Aluminum tetrahedral cluster

The tetrahedral cluster compounds can be synthesized by three methods (1) reaction of the monohalides EX (X = Cl, Br, I) with alkyllithium or Grignard reagents, (2) reduction of suitable organoelement halides of trivalent aluminum, gallium or indium, and (3) thermolysis of R2E-ER2 compounds. 

The tetrahedral Al incorporated in mesoporous silica reduces considerably the quantity of amorphous carbon, increasing the MWCNTs selectivity, due to the formation of strong Bronsted acidic sites, which allow a better dispersion of iron metallic clusters. The Fe/Al-MCM41 (10) showed the best results in CNT purity and yield. This indicates that the aluminum content and its tetrahedral structural incorporation play an important role in the CNT syntheses. 

Of particular interest are the zeolites with aluminum atoms partially substituted in their framework by boron. Calculations with optimization of the geometry of clusters have shown that the energy gains in the trigonal-to-tetrahedral state transformations in the H forms of such zeolites were 1.9 and 14.1 kcal/mol for boron and aluminum, respectively. It may therefore be concluded that H forms of boralites, if they exist, should be considerably less stable than those of zeolites. 

Structure and chemical shifts of non-framework aluminum (NFA) were studied by ab-initio methods. A T6 cluster (T=Si, AI) was used to simulate the zeolite structure. The NFA are preferentially coordinated to the oxygen atoms of the framework AIO4. For 3- and 4-coordinated species, the predicted a1 NMR results show downfield chemical shifts (70-95ppm) related to the experimentally observed ones (0-60ppm). This suggests that some of the species do not exist or that they may be coordinated with water. For 5-coordinated NFA the calculated chemical shifts was between the octahedral and tetrahedral value (45-55 ppm). 

Zeolites are crystalline microporous aluminosilicate solids with a regular three-dimensional pore structure that is stable at high temperatures. Their atomic structures are based on the three-dimensional frameworks of silica and alumina tetrahedral, where the silicon or aluminum ions are surrounded by four oxygen ions in a tetrahedral configuration. Clusters of tetrahedra form different box-like polyhedral units, further linked to build up various frameworks with different channel sizes. 

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