| Utilization of ethane as starting material for direct synthesis of acetic acid andacetaldehyde provides an attractive alternative to organic synthesis, environmentalprotect and utilization of carbon. In this thesis, the preparation, chemical component,molecular structure, chemisorption properties and catalytic performance of the bulkand supported Heteropoly compounds catalysts were investigated systematically.1. The poly-metal heteropoly acid complexes PMo9V3 and PMo9V2Nb1 wereprepared through the combination of common acidification and aether extractionmethod by transtion metal ion V(Ⅴ) and Nb (Ⅴ) modified the mono-metalheteropolyanion acid (12-molybdophosphoric acid). The kieselguhr supportedcatalysts PMo9V3 /K and PMo9V2Nb1/K were prepared by means of isovolumeimpregnaton and surface reaction.The specific surface area, chemical component,surface component, thermal stability, molecular structure, phase structure,oxy-reductive character, chemisorption properties and catalytic performance ofthese catalysts were investigated by the techniques of BET, ICP, TG-DTA, IR, UV-vis,XRD, cyclic voltammetry, TPR, temperature programmed desorption-massspectrometry (TPD-MS) and microreactor.2. The results obtained from the catalyst characterization show that: the specificsurface area of bulk and supported catalysts are about 6-10m2.g-1, the modified bulkcatalysts are still maintained the keggin structure, the supported heteropoly acidcomplexes are highly dispersed in microcrystalline on the surface of the kieselguhrsupport. The thermal decomposation temperature of the modified bulk catalysts isabout 471℃, and the thermal stability of all catalysts is in the order of the dimetalheteropolyanion acid catalyst >the poly-metal heteropolyanion acid catalyst and thesupported catalysts >the bulk catalysts. Two kinds of acidic sites, Bro|¨nsted acid sitesand Lewis acid sites, present on surface of the PMo9V3/K and PMo9V2Nb1/K catalysts.The Br?nsted acid strength of the bulk and supported PMo9V2Nb1 is the most,which correlated with the best oxidation-reduction ability at low temperature.3. There are kinds of active sites, Mo=O, V=O and M1—O—M2 have been foundon the bulk and supported poly-metal heteropolyanion acid catalysts. Ethane can bechemisorbed on these active sites to form poly-types of adsorbing states. Thehydrogen in C2H6 can be adsorbed on the oxygen sites in Nb—O-Mo or V—O-Mo bonds of the catalyst to form a molecular adsorption state. With the temperaturerising, the C-H bond in the adsorbed C2H6 on the bridge oxygen of Nb—O-Mo bondwould be broken to form dissociative adsorption states, then react with the surfaceoxygen to form the target products. It has been found that the iso-nuclear bridgingstructure Nb—O-Mo may be the active site for the oxydation of ethane at lowtemperature.4. The activity of the bulk and supported catalysts evaluated from microreactorshow that: the reaction products of ethane oxidation are acetic acid, acetaldehyde,ethene and COx at low temperature, the activity of the catalysts is in the order ofsupported catalyst > bulk catalyst and PMo9V2Nb1 catalyst > PMo9V3 catalyst.Thereaction temperature effects a obvious efficiency on the activity and selectivity of thecatalysts. With the PMo9V2Nb1/K catalyst, under a optimum conditions of 240℃ ,0.4 MPa and the composition of raw gas C2H6:O2:N2=1.5:1:4(mol%),the conversionof ethane is about 22.5% with the selectivity of 90.8% to acetic acid andacetaldehyde.5. Based on the experimental results mentioned above, a reaction mechanism ofethane selective oxydation to acetic acid and acetaldehyde on the poly-metalheteropolyanion acid catalysts has been proposed, and an elemental modle of theactive structure for the PMo9V2Nb1/K catalyst has also been established, which couldclear the activity and oxidation-reduction ability of the O2-in the iso-nuclear bridgingstructure Nb-O-Mo or V-O-Mo. |
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