Microstructures of the Sulfonic Acid-Functionalized Ionic Liquid

Subscriber access provided by READING UNIV

Article

Microstructures of the Sulfonic-Acid-Functionalized Ionic Liquid/Sulfuric Acid and Their Interactions: A Perspective from the Isobutane Alkylation Weizhong Zheng, Chizhou Huang, Weizhen Sun, and Ling Zhao J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b09755 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Microstructures of the Sulfonic-Acid-Functionalized Ionic Liquid/Sulfuric Acid and Their Interactions: A Perspective from the Isobutane Alkylation Weizhong Zheng, Chizhou Huang, Weizhen Sun∗ and Ling Zhao State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China.

ABSTRACT The all-atom force field for the concentrated sulfuric acid (98.30 wt%) was developed in this work based on the ab initio calculation. The structural and dynamical properties of sulfuric acid and mixing behaviors of sulfuric acid with ionic liquids (ILs), i.e. SFIL (1-methyl-3-(propyl-3-sulfonate) imidazolium bisulfate ([PSMim][HSO4]))

and

non-SFIL

(1-methyl-3-propyl

imidazolium

bisulfate

([PMim][HSO4])), were investigated using molecular dynamics simulation. For sulfuric acid, most of H3O+ ions were found to be located beside the HSO4− ions, forming a contact ion pair with HSO4− ions, and there existed three-dimensional hydrogen-bonding networks in the sulfuric acid. Both of ILs could be miscible with sulfuric acid with a strong exothermic character. The new strong interaction site between the sulfonic acid group of SFIL and the H2SO4 molecule through the strong hydrogen-bonding interaction was observed, which was beneficial to the catalytic activity and stability of the sulfuric acid. This observation has a good agreement with

*Corresponding author at: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. Tel: +86-21-64253027. E-mail address: [email protected] (W. Sun). 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the experimental results that the SFIL could enhance the reusability runs of sulfuric acid for the isobutane alkylation about four times more than non-SFILs. Hopefully this work would provide insights into the screening and designing of new isobutane alkylation catalysts based and sulfuric acid and SFILs.

1. Introduction Sulfuric acid is one of the most fundamental and important commodity chemicals with various applications in numerous industrial fields, such as fertilizer production, petroleum refining, metal processing, and electrochemistry, etc1. Among these applications, the concentrated sulfuric acid has been widely investigated as a catalyst for the alkylation of isobutane with C3~C5 olefins to produce the clean-burning and high-quality gasoline (alkylate) since the late 1930s, because the alkylate shows the advantages of high octane number, low-vapor pressure, and no aromatic and sulfur content2. These advantages of alkylate have motivated numerous researchers to systematically study a serial of physical and chemical factors that influence the composition and quality of alkylate, including reaction mechanism, reaction temperature, stirring speed, alkanes/olefins ratio, acidity of the acid, etc3-8. In particular, the sequence of more than 30 chemical, physical, and transfer steps was summarized and explained to provide a better understanding of the fundamentals during the alkylation by Albright et al9. Based on the classic carbonium ion mechanism, the reaction pathways and kinetic model were established by Sun et al, which could well predict the concentration variation of three key groups of alkylate,

2


Page 2 of 34

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


namely, trimethylpentanes, dimethylhexanes, and heavy ends10. In contrast to the above-mentioned studies, which were performed from the macroscopic point of view, there is very limited information available regarding the H2SO4-catalyzed alkylation at a molecular level due to the absence of the force field parameters of the concentrated H2SO4. Recently, however, the vast majority of theoretical studies have focused on the aqueous solution of sulfuric acid system using molecular dynamics approach11-18. Recently, however, the vast majority of theoretical studies have focused on the aqueous solution of sulfuric acid system using molecular dynamics approach13. Ishiyama et al. developed a flexible and polarizable molecular model to investigate the surface structure of aqueous sulfuric acid solution by molecular dynamics (MD) simulation14, 15. Hammerich et al. applied ab initio molecular dynamics simulation to study the liquid-vapor interface of sulfuric acid solution, finding that the hydronium ion exhibited a surface preference17. However, basic insight into the concentrated H2SO4 from the aspect of MD simulation still remains insufficient. As a novel acid catalyst or dual solvent-catalyst, the sulfonic acid-functionalized Brønsted acidic ionic liquids (SFILs) have been widely applied in alkylation19, esterification20-22, polymerization23, cellulose hydrolysis24, 25, and biodiesel synthesis26, 27

because of their chemical stability, adjustable acidity, high catalytic activity, and

high recyclability28. Especially for the isobutane alkylation, the SFILs, as a co-catalyst, could obviously improve the catalytic activity and stability of the strong acid catalyst. 3



In the pioneered work, Tang et al. coupled the Brønsted acidic ionic liquids (BILs) with strong acids to catalyze the alkylation with an excellent catalytic performance observed29. The binary mixtures of BIL/triflic acid were studied as a synergistic catalyst for the isobutane alkylation, showing that the research octane number (RON) could reach up to 97.3 and the lifetime up to 36 times30. They attributed the lifetime enhancement to the formation of anionic clusters between the BIL and triflic acid linked by strong hydrogen bonds, which is helpful to maintain the acidity of strong acid and further enhance its reusability. Furthermore, from our recent work, it was found that the addition of the SFILs into the concentrated H2SO4 could dramatically enhance its catalytic activity and stability for the isobutane alkylation. Specifically, at the optimized conditions the binary mixtures showed an excellent catalytic performance with the selectivity of C8-alkylate more than 75.7%, the RON up to 95.66 and, more importantly, the reusability up to 24 runs, outclassing the pure H2SO4. The prolonged reusability can be ascribed to the formed clusters between the sulfonic-acid groups and sulfuric acid connected by strong hydrogen bonds, which was further verified by both

1

H-NMR spectra and density functional theory

calculation. Therefore, the proton microenvironment and nanostructure in the binary mixtures of SFIL/sulfuric acid system play an important role in determining the proton transfer, catalytic activity and stability. However, the fundamental understanding of the proton microenvironment and nanostructure in the binary mixtures of SFIL/sulfuric acid still remains unclear, which hinders the design and 4


Page 4 of 34

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


application of SFILs. In the present work, the force field for the sulfuric acid at a typical concentration of 98.30 wt% for the isobutane alkylation was parameterized using ab initio calculation. These parameters were incorporated into the Optimized Potentials for Liquid Simulations All Atom (OPLS-AA) force field. Subsequently, the classical molecular dynamics simulations were carried out to characterize structural and dynamical properties of the sulfuric acid. The validation of the force field developed in this work was confirmed from density, surface tension, microstructures, hydrogen bonds and diffusion coefficients. In addition, the binary mixtures of SFIL (1-methyl-3-(propyl-3-sulfonate) imidazolium bisulfate, [PSMim][HSO4])/sulfuric acid were investigated at the mole ratio of IL to sulfuric acid from 0 to 1. The 1-methyl-3-propyl imidazolium bisulfate ([PMim][HSO4])/sulfuric acid binary mixtures with the same mole ratios were also studied as a benchmark for comparison.

