Gallery height

The gallery height in LDHs is normally estimated [1] by subtracting the thickness of the brucite-Hke layers (assumed [198] to be 0.48 nm) from the... 

Many workers have shown [3,19] that the gallery height in LDHs containing long chain aliphatic carboxylate, dicarboxylate, sulfonate or sulfate guests increases as the chain length increases. For the case of o, 6t)-dicarboxylate anions OOC(CH2)nCOO , the basal spacing shows a mean increase of 0.127 nm/CH2 from n = 3 to 12 as shown in Fig. 21 [213] for Mg/Al LDH... 

Figure 2 illustrates the X-ray powder diffraction patterns for the as -synthesized and the calcined heterostructures. Both diffraction patterns show a 001 reflection indicating the presence of a layered structure with a basal spacing 36.8 A respectively 32.9 A. However, a decrease of 4 A in the gallery height, along with a small loss of crystallinity, can be observed upon calcination. 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

The large difference in gallery heights for Cr3 53 and Cr1 88-montmorillonites leads to dramatic differences in catalytic reactivity (17). Figure 4 illustrates the conversion of cyclohexane to benzene over both materials at 550°C as a function of reaction time. Both catalysts were pre-reduced under H2 in a continuous flow reactor at 500°C, followed by reaction with cyclohexane (weight hourly space velocity = 3, contact time = 6 sec, He carrier gas.) The clay remains intact at these reaction temperatures as evidenced from the thermal data (17). 

Little or no reactivity is observed for Cri.88" montmorillonite, but the Cr3 53 derivative with a la rge gallery height is much more reactive. Also, it should be noted that, as expected, pure Na+-montmorillonite showed no catalytic activity. The decrease in reactivity with increasing reaction time for Cr3 53-montmorillonite most likely arises from the formation of coke and the concomitant occlusion of gallery pores. 

Figure 12 Free energy per unit area (a) and overall polymer density (b) as a function of the gallery height H for the Xsc= 84 system. Calculations are based on the compressible lattice SCFT model.

Figure 13 Calculated density profiles for the sc= — 84 system at various gallery heights H (a) H = 2.4nm (6 lattice units) (b) H=7.2nm (18 lattice units) (c) H— 24 nm (60 lattice units).

Schematic 2. Schematic representation of the system components before and after the intercalation takes place. The changes in entropy and free energy as a function of the change in gallery height are shown in Fig. 6 and 7.

Fig. 6. The change of entropy per area versus the change in gallery height, for the polymer and the surfactant (octadecylammonium) functionalized surface based on the thermodynamic model presented in [26].

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