Adsorbents hectorite

The evolution of hydrogen was observed when an aqueous suspension containing chlorophyll a adsorbed hectorite-PVP, 2-ma-captoethanol, and... 

The evolution of hydrogen was observed when an aqueous suspension containing chlorophyll a adsorbed hectorite-PVP, MV +, 2-mercaptoethanol, and hydrogenase was illuminated (271). and 2-mercaptoethanol were thought to be an electron carrier and electron donor, respectively. The chlorophyll a was effectively stabilized by the adsorption onto hectorite-PVP intercalation compound. 

Chiral recognition of A-[Co(phen)3]3+ has been observed in a modified /3-cyclodextrin.772 Chiral discrimination has also been seen in photoinduced energy transfer from luminescent chiral lanthanoid complexes773 to [Co(phen)3]3+ and between photoexcited [Ru(bpy)3]2+ and [Co(phen)3]3+ co-adsorbed on smectite clays.774 The [Co(bpy)3]3+ ion has been incorporated into clays to generate ordered assemblies and also functional catalysts. When adsorbed onto hectorite, [Co(bpy)3]3+ catalyzes the reduction of nitrobenzene to aniline.775 The ability of [Co(phen)3]3+ to bind to DNA has been intensively studied, and discussion of this feature is deferred until Section 6.1.3.1.4. 

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

ESR has also been used in the characterization of species adsorbed on pillared clays, i.e. smectites with hydroxy-aluminium interlayers. Adsorption of Cu(II) on hydroxy-aluminium hectorite produced mobile hexaaquacopper(II) and Cu(II) chemisorbed to... 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Fe(III) and Cu(II) adsorbed on smectites (hectorite), either as hydrated or partially hydrolyzed ions, have a high degree of... 

Table 1. Effect of hydrothermal temperature on the methylene blue(MB) adsorbed, interlayer silicate content and 001 spacing of silicate-bearing hectorites...

Calculated based on the precursory synthetic hectorite. Calculated from MB adsorbed, Calculated from thermogravimetric data. 

Figure 6). The lqjnipescence quantum yield ( ) in solution for excited [Ru(bpy)J2 ] Is 4%. The same quantum yield is observed for adsorbed Ru(bpy)j in deh drgted hectorite. The luminescence quantum yield or [Cr(bpy)3 ] is 0.1% in solution and also in hydrated ( 16% H20 w/w) hectorite. Upon hydrating the hectorite film loaded with the Ru complex, increases slightly, whereas dehydrating the film loaded with the Cr complex increases by a factor of 100. In order to understand these opposite behaviors, the chemical reactivity of Cr(bpy)3 and of Ru(bpy>3 with respect to water must be recalled. ...

This concentration effect shows up in the absorption and emission spectra of the adsorbed dyes, because it affects the relative amounts of monomers and dimers. This has been shown by a study of the absorption spectra of adsorbed methylene blue and proflavine(4,5). For Na+- and Ca2+-hectorite and barasym the absorption spectra can be quantitatively interpreted in terms of the following equilibrium on the surface... 

In this way fluorescence spectroscopy of adsorbed dyes is a useful tool to characterize clays in aqueous suspension. From figures 5-8 it is noted that (1) laponites behave differently from hectorites, except Ca2+-L (2) PFH+ is the dominant species on Cs-H and Cs-L, because the fluorescence intensity is almost independent of the loading and thus no quenching due to dimers occurs (3) the difference between neutral BS suspensions and BS suspensions at pH = 9 is the presence of PFH22+ in the former case. 

According to Cool and Vansant (1996), pores between 0.7 and 1.1 nm are probably present in all pillared clays, whereas the narrow and wider pores are particular features of the Zr-laponite and Zr-hectorite. A relatively high adsorption affinity (i.e. the low pressure capacity) of Zr-laponite for cyclohexane was attributed to the presence of a large number of narrow pores, giving rise to enhanced adsorbate-adsorbent interactions. 

Visible light-induced cleavage of water has been reported for a mixed colloidal clay system consisting of a mixture of sepiolite clay-Ru02-Ru(bpy)3 + colloid (for O2 production) and Al,cEui (OH)3-Pt colloid (for H2 production) [165]. A turnover number of 20 with respect to the sensitizer, Ru(bpy)3 +, was observed. It was reported that the gas (H2 + O2) evolution displayed a damped oscillatory behavior. Photo-oxidation of water by tran5-diaqabis-(2,2 -bipyridine)ruthenium(2- -) adsorbed on the surface of hectorite clay has also been reported [166]. 

Strontium adsorption onto soil minerals is an important retardation mechanism for Sr " ". Chen et al. (1998) investigated the adsorption of Sr " " onto kaolinite, illite, hectorite, and montmorillonite over a range of ionic strengths and from two different electrolyte solutions, NaNO3 and CaCb- In all cases, the EXAFS spectra suggested Sr adsorbed to clay minerals as an outer-sphere mononuclear complex. Sahai et al. (2000) also found that on amorphous silica, goethite, and kaolinite substrates, Sr"+ adsorbed as a hydrated surface complex above pH 8.6. On the other hand, Collins et al. (1998) concluded from EXAFS spectra that Sr " " adsorbed as an inner-sphere complex on goethite. 

In a subsequent work, Woessner [57] used deuterium TiS of D O adsorbed at different saturation levels on a hectorite sample to monitor water mobility in the vicinity of a clay surface. It is clear from the data that the first two monolayers of water on the clay sheets experience restricted motion. 

Resing and coworkers performed C NMR measurements on a benzene/ hectorite system [60]. A single-crystal" study was performed in essence, since the clay platelets were preferentially oriented and then the clay/adsorbate system was rotated in the magnetic field. The benzene molecules were found basically to stand on edge in the interlayer space and undergo rotation about their hexad axes. They also seem to rotate about an axis normal to the clay layers, but this motion is frozen out at 77 K. 

Fyfe et al. [61] studied p-xylene and y-butyrolactone in hectorite in 1981 using H and C NMR with no MAS. The relatively good resolution of these spectra indicates a high degree of mobility of the adsorbates in all cases. The C chemical shifts are nearly identical to those in solution spectra of intercalants, suggesting that magnetic susceptibility effects are not large, at least in this clay. 

Tennakoon and coworkers [64] also did a later study of some low-Fe hectorite and montmorillonite samples. These experiments included Si and Al MAS-NMR for examination of the clay sheets, C NMR of adsorbates on the clays, and also H NMR for examination of structural hydroxyls. Neither the H nor the C measurements of the adsorbate was made under MAS. Adsorption of 1-hexene on the Al -exchanged hectorite sample followed by high-resolution" C NMR revealed no olefinic C resonances. Inspection of the AI MAS spectrum indicates an apparent change in the ratio of tetrahedral Al to octahedral Al. FI NMR of a deuterated sample (with D O replacing water of hydration of the clay) reveals that the olefin is indeed intact and therefore the olefinic end must be bonding to some site in the clay (Fig. 9). The main evidence for this would seem... 

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