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\begin{document}

\title{Effects of Deep Eutectic Solvents on H2SO4-catalyzed Alkylation:
Combining Experiment and Molecular Dynamics Simulation}



\author[1]{Zhihong Ma}%
\author[1]{Jialei Sha}%
\author[2]{Weizhong  Zheng}%
\author[1]{Weizhen Sun}%
\author[1]{Ling ZHAO}%
\affil[1]{Affiliation not available}%
\affil[2]{East China University of Science and Technology}%


\vspace{-1em}



  
  \date{June 16, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{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.%
\end{abstract}%



\sloppy


\textbf{Effects of Deep Eutectic Solvents on
H\textsubscript{2}SO\textsubscript{4}-catalyzed Alkylation: Combining
Experiment and Molecular Dynamics Simulation}

\textbf{Zhihong Ma\textsuperscript{1}, Jialei Sha\textsuperscript{1},
Weizhong Zheng\textsuperscript{*,1}, Weizhen Sun\textsuperscript{*,1},
and Ling Zhao\textsuperscript{1,2}}

\textsuperscript{1}State Key Laboratory of Chemical Engineering, School
of Chemical Engineering, East China University of Science and
Technology, Shanghai 200237, China

\textsuperscript{2}School of Chemistry \& Chemical Engineering, XinJiang
University, Urumqi 830046, China

\textbf{Abstract}

To enhance the catalytic performance of
H\textsubscript{2}SO\textsubscript{4}-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
H\textsubscript{2}SO\textsubscript{4} 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
C\textsubscript{8} 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
H\textsubscript{2}SO\textsubscript{4}. MD results reveal that the phenyl
molecules of DESs additives play a major role in enhancing interfacial
properties of H\textsubscript{2}SO\textsubscript{4} 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
H\textsubscript{2}SO\textsubscript{4} alkylation.

\textbf{Keywords} : H\textsubscript{2}SO\textsubscript{4} alkylation,
DESs, Liquid/liquid interface, MD simulation.

\textbf{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 compounds\textsuperscript{1-4}. The commercial
alkylation process usually uses liquid acid as catalyst, including
concentrated sulfuric acid (H\textsubscript{2}SO\textsubscript{4}) and
hydrofluoric acid (HF)\textsuperscript{5,6}. However, HF suffers from
high toxicity and volatility once it releases or forms aerosol, which
can result in large potential danger\textsuperscript{3,7}. The solid
acid as an environmentally friendly alternative shows a good selectivity
and catalytic activity\textsuperscript{8-11}. However, the disadvantages
of easy deactivation by coking and difficult regeneration restrict its
industrial application\textsuperscript{1,11-13}. Ionic liquids (ILs) are
also the promising alternative to catalyze C4 alkylation, but the
disposal of spent salt is too difficult. Currently,
H\textsubscript{2}SO\textsubscript{4} is still the dominant catalyst for
the industrial alkylation process. However, the drawbacks of equipment
corrosion and high acid consumption of
H\textsubscript{2}SO\textsubscript{4} alkylation process motivate
researchers to develop various additives, such as aromatics,
surfactants, and ILs, to improve the catalytic activity of
H\textsubscript{2}SO\textsubscript{4}\textsuperscript{6,10,13,14}.

It is well-confirmed that surfactants as the additives of
H\textsubscript{2}SO\textsubscript{4} alkylation can efficiently enhance
the quality of alkylate\textsuperscript{15,16}. Chen et al. reported
several cationic, anionic, and amphoteric surfactants as additives in C4
alkylation in 2003\textsuperscript{17}. Our recent work gave a detailed
investigation about the effect of surfactants on the catalytic
performance and interfacial features of
H\textsubscript{2}SO\textsubscript{4} alkylation using experiments and
molecular dynamics (MD) simulation\textsuperscript{15,18}. 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
corrosion\textsuperscript{15}. ILs have been proved as potential
catalysts or additives for C4 alkylation\textsuperscript{4,19-21}. 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 CuAlCl\textsubscript{4} complexes
show better catalytic performance\textsuperscript{22}. In addition,
Br\selectlanguage{ngerman}ønsted 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{]}{[}HSO\textsubscript{4}{]}) coupled with strong acid were
investigated by Tang et al\textsuperscript{23}. 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
component\textsuperscript{24,25}. 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 ILs\textsuperscript{24-26}. 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{]}\textsubscript{3}{[}TAU{]}/PEG-200 catalytic system displays
the encouraging increase in catalytic activity and selectivity with the
C\textsubscript{8} selectivity up to 85.54\%\textsuperscript{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 H\textsubscript{2}SO\textsubscript{4} alkylation. In
addition, the behaviors of DESs additives at the
H\textsubscript{2}SO\textsubscript{4}/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
H\textsubscript{2}SO\textsubscript{4}/C4 hydrocarbons interface in our
previous papers\textsuperscript{7,15,18,28-32}.

