According to the definition of the World Health Organization (WHO), volatile organic compounds (VOCs) are organic compounds with a boiling point of 50 to 260°C. They are precursors of photochemical oxidants. Some pollutants are easily harmful to the human nervous system and may even cause cancer [1]. Among them, acetone is a common typical compound in pharmaceutical, pesticide, chemical, and other industries [2]. Therefore, it is very important to control the concentration of acetone in the air [3]. A variety of acetone treatment technologies have been developed, including gas adsorption [4], catalytic oxidation [5], plasma catalytic oxidation [6], and so on. Generally, the adsorption method is widely used to capture the acetone on the surface of the adsorbent [7], and the most commonly used adsorbent is activated carbon (AC) because activated carbon has the characteristics of developed pore structure and high specific surface area, and low cost, surface reaction, high activity and an excellent adsorbent [8]. However, the structure and surface chemistry of activated carbon is very complex, which easily affects the adsorption capacity of acetone [9].
There have been many studies discussing the influence of the adsorbent pore structure and the chemical properties of the carbon surface on the adsorption of VOCs. For example, Gil et al. [10] used alkaline agents to modify the adsorption material to develop a microporous carbon material. The research results showed that the developed carbon material has a developed microporous structure, which can increase the adsorption capacity of toluene from 27 mg/g to 288 mg/g. Shu et al. [11] used ultrasonic oscillation to prepare iron-loaded activated carbon with different texture characteristics but similar surface functional groups. The results of the study showed that micropores in the range of 0.7-2.0 nm can enhance adsorption [11]. Qi et al. have synthesized spherical porous carbon with a controllable pore structure and proposed that carbon spheres with similar microporosity on the surface chemically play a key role in the VOCs adsorption reaction. A study by Almazan et al. [12] found that the existence of micropores of appropriate size plays a very important role in pollutant treatment, and oxygen-containing groups can inhibit the adsorption of alkanes and benzene. Ghimbeu et al. [13] studied the interaction between oxygen-containing groups and ethanol, and learned that oxygen-containing groups can enhance the adsorption capacity. Liang et al. [4] performed molecular adsorption simulations, and their results confirmed that the addition of oxygen-containing functional groups can increase the adsorption capacity of acetone [4]. It can be seen that the physical and chemical interaction between oxygen-containing groups and pollutants on carbon materials is quite complicated [14]. Some pollutants are physically adsorbed in the pores and easily desorbed due to the airflow, but some can be chemically adsorbed by combining functional groups on the material with the pollutants [15]. Many researchers have studied the physical and chemical adsorption of VOCs, such as toluene, xylene, and alkyl halides, and explored the adsorption characteristics of strongly polar VOCs (such as acetone) and non-polar or weakly polar VOCs, and the characteristics of different pollutants relationship [16].
In recent years, many researchers have developed adsorption materials with higher adsorption capacity, such as metal-organic framework (MOF). MOF is a highly ordered crystalline material composed of metal ions and organic multifunctional ligands [17]. This material has a high specific surface area, multiple functional groups, and adjustable porosity, and can have a wide range of potential applications in gas separation, gas storage, catalysis, chemical sensing, and drug delivery [18]. In order to improve the adsorption capacity of MOF, many researchers have recrystallized cyclodextrin (CD) in the presence of alkali metal cations, and successfully prepared a series of non-toxic α-, β- and γ-cyclodextrin-based MOF (Cyclodextrin metal-organic framework, CD-MOF) [19]. CD-MOFs have good application prospects in the field of gas adsorption and separation. For example, by forming a carbonic acid group, CO2 binds firmly and reversibly with the free primary alcohol group in γ-CD-MOF [20]. Formaldehyde can be adsorbed through the hydrogen bonding of the hydroxyl group in γ-CD-MOFs [21]. γ-CD-MOF can also be used to adsorb sulfur hexafluoride, and the packaging capacity can reach 2.7 ± 0.5% (w/w) after 12 hours of treatment at 1.2 MPa [22]. Al-Ghamdi et al. (2016) used γ-CD-MOF for acetaldehyde adsorption, and its adsorption capacity can be as high as 53 µg/g [23]. Li et al. (2018) used γ-CD-MOF to separate the mixture of CO2 and acetylene (C2H2) and learned that high-purity C2H2 can be obtained by γ-CD-MOF to adsorb CO2 through selective adsorption [24].
Therefore, this study is based on the carbohydrate metal-organic framework of the biocompatible cyclic oligosaccharide cyclodextrin, and the developed CD-MOFs can be biodegraded and reused, and are less harmful to the environment than other metal-organic frameworks. Therefore, the purpose of this study is to explore the CD-MOF treatment of volatile organic compounds and the treatment mechanism.