Understanding Structure-Property Relationship of SO3H

Oct 24, 2018 - Understanding Structure-Property Relationship of SO3H-Functionalized Ionic Liquids together with Sulfuric Acid in Catalyzing Isobutane ...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Understanding Structure−Property Relationship of SO3H‑Functionalized Ionic Liquids together with Sulfuric Acid in Catalyzing Isobutane Alkylation with C4 Olefin Weizhen Sun,† Weizhong Zheng,† Wenxiu Xie,† and Ling Zhao*,†,‡ †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China School of Chemistry & Chemical Engineering, XinJiang University, Urumqi 830046, China

‡

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S Supporting Information *

ABSTRACT: In this work, the SO3H-functionalized ionic liquids (SFILs) ([CnPSIm][HSO4], n = 1, 2, 4, 6, and 8) were investigated as cocatalyst mixed with the concentrated H2SO4 for the isobutane alkylation. The SFILs with longer alkyl chain show a better catalytic performance with the C8 selectivity up to 75.73 and RON up to 95.66, respectively. The better catalytic performance can be attributed to the better dispersion of isobutane in the SFIL/H2SO4 system led by the SFILs with longer alkyl chain, which is further correlated with the nanostructured-aggregation feature of the longer alkyl chain confirmed by MD simulation. In addition, the reusability of SFIL/H2SO4 mixture can reach up to 22 runs, outclassing the pure H2SO4. The MD simulation, quantum chemistry calculations, and 1H NMR spectra revealed that cationic clusters are formed by the strong hydrogen bonds between the sulfonic acid group and the H2SO4, which is beneficial to the longer lifetime of the SFIL/H2SO4 mixtures. Hopefully, the useful information in this work will provide valuable insights into the screening and design of novel SFILs for the H2SO4 alkylation. HCl with traces of H2O.12 On the contrary, Brønsted acidic ionic liquids (BILs) have been regarded as a promising candidate, playing a dual solvent−catalyst role in extensive applications for considerable chemical processes due to their excellent properties, including tunable acidity, high catalytic activity, and better dissolving capacity for a wide range of organic and inorganic compounds.13−15 Despite the numerous advantages of the BILs, little work has focused on the BILs as additive or cocatalyst for the isobutane alkylation. In the pioneering work, both sulfonic-acid-functionalized Brønsted acidic ionic liquids (SFILs) and non-SFILs as cocatalyst were used to catalyze the strong acid (H2SO4 and TfOH) alkylation by Tang and co-workers, and a good catalytic performance was obtained, but only with emphasis on the optimization of the addition amount of the BILs and the corresponding catalytic performances of the non-SFILs.12 Subsequently, Wang and coworkers investigated the mixed catalyst system of non-SFIL ([N222H]CF3SO3) and TfOH for the alkylation with the C8 selectivity up to 86.23%, the research octane number (RON) up to 97.3, and the reusability up to 36 runs.16 The higher reusability was ascribed to the formation of anionic clusters between the TfO− and TfOH linked by strong hydrogen bonds, and the anionic clusters were considered as a buffer to

1. INTRODUCTION The isobutane alkylation with C3−C5 olefins for the production of the alkylate is an important process in petroleum industry, since the outstanding advantages, including high octane value, low vapor pressure, and no olefin and aromatics content, allow the alkylate to be an ideal blending component for gasoline.1 The current commercial processes mainly use the concentrated sulfuric acid (H2SO4) and hydrofluoric acid (HF) as catalysts.2 Actually, both catalysts have several drawbacks, such as serious environmental pollution and equipment corrosion. Particularly, the severe drawback of the HF, when it releases and forms aerosol, makes the alkylation units prefer to choose the H2SO4 as catalyst.3 Nevertheless, the biggest challenge using the H2SO4 for the alkylation is the huge acid consumption with the amount of spent acid as much as 0.5−0.6 lb/gallon. To cope with these problems, ionic liquids (ILs), a novel kind of materials that are liquid phase at room temperature, have attracted numerous attentions as a novel and promising candidate for the isobutane alkylation owing to their unique properties, such as negligible vapor pressure, high chemical stability, strong Brønsted and Lewis acidity, and adjustable combination of cations and anions. During the past decade, considerable efforts regarding to the chloroaluminate-based ILs as the Lewis acidic catalyst for the alkylation have been conducted, confirming the advantages of the ILs over H2SO4.4−11 However, the chloroaluminate-based ILs are extremely sensitive to moisture and atmosphere, forming © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

August 15, 2018 October 22, 2018 October 24, 2018 October 24, 2018 DOI: 10.1021/acs.iecr.8b03923 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Optimized molecular structures of SFILs, concentrated H2SO4, and isobutane for experiments and simulations.

