Applications and Research Progress of Metal Catalysts

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1.1 Introduction Catalytic technology is an important part of the chemical industry, and it finds wide applications in fields such as petroleum refining, chemistry, polymer materials, and pharmaceuticals. For a long time, traditional industrial catalysts have been characterized by high costs and poor selectivity. Moreover, the reactions require harsh conditions such as high temperature and pressure, which not only results in high energy consumption and low efficiency but also leads to the generation of toxic gases, thereby having an adverse impact on the environment. Therefore, developing new catalysts that are green, efficient, and environmentally friendly has always been a goal pursued by people. Metal catalysts generally exist in the form of metal crystals, which have various crystal structures; this provides many different adsorption sites for chemical reactions. At the same time, due to the attraction between these sites, it is possible to promote interactions between the substances being adsorbed, thereby facilitating reactions. This is an advantage of metal catalysts, but it is also a drawback, as different adsorption sites can lead to different reactions, thereby reducing the selectivity of the metal catalysts. This metal catalyst also has a very important feature: it can be effectively decomposed and absorbed, allowing it to participate in various reactions. Metal-supported catalysts are widely used in industry. 2 Advances in Metal Catalyst Research 2.1 Advances in New Technologies for the Preparation of Supported Metal Catalysts Over 90% of the chemical reactions in modern chemical industry and petroleum refining are carried out with the help of catalysts. There are various types of catalysts. Supported catalysts, due to their advantages such as recyclability, reusability, high atom economy, high activity, good selectivity, and stability, have been widely used in fields such as environmental catalysis, fine chemical industry, and petrochemical industry. Such catalysts are typically prepared by methods such as impregnation, precipitation, ion exchange, and melting. In recent years, many scholars at home and abroad have conducted extensive research on the preparation of novel support catalysts. Wu Shihua et al. used SMAI technology to prepare Au/TiO2 catalysts with different gold contents and high dispersion under vacuum conditions via the impregnation method. Studies have found that as the gold content increases, the particle size of the gold particles gradually increases, and the particle size distribution becomes narrower (18–35 nm). The Au/TiO2 catalyst with 18 nm gold particles exhibits the highest activity at 323 K. Wu Shihua and colleagues used SMAI technology to prepare three Pd-Cu/C bimetallic catalysts with different Pd contents in a vacuum reaction system at 0.13 Pa, using metal palladium (spectrally pure) and metal copper with a purity of 99.9% in specific proportions. The results showed that both Pd and Cu were in the zero-valent state and had an average particle size of less than 5 nm. Upon the addition of subisopropylpropanenitrogen, the catalyst activity increased as the Cu content increased, whereas the selectivity for hexacarbon compounds remained unchanged. 2.2 Research progress on supported metallocene catalysts: Studies by Wang Xi and colleagues have shown that loading Et(Ind)2ZrCl2 and MAO on SiO2 can reduce the aluminum-to-zirconium ratio of the catalyst to 215–220. At 50°C and a pressure of 0.6 MPa, with a molar ratio of propylene to ethylene of 2:1, the maximum polymerization activity can reach 1.29×107 g (mol·h)-1; moreover, the molar content of propylene in the polymer is 40.0%, and the molecular weight distribution is 3.19. Kuo-Tseng Li et al. compared the copolymerization properties of nanoscale SiO2 and microscale SiO2 loaded into Cp2ZrCl2/MAO. The results showed that at 60°C, the catalytic activity of the nanocatalyst was 4.35 times higher than that of the microscale catalyst, mainly due to its larger specific surface area, the absence of internal diffusion barriers, and better dispersion of active sites. 2.3 Research Progress on Polystyrene-Loaded Catalysts for Olefin Polymerization. Research on catalyst loading is of great significance for promoting the rapid development of China’s polyolefin industry. Compared with traditional inorganic materials, it features a clear structure and the ability to significantly reduce the inorganic ash content in polymers, and is gradually gaining attention. KalitaA et al. for the first time prepared three Z-N type catalysts with different TiCl4/PS-PMMA mass ratios using a copolymer of styrene and methyl methacrylate (PS-PMMA) as a carrier, and based on this investigated the coordination between the carbonyl oxygen and titanium in the catalyst. When using this catalyst for the production of polyethylene at normal pressure at 50°C, its activity can reach 0.60–1.01 kg/(gh). Although it is lower than that of the Z-N catalyst loaded with MgCl2, its stability is much better than that of conventional Z-N catalysts. 