Therefore, in this paper, the effects of DESs additives on the catalytic
performance and interfacial properties of
H\textsubscript{2}SO\textsubscript{4}-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-NH\textsubscript{2}OH\selectlanguage{english}*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
H\textsubscript{2}SO\textsubscript{4} alkylation.

\textbf{2. METHODOLOGY}

\textbf{2.1 Materials}

All chemicals (AR grade) were purchased commercially and used as
received. H\textsubscript{2}SO\textsubscript{4} (A.R, 98 wt\%) was
bought from Sinopharm Chemical Reagent Co., Ltd. Choline~chlorides and
carbon tetrachloride (\selectlanguage{english}[?]99.5\%) were purchased from Shanghai Titan
Scientific Co., Ltd. Phenol (\selectlanguage{english}[?]99.0\%) and benzoic~acid (99.5\%) were
purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.
P-toluenesulfonic~acid (99.0\%) and hydroxylamine~hydrochloride (98.5\%)
were purchased from Shanghai MackLin Biochemical Technology Co., Ltd.
Isobutane/2-butene mixture (86.9/13.1 wt\%\selectlanguage{english})was obtained from Shanghai
Weichuang Standard Gas Analysis Technology Co., Ltd. Nitrogen (99.99\%)
gas was purchased from Air Liquide (Shanghai) Gas Co., Ltd.

\textbf{2.2 Preparation of DESs}

ChCl was chosen as the HBA, and phenol (Pho), p-toluenesulfonic~acid
(TsOH), benzoic~acid (BOA) and hydroxylamine~hydrochloride
(NH\textsubscript{2}OH\selectlanguage{english}*HCl) were selected as HBD, respectively. The
synthesis of DESs was carried out in a glass flask. The acceptor and
donor of the hydrogen bond were mixed according to molar ratio of 1/2,
1/1, 1/2, 1/2 to prepare ChCl-Pho (1:2), ChCl-TsOH (1:1), ChCl-BOA
(1:2), and ChCl-NH\textsubscript{2}OH\selectlanguage{english}*HCl (1:2), respectively, with the
structures shown in Figure 1\textsuperscript{33-35}. The mixtures were
heated with stirring for 1 h after the appearance of homogeneous liquid
phase in the oil bath.

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

Figure 1. Structures of different DESs.

\textbf{2.3 Experimental process}

For the typical alkylation process, the reaction took place in a batch
reactor with 1 L volume equipped with a cooling system. At first, the
180 ml DESs/H\textsubscript{2}SO\textsubscript{4} mixture was put into
the reactor, in which the ratio of DESs to
H\textsubscript{2}SO\textsubscript{4} was set to be 0.5, 1.0, 3.0, 5.0
and 8.0 wt\%, respectively. Subsequently, the N\textsubscript{2} gas was
added to purge the reactor by three times to remove the air, after which
the cooling system turned on to keep reaction temperature at the given
temperature. The pressure in the reactor was controlled at 0.5 MPa to
maintain the C4 reactants as liquid phase. Then, 120 mL of
isobutane/2-butene mixture with a given ratio were quickly injected into
the reactor and the agitation started to work at the stirring rate of
3000 rpm to achieve a well-distributed dispersion between C4 reactants
phase and acid phase. After the reaction stopped, the products were
collected and analyzed by GC after being extracted and separated from
acid phase using carbon tetrachloride. The details of the analysis of
the products displayed on our previous work\textsuperscript{18}.