properties in terms of the isobutane solubility and diffusion for the alkylation.21 To the best of our knowledge, however, there is no literature investigating the microscopic structure characteristics of the SFIL/H2SO4 mixtures, especially at the molecular level, and the relationship between the structure features and catalytic performance, although these information is of essencial importance to the design and application of SFILs for the alkylation. In this present work, a series of SO3H-functionalized ionic liquids, i.e., [CnPSIm][HSO4] (where n = 1, 2, 4, 6, and 8), mixed with the H2SO4 as synergistic catalysts for isobutane alkylation have been studied to assess their catalytic performance and reusability. Furthermore, the microscopic structure characteristics of the SFIL/H2SO4 mixtures were investigated using MD simulation and DFT calculation to further correlate with the corresponding catalytic performance.

maintain the acidity of strong acid during the alkylation process, which was helpful to enhance its reusability. Therefore, it is necessary to deeply investigate the catalytic performances of SFILs as cocatalyst for the H2SO4 alkylation, especially the structure−property relationship. The fundamental understanding of the structure−property relationship for a material is of great importance to its further optimization and design. In terms of the H2SO4 alkylation, it is still challenging to deal with two key issues, i.e., the low dissolution of isobutane in H2SO4 and the short lifetime of H2SO4.17−19 The higher dissolution of isobutane promotes the formation of carbonium ions and facilitates a faster hydride transfer from isobutane to C8+ ions, further contributing to a higher quality of alkylate.10 The long lifetime of H2SO4 means the low acid consumption, which is benificial to the reduction of environmental pollution. The additive or cocatalyst, which can simultaneously enhance the isobutane dissolution and the lifetime of H2SO4, will be the priority for the H2SO4 alkylation. From this point, the SFIL may be a good choice as a cocatalyst. This is because that the ILs is well-accepted to present a higher isobutane solubility due to the existence of nonpolar cations,1,20−22 and the introduction of sulfonic acid groups can change the proton microenvironment and nanostructure of the IL/H2SO4 mixture system, which has an important influence on determining proton transfer, catalytic activity and stability.23 However, it is extremely difficult to investigate the structure characteristics of the SFILs, especially the SFIL/ H2SO4 mixture system using the experimental methods, due to the lack of effective in situ experimental observation techniques. Fortunately, molecular simulation is suitable to probe the relationship between microscopic structure and macroscopic property.24 In fact, the liquid/liquid interfacial properties between the common ILs and C4 hydrocarbons (isobutane and 2-butene) were investigated in detail using molecular dynamics (MD) simulation in our recent work, and a good correlation between the interfacial properties and the IL catalytic performance was obtained.1,20 Furthermore, on the basis of the solvation free energy calculation, the common imidazolium-based ILs were further investigated for the isobutane alkylation, and the structure characteristics of the common ILs were well-correlated with the corresponding

2. EXPERIMENTAL AND SIMULATION SECTIONS 2.1. Materials and Instruments. All chemicals in experiments were obtained commercially and used as received directly. N-Alkyl imidazole was purchased from Energy Chemical and 1,3-propyl sultone was supplied by Aldrich. The concentrated H2SO4 (98.03 wt %) was purchased from Shanghai LingFeng chemical agent Co., Ltd. The NMR (1H NMR and 13C- NMR) spectra was obtained by an AVANCE III 400 MHz NMR spectrometer. The UV− vis spectra was recorded on a CARY 500 spectrometer to measure the acid strength of the SFILs. The measurement of moisture content of ionic liquids was carried on an Automatic Karl Fischer moisture analyzer. 2.2. Synthesis and Characterization of SO3H-Functionalized Ionic Liquids. In the present work, five SO3Hfunctionalized ILs [CnPSIm][HSO4] (where n = 1, 2, 4, 6, and 8), namely, [MPSIm][HSO4], [EPSIm][HSO4], [BPSIm][HSO4], [HPSIm][HSO4], and [OPSIm][HSO4], were synthesized according to the previous literatures, respectively, with the optimized molecular structures as shown in Figure 1.25−27 The detailed synthesis processes of SO3H-functionalized ILs and the acid strength represented by Hammett acidity function (H0) as well as the measured process were included in the Supporting Information. The structures of the B

DOI: 10.1021/acs.iecr.8b03923 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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was scaled down to +0.8 and −0.8 e, respectively, following the similar strategy as our recent work.23

