Abstract
To enhance the catalytic performance of H2SO4-catalyzed alkylation, various catalytic additives have drawn considerable attention. Herein, the effects of deep eutectic solvents additives (DESs) on catalytic performance and interfacial properties of H2SO4 alkylation were systematically investigated using experimental methods and molecular dynamics (MD) simulation. Experimental results indicate that DESs additives with the optimal concentration about 1.0 wt% can efficiently improve C8 selectivity and research octane number (RON) of alkylate. However, DESs additives contribute less to the quality of alkylate at low temperature and to the lifetime of H2SO4. MD results reveal that the phenyl molecules of DESs additives play a major role in enhancing interfacial properties of H2SO4 alkylation, including enlargement of interfacial thickness, promotion of isobutane relative solubility and diffusion to butene, which is probably the main reason for the better quality of alkylate. This work gives a good guideline for the design of novel DESs for H2SO4 alkylation.
Keywords : H2SO4 alkylation, DESs, Liquid/liquid interface, MD simulation.
Introduction
The alkylate, produced by isobutane alkylation (C4 alkylation) with C3-C5 olefins using strong acid as catalyst, is an ideal blending component of the gasoline pool, owing to its numerous advantages, such as high RON, low Reid vapor pressure, free of sulfur, absence of aromatics and alkenes compounds1-4. The commercial alkylation process usually uses liquid acid as catalyst, including concentrated sulfuric acid (H2SO4) and hydrofluoric acid (HF)5,6. However, HF suffers from high toxicity and volatility once it releases or forms aerosol, which can result in large potential danger3,7. The solid acid as an environmentally friendly alternative shows a good selectivity and catalytic activity8-11. However, the disadvantages of easy deactivation by coking and difficult regeneration restrict its industrial application1,11-13. Ionic liquids (ILs) are also the promising alternative to catalyze C4 alkylation, but the disposal of spent salt is too difficult. Currently, H2SO4 is still the dominant catalyst for the industrial alkylation process. However, the drawbacks of equipment corrosion and high acid consumption of H2SO4 alkylation process motivate researchers to develop various additives, such as aromatics, surfactants, and ILs, to improve the catalytic activity of H2SO46,10,13,14.
It is well-confirmed that surfactants as the additives of H2SO4 alkylation can efficiently enhance the quality of alkylate15,16. Chen et al. reported several cationic, anionic, and amphoteric surfactants as additives in C4 alkylation in 200317. Our recent work gave a detailed investigation about the effect of surfactants on the catalytic performance and interfacial features of H2SO4 alkylation using experiments and molecular dynamics (MD) simulation15,18. In spite of the good performance of surfactants for C4 alkylation, the commercial surfactants, such as sodium dodecyl benzene sulfonate (SDBS) and hexadecyltrimethylammonium bromide (CTAB), contain the sodium and bromine elements, which can accelerate equipment corrosion15. ILs have been proved as potential catalysts or additives for C4 alkylation4,19-21. The chloroaluminate-based ILs with Lewis acidity were the most frequently studied to enhance the C4 alkylation. Liu et al. reported that chloroaluminate-based ILs containing CuAlCl4 complexes show better catalytic performance22. In addition, Brønsted acidic ILs (BILs) were also demonstrated to possess excellent catalytic activity for C4 alkylation. For example, 1-(3-sulfopropyl)-3-methyl-imidazolium hydrogen sulfate and 1-(3-sulfobutyl)-3-methylimidazolium hydrogen sulfate ([MBSIm][HSO4]) coupled with strong acid were investigated by Tang et al23. However, despite the efficient improvement of ILs, the high cost of raw material and complicated preparation process inhibit its further development to some extent.
More recently, DESs have been attracting numerous attentions, which consist of hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) via hydrogen-bonding interaction, and thus the melting point is obviously lower than either individual pure component24,25. The DESs exhibit excellent physical and chemical properties, such as low vapor pressure, relatively wide liquid range, non-flammable, conductivity, non-toxicity, sharing similar properties as ILs24-26. Moreover, DESs can be prepared easier with lower cost of raw materials, resulting in that DESs has emerged as the promising alternatives to ILs. Yu et al used acidic DES, consisting of trifluoromethanesulfonic acid (TfOH) and taurine (TAU), as the catalyst for C4 alkylation. They found that [TfOH]3[TAU]/PEG-200 catalytic system displays the encouraging increase in catalytic activity and selectivity with the C8 selectivity up to 85.54%27. However, the applications of DESs as additives in C4 alkylation still remain seldom. More importantly, there is no systematical report regarding to the effect of DESs additives on the catalytic and recycle performance of H2SO4 alkylation. In addition, the behaviors of DESs additives at the H2SO4/C4 hydrocarbons interface are still insufficient. Fortunately, molecular dynamics (MD) simulation is a powerful tool to probe the microscale interfacial properties, which has been well proved to be able to efficiently reveal the interfacial behaviors of ILs and surfactants additives at the H2SO4/C4 hydrocarbons interface in our previous papers7,15,18,28-32.
Therefore, in this paper, the effects of DESs additives on the catalytic performance and interfacial properties of H2SO4-catalyzed C4 alkylation were studied in details using experimental methods and MD simulation. The investigated DESs include choline chlorides-phenol (ChCl-Pho (1:2)), choline chlorides-p-toluenesulfonic acid (ChCl-TsOH (1:1)), choline chlorides-benzoic acid (ChCl-BOA (1:2)), and choline chlorides-hydroxylamine hydrochloride (ChCl-NH2OH•HCl (1:2)). The effect of DESs concentration, reaction time, temperature and recycle times on the C4 alkylation were investigated systematically. Additionally, MD simulations were used to reveal the enhancement of (ChCl-Pho (1:2)), ChCl-TsOH (1:1), and ChCl-BOA (1:2) on the interfacial properties of H2SO4 alkylation.