Synthesis, characterization and catalytic testing of layered double hydroxide materials pillared by heteropoly oxometalate anions.

摘要

Layered double hydroxide (LDH) materials pillared with polyoxometalate (POM) anions were synthesized via two routes: the organic anion precursor method and the LDH-OH/LDH-adipate precursor method. Well ordered [M 2+ _RM3+(OH)₂][POM].zH ₂O (M2+ = Mg2+ or Zn2+, M3+ = Ga3+ or Al3+, R = 2, 3, 4 and POM = H₂W₁₂O₄₀6− or PW₁₁O₃₉7−) pillared LDH phases were obtained, however significant differences in the chemical composition and microtextural properties of the products were noted. Most notably, pillaring resulted in the creation of accessible internal surface area only for the products synthesized by the LDH-OH/LDH-adipate method. In contrast, the organic anion precursor method produced layered but non-porous products in which the gallery region was completely filled by anions.; Ion exchange of the LDH-adipate precursor produced a series of Mg _RAl-H₂W₁₂O₄₀6− materials with final R-values of 1.75, 2.31 and 3.51. The pillared products all had BET-N₂ surface areas in excess of 110 m2g −1 and micropore volumes greater than 0.026 mLg−1 . The micropore size distribution plots obtained from argon adsorption isotherms indicate that the micropores become smaller as the surface charge density of the LDH increases; variation of pore size with layer composition has not previously been reported. In addition, the method was also successfully applied for the first time to other layer types, resulting in microporous Mg_RGa-POM and Zn_RAl-POM materials with properties similar to their Mg_RAl-POM analogues. The catalytic activity of pillared and unpillared LDH materials prepared via the LDH-OH/LDH-adipate method was tested using the decomposition of 2-propanol as a probe reaction. Differences were noted for materials of different chemical composition or for identical samples that had been subjected to different activation conditions. Only acid sites associated With the POM pillars were present after activation at 200°C, resulting in 100% selectivity for dehydration to propene. After activation at 550°C, however, the layers containing Zn2+, and to a lesser degree those containing Ga3+, also became active for dehydrogenation to acetone. Clearly, the strength and number of acid sites may be varied by changing the layer or pillar composition, or by using different activation conditions, suggesting that the LDH-OH/LDH-adipate precursor route offers a great deal of flexibility for synthesizing catalysts with modifiable activity.