synthesized SFILs were confirmed using the NMR (1H NMR and 13C NMR) spectra with the results also listed in the Supporting Information. 2.3. Alkylation Procedure. The isobutane alkylation was performed in a batch reactor equipped with a cooling system to control the reaction temperature as depicted in our previous work.28 The autoclave pressure was set to be 0.5 MPa to keep isobutane and olefine as liquid phase. The stirring rate maintained at 3000 r/min during the process to ensure an excellent mixing of hydrocarbon and catalyst. All experiments were conducted at 281.2 K, which is consistent with the temperature range of industrial interest. In a typical alkylation reaction, 210 mL of catalyst prepared in advance was added into the batch reactor. Subsequently, the air inside the reactor was removed by nitrogen (N2) three times, and then the pressure was set to 0.5 MPa with N2. Next, 140 mL of mixtures of isobutane and trans-2-butene with molar ratio of 8:1 were quickly injected into the reactor when the temperature reached the desired value. Then, the reagents were stirred by the agitator at a speed of 3000 r/min for a certain time. In the reaction process, the cooling device switch was adjusted to keep reaction temperature stable in the set value. During the reaction, the samples were taken out and analyzed by GC after extraction and separation according to the previous work.28 For cycle experiments, the catalysts were separated immediately and reused directly in the next run under the same reaction conditions. 2.4. Simulation Method and Procedure. For all the molecules, the structures were optimized by the B3LYP/6311(G) method, as shown in Figure 1. Initially, the simulation boxes consisted of 428 H2SO4 molecules, 42 bisulfate ions, 42 hydronium ions, 30 cation−anion pairs of SFILs, and 30 isobutane molecules with the approximate size of 4.0 × 4.0 × 4.0 nm3 using Packmol software, corresponding to the mole ratio of SFIL to H2SO4 and isobutane to catalyst is 5.5 and 5.2%, respectively. Then, energy minimization were performed by 2000 steps for the built boxes. For the resulting boxes, the annealing was conducted from 281 to 500 K, and then back to 281 K by 8 ns under canonical (NVT) ensemble. To further equilibrate the obtained boxes, the isothermal−isobaric (NPT) ensemble was performed for 30 ns with the last 5 ns for data analysis. It should be noted that in order to gain a full equilibration of the system, the number density distribution of each moiety was sampled every 1 ns, and no apparent density deviations could be found after 20 ns. All the simulations were run by GROMACS 4.5 software with the temperature and pressure controlled at 281 K and 1 atm using the Hoover-Nose thermostat and Parrinello− Rahman barostat. The three directions of these boxes were applied by periodic boundary conditions (PBC). Particle Mesh Ewald method (PME) was used to deal with the long-range electrostatic interaction with Lennard-Jones and Coulomb interaction cut-off at 1.2 nm using Lorentz−Berthelot (LB) mixing rules. On the basis of Maxwellian distribution, the initial atomic velocities were generated, along with the LINCS algorithm to treat all covalent bonds related to hydrogen atoms. In addition, the interaction between SFILs, H2SO4, and isobutane was described using all-atom force field (OPLS-AA). Specifically, the potential parameters of the concentrated H2SO4 were taken from our recent work.23 The potential parameters of the ILs and sulfonic acid groups were taken according to the previous work reported by Lopes et al.29−33 It should be noted that the total charge of the cation and anion

3. RESULTS AND DISCUSSION 3.1. Acid Strength of the SO3H-Functionalized ILs. Acid strength is an important parameter in the isobutane alkylation. According to the alkylation reaction mechanism, acidity is the impetus of alkenes protonation, which has a tremendous influence on the quality of alkylate.34,35 An extremely high acidity can cause the cracking of alkylates, while the low acidity is inclined to result in the consequences of alkenes polymerization.36 To characteristic the acid strength and obtain the value of H0, the UV−vis spectroscopy and Hammett indicator were utilized in this work. The UV absorption spectra of SO3H-functionalized ionic liquids in ethanol is shown in Figure S1. It can be seen that the maximum absorbance peak is observed at the wavelength of 373 nm, and the peak intensity increases with the prolonged alkyl chain length, but corresponds to a decreased acid strength. Furthermore, the value of H0 was determined by different absorbance at 373 nm and listed in Table 1. Table 1. H0 of SO3H-Functionalized Ionic Liquids entry 1 2 3 4 5 6

ILs

Amax

[I] (%)

[IH+] (%)

H0

22.50 7.50 5.83 3.33 1.67

−12.0 1.52 1.59 1.60 1.61 1.62

H2SO437

pure [MPSIm][HSO4] [EPSIm][HSO4] [BPSIm][HSO4] [HPSIm][HSO4] [OPSIm][HSO4]

