High fraction of penta-coordinated aluminium in amorphous and crystalline aluminium borates.
27Al and 11B magic-angle spinning nuclear magnetic resonance (MAS NMR) results are reported on aluminium borates of composition Al2(1 - x)B2xO3 (0 < or = x < or = 0.7). It is shown that starting from liquid precursors with hexa-coordinated Al and tri- and tetra-coordinated B, porous aluminium borates are obtained containing high fractions of penta-coordinated Al. The presence of boron in the crystalline phases stabilizes penta-coordinated Al, existing already in their amorphous precursors, and at the same time distorts the surroundings of the hexa-coordinated Al. The reversible effect of air exposure on low-coordinated Al (Al4C, Al5C) and B (B3C) proved that at least these species are situated at the surfaces of the pores. The A9B2 phase still contains a small fraction of tetra-coordinated B. The 27Al and 11B MAS NMR spectra of A9B2 show a well-resolved powder spectrum with no indication of broadening due to a distorted configuration.
Simon
S
S
Department of Molecular Spectroscopy, Faculty of Science, University of Nijmegen, Netherlands.
van Moorsel
G J
GJ
Kentgens
A P
AP
de Boer
E
E
D017607:Aluminum Compounds / Q000737:chemistry*; D001888:Boric Acids / Q000737:chemistry*; D003460:Crystallization; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D012996:Solutions; D014867:Water; D014961:X-Ray Diffraction
D017607:Aluminum Compounds; D001888:Boric Acids; D012996:Solutions; D014867:Water