Catalysts for synthesizing dimethyl ether

The so-called two-step method involves using the commonly employed industrial process to first synthesize methanol from syngas, and then dehydrating methanol to produce dimethyl ether. The methanol dehydration process in the two-step process also includes two methods: the gas-phase method and the liquid-phase method. The liquid-phase method generally uses concentrated sulfuric acid or mixed acids as catalysts, and methanol is produced through catalytic dehydration in a stirred-tank reactor. The liquid-phase method is characterized by a relatively low reaction temperature and good reaction efficiency, but it presents problems such as equipment corrosion and pollution from residual liquids and wastewater. The gas-phase method uses solid acid catalysts, and the synthesis takes place in a fixed-bed reactor. It is simple to operate, with no corrosion or waste liquid generated during the synthesis process; however, the reaction temperature is high, so proper consideration must be given to the efficient utilization of thermal energy during design. In terms of plant scale capacity, the production capacity of a single unit in liquid-phase synthesis plants is relatively low, usually only ranging from a few hundred to a few thousand tons per year; therefore, it is not very suitable for use in large-scale manufacturing facilities ； The gas-phase method can achieve a production volume of tens of thousands of tons per unit per year, making it more suitable for large-scale two-step processes.

There are one-step and two-step methods for the production of dimethyl ether. The one-step method involves the direct synthesis of dimethyl ether from feed gas, while the two-step method involves synthesizing methanol from syngas and then producing dimethyl ether through dehydration. ● One-step process: In this method, syngas is produced through the conversion of natural gas or gasification; this syngas then enters a synthesis reactor, where the reactions for methanol synthesis and methanol dehydration take place simultaneously, along with a shift reaction. The product is a mixture of methanol and dimethyl ether, which is separated into dimethyl ether using a distillation unit, while the unreacted methanol is returned to the synthesis reactor. The one-step process often employs bifunctional catalysts, which are generally composed of a physical mixture of two types of catalysts; one of these is a catalyst for methanol synthesis, such as Cu-Zn-Al(O)-based catalysts, BASFS3-85, and ICI-512 ； Another category consists of methanol dehydration catalysts, such as alumina, porous SiO2-Al2O3, Y-type molecular sieves, ZSM-5 molecular sieves, mordenite, etc. ● Two-step method: This method is carried out in two steps, namely, methanol is first synthesized from syngas, and then dimethyl ether is produced by dehydrating methanol in the presence of a solid catalyst. Domestically and internationally, ZSM-5 molecular sieves made with γ-Al2O3/SiO2 are commonly used as dehydration catalysts. The reaction temperature is controlled at 280–340°C, with a pressure of 0.5–0.8 MPa. The one-way conversion rate of methanol is between 70% and 85%, while the selectivity for dimethyl ether is greater than 98%. The one-step synthesis of dimethyl ether eliminates the intermediate steps involved in methanol synthesis. Compared with the two-step method, it features a simpler process flow, fewer pieces of equipment, lower investment costs, and reduced operating expenses, thereby lowering the production cost of dimethyl ether and improving economic efficiency. Therefore, the one-step synthesis of dimethyl ether is a hot topic in research both domestically and internationally. Representative one-step processes developed abroad include the Danish Topsφe process, the American Air Products process, and the Japanese NKK process. The two-step method for synthesizing dimethyl ether is currently the main process used for its production both domestically and internationally. This method uses high-purity methanol as raw material, produces few by-products during the dehydration reaction, results in a dimethyl ether purity of 99.9%, features a mature process design, wide applicability of the equipment used, and simple post-treatment requirements. It can be implemented in methanol production plants, as well as in other facilities that have good infrastructure and are not dedicated to methanol production. However, this process requires steps such as methanol synthesis, methanol distillation, methanol dehydration, and dimethyl ether distillation, resulting in a long production sequence and thus higher equipment investment. However, the vast majority of large-scale dimethyl ether production projects announced abroad currently use the two-step process technology, indicating that this approach possesses strong overall competitiveness. 