0.93 1.11 1.13 1.16 1.18

77.50 92.50 94.17 96.67 98.33

Obviously, the H0 value is observed as follows: [MPSIm][HSO4] < [EPSIm][HSO4] < [BPSIm][HSO4] < [HPSIm][HSO4] < [OPSIm][HSO4], indicating that the SFILs with longer alkyl chain present a lower acid strength. However, the acid strength of SFILs is much lower than the optimal H0 range for the isobutane alkylation (−8.1 to −12.7). Therefore, the SFILs were mixed with H2SO4 as a cocatalyst to enhance its catalytic performance. 3.2. Reaction Time. In order to obtain an excellent catalytic performance, the effect of reaction time on the distribution of alkylate using [MPSIm][HSO4]/H2SO4 as the catalyst is shown in Figure 2. It is clear that the selectivity of both C8 and trimethylpentanes (TMP) components increases sharply, and the selectivity of C9+ components goes down rapidly before 5 min, indicating that the isobutane alkylation is a fast reaction. Then, the selectivity values slow down and gradually tend to be stable. The research octane number (RON) follows a similar trend as the selectivity of C8 components. To ensure the quality of alkylate, the reaction time of 25 min was adopted in this work. 3.3. Mass Ratio of ILs to H2SO4. In order to ensure the optimal addition amount of SFILs, the effect of addition amounts of [MPSIm][HSO4] into H2SO4 on the quality of alkylate was shown in Figure 3. Compared to that of pure H2SO4, the [MPSIm][HSO4]/H2SO4 mixture shows a better catalytic performance with a higher selectivity of C8, a better selectivity of TMP, and a larger RON. When 5 wt % [MPSIm][HSO4] is added, the catalytic performance is almost the same with pure H2SO4. The mass ratio of 10 wt % of [MPSIm][HSO4] to mixed catalyst shows the best catalytic performance with the selectivity of C8 up to 57.2% and the C

DOI: 10.1021/acs.iecr.8b03923 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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the corresponding catalytic performance, remains unclear. To better understand the structure−property relationship, it is of great importance to discuss the experimental results of the SFILs with different alkyl chain lengths more deeply. For the isobutane alkylation, it is well-accepted that the acidity of catalysts and the solubility of isobutane as well as the degree of emulsification play a crucial role in the quality of alkylate. Acid strength, as discussed earlier indicates, that the SFILs acidity decreases with the increasing alkyl chain length, but the catalytic performance follows an opposite order. Thus, the acidity of SFILs is not the key factor to affect the catalytic activity in this work. In addition, the imidazolium-based IL viscosity rises with the increase of alkyl chain length in cations due to the enhancement of van der Waals forces and hydrogen bonds.39 In this sense, the emulsification degree between the catalyst phase and C4 hydrocarbon phase decreases under the same stirring speed as the alkyl chain length of the SFILs prolongs, which is actually unfavorable for the alkylation. However, from our recent simulation results related to the interfacial properties between the SFILs/H2SO4 mixtures and C4 hydrocarbons,40 the interface tensions become much smaller with the increase in the alkyl chain length of the SFILs. The great decrease of the interfacial tension led by the SFILs with longer chains contributes much more to the better emulsification. Therefore, it can be inferred that the reason why the quality of alkylate is greatly improved by the addition of the SFILs with the longer alkyl chain can be ascribed to the higher solubility or better dispersion of isobutane in the SFILs with longer alkyl chain. To deeply probe the underlying mechanism of the better catalytic performance of the SFILs/H2SO4 mixed catalyst, the dispersion behaviors of isobutane in the SFIL/H2SO4 systems at the molecular level were investigated with the equilibrated snapshots shown in Figure 4. It can be seen that isobutane tends to assemble strongly around each other in the pure H2SO4 system owing to the large polarity difference. This assembly of isobutane in H2SO4 is similar as the dispersion behaviors of isobutane in the acid-continuous phase at the experimental conditions. Compared to that of pure H2SO4, the introduction of the SFILs can contribute to a better dispersion of isobutane in H2SO4, indicating that the SFILs can promote the isobutane dissolution. In particular, the dispersion behaviors of isobutane become much stronger with the increased alkyl chain length of the SFILs, and the dispersed isobutane tends to be located around the long alkyl chain of the cations, which confirms the great contribution of the SFILs, especially with longer alkyl chain, to the dispersion of isobutane in the H2SO4. In order to further assess the dispersion behaviors of isobutane in the SFIL/H2SO4 system, center-of-mass radial distribution functions (RDFs) between isobutane molecules were calculated, as shown in Figure 5. For all the systems, there is a maximum peak for the RDFs located at 5.46 Å with a strong intensity up to 8, which suggests an assembly behavior of isobutane in the SFIL/H2SO4 mixture, consistent with the results in Figure 4. Furthermore, the peak intensity of the RDFs decreases gradually from the pure H2SO4 to the SFILs with shorter alkyl chain to the ones with longer alkyl chain. The weaker intensity of the RDFs corresponds to the better dispersion of isobutane. Therefore, it can be concluded that the SFILs with longer alkyl chain is more favorable for the isobutane dispersion in the H2SO4.

Figure 2. Effect of reaction time on the distribution of alkylate. Reaction conditions: catalysts, [MPSIm][HSO4](5 wt %)/H2SO4; reaction temperature, 281.2K; stirring rate, 3000r/min; volume ratio of H2SO4/hydrocarbon, 1.5:1; mass ratio of I/O, 8:1.

Figure 3. Effect of the mass ratio of [MPSIm][HSO4] to mixed catalyst on the quality alkylate. Reaction conditions: reaction time, 25 min; reaction temperature, 281.2 K; stirring rate, 3000 r/min; volume ratio of H2SO4/hydrocarbon, 1.5:1; mass ratio of I/O 8:1.

