Ammoxidation Of Ethane To Acetonitrile Over Various Catalysts

There is considerable interest in converting light alkanes directly to higher value organic chemicals because of the low cost and abundance of the alkanes. Recently, more attention has been drawn on studying and developing of novel technical route for the use of ethane, such as the catalytic ammoxidation of ethane to acetonitrile, which is mainly stimulated by the great success of acrylonitrile process via propane ammoxidation. Several catalyst systems, including Cr-Nb-Mo, Nb-Sb/Al2O3 and metal ions exchanged zeolites, have found to be active for the ammoxidation of ethane. Among them, ion-exchanged Co-ZSM-5 zeolites exhibit the best catalytic performance. However, further work is still required in order to study the influence of preparation methods, type of supports, reaction conditions etc on the catalytic performance, and to clarify the nature of active sites for understanding the reaction mechanism. Besides, the catalytic performance of the reported catalysts is still unsatisfied concerning future practical application. Therefore, it still is a highly attractive and challenging research subject to search new catalysts that have unique activity, selectivity for the ammoxidation of ethane. In this work, various ion-exchanged zeolites and modified ion-exchanged zeolites were prepared by different methods, and the catalytic performance was investigated for the purpose to improve the catalytic properties of the catalysts and to find some novel effective catalysts for the ethane ammoxidation to acetonitrile. The physical and chemical natures of catalysts were characterized and studied by XRD, BET, TPR and XPS etc. techniques. The relationship between the structure, acid-base and redox properties of the catalysts and the catalytic performance was built, and the nature of active center and the reaction mechanism were also discussed. The main experimental results and conclusions are as follows: 1. Catalytic performance of ethane ammoxidation over ion-exchanged Co-ZSM-5 zeolites Various Co-ZSM-5 zeolites with different Si/Alratio and crystal size were prepared by ion-exchanged method and the catalytic performance of ethane ammoxidation was investigated. The activities of Co-ZSM-5 decrease with the increase of Si/Al ratio. The main reason is due to the fact that increasing Si/Al ratio can decrease the exchanged amounts of Co ions, which are the main active centers for the ammoxidation reaction. Besides, the change of Si/Al ratio can also affect the selectivity of products. Moreover, Co-ZSM-5 with small crystal size exhibits higher acetonitrile selectivity than the Co-ZSM-5 with big crystal size. Under the same preparation condition, the Co ions are easily exchanged with the Na+ ions located on the zeolites with small crystal size. The existence of co-cations in the Co-ZSM-5 zeolites can considerablyinfluence the reaction performance of the catalysts. With the change of Co contents, Co-HZSM-5 and Co-NaZSM-5 exhibit quite different selectivity to ethylene and CO2. Characterization of the catalysts by TPD, TPR, IR and UV-Vis suggest that the presence of co-cations (Na+, H+) could affect the acid-base and redox properties, which should be the main reason for changing the catalytic performance of the catalysts. By comparing the reaction results of ethane ammoxidation over Na, Ba and H-form Co-ZSM-5, and the state of Co2+ ion species located in the ZSM-5 zeolites, we can concluded that the α-type Co2+ ions should be the main active centers for the ammoxidation reaction. The introduction of moderate amount of B ions can increase the activity and the selectivity of acetonitrile. The results of TPR suggested that the presence of B ions can enhance the reducibility of Co2+ ions and increase the amount of α-type Co2+ ions, hence resulting in the improvement of catalytic performance. 2. Catalytic performance of ethane ammoxidation over ion-exchanged Co-MCM-49 zeolites Various microporous molecular sieves and mesoporous materials were selected as support for the preparation of ion-exchanged Co-containing catalysts. It was found that the pore size of the support and the distribution of framework Al species have considerable effect on the catalytic performance of ethane ammoxidation. The catalytic activity of ethane ammoxidation on microporous zeolites is generally higher than that on mesoporous zeolites. Among the samples investigated, Co-MCM-49 exhibits the highest catalytic activity. The particular pore system of MCM-49 support should be suitable to obtain the Co-ions specieswith high activity/selectivity for the ammoxidation of ethane. Various factors, such as Co contents, reaction temperatures, space velocities (GHSV) and the compositions of feed gas, can considerably influence the catalytic performance of Co-MCM-49 zeolites for ethane ammoxidation. In comparison with the result of oxidation of ethane, we found that the presence of ammonia in the reaction mixture leads to the considerable increase in selectivity to total C2 (ethylene and acetonitrile) and suppress effectively the formation of COx. For the ethane ammoxidation over Co-MCM-49 catalyst, the conversion of ethane can reach to 49.8% with a yield of 17.6% to acetonitrile and 39.3% to total C2 under an optimal reaction condition. These results are very close to the best result of Co-ZSM-5 reported in references. 3. Catalytic performance of ethane ammoxidation over Sb2O3/Co-ZSM-5 and Sb2O3/Co-MCM-49 zeolites High-pressure-hydrothermal exchanged method was carried out to prepare ion-exchanged Co-ZSM-5 and Co-MCM-49 zeolites for obtaining the sample with high Co content. The results of XRD and BET suggested the crystalline level ZSM-5 zeolites decrease slightly after the exchanged process. The Co-ZSM-5 catalysts prepared by this novel route showed higher catalytic performance for ethane ammoxidation than that of the catalyst prepared by conventional aqueous ion-exchanged method. In order to further enhance the catalytic performance of ethane ammoxidation over the catalysts prepared by high-pressure-hydrothermal exchanged method, different kinds of oxides, such as Sb2O3, V2O5, MoO3, Ga2O3, ZnO, WO3, Nb2O5,...