Density Functional Theory Investigation for Catalytic Mechanism of

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Density Functional Theory Investigation for Catalytic Mechanism of Gasoline Alkylation Desulfurization over NKC‑9 Ion-Exchange Resin Yu Liu, Bolun Yang, and Chunhai Yi* Department of Chemical Engineering, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, P.R. China S Supporting Information *

ABSTRACT: The molecular level understanding of the mechanism about the 3-methylthiophene (3MT) alkylation with isobutylene (IB) as well as the side reaction of IB dimerization over NKC-9 cation exchange resin has been investigated using the density functional theory (DFT) of quantum chemical method. A model of benzene sulfonic acid was used to represent the cation-exchange resin catalyst. Two different reaction mechanism typesstepwise scheme and concerted scheme have been evaluated. Activation energies of each reaction path which were obtained from the DFT results have been improved by singlepoint MP2 calculations. In the stepwise mechanism, both 3MT alkylation and IB dimerization proceed by adsorption and protonation of the IB to form a sulfonic ester intermediate, and then by C−C bond formation between the sulfonic ester intermediate and another 3MT or IB to give the reaction products. The second step is rate-determining and has activation barriers of 148.41 kJ/mol for 3MT alkylation and 160.52 kJ/mol for IB dimerization. In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C−C bond formation. The activation barrier is calculated to be 169.10 kJ/mol for 3MT alkylation, and that for IB dimerization is 174.02 kJ/mol. The results revealed that the reaction mechanism of 3MT alkylation was very similar to that of IB dimerization, and the stepwise mechanism dominated both the 3MT alkylation and IB dimerization. Moreover, 3MT alkylation is more easily occurs than IB dimerization during gasoline alkylation desulfurization.

1. INTRODUCTION Environmental pollution due to the exhaust gas from the burning of fossil fuel has been a concern worldwide in recent years. Because of increasingly stringent regulations on fuel specifications of sulfur compounds, deep desulfurization of gasoline is receiving more and more attention in the research community.1−4 The olefinic alkylation of thiophenic sulfur (OATS) technology developed by British Petroleum is a good alternative desulfurization technology compared to the conventional catalytic hydro-desulfurization process. In this process, a catalytic alkylation reaction between thiophenes and olefins in the fluid catalytic cracking (FCC) gasoline will take place in the presence of an acidic catalyst to elevate the boiling points of the sulfur compounds, and then these high-boiling-point sulfur components will be easily separated by distillation operation.5 Owing to the significant advantages to lessen hydrogen consumption, moderate operation conditions, and reduce octane number loss, the OATS process has received considerable attention of researchers in the past decade.6−12 However, the efficiency and industrialization of the OATS process is still limited by the two side reactionsalkylation of aromatic hydrocarbons and olefin oligomerization, as reported by I. V. Babich et al.13 Although the aromatics alkylation could be neglected under the same employed conditions,14 the olefin oligomerization cannot be ignored for the reason that there is greater excess of olefins than sulfur compounds in real gasoline feed. As all we know, too much olefin oligomerization would lead to a decline in the yield of overhead gasoline in the distillation column. Besides, the olefin oligomers are liable to adhere on the active sites of catalyst and to block the catalyst pores, which can cause the deactivation of solid acid catalyst. Thus, to realize the large-scale © 2013 American Chemical Society

industrialization of FCC gasoline desulfurization by OATS technology, the selectivity of the solid acid catalyst must be improved. How to prolong the lifetime of the catalyst and to enhance the yield of overhead gasoline by improving the catalyst selectivity, without affecting its highly catalytic activity for thiophenic sulfur compound alkylation, is the first and foremost problem. As is well-known, the mechanisms involving in the reactions on the solid acid catalyst are closely relevant to the deactivation, selectivity, and activity of the catalysts. Hence, an improved understanding of how reactant molecules such as alkenes and thiophenic sulfurs interact with acid sites of the OATS catalysts is very beneficial to solve the problem of catalyst deactivation and gasoline yield decline. As noted by Supawadee Namuangruk, theoretical investigations can offer a practical means to elucidate the reaction mechanism at the molecular level,15 which can be used as the guidance to develop novel solid acid catalysts with superior catalytic performance, to optimize the reaction conditions for reducing the side reactions, and to prolong the lifetime of the catalyst. Benoit Dupuy et al.16 studied the alkylation of 3-methylthiophene with 2-methyl-1pentene over HY, HBEA, and HMCM-22 acidic zeolites experimentally and theoretically; however, the zeolite framework was not taken into account in the computation. Yonghong Li et al.14 reported the catalytic mechanism of alkyalation desulfurization over MCM-41 supported phosphoric acid (SPAM) catalyst Received: Revised: Accepted: Published: 6933

