Aluminophosphate

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

Additional to tire aluminosilicate-based zeolites, a number of otlier crystalline microporous tliree-dimensional oxides have been syntliesized [25]. Most prominent among tliese are tire aluminophosphates (ALPO series) [26,... 

The introduction of phosphate into the framework has led to an extraordinary expansion of the possible pore diameters in framework molecular sieves. The discovery of VPI-5, the first 16-membered-ring aluminophosphate framework, was a landmark (8). Although it is a neutral framework, it... 

There are numerous stmctures that are similar to 2eofites, such as aluminophosphate molecular sieves, AlPOs, but these have not found catalytic apphcations. Zeofites can be modified by incorporation of cations in the crystalline lattice which are not exchangeable ions, but can play catalytic roles. For example, sificahte, which has the stmcture of ZSM-5 but without Al, incorpora ting Ti in the lattice is a commercial catalyst for oxidation of phenol with H2O2 to give diphenols the catalytic sites may be isolated Ti cations (85). 

Other materials, closely related to zeoHtes, with zeo-type structures are silico-aluminophosphates (SAPOs) and aluminophosphates (ALPOs) [6]. 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

An example of novel basic catalysts the aluminophosphate oxynitrides or "AlPONs"... 

New aluminophosphate oxynitrides solid basic catalysts have been synthesised by activation under ammonia of an AIPO4 precursor. When the nitrogen content increases, XPS points out two types of nitrogen phosphorus bonding. The conversions in Knoevenagel condensation are related to the surface nitrogen content. Platinum supported on aluminophosphate oxynitride is an active catalyst for isobutane dehydrogenation. 

Nitridation of the oxide precursor was performed under pure ammonia flow. Different aluminophosphate oxynitrides AlPONs with variable nitrogen contents were obtained by modif3ring the time and/or the temperature of nitridation (Table 1). 

Four mmoles of malononitrile and benzaldehyde were introduced in a batch stirred tank reactor at 323 K with toluene as solvent (30 ml). Then 0.05 g of aluminophosphate oxynitride was added. Samples were analysed by gas chromatography (Intersmat Delsi DI200) using a capillary column (CPSilSCB-25 m). Care was taken to avoid mass or heat transfer limitations. Before the reaction no specific catalyst pretreatment was done. 

To evaluate properties of basic catalysts, the Knoevenagel condensation over aluminophosphate oxynitrides was investigated [13]. In this reaction usually catalysed by amines, the solid catalysts function by abstraction of a proton from an acid methylene group, which is followed by nucleophilic attack on the carbonyl by the resultant carbanion, re-protonation of oxygen and elimination of water. The condensation between benzaldehyde and malononitrile is presented below. 

A novel basic support and catalyst have been prepared by activation of aluminium phosphate with ammonia. Fine control of time and temperature allows to adjust the 0/N ratio of these oxynitride solids and thus to tune the acid-base properties. The aluminophosphate oxynitrides are active in Knoevenagel condensation, but a basicity range can not yet determined. Supporting Pt or Pt/Sn on AlPONs allows to prepare catalysts that are highly active and selective in dehydrogenation reactions. 

Although this account of gelation is made with reference to organic polyelectrolytes, it is of wider application and may be applied to phosphoric acid cements. Orthophosphoric acid solutions used in these cements contain aluminium, and soluble aluminophosphate complexes are formed. Some appear to be multinuclear and there is evidence for polymers based on the bridging Al-O-P unit. These could be termed polyelectrolytes (Akitt, Greenwood Lester, 1971 Wilson et al., 1972 O Neill et al., 1982). 

Sveshnikova, V. N. Zaitseva, S. N. (1964). Aluminophosphates as polyelectrolytes. Russian Journal of Inorganic Chemistry, 9, 672-5. 

The addition of aluminium to the liquid slowed down the reaction. An amorphous cement was formed and there was no crystallization in the bulk of the cement. However, after some time crystallites were formed at the surface. Thus, the presence of aluminium exerts a dedsive influence on the course of the cement-forming reaction. This effect is to be attributed to the formation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1). These complexes may delay the predpitation of zinc from solution and also introduce an element of disorder into the structure, thus inhibiting crystallization. It is significant that zinc, which does not form complexes, has little effect on the nature or speed of the reaction. 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

As we have seen in Section 6.2, there is some evidence for supposing that zinc phosphate cements contain an amorphous aluminium phosphate or zinc aluminophosphate phase. Also, as we shall see in Section 6.5, amorphous aluminium phosphate is the binding matrix of dental silicate cement. 

The correlation of phosphate precipitation with decrease of conductivity (Wilson Kent, 1968), increase in pH (Kent Wilson, 1969) and hardness (Wilson et al, 1972) is shown in Figure 6.16. These results demonstrate the relationship between the development of physical properties and the underlying chemical changes, but there are no sharp changes at the gel point. Evidence from infrared spectroscopy (Wilson Mesley, 1968) and electron probe microanalysis (Kent, Fletcher Wilson, 1970 Wilson et al, 1972) indicates that the main reaction product is an amorphous aluminophosphate. Also formed in the matrix were fluorite (CaF ) and sodium acid phosphates. 

Frache, A., Gianotti, E. and Marchese, L. (2003) Spectroscopic characterisation of micro-porous aluminophosphate materials with potential application in environmental catalysis, Catal. Today, 77, 371. 

Aza macrocycles have also been used as templating agents in the formation of zinc and other metal cation-containing aluminophosphates via hydrothermal synthesis.705 The zeolite-like structures have been formed in the presence of cyclam- and hexaaza-based macrocycles. 

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

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