\textbf{2.4 MD simulation details}

The potential parameters and models of the concentrated
H\textsubscript{2}SO\textsubscript{4} and C4 reactants for MD
simulations were from the same as our previous
work.\textsuperscript{6,7} The C4 reactants model includes equivalent
2-butene and isobutane with size of 6.0x6.0x4.0 nm\textsuperscript{3}.
The size of H\textsubscript{2}SO\textsubscript{4} model is 6.0x6.0x8.0
nm\textsuperscript{3}, containing 2848
H\textsubscript{2}SO\textsubscript{4} molecules, 84 hydronium ions, and
84 bisulphate ions. For the interfacial model, the sandwich-like models
were constructed with pure H\textsubscript{2}SO\textsubscript{4} model
in the middle and pure C4 reactants model in the both sides. 2 nm gap
between pure H\textsubscript{2}SO\textsubscript{4} model and pure C4
reactants model was used to place DESs. The number of ChCl-Pho (1:2) in
the gap are 6, 15, and 24, corresponding to the concentration of 1.0
wt\%, 3.0 wt\%, and 5.0 wt\%, respectively. For ChCl-TsOH (1:1), the
number is 6, 16, and 26, respectively. For ChCl-BOA (1:2), the number is
6, 12, and 21, respectively. Initially, the energy minimization of 5000
steps was carried out, followed by 8 ns canonical (\emph{NVT} )
simulations at 298.2 K with nose-hoover thermostat. The
isothermal-isobaric (\emph{NPT} ) ensemble was further conducted for 10
ns to fully equilibrate configures. During the simulation, the number
density distribution of each moiety was calculated every 2 ns, and the
configures were fully equilibrated after there are no apparent
deviations for the density. At last, 6 ns \emph{NVT} was carried out for
data collection of interest with the time steps of 2.0 fs. The
semi-isotropic Parrinello Rahman barostat method on \emph{z} axis was
used with 2.0 ps relaxation time at 1 bar for all the \emph{NPT}ensemble
to keep the interface unchanged\textsuperscript{15}.

For all the MD simulations, GROMACS software was employed with periodic
boundary conditions (PBC) in three dimensions\textsuperscript{36}. The
Lennard-Jones interaction and Coulombic interaction were cut off at 1.2
nm. All the covalent bonds related to hydrogen atoms were treated using
the LINCS algorithm. Maxwell distribution was employed to obtain the
initial atomic velocity. The particle grid Ewald method was used to
treat long-range electrostatic interactions. The OPLS-AA was used to
treat the interaction between the DESs,
H\textsubscript{2}SO\textsubscript{4} and C4
hydrocarbons\textsuperscript{6,7,29}. At the same time, the charges of
the DESs were refitted based on the ChelpG method at the Basis set of
6-311++G(d,p) level with Gaussian 09 program\textsuperscript{37}.

\textbf{3. Results and discussions}

\textbf{3.1 Effect of DESs on reaction time}

The effects of ChCl-Pho (1:2), ChCl-TsOH (1:1), ChCl-BOA (1:2) and
ChCl-NH\textsubscript{2}OH\selectlanguage{english}*HCl (1:2) additives on the alkylate
components as a function of time were initially investigated, as shown
in Figure 2. At the initial time, the C\textsubscript{9}+ components are
relatively high with the low conversion of C\textsubscript{8} key
components. With the extended reaction time, the RON and
C\textsubscript{8} key components increase gradually, and the LEs
(C\textsubscript{5}-C\textsubscript{7}) and C\textsubscript{9}+
components present a decreasing trend. The RON and C\textsubscript{8}
approach a plateau basically after 15 min, as same as the reaction time
of H\textsubscript{2}SO\textsubscript{4}system without any additive (see
Figure S1). Among the reaction, the trimethylpentanes (TMPs) with higher
RON (\textgreater{}100.0) grow in pace with the C\textsubscript{8}
components, leading to the higher RON of alkylate. Hence, the addition
of DESs have no effect of the reaction time of
H\textsubscript{2}SO\textsubscript{4} alkylation in contrast to the pure
H\textsubscript{2}SO\textsubscript{4} system. The reaction time of 15
min was taken for the following experiments.

