Describe the development of catalysts for methanol synthesis?

Catalysts for methanol synthesis can be roughly divided into two major categories. One category is copper-based catalysts in which copper is the main active component ； Another category is supported catalysts that use precious metals as active components. Mainly include ； 1 ； Zinc-chromium catalyst------ The zinc-chromium (ZnO/Cr2O3) catalyst is a high-pressure solid catalyst that was first developed by the German company BASF in 1923. Zinc-chromium catalysts have low activity; to achieve higher catalytic activity, the operating temperature must be between 590 K and 670 K. To achieve a high conversion rate, the operating pressure must be 25 MPa–35 MPa; therefore, it is known as a high-pressure catalyst. Zinc-chromium catalysts are characterized by: a) good heat resistance, enabling them to withstand overheating conditions with temperature differences of over 100°C; b) insensitivity to sulfur; c) high mechanical strength; d) a long service life, wide range of applications, and easy operation and control. Compared to copper-based catalysts, they have lower activity, lower selectivity, and make distillation more difficult due to the complexity of impurities in the product. Since the mass fraction of Cr2O3 in such catalysts is as high as 10%, it becomes one of the major sources of chromium pollution. Chromium is toxic to the human body, and such catalysts are being phased out gradually. 2 ； Copper-based catalysts—— Copper-based catalysts are catalysts for methanol synthesis at low temperatures and pressures. Their main component is CuO/ZnO/Al2O3 (Cu-Zn-Al), and they were developed successively by the British company ICI and the German company Lurgi. The operating temperature for low (medium) pressure copper-based catalysts is 210°C–300°C, with a pressure of 5 MPa–10 MPa; this is much lower than the temperature used in traditional synthesis processes, which is favorable for the equilibrium of the methanol reaction. Its characteristics are: a) good activity, with a one-step conversion rate of 7%–8%; b) high selectivity, exceeding 99%, and its impurities consist only of trace amounts of methane, dimethyl ether, and methyl formate, allowing for the production of high-purity methanol; c) poor heat resistance and sensitivity to sulfur. 3 ； Palladium-based catalysts ----- Since copper-based catalysts can achieve a selectivity of over 99%, the focus in the development of new catalysts is on further improving their activity, enhancing their thermal stability, and extending their service life. Most research on new catalysts is based on transition metals, precious metals, etc., but compared to traditional (or conventional) catalysts, their activity is not ideal. For example, catalysts whose main catalytic component is the precious metal palladium show only a modest increase in activity, and the selectivity of some of these catalysts even decreases. 4 ； Molybdenum-based catalysts – Copper-based catalysts are important catalysts in the methanol synthesis industry. However, the presence of small amounts of H2S, CS2, Cl2, etc. in the feed gas can easily lead to catalyst poisoning; therefore, there is an increasing interest in the development of sulfur-resistant catalysts. Zhang Jiyan from Tianjin University developed a MoS2/K2CO3/MgO-SiO2 catalyst for the synthesis of sulfur-containing methanol. Under conditions of a temperature of 533 K, a pressure of 8.1 MPa, an space velocity of 3000 h-1, a ratio of φ(H2) to φ(CO) of 1.42, and a mass concentration of sulfur compounds of 1350 mg/L, the conversion rate of CO was 36.1% while the selectivity for methanol was 53.2%. Although this catalyst has a high one-pass conversion rate, its selectivity is only 50%, and the downstream treatment of by-products is complex; it is still a long way away from industrial application. In addition, there are also Cu-Zn-Al-V series catalysts. The Cu-Zn-Al series of copper-based catalysts includes types such as C207, C301, C3011, NC5011, C306, and C307, represented by the Research Institute of Nanhua Group. The Cu-Zn-Al-V series of copper-based catalysts includes types such as CN J202, C302, C3021, C3022, CN J206, and XNC98, represented by the Southwest Chemical Industry Research Institute. Copper-based catalysts are currently widely used. Future research directions for copper-based catalysts will focus on low temperature, low pressure, high activity, high selectivity, as well as energy efficiency and environmental protection.