Montmorillonite benzene adsorption

Thermodynamic parameters for acid-activated montmorillonite-benzene complexes have been determined by a group of Russian authors." It is concluded that a phase transition resembling fusion of the guest species occurs in the intercalated complex. The phase-transition temperature increases to that of the melting point of bulk-phase benzene (5.5 C) on attainment of the limiting degree of adsorption. 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

Rogers, R.D., McFarlane, J.C., and Cross, A.J. Adsorption and desorption of benzene in two soils and montmorillonite clay. 

Figures 2.21 and 2.22 refer to the adsorption of low molecular weight aliphatic alcohols from alcohol + benzene mixtures on montmorillonite. This adsorbent Is a so-called swelling clay mineral, meaning that it consists of packages of thin (aluminosilicate) layers that, under certain conditions, swell to give ultimately a dispersion of the individual sheets. Upon this swelling the specific surface area increases dramatically, it can readily reach several hundreds of m g" On adsorption from solution the swelling is determined by the extent to which one or both of the component(s) penetrate(s) between these sheets. In other words, we are dealing here with a non-inert adsorbent. The gas adsorption equivalent has been illustrated in fig. 1.30.

Figure 2.21. Excess adsorption of alcohols from alcohol + benzene mixtures on montmorillonite. (Redrawn from I- Dek y, F. Sz to and L.G. Nagy. Progr. Colloid Polym. Sci. 65 (1978) 125.)...

When the amount of modifier on the montmorillonite surface increases, a regular decrease in differential heats of adsorption is observed, with the simultaneous increase of the specific retention volumes per unit area of external surface and Henry s constants for the hydrocarbons considered (Tables 7, 8). At the same time, an increase in the amount of long-chain cationic surfactants on the kaolinite surface leads to the decrease in both differential heat of adsorption and Henry s constant for benzene (Table 8), not surprisingly, because of a more complete covering of the kaolinite surface hy the presorbed modifying layer and a decrease in the charge of the polar NHj groups of the modifier [41]. 

Specific retention volumes V , (cm /g), Henry s constants Ki (mm) and differential heats of adsorption Qa (kJ/mol) of benzene on organo-substituted kaolinite and montmorillonite samples with different amounts of presorbed modifier a (meq/g) at 120°C... 

The decrease of Vm and Ki values for the adsorption of benzene on the cetylpyridi-nium montmorillonite prepared from the heated Li-form (samples 5, 6) relative to the modified material based on initial Li-montmorillonite (sample 7) can be explained by the closing of the interlayer gaps of the thermoprocessed mineral. This prevents most active sites of the external surface, the micropores on side faces, from taking part in the adsorption-separation processes. 

The adsorption of thiophen on montmorillonites and on other catalysts has been investigated. Several papers of an analytical nature and on the separation of thiophen from benzene have appeared. ... 

Mortland, Pinnavaia and their co-workers have found that the adsorption of certain aromatic molecules on montmorillonites whose exchangeable cations are saturated with transition metal ions leads to the formation of colored complex(2-4). The IR spectrum of benzene complex formed under a dry atmosphere Indicates the chemical state of the adsorbed benzene is remarkably different from that of liquid benzene (2,3), while adsorbed anisole is reported to dimerize to 4,4 -dimethoxy-biphenyl(5). The essential step in the formation of these colored complexes has been shown to involve an electron transfer from the adsorbed organic molecules to the interlayer transition-metal ion, notably by ESR(4,6). However, the detailed spectroscopic information concerning the structure of adsorbed species have not been sufficient. 

In order to understand the structure and the nature of these colored compounds, largely benzene derivatives, we investigated the adsorption of these aromatic molecules on transition-metal, Cu(II), Fe(III), Ru(III) ion-exchanged montmorillonites using resonance Raman... 

Clay complexes with organic cations may absorb neutral organic molecules in the interlayer space. This process is accompanied by a separation of the silicate layers and, generally, with a change of orientation of the organic cation. Thus, adsorption of benzene or chlorobenzene in pyridinium-montmorillonite changes the disposition of the pyridinium ion from parallel to normal to the silicate layers, and an increase of from 1.25 to 1.50 nm is observed. IR studies of the dichroism of specific IR absorption bands show that the pyridinium cations have their N—H groups directed to the layer surface (C2 axis perpendicular to the layers). For chlorobenzene, the molecules also adopt a perpendicular orientation but with the C—Cl bond axis (C2 axis) parallel to the layers (30). 

Clays.—The adsorption properties of clays have been extensively studied in the past and the situation up to 1976 is dealt with in van Olphen s book. A major feature of recent work has been the study of the effect on their adsorption characteristics of modification of clays by exchangeable organic cations. The earlier work of Dekany and his collaborators, outlined in a previous Report (Vol. 3, p. 147) and a publication in Hungarian, has been continued. They have been concerned mainly with the adsorption of benzene + alkane and benzene + alcohol mixtures on montmorillonites and kaolinites treated to varying extents with hexadecylpyridinium chloride (HDPCl) to form HDP-complexes of known HDP-content and having a partially organophilic surface. 

The surface excess isotherms for methanol adsorption from benzene on untreated montmorillonites and kaolinite were both of type II in the Schay classification. " Pretreatment of montmorillonite with water+isopropanol or with methanol (50 h contact with boiling liquid) lowered the initial part of the isotherm indicating that pretreatment had covered some of the more active sites, but the final slope as x (alcohol) 1 was unchanged showing that the... 

Figure 6 Surface excess isotherms for the adsorption from methanol (l) +benzene (2) by HDP-modified montmorillonites. Points, experimental lines calculated from equations (116) and (119). Fraction of ion exchange by hexadecylpyridinium ions I, 0.326 II, 0..536 III, 0.635 VI, 0.842 (Redrawn from Acta Phys. Chem., Acta Sci. Hung., 1977, 23, 485).

The adsorption of benzene, phenol, and two chlorobenzenes from dilute aqueous solution on montmorillonites treated with tetra-alkylammonium ions was studied by McBride, Pinnavaia, and Mortland in work related to pollutant control. Maximum adsorption of benzene and phenol occurred on the tetramethylammonium complex and was three times larger than on a natural soil. However the soil (which contained organic matter) was a better adsorbent for the chlorobenzenes presumably because these molecules were too large to penetrate the interlamellar spaces of the montmorillonite. 

TMA montmorillonite adsorbs aromatic hydrocarbons in the order benzene > toluene > p-xylene, ethyl benzene > o-xylene > dichlorobenzene [98]. The adsorption of benzene by TMA montmorillonite is not influenced by the presence of toluene but adsorption of toluene decreases when benzene is added [99]. 

FIG. 17 Determination of the adsorption capacities from Eq. (15). Curve 1 Na-iUite curves 2-4 HDP-illites curve 5 Na-montmorillonite in the methanol) l)-benzene(2) mixture. 

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