Preparation And Research On Si-Al Based Catalyst For DME Synthesis In Slurry Reactor

At present, the excessive development and utilization of fossil fuels has led to the increasingly serious energy crisis and environmental problems. Because of this, the exploitation and utilization of renewable energy has received more attention and concern. Dimethyl ether (DME) has been widely known as a clean alternative fuel due to its similar physical properties with liquefied petroleum gas (LPG). DME has extensive sources, which can be synthesized from natural gas, crude oil, residual oil, coal and biological debris and other resources. So DME is known as the clean energy of the 21st century, which is a kind of important materials to solve the energy and environmental problems in the future.In the process of the synthesis of dimethyl ether, one-step synthesis of DME in slurry reactor is characterized by its process simplicity and the low production cost, which has become a new technology and main research field at home and abroad. However,the catalysts prepared by traditonal methods can not be well adapted to the slurry bed reactor.The poor stability of the catalyst is the main obstacle for its extensively industrial application. Complete liquid-phase technology is aimed at the problems of catalysts used in slurry bed reactor and based on the concept that’the formation and growth of all materials are finished in the course of adapting to surroundings. This technology can make the catalyst materials more rheology and stability. Subsequently, the research in our group showed that Cu-Zr, Cu-Zn-Al, Cu-Zn-Al-Zr catalysts prepared by this method and used in the slurry reactorshowed the excellent stability. After that, our group introduced Si-component into the catalyst Cu-Zn-Si system, which improved the catalytic activity and reduced costs. It was found thatthe catalyst was proved higher activity and the selectivity of dimethyl ether with the deactivation. Previous research showed that the loss of Cu component in the reaction was the main reason for its deactivation.In this paper,on the basis of the analysis and summary of the previous research and experiments, changing the preparation of the catalystand the silicon source and conditions were studied in detail. Eventually, we got the better catalytic performance and excellent stability of the catalyst.The catalysts before and after reaction were characterized by XRD, H2-TPR, NH3-TPD-MS,XPS techniques, etc. The main conclusions are drawn as follows:1. Catalyst precursors were prepared by flow coprecipitating method, the time and its pH should be rational controlled. When t= 2h, pH= 7-7.2, the catalytic performance is the best.2. The selection of silicon source has a great influence on the catalyst. The addition of the silicon was not the crititical reason for the deactivation of the Si-Al catalyst, but the introduction of the silicon source itself was. The introduction of the fresh silica must match with the reaction system to improve the catalytic activity and stability.3. The silicon sol itself is in a metastable state, which would make the dehydration polymerization occur with the storage time.We should use the alkaline silica which is placed within eight months to ensure the stability of the catalyst.4. The addition of Si species would enhance the interaction between copper and the otheractive ingredients,and promote the dispersion of Cu species on the catalyst surface. At the same time,the size of copper grains was reduced.5. The introduction of Si species would interact with Al components, thereby affecting the dehydration properties of catalyst.6. Surface acid properties of catalysts were adjusted by adding silicon, which improve dehydration properties to greatly reduce the selectivity of hydrocarbons and increase the selectivity of dimethyl ether. When the atomic ratio of Si / Al in the catalyst reached 2, the catalytic performance was relatively higher.