The calcium catalyst refers to an elemental calcium or calcium compound which has a catalytic function. Calcium is a kind of alkaline earth metal which is soft in texture and silvery white at normal temperature. Calcium is located in the fourth cycle, Group IIA of the chemical periodic table. It is chemically active and forms an oxide or nitride film on the surface of the air to slow down further corrosion. It can be combined with oxygen to form calcium oxide or combined with nitrogen to form calcium nitride Ca3N2. Also, it could be combined with fluorine, chlorine, bromine, iodine, etc. to form the corresponding halide, or combined with hydrogen to form calcium hydride under the action of 400 ℃. It reacts with water at room temperature to form calcium hydroxide and emits hydrogen. It reacts with dilute sulfuric acid such as hydrochloric acid to form salt and hydrogen, and reacts with carbon at high temperature to form CaC2. The types of calcium compounds are very rich, and different calcium compounds are often used as catalysts in different fields.
Figure 1. Calcium oxide catalyst
Due to the rich variety of calcium catalysts and good catalytic performance, they are used in organic synthesis and environmental protection.
- Organic Synthesis: Biodiesel has the advantages of degradable regeneration, non-toxicity, high flash point and high cetane number. It is an environmentally friendly green fuel. The development and application of biodiesel is of great significance for ensuring the steady development of the world and reducing the pollution caused by fossil energy to the global environment. Most of the transesterification reactions in the traditional biodiesel production process use homogeneous catalysts. However, its subsequent processing is complicated and can presents large environmental pollution. The magnetic nanosolid calcium aluminate catalyst has a large specific surface area and is in sufficient contact with the reactants to further increase the catalytic activity. At the same time, since the magnetic nano-calcium aluminate has magnetic properties, it can be recovered by means of an external magnetic field, which further simplifies the subsequent processing. Therefore, the magnetic nano-calcium aluminate catalyst can be used as a heterogeneous catalyst for the preparation of biodiesel. In addition, calcium catalysts can also be used in asymmetric reactions in the field of organic synthesis, such as asymmetric 1,4 addition reactions, asymmetric hydroamination reactions, and the like.
- Environmental Protection: Activated carbon is an indispensable product in national economic construction and people's daily life. It is widely used in environmental protection, food, medicine and other fields. Generally, activated carbon requires more mesopores, while micropores and macropores are less, which makes it easier to adsorb organic contaminants with larger molecular diameters. Calcium oxide has the characteristics of reducing the fusion of coal pyrolysis, and can catalyze dealkylation and dehydrogenation, that is, can catalyze carbonization. Compared with the preparation of activated carbon by the metal compound of alkali metal such as sodium or potassium, the preparation of activated carbon by calcium oxide is simpler, and the prepared activated carbon is also better. Therefore, calcium catalysts are widely used in the preparation of activated carbon. NOx is one of the main components of automobile exhaust, and it mainly includes various oxides such as N2O, NO, NO2, N2O3, N2O4, and N2O5. The NOx in the atmosphere usually exists in the form of NO and NO2. NO can bind to hemoglobin in the blood, reducing blood oxygen production capacity, while NO2 can induce photochemical smog and acid rain. Therefore, effective control of NOx emissions is the key to air pollution control. The NOx tail gas treatment technology mainly includes catalytic decomposition method, storage reduction method and selective catalytic reduction method. Currently, selective catalytic reduction is widely used. The rare earth perovskite-type composite oxide catalyst (such as Ca2Fe2O5) has strong ability to eliminate NO, and has the advantages of low price and high temperature stability. Therefore, the perovskite-type composite oxide can be used as a catalyst in the selective catalytic reduction method for the removal of NOx.
Figure 2. Perovskite-type composite oxide catalyst
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- Wixey, James S. (2011), "Chiral calcium catalysts for asymmetric hydroamination/cyclisation." Chemical Communications (Cambridge, United Kingdom 47(19), 5449-5451.
- Matos, J. (2006), "Methane Transformation in Presence of Carbon Dioxide on Activated Carbon Supported Nickel-calcium Catalysts." Catalysis Letters 109(3-4), 163-169.
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