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

Author： Bautista F.M. Campelo J.M. Garcia A. Luna D. Marinas J.M. Moreno M.C. Romero A.A. Navio J.A. Macias M.

Publisher： Academic Press

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.173, Iss.2, 1998-01, pp. : 333-344

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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.