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.