SYNTHETIC AND CATALYTIC STUDIES OF INORGANICALLY PILLARED AND ORGANICALLY PILLARED LAYERED DOUBLE HYDROXIDES
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abstract
By using freshly prepared or thoroughtly wet layered double hydroxides (LDHs), or by coprecipitating the LDHs in the presence of the pillaring anions, we have synthesized and performed a catalytic study on a large number of pillared HT anionic clays. The metal cations used in the layers include A1 3+ , Cr 3+ , Fe 3+ , Mg 2+ , Ni 2+ , Zn 2+ , Co 2+ , Fe 2+ , Cue 2+ , Ba 2+ , and Ca 2+ . Pillaring was achieved with an extensive variety of heteropolyoxometalates having the Keggin structure, as well as some other larger heteropolyoxometalate anions. The product was usually a mixture of one phase with an ideally pillared structure and another phase containing defects in the layers; defects that were created during the pillaring reaction. Hexacyanoferrate(111) was also used as a pillaring anion in a clean reaction that provides an example of LDHs pillared with organometallic species. The organic pillars, [O 3 PC 6 H 4 PO 3 ] 4- , [O 3 PC 6 H 4 C 6 H 4 Po 3 ] -4 , and [O 3 PC 6 H 4 C 6 HC 6 H 4 PO 3 ] 4- , first formed intercalates with d-spacings exactly equal to the sum of the height of the pillar and the thickness of the LDH layer. These intercalates, in turn, dehydrate easily to yield lower d-spacing compounds in which the organic pillars are, presumably, fixed onto the layers through POM bonds. The catalysis tests of polyoxometalate-pillared LDHs on isopropanol showed high levels of conversion, generally enhanced acid activity, and a gradual change in selectivity with respect to dehydrogenation versus dehydration. The selectivity data are presented in correlation with the composition of the LDH layers. 1995.
