Effect of phosphate precursor and organic additives on the structural and catalytic properties of amorphous mesoporous AlPO4 materials

Amorphous mesoporous AlPO4 materials were prepared by a gel precipitation method using different phosphate precursors [H3PO4, (NH4)H2PO4, and (NH4)(2)HPO4], 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 AlPO4 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, Al-27 and P-31 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 AlPO4 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 AlPO4 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.