TSLS complex

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

A schematic representation of a TSLS complex is provided in Figure 1. On the basis of preliminary XRD and stochiometric studies, it appears that the imogolite tubes are in van der Waals contact, most likely in a log-jam-like array in the layer silicate galleries. Although the tubes stuff the galleries, two unique adsorption environments are available, namely, the intra-and inter-tube pores designated A and B in Figure 1. 

In the present work we examine the microporosity of a TSLS complex formed from synthetic imogolite and natural montmorillonite. Nitrogen adsorption and desorption isotherms are reported and analyzed in terms of microporous volume and surface area. Also, the adsorption isotherm for an organic adsorbate, m-xylene, is reported. Preliminary FTIR results for the chemisorption of pyridine and catalytic studies of the dealkylation of cumene suggest that TSLS complexes are promising microporous acids for shape selective chemical conversions. 

An equivalent surface area of 460 m g was determined from the monolayer volume, Vj. The value obtained for the dimensionless energetic constant, C=260, was characteristic of a microporous material. Although the BET surface area may not be a physically precise quantity due to the fact that the nitrogen molecule does not exhibit the same cross-sectional area in a microporous environment as on a flat surface, the BET value is useful for comparisons of relative porosities among a related class of adsorbents. For instance, smectite clays pillared by metal oxide aggregates typically exhibit BET surface areas in the range 150 - 400 m /g. Thus, the TSLS complex is among the more porous intercalated nanocomposites derived from smectite clays. 

The total pore volume (Vq) of the TSLS complex was assessed by applying the Dubinin-Radusikevich equation to the nitrogen adsorption data ... 

Figure 1. Schematic illustration of the TSLS complex formed from imogolite and Na" "- montmorillonite. The larger filled circles represent hydroxyl groups, the open circles are oxygens and the small filled circles are the positions of metal ions such as silicon and aluminum. A and B denote two possible types of micropores.

Figure 2. Isotherms for the adsorption and desorption of nitrogen on the TSLS complex at -196 C.

In order to examine the adsorption of an organic adsorbate by the TSLS complex, the adsorption isotherm for m-xlene (kinetic diameter, 5.6A) was obtained at 25°C. As shown in Figure 5, the shape of the isotherm was again type... 

The acidic functionality of the TSLS complex was demonstrated in the catalytic dealkylation of cumene. For this model reaction at 350 C the conversion of cumene to benzene was monitored as a function of time on stream (see Figure 8). Included in the study for comparison purposes... 

FTIR spectrum (1700 - 1300 cm ) for pyridine adsorbed on the TSLS complex. 

Figure 8. Catalytic dealkylation of cumene at 350 C over the imogolite-montmorillonite TSLS complex and alumina pillared montmorillonite (APM).

Acid-catalyzed conversion, propylene, 83,85/ Acidic surfaces, decavanadate species, 326 Acidification, terephthalate-pill ed hydrotalcite, 142 Acidity catalysts, 244 TSLS complex, 119-127 Activation... 

Adsorption environments, TSLS complex, 120,12 Adsorption measurements, procedures, 90 Adsorption sites, distinguishing between, 93-94 Aging, effect on particle size of titanium dioxide sols, 200-201,203/... 

Figure 3. Structure of a TSLS complex formed by intercalation of an imogolite monolayer in the galleries of Na" -montmorillonite.

Figure 4. Plot of the N2 adsorption data for a TSLS complex according to the Langmuir equation.

The regular microporosity of a TSLS complex was indicated by the fit of the N2 adsorption isotherm [12,13] to the Langmuir equation. The Langmuir... 

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