Aluminophosphate molecular hydrophilicity

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

A new family of crystalline molecular sieves, 2) having aluminophosphate frameworks was synthesized. Strict alternation of A1 and P on the tetrahedral nodes yields neutral oxygens in contrast to the aluminosilicate zeolites, and non-framework cations are not needed for charge balance. Whereas a microporous silica (silicalite, 3 ) with neutral oxygens is hydrophobic, the aluminophosphate sieves are moderatley hydrophilic. 

Calorimetric measurements at 303 K of the adsorption heats for polar molecules (water, methanol, ethanol) on VPl-5 and AIPO4-11 were performed byjanchen et al. [268], and the results were compared with the corresponding data on AIPO4-5, two silica molecular sieves, and Na-X. Prom the measured data, it followed that the adsorption heats on the investigated aluminophos-phates were smaller than on the hydrophilic Na-X and larger than on the hydrophobic Si02 molecular sieves. Electrically neutral aluminophosphates exhibited a medium hydrophihc character. The stepwise course of the heat curves and isotherms of the polar molecules on VPl-5 may be related to the structure of the as-synthesized VPl-5 as an aluminophosphate dihydrate phase, in accordance with MAS NMR results. 

The metal cations can also be immobilized onto supports by cation exchange. Already classic examples are those of metal-zeolites, metal-acidic clays, or metal-aluminophosphates (APO) [5,6,38,70-72]. Fe-substituted molecular sieves can be considered as very good examples for this variety of catalysts and the oxidation of pinacol to pinacolone evidenced the effect of the crystalline support structures on the catalytic activity. Thus, for a series of APOs and silicalites, the activity decreased in the following order APO-5> APO-11 > APO-8>VPI-5 > silicalite-1. Since the catalytic activity was independent of the pore diameter of these supports, the liquid-phase oxidation was considered to proceed mainly on the outer surfaces of the catalysts. The hydrophilicity of the aluminophos-phate surface was in the fevor of catalyzing the pinacol reaction, which in fact corresponds to the case of polar reactants and less polar products. Moreover, a high pinacol conversion was achieved by using solvents of low polarity [70]. 

The binary altuninophosphate molecular sieve family AIPO4 exhibits constant chemical composition with an A1 to P ratio of unity. This fixed composition provides electrically neutral framework structures. Consequently, there are no extra framework cations or protic acidity. The binary aluminophosphates are mildly hydrophilic, in contrast to the strongly hydrophilic zeolites and hydrophobic silica molecular sieves. The AIPO4 crystals generally have excellent thermal and hydrothermal stability, similar to that of stable zeolites. 

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