2.1 Major foreign process technologies (1) Topsφe process The one-step syngas production process of Topsφe is a new technology developed specifically for natural gas feedstocks. The gas generation section of this process uses an autothermal converter (ATR). An autothermal converter consists of three parts: a high-pressure reactor lined with a refractory lining, a combustion chamber, and a catalyst bed. Dimethyl ether synthesis employs multi-stage adiabatic reactors with built-in interstage cooling to achieve high conversion rates of CO and CO2. A mixed bifunctional catalyst for synthesizing dimethyl ether from methanol and for its dehydration using a catalyst. The synthesis of dimethyl ether is carried out in spherical reactors, with a single unit capable of producing 7,200 tons of dimethyl ether per day. The operating conditions selected for the Topsφe process are 4.2 MPa and 240–290°C. At present, no commercial plant for this process has been built yet. In 1995, Topsφe built a pilot plant with a capacity of 50 kg/d in Copenhagen, Denmark, to test the process performance. (2) Air Products’ new liquid-phase dimethyl ether (LPDMETM) process: With funding from the U.S. Department of Energy, as part of a program to develop technologies for clean coal and alternative fuels, Air Products has developed a new liquid-phase dimethyl ether process, abbreviated as LPDMETM. The main advantage of the LPDMETM process is that it replaces traditional gas-fixed-bed reactors with slurry bubble column reactors. The catalyst particles are in the form of fine powder, and a slurry is formed with them using an inert mineral oil. The high-pressure syngas feed is injected from the bottom of the tower and bubbled, ensuring thorough mixing between the solid catalyst particles and the gas feed. Mineral oil is used to ensure better mixing, isothermal operation, and easy temperature control. The dimethyl ether synthesis reactor uses built-in cooling tubes to remove heat while generating steam. The catalyst in the slurry-phase reactor can be easily loaded and unloaded without the need to shut down the operation. Furthermore, since it is an isothermal operation, there is no issue of hot spots in the reactor, and the catalyst deactivation rate **decreases**. Typical reactor operating parameters are: pressure of 2.76–10.34 MPa, with 5.17 MPa recommended ； Temperature: 200–350°C, with 250°C recommended. The catalytic amount is 5% to 60% of the mass of the mineral oil, with 5% to 25% being the optimal range. This process has an advantage over natural gas-based syngas when using CO-enriched coal-based syngas. However, a higher yield can also be obtained using natural gas as a raw material. Air Products has tested this process in a pilot plant with a capacity of 15 tons per day, and the results were satisfactory; however, no large-scale commercial facility has yet been built. (3) The new one-step liquid-phase process by NKK Corporation in Japan: In addition to Air Products, NKK Corporation in Japan has also developed a new process for the one-step synthesis of dimethyl ether from syngas using a slurry reactor. Natural gas, coal, LPG, etc. can be used as raw materials. The first step in the process is gas generation; the syngas is cooled and compressed to 5–7 MPa, after which it enters a CO2 absorption tower to have the CO2 removed. The decarburized feed syngas is passed through an activated carbon adsorption tower to remove sulfides, after which it is heated to 200°C and fed to the bottom of the reactor. Syngas is bubbled through a slurry in the reactor composed of catalyst and mineral oil, to produce dimethyl ether, methanol, and CO2. The reactor product is cooled and fractionated to separate it into dimethyl ether, methanol, and water. The unreacted syngas is recycled back to the reactor. Through distillation, a highly pure dimethyl ether product (95%–99%) can be obtained from the top of the tower, while a crude product consisting of methanol, dimethyl ether, and water is obtained from the bottom of the tower. A semi-industrial plant for the one-step production of dimethyl ether from syngas with a capacity of 10,000 tons per year has been built in Niigata using NKK technology. 2.2 Domestic process technology and research status: Around the 1990s, China began to develop the process technology for producing dimethyl ether using the gas-phase methanol method (two-step process), as well as the corresponding catalysts, and industrial production facilities were quickly established. In recent years, with the upsurge in dimethyl ether production, China’s two-step dimethyl ether production technology has seen further development, and it has now approached or reached the advanced level of foreign technologies. Shandong Jiutai Chemical Technology Co., Ltd. (formerly Linyi Luming Chemical Co., Ltd.) has developed a process for the catalytic production of dimethyl ether using a liquid-phase composite acid dehydration method, for which it holds independent intellectual property rights. A production facility with an annual capacity of 5,000 tons has been built, and more than a year of operational experience has shown that this technology is mature and reliable. The company’s second 30,000-ton/year plant will also come online. The dimethyl ether production technology developed by Shandong Jiutai has passed the evaluation conducted by the Shandong Provincial Department of Science and Technology, and has been recognized as reaching international standards. In particular, the development of liquid-phase composite acid dehydration catalysts and