Basal montmorillonites

Fig. 2. Diagram showing the intercalation of compact quaternary ammonium cations, such as trimethylphenylammonium (TMPA) into different smectites, giving rise to type I organoclays with a basal spacing of about 1.5 nm. SWa is a high-charge nontronite (iron-rich smectite) and SAz is a high-charge montmorillonite, while SAC is a low-charge montmorillonite. After Jaynes and Boyd (1991b).

The same evolution of the basal spacing (d 001) for the pillared montmorillonite in which the Li has been introduced after the Zr is illustrated in fig. 5. It has to be mentioed that, after saturation of the solids by ethylene glycol, the interlayer distance of the samples calcined at 400°C is always slightly higher than before saturation. 

A small increase of the (d 001) basal spacing is observed for the Li containing Zr pillared clays. However, the thermal stability of these solids drastically decrease. At high temperature, the collapse of the strucutre is also supported by the decrease of the surface area which is, at 700°C, almost identical to those measured for the montmorillonite. Different hypothesis may be proposed to explain the increase of the interlayer distance at low temperature (i) a better polymerization of the intercalated complex (ii) a modification of the distribution of the pillars (iii) a lower interaction between the pillar and the silica layer. The first hypothesis may easily be eliminated since the small variation of the height of the pillars (less than 1 A) cannot be explained by structural changes of the... 

The diffusion of Li+ in the octahedral cavities of the Na+montmorillonite allows to control the density of the pillars of the Zr pillared montmorillonite. The solids, stable up to 300°C, have larger surface area basal distancy than the pure Zir montmorillonite. The distance between the pillars increases while the interaction strength between the pillars and the clay layer decreases. 

Titanium-pillared montmorillonite may be used as a heterogeneous catalyst for the Sharpless asymmetric epoxidation of allylic alcohols (Scheme 20) (46). The enantiomeric purities of the epoxy products are comparable with those achieved using homogeneous Ti isopropoxide with molecular sieves as water scavengers (Chapter 4). Since basal spacing of the recovered catalyst after the reaction is unaltered, the catalyst can be recycled. 

K is obtained from associated K-feldspars and micas. The layer charge is increased by the reduction of iron in the octahedral sheet and incorporation of Al, entering through the ditrigonal holes in the basal oxygen plane, into the tetrahedral sheets (Weaver and Beck, 1971a Pollard, 1971). Weaver and Beck have presented evidence that indicates mixed-layer clays formed in this manner contain 20—30% chloritic layers and are actually mixed-layer illite-chlorite-montmorillonite clays. 

The catalytic and structural properties of two chromia-pillared montmorillonites were compared in an effort to establish structure-reactivity relationships in these materials. The basal spacings of pillared products, prepared by reaction of Na+-montmorillonite with base-... 

Evidence that difference in reactivity for Cr3 53 and Crx 88-montmorillonite is due largely to differences in gallery accessibility is provided by the adsorption data in Figure 5. The Cr3 53 derivative, which retains a basal spacing near 21 A after reaction, is capable of rapidly adsorbing cyclohexane. However, the Cr1 88 derivative with a basal spacing near 13.7 A adsorbs very little cyclohexane. 

A DFT calculation of the chemical states of Cs+ cation adsorbed on smectites (montmorillonite, montmorillonite-beidelite, Fe-montmorillonite, nontronite) [98] shows agreement with results of MD simulation study [78] that Cs+ ion is adsorbed strongly on a basal oxygen hexagonal hole. 

These speciation concepts are illustrated in Fig. 3 for the idealized basal-plane surface of a smectite, such as montmorillonite. Also shown are the characteristic residence-time scales for a water molecule diffusing in the bulk liquid (L) for an ion in the diffuse swarm (DI) for an outer-sphere surface complex (OSQ and for an inner-sphere surface complex (ISC). These time scales, ranging from picosecond to nanosecond [20,21], can be compared with the molecular time scales that are probed by conventional optical, magnetic resonance, and neutron scattering spectroscopies (Fig. 3). For example, all three surface species remain immobile while being probed by optical spectroscopy, whereas only the surface complexes may remain immobile while being probed by electron spin resonance (ESR) spectroscopy [21-23]. 

Simulation of the ESEM pattern for Cu2+-doped Mg-montmorillonite leads to a coordination number of six and a Cu2+-D distance of 0.29 nm [41]. X-ray diffraction shows that the smectite layers are about 1.04 nm apart when the ESR lineshape becomes isotropic with a single peak. This large basal-plane spacing and the ESEM data suggest a diffuse-layer Cu(H20)g+ species that tumbles sluggishly. Copper-doped Mg-hectorite whose layers are about 0.54 nm apart yields an ESR spectrum like those for beidellite and montmorillonite at low relative humidity, whereas with the layers 1.04 nm apart, the spectrum is again isotropic [39]. Figure 8 illustrates the three Cu2+ surface complexes that appear successively as a smectite... 

