STRUCTURAL AND TEXTURAL CHARACTERIZATION OF ALPO4-B2O3 AND AL2O3-B2O3(5-30 WT-PERCENT B2O3) SYSTEMS OBTAINED BY BORIC-ACID IMPREGNATION

A series of AlPO4-B2O3 (APB) and Al2O3-B2O3 (AlB) systems with various boria loadings (5-30 wt%) were prepared by impregnation of AlPO4 with boria from a boric acid solution (methanol or water) and characterize d by XRD, SEM-EDX, TG/DTA, XPS, DRIFT and nitrogen adsorption. B-11, A l-27, and P-31 MAS NMR were used to determine the local structure. DTA and XRD analysis showed that crystallization occurred in all cases ab ove 900 K, yielding alpha-cristobalite-AlPO4 in APB systems, and alumi num berate in AlB systems. Also, boron oxide remained amorphous (XRD) for thermal treatments in the range 473-1073 K. SEM showed a non-homog eneous distribution in morphology, texture and particle size. Also, ED X and XPS indicated a surface enrichment in boron for APB systems whil e for AlB ones the reverse is true. Moreover, Al-27 MAS NMR showed the simultaneous presence of four and six-coordinate Al atoms in AlB syst ems while in APE ones, Al atoms appeared only in AI(OP)(4) environment s. P-31 MAS NMR only identified P atoms with tertrahedral coordination . Besides, the presence of boria did not change the local structure of the starting AlPO4 or Al2O3 support. B-11 MAS NMR identified both tet rahedral(BO4) and trigonal (BO3) species as being present; the concent ration of tetrahedral oxygen coordinated berate (BO4) species were fou nd to increase with boria loading. Furthermore, DRIFT spectroscopy sho wed that the incorporation of boron results in the appearance of B-OH groups and B-O stretches of a berate phase where both Trigonal and tet rahedral boron exist on the surface of APB and AlB samples. Besides, t rigonal berate species predominate and tetrahedral berate species incr ease 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. (C) 1998 Academic Press.