On the nature of selective olefin oxidation catalysts derived from molybdate- and tungstate-intercalated layered double hydroxides

Catalysts derived from Mg(2)Al layered double hydroxides (LDHs) intercalated by polyoxometalates (POMs) of molybdenum and tungsten have been previously reported to exhibit substrate shape selectivity in condensed phase olefin oxidations. On the basis of X-ray powder diffraction, Raman spectroscopy, and nitrogen adsorption studies of these and related Zn(2)Al-LDH-POM systems, the present work elucidates the catalyst precursors in terms of mixtures of salt-like and interlayered LDH phases containing Mo(7)O(24)(6-) and W(7)O(24)(6-). Under the conditions used to dry the precursors prior to catalysis (120 degrees C), the LDH components are transformed into layered decomposition products that act as the active catalysts. Substrate selectivity based on the preferred access to the intragallery sites of pillared LDH phases is precluded under catalytic conditions. Even a structurally stable Zn(2)Al-H(2)W(12)O(40)(6-)-LDH catalyst lacks the intragallery microporosity needed for shape selective substrate access to intragallery active sites under condensed phase reaction conditions. Other factors, perhaps selective adsorption based on substrate polarity, are responsible for the observed selectivity. (C) 1998 Elsevier Science B.V.