Intercalation chemistry of layered double hydroxides: Materials engineering at a molecular level

The pillaring chemistry of Mg{dollar}{bsol}sb3{dollar}Al layered double hydroxide (LDH) was studied with several types of robust polyoxometalates (POMs) with high molecular dimension and charge to prepare new supergallery LDH intercalates potentially useful for shape-selective adsorption and catalysis. When pre-swelled with adipate anions, LDH hydroxide precursors afforded highly crystalline pillared product at 100{dollar}{bsol}sp{bsol}circ{dollar}C with POMs of Keggin ({dollar}{bsol}alpha{dollar}-{dollar}{bsol}rm H{bsol}sb2W{bsol}sb{lcub}12{rcub}O{bsol}sb{lcub}40{rcub}{bsol}sp{lcub}6-{rcub}{dollar}), Dawson ({dollar}{bsol}alpha{dollar}-{dollar}{bsol}rm P{bsol}sb2W{bsol}sb{lcub}18{rcub}O{bsol}sb{lcub}62{rcub}{bsol}sp{lcub}6-{rcub}{dollar}) and Finke ({dollar}{bsol}rm CO{bsol}sb4(H{bsol}sb2O){bsol}sb2(PW{bsol}sb9O{bsol}sb{lcub}34{rcub}){bsol}sb2{bsol}sp{lcub}10-{rcub}{dollar}) structures. Polyoxometalates of a lacunary Dawson ion ({dollar}{bsol}alpha{dollar}-{dollar}{bsol}rm P{bsol}sb2W{bsol}sb{lcub}17{rcub}O{bsol}sb{lcub}61{rcub}{bsol}sp{lcub}10-{rcub}{dollar}), Finke ions ({dollar}{bsol}rm Zn{bsol}sb4(H{bsol}sb2O){bsol}sb2(AsW{bsol}sb9O{bsol}sb{lcub}34{rcub}){bsol}sb2{bsol}sp{lcub}10-{rcub}{dollar} and {dollar}{bsol}rm WZn{bsol}sb3(H{bsol}sb2O){bsol}sb2(ZnW{bsol}sb9O{bsol}sb{lcub}34{rcub}){bsol}sb2{bsol}sp{lcub}12-{rcub}{dollar}, a doubly condensed Dawson ion ({dollar}{bsol}rm P{bsol}sb4W{bsol}sb{lcub}30{rcub}Zn{bsol}sb4(H{bsol}sb2O){bsol}sb2O{bsol}sb{lcub}112{rcub}{bsol}sp{lcub}16-{rcub}{dollar}) and macro inorganic polyoxocryptates ({dollar}{bsol}rm NaSb{bsol}sb9W{bsol}sb{lcub}21{rcub}O{bsol}sb{lcub}86{rcub}{bsol}sp{lcub}18-{rcub}{dollar}, {dollar}{bsol}rm NaP{bsol}sb5W{bsol}sb{lcub}30{rcub}O{bsol}sb{lcub}110{rcub}{bsol}sp{lcub}14-{rcub}{dollar} and {dollar}{bsol}rm NaAs{bsol}sb4W{bsol}sb{lcub}40{rcub}O{bsol}sb{lcub}140{rcub}{bsol}sp{lcub}27-{rcub}{dollar}) also have been successfully pillared into Mg{dollar}{bsol}sb3{dollar}Al LDH, using the LDH hydroxide precursor derived by a thermal decomposition and layer reconstitution procedure of a Mg{dollar}{bsol}sb3{dollar}Al LDH carbonate. Incorporation of the macromolecular oxide POM ions with oxygen numbers up to 140 and charge to 27{dollar}{bsol}sp{lcub}-{rcub}{dollar} into the LDH gallery resulted in crystalline LDH-POM intercalates stable up to 250{dollar}{bsol}sp{bsol}circ{dollar}C. Topotactic pillaring reactions of Mg-Al LDH with POM ions exhibited considerable increase in gallery height from {dollar}{bsol}sim{dollar}4.0 A of the LDH carbonate up to {dollar}{bsol}sim{dollar}17 A. Gallery orientation of the POM intercalants were mainly determined by H-bonding interactions between the oxygen framework of the POM pillar and LDH layer hydroxyls. Nitrogen adsorption/desorption studies on the LDH intercalates showed that access to the gallery micropores was achieved upon POM pillaring.; A new synthetic route to POM pillared LDHs also is presented based on the atopotactic reactions of the LDH carbonate with glycerol or triethyleneglycol at 120 to 130{dollar}{bsol}sp{bsol}circ{dollar}C and subsequent ion exchange reaction of the polyol-solvated LDH intercalates with the pillaring POM ions.; LDHs interlayered with silicate anions were prepared by reaction of tetraethylorthosilicate with LDH hydroxide precursors. In addition to the study on the textural properties of the obtained LDH intercalates, acid/base catalytic properties of the LDH-carbonates, silicates, and polyoxometalates were also studied using 2-methyl-3-butyn-2-ol as a reactive probe.