2 Simulation details 2.1 Force Fields for H2SO4 and ILs The energy minimum structures of H2SO4 molecule, bisulfate ion, hydronium ion, 1-methyl-3-propyl imidazolium and 1-methyl-3-(propyl-3-sulfonate) imidazolium cation are shown in Figure 1. The all-atom force field (OPLS-AA) was employed to describe the interaction among these species with the total potential energy being shown in Eq.1

5



Vtotal =

Page 6 of 34

n = 0 −5 kb k (r − r0 )2 + ∑ θ (θ − θ 0 )2 + ∑ Cn ( cos(ϕ − 180° )) bond 2 angle 2 dihedral

∑

(1)

 σ σ  1 qi q j +4ε ij  ( ij )12 − ( ij )6  +  r  rij  4πε 0 ε r rij  ij

where kb and kθ are force constants of bonds and angles, respectively. Cn and φ correspond to dihedral coefficient and dihedral angle. εij and σij are the Lennard-Jones parameters for atom pairs, and qi is the atomic charge on atom i. ε0 and εr represent the permittivity of vacuum and relative dielectric constant, respectively. In this work, the molecular geometries of H2SO4 molecule, bisulfate 1-methyl-3-propyl

imidazolium

cation,

and

ion,

hydronium

ion,

1-methyl-3-(propyl-3-sulfonate)

imidazolium cation were optimized at the B3LYP/6-311++g(d,p) level, respectively. Based on the optimized structures, the atomic charges were derived at the MP2/ aug-cc-pvtz level using the CHelpG method. The derived total charge of cation and anion for the ILs was scaled to +0.8 e and -0.8 e, respectively, due to the overestimated electrostatic interaction and consequently the underestimated dynamic properties caused by unit chargeunit charge31. This strategy has been successfully applied in common ILs32-34 and pyridinium-based SFILs35. In addition, the force field parameters for the ILs and several functional groups were adopted according to the work of Lopes et al36-39.

6


Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Equilibrium geometry and atomic labels for the constituent species.

For the concentrated sulfuric acid, however, the classical force field parameters were not currently available because its composition strongly depends on concentration and temperature due to the inherently complicated dissociation according to the first and second dissociation equilibria: −

H 2 SO 4 + H 2 O ↔ HSO 4 + H 3 O −

2−

HSO 4 + H 2 O ↔ SO 4 + H 3 O

+

(2)

+

(3)

Furthermore, for the isobutane alkylation, the acidity of sulfuric acid has a decisive effect on the quality of the alkylate and the Harnett acidity (H0) should be lower than -8.0, corresponding to the mass fraction of the sulfuric acid more than 90 %6. Therefore, the force field parameters of the concentrated sulfuric acid at a typical concentration of 98.30 wt% for the isobutane alkylation were chosen to be fitted on the basis of the force-field functional form (Eq.1). It has been confirmed by experiment using Raman spectroscopy6, 40 and model calculation18, 41, 42 that at very high concentration, the second dissociation of sulfuric acid is extremely suppressed and thus the reminder in the concentrated sulfuric acid system are the undissociated H2SO4 molecule, bisulfate ion and hydronium ion. All equilibrium parameters for 7



Page 8 of 34

bonds and angles of H2SO4 molecule and bisulfate ion were directly obtained from the optimized structures, and their force constants were extracted from the Hessian matrix by the VFFDT software developed by Zheng et al43. It should be pointed out that there exist two minimum energy geometries for H2SO4 molecule, i.e. trans-H2SO4 and cis-H2SO414, 44, 45. Thus, the parametrization of the dihedral angles related to H2SO4 molecule requires particular attention. The dihedral angle for Hs-O-Ss-O, Hs-O-Ss-Os and Hb-O-Sb-Ob were scanned in a step of 10° using the DFT based on B3LYP/6-311++g(d,p) method. Afterwards, the scanned data were fitted and regressed using the Rychert-Bellemans function (Equation 4) embedded in the OPLS-AA force field, and then the dihedral parameters could be obtained. 1 Vrb = [ f1 (1 + cos φ ) + f 2 (1 + cos 2φ ) + f 3 (1 + cos 3φ ) + f 4 (1 + cos 4φ )] 2

(4)

This procedure is similar to the nonlinear least-squares fits from I-NoLLS program46, 47

. The effect of two isomers of H2SO4 molecule on its dihedral angle could be

captured by the above method, which was confirmed by Yosa et al48. For van der Waals interactions, Lennard-Jones potentials were initially and directly derived from model 2 in Reference49. This model could well reproduce the experimental density of the concentrated sulfuric acid, but greatly overestimated the surface tension and significantly slow down dynamic properties due to the strong intermolecular interaction caused by the Lennard-Jones potentials. Particularly, the predicted surface tension could be comparable to that of the pure water at the same conditions. This may be ascribed to the fact that the Lennard-Jones potential model was developed 8


Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


based on the ab initio binding energies of sulfate anion in water. Therefore, in this work, the L-J parameters of S and O atoms concerning H2SO4 molecule and bisulfate ion were correspondingly modified using trial and error method on the basis of the experimental density, surface tension, and the self-diffusion coefficient of the concentrated H2SO4, which has covered the bulk, interfacial and dynamic properties of the concentrated H2SO4. The validity of the parameters will be justified by the predicted results. The force-field parameters for hydronium ion were taken from the work of Jang et al50. All the force field parameters for the H2SO4 molecule, bisulfate ion, and hydronium ion were included in the Supporting Information.

2.2 MD Simulation Details All MD simulations were carried out using GROMACS software. Based on the refined electrolyte NRTL model, the 98.30 wt% sulfuric acid is composed of H2SO4 molecule, bisulfate ion, and hydronium ion with the estimated ratio of 10:1:1 at 298.2 K and 1 bar51. For the pure sulfuric acid, the simulated box containing 428 H2SO4 molecules, 42 bisulfate ions and 42 hydronium ions was built with the size of 3.51×3.51×3.51 nm3. For the IL/sulfuric acid mixtures, the box consisted of the concentrated sulfuric acid and IL with a total number of 512, in which the number of ion pairs of the IL is 51, 102, 154, 205, 256, 307, 358, 410 and 461 with the size between 3.51×3.51×3.51 ~ 5.55×5.55×5.55 nm3, corresponding to the mole ratio of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9, respectively. For comparison, the box containing 512 ion pairs of pure IL with the size about 5.55×5.55×5.55 nm3 was built

9



as a benchmark. Newton’s motion equation was integrated using the leapfrog algorithm with a time step of 1.5 fs. Periodic boundary conditions (PBC) were utilized in three directions. For all the simulated boxes, the energy minimization was initially simulated by 5000 steps. Subsequently, an 8 ns simulated annealing was taken by heating the initial temperature to 500 K and cooling back to the initial temperature under the canonical (NVT) ensemble with the Hoover-Nose thermostat. Then, MD simulations lasted for 40 ns under NPT ensemble with the Parrinello-Rahman barostat to reach equilibrium. Finally, 10 ns production run under NVT ensemble were used to collect the data of interest. To calculate the surface tension of the concentrated sulfuric acid, the above equilibrated boxes were doubled along the z direction, creating two gas-liquid slabs with the sulfuric acid in the middle and the vacuum phase on both sides. Afterwards, the interface system was equilibrated under the NVT ensemble for another 20 ns. For all MD simulations, the Lennard-Jones interaction was truncated at a radius of 1.2 nm, and the Coulombic interaction was treated using the particle-mesh Ewald (PME) summation with a cutoff of 1.2 nm.

