Adsorbent boehmite

Gelatinous boehmite, called alumina gel in commeicial use, is used in the piepaiation of adsorbents, desiccants (qv), catalysts, and catalyst supports (see Catalysts, SUPPORTBd). A significant amount is used in pharmaceutical preparations. 

When the gibbsite is dehydrated a structural collapse occurs with a large increase in surface area. The boehmite sample has a nominal surface area of 325 m /g. The infrared spectrum of the boehmite shows distinct structure in the OH stretching region, with two peaks located at 3090 and 3320 cm". There are three features at 1648, 1516 and 1392 cm" that are due to adsorbed water and carbonate, which are removed upon heating the boehmite to 350 0 in hydrogen. 

Further dehydration of boehmite at 600 0 produces y-alumina, whose spectrum is shown in Figure 3b. There is a loss in surface area in going from boehmite to y-alumina. The sample shown here has a surface area of 234 m /g (this sample was obtained from Harshaw A23945 the calcined Kaiser substrate gave an identical infrared spectrum). The y-alumina sample shows two major differences from o-alumina. First, there is a more intense broad absorption band at 3400 cm" due to adsorbed water on the y-alumina. Second, the y-alumina does not show splitting of the phonon bands between 400 and 500 cm" as was observed for o-alumina. The y-alumina is a more amorphous structure and has much smaller crystallites so the phonon band is broader. The y-alumina also shows three features at 1648, 1516 and 1392 cm" due to adsorbed water and carbonate. 

The features due to adsorbed water and carbonates observed on the boehmite and y-alumina deserve further attention as they differ from results published by previous investigators. Figure 4 shows a series of difference spectra for adsorption on y-alumina. Spectra were taken after drying the y-alumina at 350 C, cooling to room temperature and carrying out room temperature adsorption. The spectra are the difference of the sample before and after adsorption. Spectrum 4e is the spectrum for the as received alumina differenced with the dried alumina. The positive band at 3400 cm" is due to adsorbed water, and the small negative feature at 3740 cm" is due to isolated hydroxyls on the dried surface. Besides the three... 

Berrodier, I., Farges, F., Benedetti, M., Winterer, M., Brown Jr, G. E., Deveughele, M. (2004). Adsorption mechanisms of trivalent gold on iron- and aluminum-(oxy)hydroxides. Part 1 X-ray absorption and Raman scattering spectroscopic studies of Au(III) adsorbed on ferrihydrite, goethite, and boehmite. Geochimica et Cosmochimica Acta, 68(14), 3019-3042. doi 10.1016/j.gca.2004.02.009... 

Such an effect might be expected when boehmite supported cobalt is being calcined, viz. during the phase transition AIO(OH) - y-Al203. Figure 7 shows spectra of pyridine, adsorbed on the sample CoMo-124 B, which has been prepared in this way. Spectra for MoCo-122, -123 and -124, containing 2, 3 and 4 wt% CoO resp. are shown for comparison. All these catalysts have had a final calcination of 650°C. Comparison of the spectra of CoMo-124 B and MoCo-124 indicates that the intensity of the 1612 cm l band, which is introduced by the interaction of the cobalt ions and the molybdate layer, is lower for CoMo-124 B than for MoCo-124. The spectrum for CoMo-124 B resembles that of CoMo-123, indicating that a part of the cobalt ions does not participate in this interaction. 

Some bulk cobalt aluminate formation is expected to take place for the boehmite based catalyst, owing to a Hedvall effect (24).The spectrum of adsorbed pyridine on CoMo-124 B shows indeed a weaker 1612 cm l band, comparable with the intensity of this band for the MoCo-123 catalyst. This indicates that about 25 % of the cobalt ions has disappeared in the bulk of the alumina. (Figure lid). 

A fibrillar type of boehmite (i.e. a du Pont product named Baymal colloidal alumina) was first described by Bugosh etal. (1962). The BET area was reported to be 275 m2g-1. The discrete boehmite fibres of about 5 ran diameter and 100 nm length were protected by adsorbed acetate groups and were thus easily dispersible. 

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Nuclear magnetic resonance (NMR) spectroscopy can be applied to aqueous samples and can distinguish between inner- and outer-sphere ion surface complexes. The adsorption behavior of the cations Cs+ and Na" " was studied on the surfaces of silica, boehmite, kaolinite, and illite (Kim and Kirkpatrick, 1997). Cesium was adsorbed both as inner-and outer-sphere surface complexes and in the diffuse layer, while Na was adsorbed only as outer-sphere surface complexes and in the diffuse layer. The adsorbed Na ions were fully hydrated, while the Cs ions had direct contact with the surface oxygen atoms. 

Weesner, EJ. and Bleam, W.E, X-ray absorption and EPR spectroscopic characterization of adsorbed copper(ll) complexes at the boehmite (AIOOH) surface, J. Colloid Interf.Sci., 196, 79, 1997. 

Furthermore, supporting this idea, the amount of adsorbed molybdates reaches 1.5 Moat./nm at the plateau such a value corresponds to the number of pi-OH2 sites located on the boehmite lateral surfaces as determined by Raybaud et al. [13]. 

Fig. 9. Raman spectra of adsorbed species at [AHM]i= 50 lO mol/1 a) solid dried b) calcined upon 773K and rehydrated lines of boehmite...

At low molybdenum loading, tetrahedrally coordinated molybdate monomers preferably adsorb on lateral surface of the boehmite particles. 

Finally, at high molybdenum loading, a third adsorption regime occurs. Adsorbed monomers are not observed by LRS. Assuming unlimited dissolution, is consistent with the fact that Anderson-type polyanion formation and further adsorption on boehmite basal planes occurs. 

The term alumina hydrates or hydrated aluminas is used in industry and commerce to designate aluminum hydroxides. These compounds are tme hydroxides and do not contain water of hydration. Several forms are known a general classification is shown in Figure 1. The most well-defined crystalline forms ate the trihydroxides, Al(OH)3 gibbsite [14762-49-3], bayerite [20257-20-9], and nordstrandite [13840-05-6], In addition, two aluminum oxide—hydroxides, AIO(OH), boehmite [1318-23-6] and diaspore [14457-84-2], have been clearly defined. The existence of several other forms of aluminum hydroxides have been claimed. However, there is controversy as to whether they ate truly new phases or structures having distorted lattices containing adsorbed or intedameUar water and impurities. 

This interpretation has been supported by a study of oxalate sorption on corundum modelled by the CD-MUSIC model involving ATR-IR spedroscopy (Johnson et al., 2004). A mononuclear bidentate complex was found up to 14 pmol/m, whereupon oxalate additionally adsorbed as an outer-sphere complex. Sorption of oxalate has also been studied on boehmite and corundum by Yoon et al. (2004) The peaks assigned to the inner-sphere complex in previous works (near 1286,1418,1700 and 1720 cmi) were claimed to arise from the presence of several species. Evidence for this phenomenon comes from the observation that peaks at 1286 and 1418 cm-i are shifted to 1297 and 1408 cm-i as the oxalate surface coverage increases. The authors finally postulated the existence of two species species "A" at 1286 and 1418 cmi, and species "B" at 1297 and 1408 cm-i, respectively, which were... 

Yoon, T. H., Johnson, S. B., Musgrave, C. B., Brown, G. E (2004). Adsorption of organic matter at mineral/water interfaces I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces, Geochim. Cosmochim. Acta Vol. 68, No. 22, pp. 4505-4518,0016-7037. 

Dogu and Smith (1972) used pellets made by compressing porous Boehmite particles. They provided the following data about the system of non-adsorbing helium in nitrogen at 24 and 1 atm. 

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