crystal structure, the dynamics of the sorbing molecules or the interaction between zeolite pore and sorbed molecule. The crystals and/or sorbed phase after sorption may be stmcturally different from the bulk diase and/or unfilled zeolite. The pore sizes determined in an X-ray analysis may be different from those present during soiption. Thus, it is preferred to study zeolite morphology by a combination of structural and sorption analysis. In this manner, it is possible to study both the state of the zeolite crystals and the state of the sorbed phase and to infer how these influence the amount of sorption of gas phase molecules and the effective micropore size. 

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