2.3 Validation of the Force Field To confirm the validation of the force field, the density is probably the most fundamental property to be considered. Thus, the predicted density of sulfuric acid at 298.2 K and 1.0 bar is 1.792 g/cm3, which well reproduces the experimental value52 of 1.831 g/cm3 with the calculated deviation of 2.130 %.

10


Page 10 of 34

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Another powerful factor to validate the force filed is the surface tension (γ), which is calculated from the diagonal components of the pressure tensor, Pxx, Pyy, and Pzz using this formula

L 2

γ = ( )[ Pzz −

( Pxx + Pyy ) 2

]

(4)

The simulated γ is 56.157 mN/m, which is well comparable to the experimental data53, 54

of 53.827 mN/m with the corresponding deviation less than 4 %, indicating that the

force field parameters in this work can satisfactorily predict the surface properties of the sulfuric acid.

3 Results and discussion 3.1 Microstructures of the pure sulfuric acid To obtain insight into the microstructure of the sulfuric acid, the center-of-mass radial distribution functions (RDFs) between different species were calculated, as shown in Figure 2. It is clear that the maximum peak for HSO4−-H3O+ is located at 3.96 Å with a dramatic intensity nearly up to 40, which suggests that most of H3O+ ions are located around the HSO4− ions, forming a contact ion pair with HSO4−. Similar finding has been reported by Choe et al. based on the first-principles molecular dynamics calculation on aqueous sulfuric acid solution13. They found that the coordination number of H3O+ around HSO4− is about 2 in the aqueous sulfuric acid solution with the concentration of 64.52 wt%. However, the HSO4− is coordinated by four H3O+ in the concentrated sulfuric acid in this work. It should be noted that the coordination number is calculated by the integral of RDFs of HSO4−-H3O+ from zero 11



to the first minimum in Figure 2. This difference may be ascribed to the strong interaction between H3O+ and H2O in the aqueous sulfuric acid solution13, which correspondingly weakens the coordination number of H3O+ around HSO4−.

Figure 2. RDFs between different species in the concentrated sulfuric acid.

In order to further study the organization of the sulfuric acid, the site-site RDFs for the atoms in H2SO4 and H3O+, HSO4− and H3O+, and H2SO4 and HSO4− is shown in Figure 3, where the subscript s, b, and h correspond to the H2SO4 molecule, HSO4− and H3O+ ions, respectively. From Figure 3a, it is noteworthy that the first peak of Hh-Os is located at 2.62 Å. Similar behavior for RDF of Oh-Hs with the location of the first peak at 2.68 Å is observed, as shown in Figure 3b. These observations suggest that there exists a hydrogen-bonding interaction between H3O+ ion and H2SO4 molecule. The relatively high magnitude of the first peak of Oh-Os and Oh-Ss (see Figure 3b) implies that H2SO4 molecule is inclined to be coordinated by H3O+ ions. Particularly, the RDF of Hh-Ob presents a greatly sharp peak at 1.66 Å with a dramatic intensity, which suggests that there is a strong hydrogen-bonding interaction between H3O+ and HSO4− (Figure 3c). This behavior is in good agreement with the above RDF

12


Page 12 of 34

Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


result of HSO4−-H3O+. Correspondingly, a sharp peak for the RDF of Oh-Ob is located at near 2.74 Å due to the contact ion pairs between HSO4− and H3O+ (Figure 3d). This finding is comparable to the trend reported by Choe et al. that the location of the first peak for Oh-Ob changed from 2.6 to 2.7 Å, when the concentration of the sulfuric acid increased from 7.84 to 64.52 wt% in aqueous sulfuric acid solution13. In Figure 3e, the first peak of Hs-Ob is placed at 1.76 Å, which is in good agreement with the quantum chemical computation 55, 56. Moreover, the corresponding peak for Os-Hb is located at 2.02Å. These also indicate that a relatively strong hydrogen-bonding interaction is formed

between

H2SO4

and

HSO4−.

As expected,

the

three-dimensional

hydrogen-bonding networks in the concentrated sulfuric acid are observed from the above site-site RDFs.

Figure 3. Radial distribution functions for the atoms in H2SO4 and H3O+, HSO4− and H3O+, and

H2SO4 and HSO4−.

The microstructure can also be analyzed by examining the coordination number, which is the average number of specific sites or atoms within a sphere of radius r 13



Page 14 of 34

about some other central sites or atoms. The coordination numbers can be calculated by the integral of RDFs from zero to the first minimum57. Table 1 presents the coordination number by integral of RDFs in Figure 3. It is evident that the HSO4− is surrounded by more H3O+ due to the strong electrostatic interaction, well consistent with the conclusion of RDFs. Table 1. Coordination Numbers for site-site RDFs in sulfuric acid

pair H2SO4-H3O+

HSO4−-H3O+

H2SO4-HSO4−

type Hh-Ss

Hh-Os

Hh-Hs

Oh-Ss

Oh-Os

Oh-Hs

0.36

0.12

0.46

0.32

0.10

0.02

Hh-Sb

Hh-Ob

Hh-Hb

Oh-Sb

Oh-Ob

Oh-Hb

2.96

0.95

1.74

3.89

1.33

1.39

Hs-Sb

Hs-Ob

Hs-Hb

Os-Sb

Os-Ob

Os-Hb

0.05

0.01

0.71

0.75

0.35

0.07

3.2 Hydrogen bond of the pure sulfuric acid The hydrogen bond plays an important role in the properties of the sulfuric acid. For example, the hydrogen bonds are responsible for the strong acidity, high catalytic activity, and proton transfer, all of which are crucial to the composition and quality of alkylate. Therefore, in this work, the average number of hydrogen bonds between different moieties in the sulfuric acid was calculated in term of the criteria that H-O distance is less than 2.5 Å and O-H···O angle is more than 150°. Table 2 shows the average number of hydrogen bonds for H-O between different moieties in the sulfuric acid. It is clear that the hydrogen-bonding networks in the sulfuric acid can be observed. The average number of hydrogen bonds for different pairs follows the order HSO4−-H3O+ > H2SO4-H2SO4 > HSO4−-HSO4− > H2SO4-HSO4− > H2SO4-H3O+ >

