Aluminophosphate gel

The precursor to the AIPO4 5 molecular sieve crystallizes at 150°C from a hydrothermal system containing an aluminophosphate gel and tetrapropylammonium hydroxide. Its ideal chemical composition is TPAOH I2AIPO4. Removal of the TPAOH by calcination at 400-600°C produces a molecular sieve for which absorption studies are consistent with unconnected pores bounded by 12-rings. The AlPOtt 5 sieve is thermally stable to 1000°C above which it transforms to the AlPO. analog of tridymite. 

In 1982, workers from Union Carbide (ref. 4) announced the synthesis of a new microporous oxide system based on the aluminophosphates. The synthesis of this system resulted from the application of an organic base to an aluminophosphate gel followed by hydrothermal treatment. The resulting aluminopRosphate structures have indeed been shewn capable of isomorphous substitution (refs 5-8) and the range of structures and substituting elements continues to grew at a nuch faster rate than that for the conventional silicate systems. The substituted aluminophosphates have been shown to be catalytic and could provide a complementary series of shape selective catalysts. 

In a typical synthesis, 4.2 g. pseudoboehmite (Catapal B, 73% AI2O3) was dispersed in 15.0 g. water. 6.93 g. orthophosphoric acid (85%, S. D. Fine Chemicals) was diluted in 9.7 g. water and then added to a dispersed pseudoboehmite in water. The aluminophosphate gel thus obtained was aged for 24 hours and 3.91 g n-DBA (99%, Riedel-de Haen) was added to the aged gel. The mixture was stirred for 10 minutes and then crystallized in a stainless steel autoclave between 415-418K for 24 hours. After crystallization the autoclave was quenched... 

The crystallization was carried out in teflon bottles placed in stainless steel autoclaves. The mixtures were heated to the reaction temperatures under autogeneous pressure. For the preparation of the batches two basic reaction mixtures were used an aluminophosphate gel and another gel, containing the silica and the template. The standard reaction mixture to prepare a SAPO- (AlPOjj-) structure had the following molar composition ... 

Values for Na20, K20, and H20 are slightly higher than the values shown and undetermined because of unknown quantities absorbed on the precipitated hydrous aluminophosphate gel. 

Though crystallization of the aluminophosphate materials generally prefers acidic conditions, the use of fluoride as a mineralizing agent has been found to offer further flexibility in the synthesis of AIPO4 phases. Generally, the introduction of fluoride to an aluminophosphate gel results in a more rapid formation of the crystalline product. Fluoride ion addition may also increases the size of the crystals produced [82]. In some cases centimeter long crystals have been obtained [83]. 

The aluminophosphate-based molecular sieves are synthesized by hydrothermal crystallization from reactive aluminophosphate gels containing all the framework elements and an organic template. The organic template plays a structure-directing role, and upon synthesis it is incorporated in the structural voids of the crystal. 

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. 

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. 

Solid-state NMR spectroscopy was used for studying the formation of cubic mesoporous aluminophosphate thin films and powders. The analysis of the initial gel, the as-deposited materials and the thermally-treated materials elucidated the changes in the coordination of phosphorus and aluminium atoms and thus revealed how the framework formation and condensation proceeds. The consolidation process in thin films was different than the process in powders. Most probably this could be attributed to the effect of glass substrate. 

Aluminide alloys, 13 530 Aluminium powder, 10 738. See also Aluminum entries Aluminohydride derivatives, 13 624 Aluminohydrides, 13 621-624 Aluminophosphate zeolites, 14 98 Aluminosilicate gels, 16 830 Aluminosilicate glass, matrix for... 

The extensive size of organic amine as structure-directing templates or pore filling agents, coupled with a new gel chemistry resulted in the discovery of a third generation of zeolites containing Al3+ and P5+ as lattice atoms (1982). These aluminophosphate materials are a family of molecular sieves as shown in figure 7.10. 

A typical synthesis procedure involves the following steps (i) alumina is slurried in water, (ii) phosphoric acid is diluted in water, (iii) the phosphoric acid solution is added to the alumina slurry, (iv) the aluminophosphate precursor mixture is aged at ambient conditions, (v) an organic is added to the precursor mixture and aged with rapid agitation to form the final gel, which (vi) is charged into the autoclave and heated. The gel composition can be written as... 

The precursor gel is vigorously agitated just prior to the addition of TBAOH. When the TBAOH is combined with the aluminophosphate mixture the pH instantly rises to around 5. The final pH is dependent upon the degree of mixing during addition of TBAOH. Incomplete mixing produces pH s below 5 and can lead to the formation of impure TBA-VPI-5 (with small amounts of H3 present). 

The initial gel pH ranged from 3.5 - 4.5, increasing with Mg content. At 100°C and a 2 day reaction time (Figure 4), the lowest Mg concentration (0.05) yields primarily template-free aluminophosphate hydrate structure-types such as metavariscite and variscite (A1P0 2 H20> and H3 (A1P0 1.5 H20> (20.21). 

Similarly, zeotype molecular sieves are synthesized by mixing the basic ingredients with the organic template, e.g. aluminophosphates are prepared from alumina and phosphoric acid. Other main group or transition elements can be incorporated into the framework by adding them to the initial sol-gel. Alternatively, different elements can be introduced by post-synthesis modification (see later), e.g. by dealumination followed by insertion of the new elements into the framework position [31]. 

In reporting the results of a spectroscopic study of aluminium phosphate in 1971, Peri drew attention to the isostructural nature of A1P04 and Si02 and the likely value of A1P04 as an adsorbent and catalyst support. Stable high-area A1P04 gels could readily be prepared in 1971, but at that time there was no indication in the open or patent literature that zeolitic forms of aluminophosphate could be synthesized. 

The aluminum in the zinc phosphate cements was considered very important, van Dalen [21] recognized its importance first. The reaction of zinc oxide and phosphoric acid was greatly moderated by aluminum. This effect was attributed to formation of an aluminum phosphate gelatinous coating on zinc oxide particles. In fact, Wilson and Nicholson believe that the gelatinous substance may even be zinc aluminophosphate phase [3], which subsequently crystallizes into hopeite and aluminophosphate amorphous gel (AlP04-nH20). 

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