RON up to 92.2. After the addition amount of [MPSIm][HSO4] exceeds 10 wt %, the quality of alkylate begins to deteriorate. For the SFILs with longer chains, the addition amounts follow the similar trend as the shorter one.38 This result can be interpreted by the following two reasons. First, considerable amounts of SFILs reduce the acidity strength of the catalyst that causes alkenes polymerization. Second, a greater viscosity is exhibited when more SFILs are added since they are much more viscous than H2SO4, which have an important effect on the mixing degree. Accordingly, in the following experiments, the mass ratio of 10 wt % was adopted. 3.4. Dispersion Behaviors of Isobutane in SFILs/ H2SO4 System. From our previous work,20 five different SFILs ([CnPSIm][HSO4], where n = 1, 2, 4, 6, and 8) have been investigated as cocatalyst for the alkylation to probe the effect of alkyl chain length on the catalytic performance, because the alkyl chain length of the cations is one of the most key factors to determine the structure characteristics of imidazolium-based ILs. It can be concluded that the catalytic performance was obviously increased with the prolonged alkyl chain length of the SFILs. However, the understanding about the structure characteristics of the SFIL/H2SO4 mixture, especially the relationship between the structure features and D

DOI: 10.1021/acs.iecr.8b03923 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Snapshots of the isobutane dispersion behaviors in different simulation systems. (a) Pure H2SO4, (b) [MPSIm][HSO4], (c) [EPSIm][HSO4], (d) [BPSIm][HSO4], (e) [HPSIm][HSO4], and (f) [OPSIm][HSO4]. Molecules in red color represent isobutane, and molecules with the blue (N), gray (C), yellow (S), and red (O) atoms stand for the cations of the SFILs. The concentrated H2SO4 and anions are removed, and the snapshots including the concentrated H2SO4 and anions are shown in Figure S2.

From the H2SO4 alkylation point of view, strong agitation is always needed to obtain good emulsification between the H2SO4 phase and C4 hydrocarbon phase. Actually, the agitation has almost no influence on the intrinsic solubility of isobutane in the H2SO4. However, the introduction of the SFILs, especially with longer alkyl chain, can promote the isobutane dispersion in the H2SO4, that is, the SFILs can facilitate the solubility of isobutane in H2SO4, which is beneficial to the quality of alkylate, in good agreement with the experimental and previous MD simulation results.38 3.5. Structure Characteristics and Interaction Energy. One of the most outstanding structure characteristics of the ILs that determines the dissolution of a wide range of polar or nonpolar molecules is the nanostructured aggregation, which generally contributes to the nanosegregation of the polar and nonpolar regions.41 Figure 6 shows the nanostructuredaggregation in the SFILs represented by the intermolecular atom−atom RDFs between the equivalent atoms along the

Figure 5. RDFs of isobutane molecules in different simulation systems.

Figure 6. Site−site RDFs of the atoms of SFILs in different simulation systems. (a) Cm−Cm and (b) Sp−Sp. E

DOI: 10.1021/acs.iecr.8b03923 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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recycling experiments with [OPSIm][HSO4]/H2SO4 as catalyst were conducted under the optimized conditions with the experimental data referring to our previous work,23 and we briefly introduce it here to just support our findings about ionic clusters in this mixed catalyst. Compared to that of pure H2SO4, the lifetime of H2SO4 with the SFILs as cocatalyst is prolonged up to 22 times, which means that the SFILs actually function as a butter agent and resist the acidity change of H2SO4. This is probably attributed to the formation of ionic clusters between the sulfonic acid group and H2SO4 by the hydrogen-bonding interaction. It has been believed that the strong hydrogen-bonding anion clusters like [CF3SO3(CF3SO3H)n]−(n = 1 and 2)16 exhibit a buffering function to resist the change of TfOH and prolong the lifetime of TfOH. To explore this important behavior at an atomic and molecular level, the MD simulation and quantum chemistry calculation were performed. For MD simulation, the intermolecular atom−atom RDFs between the protons and oxygen atoms on the SFILs and H2SO4 are shown in Figure 8. In general, a strong hydrogen

