Tetrahedrally coordinated aluminum

Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

Fig. 22-3. A tetrahedral complex aluminum with coordination number 4.

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

Zeolite structures typically consist of silicon and aluminum finked by tetrahedrally coordinating oxygen atoms. However, similar structures as found for these aluminosilicates can be formed by substitution of the aluminum by other elements (e.g., Ga in gallosilicates or Ti in titanosilicates). Even the substitution of both Si and A1 is possible, as for example in aluminophosphates or... 

The micas have layer structures in which silicate sheets are combined with aluminate units the aluminum ions can be octahedrally as well as tetrahedrally coordinated. For example, the mica muscovite contains both octahedral and tetrahedral Al3+ ... 

Figure 4 shows the27 A1 NMR spectrum of a calcined sample. There are three peaks visible, a peak due to octahedrally coordinated aluminum (Oh), a peak due to tetrahedrally coordinated aluminum (Td) and a peak in between due to highly distorted tetrahedral sites. The tetrahedrally coordinated aluminum can be assumed to be incorporated into the aluminosilicate network while the octahedrally coordinated aluminum is occluded in the pores or exists as an amorphous by product. 

The influence of calcination and different hydrothermal treatments on the coordination of aluminum in MCM-41 molecular sieves is investigated by 27A1 MAS NMR and FTIR spectroscopy using ammonia as probe molecule. Samples are further characterized by XRD and 29Si MAS NMR spectroscopy. It is found that non-tetrahedral Al species formed during calcination can be reversible transformed into tetrahedral coordinated framework Al by hydrothermal treatment. Re-inserted Al gives rise to Bronsted acidity. 

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. 

These NMR results are confirmed by the IR spectra of ammonium-exchanged Al-MCM-41. In the spectral range of NH deformation modes, a vibration band at ca. 1450 cm 1 is observed which is assigned to the deformation mode of ammonium ions (BS band). Its presence evidences the incorporation of A1 into tetrahedrally coordinated framework positions, because ammonium ion exchange requires a negatively charged framework. The intensity of the NH4+ deformation mode increases with increasing aluminum content until a Si/Al (gel) ratio of 7.3. 

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