Theoretical Study on the Conversion Mechanism of Biobased 2,5

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11111−11120

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Theoretical Study on the Conversion Mechanism of Biobased 2,5Dimethylfuran and Acrylic Acid into Aromatics Catalyzed by Brønsted Acid Ionic Liquids Zhaoyang Ju,†,‡ Xiaoqian Yao,*,‡ Xiaomin Liu,‡ Lingli Ni,‡ Jiayu Xin,‡ and Weihua Xiao*,†

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†

Biomass and Bioresource Utilization Laboratory, College of Engineering, China Agricultural University, Beijing 100083, People’s Republic of China ‡ CAS Key Laboratory of Green Process and Engineering, Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: The use of 2,5-dimethylfuran (DMF) and acrylic acid (AA) as the reactants to produce aromatics catalyzed by Brønsted acid ionic liquids (ILs) has been achieved successfully under mild conditions. The whole conversion process including Diels−Alder (D−A) cycloaddition, ring-opening/decarboxylation, and dehydration was investigated by density functional theory (DFT) calculations which were also verified by experiments. Two pathways for the activation of DMF were proposed: unprotonated oxygen of the DMF (UPOD) and direct protonated oxygen of the DMF (POD). In the UPOD pathways, two routes (endo and exo), in which the overall rate was limited by the ring-opening step, produce p-xylene or 2,5-dimethylbenzoic acid (DMBA), respectively, whereas in POD pathways, the limiting step of DMBA production was attributed to the D−A cycloaddition. The role of Brønsted acid ILs was mainly reflected in the proton transferability. The present study provides basic aids to understand the mechanism of converting furanics and AA into aromatics catalyzed by Brønsted acid ILs. be obtained from dehydration of biomass-derived lactic acid.20 There are more and more studies on the production of aromatic compounds from DMF and AA.21,22 For the synthesis of aromatics, various catalysts have been developed such as metal-based catalysts23 and organocatalysts.24 New challenges such as high temperature and multiple catalytic steps still need to be addressed. As a new kind of highly efficient green media, ionic liquids (ILs) have gained rapid development in recent years.25−27 Furthermore, ILs can also show a great efficiency on various D−A reactions with high selectivity and fast reaction rate.28−30 Recently, Ni et al. found that Brønsted acid ILs, such as 1-butyl-3methylimidazolium hydrogen sulfate ([Bmim]HSO4), can show a high activity to directly synthesize PX and 2,5dimethylbenzoic acid (DMBA) from biobased DMF and AA under mild conditions.31 The main reaction mechanism of proceeding via D−A reaction and subsequent conversion of the cycloadduct to aromatics is shown in Scheme 1. When the literature on reaction mechanisms is summed up, most

1. INTRODUCTION Utilization of renewable resources has become an attractive choice for sustainable production of valuable chemicals.1,2 In recent decades, there are many types of research projects on biomass conversion to fuels and platform chemicals.3−5 The major biorenewable chemicals of interest are aromatics [particularly p-xylene (PX)], which can serve as the starting material to produce poly(ethylene terephthalate) (PET).6−8 PET can be used to make various kinds of containers related to our daily life, such as drinking bottles, barrels of edible oil, and so on.9−12 2,5-Dimethylfuran (DMF), which has a conjugated diene structure, is the mainly sugar-derived chemical feedstock from lignocellulosic material.13,14 It provides a promising strategy from the biomass-derived furanics and dienophiles via Diels−Alder (D−A) reaction to produce aromatic skeleton structures. The previous production of PX is from biomassderived DMF and the alkenes (usually ethylene or propylene).15−17 However, there are many disadvantages to employing alkenes as dienophiles. The alkenes are usually isolated from fossil sources, and this batch synthesis requires high pressure to store the gas, which makes this process less safe.18,19 These days, the use of other dienophiles which are easier to handle has been explored to synthesize aromatics. Acrylic acid (AA), which is a liquid at room temperature, can © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11111

March 22, 2019 May 16, 2019 June 4, 2019 June 4, 2019 DOI: 10.1021/acs.iecr.9b01585 Ind. Eng. Chem. Res. 2019, 58, 11111−11120