sulfopropyl group (Sp) and the alkyl chain (Cm) with the atom labels included in Figure 1. From Figure 6a, it is clear that the first peak of the RDFs for the terminal carbon atoms (Cm) is located at 4.0 Å, suggesting the nanostructured aggregation feature of the alkyl chain. Moreover, the peak intensity of the RDFs increases obviously with the prolonged length of the alkyl chain, especially for the SFIL with alkyl chain longer than C4. This observation indicates that the longer alkyl chain of the SFILs possess a stronger nanostructured aggregation feature, presenting larger nonpolar domains, which is consistent with the previous findings.42,43 In addition, the first peak of the RDFs for the terminal sulfur atoms (Sp) is presented at 4.9 Å, indicating that the sulfopropyl groups also show the nanostructured aggregation features (Figure 6b). For the SFILs with the alkyl chain shorter than C4, the peak intensity declines with the increased alkyl chain length. However, the peak intensity of the RDFs (Sp− Sp) becomes much stronger for the SFILs with longer alkyl chain length, following a similar trend as that of the alkyl chain. Therefore, it is confirmed that the SFILs with longer alkyl chain show a stronger nanostructured aggregation behavior and provide a larger nonpolar domain. This structure features can facilitate the dispersion of isobutane in the H2SO4, which can explain well the dispersion behaviors shown in Figure 4. The dispersion behaviors of isobutane in SFILs are partly determined by the interaction between isobutane and SFILs. The van der Waals and electrostatic contributions to the total energy for isobutane−cation and isobutane−anion interaction energy is shown in Figure 7. For all the simulation systems, the

Figure 8. Site−site RDFs of the atoms between [OPSIm][HSO4] and H2SO4 molecules, as well as HSO4− ions. Hm is the imidazole ring proton. Hp is sulfonic acid group proton. Hb is the proton of anions. Hs represents the proton of H2SO4 molecules. Ob is oxygen atom of anions. Os is oxygen atom of H2SO4 molecules. Op stands for oxygen atom of the sulfonic acid groups.

Figure 7. Interaction energy between isobutane and SFILs in different simulation systems. Yellow represents total potential energy, and blue stands for van der Waals energy. Red represents electrostatic energy. The filled bars represent the (isobutane + cation) interaction energy, and the patterned bars represent the (isobutane + anion) interaction energy.

bond is identified with the distance between the proton on the donor group and the acceptor atom shorter than 2.7 Å and the angle larger than 90°.44 The first maximum peak of RDFs for Hm−Ob and Hp−Ob is located at 2.32 and 2.58 Å with a strong intensity, respectively, suggesting the strong hydrogen-bonding interaction between the cations and anions of the [OPSIm][HSO4]. This observation is consistent with the existence of the hydrogen bonds, even three-dimensional networks, in the common ILs,45 BILs, and SFILs with short alkyl chain.23 The first maximum peaks of RDFs are located at a distance of 2.43 Å for Hm−Os and 2.67 Å for Hp−Os, respectively, which indicates that the cations of [OPSIm][HSO4] can form the hydrogen bonds with H2SO4 molecules. In addition, the atoms of the sulfonic acid groups (Op) are interacted with the proton of the anions and H2SO4 molecules by the weak hydrogenbonding interaction. Therefore, from the MD results, the SFILs can form the strong hydrogen-bonding interaction with the anions and H2SO4 molecules, indicating that the introduction of the SFILs can increase the number of hydrogen bonds, enhance the hydrogen-bonding networks, and change the proton microenvironment of the H2SO4.

total energy between isobutane and cation is greatly larger than that between isobutane and anions, and the van der Waals energy contributes significantly to the total energy between isobutane and cation, which suggests that the van der Waals interaction between isobutane and cation plays a decisive role in the dispersion of isobutane. Furthermore, the van der Waals energy increases obviously with the prolonged alkyl chain length of the SFILs. The strong van der Waals interaction can promote the dispersion of isobutene. Therefore, from the interaction point of view, it can be concluded that the SFILs with longer alkyl chain show a stronger interaction between isobutane, and then lead to a better dispersion behaviors in the H2SO4, further contributing to a better catalytic activity. 3.6. Characterization of Ionic Clusters by Quantum Chemistry Calculations and 1H NMR. To assess the contribution of the SFIL to the lifetime of H2SO4, the F

DOI: 10.1021/acs.iecr.8b03923 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. Optimized structures of ionic clusters [[OPSIm](H2SO4)n]+ (n = 0, 1, 2, and 3). Symbols: white: H, red: O, yellow: S, bule: N, gray: C.

