Structural hydroxyls, adsorption

The pH dependence of the adsorption of Co on montraorillonite in the pH range 5-6 has been interpreted (14,24) as due to the behaviour of structural hydroxyl groups on the clay or to hydroxy-Al compounds. In other work, adsorption of Cd, Co and Sr on to montmorillonite from solutions with widely different salt... 

Hydrated iron oxides can adsorb heavy metals. These adsorption properties arise from the presence of structural hydroxyl groups on their surface, which exhibit amphoterism (56) ... 

The formation of structural hydroxyl groups in the presence of divalent cations has been explained on the basis of a hydrolysis mechanism (148) involving water initially coordinated to the metal ions (210, 214-216). The formation of a nonacidic hydroxyl group on the metal ion and an acidic hydroxyl on the zeolite framework by dissociation of the water molecule is consistent with the observed IR spectra and pyridine adsorption experiments. Further calcination at higher temperatures results in dehydroxylation and formation of Lewis acid sites at tricoordinate aluminum atoms in the zeolite framework (149). 

Calculations of the adsorption of organic molecules on active sites of zeolites, including the structural hydroxyls, are still very limited. A wide field of applications opens here for the quantum-chemical methods. 

Senchenya et al. (96) have treated the adsorption of ethanol on a structural hydroxyl group (Fig. 14) using a CTP scheme and the CNDO/BW method. The separation of a molecule and cluster with respect to the z axis was optimized, the optimal values being r = 1.19 A and R = 1.28 A The adsorption energy was 23.2 kcal/mol, which was close to the experimental value (97). Note that this was essentially the two-point adsorption involving both acid and base sites. This case is quite similar to the above propylene adsorption (90). There is also no definite trend toward proton transfer from the hydroxyl group of a zeolite to the alcohol molecule. The carbocation state is also predicted to be activated. This, in turn, increases relative efficiency of the synchronous mechanism (with the same recommendation for its experimental examination). The estimation (96) of the energetics of the intermediate structures of the synchronous mechanism showed that such a mechanism is quite realistic. 

The strong interaction of this halide with the surface inhibits the hydroxyl adsorption and later the oxide formation. However, a hysteric effect from the 3/7 of the ML structure associated to a transition from the ordered to disordered structure can be seen [54]. This does not occur with the (3 x 3) structure as demonstrated by the diffraction patterns. 

Zeolite properties are being studied by nearly every type of modern scientific discipline, and they are being utilized in many new chemical engineering processes. Important advances include detailed basic information on cations in zeolites, more understanding of the mechanism of zeolite formation, the formation and character of structural defects and hydroxyl groups, the role of zeolite structure in adsorption and catalysis, and the increasing technology of the use of molecular sieve zeolites in catalysis and adsorption. 

Cation-Containing Zeolites. Zeolite surface structural groups have been studied extensively because of their importance in adsorption and catalysis. From a consideration of the theoretical structure, only absorption bands due to —Si—O— and —Al—O vibrations would be expected. The earliest work has been reviewed previously (7, 74, 75). In general, except for the initial study of Bertsch and Habgood (9), at least 3 types of structural hydroxyl groups were detected. 

Hence, the zeolite surface groups which are probably important as adsorption and catalytic sites are the structural hydroxyl groups, the tricoordinated aluminum sites, and the exchangeable cations. 

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... 

Between processes of adsorption and chemisorption there is a close relationship, since in the process of evolution of adsorbed species one can observe formation of transient complexes that are products of reactions proceeding by the Langmuir-Hinshelwood mechanism. According to the accepted classification of reactions in a surface layer of SiOi they are subdivided into two main classes, namely reactions with substitution of structural hydroxyl protons (SeI processes) and reactions with substitution of OH groups at silicon atoms (SnI processes). Reactions of heterolytic decomposition of siloxane bonds proceeding by the AdN3 mechanism form a separate class. 

An adsorption band of 3680 cm corresponds to such a complex. The hexagonal cavities ( hexagons ) with concentration comparable with content of structural hydroxyl groups can serve as channels for an-out-of activation penetration of H2O molecules into the subsurface layer. The occupation of hexagon with water molecule creates a potential barrier at the month of surface six-link cycles for the penetration of the next water molecules into three nearest hexagons [123]. Thus, the concentration of primary water adsorption sites is at least ten... 

At the consequent stages of water adsorption structural hydroxyl groups are involved into the process of cluster formation. The dehydration of Si02 samples is characterised by essentially reverse sequence of corresponding process. 

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

In the absence of TCE and chlorine, the possible active species are holes (h+), anion vacancies, or anions (02 ), and hydroxyl radicals (OH ). At constant illumination and oxygen concentration, we may expect h+, and O2 concentrations to be approximately constant, and the dark adsorption to be a dominant variable. If kh+, or ko2- does not vary appreciably with the contaminant structure, the rate would depend clearly on the contaminant coverage as shown in Figme 2a, and the reaction would therefore occur via Langmuir-Hinshelwood mechanism. (Note only rates with conversions below 95% are correlated here (filled circles), as the 100% conversion data contains no kinetic information). This rate vs. d>r LH plot is smoother than those for koH or koH suggesting that non-OH species (holes, anion vacancies, or O2 ) are the active species reacting with an adsorbed contaminant. 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Zinc adsorption can occur via exchange of Zn2+ and Zn(OH)+ with surface-bound Ca2+ on calcite (Zachara et al., 1988). Zinc and Ni form surface complexes on calcite as hydrate until they are incorporated into the structure via recrystallization (Zachara et al., 1991). The selectivity of metal sorption on calcite is as follows Cd > Zn > Ni (Zachara et al., 1991). The easily reducible oxide bound metals are primarily from Mn oxides (Chao, 1972 Shuman, 1982 and 1985a). At pH > 6, Zn sorption on Mn oxide abruptly increases because of hydroxylation of the ions (Loganathan et al., 1977), and a high soil pH in arid soil may favor Zn sorption on Mn oxides due to a great... 

---