ALUMINIUM DISTRIBUTION

Aluminium distribution in A and B1 intermediate and final phases Various compositional zonings have been reported in silica-rich ZSM-5 zeolites prepared under particular conditions (11-13,19,26, 27,31,32,59,68) while other studies provided evidence for homogeneous A1 distribution throughout the crystallites (69-71). Obviously, the distribution of aluminium in ZSM-5 must depend on its mechanism of crystallization. 

Cruz, ).M., Corma, A., and Eornes, V., Framework (1989) Framework and extra-framework aluminium distribution in (NHdjSiFs-dealuminated Y zeolites relevance to cracking catalysts. Appl. Catal, 50, 287. 

Microprobe analysis of HZSM-5 samples from a Li-NH -TPA system (sample A), see Figure 7, gave rise to a characteristic compositional zoning of A1 in the outer rim of the crystals. HZSM-5 from alkaline-free processes (sample B), see Figure 8, had a more homogeneous aluminium distribution throughout the crystals. 

Si Al ratio of the outer layers (ca. 10 A) of the zeolite crystals. The extent to which the surface composition differs from the bulk composition appears to depend on preparation conditions, and all three possible situations (silicon rich surface, silicon deficient surface and surface composition equal to bulk composition) have been reported (refs. 12-14). Variations in aluminium distribution have also been probed by high resolution scanning electron microscopy (ref. 15) and energy dispersive X-ray analysis (ref. 16). 

Crapper DR, Krishnan B, Dalton AJ Brain aluminium distribution in Alzheimer s disease and experimental neurofibrillary degeneration. Science 180 511-513,1973 Crapper DR, Krishnan SS, Quittkat S Aluminium, neurofibrillary degeneration and Alzheimer s disease. Brain 99 67-80,1976... 

On the basis of our recent investigations concerning the realuralnatlon of dealumlnated zsM-5 zeolites in alkaline me-diuihf the provable phosphorus Insertion Is coupled with the presence of lattice defects In the activated zeolites. The concentration of lattice defects and their distribution depends on the aluminium distribution in the parent zeolite and the kind of pretreatment. 

It is apparent that more detailed information is needed of aluminium distribution in the plasma of normals and patients on chronic intermittent hemodialysis. Such information would clarify the variability of reports on aluminium loading during hemodialysis. Recent work in the authors laboratory (King et al., 1982 1979) has been directed towards an attempt to separate aluminium species in plasma into more than just ultrafiltrable and protein-bound fractions. The approach used to define the plasma distribution and binding of aluminium was to employ gel filtration under equilibrium conditions which was a technique used previously in our laboratory for studying the distribution of calcium in plasma CToffaletti et al., 1977). 

P-19 - Aluminium distribution in high silica pentasil ring zeolites... 

Monte Carlo calculations by Soukoulis indicate that the aluminium distribution in zeolite frameworks is a function of synthesis temperature. The location of aluminium within the framework has also been addressed using cluster calculations and ab... 

Solid-state NMR can, indirectly, provide information on aluminium distribution and the relative intensities of the Si(nAl) configurations suggest interesting differences in the distribution of aluminiums in synthetic products as compared with those produced by secondary synthesis. Relative populations of Si(lAl) are higher, and those of Si(OAl) are lower, for zeolites prepared by primary synthesis than for zeolites produced by secondary synthesis. 

Consequently, diese results do reflect differences in aluminium ordering the increased Si(lAl) and reduced Si(0Al) suggesting that isolated aluminium sites are relatively more numerous for products made by primary synthesis than they are for materials of similar compositiai made by secmidary synthesis. It is of interest to know whether these differences in aluminium distribution are reflected in catalytic activity. From preliminary studies of n-hexane cracking at low conversion it does appear diat the synthetic products are the more active and it is tempting to ascribe this to differences in aluminium ordering. However, this conclusion should be taken as tentative until other factors which can infiuence rates are exduded [59]. 

Equation 4.1 implies that two prerequisites are fulfilled the critical aluminium content has to be known and the aluminium distribution throughout the whole sample has to be uniform, so that the critical aliuninium content takes the same value at the centre and at the periphery of the sample. 

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