For the quantum chemistry calculation, the optimized structures of the [[OPSIm](H2SO4)n]+(n = 0, 1, 2, and 3) obtained at the B3LYP/6-31++G** level are displayed in Figure 9 with only the lowest-energy conformers considered. For [[OPSIm](H2SO4)n]+(n = 1), it can be seen that two lowest-energy conformers with the hydrogen bond exist between the cation of ILs and H2SO4. The hydrogen bonds between the oxygen of H2SO4 and the hydrogen attached to the carbon atom between two N atoms and the hydrogen on the propyl-SO3H groups are formed with the distances of 2.162 and 2.584 Å, respectively, which agrees well with the previous finding that the hydrogen attached to the carbon atom between two N atoms can form the hydrogen bond with the oxygen of anion for different ILs.44,46−48 However, a stronger hydrogen bond with the distance of 1.714 Å is observed between the sulfonic acid group on the cation and H2SO4. Therefore, compared to that between the oxygen of H2SO4 and the hydrogen attached to the carbon atom between two N atoms, this stronger hydrogen bond between the sulfonic acid group and H2SO4 plays a more vital role in resisting the acidity change and improving the reusability of H2SO4 based on the fact that the [OMIm][HSO4] exerts no effect on the reusability of H2SO4,12 while the reusability of H2SO4 is dramatically prolonged after the introduction of the sulfonic acid group. Furthermore, it is also found that the hydrogen bonds exist between the sulfonic acid group and two H2SO4 and three H2SO4 to form [[OPSIm](H2SO4)n]+(n = 2, 3), respectively. Therefore, it is safe to conclude that the SFILs act as a buffering agent and can efficiently improve the reusability of H2SO4, due to the ionic clusters of the [[OPSIm](H2SO4)n]+(n = 1, 2, and 3) resulted from the strong hydrogen bonds between the sulfonic acid groups and the H2SO4. Anionic clusters [HSO4(H2SO4)n]− (n = 0, 1, and 2) have been confirmed according to the previous literature.40−51 To verify the existence of cationic clusters [[OPSIm](H2SO4)n]+ predicted by quantum chemistry calculations in this work, the 1 H NMR spectra of [OPSIm][HSO4]/H2SO4 with various molar ratio were determined, which is shown in Figure 10. For xH2SO4 = 0.67, the chemical shift in downfield δ = 11.6 ppm corresponds to the proton located on [[OPSIm](H2SO4)n]+ (n = 1), while δ = 8.4 ppm corresponds to the proton in imidazole

Figure 10. 1H NMR spectra of [OPSIm][HSO4]/xH2SO4 system (600 MHz, 25 °C, neat) for xH2SO4 = 0.67, 0.75, and 0.8.

ring between two nitrogen atoms. For xH2SO4 = 0.75, the chemical shift at 11.56 ppm integrates to the proton of [[OPSIm](H2SO4)n]+ (n = 2). The formation of hydrogen bonds between sulfonic acid groups and H2SO4 molecules weakens the interaction of N−H, consequently, an upfield signal at 8.27 ppm integrates to N−H of imidazole ring. For xH2SO4 = 0.80, the single peak at 10.99 ppm is in agreement with the proton of [[OPSIm](H2SO4)n]+ (n = 3), where chemical shift of N−H appears at an upfield signal 8.15 ppm for the same reason. The cationic clusters [[OPSIm](H2SO4)n]+ are proved since the single peak changes upfield when the molar ratio of H2SO4 is over 0.75, whereas anionic clusters [HSO4(H2SO4)n]− (n = 0, 1, and 2) exist in the catalyst system alone since the maximal value of n in [HSO4(H2SO4)n]− is 2 and the single peak cannot change when the molar ratio of H2SO4 is over 0.75 (n = 2).52 Moreover, the average bond length obtained by quantum chemistry calculations are 1.714, 1.718, and 1.728 Å when n = 1, 2, and 3 in [[OPSIm](H2SO4)n]+, which is in accordance with the spectra result that the upfield chemical shift is observed from 1H NMR spectra for the reason that the strong hydrogen bonds make the chemical shift downfield. To sum up, it is reasonable to conclude that the structures of the SFILs have a close relationship with the catalytic G