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Industrial & Engineering Chemistry Research Scheme 1. Conversion of DMF and AA to PX and DMBAa

a dispersion correlation.40 The same level of theory was used in the mechanism of DMF and ethylene studies and had gotten reasonable results.32,34 The vibrational frequencies calculated with the same level were also used to verify whether the structures were stationary points (no imaginary frequency) or TSs (only one imaginary frequency). To check the relative saddle point links connecting each TS to reactant and product of the proposed mechanism, the intrinsic reaction coordinate (IRC) pathways41 had been traced using the second-order González−Schlegel integration method.42 Furthermore, to search for the TS structures which were not easy to find, the potential energy surface (PES) scans43 were also performed in this study. All reported Gibbs free energies which were corrected by zero point had been calculated in experimental temperature (T = 298.15 K and P = 100 kPa). The Gibbs free energy (G) of the system can be defined as follows:

a

Refs 21 and 31.

research focused on the reaction of ethylene and furan compounds to PX, while few studies about AA and DMF were reported. In 2012, Williams et al. proposed a renewable strategy to produce PX from DMF and ethylene catalyzed by H−Y zeolite catalysts.18 They thought the dehydration of D−A cycloadduct can be effectively catalyzed in the presence of Brønsted acid, and the limiting step was ultimately attributed to initial cycloaddition. In addition, Nikbin et al. presented the energetics of DMF and ethylene in the condition of uncatalyzed and Brønsted/Lewis acid catalyzed reactions.32 They concluded that ethylene and DMF can directly undergo the D−A cycloaddition without Brønsted acid catalyst. Another reaction mechanism had been proposed for the production of a bicyclic adduct from DMF and ethylene in which the oxygen of DMF was protonated in the presence of Brønsted acids.33,34 The ring opening of the cycloadduct can be effectively catalyzed by Brønsted acids according to the density functional theory (DFT) calculations. Most studies about the reaction mechanism are based on ethylene as a reactant. However, because of the asymmetry of AA, it will render a big difference in the reaction pathways. In this work, the detailed mechanism on the reaction of DMF and AA into aromatics catalyzed by Brønsted acid ILs has been probed by DFT calculations. Besides, we also investigated the role of Brønsted acid ILs and the effect of different catalysts in the reaction process. Two kinds of mechanisms were presented for converting DMF and AA to PX and DMBA with a six-membered ring structure: (1) unprotonated oxygen of the DMF (UPOD) and (2) protonated oxygen of the DMF (POD). Because of the asymmetric structure of AA, there were two different D−A reaction pathways (named endo and exo) for DMF and AA. Two routes were probed in UPOD and POD pathways, respectively. In addition, gas chromatography−mass spectrometry (GC−MS) experiments were adduced to verify our theoretical predictions about pathways in Brønsted acid ILs catalytic systems. The content of this study is as follows: In section 2, computational methods employed here are briefly explained. The DFT results for the whole reaction mechanism and the pathways catalyzed by different catalysts are presented in section 3, and the results for the D−A reaction, ring-opening, and dehydration steps are discussed there in detail. Section 4 concludes.

(1)

G = H − TS

where the H, T, and S are the enthalpy, temperature, and the entropy of the system, respectively. The active energy (Ga), which is the energy barrier, and reaction energy (Gr) of the systems are defined as follows: Ga = GTS − G R ,

Gr = Gp − G R

(2)

where GTS, GR, GP represent the Gibbs free energies of transition state, reactant, and product.

3. RESULTS AND DISCUSSION 3.1. Process of UPOD. According to the studies about the reaction of ethylene as a reactant, it is crucial to efficiently convert DMF and ethylene to PX and make more valuable chemicals from biomass-derived sugars.18,44 On the basis of previous experimental studies on the conversion of furan and AA into aromatics catalyzed by Brønsted acid ILs,21,31 it is generally recognized that the detailed process to produce PX or DMBA can be described as a sequence of (1) D−A cycloaddition, (2) ring-opening/decarboxylation, and (3) dehydration events. Because of the asymmetry of AA, we name two kinds of D−A reaction pathways (Figure S1) between the ring of DMF and CC of AA route endo and route exo where the carboxyl of AA and the oxygen of DMF are in the same and different sides, respectively. This will lead to a big difference in the subsequent reaction process. 3.1.1. Diels−Alder Cycloaddition. The D−A reaction is the most useful organic reaction to build regio- and/or stereoselectivity with six-membered cyclic structures in the organic synthesis area.45,46 In order to assist the DFT calculations to explain the conversion mechanism, various compounds are detected by the GC−MS experiments which are depicted in Figures S2−S4. The fragment of the m/z 169 peak, which corresponds to a molecular formula of C8H12O3, is the evidence of the intermediate cycloadduct. However, the C8H12O3 compounds are unstable based on the experiment. Due to the asymmetric structure of AA, the optimized geometries of reactant, transition state, and product for the D−Aendo and D−Aexo reaction are depicted in Figure 1. In the D−Aendo TS structure, the bond lengths of C1−C1′ and C2′− C2 are 1.974 and 2.332 Å, respectively. It means that the C−C bond length which is around the carboxyl is much longer than that of the other side in the TS structure. Additionally, this reaction is needed to overcome a 24.0 kcal/mol energy barrier and is exothermic by 0.3 kcal/mol. In the D−Aexo reaction, the reversed AA is added in DMF with a 23.4 kcal/mol energy

