Studies On The Preparation Technology Of Catalyst For DME Synthesis In Slurry Reactor

DME is an environmentally benign, non-toxic, biodegradable product with physical properties similar to LPG. The current key markets for DME as a fuel are LPG blend stock, transportation fuel as diesel substitute, fuel for power generation using gas turbines, and chemical intermediate for olefins and gasoline production. Nearly all of the DME is produced in China from coal-derived methanol via the well-known catalytic dehydration process where two molecules of methanol react to form one molecule of DME and one molecule of water. Currently in China, the largest market for the use of DME is as a blend fuel with LPG for residential cooking and heating purposes, more than 90% of DME produced in China is blended with LPG. Currently, efforts are underway to commercialize DME as a high-quality diesel alternative.Direct synthesis of DME from syngas (called STD process) is considered to be a promising process. As STD process is a strong exothemic reaction process, the traditional fixed bed reactor is not appropriate because of its limited heat removed capacity, in which aspect the slurry reactor has many merits in caloric transfer, energy utility and catalyst replacing on line and so on, and has been successfully applied in F-T synthesis process. However, the deactivation of catalysts for one-pot synthesis of DME in the slurry reactor has not been effectively overcome yet. Based on the analysis of characteristics of slurry bed catalyst, we invented a novel technology for the preparation of slurry catalysts named complete liquid-phase technology, and the technology successfully prevents the deactivation, but the activities of the catalysts are not satisfying compared with that of those prepared by the conventional methods, such as coprecipitation or impregnation. In the present work, the complete liquid-phase technology is obviously improved in order to greatly promote catalyst activity.In this work, we improve the preparation of catalyst precursors of slurry catalysts in the complete liquid-phase technology. In order to get excellent catalysts, we design some special steps to adjust the rate of hydrolysis/condensation in sol-gel system; the different kinds of alcohols as well as different amounts of them are systematically investigated; a series of catalysts are prepared by the same method in order to repeat the best one; the impact of alcoholysis time on the catalytic activity is also investigated. These slurry catalysts are characterized by XRD、H2-TPR、NH3-TPD-MS、N2 adsorption、 XPS and TEM-EDX respectively. The main conclusions are as follows:(1) The modified aluminium alkoxide prepared by alcoholysis reaction is used as aluminium sources in the complete liquid-phase technology for the preparation of slurry catalysts, combining some special synthetic steps to adjust the rate of hydrolysis/condensation. These catalysts have excellent performance, the best one has the highest CO conversion and dimethyl ether selectivity, reaching 62.6% and 62.5% respectively.(2) The pre-alcoholysis of AIP is one of the most important steps in the catalyst synthesis, which not only can change the textural and surface properties of the catalysts, but also have a significant influence on the physicochemical properties of active components, such as reducibility, dispersity, particle size and so on, which directly influence the performances of catalysts.(3) Controlling the rate of hydrolysis/condensation through pre-alcoholysis of AIP can obtain small active metal size, large surface area and pore volume, proper surface acidity and strong interaction between catalyst components, all of which are beneficial to the catalyst performance.(4) The fresh AIP is not suitable for the process of pre-alcoholysis, only after placing a long time (destroying its crystal structure), can it reach the good results.(5) The aluminium sources obtained by pre-alcoholysis must undergo aging process (one day) before the next synthetic steps. Or it will never work effectively.(6) The time of pre-alcoholysis has a great influence on the activity of catalyst, and two hours are the best. More than 4 hours of pre-alcoholysis are not benefit of activity due to destruction of the crystal face of aluminum species.(7) The ZnO particles serve as spacers between the active copper nanoparticles to make them stabilized, reducing the dosage of Zn will lead to catalyst deactivation for the loss of copper components.