Looking at the Future Development of the Formaldehyde Industry through the Improvement of Formaldehyde Catalysts

Thread Content

Tangda 1 Overview: The performance of formaldehyde catalysts is an important factor affecting chemical reactions, and the quality of these catalysts has a direct impact on the efficiency of production. The general requirements for catalyst performance are: high catalytic activity, good selectivity, strong mechanical strength, good thermal stability, a certain degree of resistance to poisoning, low self-degradation, and ease of regeneration. China’s formaldehyde industry was established in the 1950s at the Shanghai Solvent Factory, where Soviet technology was used. Pumice silver served as a catalyst, the system pressure was below atmospheric pressure, and the oxidation temperature ranged from 650 to 750°C. Methanol was fed into the evaporator after being diluted with water, using a production process that did not involve any steam for mixing. With the successful development of new processes for producing polyoxymethylene resin and aldehyde-method synthetic rubber in our country, new requirements were put forward regarding the concentration of formalin solutions in the mid-1960s. Iron-molybdenum oxide catalysts began to be used in formalin production, with manufacturers such as Jilin Shijingou Integrated Chemical Plant, Tianjin No. 2 Petrochemical Plant, and Henan Anyang Plastic Factory being among the first to adopt them. In 1977, the Department of Chemistry at Fudan University in China, in collaboration with Shanghai Solvent Factory, successfully developed a new generation of formaldehyde catalyst—electrolytic silver. This catalyst features high activity, good selectivity, low consumption per unit of product, ease of production, and no environmental pollution, and its manufacturing process has been used to this day. Since the beginning of the 21st century, various formaldehyde manufacturers have increased the production capacity of their oxidizers, exceeding the capacities specified in traditional designs; this has led to higher demands regarding the catalyst’s lifespan, activity, and resistance to poisoning. 2 Differences in the performance of the three catalysts and their production processes 2.1 Pumice silver and modified pumice silver catalysts The formaldehyde synthesis process using pumice silver involves a gas-solid multiphase catalytic reaction, and it is one of the most widely used catalytic oxidation catalysts in industry. Pumice silver uses natural pumice as a carrier; after preliminary sorting, acid washing, and water washing to remove the pumice particles with high density and small pores that are not suitable for use, it is crushed and screened to produce irregular spherical particles with a diameter of 3–7 mm. These particles are then baked in a muffle furnace to remove moisture and impurities, at a temperature of around 350–400°C; after 1–2 hours, they are taken out and ready for use. Prepare an aqueous solution of AgNO3 at 12%–13%, fully submerge the treated pumice in it, mix thoroughly, and gradually heat the solution to allow AgNO3 to evaporate slowly. Once AgNO3 has penetrated the pumice, dry it using a muffle furnace, maintaining a temperature of 500–550°C to promote the decomposition of AgNO3. Finally, raise the temperature to 700–750°C for thermal setting; after holding this temperature for 2 hours, remove the sample and let it cool naturally to room temperature before use. The pumice silver produced in this way contains only 35%–42% silver as the active component; this is because during the production process, AgNO3 decomposes to produce harmful gases such as nitrogen oxides. The use of nitric acid for acid treatment can cause burns to people, and the activity of the catalyst decreases after regeneration. As a result of low conversion rates and high consumption of methanol in production, this method has been phased out. Currently, only a few manufacturers still use it as a catalyst due to their use of crude methanol as raw material. Pumice silver catalysts exhibit strong resistance to poisoning, a wide range of temperature tolerance, and it is possible to incorporate other active elements such as catalyst promoters during their preparation; this modified version of pumice silver can improve the performance of the catalyst. To achieve high conversion rates in production, longer residence times and higher reaction temperatures are employed, with the catalyst layer thickness typically ranging from 100 to 150 mm. 2.2 Iron-molybdenum oxide catalysts: China uses the iron-molybdenum method for the production of formaldehyde. Although this method was put into use in the 1960s, technological progress was slow; due to issues with catalyst performance and process control, the level of production remained low. It was not until the 1990s, with the introduction of foreign equipment and technology, that the situation improved, and the production level of formaldehyde using the iron-molybdenum method in China became comparable to that of the silver method. Iron-molybdenum