2.4 Research Progress on the Catalytic Reduction of Nitrate Nitrogen in Water by Supported Transition Metal Catalysts. With the rapid development of industry and agriculture, the widespread use of pesticides and nitrogen-containing insecticides, as well as the discharge of large amounts of high-concentration nitride wastewater from industrial processes, the global water environment is deteriorating increasingly. At present, due to the severe impact humans have had on the natural nitrogen cycle, nitrate pollution in surface and groundwater is a global problem. In the 1960s, similar reports emerged in both the United States and Europe, suggesting that the extensive use of nitrogen was the main cause of elevated levels of nitrate nitrogen in natural water bodies. Surveys show that nearly one-quarter of cases in the United States are health risks resulting from excessive nitrogen intake. In the UK, 125 drinking water sources that supply 1.8 million people all have nitrate levels above the standard ； At the same time, nitrate contamination in drinking water is also a problem in cities and towns across our country. Nitrate nitrogen has stable chemical properties, and it is difficult to remove using denitrifying microorganisms in natural water bodies and self-purification methods. In recent years, many scholars have conducted extensive research on the treatment of NO3- in wastewater. Currently, methods for removing nitrate nitrogen from water include adsorption, ion exchange, reverse osmosis, biological denitrification, and chemical reduction. Among them, physical methods generate wastewater with high concentrations of nitrates, requiring secondary treatment and incurring high costs ； Although biological nitrogen removal technology can convert nitrogen in water bodies into harmless forms, it has a weak capacity to withstand shock loads, a long startup time, a complex process, and causes severe secondary pollution. Since Vorlop and Tacke were the first to use metal catalysts to selectively reduce nitrate nitrogen to harmless nitrogen gas, catalytic reactions of transition metals on NO3- have become a topic of intense research. Using transition metal catalysts, the vast majority of nitrate nitrogen in water is converted into nitrogen through catalytic hydrogenation, featuring mild conditions, rapid efficiency, and no secondary pollution. Currently, this method has received widespread attention from countries around the world. TangT et al. used copper as a catalyst and combined it with iron powder to obtain a bimetallic catalyst. This study employs a method that combines catalytic reduction with physical ion exchange resins; using D407 ion exchange resin as the carrier and a copper-iron-based transition metal catalyst, it is possible to effectively reduce the levels of nitrate and nitrite on the resin at an optimal pH of 3, thereby extending its service life. PizarroA H et al. combined palladium with indium oxide to carry out denitrification using hydrogen as a reducing agent. Using nitrate and nitrite as raw materials, hydrogen reduction was carried out at room temperature, achieving good reduction results. Loading an indium oxide catalyst based on palladium on alumina can improve the overall stability of this catalyst. This catalytic process also requires being carried out in an acidic environment, that is, by mixing carbon dioxide and hydrogen into water to lower the pH. As a reducing agent, H2 does not cause secondary pollution, and thus has received widespread attention from scholars at home and abroad. 2.5 Research Progress on Metal Catalysts in the Flame Retardation of Polymer Degradation to Char. In recent years, polymer fires have become a major cause of urban fires. Therefore, there is an increasing emphasis on the flame-retardant properties of polymers. Although flame-retardant technologies for polymer materials have made significant progress in recent years, flame-retardant polymers release harmful gases during combustion and also exhibit severe droplet formation, posing hazards to humans. Modifying polymers to improve their properties such as resistance to melt droplets and smoke suppression is key to ensuring the safety of people and the environment. Li et al. investigated the preparation method of a novel organic-inorganic hybrid polyphosphazene-modified manganese hypophosphite (PZS-MnHP) to improve the flame-retardant and melt-drop resistance properties of polyesters. Studies have found that PET containing 5% PZS-MnHP can significantly reduce fire hazards; its peak heat release rate is 57.0% lower than that of PET, and its limiting oxygen index is 25.3%. At the same time, the presence of PZS-MnHP accelerates the formation of expanded carbon, blocks external oxygen and heat, and significantly reduces the formation of droplets. Cheng et al. added ferrocene-based polymers (PDPFDE) to flame-retardant polypropylene (PP/EG). The test results showed that the PP3 sample (with 20% EG and 5% PDPFDE added) achieved the UL94 V-0 rating. Compared to pure PP, its limiting oxygen index increased from 17% to 28.8%, the peak heat release rate decreased by 82.7%, and the heat release amount decreased by 41.0%. There is a synergistic effect between EG and PDPFDE. PP3 can also produce highly graphitized and dense carbon residues. The use of metal catalysts in flame-retardant polymer materials enables the prevention of heat transfer, the blocking of oxygen, the reduction of melting droplet formation, and the limitation of the release rate and total amount of flammable gases. This achieves three key functions: flame retardancy, resistance to melting droplets, and smoke suppression, meeting the trends toward sustainable development and offering broad application prospects. Currently, although metal catalysts are widely used in flame-retardant polymer systems, aspects such as the relationship between their catalytic char formation and flame-retardant effects, the selection of polymer and catalyst types, and the catalytic flame-retardant mechanism still require further investigation. 