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

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-NH\textsubscript{2}OH\selectlanguage{english}*HCl (1:2) (d) . Reaction conditions:
temperature 281.2 K, stirring rate 3000 r/min, mass ratio of
DESs/H\textsubscript{2}SO\textsubscript{4} 1.0 wt\%, volume ratio of
H\textsubscript{2}SO\textsubscript{4}/hydrocarbon 1.5:1, volume ratio of
I/O 12:1.

\textbf{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 C\textsubscript{8} 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 C\textsubscript{9}+ components decrease slightly, and
C\textsubscript{8} and TMPs increase obviously, which demonstrates that
ChCl-Pho (1:2) has a promoting effect on the generation of
C\textsubscript{8} 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
C\textsubscript{9}+ 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 H\textsubscript{2}SO\textsubscript{4} 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 C\textsubscript{9}+ components and the decrease of
key components dramatically\textsuperscript{38}.

When ChCl-TsOH (1:1) is used as additives, the change of LEs,
C\textsubscript{8} and C\textsubscript{9}+ 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 C\textsubscript{8} 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
alkylation\textsuperscript{39}. 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 C\textsubscript{8} 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.

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

Figure 3. Alkylate components as a function of DESs concentrations. (a)
and (c) for ChCl-Pho (1:2). (b) and (d) for ChCl-TsOH (1:1). Reaction
conditions: temperature 281.2 K, stirring rate 3000 r/min, reaction time
15 min, volume ratio of
H\textsubscript{2}SO\textsubscript{4}/hydrocarbon 1.5:1, volume ratio of
I/O 12:1.

\textbf{3.3 Effect of temperature}

C4 alkylation is an exothermic reaction. The low temperature is
beneficial to the formation of C\textsubscript{8} key components with
higher RON\textsuperscript{40,41}. Thus, the effect of reaction
temperature on C4 alkylation from 281.2 K to 269.2 K with ChCl-Pho (1:2)
additive was listed in Table 1. The difference of alkylate components
between DESs-aiding systems and pure
H\textsubscript{2}SO\textsubscript{4} system as a function of
temperature are shown in Figure 4. For the system with 1.0 wt\% ChCl-Pho
(1:2), it is clear that the targeted C\textsubscript{8} components and
the RON increase obviously from 83.77\% at 281.2 K to 86.07\% at 274.2
K, 97.39 at 281.2 K to 98.27 at 274.2 K, respectively, which is mainly
attributed to lower reaction temperature. This tendency is in good
agreement with our previous work, in which, lower temperature leads to a
great increase in C\textsubscript{8} contents and significant decrease
in LEs, HEs and DMHs components\textsuperscript{15}. However, in
ChCl-Pho (1:2)-aiding system, when the temperature is below 274.2 K, the
quality of alkylate is not improved as expected. The C\textsubscript{8}
contents dramatically decrease and HEs sharply increase at the 269.2 K.
The C\textsubscript{8} key components and the RON decrease obviously,
and the content of C\textsubscript{9}+ components dramatically increase.

Table 1. Temperature-dependent alkylate components for
H\textsubscript{2}SO\textsubscript{4} alkylation with ChCl-Pho (1:2) as
additive.\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Additive\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Additive\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Alkylate composition (wt\%)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Alkylate composition (wt\%)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Alkylate composition (wt\%)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Alkylate composition (wt\%)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Alkylate composition (wt\%)\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Alkylate composition (wt\%)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Temp /K\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
C5\textasciitilde{}C7\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
C8\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
TMP\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
C9+\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
DMH\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
RON\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
Pure H\textsubscript{2}SO\textsubscript{4}\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
281.2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
11.05\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
74.50\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
67.56\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
14.45\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
6.94\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
95.80\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
ChCl-Pho (1:2)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
281.2\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
6.46\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
83.77\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
76.98\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
9.77\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
6.79\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
97.39\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
277.2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
6.07\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
85.65\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
79.45\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
8.28\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
6.20\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
98.05\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
274.2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
5.62\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
86.07\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
80.15\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
8.30\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
5.92\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
98.27\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
273.2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
4.65\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
84.21\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
78.67\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
11.14\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
5.54\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
97.86\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
269.2\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
6.67\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
71.20\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
67.31\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
22.12\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
3.89\strut
\end{minipage} & \begin{minipage}[t]{0.10\columnwidth}\raggedright\strut
95.74\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{a} Reaction conditions: ChCl-Pho (1:2) concentration
1.0 wt\%, stirring rate 3000 r/min, reaction time 20 min, volume ratio
of H\textsubscript{2}SO\textsubscript{4}/hydrocarbon 1.5:1, volume ratio
of I/O 12:1.