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compounds involved in our studies contain only C, H, O, S atoms, we anticipate that the effect of the basis set superposition error (BSSE) will be relatively small.22 NKC-9, prepared by sulfonation of polystyrene cross-linked with divinyl benzene, is a commonly used cation exchange resin catalyst. For simplicity and cost-effectiveness, this catalyst can be modeled by benzene sulfonic acid as it shares the same functional group as NKC-9 (see Figure 1). Moreover, this type

based on quantum chemical calculations; the SPAM catalyst was represented as a cluster containing the acidic active group -PO2H. Nevertheless, there is little literature available on understanding the mechanism of desulfurization over an acidic resin catalyst, which is a type of effective and important catalyst during alkylation desulfurization reactions. In the present work, a systematical theoretical study was conducted to gain insights into the reaction mechanism of alkylation desulfurization over acidic resin catalysts. The geometry optimizations and characterizations of the stationary points were carried out using hybrid density functional theory (DFT) for the reason that the DFT method was very useful for elucidating the mechanism of reactions involving solid acid catalysts,17,18 and afterward single-point calculations on these geometries were performed using the correlated MP2 method. On the basis of previous experimental results,19 3-methythiophene (3MT) and isobutylene (IB) was used to mimic the thiophenic sulfur compound and olefin, respectively. A macroporous acidic cation exchange resin, NKC-9 was selected as the alkylation catalyst because of its good catalytic performance for alkylation of thiophene sulfur compounds with olefins.20 Under the premise of explaining the catalytic mechanism for the main and side reactions in alkylation desulfurization concisely and effectively, the monoalkylation of 3MT with IB and the self-dimerization of IB were studied. An overall discussion of the proposed mechanism is summarized. The determination which is the prevailing mechanism has been concluded, and the results can be served as the basis for the design and development of solid acid catalyst in OATS process.

Figure 1. Optimized geometries of the model of NKC-9 for the theoretical calculation.

of model has recently been used for studying the alkylation of phenol with olefins catalyzed by styrene resin23 and has proved to be an accurate and practical model for exploring the structure, adsorption, and mechanisms of reactions taking place inside the styrene ion-exchange resin pores.24 Therefore, we used this model to study the alkylation and dimerization over NKC-9 catalyst. The sulfonic acid group in the benzene sulfonic acid is the active group. Although the aromatic ring in the resin has no catalytic activity, the aromatic ring is the electron-donating group and can affect the sulfonic acid group connected on it. The QST3 method in Gaussian software, which is based on the STQN (Combined Synchronous Transit and Quasi-Newton) method, is used for finding the transition states. For all stationary points, vibrational frequencies were calculated at the same level of optimizations to ensure that the correct number of imaginary frequencies was at hand, that is, one imaginary frequency for transition states and zero for energy minima. The normal vibrational modes corresponding to the imaginary frequencies of the transition states were visualized to confirm that they indeed corresponded to the expected motion of atoms. Meanwhile, zeropoint energies (ZPE) correction and thermodynamic parameters can be obtained in frequencies calculation. When the zero-point correction is included, all the adsorption and activation energy became slightly lower. Internal reaction coordinate (IRC) calculations, as implemented in Gaussian 09, were in some cases carried out. Such calculations follow reaction paths in both forward and reverse directions from a given transition state in order to investigate the minima connected by the transition state. In the following, when energies are discussed, we refer to the B3LYP/ 6-31G (d,p) + ZPE values, unless otherwise stated.

