Study On The Catalysts For Selective Oxidation Of Propylene To Acrolein

Acrolein is an important organic intermediate. The most extensive application on market is to synthesize acrylic acid and methionine. Stepwise co-precipitation method is used to prepare Mo-Bi-based catalysts. The acrolein selectivity measured over the catalysts is higher and the yield of byproduct acrylic acid is lower. So as-produced product, acrolein, can properly used for the synthesis of methionine. The catalysts composition was represented by the following formula:Mo12Bi1FeaCobMncKdSieOx. During the catalyst preparation, citric acid and ethylene glycol, which were used as combustion agents, were added to the mixing precipitate of active components to produce the catalyst precursor, followed by drying and calcining to produce the said catalyst. In the course of the calcination of the catalyst, the combustion agents were burned up, left add place over in the catalyst, which could effectively adjust the surface area, pore volume and other physical parameters, as a result, improving the dispersion of active species and the selectivity of the catalyst. The main works of the thesis were as follows:1.Stepwise co-precipitation was used to prepare the catalysts. Taking Mo12Bi1components as catalyst substrate, Fe, Co, K and other elements were added to the catalyst substrate to produce Mo12Bi1FeaCobMncKdSieOx catalyst. The effects of these promoters on catalytic performances of the catalyst for propylene oxidation were investigated. It was found that the addition of Fe alone and Co alone could only improve the selectivity toward acrolein and the conversion of propylene, respectively; but the yield of the desired product acrolein was almost kept unchanged. While Fe and Co were added simultaneously, the activity and selectivity of the catalyst was improved significantly. The conversion of propylene increased from32.0%to87.5%and the selectivity to acrolein increased from59.5%to84.3%. XRD characterization results showed new compounds Fe2(MoO4)3, CoMo04, Co6Mo12Fe4Bi1.5Ox can be formed by adding Fe and Co simultaneously. No new compound phase was found when K was added to the catalyst, but the addition of K could reduce the catalytic acidity and the selectivity to acrylic acid, while increased the selectivity toward acrolein.2. The addition methods of Bi and Mn were optimized. The catalytic activity was further enhanced owing to the addition of Bi2Mo3O12or Bi2Mo2O9; the former was found to be superior to the later. The XRD characterization results showed that Bi2Mo3O12is in favor of producing more Co6Mo12Fe4Bi1.5Ox, which could improve the catalytic performance of the catalyst of the reaction. The addition of Mn improved the activity further. Bi2Mo3O12crystalline phase was produced more, when Mn was introduced in the form of Mn(NO3)2. More Bi2Mo3O12and Co6Mo12Fe4Bi1.5were produced when Mn(NO3)2was replaced by MnMoO4. The yield of acrolein was enhanced from82.6%to88.0%. CoMoO4was easily reduced because of the existence of MnMoO4. We speculate too many CoMoO4is bad for the reaction.3. As binders in the molding process, the silica sol and fine silica powder were simultaneously added to the catalyst, which were found to have a little improvement for the conversion of propylene and to inhibit further oxidation of acrolein to acrylic acid. The binder also increased the strength of molded catalysts and the smoothness of surface and so on. The BET characterization results showed the binder increased the catalytic specific surface area and pore volume.4. The catalyst preparation and the catalytic reaction conditions were optimized. The stepwise co-precipitation of the active components was carried out at ambient temperature; the optimal calcination temperature was500℃. When the space velocity was1200h-1, the catalytic activity was found to be the highest. The conversion of propylene was as high as96.7%and the selectivity to acrolein is91.0%.5. During the preparation of the catalysts by stepwise co-precipitation method, citric acid and ethylene glycol were chosen as combustion agents. The addition of citric acid and ethylene glycol could improve the catalytic activity and selectivity of the catalysts. When30g of citric acid was added to the catalyst, the selectivity toward acrolein was raised by5.2percent. The TG characterization results showed that the citric acid changed the catalytic heat effect, reduced the decomposition temperature of NH4+and NO3-and made the molding process simplified. The addition of ethylene glycol improved the selectivity of the catalysts. BET characterization results showed that the catalytic pore volume was bigger. NH3-TPD characterization results showed that the catalyst basicity increased, making the adsorption of product on the catalyst surface weaker. While, excessive amounts of EG added give rise to the reduction of the catalytic activity, because the active sites may be covered. So, the catalyst CAT-EG10was found to be better than others.6. The catalyst was evaluated in a static bed reactor filled with2mL of the catalyst. The thermal stability test indicates that after200h reaction, the conversion of propylene remained at about93%, the selectivity to acrolein was about between93.5%-96%. The IR and elemental analysis characterization results showed that the surface construction and the chemical composition of the catalyst remained nearly unchanged after reaction. Comparing to industrial catalysts, the results show that C425catalyst has higher selectivity, larger strength, and surface area as well as pore volume.