Effect of Phosphate Precursor and Organic Additives on the Structural and Catalytic Properties of Amorphous Mesoporous AlPO₄ Materials

Amorphous mesoporous AlPO 4 materials were prepared by a gel precipitation method using different phosphate precursors [H 3 PO 4, (NH 4 )H 2 PO 4, and (NH 4 ) 2 HPO 4 ], both in the presence and absence of an aluminum-complexing assisted gel agent (1,2-ethanediol, 1,2- and 1,3-propanediol, and 2-methyl-2,4-pentanediol), followed by calcination in the 773−1273 K range. TG/DTA, XRD, and laser-Raman spectroscopy showed that the method assisted by an organic additive resulted in AlPO 4 with higher thermal stabilities (crystallization was delayed), as well as high surface areas and pore volumes even after calcination at 1073/1273 K. In all cases, 27 Al and 31 P MAS NMR spectroscopy only indicated the formation of tetrahedral Al(OP) 4 and P(OAl) 4 units. Moreover, surface areas and pore volumes depended on the organic additive used. Thus, the AlPO 4 obtained in the presence of 1,3-propanediol and 2-methyl-2,4-pentanediol possessed higher thermal stabilities that resulted in larger surface areas and pore volumes. Surface acidity measurements, through chemisorption (measured by gas-chromatography) of pyridine and 2,6-dimethylpyridine at 473−573 K, indicated that the AlPO 4 obtained in the presence of 1,3-propanediol and 2-methyl-2,4-pentanediol exhibited surface acid sites even after calcination at 1273 K. Cyclohexene skeletal isomerization, which is a model reaction catalyzed by relatively strong acid sites, was also studied for further characterization of acidic properties. Good correlations between catalytic performance and acidic properties were found.