Mixed framework structures

The new phases were discovered by the combination of exploratory synthesis and a phase compatibility study. As commonly practised, the new studies were initially made through the chemical modification of a known phase. Inclusion of salt in some cases is incidental, and the formation of mixed-framework structures can be considered a result of phase segregation (for the lack of a better term) between chemically dissimilar covalent oxide lattices and space-filling, charge-compensating salts. Limited-phase compatibility studies were performed around the region where thermodynamically stable phases were discovered. Thus far, we have enjoyed much success in isolating new salt-inclusion solids via exploratory synthesis. 

Mixed framework structures containing octahedra and tetrahedra. Silicates,... 

In addition to the influence of neighbors on 29Si chemical shifts, the geometrical effects (such as Si-O-T angles) already described above are also evident of mixed frameworks with elements other than Si on tetrahedral positions. This is reflected by the broadness of the bars shown in Fig. 1. Multinuclear NMR investigations on a large set of sodalite structures with various framework compositions show that T-O-T bond angle (T = Si, Al, Ga) and dTT distance chemical shift dependences exist, and mutual correlations between chemical shift of these NMR nuclei can be observed [68],... 

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

For the Et.N /(n-Pr) N cation pair the formation of the ordered structure of BOR-C is consistent with expectation, the same structure being formed in the presence of the individual cations. The (n-But) N cation favors the formation of disordered framework structures, evident from the synthesis of BOR-D, when mixed witlji Me N and Et.N but not (n-Pr) N. In the latter case, the (n-Pr) N cation is greatly favored in the crystallization process and the... 

The positions of non-framework cations in aluminosilicate zeolites can control or fine-tune their sorptive and catalytic properties. Measurement, however, requires careful and usually protracted analyses of accurate single crystal or powder dif action data. In cases for which extensive experimental data are available, statistical mechanics analyses can yield insight into relative site energies [53-55] etirlier analyses have also attempted to quantify the relative importance of short and long-range interactions in controlling site occupancy patterns [56]. Earlier atomistic simulations in this area [57-62] had mixed results. Recent developments in methods and interatomic potentials have allowed non-framework cation positions to be simulated based solely on a knowledge of the framework structure in zeolite systems for which validatory experimental data are available [113]. 

The structures of ulrichite, Cu[Ca(H20)2(U02)(P04)2](H20)2, and of the mixed-valence synthetic compound [U (U02)(P04)] also exhibit substitution for uranyl in the uranophane sheet-anion topology (Fig. 46). In ulrichite, CaOs polyhedra alternate with uranyl pentagonal bipyramids, and the sheets are connected by Jahn-Teller-distorted Cu-centered octahedra [177]. In the structure of [U" (U02)(P04)], both and are in sevenfold coordination to form pentagonal bipyramids. Dimers of polyhedra alternate with dimers of uranyl pentagonal bipyramids along the uranium phosphate chains that make up the sheet (Fig. 46). The polyhedra also share vertices with tetrahedra of adjacent sheets to form a framework structure [178]. 

One feasible method for the exploration of chiral open-framework compounds is the use of chiral chemical units as primary building blocks by coordinating with metal or other assembly methods to form 2-D layer or 3-D open-framework structures with optical activity. A notable example is the enantiomerically pure zinc phosphonate based on a mixed phosphonic acid-phosphine oxide chiral building block reported by Bujoli and coworkers in 2001.[91] The reaction procedures are shown as follows. 

Mixed-valence transition metal phosphates have been found for Ti, V, and Nb. Their crystal chemistry and properties are more closely related to those of the Mo analogs in that they have mixed frameworks of MOe octahedra that are partially or wholly isolated from one another by intervening phosphate groups and the d electrons do not appear to be delocalized. A particularly rich chemistry exists for the vanadium analogs. Details of their structural properties have been reviewed by Borel and coworkers. ... 

The high Ti 2p BE assigned to framework Ti by most groups is probably not linked either with the framework structure or with the tetrahedral Ti coordination Similar values have been measured also with Ti02-Si02 mixed oxides prepared by precipitation [145] or sol-gel techniques [146] and glasses [147,148], and with Ti-Si minerals containing Ti in octahedral coordination [136,139]. The... 

The addition of organic cations resulted in the preparation of an interrupted framework structure which contains beryUium ions coordinated to water and/or P-OH groups [ 167]. In a mixed NH4/di-isopropylamine system the beryllophosphate obtained did not incorporate the organic moiety but did take up the ammonium cation [168]. Thermal treatment of this material showed a decomposition of the ammonium cation around 600 C. 

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