Disadvantages and Improvement Options for Copper-Manganese Composite Catalysts

Copper-manganese composite catalysts (often in the form of spinel structures such as CuMn₂O₄) have been widely used in the catalytic combustion of volatile organic compounds (VOCs), low-temperature carbon monoxide (CO) oxidation, and certain selective oxidation reactions due to their excellent low-temperature catalytic activity and relatively low cost. However, their industrial application has exposed several key and interrelated shortcomings:

Poor thermal stability: Under prolonged high temperatures or frequent temperature fluctuations, the active components (copper and manganese oxides) of the catalyst tend to sinter and grow, resulting in a sharp decrease in specific surface area and a reduction in active sites, leading to irreversible catalyst deactivation. This limits their service life under conditions with high reaction heat or requiring high-temperature regeneration.

Weak resistance to poisoning, especially to sulfur and chlorine: Trace amounts of sulfur dioxide (SO₂), hydrogen chloride (HCl), or organic chlorine/sulfur compounds in the feed gas can strongly chemically adsorb to the catalyst's active sites, forming stable sulfates or chlorides. These substances can cover or permanently occupy the active sites, leading to chemical deactivation of the catalyst. Copper-manganese catalysts are particularly sensitive to these poisons.

Inadequate hydrothermal stability (poor moisture resistance): Water vapor is a common component in many industrial waste gases. Water molecules compete with reactant molecules (such as CO and VOCs) for adsorption on the catalyst surface. Especially at low temperatures, this competitive adsorption can significantly inhibit the reaction rate. More seriously, in a high-temperature, water-containing atmosphere, water vapor accelerates the migration and crystallization of active components (i.e., hydrothermal sintering) and interacts with manganese species, leading to phase changes and exacerbating physical and chemical deactivation of the catalyst.

Mechanical strength needs improvement: Copper-manganese catalysts prepared by certain preparation methods lack sufficient strength. They are prone to pulverization during loading, airflow impact, and startup and shutdown of industrial reactors, increasing bed pressure drop and even causing reactor blockage, affecting the long-term stable operation of the device.

These shortcomings do not exist in isolation; rather, they interact with each other, collectively restricting catalyst performance. For example, sulfur poisoning and the presence of water can synergistically exacerbate deactivation at high temperatures.

Optimization and Improvement Plans Based on Existing Conditions
To address the above shortcomings, we can approach "system optimization" by improving the catalyst's "external environment" to maximize its strengths and mitigate its weaknesses, fully exploiting its low-temperature activity advantages.

1. Optimizing Reaction Conditions and Creating a Mild Reaction Environment
Reaction conditions are the primary factor directly affecting catalyst life. The key to optimization lies in avoiding prolonged exposure of the catalyst to harsh conditions.

Precisely Control Reaction Temperature: Establish a temperature-zone control system to ensure that the reaction occurs within the catalyst's efficient temperature window, avoiding local overheating. For example, for highly exothermic reactions, multi-stage air intake or intercoolers can be used to maintain a relatively stable and appropriate range (e.g., 200-350°C), effectively preventing sintering and deactivation caused by high temperatures. Using advanced control systems to monitor hotspot bed temperatures in real time and adjust preheat temperature or feed concentration in a timely manner is key to extending catalyst life.

Optimizing the GHSV: Excessively high GHSV results in insufficient contact time between reactants and catalyst, reducing conversion, and causing increased airflow, which in turn increases wear. Excessively low space velocity can lead to localized overheating. The optimal space velocity range should be determined experimentally based on the actual feed gas concentration, ensuring conversion while also balancing thermal management and minimizing mechanical losses. When the feed gas concentration fluctuates significantly, an adjustable bypass system can be considered to stabilize the load entering the catalyst bed.

II. Enhance gas pretreatment to eliminate poisons at the source
The most effective anti-poisoning strategy is to remove as many poisons as possible before the reactants enter the catalyst bed.

Deep desulfurization and dechlorination: To address sulfur and chlorine poisoning, a pre-cleaning unit is essential. Depending on the concentration and type of poisons in the feed gas, a combination of dry methods (such as zinc oxide and activated carbon desulfurizers) and wet methods (alkaline washing) can be used. For complex waste gases containing organic sulfur and organic chlorides, a hydrolysis unit may be required to convert them into easily removable H₂S and HCl before absorption. While this increases the initial investment, it is crucial for protecting expensive catalysts and ensuring long-term stable operation of the unit, making it economical from a lifecycle cost perspective.

Fine dust and mist removal: Dust particles and oil mist in the airflow can physically clog catalyst pores and coat active surfaces. High-efficiency filters (such as bag filters or ceramic fiber filters) and demisters should be installed before the catalyst bed to ensure the cleanliness of the gas entering the catalyst bed. Regular inspection and replacement of filter elements are essential for ensuring effective pretreatment.

III. Active Moisture Control and Moisture Management to Mitigate Hydrothermal Deactivation
The effects of water vapor cannot be completely avoided, but measures can be taken to minimize their negative impact.

Inlet Air Pre-drying: Adding a pre-drying step to the gas pretreatment unit is a direct and effective method. Dehumidification can be achieved through condensation cooling or using an adsorption dryer (such as molecular sieve or silica gel). For continuous production processes, a dual-tower adsorption drying system can be used, with one tower operating and the other regenerating, to achieve continuous dehydration. Keeping the inlet air dew point low (e.g., below -20°C) can significantly alleviate competitive adsorption issues at low temperatures.

Raising the reaction temperature to "use dryness to counteract moisture": Within the permitted range of the process, the reactor inlet temperature should be appropriately raised to keep the bed temperature well above the condensation point of moisture and within a temperature range where competitive adsorption of water vapor is less affected. Although this will increase energy consumption, it may be a better option than catalyst deactivation due to moisture poisoning at low temperatures. The key lies in finding a balance between energy consumption and activity.

Manipulating the local "microenvironment" within the bed: One ingenious approach is to physically mix or layer a certain proportion of highly hydrophobic materials or desiccants (such as hydrophobic molecular sieves or hydrophobically treated alumina balls) at the front end of the catalyst bed. These materials preferentially adsorb moisture from the inlet air, creating a relatively dry local reaction environment for the copper-manganese catalyst in the downstream stage. Furthermore, ensuring good reactor insulation to prevent condensation on the reactor walls due to low temperatures is also a crucial engineering detail.

In summary, improvements to copper-manganese composite catalysts should not be limited to the development of the catalyst itself. By combining optimized operating conditions (stabilizing temperature and maintaining appropriate airspeed), enhanced pretreatment (removing sulfur and chlorine, and removing dust), and proactive moisture control (pre-drying, temperature adjustment, and localized protection), we can systematically improve the operating environment and effectively address shortcomings in thermal stability, poisoning resistance, and moisture resistance. These measures, based on existing technologies and equipment, can significantly enhance the practical performance and service life of copper-manganese catalysts through refined process management and appropriate system modifications, offering exceptionally high engineering value.