oxide catalysts are used in the process of converting methanol directly into formaldehyde, and this conversion takes place in the presence of an excess of air, at a volume ratio of around 94%. Almost all of the formaldehyde is oxidized in this process. The catalyst is a mixture of iron and molybdenum oxides, and it is placed in the tubes of a tubular oxidizer in sheet, spherical, or granular form. During operation, the catalyst is heated to 260°C by heat transfer oil circulating between the tubes; after the oxidation reaction occurs, the heat is removed by the same heat transfer oil. This system operates quite stably, with excellent reproducibility of performance and good resistance to oxidation. It allows reactions to take place at lower temperatures (300–380°C), thereby reducing the occurrence of side reactions. It also offers high selectivity, enabling the direct production of high-concentration formaldehyde at around 55%, without the need for distillation. Since the catalyst is installed inside the tubes, it is possible to maintain uniformity within the catalyst bed, preventing issues such as cracks or displacement. As a result, the catalyst can last for more than a year. Ammonium molybdate and ferric nitrate, the raw materials, are prepared into solutions of certain concentrations in a specific molar ratio; the pH value is adjusted to initiate the precipitation reaction. The resulting material is then processed through steps such as standing for aging, filtration, washing, drying and grinding, shaping, and calcination for activation. The formed catalyst is cylindrical or annular in shape, with a particle size generally ranging from 3 to 5 mm. The active components are MoO3 and Fe2(MoO4), and the molar ratio of molybdenum to iron being between 2 and 3 is the most representative. The ferro-molybdenum process operates within the lower explosion limit of methanol, which results in relatively large equipment, high initial investment costs, and electricity consumption that is nearly twice that of other processes ； Since there is no hydrogen in the exhaust gases, their calorific value is relatively low, resulting in poor utility value ； Furthermore, catalysts are expensive and cannot be regenerated or reused. At present, there are still relatively few manufacturers in China that use this method. However, this process can directly produce the urea-formaldehyde pre-condensate (UFU) used in resins. As the demand for high-concentration formalin solutions (over 50%) increases, the iron-molybdenum method is widely applied in large-scale formalin production facilities (60 kt/year). 2.3 Electrolytic silver catalysts: Electrolytic silver catalysts are metal-based catalysts that were developed independently in China in the mid-1970s; they are sometimes also referred to as sponge silver. It uses metals such as platinum or titanium-ruthenium as the anode, and pure silver plates as the cathode. In an electrolytic cell filled with an electrolyte containing silver nitrate, a reduction reaction occurs at the cathode, generating electrons that result in the deposition of metallic silver. This electrolytically produced silver is then washed, filtered under vacuum, dried, and heat-treated; after that, it is granulated and screened to produce electrolytic silver catalysts. The electrolysis process involves primary electrolysis and secondary electrolysis. In primary electrolysis, a lower current density (6–7 A/dm2) is used to enable slow silver deposition; the main purpose of this step is purification, and it is generally carried out when using pure silver for the first time or when there is severe contamination by catalysts ； Secondary electrolysis uses a relatively high current density (12–14 A/dm2) with the aim of obtaining a porous catalyst with a large specific surface area while purifying it, thereby enhancing its activity; this method is commonly used in practical production to regenerate deactivated silver. Compared to pumice silver catalysts, electrolytic silver catalysts feature a simpler preparation method, easier regeneration, no generation of harmful gases, lower silver consumption, as well as higher conversion and selectivity; the specific consumption of methanol can generally reach 440–460 kg. However, it is highly sensitive to iron; when the surface of the silver catalyst contains iron impurities, this not only reduces the catalyst’s activity but also promotes the complete combustion of methanol, leading to increased methanol consumption and difficulty in controlling the bed temperature, thereby causing what is known as a \"temperature spike\". In the mid-1980s, China’s formaldehyde industry promoted and publicized a \"silver catalyst purification process technology\" developed by the Shanghai Institute of Materials. This process used pure oxygen at high temperatures (650°C) to oxidize impurities such as iron, copper, cobalt, manganese, and chromium present in the deactivated silver, converting them into metal oxides. Organic impurities like carbon deposits in the catalyst were also oxidized into