2. COMPUTATIONAL DETAILS The basic reactions are shown in formulas 1 and 2. The thiophenic sulfur alkylation and olefin dimerization were described as two different mechanisms: stepwise and concerted. The stepwise mechanism involves formation of a sulfonic ester complex in the first step and the reaction of the sulfonic ester with another thiophene/alkene molecule to form an alkylated thiophene or alkene dimer. The concerted mechanism involves simultaneous protonation and reaction of two reactant molecules to give the alkylation or dimerization product in one step. Details of these two mechanisms are presented herein with all stables, intermediates, and transition states computed.

All quantum chemical calculations were performed with the Gaussian 09 programs package.21 The B3LYP hybrid density functional combined with 6-31+G(d,p) basis sets were employed for all geometry optimizations of reactants, products, intermediates, and transition states (TS). The ultrafine integration grid was used to ensure the convergence of calculation. Additionally, single point electronic energies were calculated for the optimized geometries using the MP2/6-311G +(d,p) level to obtain more realistic values. Particularly, since all

3. RESULTS AND DISCUSSION 3.1. Stepwise Mechanism. In this pathway, one IB molecule is initially adsorbed on the active site of NKC-9 catalyst through a π bond. Then, the adsorbed IB is protonated and a sulfonic ester 6934

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Figure 2. (a) Calculated energy profile for the stepwise mechanism in IB protonation. (b) π-complex of 3MT on the NKC-9 catalyst. (c) Vibrational movement corresponding to the imaginary frequency at the transition structure in IB protonation.

adsorption are about 0.004 kJ/mol, so we consider it negligible and no further corrections are made. As shown in Figure 2a, all the ΔEads values are negative, which indicates that the lower the value is, the easier the adsorption will take place. It seems that 3MT may compete with IB to adsorb on NKC-9 catalyst at the initial stage of adsorption, as the calculated adsorption energy of the π- complex of IB was only a little lower than that of 3MT, 2 kJ/mol. Nevertheless, the intermediate product of 3MT with benzene sulfuric acid was unable to generate, and a physically meaningful transition state for the formation of alkylated 3MT could not be obtained in this case. Besides, the content of thiophenic compounds was very tiny compared with the content of alkenes in real FCC gasoline feed. It can be assumed that almost all of the acidic active sites on the catalyst surface were occupied by IB in the form of π-complexes at the initial stage of adsorption, then the physisorbed IB could be transformed into a stable sulfonic ester intermediate on the NKC-9 catalyst (σ- complex of IB on the NKC-9) by passing over the activation barrier. The energy barrier and apparent activation energy for this step are calculated to be 84.03 and 66.33 kJ/mol, respectively. As shown by the physisorption of IB on the NKC-9 catalyst in Figure 2a and the related geometric parameters listed in Supporting Information, Table S1, the optimized geometric parameters of the weakly adsorbed π complex are very similar to those of the isolated structures. The C1C2 bond length was slightly stretched from 1.340 to 1.346 Å, and the Brønsted acid bond O1−H1 is slightly lengthened from 0.973 to 0.988 Å, indicating that the adsorption slightly weakened the double bond and the acid bond, which may lead to the protonation of IB. The acidic proton H1 points to the center of the C1C2 bond of adsorbed IB with almost equal H1−C1 and H1−C2 distances, and was inclined to approach the C2 atom on the double bond, which was in line with the general chemical theory. By comparison, the physisorption had relatively little impact on the geometric structure of 3MT (Figure 2b), as the extensions of the bonds O1−H1 and C6−C7 on the π-complex of 3MT were shorter than those on the π-complex of IB, 0.004 and 0.002 Å, respectively. Moreover, the other geometric structures of 3MT were almost unchanged. This may be caused by the aromatic character of the thiophenic ring, which led

intermediate is formed. Subsequently, another 3MT or IB is adsorbed onto the intermediate and interacts with the isobutyl group to give the alkylated 3MT or IB dimer (eqs 3−6): C4 H8 + H−SO3−Ph → C4 H8 ··· H−SO3−Ph

(3)

