Raman microscopy study of basic aluminum sulfate. Part II : Raman microscopy at 77 K

Abstract

Listen

Translate article

The tridecameric aluminum polymer [AlO4Al12(OH)24(H2O)12]7+ was prepared by forced hydrolysis of Al3+ up to an OH/Al molar ratio of 2.2. Upon addition of sulfate the tridecamer crystallised as the monoclinic basic aluminum sulfate Na0.1[AlO4Al12(OH)24(H2O)12](SO4)3.55. These crystals have been studied using Raman microscopy at 300 and 77 K and compared to Na2SO4·xH2O and Al2(SO4)3·xH2O. The Raman spectrum of basic aluminum sulfate is dominated by two broad bands, which are assigned to the ν2 and ν4SO42− triplets at 446, 459 and 496 cm−1 and 572, 614 and 630 cm−1. The ν1 is observed as a single band at 990 cm−1, partly overlapped by the ν3 triplet at 979, 1009 and 1053 cm−1 of the sulfate group in the Al13 sulfate structure. Furthermore the band at 726 cm−1 is assigned to an Al–O mode of the polymerised Al–O–Al bonds in the Al13 Keggin structure. For the first time the OH-stretching region of the basic aluminum sulfate has been reported. The 77 K spectrum shows the presence of 3 crystal water bands at 3035, 3138 and 3256 cm−1 accompanied by 3 Al–H2O bands at 3354, 3418 and 3498 cm−1 and 4 Al-OH bands at 3533, 3584, 3671 and 3697 cm−1.