## 3 Research Progress on Adsorption and Separation of Hydrocarbon Molecules
### n-Alkanes  
Straight or branched chain (acyclic structure) n-paraffins or isoparaffins are the main components in petroleum fractions. Octane number is an important measure of the anti-knock ability during gasoline ignition. Under normal circumstances, due to the low octane numbers of n-paraffins and high octane numbers of isoparaffins, aromatic hydrocarbons, and napthenes, the separation of n-paraffins is beneficial to improve the octane number of gasoline [31]. According to the above-mentioned, a molecular sieve is the adsorbent to achieve selective adsorption depending on the molecular size of different components in the mixture and the affinity with the acid site and the internal diffusion effect [32; 33; 34]. It can adsorb alkane cations such as C\({}_{3}\)H\({}_{7}\)\({}^{-}\), and basic sites are used to adsorb H\({}^{+}\). Among many kinds of molecular sieves, the effective pore size of molecular sieve 5A between the kinetic diameters of n-alkanes and isoparaffins is widely used in the separation of n-alkanes. [PERSON] [35] used molecular sieve 5A as an adsorbent for the adsorption and desorption of n-alkanes in naphtha. A series of experiments were carried out under different conditions to explore the influence of various factors on the adsorption and desorption performance. Adsorption experiments were carried out at different temperatures to explore the effect of temperature on the adsorption and desorption performance. Figure 1 shows the content of n-alkanes in the raffinate after the adsorption is completed. It can be seen that when the temperature is lower than 180 \({}^{\circ}\)C, the content of n-alkanes in the raffinate of the reaction system gradually decreases with increasing of the adsorption temperature, and this indicates that in this temperature range, the adsorption performance will increase with the increase of adsorption temperature. Through the desorption tests at different temperatures, Figure 2 shows the content of n-paraffins in the desorption liquid during the desorption process carried out at different temperatures. It can be seen that in the whole temperature range, the desorption efficiency is positively correlated with the temperature, which is basically consistent with the adsorption rule. Figure 3 shows the effect of different space velocities on the adsorption and desorption performance, it can be seen that the molecular sieve has an obvious difference in the adsorption effect of the adsorbate at different space velocities and molecular sieve 5A exhibits obvious desorption effect of alkane at a 12 h\({}^{-1}\) space velocity. In order to explain the effect of space velocity on the adsorption capacity more conveniently, the researchers also studied the relationship between the oil/molecular sieve ratio and the content of n-paraffins (carbon chain length 6-9) in the system after the adsorption is completed under different space velocities. The results obtained are shown in Figure 4. This means that to achieve the same separation effect, the minimum amount of molecular sieve is required when the space velocity is 0.0025 h\({}^{-1}\), which indicates that the optimal space velocity for adsorption is 0.0025 h\({}^{-1}\). During the whole cycle of adsorption-desorption of n-alkanes, the optimum temperature for molecular sieve 5A was 180 \({}^{\circ}\)C, the optimum space velocity for adsorption was 0.0025 h\({}^{-1}\), and the optimum space velocity for desorption was 12 h\({}^{-1}\). Figure 3: Relationship between desorption time and n-paraffin content at different space velocities [35]. (**a**) 25 h\({}^{-1}\) (**b**) 18 h\({}^{-1}\) (**c**) 12 h\({}^{-1}\) (**d**) 7 h\({}^{-1}\) (**e**) 3.5 h\({}^{-1}\). Space velocities: The amount of hydrocarbons passing through 5A catalyst per unit time. Figure 2: The content (%) of n-alkanes in the desorbed liquid at different temperatures [35]. In addition, [PERSON] [36] used pulse experiments to study the adsorption and separation of n-alkanes between 10 and 16 carbon atoms by molecular sieve 5A.