C4 H8 ··· H−SO3−Ph → C4 H 9−SO3−Ph

(4)

C4 H 9−SO3−Ph + 3MT → C4H 9 − 3MT + H−SO3−Ph (5)

C4 H 9−SO3−Ph + IB → IB dimer + H−SO3−Ph

(6)

3.1.1. Adsorption on the Catalyst. Figure 2a shows the calculated energy profiles of the adsorption of the reactants and the transformation from the π- to the σ-complex of IB on the NKC-9 catalyst. The adsorption energy, ΔEads, for different adsorption status of reactants on the NKC-9 was calculated by eq 7, ΔEads = Eadsorbate·NKC − 9 − (E NKC − 9 + Eadsorbate)

(7)

where Eadsorbate·NKC‑9 represents the total energy of the reactant adsorbed on the NKC-9 catalyst. ENKC‑9 and Eadsorbate represent the energies of the separated NKC-9 and the adsorbate structure, respectively. It is noteworthy at this point that adsorption energies calculated as the difference between the total energy of the complex and the sum of the energies of the separated fragments are always an overestimation due to the basis set superposition error (BSSE).22 This error is associated with the fact that the basis sets commonly used to calculate the energies of both the complex and the separated fragments are far from being saturated, and therefore, in the complex, each fragment tends to use the basis functions of the other fragment to lower its energy. The CP method25 allows an estimation of the BSSE defined as BSSE = E(A)A + E(B)B − E(A)AB − E(B)AB

(8)

where E(X)XY is the total energy of fragment X at its equilibrium geometry in the complex that is computed with a basis set formed by adding all basis functions of the other fragment without its electrons and nuclei (ghost functions) to the basis set of fragment X. The values calculated with the CP method26 for the BSSE in IB 6935

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Figure 3. (a) Calculated energy profile for the stepwise mechanism of 3MT alkylation. (b) Vibrational movement corresponding to the imaginary frequency at the transition structure of 3MT alkylation.

Figure 4. (a) Calculated energy profile for the stepwise mechanism of IB protonation. (b) Vibrational movement corresponding to the imaginary frequency at the transition structure of IB protonation.

3MT to be physisorbed on the acid site of catalyst easily and was not beneficial to the thiophene protonation. The catalyst proton H1 tended to be close to the C6 atom on the α site of 3MT because of the electron-donating effect of the S atom. On the basis of the above analysis, the weakly adsorbed π complex can be further activated via protonation of the adsorbed IB molecule by the acidic proton on the catalyst. Because the carbenium ion of protonated IB is not stable, it can rapidly transform into a sulfonic ester intermediate (σ-complex of IB on the NKC-9, Figure 2a) by forming the covalent bond C1−O2 to one of the bridging oxygen atoms on the catalyst. The calculation showed that it needed to form a transition state (Figure 2a) during the transformation process from π- to σ- complex of IB on the NKC-9 catalyst. There was only one imaginary frequency at −357.5 cm−1 associated with the transition state (Figure 2c), which corresponds to the movements along the reaction coordinate where the acidic proton H1 on the catalyst was moving toward the C2 atom of IB, while the C1C2 bond was elongating from 1.346 to 1.417 Å and the other carbon atom C1 was moving toward the adjacent O2 atom on the NKC-9 to form a covalent bond C1−O2. At the transition state, the H1 proton is

located closer to the IB carbon atom (C2−H1 1.215 Å) than the resin oxygen atom (O1−H1 1.488 Å), and significant lengthening of the C1C2 bond from 1.346 to 1.417 Å indicates formation of the carbenium ion. The formation of the sulfonic ester complex was accompanied by structural changes in the active site of NKC-9 (Supporting Information, Table S1) for the fracture of acid bond O1−H1 and the formation of covalent bond C1−O2. The bond O1−S1 and dihedral angle O1−S−C5−C6 were decreased by 0.165 Å and 71.9°, respectively, while the bond O2−S and bond angle O1−S1−O2 were increased by 0.002 Å and 2.11°, respectively. 3.1.2. Reaction on the Catalyst. In the next step, another 3MT or IB molecule can react with the reactive sulfonic ester intermediate formed inside the catalyst pore. The calculated energy profile for the reaction between the sulfonic ester complex and 3MT molecular to produce alkylated 3MT and the optimized structures of the involved stationary points are displayed in Figure 3. Correspondingly, the major structural parameters of the geometric structures are tabulated in Supporting Information, Table S2. These revealed that further progress of alkylation reaction required the physisorption of another reactant 6936