Furthermore, the difference of alkylate components and
C\textsubscript{8} components of
H\textsubscript{2}SO\textsubscript{4}alkylation with ChCl-Pho (1:2)
additive as a function of temperature. From Figure 4a, compared with the
pure H\textsubscript{2}SO\textsubscript{4} system at 281.2 K, the
C\textsubscript{8} contents and TMPs increase 9.27\%, 11.15\%, 11.57\%
and 9.71\%, 9.42\%, 11.89\%, 12.59\% and 11.11\% at 281.2 K, 277.2 K,
274.2 K, and 273.2 K, respectively. In Figure 4b, 2,2,4-TMPs, 2,3,3-TMPs
and 2,2,3-TMPs follow the similar trend as the
C\textsubscript{8}components, which suggests the change of the reaction
temperature mostly impacts on the TMP components with higher RON.

\textbf{Hosted file}

\verb`image4.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

Figure 4. The difference of alkylate components between
H\textsubscript{2}SO\textsubscript{4} alkylation with ChCl-Pho (1:2) as
a function of temperature (a). Temperature-dependent
C\textsubscript{8}components for
H\textsubscript{2}SO\textsubscript{4}-catalyzed alkylation with ChCl-Pho
(1:2) additive (b). Reaction conditions: ChCl-Pho (1:2) concentration
1.0 wt\%, stirring rate 3000 r/min, reaction time 20 min, volume ratio
of H\textsubscript{2}SO\textsubscript{4}/hydrocarbon 1.5:1, volume ratio
of I/O 12:1.

\textbf{3.4 Lifetime of H\textsubscript{2}SO\textsubscript{4} with DESs}

Based on the optimal reaction results at 281.2 K, ChCl-Pho (1:2) and
ChCl-TsOH (1:1) were chosen to probe the effect of DESs on the recycling
capability for the H\textsubscript{2}SO\textsubscript{4}. The recycling
run of H\textsubscript{2}SO\textsubscript{4}/ChCl-Pho (1:2),
H\textsubscript{2}SO\textsubscript{4}/ChCl-TsOH (1:1) and pure
H\textsubscript{2}SO\textsubscript{4} are shown in Figure 5(a), Figure
5(b) and Figure S2, respectively. From Figure 5(a), the RON and
C\textsubscript{8} components slightly increase initially and then
decrease slowly. The C\textsubscript{9}\textsuperscript{+} components
increase with the increased recycle times. The first 3 runs can be
defined as catalytic activity strengthening stage. The RON and the
C\textsubscript{8} contents increase from 93.62 and 63.83\% to 94.50 and
68.07\%, respectively. In the initial strengthening stage, the acidity
of catalyst is still high to promote the main reactions. The trace
amount of side reactions products of H\textsubscript{2}O, esters and
acid soluble oil (ASO) has a positive effect on C4 alkylation, which is
the reason why the quality of alkylate is improved in first 3
times\textsuperscript{42}. Nevertheless, the
4\textsuperscript{th}-11\textsuperscript{th} recycles step into the
gradual deactivation stage. The C\textsubscript{8} contents and RON
decrease from 66.00\% and 93.78 to 51.23\% and 90.90, respectively. In
this stage, the notorious side reactions products of
H\textsubscript{2}O, esters and ASO gradually accumulate with the
increase of recycle times and leads to the gradual decrease in the acid
strength and the quality of the alkylate. After
11\textsuperscript{th}runs, the catalyst tends to be inactive and the
C\textsubscript{9}+ components dramatically increase from 36.77\% to
53.55\%.