2. COMPUTATIONAL METHOD All the DFT calculations were performed with the Gaussian 16 program,35 and the figures were generated by using CYL view.36 The molecular geometries of reactants, transition states (TSs), and products were optimized using the M06-2X-D3 hybrid DFT with the 6-311+G** basis set.37,38 Recently, Zahn et al. had reported that it could give reliable results for ILs by using the Minnesota family of the M0X type density function.39 In the M06-2X-D3 method, the D3 term represents 11112

DOI: 10.1021/acs.iecr.9b01585 Ind. Eng. Chem. Res. 2019, 58, 11111−11120

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Industrial & Engineering Chemistry Research

structures and interaction energies are listed in Figure S8 and Table S1. The interaction between anion and cation (ΔECA) is about 80−90 kcal/mol, which is higher than that of the P1/P1′ cation (about 10−20 kcal/mol). Furthermore, from the structures of P1/P1′-IL systems (Figure S8), the cation mainly form H-bonds with the anion due to the strong electrostatic interaction. It indicates that the cation mainly forms H-bonds with the anion and the anion will mainly affect the reaction.51,52 Because the H of C2′ can be protected in the D−Aendo reaction, two kinds of pathways (route endo and route exo) in the ring-opening step are presented in Scheme 2. In the route Scheme 2. Two Kinds of Pathways (Route Endo and Route Exo) in the Ring-Opening Step Figure 1. Potential Gibbs free energy (kcal/mol) profiles with the structures of reactants, transition states, and products for the D−Aendo and D−Aexo reaction pathways (bond distances in angstroms).

barrier. It indicates that the D−Aexo is easier to occur than the D−Aendo. On the other hand, the bond lengths of C1−C1′ and C2′−C2 are 1.989 and 2.264 Å in the D−Aexo TS structure. To analyze the correlation between bond distances and energy values in the D−A reaction process, the bond distances (C1− C1′, C2−C2′) and relative energy are shown in Figure S5. In the D−Aendo reaction, the bond length of C2−C2′ which is neighboring the carboxyl is longer than that of C1−C1′. Initially, the bond length of C1−C1′ is a bit longer than that of C2−C2′ in the D−Aexo reaction. However, as the reaction proceeds, the bond length of C2−C2′ is longer than that of C1−C1′. Besides, the quantitative molecular surface analysis of electronic potential (ESP)47 of P1/P1′ has been calculated and is shown in Figure S6; the difference in the bond length of C1−C1′ and C2′−C2 mainly accounts for the physicochemical property of the substituent (carboxyl) group. Meanwhile, the Mayer bond order analysis48 has also been made which is listed in Figure S7. It can be concluded that the C−C bond length of which neighboring functional group is longer than the other side. The calculated energy barrier of the D−A reaction agrees with the experimental results (17.8 kcal/ mol),31 which indicates the M06-2X-D3/6-311+G** method is acceptable and meets the requirements of our calculations regardless of the errors between DFT calculations and experiment. Furthermore, as we can see from the P1 in Figure 1, the most interesting thing is that the hydrogen of C2′ plotted by the black circle can be hidden, and it will make a big difference in the subsequent ring-opening step. 3.1.2. Ring-Opening Protonation and Decarboxylation. Recently, the decarboxylation reaction which was the source of CO2 from organics has been a hot topic. Some studies reported the decarboxylation of 2,4-dimethoxybenzoic acid which has a relatively high energy barrier and should not favor the formation of CO2 in the acid condition.49 The water can be added in the carboxyl to form carbonic acid occurring together with the C−C bond cleavage supplemented with DFT calculations. However, the reaction barrier of decarboxylation reactions is relatively high no matter what to form CO2 or carbonic acid in the acid condition.50 As the D−A reaction takes place, there are two different spatial isomerizations (P1 and P1′). To search for the stable combination, 16 different kinds of geometries had been calculated by placing the anions and cations at different positions of P1 and P1′, and the