carbon dioxide and removed. After oxidation, the catalyst was boiled in hydrochloric acid to convert the metal oxides into salts, which were then precipitated out; finally, it was washed with water until it was clean. During the pickling process, silver metal reacts with chloride ions in hydrochloric acid to form AgCl precipitates, which accumulate in the pores on the surface of the catalyst and affect its activity. These precipitates can be removed by neutralization with ammonia. After ammonolysis, NH4Cl remains in the pores of the catalyst, so further purification is required through sublimation; this process also loosens the catalyst and increases its specific surface area, thereby achieving further activation. The purified catalyst exhibits significantly improved mechanical strength in actual production, activity comparable to that of new catalysts, as well as good resistance to poisons and adaptability to temperature. The equipment required for this method is simple, and the raw materials are common and readily available; it constitutes an effective treatment method for formaldehyde manufacturers that do not possess electrolysis facilities. 3 Brief introduction to other methods for producing silver catalysts: (1) A process that uses molten zinc as a catalyst to directly dehydrogenate methanol to produce formaldehyde with an extremely low water content has attracted widespread attention from researchers around the world. The catalysts that have been extensively studied are mainly a series of oxides, such as CuO/SiO2 and ZnO/SiO2. These catalysts are prepared using the sol-gel method: Al(NO3)3 is added to an silver nitrate solution, the silicon-to-aluminum ratio is controlled, ethyl silicate is added, the sol is solidified by heating, the solvent is removed, and the resulting material is then calcined in a muffle furnace at 750–800°C. After cooling to room temperature, it is crushed into particles with a particle size of 40–60 mesh, thereby yielding the Ag-SiO2-Al2O3 composite catalyst. (2) Crystal silver is an electrolytic silver catalyst with a bright, glossy appearance and a crystalline granular structure. During the electrolysis process, it is important to ensure an optimal match between voltage and current, adjust the concentration of silver nitrate, lower the temperature of the electrolyte, and modify the stripping time. The key lies in adding stabilizers and activators to the electrolyte; this results in a catalytic material in crystalline form, namely microcrystalline aggregates. Such crystals are composed of tens of thousands of small crystals, possess a certain degree of hardness and specific surface area, which reduces the thermal contraction rate of the catalyst. It has good thermal stability, low bed resistance, and prevents cracks from occurring due to shutdowns. Its lifespan can exceed 3 months, making it a new type of catalyst with excellent performance – one that is currently being widely adopted in the domestic industry. 4 Conclusion Formaldehyde is the simplest aldehyde in the aliphatic series; it possesses highly reactive chemical properties and can be used to synthesize a variety of compounds. It is one of the important bulk basic chemical raw materials, widely applied in industries such as chemicals, pharmaceuticals, dyes, and agriculture. It is primarily used as a raw material for urea-formaldehyde resins, phenol-formaldehyde resins, and melamine-formaldehyde resins. Additionally, it is used in the production of chemical products such as nylon, 1,4-butanediol, polyformaldehyde, trimethylolpropane, pentaerythritol, urotropin, pyridine, and isoprene. Formaldehyde can also be utilized in the synthesis of biological fragrances, synthetic **, synthetic chelating agents, synthetic additives, and important intermediates. After more than 40 years of development, China’s formaldehyde industry has made significant progress in terms of plant design, catalyst improvement, as well as waste heat utilization and process control. This has enabled the consumption of raw material methanol to approach the advanced levels seen in similar international processes, resulting in improved product quality and an expanding production scale. Currently, there are nearly 300 enterprises in China, with a total production capacity of 7 million tons; supply and demand are generally in balance. However, overall, the scale of formaldehyde production facilities in China remains small, with many facilities scattered across different locations. The pace of technological development is highly uneven, and progress in the production technologies and variety development of downstream derivative products is slow, which is a concerning situation. It is therefore necessary to accelerate research and development efforts, focusing on the development of production technologies for high-concentration formaldehyde and low-alcohol methanol, so as to meet market demands and enable formaldehyde products to play a greater role in China’s national economic development. This post was last edited by lcs000212 on 2007-12-2 16:47]