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N-butane Oxidative Dehydrogenation Of Metal Molybdate Catalysts

Posted on:2005-09-08Degree:MasterType:Thesis
Country:ChinaCandidate:Y F LiuFull Text:PDF
GTID:2191360122497466Subject:Applied Chemistry
Abstract/Summary:PDF Full Text Request
The selective oxidation of light saturated hydrocarbons into more valuable unsaturated hydrocarbons and organic compounds-containing oxygen has become one of the challenging research subjects in heterogeneous catalysis. With the increasing desire of butene, the n-butane oxidative dehydrogenation (ODH) being as a vital reaction of the selective oxidation has drawn more and more attention throughout of the world. Compared with the direct dehydrogenation of n-butane to n-butene, the oxidative dehydrogenation of n-butane has many advantages, such as: it is quite thermodynamically favorable due to the formation of water in the reaction; the presence of oxygen in the feed can reduce the formation of coke and the reaction can be performed at relative low temperature being beneficial for the save of energy, and so on. Thus, it is becoming increasingly important not only in academic research but also in industrial utilization.The molybdates of the first transitional period metal (Cr, Mn, Fe, Co, Ni, Cu, Zn) and Mg are prepared by citrate method for the first time and their catalytic performance for the ODH reaction of n-butane to butenes is also studied. It shows that CoMoO4 and MgMoO4 have superior catalytic property for n-butane ODH reaction, over which the selectivity to C4 olefins (butenes and butadiene) can reach 80% and 75%, respectively. The XRD patterns suggest the metal molybdates prepared by citrate method to be monophasic. The results of NH3-TPD and H2-TPR indicate that the redox property of the catalysts plays a key role in the n-butane oxidative dehydrogenation and their catalytic performance has no relationship with the acid-base character of the catalysts. The possible reaction network for the oxydehydrogenation of n-butane, in which the CO is produced only by the consecutive reaction of G4 olefins whereas the butadiene and CO2 are also formed in the initial parallel comparative (with the formation of butene) reaction, is proposed. Further research shows that over metal molybdates the activation energy for the oxydehydrogenation reactions is higher than that to the formation of carbon oxides.The BET result indicates that cobalt molybdate has a specific surface area less than 6 m2/g. In order to increase the catalytic efficiency of CoMoO4, three types of CoMoO4/SiO2 catalysts are prepared by different methods: in citrate method impregnating SiO2 particle(l) and silica gel (2); in precipitation method depositing CoMoO4 on silica gel(3). The catalytic performance of the supported cobalt molybdate catalysts for the ODH of n-butane is originally investigated in this work. Among the supported cobalt molybdate catalysts, the one with CoMoO4/SiO2=0.4 catalyst prepared by method (1) shows the highest catalytic activity and leads to the best yield in C4 olefins. The characterization of XRD and TG/DTA on CoMoO4/SiO2 catalysts indicate that no molybdenum or cobalt containing phase except CoMoO4 phase presents over the SiO2 support and it can be deduced that there are cooperative action In the n-butane ODH reactionbetween CoMoO4 and Si-OH, which results in the substantial decrease of the C4 olefins selectivity. Compared with the catalyst prepared by the traditional precipitation method, CoMoO4 is more dispersive on the SiO2 support in the catalyst prepared by citrate-sol-gel method that exhibits better catalytic activity for n-butane ODH reaction. The time on stream study shows that the SiO2-suppported CoMoO4 is quite stable hi the n-butane oxidative dehydrogenation reaction.
Keywords/Search Tags:oxydehydrogenation, n-butane, metal molybdates, citrate method, butene, silica
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