(651b) Synthesis Of Mo-V-Te-Nb-O Mixed Oxide Catalyst By Microwave-Assisted Hydrothermal Method
Direct ammoxidation of propane is a highly desirable process to produce acrylonitrile, an important chemical intermediate for the manufacture of synthetic fibers, resins and rubber (1). The current process for making acrylonitrile use propylene as the feed which is more costly and less abundant than propane. Several studies have examined a number of catalyst systems for the direct ammoxidation of propane and the Mo-V-Te-Nb-O mixed oxide system is the most promising candidate reported to date (2-21). This catalyst system is made up of two major phases, so-called M1 and M2 possessing an orthorhombic and pseudo-hexagonal structures, respectively, with the M1 phase being more active and selective in propane ammoxidation (22-33). The reproducibility and scalability of present methods for the synthesis of pure, highly crystalline M1 phase are poor and new synthesis methods are needed. The dry-up method originally reported for the preparation of this catalyst system provides poor control over nucleation and growth of the desired M1 phase and often leads to phase mixtures. Hydrothermal synthesis method is more preferred since essentially pure M1 phase catalysts can be obtained by this method (33-36). However, hydrothermal synthesis requires 24-78 hours at 175 0C in order to obtain highly crystalline M1 phases.
Recently we have succeeded in obtaining pure, monophasic 4-component M1 phase catalysts by microwave irradiation of the synthesis mixture after only 2h. Moreover, microwave-assisted synthesis methods are environmentally friendly, rapid, simple and more energy efficient as compared to conventional methods (37). For this synthesis, metal precursors were mixed in deionised water heated at 80 0C, in the required ratio and the final solution was then transferred to Teflon vessel (HP-500), sealed and then kept in a microwave accelerated reaction system, MARS 5 (CEM, USA). The Teflon container was then irradiated with microwaves to a constant temperature for different time, while being rotated inside the oven. After the microwave treatment, the dark powder obtained was filtered, washed with deionised water several times and dried for 12 h at 90 0C. The powder was then calcined under pre-purified nitrogen flow (20 ml min-1) for 2 h at 600 0C. After calcination at 600 0C in N2, highly crystalline material is obtained possessing pure orthorhombic M1 phase structure. The crystallinity of the material is a function of temperature and irradiation time. Microwave irradiation at 175 0C for 2h produced samples with similar crystallinity to that obtained by the hydrothermal treatment at 175 0C for 48h. The SEM images confirmed the typical crystal morphology of the M1 phase.
The Mo?V?Nb?Te?O catalysts obtained by microwave-assisted synthesis were investigated in the ammoxidation of propane. The results show that the performance of this catalyst is significantly better than that of the Mo-V-O and Mo?V?Te?O catalysts and comparable to that of a 4-component Mo-V-Te-Nb-O system prepared by conventional hydrothermal synthesis for 48h. Further details of this work will be presented and discussed at the conference.
The authors acknowledge the financial support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG02-04ER15604. We thank Prof. Junhang Dong (University of Cincinnati) for the access to the MARS 5 microwave system. We also acknowledge the Geology Department, University of Cincinnati, for the access to the XRD equipment.
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