14


Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H3O+-H3O+. This indicates that hydrogen bonds form more easily for HSO4−-H3O+ pair, which agrees well with the result of RDFs for HSO4−-H3O+ pair. Furthermore, the number of donated hydrogen bonds per water, represented by the sum of the average number of hydrogen bonds for HSO4−-H3O+, H2SO4-H3O+ and H3O+-H3O+ in this work, is 2.58, which is in good agreement with the conclusion from ab initio molecular dynamics simulation reported by Niskanen et al. that the number of donated hydrogen bonds per water increases to 2.5, when the concentration of sulfuric acid increases up to 18.0 M (94 wt%)18. Table 2. Average number of hydrogen bonds for H-O between different moieties in the sulfuric

acid. hydrogen bond

H2SO4-H2SO4

HSO4−-HSO4−

H3O+-H3O+

O-H···O

1.34

0.34

0.00

−

HSO4 -H3O O-H···O

+

2.42

H2SO4-HSO4 0.19

−

H2SO4-H3O+ 0.16

3.3 Diffusion property of the pure sulfuric acid A self-diffusion coefficient (Dself) of the species in the concentrated sulfuric acid was calculated using the Einstein relationship

6Dself t = lim ri (t ) − ri (0)

2

t →∞

(5)

where ri stands for the center of mass position of atom i., The mean square displacements (MSD) of H2SO4 molecule, HSO4− and H3O+ ions are shown in Figure 4. On the basis of the slope of MSD profiles, the corresponding self-diffusion coefficients of three species are 0.1390×10-5, 0.0095×10-5, and 0.0094×10-5 cm2/s, respectively. It is obvious that the diffusion of the H2SO4 molecule is about two orders of magnitude larger than that of HSO4− and H3O+ ions that possess a comparable 15



Page 16 of 34

self-diffusion coefficient because of the existence of the strong ion pair between them. In addition, the predicted self-diffusion coefficient of the concentrated sulfuric acid is approximately calculated by

DSA = xH 2SO4 DH 2SO4 + xHSO − DHSO − + xH O + DH O + 4

4

3

(6)

3

where x is the mole fraction. Thus, the predicted self-diffusion coefficient of the concentrated sulfuric acid from the simulation is 0.125×10-5 cm2/s. If the Stokes-Einstein relation is assumed to be applied to the concentrated sulfuric acid system, the self-diffusion coefficients can be estimated from the formula

DSA = DH 2O

ηH O η SA

(7)

2

where η is the viscosity. The self-diffusion coefficient of water at 298.2 K and 1 bar is 2.3×10-5 cm2/s 58. The viscosity of water and the concentrated sulfuric acid at 298.2 K and 1 bar is 0.9 and 21.5 cP, respectively. The estimated self-diffusion coefficient for the sulfuric concentrated acid is 0.095×10-5 cm2/s. It is clear that a good agreement between the simulated self-diffusion coefficient and the estimated one using the above formula can be quite satisfactory at the same conditions. Hence, the force field parameters of the sulfuric acid we fitted are reasonably believable from the dynamic properties.

16


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Mean square displacement of the H2SO4 molecule, HSO4− and H3O+ ion.

3.4 Density and heat of mixing for IL/sulfuric acid mixtures The density and heat of mixing for the IL/sulfuric acid binary mixtures obtained from MD simulations are presented in Figure 5. The density of the pure [PMim][HSO4] and [PSMim][HSO4] is 1.330 and 1.499 g/cm3, respectively, which are comparable to the respective experimental values of 1.350 and 1.520 g/cm3 measured based on the method similar to the previous paper59. Mixtures of the sulfuric acid with the increased concentration of both ILs exhibit a monotonic decrease in density. Furthermore, the mixing of the sulfuric acid with the [PMim][HSO4], as well as the [PSMim][HSO4], show an exothermic character. Compared to the [PMim][HSO4]/H2SO4 systems, the [PSMim][HSO4]/H2SO4 systems have a higher mixing heat, especially at the IL mole fraction of 0.3 or 0.4. This indicates that the mixing of the sulfuric acid with the [PSMim][HSO4] probably presents a stronger exothermic behavior, which may be due to the increase in the polarity of the [PSMim][HSO4] after the introduction of the sulfonic acid groups.

17



Figure 5. Density and heat of mixing of binary mixtures of IL/sulfuric acid at different mole

fraction of IL. (a) [PMim][HSO4]/sulfuric acid system. (b) [PSMim][HSO4]/sulfuric acid system.

3.5 Bulk structure for IL/sulfuric acid mixtures To probe the effect of [PMim][HSO4] and [PSMim][HSO4] on the bulk structure of the sulfuric acid, the representative RDFs of H-O and H-S for HSO4−-H3O+ and H2SO4-HSO4− pairs are calculated as a function of the mole fraction of IL, as shown in Figure 6 and Figure 7, respectively. It is obvious that the addition of the ILs produce no effect on the location of the first maximum peak of RDFs, which suggests that both [PMim][HSO4] and [PSMim][HSO4] can be well miscible with the sulfuric acid. However, the corresponding peak intensities are changed with the addition of ILs. Specifically, for both systems the first peak intensities of Hh-Ob and Hh-Sb for HSO4−-H3O+ pairs decrease with the increase in mole fraction of IL (see Figure 6a, 6b and Figure 7a, 7b), while the first peak intensities of Hs-Ob and Hs-Sb in H2SO4-HSO4− pairs show an opposite trend (see Figure 6c, 6d and Figure 7c, 7d). It is well-known that the weak acidity of the BILs leads to a quick decrease in the acidity of sulfuric acid with the increased addition of IL. According to Grotthuss proton hopping mechanism, the structure characteristics between the H2SO4 molecules, 18


Page 18 of 34

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bisulfate ions and hydronium ions play an important role in the proton hopping, and further affect the acidity of the sulfuric acid. The change tendency of the acidity of sulfuric acid follows the same trend as the first maximum peak intensities of Hh-Ob and Hh-Sb in HSO4−-H3O+ pairs as a function of the concentration of IL. Hence, we can conclude that the intensity of hydrogen-bonding interaction in HSO4−-H3O+ pairs is of great importance to the acidity of sulfuric acid. In addition, the distinct difference in the effect of [PMim][HSO4] and [PSMim][HSO4] on the structure of the sulfuric acid is observed, especially at a low concentration of IL. The [PMim][HSO4] dramatically change the first peak intensities of the representative RDFs of the sulfuric acid, even at the mole fraction of 0.1 (Figure 6), whereas the [PSMim][HSO4] shows a gradual change in these first peak intensities with the increase in mole fraction of IL (Figure 7). This indicates that the [PSMim][HSO4], when added into the sulfuric acid, can lead to a smaller change tendency of the structure of the sulfuric acid with the increase in the IL mole fraction compared to the [PMim][HSO4], which is due to the new hydrogen-bonding interaction between the sulfonic acid group and the sulfuric acid (see Figure 10). Therefore, we can safely say that compared to the [PMim][HSO4], the [PSMim][HSO4] is more beneficial to the stability of the structure and acidity of the sulfuric acid due to the introduction of the sulfonic acid group.

