Selective oxidation of propylene to acrolein over vanadium and niobium doped bismuth molybdates

Selective oxidation catalysis is of vital significance to the well being of society, since it produces about 25% of the most important industrial organic chemicals and intermediates. Examples include acrolein, acrylic acid, and acrylonitrile, which are all based on propylene-fed processes catalyzed by multicomponent derivatives of bismuth molybdates. Besides looking for more efficient catalysts to convert propylene, a lot of research efforts are also in progress aimed at replacing propylene with the more abundant and less expensive propane. However, due to the difficulties in activating propane and suppressing side reactions, rare catalysts have been found that perform at commercially viable levels for selective oxidation of propane yet.;Among the materials developed for this purpose, the so-called M1 catalyst (Mo7.8V1.2NbTe0.94O28.9, Mitsubishi Chemical Corporation) is considered one of the most promising candidates. Up to 62% yield for selective ammoxidation of propylene to acrylonitrile has been reported over this material, but its detailed mechanism is still under study. Particularly, V and Nb have been proposed to be important elements for propane activation by our group and others, and propylene is the first intermediate in the reaction. These mechanisms lead to the rational strategy of combining propane activating features with propylene oxidation activity for designing potential propane oxidation catalysts. Preliminary results showed that adding a small amount of V into beta-phase bismuth molybdate effectively increased the activity for propylene oxidation, but such materials showed no activity for propane oxidation at relatively low temperature. This research is geared towards further exploring and understanding the structure-activity relationships for vanadium- and niobium-doped bismuth molybdates, especially the roles of dopant elements, which will give insight into how to develop a good catalyst system for selective oxidation of propane or other paraffin feedstocks. To fulfil the objectives, the entire work consists of several investigation steps as follows: (1) Synthesis of materials by coprecipitation. (2) Characterization of samples by X-ray diffraction, electron microscopy, total surface area measurement, and X-ray photoelectron spectroscopy. (3) Catalyst testing in a fixed-bed flow reactor equipped with gas chromatograph for product analysis.;beta-Bi2Mo2O9 was chosen as the parent catalyst and three groups of catalysts were synthesized via coprecipitation methods by doping V, Nb, and V & Nb separately. Each group consisted of several samples with different dopant contents ranging from trace quantity to higher, in order to maintain the beta structure as well as to identify the approximate solubility limit of each dopant. Synthesis conditions were selected based on the previous studies.;X-ray diffraction showed that with small amounts of dopants, the beta structure was well maintained. The solubility limit of each dopant was roughly identified and beyond the limits, new phases such as a new ss-variant phase were created. Total surface area measurement by nitrogen adsorption and BET method showed that all catalysts had low surface areas (ca. 1 m2/g), with small differences among them, in accordance with previous studies. Surface morphology from SEM images indicated that doping V and Nb together generated more inhomogeneous particle size distributions than doping solely with V or Nb. Surface compositions of some V-doped samples were investigated by XPS and measured Bi/Mo ratios were near the expected stoichiometric values.;Reaction studies were carried out in a fixed-bed reactor by performing selective oxidation of propylene to acrolein for each group of catalysts. The results showed that for V-doped bismuth molybdates, the activity and selectivity both increased by addition of small amounts of V, but differences among all samples decreased with more oxygen in the feed. It was demonstrated that in no case was the conversion of propylene limited by the availability of gas phase oxygen. Selectivity was observed to be a weak function of both catalyst composition and feed composition. For V/Nb-doped bismuth molybdates, activities were relatively low compared to V-doped samples. Selectivities were also higher than for the parent phase. Nb-doped samples showed comparable conversions to the parent phase with slightly improved selectivies.;Combining all results together, it was found that the activity of each sample was highly related to the corresponding phase, and generally materials with the beta structure showed higher activity than those with a beta-variant structure. The reduced activity could be related to the new phase formed for those samples.;The present work provides more detailed insight into the effects of Group V transition metals on catalytic performance when added into bismuth molybdates for selective oxidation of propylene to acrolein. This work also advances the knowledge of phase diagrams for Bi-Mo(V, Nb)-O systems and extends the foundation for the development of multicomponent metal oxides which retain the structure of the parent bismuth molybdates with potential application in paraffin selective oxidation. (Abstract shortened by UMI.)...