2.6 Research Progress on Low-Temperature Denitration Using Transition Metal Catalysts: With the acceleration of urbanization and the rapid economic development in China, air pollution has become an increasingly serious issue. NOx is a major cause of acid rain, fog, photochemical smog, the greenhouse effect, and ozone layer damage, posing various acute or potential threats to human health. In recent years, many flue gas end-treatment technologies have been employed domestically and internationally to control the emissions of NOx from industrial waste gases, among which the NH3 selective catalytic reduction (NH3-SCR) technology based on V2O5-WO3/TiO2 catalysts is the most widely used. In domestic NH3-SCR denitration processes, V2O5-WO3/TiO2 is the primary catalyst used in this technology at present. However, this catalyst still has several issues that need to be addressed, including poor heat resistance, insufficient activity at low temperatures, low N2 selectivity at high temperatures, and the biological toxicity of V2O5. Liu et al. developed a CeO2-WO3-ZrO2 composite catalyst, which can significantly improve the hydrothermal stability of Ce-based catalysts. However, at present, the problem of sulfide poisoning caused by Ce-based materials in low-temperature flue gas desulfurization technologies remains very common. Xu et al. found that in low-temperature environments, ammonium bisulfate deposits on the surface of the catalyst, forming CeSO4, which leads to irreversible sulfur poisoning of the Ce-based catalysts. 2.7 Research Progress on Metal Catalysts Metallocene catalysts have become the next generation of catalysts following traditional Ziegler-Natta catalysts, thanks to their single active center and high catalytic activity. The use of metallocene catalysts enables the production of polymers with good structure and molecular weight. However, during homogeneous polymerization, the morphology of the polymer particles is poor and the reaction vessel tends to get clogged; therefore, it is necessary to load the catalyst onto these particles. Li Wei used the phase transformation method, along with a two-reactor series reaction process using industrial slurry in the laboratory, to prepare blends of polyethylene with extremely high molecular weights and extremely low relative molecular weights. The experimental results show that in the early stage of the reaction, the main reactant is (n-BuCp)2ZrCl2; after 2 hours of reaction, the TiCl3 catalyst becomes involved, granting it an activity duration of 8 hours, which is much longer than that of the carrier catalyst alone. Moreover, the desired blend can be controlled by adjusting the polymerization times for the first and second stages of the reaction. Wang Xiong utilized various inorganic functional monomers to prepare porous polymer materials, whose pore structures, morphologies, and packing densities varied greatly. Therefore, by designing and adjusting the porous structure, it is possible to obtain materials with favorable properties such as structure, particle size distribution, specific surface area, pore volume, and packing density. 3 Conclusion In the petrochemical industry, it is crucial to develop new reaction pathways and processes in order to reduce environmental pollution and energy consumption. The main function of a catalyst is to reduce the activation energy of chemical reactions and accelerate them, which is why it is widely used in fields such as petroleum refining, chemicals, pharmaceuticals, and environmental protection. Advances in catalytic technology are the most powerful driving force behind the development of these industries. The emergence of a new catalyst or new catalyst technology will bring about a revolution, yielding significant economic and social benefits. Currently, humanity is facing many serious problems: the continuous depletion of resources, the comprehensive utilization of resources, resource conservation, as well as development in industries, transportation, and daily life. Economic growth has led to the spread of environmental pollution and the degradation of the natural environment. Therefore, it is necessary to establish and develop an ecological industry that achieves complete recycling of resources, enabling their circular use from production to consumption. The resolution of all these problems relies on catalysts and catalytic technologies. Therefore, countries around the world, especially developed ones, attach great importance to the development of catalysts and consider catalytic technology to be a key technology for the new century.