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Figure 5. (a) Calculated energy profile for the converted mechanism of 3MT alkylation. (b) Vibrational movement corresponding to the imaginary frequency at the transition structure of 3MT alkylation.

imaginary frequency (−107 cm−1) shows that the reaction involves concerted bond breaking of C1−O2 and formation of the bond between C1 and C7. The transition state structure shows that the covalent bond of the sulfonic ester with the resin oxygen atom is broken with a large increase of the C1−O2 bond length from 1.499 to 3.926 Å, and C1 is located about midway between the resin oxygen atom O2 and C7 of IB, where a new bond is forming. The CC bond length of the other IB molecule is also slightly increased from 1.34 to 1.341 Å. The activation barrier for this step is calculated to be 160.5 kJ/mol. 3.2. Concerted Mechanism. Alternatively, the alkylation and dimerization mechanism can be considered to proceed in a concerted manner. In this mechanism, protonation and C−C bond formation between another 3MT or IB molecule and the π-bonded adsorbed isobutylene molecule occur simultaneously to give the alkylation and dimerization product without formation of the sulfonic ester intermediate [eqs 3, 9, and 10]:

(3MT) to the sulfonic ester group of IB on NKC-9 catalyst to form a larger complex, and the adsorption reduced the total energy of the system about 5.8 kJ/mol. During the adsorption, 3MT was diffused into the vicinity of the sulfonic ester group and formed a weak interaction with it. The covalent bond O2−C1 increased by 0.002 Å, while the bond O2−S1 on the active site of the NKC-9 catalyst decreased by 0.003 Å. After the reactant 3MT was adsorbed on the sulfonic ester complex, the alkylation between the adsorbed 3MT and sulfonic ester group started by adsorbing enough energy. The 3MT alkylation involved the forming of a covalent bond C1−C7 between the carbon atoms of IB and 3MT, and the proton giving back to the adjacent oxygen atom O1 on the catalyst by breaking the proton H2 on the electron-rich site of 3MT (α site). The whole process was clearly displayed by the vibrational motion corresponding to the single imaginary frequency (−152 cm−1) at the transition state of 3MT alkylation in Figure 3. During the transformation, the covalent bond C1−O2 of the sulfonic ester complex was broken by stretching to form a carbonium ion-like transition state that was not stable and continually attacked the adsorbed 3MT until the bond C1−C7 between isobutyl carbocation and 3MT began to form and the proton H2 on 3MT left toward the active site of NKC-9 catalyst. The calculated activation energy (Eact) for the process was 148.4 kJ/mol. Subsequently, the alkylated 3MT was desorbed from the NKC-9 catalyst endothermically, which required the desorption energy (ΔEdes) of at least 13.9 kJ/mol. The energy profile of the reaction between the sulfonic ester intermediate and another IB molecule to form the IB dimer is shown in Figure 4, and geometric parameters are listed in Supporting Information, Table S3. The other IB molecule is adsorbed on the sulfonic ester intermediate and forms a complex. This complex is more stable than the sulfonic ester intermediate and has an adsorption energy of −2.48 kJ/mol. The coadsorption of another IB molecule to some degree weakens the covalent sulfonic ester bonds, as seen from the lengthening of the C1−O2 bond from 1.498 to 1.499 Å. Then, the reaction proceeds by the breaking of the covalent bond between the sulfonic ester species and the catalyst sites and formation of a new C−C bond to the other IB molecule. The transition state for this reaction step has been identified, and the vibrational motion (Figure 4b) associated with the

C4 H8 ··· H−SO3−Ph + 3MT → C4 H 9−3MT + H−SO3−Ph

(9)