condensation separation technologies effectively overcomes the high purification costs and large capital investment associated with one-step synthesis and gas-phase dehydration, enabling continuous reaction and dehydration processes. This reduces equipment corrosion and investment costs, with an overall recovery rate of over 99.5% and a product purity of at least 99.9%; moreover, the production cost is significantly lower compared to the gas-phase method. In August 2003, the 10,000-ton-per-year dimethyl ether production plant developed through a partnership between Lutianhua and the Japanese company Toyo Engineering successfully underwent trial operation. This device features a rational process flow and optimized operating conditions; it offers high product purity, low material consumption, and low energy consumption. It is at the advanced level in China in terms of process technology, product quality, and the automated operation of hardware equipment. In recent years, China has also been actively developing technologies for the production of dimethyl ether from syngas in a single-step process, and various research institutes and universities have made significant progress in this area. Lanhua Research Institute, Lanhua Fertilizer Plant, and Lanzhou Institute of Chemical Physics jointly carried out a 5 mL pilot-scale study on the production of dimethyl ether from syngas, focusing on process research as well as the preparation of catalysts and the evaluation of their activity and lifespan. The experiment yielded good results: CO conversion rate > 85% ； Selectivity > 99%. Two long-term (500 h, 1000 h) tests showed that the developed catalyst exhibits good stability in synthetic gas derived from industrial feedstocks ； The selectivity of dimethyl ether for organic compounds >97% ； CO conversion >75% ； Dimethyl ether product purity >99.5% ； The overall yield of dimethyl ether was 98.45%. The Dalian Institute of Chemical Physics, Chinese Academy of Sciences, conducted systematic research on the direct conversion of syngas into dimethyl ether using composite catalyst systems. Catalysts of types SD219-Ⅰ, SD219-Ⅱ, and SD219-Ⅲ were selected, all of which exhibited good catalytic performance: the CO conversion rate reached 90%, and the selectivity of the resulting dimethyl ether among oxygen-containing organic compounds was close to 100%. Tsinghua University has also carried out research on one-step dimethyl ether production; using a LP+Al2O3 bifunctional catalyst in a slurry bed reactor, at temperatures of 260–290°C and pressures of 4–6 MPa, the one-pass conversion rate of CO reached 55%–65%, while the selectivity for dimethyl ether was 90–94%. Currently, domestic institutions such as Zhejiang University, Shanxi Coal Chemistry Research Institute, Southwest Chemical Industry Research Institute, and East China University of Science and Technology are also engaged in research on the one-step synthesis of dimethyl ether from syngas. Hangzhou University uses a self-developed dimethyl ether catalyst to synthesize dimethyl ether in the gas phase from the existing semi-water gas in ammonia plants, under specific reaction temperatures, pressures, and space velocities. The one-way conversion rate of CO reaches 60%–83%, with a selectivity of 95%. This technology has now been applied at Hubei Tianli Company to build an industrial plant with an annual production capacity of 1,500 tons of dimethyl ether. This device can produce both alcohol-ether fuels and high-purity dimethyl ether with a purity of over 99.9%, achieving a CO conversion rate of 70%-80%. This is the first set of industrial production facilities in China to produce high-purity dimethyl ether directly from syngas via a one-step process. For the two-step dimethyl ether production process, whether using the gas-phase or liquid-phase method, domestic technology has reached an advanced, mature, and reliable level, providing all the necessary conditions for building large-scale production facilities. The domestic-developed one-step gas-phase process for producing dimethyl ether from syngas is now largely mature, with kiloton-scale plants already built. However, for the construction of large-scale dimethyl ether plants, domestic technology still needs to be tested in practice.

The composite acid method does not suffer from corrosion issues due to the use of reactive distillation; the main challenge is controlling the process parameters. Its advantage is that there is little non-condensable gas, and it is easy to achieve a purity of 99.95%. The solid acid method requires a higher reaction temperature than the liquid composite acid method, which leads to more non-condensable gas being produced, but it offers a larger capacity per tower. Each of these two processes has its own advantages and disadvantages. I have experience with both processes: for composite acids, the scale ranges from 1,000 to 5,000 tons, while for solid acids, it ranges from 10,000 to 50,000 tons. I have not implemented the one-step syngas process; theoretically it is a viable approach, but industrialization is still some time away. I believe there is still work to be done in fully utilizing counterheat optimization within the existing processes.