Pillaring of Layered Double Hydroxides With Cage-Like Polysilicates: a Possible New Class of Base Catalysts or Catalyst
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polysilicates and their derivatives, which may be used as pillars for LDHs. As can be seen, these anions are all highly charged, which may make them excellent pillars even for those LDHs with high layer charge densities. Table I. Typical Cage-Like Polysilicates and Their Derivatives Structures Si 6 0 15 6 AISi5 O 15 7-
Silicatesa Aluminosilicatesb
Si 8 0 208AlnSisgnO 20 (8+n)"
Germanosilicatesc
GenSi8gnO2O8 " (n=1,2; 3 isomers when n=2)
(n=1-4; 2 isomers when n=2)
Si 10 0 25 10"
GenSi6.nO156" (n=1,2)
a. Yu. I. Smolin, et al., Soy. Phys. Crystallogr. 24, 19(1979); 29, 421(1984); D. Hoebbel,
et al., Z. Anorg. AUg. Chem. 484, 7(1982);
b. G. Engelhardt, et al., Z. Anorg. AUg. Chem.
484, 22(1982); c. C.T.G. Knight, et al., JACS, 30(1986); 1632(1987).
EXPERIMENTAL MgnAI(OH)2n+2CI'xH20 (MgnAl-Cl, n=2-5) LDHs were prepared using the "coprecipitation "method 2 . Briefly, a mixture solution (100ml) of MgCl2-6H20 and A1CI .6H 0, 3 2 and a NaOH solution were simultaneously and slowly added into a reaction flask containing 100ml H 2 0. The rate of addition of the two solutions was adjusted so that the reaction pH was maintained as close to 10 as possible and after the addition of Mg-Al solution, the final pH was raised to 10.2 by the addition of more base. The resulting white slurries were then boiled for 24 hours before being filtered and washed. The products were either dried at 70'C for 3 days for physical measurements or resuspended into 800mn H 20 as stock slurries for pillaring reactions. The total concentration of the metal salt reagent solution (Mg 2 + + A13+) is I mol/l but the molar ratio of Mg/Al changes from 2 to 5 as required by the idealized formula given above. In order to avoid contamination with C0 3 2 -, the NaOH solution was freshly prepared each time by diluting a saturated stock solution with 10 volumes of distilled H2 0. C02 was removed from the distilled H20 by boiling with N2 bubbling for several hours before use and all of the operations were carried out under an atmosphere of N2. The preparation of [N(Me) 4 ]8Si 8 O 2O0xH20 (TMASi8) crystals closely followed the literature method: 36.2g (0.2 mols) of [N(Me) 4 ]OH-5H20 (TMAOH) was dissolved in water and mixed with 40g Ludox solution (0.2 mols in SiO 2 ). More water was then added to give a total volume of 200ml. The cloudy mixture was stirred at room temperature for about a week till it became completely clear. The solution was filtered and its volume reduced to about 3/4 in a vacuum desiccator before crystallization was carried out by slowly evaporating the solution over concentrated H2S0 4 . Crystals were collected by filtration. To carry out the intercalation via ion exchange, 50ml of the stock suspension of LDHs was treated in a ultrasonic bath for about a hour and then slowly added to a freshly made solution of lOg TMASi 8 crystals and 250ml H2 0. The mixture was stirred for 3 hours at room temperature and the solid then isolated and washed by centrifugation. The products were dried at 70'C for three days except the oriented film sampl
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