The lifetime performance of
H\textsubscript{2}SO\textsubscript{4}catalyst with ChCl-TsOH (1:1)
additives is similar as that with ChCl-Pho (1:2) as additives. The first
2 recycle times is the stage of catalytic activity strengthening. After
the 10 runs, the activity of catalyst dramatically decreases with RON
down to 84.90 at 16 runs. The difference is the lifetime test with
ChCl-TsOH (1:1) additives has a relatively stable high catalytic
activity stage from 3\textsuperscript{th} to 10\textsuperscript{th}. In
high catalytic activity stage, the performance of the catalyst remains
stable, the RON and the C\textsubscript{8} contents is still above 93.0
and 63.00\%, respectively. In comparison to lifetime test of the pure
H\textsubscript{2}SO\textsubscript{4}, however, the addition of ChCl-Pho
(1:2) and ChCl-TsOH (1:1) do not improve the catalytic lifetime of
H\textsubscript{2}SO\textsubscript{4}, although they show the effective
catalytic performance,

\textbf{Hosted file}

\verb`image5.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

Figure 5. Alkylate components as a function of recycle times. (a)
ChCl-Pho (1:2) at 1.0 wt\%. (b) ChCl-TsOH (1:1) at 1.0 wt\%. Reaction
conditions: temperature 281.2 K, stirring rate 3000 r/min, reaction time
15 min, volume ratio of
H\textsubscript{2}SO\textsubscript{4}/hydrocarbon 1.5:1, volume ratio of
I/O 8:1.

\textbf{3.5 Distributions of DESs at the interface}

From the above experimental results, it is clear that the introduction
of DESs can effectively increase the RON of alkylate, but the lifetime
of H\textsubscript{2}SO\textsubscript{4} catalyst was almost not
promoted. It can be inferred that the better performance of alkylate is
probably due to the intensification of acid/hydrocarbons interface,
rather than the tunability of
H\textsubscript{2}SO\textsubscript{4}microenvironment. However, the
effect of DESs on the acid/hydrocarbons interface remains unclear to a
great extent. Thus, the acid/hydrocarbons interfacial behaviors with the
addition of ChCl-Pho (1:2), ChCl-TsOH (1:1), and ChCl-BOA (1:2),
respectively, were further investigated using MD simulations in details.

Initially, the equilibrated snapshots of DES distributions at different
concentrations were presented in Figure 6. For all the systems, ChCl
molecules were clearly observed with a better dispersion in
H\textsubscript{2}SO\textsubscript{4} phase, while the phenyl molecules,
such as Pho, TsOH, and BOA molecules, tend to aggregate in the
interfacial regions. Quantitatively, the mass density profiles
along\emph{z} axis with DESs at different concentrations were displayed
in Figure 7 and Figure S3. For the density profiles of different
moieties of DESs, the distinct difference is that the phenyl molecules
present an obvious peak in the interfacial regions, and the intensity of
the peak becomes much larger with the increased DES concentrations.
Conversely, ChCl molecules show a uniform distribution in the
H\textsubscript{2}SO\textsubscript{4} phase along \emph{z} axis. One can
conclude that the phenyl molecules play the essentially important role
in the tunability of acid/hydrocarbons interface. In addition, there is
a larger peak intensity of TsOH molecules in the interfacial regions
compared to Pho and BOA molecules at the same concentrations, suggesting
the stronger aggregation of TsOH molecules at the interface. Moreover,
the TsOH molecules stay closer to the
H\textsubscript{2}SO\textsubscript{4} phase than Pho and BOA molecules,
probably due to the strong interaction between SO\textsubscript{3}H
groups and H\textsubscript{2}SO\textsubscript{4}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Figure 6. Equilibrated snapshots of DESs distributions with different
concentrations at acid/hydrocarbons interface. (a), (b), and (c)
ChCl-Pho (1:2) at 1.0 wt\%, 3.0 wt\%, and 5.0 wt\%, respectively. (d),
(e), and (f) ChCl-TsOH (1:1) at 1.0 wt\%, 3.0 wt\%, and 5.0 wt\%,
respectively. (g), (h), and (i) ChCl-BOA (1:2) at 1.0 wt\%, 3.0 wt\%,
and 5.0 wt\%, respectively.