19



Figure 6. Radial distribution functions for (a) Hh-Ob, (b) Hh-Sb, (c) Hs-Ob, (d) Hs-Sb as a function

of the mole fraction of IL in [PMim][HSO4]/sulfuric acid system.

Figure 7. Radial distribution functions for (a) Hh-Ob, (b) Hh-Sb, (c) Hs-Ob, (d) Hs-Sb as a function 20


Page 20 of 34

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the mole fraction of IL in [PSMim][HSO4]/sulfuric acid system.

The representative snapshots for the [PMim][HSO4]/sulfuric acid systems and the [PSMim][HSO4]/sulfuric acid systems with different mole fractions of ILs from the MD trajectories are shown in Figure 8, in order to obtain a visual description of the mixing of the IL and sulfuric acid. It is clear that the [PMim] and [PSMim] cations of IL are uniformly distributed into the sulfuric acid, even at the low concentration of IL, meaning that the miscible behavior between [PSMim][HSO4], as well as [PMim][HSO4], and the sulfuric acid exists.

Figure 8. Representative snapshots for the [PMim][HSO4]/sulfuric acid systems (1) and

[PSMim][HSO4]/sulfuric acid systems (2) with different mole fractions of IL. (a) MIL=0.2, (b) MIL=0.4, (c) MIL=0.6, (d) MIL=0.8, where Red represents cations of IL, Blue represents H2SO4 molecules and Green represents HSO4− and Purple represents H3O+ ions.

To further probe the structure of the pure IL and the IL/sulfuric acid mixtures, a serial of site-site RDFs were calculated, as shown in Figure 9. For both the pure [PMim][HSO4] and [PSMim][HSO4] (see Figure 9a and 9b), the value of the first maximum peak of RDFs between the oxygen atom of anions (Ob) and the carbon 21



atoms of cations (C2, C9) are more than 2.4, indicating the preferential location of HSO4- around these atoms of cations. The first maximum value of RDFs is observed to decrease following the order of C2-Ob >C9-Ob >C6-Ob >C4-Ob >C5-Ob, which indicates the anions prefer to locate near the carbon atoms (C2) attached to two nitrogen atoms and the methyl chains (C9), and simultaneously exhibit lesser probability to be present near the carbon atoms (C5) due to the steric hindrance caused by propyl (or propyl sulfonic) chains. This observation is very close to earlier finding in a common IL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMim][PF6])33. The first peak of RDFs between the Ob atom of anions and the C8 atom of cations also exhibits a rather strong intensity with the corresponding value up to 1.5. It should be noted that compared to the [PMim][HSO4], for the [PSMim][HSO4] the first maximum peak of RDFs between the oxygen atom of the sulfonic acid group (O25) and the oxygen atom of anions (Ob) is more than 2.0, indicating that a new strong interaction site appears between the sulfonic acid group of cations and the anions due to the introduction of sulfonic acid group into the terminal position of the alkyl chains of [PMim][HSO4]. For the [PMim][HSO4]/sulfuric acid mixture and the [PSMim][HSO4]/sulfuric acid mixture systems, the RDFs between the oxygen atom of anions (Ob) and different heavy atoms of cations (see Figure 9c and 9d) follow almost identical behaviors as the corresponding pure IL, except for a lower peak intensity, which illustrates that the sulfuric acid produces no effect on the microstructure of the ILs. At the same time, the preferential location of H2SO4 around 22


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the cations of [PMim][HSO4] and [PSMim][HSO4] is also found (see Figure 9e and 9f). However, the RDFs for the oxygen atom of H2SO4 (Os) and the heavy atoms of the cations present a sharp contrast to those for the oxygen atom of anions (Ob) with the corresponding atoms of the cation. The intensity of the first peak of RDF for C8-Os in the [PMim][HSO4]/sulfuric acid mixture system is lower than that of C9-Os, but higher than others. On the contrary, the [PSMim][HSO4]/sulfuric acid mixture shows a rather high intensity of the first peak for O25-Os which is just lower than that of C9-Os. These behaviors not only indicate that the H2SO4 molecules show a large probability to be present near the methyl chains of the cations, but also illustrate that there exists a strong interaction between the sulfonic acid group and the H2SO4.

Figure 9. Site-site RDFs of the pure IL and IL/sulfuric acid mixture system with IL mole

fraction of 0.3 between the oxygen atoms on HSO4− and H2SO4 and the heavy atoms on the cation. (a) pure [PMim][HSO4], (b) pure [PSMim][HSO4], (c) and (e) [PMim][HSO4]/sulfuric acid, (d) and (f) [PSMim][HSO4]/sulfuric acid.

3.6 Microenvironment of proton for IL/sulfuric acid mixtures The proton microenvironment in the pure IL and the binary mixtures of the

23



IL/sulfuric acid determines some unique properties, such as the acidity, catalytic activity and proton transport 60, 61. Therefore, the microenvironment of the proton in the pure IL and the binary mixtures of IL/sulfuric acid have been investigated by checking the RDFs of the imidazole ring proton (H10) and the sulfonic acid proton (H26) with the oxygen atom of the anions (Ob) and the H2SO4 molecule (Os), and the proton of the anions (Hb) and the H2SO4 molecule (Hs) , as well as hydronium ion (Hh), with the oxygen atom of the sulfonic acid groups (O25), as shown in Figure 10. From Figure 10a, the first maximum peak of RDFs for H10-Ob is located at a distance of 2.30 Å for the pure [PMim][HSO4], and 2.32 Å for the [PMim][HSO4]/sulfuric acid system with a relatively lower intensity. This behavior illustrates that the hydrogen-bonding interaction exists in both systems, which agrees well with the fact that the hydrogen bonds, even the three-dimensional networks, could be found in the common ILs62, 63. The first maximum peak of RDF of H10-Os is located at a distance of 2.50 Å, indicating that the H2SO4 can also form the hydrogen-bonding with the cations of the [PMim][HSO4]. The similar phenomenon is also observed for the RDFs of the pure [PSMim][HSO4] and [PSMim][HSO4]/sulfuric acid systems (Figure 10b). In addition, the RDFs between the sulfonic acid proton (H26) and the oxygen atom of the anions (Ob), as well as the H2SO4 molecules (Os) show that the hydrogen-bonding interaction is formed between the H2SO4 and the sulfonic acid group of cations of the [PSMim][HSO4] for the pure and binary mixture systems (Figure 10c). In particular, the first maximum peak of RDFs between the proton of the H2SO4 molecule (Hs) and 24


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the anions (Ob) and the oxygen atom of the sulfonic acid groups (O25) is located at a distance of 1.86 and 2.08 Å, respectively, which indicates that the sulfonic acid groups tend to form a stronger hydrogen-bonding interaction with the H2SO4 molecule than the anions. There is a similar interaction between the hydronium ion and the sulfonic acid groups. Therefore, compared to the [PMim][HSO4], the [PSMim][HSO4] provides the new strong interaction site between the sulfonic acid groups and the H2SO4 for proton microenvironment of IL/sulfuric acid mixtures after the introduction of the sulfonic acid group. The new microenvironment of proton between the sulfonic acid group and the H2SO4 molecule is useful to slow down the acidity change of the sulfuric acid. For example, the sulfonic acid functionalized ionic liquids, when used as a co-catalyst for sulfuric acid-catalyzed isobutane alkylation, can enhance the reusability of the sulfuric acid compared to non-sulfonic acid functionalized ionic liquids, which will be shown in the following section 3.7.