C4 H8 ··· H−SO3−Ph + IB → IB dimer + H−SO3−Ph (10)

The energy profile of the 3MT alkylation reaction is shown in Figure 5a, and selected geometrical parameters are listed in Supporting Information, Table S4. The initial step starts with the adsorption of an IB molecule to give the weak π-adsorption complex. Then, another 3MT molecule is weakly coadsorbed onto the π complex by interaction of a carbon atom with the oxygen atoms of the resin catalyst. The C7−O2 and C7−O1 distances are calculated to be 4.09 Å and 3.96 Å, respectively. These weak interactions result in a small binding energy of −7.22 kJ/mol, which is lower than the binding energy of the π complex of −17.71 kJ/mol. The next step is the concerted protonation of the π-adsorbed IB molecule by the resin proton and simultaneous formation of a C−C bond between the 3MT and IB molecules. At the transition state, the acidic proton of the resin catalyst has partially protonated the carbon atom of the IB molecule, as indicated by the shorter distance of H1 to the IB carbon atom than to O1 of the resin, while the C−C bond between the 3MT and IB molecules is forming. The C1C2 and C7C8 bond lengths are increasing and the new 6937

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dimerization also started from the adsorption of an IB molecule to give the weak π-adsorption complex, then, another reactant molecule IB was physically adsorbed on the π complex by interaction of a carbon atom with another oxygen atom of the resin catalyst (Figure 6a). An isobutyl carbocation was also found in the transition state for the self-dimerization of IB (Figure 6a). The vibrational motion corresponding to an imaginary frequency (−116 cm−1) described the formation of a new covalent bond C2−C7 between the carbocation and the least substituted carbon atom C7 on the double bond of IB. The calculated activation energy (Eact) for the process was 174 kJ/mol. Subsequently, the IB dimer was desorbed from the NKC-9 catalyst endothermically, which required the desorption energy (ΔE) of at least 15 kJ/mol. For convenient comparison, the complete energetic profiles of both possible mechanisms for 3MT alkylation and IB dimerization are shown on the same diagram (see Figure 7 and 8). In the 3MT alkylation reactions, as shown in Figure 7, the protonation of the adsorbed IB has an activation barrier of 84.03 kJ/mol, and the alkylation of the coabsorbed 3MT with the sulfonic ester intermediate is a rate-determining step with an activation barrier of 148.41 kJ/mol in the stepwise mechanism. While in the concerted mechanism, the activation barrier is calculated to be 169.10 kJ/mol, which is significantly higher than the energy barrier of the rate-determining step of the stepwise mechanism. Furthermore, for the dimerization of IB as shown in Figure 8, the activation barrier for the dimerization step in the stepwise mechanism is 160.52 kJ/mol, which is the rate-determining step. As similar to the 3MT alkylation reaction, the activation barrier of IB dimerization in the concerted mechanism is 174.02 kJ/mol, which is higher than the energy barrier of the rate-determining step in the stepwise mechanism. For both 3MT alkylation and IB dimerization, the relative energy of the transition state in the concerted mechanism is higher than that of the transition states in the stepwise mechanism. Thus, from the energetics of the reactions and relative stability of transition states, it can be concluded that the stepwise mechanism should dominate the overall reaction of the alkylation of 3MT and dimerization of IB, which is in agreement with the experimental results of our previous work. From the above study, it can be found that the reaction paths of the 3MT alkylation and IB oligomerizaiton are similar. Therefore, the side reaction, olefin oligomerization was difficult to avoid during alkylation desulfurization. However, there are still some obvious differences between thiophenic compounds alkylation and alkene oligomerization. The energy change after the adsorption of 3MT on the sulfonic ester complex was lower than that of IB, indicating that 3MT was more inclined to be