Green color represents the
H\textsubscript{2}SO\textsubscript{4}molecules in line style. Blue color
represents isobutane molecules in VDW style. Brown color means 2-butene
molecules in VDW style.

\textbf{Hosted file}

\verb`image7.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

Figure 7. Mass density profiles along \emph{z} axis of the simulated box
with different DESs at different concentrations. (a), (b), and (c)
ChCl-Pho (1:2) at 1.0 wt\%, 3.0 wt\%, and 5.0 wt\%, respectively. (d),
(e), and (f) ChCl-TsOH (1:1) at 1.0 wt\%, 3.0 wt\%, and 5.0 wt\%,
respectively.

\textbf{3.6 Interfacial thickness}

The capillary-wave theory can well describe the density of component A
and B in the interfacial system\textsuperscript{43},

\textbf{Hosted file}

\verb`image8.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

(1)

\textbf{Hosted file}

\verb`image9.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

(2)

\textbf{Hosted file}

\verb`image10.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

\textbf{Hosted file}

\verb`image11.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

\textbf{Hosted file}

\verb`image12.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

\textbf{Hosted file}

\verb`image13.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

\textbf{Hosted file}

\verb`image14.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/default-figure}

\end{center}
\end{figure}

\textbf{Hosted file}

\verb`image15.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

\textbf{Hosted file}

\verb`image16.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

\textbf{Hosted file}

\verb`image17.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

andmean the position of the Gibbs dividing surface of component \emph{A}
and\emph{B} , respectively.and represent component \emph{A} and \emph{B}
bulk density, respectively.

is the contribution of thermal fluctuations to interfacial thickness.
Using equation (1) and (2), the parameters,, and can be calculated.
Thus, the interfacial intrinsic thickness () is obtained,

\textbf{Hosted file}

\verb`image18.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

(3)

\textbf{Hosted file}

\verb`image19.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

The total interfacial thickness () consists of the intrinsic width and
thermal fluctuations using equation (4),

\textbf{Hosted file}

\verb`image20.wmf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

(4)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image22/default-figure}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image23/default-figure}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image24/default-figure}

\end{center}
\end{figure}

For the DESs-aiding systems, the density profiles of
H\textsubscript{2}SO\textsubscript{4} and MixC4 (isobutane + butene) are
shown in Figure 8 and Figure S4, respectively. It is clearly seen that
there is a good fitting for all the systems. The obtained

for different DESs as a function of concentrations as shown in Figure
8d. The of pure H\textsubscript{2}SO\textsubscript{4} system is taken
from our recent work as a benchmark\textsuperscript{15}. The results
indicate that the addition of DESs can significantly increase

in comparison to the pure H\textsubscript{2}SO\textsubscript{4} systems.
Furthermore, the interfacial thickness rises along with the
concentration of DESs. From experimental point of view, the enlargement
of the interfacial thickness provides the larger interfacial reaction
zones, which is better for higher-quality alkylate. At the same
conditions, the ChCl-TsOH (1:1) additive can contribute more to the
interfacial thickness compared to ChCl-Pho (1:2) and ChCl-BOA (1:2)
additives.

\textbf{Hosted file}

\verb`image21.emf`  available at \url{https://authorea.com/users/435862/articles/538597-effects-of-deep-eutectic-solvents-on-h2so4-catalyzed-alkylation-combining-experiment-and-molecular-dynamics-simulation}

Figure 8. The fitting results of H\textsubscript{2}SO\textsubscript{4}
and MixC4 mass density for the calculation of interfacial thickness in
the DES-aiding systems with different concentrations. (a), (b), and (c)
ChCl-Pho (1:2) at 1.0 wt\%, 3.0 wt\%, and 5.0 wt\%, respectively. The
obtained interfacial thickness for different DESs as a function of
concentrations (d).

