## 3 Research Progress on Adsorption and Separation of Hydrocarbon Molecules
### Aromatic Hydrocarbons
#### 3.2.1 Carbon Materials
In addition to the micropore effect, there is also a sieving effect in the adsorption process of organic aromatic compounds--the width of the pores determines the entry of the adsorbate into the pores, and it changes with the shape of the adsorbent [48]. Specifically, for cylindrical or ellipsoid adsorbents, the second width of the adsorbate, such as the size of the PNT molecule (11.7 A \(\times\) *8.0 A \(\times\) 3.4 A), determines whether it can enter the adsorbent, such as CNTs and ACF10, while for interstitial adsorbates, the factor that determines adsorbate entry is the smallest size, such as HD4,000. In ACF10, since 70% of the pores are smaller than 1 nm, and the second size of PNT is 0.8 nm, the sieving effect is more obvious in the adsorption effect of ACF10, which is also consistent with the fitting results of the Freundlich model above. [PERSON] [49] also studied the separation behavior of different carbon materials (activated carbon AC, carbon nanotube MWCNT, and graphene oxide GO) for thiophene and dibzenothiophene DBT mixed into n-hexane, and studied the effect of stirring speed, adsorbent mass loading, contact time and adsorption kinetics and initial organosulfide concentration on adsorption. The results show that the adsorption capacity of graphene oxide and carbon nanotubes is weaker than that of activated carbon adsorption desulfurization, but compared with activated carbon, their adsorption speed is faster and the volume quality is lower, which make it can be used for adsorption desulfurization of automobiles and fuel cells, so there is more practical economic value. The author also uses the Freundlich model to fit the adsorption kinetics to judge the applicability of the adsorbent to the adsorbate. For DBT, the obtained isomerism value (1/n) exceeds 1, indicating that carbon nanotubes and the adsorption effect of graphene oxide on it are strong, which are 23.42 and 22.73 mg/g, respectively. The isomerism value (1/n) of thiophene is less than 1, indicating that these adsorbents are not suitable for the adsorption of thiophene. Activated carbon (AC) is the most widely reported carbon adsorption material, which has been widely commercialized due to its multiple sources and high economic value. [PERSON] et al. [50] reported the synthesis of magnetically activated carbon nanocomposites (MNPsGTAC) from green tea waste, with a specific surface of 118.8 m\({}^{2}\) g\({}^{-1}\) and a particle size of 8.6 nm, which were used to evaluate the adsorption efficiency of four priority polycyclic aromatic hydrocarbons (PAHs), and the catalyst could be completely recovered through acetonitrile. For mineral springs, the removal rates of polycyclic aromatic hydrocarbons are 86-98% and 72-89%, respectively. For tap water and river water, the polycyclic aromatic hydrocarbons are completely removed, showing high adsorption efficiency. However, from the above discussion, the adsorption efficiency of graphene and carbon nanotubes is higher than that of AC, which is mainly due to higher specific surface area, stronger hydrogen bonds, and \(\pi\)-\(\pi\) interaction with pollutants. For example, naphthalene molecules contain C-C or two benzene rings containing \(\pi\) electrons. Each carbon atom in the carbon nanotube structure has a \(\pi\) electron orbital perpendicular to the surface of the carbon nanotube, so it can form \(\pi\)-\(\pi\) bonds with CNTs. In addition, there are other adsorption sites, such as the defects of adsorbed polycyclic aromatic hydrocarbons, and the gaps and grooves between CNTs bundles, which can be effective sites for adsorbing aromatic hydrocarbons [51]. [PERSON] et al. [52] reported on the efficient adsorption of phenanthrene in water by magnetic carbon nanotubes (MCNMs) and proved that the adsorption process of phenanthrene on these MCNMs is a spontaneous, exothermic, and entropy reduction process, which can still be recovered and reused after the treatment of n-hexane. [PERSON] et al. [53] used graphene plate metal-organic network nanocomposite to remove naphthalene.