3.7 Interfacial compositions and diffusion
To further study the effect of DESs on the interfacial behaviors of the MixC4 reactants, the interfacial compositions and diffusion coefficients of MixC4 reactants for different DES systems at different concentrations were calculated, as listed in Table 2. In the interfacial regions, there are almost no ChCl molecules, while the ratio of phenyl molecules increases as the concentrations of DESs. This observation is in good agreement with the finding from density profiles. Due to the higher ratio of phenyl molecules at higher concentration of DESs, the ratio of both isobutane and butene gradually declines as a function of concentration. However, the key point factor for alkylation is the ratio of isobutane to butene (I/O ratio) in the interfacial reaction zone. Meaningfully, the I/O ratio of all the systems with DESs additives are quite higher than that of pure H2SO4system (0.850)15. This strongly indicates that the DESs additives can significantly improve the I/O ratio, which can restrain the oligomerization of butene, promote the hydride transfer, and thus lead to higher-quality of alkylate. For ChCl-Pho (1:2)-containing systems, the I/O ratio rises with the concentrations, while the opposite trend is found for ChCl-TsOH (1:1)- and ChCl-BOA (1:2)-containing systems. For the dynamics, the diffusion coefficients ratio of isobutane to butene is also generally larger than that in pure H2SO4 system (0.858)15. Thus, the introduction of DESs can promote the diffusion of isobutane with respect to butene, which is also helpful to restrain the oligomerization of butene, and promote the hydride transfer, resulting in higher-quality of alkylate. For different DESs, the diffusion coefficient ratio of isobutane to butene follows similar trend as the I/O ratio as a function of concentrations. To sum up, one can safely say that the DES additives can efficiently promote the solubility and diffusion of isobutane with respect to butene, and thus make positively contributions to the quality of alkylate.
Table 2. The interfacial compositions and interfacial diffusion coefficients of isobutane and butene, and the ratio of isobutane to butene for different DES systems at different concentrations.