Structural and Textural Characterization of AIPO4–B2O3and Al2O3–B2O3(5–30 wt% B2O3) Systems Obtained by Boric Acid Impregnation

Abstract

A series of AlPO4–B2O3(APB) and Al2O3–B2O3(AlB) systems with various boria loadings (5–30 wt%) were prepared by impregnation of AlPO4with boria from a boric acid solution (methanol or water) and characterized by XRD, SEM-EDX, TG/DTA, XPS, DRIFT and nitrogen adsorption.11B,27Al, and31P MAS NMR were used to determine the local structure. DTA and XRD analysis showed that crystallization occurred in all cases above 900 K, yielding α-cristobalite-AlPO4in APB systems, and aluminum borate in AlB systems. Also, boron oxide remained amorphous (XRD) for thermal treatments in the range 473–1073 K. SEM showed a non-homogeneous distribution in morphology, texture and particle size. Also, EDX and XPS indicated a surface enrichment in boron for APB systems while for AlB ones the reverse is true. Moreover,27Al MAS NMR showed the simultaneous presence of four and six-coordinate Al atoms in AlB systems while in APB ones, Al atoms appeared only in Al(OP)4environments.31P MAS NMR only identified P atoms with tertrahedral coordination. Besides, the presence of boria did not change the local structure of the starting AlPO4or Al2O3support.11B MAS NMR identified both tetrahedral (BO4) and trigonal (BO3) species as being present; the concentration of tetrahedral oxygen coordinated borate (BO4) species were found to increase with boria loading. Furthermore, DRIFT spectroscopy showed that the incorporation of boron results in the appearance of B–OH groups and B–O stretches of a borate phase where both trigonal and tetrahedral boron exist on the surface of APB and AlB samples. Besides, trigonal borate species predominate and tetrahedral borate species increase with boron loading. Finally, the incorporation of boron produces a decrease in surface area and pore volume that becomes greater as the boron content increases.