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(2) Kranz, K. Intro to Alkylation Chemistry: Mechanisms, Operating Variables, And Olefin Interactions; DuPont STRATCO Clean Fuel Technology, 2008. (3) Feller, A.; Zuazo, I.; Guzman, A.; Barth, J. O.; Lercher, J. A. Common mechanistic aspects of liquid and solid acid catalyzed alkylation of isobutane with n-butene. J. Catal. 2003, 216 (1), 313− 323. (4) Chauvin, Y.; Hirschauer, A.; Olivier, H. Alkylation of isobutane with 2-butene using 1-butyl-3-methylimidazolium chloride-aluminium chloride molten salts as catalysts. J. Mol. Catal. 1994, 92 (2), 155− 165. (5) Huang, C.-P.; Liu, Z.-C.; Xu, C.-M.; Chen, B.-H.; Liu, Y.-F. Effects of additives on the properties of chloroaluminate ionic liquids catalyst for alkylation of isobutane and butene. Appl. Catal., A 2004, 277 (1), 41−43. (6) Yoo, K.; Namboodiri, V. V.; Varma, R. S.; Smirniotis, P. G. Ionic liquid-catalyzed alkylation of isobutane with 2-butene. J. Catal. 2004, 222 (2), 511−519. (7) Zhang, J.; Huang, C.; Chen, B.; Ren, P.; Pu, M. Isobutane/2butene alkylation catalyzed by chloroaluminate ionic liquids in the presence of aromatic additives. J. Catal. 2007, 249 (2), 261−268. (8) Schilder, L.; Maaß, S.; Jess, A. Effective and intrinsic kinetics of liquid-phase isobutane/2-butene alkylation catalyzed by chloroaluminate ionic liquids. Ind. Eng. Chem. Res. 2013, 52 (5), 1877−1885. (9) Liu, Y.; Li, R.; Sun, H.; Hu, R. Effects of catalyst composition on the ionic liquid catalyzed isobutane/2-butene alkylation. J. Mol. Catal. A: Chem. 2015, 398, 133−139. (10) Liu, Y.; Wang, L.; Li, R.; Hu, R. Reaction mechanism of ionic liquid catalyzed alkylation: Alkylation of 2-butene with deuterated isobutene. J. Mol. Catal. A: Chem. 2016, 421, 29−36. (11) Bui, T. L. T.; Korth, W.; Jess, A. Influence of acidity of modified chloroaluminate based ionic liquid catalysts on alkylation of iso-butene with butene-2. Catal. Commun. 2012, 25, 118−124. (12) 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 (2), 243−250. (13) Zhao, Y.; Long, J.; Deng, F.; Liu, X.; Li, Z.; Xia, C.; Peng, J. Catalytic amounts of Brønsted acidic ionic liquids promoted esterification: study of acidity−activity relationship. Catal. Commun. 2009, 10 (5), 732−736. (14) Amarasekara, A. S. Acidic Ionic Liquids. Chem. Rev. 2016, 116 (10), 6133−83. (15) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Evolving Structure-Property Relationships and Expanding Applications. Chem. Rev. 2015, 115, 11379−11448. (16) 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 (30), 8271−8280. (17) Corma, A.; Martinez, A. Chemistry, catalysts, and processes for isoparaffin-olefin alkylation: Actual situation and future trends. Catal. Rev.: Sci. Eng. 1993, 35 (4), 483−570. (18) Hommeltoft, S. I. Isobutane alkylation: Recent developments and future perspectives. Appl. Catal., A 2001, 221 (1), 421−428. (19) 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 (19), 4283−4289. (20) Zheng, W.; Wang, H.; Xie, W.; Zhao, L.; Sun, W. Understanding Interfacial Behaviors of Isobutane Alkylation with C4 Olefin Catalyzed by Sulfuric Acid or Ionic Liquids. AIChE J. 2018, 64 (3), 950−960. (21) Zheng, W.; Zheng, L.; Sun, W.; Zhao, L. Screening of imidazolium ionic liquids for the isobutane alkylation based on molecular dynamic simulation. Chem. Eng. Sci. 2018, 183, 115−122. (22) Zhang, Y.; Zhang, T.; Gan, P.; Li, H.; Zhang, M.; Jin, K.; Tang, S. Solubility of isobutane in ionic liquids [BMIm][PF6],[BMIm][BF4], and [BMIm][Tf2N]. J. Chem. Eng. Data 2015, 60 (6), 1706− 1714.

performance. Specifically, the prolonged chain length of the SFILs can contribute to a better dispersion of isobutane into the H2SO4, which further improves the quality of the alkylate. Moreover, the introduction of the sulfonic acid groups into the ILs induces a strong hydrogen-bonding interaction between the H2SO4 and meanwhile forms a strong ionic cluster, which can enhance the reusability of the H2SO4. Therefore, it will be feasible way to screen and design a novel IL to handle the disadvantages of the H2SO4 for the isobutane alkylation.

4. CONCLUSIONS The binary mixtures of SO3H-functionalized ILs [CnPSIm][HSO4] (where n = 1, 2, 4, 6, and 8) and sulfuric acid acted as catalyst for the isobutane alkylation were investigated by combination of the experiment, MD simulation, and quantum chemistry calculation methods. The experimental results indicate that the optimal addition amount of SFILs is 10 wt %. In addition, the catalytic performance of SFILs became better with the increase of alkyl chain length with the [OPSIm][HSO4]/H2SO4 mixture contributing to the C8 selectivity up to 75.73 and RON up to 95.66. The better catalytic performance can be attributed to the better dispersion of isobutane resulted from the SFILs with longer alkyl chain, which is further correlated with the nanostructured-aggregation feature of the longer alkyl chain confirmed by MD simulation. More importantly, the reusability of SFILs/H2SO4 mixture can reach up to 22 runs, outclassing the pure H2SO4. The MD simulation, quantum chemistry calculations, and 1H NMR spectra revealed that cationic clusters are formed by the strong hydrogen bonds between the sulfonic acid groups and the H2SO4, which is beneficial to the longer lifetime of the SFILs/H2SO4 mixture. Hopefully, the useful information in this work will provide valuable insights into the screening and design of novel SFILs for the H2SO4 alkylation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03923. UV absorption spectra and isobutane dispersion behaviors, (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86 21 64253175. ORCID

Weizhen Sun: 0000-0002-9957-3620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Natural Science Foundation of China (91434108) and the Shanghai Excellent Technical Leaders Program (14xd1425500) is gratefully acknowledged.

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