25



Page 26 of 34

Figure 10. Site-site RDFs between the proton (H10) as well as the sulfonic acid proton (H26)

and the oxygen atom on HSO4− (Ob) and H2SO4 (Os), and the proton of HSO4− (Ob), H2SO4 (Os) and H3O+ (Hh) with the oxygen atom of the sulfonic acid groups (O25) for the pure IL and IL/sulfuric acid mixture systems with the IL mole fraction of 0.3. (a) The [PMim][HSO4] systems, (b), (c) and (d) The [PSMim][HSO4] systems.

3.7 Reusability of SFIL/sulfuric acid mixture catalyst for isobutane alkylation From the above results, it can be found that there exist a strong interaction and a new microenvironment of proton between the sulfonic acid group and the H2SO4, which may be helpful to slow down the acidity change of sulfuric acid and further improve the reusability of sulfuric acid for the isobutane alkylation. To confirm this, the reusability experiments of the isobutane alkylation with 2-butene were carried out catalyzed

by

the

mixture

catalyst

of

the

sulfuric

acid

and

SFIL

(1-octyl-3-sulfopropyl-imidazolium hydrogen sulfate, [C8PSim][HSO4]), as well as non-SFIL (1-octyl-imidazolium hydrogen sulfate, [C8Mim][HSO4]) for comparison, 26


Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as shown in Figure 11. It should be noted that the data of the reusability experiment involving SFIL is taken from our recent work (this work was submitted to AIChE J (under review)). Clearly, for the SFIL system, the yield of C8 components shows a gradual decrease after 24 runs, corresponding to a sharp increase for the yield of C9+ components, which indicates the SFILs can enhance the reusability of sulfuric acid up to 24 runs. However, for the non-SFIL system, it can be found that the yield of C8 components decreases dramatically after 3 runs, and especially after 6 runs, the RON is lower than 90, meaning that the reusability of non-SFIL/sulfuric acid mixture catalyst has a range of 4-6 runs, consistent with the earlier finding29. These observations suggest that the lifetime of sulfuric acid is substantially enhanced by the addition of the ionic liquid with the sulfonic acid group, which agrees well with the simulation results in this work. In addition, the molecular simulations can help us understand the microenvironment interaction and nanostructure between the sulfuric acid and the SFILs at the molecular scale, which contributes to a better understanding of the effect of sulfonic acid groups on the reusability of the sulfuric acid for the isobutane alkylation from the point of microscopical view. In particular, it is an essentially key step to correlate the microscopical properties that the MD simulations provide with the macroscopical experimental properties. The good correlation between the microscopical information provided by the MD simulations and the macroscopically catalytic performance shown by the experiments in this work suggests that the useful information that the MD simulations provide is very 27



fundamentally important for the design and screening of the ILs for the alkylation processes, which also confirmed the strength of the MD simulations.

Figure 11. Reusability of ionic liquid/sulfuric acid mixture catalyst for the isobutane

alkylation. Filled symbols represent the SFIL system; Hollow symbols correspond to the non-SFIL system. Reaction conditions: reaction time 25 min, stirring rate 3000 r/min, mass ratio of IL/H2SO4 10%, volume ratio of H2SO4/hydrocarbon 1.5:1, volume ratio of I/O 8:1.

4. Conclusions The all-atom force field for the sulfuric acid at the typical concentration of 98.30 wt% was parameterized using ab-initio calculations. These parameters were incorporated into the Optimized Potentials for Liquid Simulations All Atom (OPLS-AA) force field, and were further validated from several key properties, such as the density, surface tension. The most of H3O+ ions are found to be located beside the HSO4− ions, forming a contact ion pair with HSO4− ions. Both the radial distribution functions (RDFs) and hydrogen bond analysis revealed there exist three-dimensional hydrogen-bonding networks in the sulfuric acid. The diffusion of the H2SO4 molecule is about two orders of magnitude larger than that of HSO4− and H3O+ ions, both of which possess a comparable self-diffusion coefficient.

28


Page 28 of 34

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The mixing of the sulfuric acid with both the [PMim][HSO4] and [PSMim][HSO4] show an exothermic characteristics with the latter presenting stronger exothermic behavior. The RDFs from the IL/sulfuric acid mixtures confirmed that both of the ILs could be miscible with the sulfuric acid. Based on the RDFs analysis, the [PSMim][HSO4] is beneficial to the stability of the structure of the sulfuric acid compared to the [PMim][HSO4] due to the introduction of the sulfonic acid group. The RDFs results suggest that a new strong interaction site appears between the sulfonic acid group of cations and the H2SO4 molecule, which is useful to slow down the acidity change of the sulfuric acid. The SFILs can improve the reusability of sulfuric acid for the isobutane alkylation, which was confirmed by our experimental results.

Supporting Information Force field parameters for H2SO4 molecule and HSO4+ ion supplied as Supporting Information.

Acknowledgements The financial support by the National Natural Science Foundation of China (91434108) is gratefully acknowledged.

Reference (1)

King, M.; Moats, M.; Davenport, W. G., Sulfuric Acid Manufacture: Analysis, Control and

Optimization. Elsevier: Oxford, U.K., 2013. (2)

Albright, L. F., Present and future alkylation processes in refineries. Ind. Eng. Chem. Res

2009, 48, 1409-1413. 29



(3)

Hofmann, J.; Schriesheim, A., Ionic reactions occurring during sulfuric acid catalyzed

alkylation. I. Alkylation of isobutane with butenes. J. Am. Chem. Soc. 1962, 84, 953-957. (4)

Li, K.; Eckert, R. E.; Albright, L. F., Alkylation of isobutane with light olefins using sulfuric

acid. Operating variables affecting physical phenomena only. Ind. Eng. Chem. Process Des. Dev 1970, 9, 434-440. (5)

Li, K.; Eckert, R. E.; Albright, L. F., Alkylation of isobutane with light olefins using sulfuric

acid. operating variables affecting both chemical and physical phenomena. Ind. Eng. Chem. Process Des. Dev 1970, 9, 441-446. (6) Corma, A.; Martinez, A., Chemistry, catalysts, and processes for isoparaffin-olefin alkylation: Actual situation and future trends. Catal.Rev. 1993, 35, 483-570. (7)