bonds between C2 and C7 are forming, leading to the formation of the alkylated 3MT. The frequency analysis (Figure 5b) shows vibrational movement of atoms at the transition state which corresponds well with the concerted mechanism as described above, and the imaginary frequency is −157 cm−1. The partially protonated transition-state structure was also reported by Svelle et al.27 for concerted dimerization of ethylene, but in their report the distance between the second ethylene molecule and the catalyst active site appears to be farther than the distance between the 3MT molecule and the catalyst active site in our study. The activation energy is evaluated to be 169.10 kJ/mol, which agrees well with the value of 170 kJ/mol reported by Yonghong Li14 at comparable level of theory [MP2/ 6-311G(d,p)//B3LYP/6-31G(d)]. It has been pointed out that for acid-catalyzed reaction the activation energy of the reaction step involving protonation depends on the degree of proton transfer at the transition state.28,29 Generally, longer O1−H1 and shorter H1−C1 distances indicate more advanced proton transfer and higher activation energy. In this mechanism, the transition state has a significant degree of proton transfer, and thus the activation energy is mainly due to removal of the acidic proton from the resin catalyst. Therefore, it is not unexpected that the activation energies obtained from the quantum calculations are comparable.

Figure 6. (a) Calculated energy profile for the converted mechanism of IB dimerization. (b) Vibrational movement corresponding to the imaginary frequency at the transition structure of IB dimerization.

The energy profile of the IB dimerization is shown in Figure 6a combined with the corresponding structures, revealing that the reaction path for the self-dimerization of IB was very similar to that for the 3MT alkylation with IB. The selected geometrical parameters are listed in Supporting Information, Table S5. The

Figure 7. Calculated energy profiles for the stepwise (bold ) and concerted (---) reaction mechanisms of 3MT alkylation. 6938

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Figure 8. Calculated energy profiles for the stepwise (bold ) and concerted (---) reaction mechanisms of IB dimerization.

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adsorbed on the intermediate than IB during the adsorption of another reactant for the further reaction. Besides, the activation energy for the alkylation of 3MT was obviously less than that for the IB dimerization, which demonstrated that the alkylation rate of thiophenic compounds with olefins was faster than the self-dimerization rate of alkenes. In a word, the adsorption rates of another reactant and the different reaction rates led us to determine that the alkylation of thiophenic sulfurs happened more easily, as compared to the dimerization of alkenes. This theoretical result conformed to the experimental outcome in our previous work19 in the trend as the reaction rate of thiophenic sulfur compounds at the initial stage of reaction was obviously faster than those of alkenes under each investigated reaction temperature. The differences between the experimental outcomes in our previous work with the present results can be explained as followed. The reaction condition for quantum chemical calculation in this paper is in a vacuum and gas phase. While the kinetics experiments were performed in high-pressure and liquid phase. The intermolecular force and electrostatic force affect the distance between the molecules and this distance is short, so experimental values of the activation energies are lower than that of theoretical values.

ASSOCIATED CONTENT

S Supporting Information *

Tables S1, S2, S3, S4, and S5 as mentioned in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-29-82663189. Fax: +86-29-82668789. E-mail: chyi@ mail.xjtu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work from the National Basic Research Program of China (973 Program, No. 2009CB219906), National Natural Science Foundation of China (No. 21276203), and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20110201130002) are gratefully acknowledged.

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4. CONCLUSIONS The main and side reactions in alkylation desulfurization over NKC-9 catalyst, 3MT alkylation, and IB dimerization, were investigated using the DFT method of quantum chemistry. Two mechanisms, stepwise and concerted have been evaluated. The activation barrier of the rate-determining step in the stepwise mechanism for 3MT alkylation and IB dimerization is 148.41 and 160.52 kJ/mol, respectively, which is far lower than that in the concerted mechanism. The DFT calculation results indicate that the stepwise mechanism would dominate the 3MT alkylation and IB dimerization. Besides, the adsorption energy of 3MT and IB in the reaction course revealed that 3MT was more inclined to be adsorbed on the sulfonic ester intermediate and the activition energy for alkylation of 3MT with IB was obviously lower than that for IB dimerization. It can be concluded that 3MT alkylation more easily occurred than IB dimerization. In addition, these mechanisms of alkylation desulfurization can be used as theoretical guidance to develop a novel solid acid catalyst with superior catalytic performance and to optimize the reaction conditions for reducing side reactions, and can serve as the basis for the industrialization of this desulfurization process. 6939

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