Adsorption and Decomposition of Chemical Warfare Agents by Metal-Organic Framework

With the discovery of chemical weapons, mankind has faced a great threat. The fact that these weapons caused mass human deaths and were relatively easy to develop made the countries obliged to take precautions in this regard. During World War I, nearly 2 million people died due to chemical weapons. Nerve agents started to be used in making chemical weapons before World War II began. Thus, the threat of chemical weapons on humanity has reached a much more critical point. Especially the development of chemical weapons and the production of nervous agents increased during World War II and during the cold war. Chemical weapons developed by the countries started to fall into the hands of terrorist groups. Sarin gas was used in the terrorist attack that took place in Matsumoto, Japan in 1994. This incident went down in history as the first terrorist attack in which sarin gas was used. These events show how critical it is to take action against chemical warfare agents. The first method of protection from the toxic effects of chemical warfare agents is to destroy the toxic effects by breaking down chemical warfare agents. In this study, the metal organic frames used for this purpose in recent years will be mentioned.

For this purpose, the Geneva protocol was signed in 1925 with the approval of 16 countries. The Geneva protocol is committed to not using chemical weapons in war. However, there is no article in the protocol that prohibits the production, transfer or storage of chemical weapons. Until the mid-1930s, phosgene and mustard were considered the most dangerous chemical warfare agents. This situation ended with the discovery of nerve agents. When trying to develop insecticide in the laboratory, tabun gas was isolated by chance. Then, tests were carried out on experimental animals using tabun gas. As a result of the studies, it was determined that the experimental animals died approximately 20 minutes after exposure to tabun gas [2]. Different criteria can be considered when classifying chemical warfare agents. Considering their mechanism of action and usage patterns, the most important criteria are physical state, toxicological feature and volatility. The classification of chemical warfare agents according to their toxicological properties is as follows; nerve agents, choking agents, blood poisoning agents, blister agents and non-lethal chemical agents.
The discovery of nerve agents is based on studies to develop more effective pesticides. Pesticides are chemicals used to destroy pests and microorganisms that damage the plant [3][4][5]. Molecular structures of both insecticides and nerve agents are based on organophosphorus compounds [5]. Organophosphorus compounds show their effects in the body by blocking nerve conduction.
These compounds are generally esters, amides and derivatives of phosphoric acid in their structure. Organophosphates have lipid solubility [6]. Therefore, they are taken into the body by the skin, mucosa, conjunctiv and respiratory tract. They are also used as a chemical agent or drug in animal and human medicine as a chemical warfare agent in nervous system studies and military purposes [7]. This group of chemicals is most commonly used in pesticides, insecticides, acaricides, etc. They are used commercially and can be easily obtained commercially [4,8].
The general chemical structures of organophosphorus compounds are as follows; The center of the molecule has a phosphate atom and the phosphate atom can double bond with oxygen or sulfur. R1-R2 in the general chemical structure represents hydrogen, alkyl (including cyclic structure), aryl, alkoxy, alkylthiole and amino groups [3,9]. X represents halogens, cyano and thiol groups and inorganic-organic acids. The general chemical structure of organophosphorus compounds is as shown in Figure   1 of organophosphorus compounds. The organophosphorous compounds show their effect on the body through the inhibition of the enzyme acetylcholine esterase. OPCs are hydrolyzed by the enzyme by binding to the serine amino acid present in the active site of the AChE (Acetylcholinesterase) enzyme. Thus, the active site of the enzyme is phosphorylated [10]. The reaction between the AChE enzyme and the OPCs takes place in two steps. First, a reversible enzyme-inhibitor complex is formed. The rate of formation of this complex depends on the structure of the organophosphorous compound, the molecular size and the alkyl groups. After formation of the recycle complex, the substitution of the alkyl group in the structure with -OH results in a non-irreversible complex. This event is called aging and shown in Figure 2 [11][12][13]. One of the most commonly used methods for neutralizing nerve agents is the removal of the R-X bond [14,15].
Organophosphorus compounds can be directly hydrolyzed with water. However, this process is slow and takes a long time [16]. In order for the nerve agents to decomposition effectively, they must first be held by the adsorbing materials developed. Metal organic frames perform perfectly for adsorption of nerve agents and toxic chemicals thanks to their porous structure [17]. The metal organic frame is a new type of material with a porous structure, a high surface area, and inorganic cores connected by organic networks [18]. Thanks to these features, they have the potential to be used in place of the currently used adsorbents. They are used commercially for gas storage, gas separation and catalytic decontamination of nerve agents [19][20][21][22]. Katz  increased. Then, the cotton fabric surface was functionalized with a metal organic frame (UiO-66-NH2). As a result of the analysis of the fabric surface with instrumental devices, it was seen that the surface was homogeneously covered with UiO-66-NH 2 . Soman and sulfur mustard were used to test the chemical warfare agent disintegration ability of the fabric functionalized using a metal organic frame. The test results have shown that the developed cotton fabric has an excellent performance in breaking down chemical warfare agents [25].