POM-pillared LDHs

The first crystalline forms of POM - pillared LDHs were reported for Zn2Al, Zn2Cr, and Ni3Al derivatives using Vjo 02g as the pillaring reagent... 

POM-pillared LDHs have also been used for the oxidation of alkenes to epoxides with H2O2 alone or more efficiently assisted by bromide. As previously discussed, a,(3-unsaturated ketones can be readily epoxidized with the help of normal LDH catalysts. So far, however, the epoxidation of nonactivated olefins requires the use of POM-LDH catalysts. For example, cyclohexene is selectively epoxidized using Ni2Al-SiWii04o" as catalyst, with the combination of oxygen and aldehyde as oxidant (487,488). The epoxidation of 2-hexene and 3-methylstyrene was also investigated by the same authors. 

Xu et al. studied the alkylation of isobutane by butene with calcined MgAl-and ZnAl-Si(W207)6 -LDHs and NiAl-P(W207)e -LDH as catalysts. These catalysts from POM-pillared LDHs show high actitivity and selectivity toward Cs compounds (501,502). 

Pillared LDHs are special in that there is freely accessible interlayer space resulting from the intercalation of large polyvalent anions, such as polyoxometalates (POMs) and phthalocyanines (Pc) (see Sec. II.D and HLA.S.b). POMs are acidic, introducing acidic properties to the basic LDH or LDO and making LDH-POM an acidic catalyst. The pillared LDHs can be also used as oxidation catalysts with intercalation of redox-active anions, such as Co-phthalocyaninetetrasulfonate (CoPcTs). The usage of pillared LDH is described according to the reaction types in the following. 

The presence of acidic sites on the POM pillars enables the LDH-POM to act as acid/base bifimctional catalysts or even as strongly acidic catalysts (251-253,468). As mentioned previously, isopropanol is a probe molecule for distinguishing between acidic and basic catalysis. For example, ZnAl-SiWiiM04o-LDH (M = Mn, Fe, Co, Ni, Cu, and Zn divalent ions) has been used as the catalyst for the decomposition of isopropanol to give propene almost exclusively (98%, reaction (B)) at 90% conversion (251). 

The pillaring of LDHs by POMs is further complicated by the fact that most LHDs are basic, whereas the POM anions are acidic. Hydrolysis reactions of the LDH and POM can result in products that are poorly ordered or that contain multiCTystalline phases. Thus, conventional ion exchange routes to pillared derivatives are usually restricted to atypical acidic LDH precursors, such as the Zn2Al-, Zu2Cr- and Ni3Al- structures discussed earlier. 

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