Figure 2. Alkylate as a function of time with ChCl-Pho (1:2) (a), ChCl-TsOH (1:1) (b), ChCl-BOA (1:2) (c) and ChCl-NH2OH•HCl (1:2) (d) . Reaction conditions: temperature 281.2 K, stirring rate 3000 r/min, mass ratio of DESs/H2SO4 1.0 wt%, volume ratio of H2SO4/hydrocarbon 1.5:1, volume ratio of I/O 12:1.
3.2 Effect of DES concentrations
In order to explore the effect of the DES amounts on the alkylate components, ChCl-Pho (1:2) and ChCl-TsOH (1:1) additives were chosen. The alkylate components as a function of DESs concentrations are shown in Figure 3. From Figure 3a, the amount of ChCl-Pho (1:2) changes from 0 wt% to 5.0 wt% with the optimal mass ratio is about 1.0 wt%, in which the C8 selectivity and TMPs components are 83.64% and 76.85%, respectively, at the temperature of 281.2 K. Meanwhile, with the amount of ChCl-Pho (1:2) increasing from 0 wt% to 0.5 wt%, the LEs and C9+ components decrease slightly, and C8 and TMPs increase obviously, which demonstrates that ChCl-Pho (1:2) has a promoting effect on the generation of C8 components to a considerable extent. When the amount of ChCl-Pho (1:2) increase from 0.5 wt% to 3.0 wt%, the RON of alkylate reaches 97.30, which is ascribed to the conversion of LEs and C9+ components to the TMPs with higher RON. However, when the amount of ChCl-Pho (1:2) reaches 5.0 wt%, the catalytic effect significantly decreases, owing to the fact that excessive additives lead to the decrease of H2SO4 acidity for C4 alkylation. As we all know, the oligomerization of butene and the growth of red oil take place with the lower acidity of catalyst, which results in the increase of C9+ components and the decrease of key components dramatically38.
When ChCl-TsOH (1:1) is used as additives, the change of LEs, C8 and C9+ components follow the similar trend as that of ChCl-Pho (1:2). The difference is that the promoted catalytic performance of ChCl-TsOH (1:1) is slightly weaker compared to ChCl-Pho (1:2). From Figure 3b, the optimal mass ratio is 1.0 wt%, in which the RON is 96.93 and the C8 selectivity and TMPs components are 80.71% and 74.00%, respectively, at the temperature of 281.2 K. According to the previous work, Liu et al found that the addition of little amount of benzene for the C4 alkylation catalyzed by ILs could buffer the strong acidity of chloroaluminate anions to adjust acidity of catalysts and enrich the isobutane at the interface, which confirms that the addition of aromatics has a positive effect in C4 alkylation39. Thus, it is inferred that the intensified catalytic performance of ChCl-Pho (1:2) and ChCl-TsOH (1:1) additives probably comes from the phenol and TsOH, the subclass of aromatics, respectively.
Figure 3c and 3d plot the effect of the ChCl-Pho (1:2) and ChCl-TsOH (1:1) on the C8 components. Clearly, there is a dramatic influence on TMP components, which are 2,2,4-TMPs, 2,3,3-TMPs, and 2,2,3-TMPs. Particularly, at the optimal ratio, the 2,2,4-TMPs, 2,3,3-TMPs, and 2,2,3-TMPs have an increasing trend, which is consistent with the improvement of alkylate quality. In addition, ChCl-Pho (1:2) and ChCl-TsOH (1:1) have a little influence on 2,3,4-TMPs and DMHs, including 2,5-DMH, 2,4-DMH, and 2,3-DMH with lower RON. Therefore, the above results illustrate that the ChCl-Pho (1:2) and ChCl-TsOH (1:1) have a great effect on the conversion of LEs and HEs to TMPs with the higher selectivity of 2,2,4-TMPs, 2,3,3-TMPs and 2,2,3-TMPs. The enhancement of ChCl-Pho (1:2) and ChCl-TsOH (1:1) additives on the quality of alkylate is inferred to be ascribed to improvement of interfacial properties, which will be explained systematically at the molecular level via MD simulation in the following sections.