## 3 Research Progress on Adsorption and Separation of Hydrocarbon Molecules
### Aromatic Hydrocarbons
#### 3.2.2 Silicon-Based Materials  
As the second most abundant element on the earth, silicon has been widely concerned because of its wide source, friendly environment, and high economic benefits [54]. Many materials with adsorption properties can be obtained by silicon precursor, such as molecular sieve, ordered mesoporous silica, gel, etc. [55; 56; 57], which usually have high specific surface area, high pore size ratio, and low density, so they can be used as good adsorbents. [PERSON]. [PERSON] [58] synthesized MTMS aerogel gel materials to separate hydrocarbon materials from petroleum, including mixed solutions of monocyclic aromatic hydrocarbons MAHs (benzene, toluene, and xylene) and polycyclic aromatic hydrocarbons PAHs (pyrene, benzofluaranthene, and benzopyrene) to obtain adsorption isotherms, They also studied the interaction between aerogels and aromatic compounds during adsorption and desorption and sought the adsorbent materials most suitable for different adsorbents. Figure 10 showed the fitting data of different aromatic compounds using different models. The adsorbate adsorption curve presented a concave profile, and with the continuous increase of adsorbate equilibrium CE, the QE value also increased sharply, which was different from the previous convex profile of benzene and toluene [59]. Therefore, the authors discussed the adsorption parameters of the two adsorption models. Notably, the fitting effect of the graph that the simulation effect of BET is better, while the isomerism value obtained by fitting the Freundlich model (1/n) is greater than 1, which indicates that there are two possibilities for the adsorption mechanism of aerogels for aromatic compounds, one is a multilayer adsorption mechanism, and the other may be dissolved in a solvent into the aerogel and adsorbed from BET. From the bet adsorption model, there is a single-layer adsorption in the early stage (for polycyclic aromatic hydrocarbons, C\({}_{\rm E}\) is \(<\) 12 mg L\({}^{-1}\), while for monocyclic aromatic hydrocarbons, CE is \(<\) 2 mg L\({}^{-1}\)), and then the QE value increases sharply with the increase of CE value, which also indicates that there is multi-layer physical adsorption [60]. Figure 10: Adsorption isotherms of different aromatic hydrocarbon compounds at 25 \({}^{\circ}\)C [58] (The solid line is the fit of the Freundlich model, and the dashed line is the fit of the BET model). Figure 11 showed the adsorption capacity relationship obtained according to the mixed aromatics solutions with different initial concentrations. With the increase of the initial concentration of aromatic compounds, the adsorption capacity of the aromatic compounds also increases. However, it should be noted that no matter how the initial content changes, the percentage of hydrocarbon adsorption of each component remains unchanged and is basically arranged in the order of Xyl > Tol > Ben >> B(a)P > B(b)F >> Pyr. The adsorption percentage of monocyclic aromatic hydrocarbons is much higher than that of polycyclic aromatic hydrocarbons, which may be due to the difference in the adsorption of the two types of aromatic hydrocarbons on the aerogel gel surface. The biggest difference between the two aromatic hydrocarbons lies in their hydrophobicity, and the increase in adsorption capacity has the same correlation with hydrophobicity. This is mainly because the surface of aerogel gel is hydrophobic, and the hydrophobicity of monocyclic aromatic hydrocarbons is greater than that of polycyclic aromatic hydrocarbons, so the adsorption percentage of monocyclic aromatic hydrocarbons is also higher than that of polycyclic aromatic hydrocarbons. The desorption process is a process that can reevaluate the performance of the adsorbent. The mechanism of adsorption can be better understood by studying the desorption process. Figure 12 showed the desorption results of different aromatics by aerogel.