\textbf{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
H\textsubscript{2}SO\textsubscript{4}system (0.850)\textsuperscript{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 H\textsubscript{2}SO\textsubscript{4} system
(0.858)\textsuperscript{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.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
\begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Systems\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Mass fraction (wt\%)\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Interfacial compositions\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Interfacial compositions\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Interfacial compositions\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Interfacial compositions\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Diffusion coefficients (\selectlanguage{ngerman}Å/ps)\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Diffusion coefficients (\selectlanguage{ngerman}Å/ps)\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Diffusion coefficients (\selectlanguage{ngerman}Å/ps)\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Phenyl molecules\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Butene\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Isobutane\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Isobutane /butene\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Butene\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Isobutane\strut
\end{minipage} & \begin{minipage}[t]{0.11\columnwidth}\raggedright\strut
Isobutane /butene\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
ChCl-Pho (1:2)\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0179\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2452\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2451\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.9999\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2505\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2150\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8583\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
3.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0380\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2313\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2412\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0426\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0. 2490\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2156\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8657\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
5.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0640\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2149\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2328\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0833\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2423\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2137\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8818\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
ChCl-TsOH (1:1)\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0166\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2400\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2563\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0679\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2552\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2295\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8991\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
3.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0490\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2305\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2460\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0674\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2447\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2102\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8590\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
5.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0808\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2098\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2238\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0670\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2418\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2041\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8441\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
ChCl-BOA (1:2)\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0230\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2259\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2601\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.1514\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2520\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2183\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8663\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
3.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0428\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2215\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2428\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0961\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2436\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2080\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8538\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
5.0\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.0707\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2178\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2289\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
1.0507\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2422\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.2066\strut
\end{minipage} & \begin{minipage}[t]{0.08\columnwidth}\raggedright\strut
0.8530\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{4. Conclusions}

The effect of several DESs on the quality of alkylate and the
acid/hydrocarbons interfacial properties were investigated via
experiments and MD simulations. It is found that the introduction of
DESs almost has no impact on the reaction time. ChCl-Pho (1:2) displays
the most outstanding catalytic performance with RON up to 97.37 and the
C\textsubscript{8} selectivity up to 83.64\%. The most suitable addition
ratio for ChCl-Pho (1:2) and ChCl-TsOH (1:1) to
H\textsubscript{2}SO\textsubscript{4} is about 1.0 wt\% with the
improvement of C\textsubscript{8} contents from 74.50 wt\% in pure
H\textsubscript{2}SO\textsubscript{4} system at 281.2 K to 83.64 wt\%
and 80.71 wt\% as well as the increase in RON from 95.80 to 97.37 and
96.93, respectively. However, the excessive addition of DESs leads to
the decrease in reaction enhancement, which is attributed to reduction
of the H\textsubscript{2}SO\textsubscript{4} acidity. 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. However, both of
them have a great effect on the conversion of LEs and HEs to TMPs. With
the addition of ChCl-Pho (1:2), the targeted
C\textsubscript{8}components and the RON increase obviously from 83.77\%
at 281.2 K to 86.07\% at 274.2 K, 97.39 at 281.2 K to 98.27 at 274.2 K,
respectively, which is mainly contributed to low reaction temperature.
The addition of ChCl-Pho (1:2) and ChCl-TsOH (1:1) show the effective
catalytic performance, but both of them can not improve the catalytic
lifetime of H\textsubscript{2}SO\textsubscript{4} catalyst.

From MD simulations, the phenyl molecules, such as Pho, TsOH, and BOA
molecules, is found to aggregate close to the interface with a higher
density peak in the interfacial regions, which plays the essentially
important role in the tunability of acid/hydrocarbons interface. The
addition of DESs can significantly increase the interfacial thickness,
promote the solubility and diffusion of isobutane with respect to butene
in comparison to the pure H\textsubscript{2}SO\textsubscript{4} system,
thus making positively contributions to the quality of alkylate.

\textbf{Acknowledgements}

The financial support by the fellowship of China Postdoctoral Science
Foundation (2020M681205 and 2021T140205), Shanghai Post-doctoral
Excellence Program (2020135), National Natural Science Foundation of
China (61988101), and 111 project (B20031) are gratefully acknowledged.

\textbf{Conflict of interest}

The authors declare no competing interests.

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