Ende, D. J. a.; Eckert, R. E.; Albright, L. F., Interfacial area of dispersions of sulfuric-acid

and hydrocarbons. Ind. Eng. Chem. Res 1995, 34, 4343-4350. (8)

Albright, L. F., Alkylation of isobutane with C3-C5 olefins: Feedstock consumption, acid

usage, and alkylate quality for different processes. Ind. Eng. Chem. Res 2002, 41, 5627-5631. (9) Albright, L. F., Alkylation of isobutane with C3-C5 olefins to produce high-quality gasolines: Physicochemical sequence of events. Ind. Eng. Chem. Res 2003, 42, 4283-4289. (10) Sun, W.; Shi, Y.; Chen, J.; Xi, Z.; Zhao, L., Alkylation kinetics of isobutane by C4 olefins using sulfuric acid as catalyst. Ind. Eng. Chem. Res 2013, 52, 15262-15269. (11) Anderson, K. E.; Siepmann, J. I.; McMurry, P. H.; VandeVondele, J., Importance of the number of acid molecules and the strength of the base for double-ion formation in (H2SO4) m· Base·(H2O) 6 clusters. J. Am. Chem. Soc. 2008, 130, 14144-14147. (12) Vchirawongkwin, V.; Kritayakornupong, C.; Rode, B. M., Structural and dynamical properties and vibrational spectra of bisulfate ion in water: A study by ab initio quantum mechanical charge field molecular dynamics. J. Phys. Chem. B 2010, 114, 11561-11569. (13) Choe, Y.-K.; Tsuchida, E.; Ikeshoji, T., First-principles molecular dynamics study on aqueous sulfuric acid solutions. J.Chem. Phys. 2007, 126, 154510. (14) Ishiyama, T.; Morita, A., Molecular dynamics simulation of sum frequency generation spectra of aqueous sulfuric acid solution. J. Phys. Chem. C 2011, 115, 13704-13716. (15) Ishiyama, T.; Morita, A.; Miyamae, T., Surface structure of sulfuric acid solution relevant to sulfate aerosol: molecular dynamics simulation combined with sum frequency generation measurement. PCCP 2011, 13, 20965-20973. (16) Sugawara, S.; Yoshikawa, T.; Takayanagi, T.; Shiga, M.; Tachikawa, M., Quantum proton transfer in hydrated sulfuric acid clusters: A perspective from semiempirical path integral simulations. J. Phys. Chem. A 2011, 115, 11486-11494. (17) Hammerich, A. D.; Buch, V., Ab initio molecular dynamics simulations of the liquid/vapor interface of sulfuric acid solutions. J. Phys. Chem. A 2012, 116, 5637-5652. (18) Niskanen, J.; Sahle, C. J.; Juurinen, I.; Koskelo, J.; Lehtola, S.; Verbeni, R.; Müller, H.; Hakala, M.; Huotari, S., Protonation dynamics and hydrogen bonding in aqueous sulfuric acid. J. Phys. Chem. B 2015, 119, 11732-11739. (19) Elavarasan, P.; Kondamudi, K.; Upadhyayula, S., Kinetics of phenol alkylation with tert-butyl alcohol using sulfonic acid functional ionic liquid catalysts. Chem. Eng. J. 2011, 166, 340-347. 30


Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Wu, X.; Han, X.; Zhou, L.; Li, A., Catalytic performance of acidic ionic liquid in esterification of benzyl alcohol with butyric acid. Indian J. Chem. A 2012, 51. (21) Fang, D.; Zhou, X.-L.; Ye, Z.-W.; Liu, Z.-L., Bronsted acidic ionic liquids and their use as dual solvent-catalysts for Fischer esterifications. Ind. Eng. Chem. Res 2006, 45, 7982-7984. (22) Wang, F.; Zhu, G.; Li, Z.; Zhao, F.; Xia, C.; Chen, J., Mechanistic study for the formation of polyoxymethylene dimethyl ethers promoted by sulfonic acid-functionalized ionic liquids. J. Mol. Catal. A: Chem. 2015, 408, 228-236. (23) Ren, H.-x.; Ying, H.-j.; Sun, Y.-m.; Wu, D.-j.; Ma, Y.-s.; Wei, X.-f., Synthesis of poly (lactic acid)-poly (ethylene glycol) copolymers using multi-SO3H-functionalized ionic liquid as the efficient and reusable catalyst. Polym. Bull. 2014, 71, 1173-1195. (24) Amarasekara, A. S.; Owereh, O. S., Synthesis of a sulfonic acid functionalized acidic ionic liquid modified silica catalyst and applications in the hydrolysis of cellulose. Catal. Commun. 2010, 11, 1072-1075. (25) Zhang, C.; Fu, Z.; Dai, B.; Zen, S.; Liu, Y.; Xu, Q.; Kirk, S. R.; Yin, D., Biochar sulfonic acid immobilized chlorozincate ionic liquid: an efficiently biomimetic and reusable catalyst for hydrolysis of cellulose and bamboo under microwave irradiation. Cellulose 2014, 21, 1227-1237. (26) Zhang, L.; Xian, M.; He, Y.; Li, L.; Yang, J.; Yu, S.; Xu, X., A Brønsted acidic ionic liquid as an efficient and environmentally benign catalyst for biodiesel synthesis from free fatty acids and alcohols. Bioresour. Technol. 2009, 100, 4368-4373. (27) Wu, Z.; Li, Z.; Wu, G.; Wang, L.; Lu, S.; Wang, L.; Wan, H.; Guan, G., Bronsted acidic ionic liquid modified magnetic nanoparticle: an efficient and green catalyst for biodiesel production. Ind. Eng. Chem. Res 2014, 53, 3040-3046. (28) Kore, R.; Srivastava, R., Synthesis and applications of novel imidazole and benzimidazole based sulfonic acid group functionalized Bronsted acidic ionic liquid catalysts. J. Mol. Catal. A: Chem. 2011, 345, 117-126. (29) Tang, S.; Scurto, A. M.; Subramaniam, B., Improved 1-butene/isobutane alkylation with acidic ionic liquids and tunable acid/ionic liquid mixtures. J. Catal. 2009, 268, 243-250. (30) Wang, A.; Zhao, G.; Liu, F.; Ullah, L.; Zhang, S.; Zheng, A., Anionic clusters enhanced catalytic performance of protic acid ionic liquids for isobutane alkylation. Ind. Eng. Chem. Res 2016, 55, 8271-8280. (31) Köddermann, T.; Paschek, D.; Ludwig, R., Molecular dynamic simulations of ionic liquids: A reliable description of structure, thermodynamics and dynamics. ChemPhysChem 2007, 8, 2464-2470. (32) Chaumont, A.; Schurhammer, R.; Wipff, G., Aqueous interfaces with hydrophobic room-temperature ionic liquids: a molecular dynamics study. J. Phys. Chem. B 2005, 109, 18964-18973. (33) Bhargava, B.; Balasubramanian, S., Refined potential model for atomistic simulations of ionic liquid [bmim][PF6]. J.Chem. Phys. 2007, 127, 114510. (34) Chevrot, G.; Schurhammer, R.; Wipff, G., Molecular dynamics simulations of the aqueous interface with the [BMI][PF6] ionic liquid: Comparison of different solvent models. PCCP 2006, 8, 4166-4174. (35) Shan, W.; Yang, Q.; Su, B.; Bao, Z.; Ren, Q.; Xing, H., Proton microenvironment and 31



