Effect on metal aluminophosphate

Several aluminophosphate molecular sieves with AEL topology structure were synthesized and modified by Pd for direct transformation reaction of -butane to isobutene. The effect of pore geometry of the molecular sieves was studied. Pd modified 10-member ring SAPO-11 and metal-substituted AlPO-l 1 and SAPO-11 showed high selectivity towards isobutene. The incorporation of metal into the molecular sieves had effect on the product distribution. Catalytic properties and the result of chemical adsorption of monoxide predicted the interaction between the transition metal for substitution and the supported palladium. 

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]. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

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