Unit layers of smectites, notably montmorillonite, can associate in stacked, roughly parallel alignment to form a quasicrystal [52]. This particle structure is stabilized by attractive interactions between the basal planes of unit layers, as mediated by adsorbed cations and water molecules. The prototypical example of a montmorillonite quasicrystal is that comprising stacks of four to seven unit layers, with Ca2+ adsorbed in outer-sphere complexes on the siloxane surfaces to serve as molecular cross-links binding the unit layers together through electrostatic forces. This kind of quasicrystal appears to form with any bivalent cation and for any smectite [21,54]. 

As indicated earlier, the isotherms in Figure 11.5 could be termed pseudo Type II isotherms, but we prefer the designation Type lib (see Figure 13.1). Such isotherms are given by either slit-shaped pores or, as in the present case, assemblages of platy particles. The fact that the montmorillonite particles are thin and flexible may be responsible for the closer proximity of the basal faces than in uncompacted kaolinite. 

The cation exchange of layer silicates significantly influences some structural and colloid chemical properties. Depending on the charge of the cation, the interlayer space contains water in different quantities (Chapter 2, Section 2.1.2). So, the basal spacing (the distance between similar faces of adjacent layers) is different for monovalent, bivalent, and trivalent cations. For example, in monovalent montmorillonite, it is about 1.2 nm, and in bivalent and trivalent montmorillonite, it is about 1.5—1.6 nm. 

Basal (d00i) Spacing, Internal and Total Specific Surface Area, and Cation-Exchange Capacity of Natural Bentonite-Montmorillonite Samples... 

Since the size of the crystal lattice (TOT layer) is well defined by chemical bond lengths, the net basal ([Pg.86]

Some basic properties, such as basal spacing (d001), internal and total specific surface area, and cation-exchange capacity (CEC) of some natural montmorillonite or bentonite with high montmorillonite samples are listed in Table 2.1. Similar characteristics of different cation-exchanged montmorillonites are given in Section 2.3. 

In our studies, the model substance (montmorillonite) was a calcium bentonite (Istenmezeje, Hungary), the characteristic features of which are given here. X-ray diffraction (intensity of the basal reflection) and thermoanalytical (weight loss upon heating) data show 91% montmorillonite content. The other constituents are 5% calcite, 3% kaolinite, 1% x-ray amorphous silicates, and a trace of quartz. The amorphous phase is silicate particles, which are not crystalline for... 

The other type of transformation process of the interlayer cation is pillaring (Chapter 1, Section 1.3.5). In this process, metal oxide chains are formed in the interlayer space upon thermal treatment of different cation-exchanged mont-morillonites. As a result, the basal spacing, that is, the size of the interlayer space, increases. The pillared montmorillonites are widely applied in catalytic reactions, as evidenced by over 600 scientific publications in the last 10 years (e.g., Fetter et al. 2000 Johnson and Brody 1988 Perez Zurita et al. 1996). 

The structural parameters of cation-exchanged montmorillonites prepared from calcium-montmorillonite (Istenmezeje) are listed in Table 2.3. As seen in Table 2.3, the basal pacing of monovalent montmorillonite is approximately 1.25 nm, and the water content is approximately 1%. It means that there is one layer of water in the interlayer space. For bivalent montmorillonite, both basal spacing (>1.5 nm) and water content (>10%) are higher, showing two layers of water molecules in the interlayer space. The basal spacing of Pb-montmorillonite is 1.254 nm, which is similar to the value characteristic of monovalent montmorillonite (1.241 nm). However, it does not mean that lead is sorbed on the surface of montmorillonite as monovalent cation since the other parameters that are determined by the distance between the layers (hydration entropy, charge/ion radius value, water content in the interlayer space) lie between the values for bivalent and monovalent cations (Foldvari et al. 1998). 

The basal spacing of lanthanoid-montmorillonites is similar to other trivalent montmorillonites and varies between 1.5 and 1.6 nm. For example, the basal spacing of Fe(III)-bentonite is about 1.6 nm (Komlosi et al. 2007 Izumi et al. 2005 Kong et al. 2005), and that of Al-bentonite is 1.576 nm the ionic radius of Fe(III) and Al is, however, much smaller. For the lighter lanthanoids, this value is above... 

The acidic destruction of montmorillonite results in the release of silicon and aluminum. The initial fast exchange of surface cations by hydrogen ions is followed by the release of aluminum and silicon. The dissolution rate of Si is higher than that of A1 and is influenced by the relative ratios of basal siloxane and edge surfaces. The shift of pH to more basic values by the ion-exchange processes and the hydrolysis of dissolved species induce the formation of secondary amorphous solids, initiating the formation of amorphous aluminosilicates (Sondi et al. 2008). 

At the same time, the concentration of manganese ions remains the same. The basal spacing (d001) of montmorillonite determined by x-ray diffraction is very similar to the newly prepared (1.51 nm) and old (1.48 nm) samples. The distribution... 

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