| Catalysis is one of the most important applicant fields of polyoxometalates (POMs) chemistry. Comparing with other inorganic acids, POMs have the following advantages as catalysts:1) POMs show bi-functional catalytic activity because POMs have both excellent redox activity and measurable and controllable acidity;2) POMs are soluble in oxygen-containing organic solvents, which can solve the big problems in solubility of catalysts;3) it is easy to modify and design the POMs catalyst to meet the need of reaction systems because POMs have the identified structure and size;4) the POMs catalysts have high selectivity of corresponding products and low corrosion to equipments without by reaction. Therefore, POMs-based catalysts have great potential in laboratory and industry. However, there are two scientific problems to solve when developing the function-oriented POMs-based catalysts:1) the design and construction of POMs-based catalysts;2) the relationship between the multi-level structure and catalytic activity of POMs. As such, we have done the following investigation to try to solve these two significant scientific problems:1. We have designed and constructed immobilized POMs-based catalyst of H4[SiW12O40]/Si02. The immobilized POM catalyst of H4[SiW12O40]/Si02can realize highly selective and efficient acetalization of various aldehydes and ketones with ethylene glycol and1,3-propanediol under solvent-free conditions. It should be noted that the turnover number (TON) of acetalization can reach as high as30600catalyzed by H4[SiW12O40]/SiO2, which is the highest among the previously reported values. The catalysis of H4[SiWi2O40]/Si02is truly heterogeneous. Successful recovery of the heterogeneous catalyst of H4[SiW12O40]/Si02can be achieved through centrifugation;and the heterogeneous catalyst can be reused for the next run over ten times without obvious decrease of catalytic activities. Furthermore, the composition and structure of H4[SiW12O40]/Si02remains unchanged during the reaction.2. We have designed and constructed the base POM catalysts of Na8H[A-PW9O34] and Na8H[B-PW9O34] by finely adjusting the lacunary sites of POMs. The base POM catalysts of Na8H[A-PW9O34] and Na8H[B-PW9O34] can realize highly selective and efficient Knoevenagel condensation and cyanosilylation of various aldehydes and ketones and synthesis of benzoxazole derivatives. It should be noted that Na8H[A-PW9O34] and Na8H[B-PW9O34] can also realize highly selective and efficient gram-scale Knoevenagel condensation and cyanosilylation of various aldehydes and ketones and synthesis of benzoxazole derivatives. The catalysis of Na8H[A-PW9O34] and Na8H[B-PW9O34] is truly heterogeneous. Successful recovery of the heterogeneous catalyst of Na8H[A-PW9O34] and Na8H[B-PW9O34] can be achieved through centrifugation, and the heterogeneous catalyst can be reused for the next run over six times without obvious decrease of catalytic activities. Furthermore, the composition and structure of Na8H[A-PW9O34] and Na8H[B-PW9O34] remains unchanged during the reaction.3. We have intercalated the acidic POMs anions into basic layered double hydroxides (LDHs) to design and construct the acid-base LDHs-POMs catalysts. The selectivity of oximes is much higher catalyzed by LDHs-POMs catalysts than that by POMs. The enhancement of selectivity is because the hydrogen bonding between POMs and LDHs can trap the surface proton of POMs to reduce the acidity of POMs. The catalysis of LDHs-POMs is truly heterogeneous. Successful recovery of the heterogeneous catalyst of LDHs-POMs can be achieved through centrifugation, and the heterogeneous catalyst can be reused for the next run over ten times without obvious decrease of catalytic activities. Furthermore, the composition and structure of LDHs-POMs remains unchanged during the reaction.4. We have designed and constructed Lewis acid-base POMs catalysts of K11[Ln(PW11O39)2](Ln=La, Y, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Er and Yb) by introducing the Lewis acid active lanthanide ions into POMs. The Lewis acid-base POM