interfacial structure of sulfonic-acid-functionalized ionic liquids. J. Phys. Chem. C 2015, 119, 20379-20388. (36) Canongia Lopes, J. N.; Deschamps, J.; Pádua, A. A., Modeling ionic liquids using a systematic all-atom force field. J. Phys. Chem. B 2004, 108, 2038-2047. (37) Canongia Lopes, J. N.; Pádua, A. A., Molecular force field for ionic liquids composed of triflate or bistriflylimide anions. J. Phys. Chem. B 2004, 108, 16893-16898. (38) Canongia Lopes, J. N.; Pádua, A. A., Molecular force field for ionic liquids III: Imidazolium, pyridinium, and phosphonium cations; chloride, bromide, and dicyanamide anions. J. Phys. Chem. B 2006, 110, 19586-19592. (39) Canongia Lopes, J. N.; Pádua, A. A.; Shimizu, K., Molecular force field for ionic liquids IV: Trialkylimidazolium and alkoxycarbonyl-imidazolium cations; alkylsulfonate and alkylsulfate anions. J. Phys. Chem. B 2008, 112, 5039-5046. (40) Walrafen, G.; Yang, W.-H.; Chu, Y.; Hokmabadi, M., Structures of concentrated sulfuric acid determined from density, conductivity, viscosity, and raman spectroscopic data. J. Solution Chem. 2000, 29, 905-936. (41) Robertson, E. B.; Dunford, H., The state of the proton in aqueous sulfuric acid. J. Am. Chem. Soc. 1964, 86, 5080-5089. (42) Que, H.; Song, Y.; Chen, C.-C., Thermodynamic modeling of the sulfuric acid-water-sulfur trioxide system with the symmetric electrolyte NRTL model. J. Chem. Eng. Data 2011, 56, 963-977. (43) Zheng, S.; Tang, Q.; He, J.; Du, S.; Xu, S.; Wang, C.; Xu, Y.; Lin, F., VFFDT: A new software for preparing AMBER force field parameters for metal-containing molecular systems. J. Chem. Inf. Model 2016, 56, 811-818. (44) Lohr, L. L., An ab initio study of the structure and torsional modes of the H2SO4 molecule. J. Mol. Struc-THEOCHEM 1982, 87, 221-227. (45) Havey, D.; Feierabend, K.; Vaida, V., Ab initio study of H2SO4 rotamers. J. Mol. Struc-THEOCHEM 2004, 680, 243-247. (46) Law, M. M.; Hutson, J. M., I-NoLLS: a program for interactive nonlinear least-squares fitting of the parameters of physical models. Comput. Phys. Commun. 1997, 102, 252-268. (47) Devereux, M.; Meuwly, M., Force field optimization using dynamics and ensemble averaged data: vibrational spectra and relaxation in bound mbco. J. Chem. Inf. Model 2010, 50, 349-357. (48) Yosa, J.; Meuwly, M., Vibrationally induced dissociation of sulfuric acid (H2SO4). J. Phys. Chem. A 2011, 115, 14350-14360. (49) Cannon, W. R.; Pettitt, B. M.; McCammon, J. A., Sulfate anion in water-model structural, thermodynamic, and dynamic properties. J. Phys. Chem. 1994, 98, 6225-6230. (50) Jang, S. S.; Molinero, V.; Çaǧın, T.; Goddard, W. A., Nanophase-segregation and transport in Nafion 117 from molecular dynamics simulations: effect of monomeric sequence. J. Phys. Chem. B 2004, 108, 3149-3157. (51) Bollas, G.; Chen, C.-C.; Barton, P., Refined electrolyte-NRTL model: Activity coefficient expressions for application to multi-electrolyte systems. AIChE J 2008, 54, 1608-1624. (52) Perry, R. H.; Green, D. W., Perry's Chemical Engineers' Handbook. McGraw-Hill: New York, USA, 1999. (53) Sabinina, L.; Turpugow, L., The surface tension of the system sulfuric acid-water. Z. Phys. 32


Page 32 of 34

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chem 1935, 173, 237-241. (54) Wang, P.; Anderko, A.; Young, R. D., Modeling surface tension of concentrated and mixed-solvent electrolyte systems. Ind. Eng. Chem. Res 2011, 50, 4086-4098. (55) Tammet, H.; Kulmala, M., Quantum chemical studies of hydrate formation of H2SO4 and HSO4. Boreal. Environ. Res 2007, 12. (56) Hou, G.-L.; Lin, W.; Deng, S.; Zhang, J.; Zheng, W.-J.; Paesani, F.; Wang, X.-B., Negative ion photoelectron spectroscopy reveals thermodynamic advantage of organic acids in facilitating formation of bisulfate ion clusters: Atmospheric implications. J. Phys. Chem. Lett. 2013, 4, 779-785. (57) Liu, Z.; Huang, S.; Wang, W., A refined force field for molecular simulation of imidazolium-based ionic liquids. J. Phys. Chem. B 2004, 108, 12978-12989. (58) Mills, R., Self-diffusion in normal and heavy water in the range 1-45. deg. J. Phys. Chem 1973, 77, 685-688. (59) Xing, H.; Wang, T.; Zhou, Z.; Dai, Y., Novel Bronsted-acidic ionic liquids for esterifications. Ind. Eng. Chem. Res 2005, 44, 4147-4150. (60) Xing, H.; Wang, T.; Zhou, Z.; Dai, Y., The sulfonic acid-functionalized ionic liquids with pyridinium cations: acidities and their acidity-catalytic activity relationships. J. Mol. Catal. A: Chem. 2007, 264, 53-59. (61) Kumar, M.; Venkatnathan, A., Mechanism of proton transport in ionic liquid doped perfluorosulfonic acid membranes. J. Phys. Chem. B 2013, 117, 14449-14456. (62) Dong, K.; Zhang, S.; Wang, D.; Yao, X., Hydrogen bonds in imidazolium ionic liquids. J. Phys. Chem. A 2006, 110, 9775-9782. (63) Liu, X.; Zhao, Y.; Zhang, X.; Zhou, G.; Zhang, S., Microstructures and interaction analyses of phosphonium-based ionic liquids: a simulation study. J. Phys. Chem. B 2012, 116, 4934-4942.

33



TOC Graphic Hydrogen bonds H2SO4 SFILs

MD simulation

Isobutane alkylation

34


Page 34 of 34

Microstructures of the Sulfonic Acid-Functionalized Ionic Liquid

Recommend Documents