Void volume intracrystalline

The NMR chemical shift of I29xe adsorbed on molecular sieves reflects all the interactions between the electron cloud of the xenon atoms and their environment in the intracrystalline void volume [1]. This nucleus therefore proved to be an ideal probe for investigating various zeolitic properties such as pore dimensions [2, 3], location of the countercations [4, 5], distribution of adsorbed or occluded phases [6-8] and framework polarisability [8, 9]. 

A Correlation of the Calculated Intracrystalline Void Volumes and Limiting Adsorption Volumes in Zeolites... 

For the acid catalysed conversion of hydrocarbons, the reaction mechanisms in absence of sterical hinderance are rather well understood, so that molecular shape-selective effects exerted by constrained environments can be isolated [8,9]. Shape-selective catalysis is also possible when other than acid functions are confined to the intracrystalline void volumes of zeolite crystals, e.g. metal [10,11], bifunctional [12] and basic functions [13]. Nowadays, catalysis on zeolites with organic substrates containing heteroatoms receives much attention. Molecular shape-selectivity seems to be superimposed on electronic factors determining the selectivities [14,15]. 

Since K+ cations block the access of the intracrystalline void volume by H2S molecules, the rate of their transformation into COS is low the conversion value given in Figure 5 is correct for an exposure time of 5 h after ca. 300 h about 65 % of H2S were transformed into COS. So-called shrunk NaA zeolites with narrowed micropore apertures behave similarly. 

SIM experiments for binary N2-O2 mixtures on Li,RE-LSX zeolite were performed in conjunction with single-component investigations. Mixture measurements are exemplified by isosteres for a sorption-phase composition of 80 % N2 and 20 % O2 as shown in Figure 24, over the entire concentration ranges for zeoUtic intracrystalline void volume up to filling secondary pore volumes of the beads, as also observed from isosteres. In those representations, each Une of symbols is one isostere measured at the respective sorption-phase concentration. As the latter concentration approaches saturation, the orresponding isostere approaches the sublimation curve of either the single component or the binary mixture. The coincidence between isostere and sublimation curve beyond saturation capacity proves that isosteric measurements were correct and thermodynamically consistent. 

In this paper only phenomena occurring on level (iii) will be discussed. In general, if sorption kinetics on porous solid particles are considered, one is dealing with complex physical phenomena which are often superimposed by external, e.g. apparatus effects. In order to obtain the correct diffusion data for the intracrystalline MS void volume, one should start therefore, from... 

It has been concluded that, for most cases, catalysis over zeolites occurs within the intracrystalline voids. Strong supporting evidence for this was provided by Weisz (71), who compared the rates of dehydration of 2-butanol over Linde lOX and 5A zeolites at relatively high temperatures and low conversion. The rate constant per unit volume of 5A was 1/lOO-l/lOOOth that for lOX, a magnitude consistent with the ratio of available surface areas for the external area of 1-5/x-sized 5A crystals and for lOX, where the internal surface area was available to the alcohol. The strong driving force for occlusion within the intracrystalline zeolite voids is exemplified by the rapid adsorption kinetics and rectangular adsorption isotherms observed for molecules whose dimensions are not close to those of the entry pores. 

Several authors have reviewed established and promising applications in this field [2-A]. The present paper is not comprehensive, but is intended to offer some perspective, specifically, concerning methods of functionalizing the intracrystalline surfaces of zeolites, the chemical and physical nature of the interactions between the occluded species and the zeolite framework, and some principles describing the formation and behavior of organometallic compounds within the internal void volumes of zeolites. 

---