Framework atoms, octahedrally coordinate

The 27Al MAS NMR spectra of as-synthesized Al-MCM-41 samples show a main signal of tetrahedral coordinated aluminum at ca. 53 ppm. The aluminum is nearly entirely incorporated into the framework. As generally observed, calcination leads to the formation of penta- Al[51 and octahedrally coordinated aluminum Al[61 on the expense of the tetrahedral framework Al[41. In the 27A1 MAS NMR spectra of calcined samples appear additional lines at ca 30 ppm and 0 ppm, respectively (Fig. 1). Only ca. 1/3 of the Al atoms remain in tetrahedral framework positions even after thermal treatment at 600°C. 

CaX and SrX, but it is much more pronounced for nickel faujasite. The Ni(II) occupies two-thirds of the SI sites and attains nearperfect octahedral coordination with six framework oxygens, with all six oxygen atoms shifting. 5 A towards Ni (Fig. 3). The remaining Ni(II) ions are distributed in four unique positions, two in SI sites and the other two in SII and SII sites (Fig. 4). 

A hydrotalcite is regarded as a very basic support. It consists of platelets in a brucite structure, with Mg2+ ions octahedrally coordinated by O atoms. Part of the Mg2+ ions are isomorphically exchanged for Al3+ ions, creating a positive charge on the framework. Between the platelets, this positive charge is compensated with CO3" or OH ions. The OH ions located at the edge... 

NMR-characterization. 27 Corbin et al. (35) were able to show by a systematic study that Z/A1 MAS NMR gives the true Si/Al ratio with a mean error of 10 %, if two conditions are met a) The amount of paramagnetic species is less than 0.05 % and b) the sample does not contain "NMR-invisible" aluminium. Chemical analyses of the samples under study showed that condition one is fulfilled. If samples contain "NMR-invisible" aluminium a difference between the concentration determined by chemical analysis and the framework aluminium concentration determined by NMR should be observed. From the absence of such a difference we conclude that "NMR-invisible" aluminium species do not exist in our samples. Also a line at the position of about 0 ppm due to octahedrally coordinated non-framework aluminium and a broad line at about 30 ppm due to tetrahedrally coordinated nonframework aluminium (36) could not be observed. The values for the concentration of framework aluminium atoms derived from the intensities of the line at about 60 ppm (see below) are in good agreement with those corresponding to the amount of alumina used in the synthesis mixtures. In conclusion, through the Al MAS NMR measurements it was possible to show that all aluminium atoms are incorporated in tetrahedrally oxygen coordinated framework positions. 

As already described, the counterion can template the formation of different oxalate frameworks. For example, the synthesis of a heterobimetallic 3D framework K" [Cr " (ox)3] was achieved with [Cu(trans[14]dien)] " (trans[14] dien = 5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclotetradeca-4-11-diene) (66). While each and Cr atom is octahedrally coordinated, each Cu " " ion coordinates an oxalate oxygen atom in a monodentate fashion producing 21 X 9 A helical channels (Fig. 4). This generates a ths net (7) structure, while the previous 3D oxalate networks consist of chiral srs nets. Magnetic studies showed an antiferromagnetic interchain interaction between the copper and chromium centers. 

Structural information on transition-metal binary fluorides - shows that from group V through to the end of each transition series the transition-metal atom is coordinated by six fluorine ligands (F) on an octahedral framework. This is so for hexafluorides, pentafluorides, tetrafluorides, or trifluorides. 

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