Pillaring of layered double hydroxide with polyoxometalates.

摘要

Pure Zn/Al layered double hydroxides (LDHs) of the type Zn{dollar}sb2{dollar}Al(OH){dollar}sb6 cdot{dollar}A, where A = Cl{dollar}sp-{dollar}, NO{dollar}sb{lcub}3sp-{rcub}{dollar}, have been prepared by an induced hydrolysis method. Several polyoxometalates (POMs) have been intercalated in the Zn/Al LDH gallery by controlled ion-exchange reactions to form new classes of two-dimensional microporous solids.; The intercalation reaction exhibits novel ion exchange selectivities. A complete exchange reaction takes place only under acidic reaction conditions. The {dollar}alpha{dollar} isomer of Keggin-type POM (e.g. {dollar}alpha{dollar}-SiV{dollar}sb3{dollar}W{dollar}sb9{dollar}O{dollar}sb{lcub}40{rcub}sp{lcub}7-{rcub}{dollar}) is more favorably intercalated than other stereo isomers. Such favorable ion exchange behavior for {dollar}alpha{dollar}-Keggin POM can be explained in terms of the commensurate relationship of oxygen layers in the POM and the LDH hydroxyl lattice. The rhombohedral stacking pattern of the hydroxyl layer, which is more thermodynamically favorable than the hexagonal stacking polytype, is retained when the {dollar}alpha{dollar}-isomer is intercalated.; The POMs intercalated in LDH galleries are hydrolytically more stable than POM in the homogeneous solution. In the solid state, the POM-pillared LDH (LDH-POM) is generally stable up to about 200{dollar}spcirc{dollar}C. An amorphous phase is formed around 300{dollar}spcirc{dollar}C, and new crystalline inorganic phases are formed around 500{dollar}spcirc{dollar}C.; FTIR studies indicate that the POMs in LDH galleries experience two major types of interactions: anion-anion repulsions and hydrogen bonding interaction with the LDH layer. These two competing interactions determine the direction of the {dollar}nu{dollar}(M=O) band shift compared to normal potassium or ammonium salts.; LDH-POMs have been found to swell in water. Non-pillared LDH intercalate do not swell in water.; LDH-POMs exhibit high surface areas, compared to non-pillared LDH derivatives. This result indicates that extra space in the gallery has been introduced by pillars.; LDH-POMs are photochemically reactive. They oxidize isopropanol to acetone under irradiation by a Hg(Xe) lamp. The reactivity of the LDH-POM is different from that for POM in homogeneous solution. The higher the charge of POM in the gallery, the more space in the gallery, and the higher the photoreactivity of LDH-POM. (Abstract shortened with permission of author.)