endo to PX, there are also two kinds of ring-opening protonation modes, which are decarboxylation through the loss of H2CO3 (RA−PB) and the loss of CO2 (R2−P2) (Scheme 3). As shown in Figure 2, one water molecule can be added to the carboxyl group in the aqueous phase with 15.4 kcal/mol energy barrier in the loss of H2CO3 (RA−PB). The second step is to produce carbonic acid and PX via the C−C bond cleavage of the carboxyl and ring opening catalyzed by HSO4− with 48.1 kcal/mol energy barrier. The ring-opening step through losing H2CO3 is exothermic by about 23.3 kcal/ mol. However, in the other way of the ring-opening step (R2− P2) catalyzed by HSO4−, the ring-opening protonation can directly occur along with the carbon−carbon bond cleavage and release CO2. Similarly, the experimental phenomenon of bubble generation during the reaction, CO2 can also be observed in the experiment detected by our group31 and GC− MS data (Figure S4). The calculated energy barrier is 41.9 kcal/mol. The energy barrier of producing H2CO3 is about 6 kcal/mol higher than that of forming protonated CO2. The ring-opening protonation and decarboxylation catalyzed by [Bmim]HSO4 in the endo pathway has also been calculated and is listed in Figure S9. The energy barrier catalyzed by [Bmim]HSO4 is 41.6 kcal/mol, which is a little bit lower than that of HSO4− (41.9 kcal/mol). From the TS2″ structure, it can be found that [Bmim]+ mainly interacts with the HSO4− and will not substantially change the reaction pathways. Likewise, the TS structure of the ring-opening step in the route exo to DMBA is also probed by DFT calculations. Because of the asymmetry of AA, the carboxyl group of the cycloadduct is retained in route exo. To find the TS structure for the ring-opening step, the three-dimensional PES scan has been carried out for the reaction which is depicted in Figure S10. On the basis of the highest energy structure, the TSs can be found, and geometries of reactants, TSs, and final products for the ring-opening protonation (R2′−P3′) in the exo pathway are shown in Figure 3. First, H transfers to the 11113


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Scheme 3. Two Kinds of Pathways for the Ring-Opening Protonation and Decarboxylation through the Loss of H2CO3 (RA− PB) and CO2 (R2−P2) in the Endo Pathway

Some studies reported that the proton-transfer ability and acidity of the various ILs are the main reason for the role of ILs in the experiment of DMF and AA to aromatics.31,53 Two kinds of HSO4− and H2PO4− anions based ILs had been made in this study. The selection criteria of ILs is mainly to compare with the experiment data.31 To investigate the proton-transfer ability in the ring-opening protonation step, we compared with the TS barrier catalyzed by different catalysts. The corresponding Gibbs free energies of endo and exo catalyzed by HSO4−, H2O, and H2PO4− are summarized in Table S2. In the route endo, the barriers of Gibbs free energies catalyzed by HSO4−, H2PO4−, and H2O are 41.9, 43.8, and 66.8 kcal/mol, respectively. It means that the order of different catalysts in the proton-transfer ability follows HSO4− > H2PO4− > H2O. In the route exo, the energy barriers catalyzed by HSO4−, H2PO4−, and H2O are 35.5, 19.1, and 50.3 kcal/mol, respectively. It can be concluded that both HSO4− and H2PO4− show a catalytic effect in the ring-opening step. These theoretical results are consistent with those obtained by the experiments.31 From Table S2, taking the free energies catalyzed by HSO4− for example, the barriers of endo and exo are 41.8 and 35.5 kcal/ mol, respectively. Comparing the barrier of the ring-opening step, the energy barrier of endo is much higher than that of exo. In addition, the water contained in the system is less effective as a proton shuttle to promote the reaction. 3.1.3. Dehydration. Substantially, the transformation of DMF and AA to aromatics needs the dehydration process.22 The geometries of dehydration in route endo and route exo are shown in Figure 4 (R3−P3) and (R4′−P4′), respectively. In the route endo, the ring-opening step is accompanied by decarboxylation and generates CO2. Because the carboxyl group cannot be separated in the route exo, the dehydration step is different in route endo than route exo. Meanwhile, the product of ring opening in route exo, which is also the precursor of PX, has a parent ion of 124 m/z in the GC−MS experiment (Figure S3). It is also consistent with the reaction of DMF and ethylene that also detected the precursor of PX by other groups.15,54,55 In the route endo, almost 30.1 kcal/mol is needed to overcome the energy barrier catalyzed by HSO4− to produce PX. This reaction is exothermic by 29.8 kcal/mol. The barrier of dehydration in route endo to PX is higher, about 3.0 kcal/mol, than route exo. As shown in Table S3, in the route endo, the free energy of dehydration without a catalyst is 48.9 kcal/mol which shows a relatively high energy barrier. The free energy barriers catalyzed by H2O, H2PO4−, and HSO4− are 42.1, 26.2, and 30.1 kcal/mol, respectively. It means the catalysts can show a catalytic effect on the dehydration functioning as a proton shuttle to promote the reaction. In the route exo, the free energy barriers catalyzed by H2O, H2PO4−, and HSO4− are 41.4, 29.8, and 27.1 kcal/mol, respectively.