catalysts of K11[Ln(PW11O39)2] can realize highly selective and efficient oximation and cyanosilylation of various aldehydes and ketones and synthesis of benzoxazole derivatives. It should be noted that K11[Ln(PW11O39)2] can also realize highly selective and efficient gram-scale oximation and cyanosilylation of various aldehydes and ketones and synthesis of benzoxazole derivatives. The catalysis of K11[Ln(PW11O39)2] is truly heterogeneous. Successful recovery of the heterogeneous catalyst of K11[Ln(PW11O39)2] can be achieved through centrifugation, and the heterogeneous catalyst can be reused for the next run over six times without obvious decrease of catalytic activities. Furthermore, the composition and structure of K11[Ln(PW11O39)2] remains unchanged during the reaction.5. We have designed and constructed a series of amphiphilic surfactant-encapsulated lanthanide-containing POMs catalysts of [(n-C10H21)(CH3)3N]11La(PW11O39)2·7H2O (DA11[La(PW11O39)2]),[(n-C12H25)(CH3)3N]11La(PW11O39)2·7H2O (DDA11[La(PW11O39)2]),[(n-C14H29)(CH3)3N]11La(PW11O39)2·7H2O (TDA11[La(PW11O39)2]),[(n-C16H33)(CH3)3N]11La(PW11O39)2·12H2O (HDA11[La(PW11O39)2]) and [(n-C18H37)(CH3)3N]11La(PW11O39)2·11H2O (ODA11[La(PW11O39)2]). We have investigated the self-assembly behaviour of these surfactant-encapsulated lanthanide-containing POMs catalysts in catalytic reaction medium to deeply understand the self-assembly behaviour of POMs-based catalysts. The experimental results reveal that the POMs lanthanide-containing catalyst of DA11[La(PW11O39)2] can form the hollow nanosphere in catalytic reaction medium, and the hollow nanosphere can catalyze efficiently and selectively the oxidation of alkenes, alkenols, sulfides, silane and alcohol. On the contrary, the lanthanide-containing POMs catalysts of DDA11[La(PW11O39)2], TDA11[La(PW11O39)2, HDA11[La(PW11O39)2] and ODA11[La(PW11O39)2] do not form any special nano structrue in catalytic reaction medium, and they can not catalyze the oxidation of alkenes, alkenols, sulfides, silane and alcohol. The above results reveal that both the structure and the composition of nanosphere are essential for the efficient catalytic reaction. Successful recovery of the POM catalyst of DA11[La(PW11O39)2] can be achieved through centrifugation by adding excess amounts of diethyl ether, and the POM catalyst can be reused for the next run over ten times without obvious decrease of catalytic activities. Furthermore, the composition and structure of DA11[La(PW11O39)2] remains unchanged during the reaction.6. We have designed and constructed a series of amphiphilic surfactant-encapsulated lacunary POMs catalysts of ([(n-C4H9)4N]9[A-PW9O34]·5H2O (TBA9[A-PW9O34]) and (n-C4H9)4N]9[B-PW9O34]·6H2O (TBA9[B-PW9O34]). We have investigated the self-assembly behaviour of these surfactant-encapsulated lacunary POMs catalysts in catalytic reaction medium to deeply understand the self-assembly behaviour of POMs-based catalysts. The experimental results reveal that the lacunary POMs catalyst of both TBA9[A-PW9O34] and TBA9[B-PW9O34] can form the dodecahedral microcrystal in catalytic reaction medium, and the dodecahedral microcrystal can catalyze efficiently and selectively the oxidation of alkenes, alkenols, sulfides, silane and alcohol. On the contrary, TBA-encapsulated other POMs catalysts do not form dodecahedral microcrystal in catalytic reaction medium, and they can not catalyze the oxidation of alkenes, alkenols, sulfides, silane and alcohol. The above results reveal that both the structure and the composition of dodecahedral microcrystal are essential for the efficient catalytic reaction. Successful recovery of the POM catalyst of TBA9[A-PW9O34] and TBA9[B-PW9O34] can be achieved through centrifugation by adding excess amounts of diethyl ether, and the POM catalyst can be reused for the next run over six times without obvious decrease of catalytic activities. Furthermore, the composition and structure of TBA9[A-PW9O34] and TBA9[B-PW9O34] remains unchanged during the reaction. |
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