Figure 2. Potential Gibbs free energy (kcal/mol) profiles with the structures of reactants, transition states, and products for the ringopening protonation and decarboxylation through the loss of H2CO3 (RA−PB) and the loss of CO2 (R2−P2) in the endo pathway (bond distances in angstroms).

Figure 3. Potential Gibbs free energy (kcal/mol) profiles with the structures of reactants, transition states, and products for the ringopening protonation (R2′−P3′) in the exo pathway (bond distances in angstroms).

oxygen of HSO4− with a large energy barrier (34.2 kcal/mol), and the reaction is endothermic by about 33.3 kcal/mol. Second, the ring of P2′ will be open with 1.9 kcal/mol when the H transfers to the oxygen of cycloadduct, and this second step is exothermic by about 43.2 kcal/mol. That means it is difficult to transfer the proton in the first step. Besides, the second step will immediately occur followed by the first proton transfer. Most interestingly, the carboxyl group cannot be removed in the ring-opening step of route exo. 11114


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(30.2 kcal/mol) to produce PX in the dehydration step. When the energy barrier of each step in route endo and route exo is compared, it can be concluded that exo is lower than endo, and the detailed reaction pathways are presented in Scheme 4 based on the lowest reaction pathway by the DFT calculations. The selectivity of PX and DMBA is related to the temperature and the amount of catalysts.31 The yield of PX decreased significantly with increasing temperature, and the pathways to DMBA were dominant at elevated temperature. In the higher amount catalysts, the yield of DMBA is higher than that of PX. Considering the complexity in actual experiments, the product distribution can be influenced by many factors such as temperature, pressure, and the amount of catalysts. The calculations provide the potential pathways to produce PX and DMBA in the mechanism. The productions from different π−π forms of DMF and AA are different. It can be concluded that one is through endo to produce PX and the other is through exo to produce DMBA. It is the reason why there are mainly two kinds of products. 3.2. Process of POD. It was reported that D−A cycloaddition can be influenced by the protonation the oxygen of DMF in the Brønsted acids condition.33 In addition, Salavati-Fard et al. also reported that the reaction mechanism for the D−A cycloadditions of DMF and maleic anhydride can be changed in Brønsted acid which modeled as a proton.56 In another reaction pathway from DMF to aromatics, the formation of protonated water molecule plays a key role in the reaction mechanism. The optimized structure of DMF and H3O+ can be found in Figure S11. The lengths of H···ODMF and H···OH2O are 1.08 and 1.34 Å, respectively. First, the H3O+ forms a hydrogen bond (H-bond) with the ring oxygen of DMF. The O−H···O H-bond is formed if the distance between O···H is less than 2.72 Å,57 which is the sum of the van der Waals radii. Besides, the binding energy is also an important criterion to evaluate different theoretical approaches for the description of H-bonded complexes. The binding energy between DMF and H3O+ has also been calculated using the M06-2X-D3/6-311+G** method and is 39.7 kcal/mol. From the above results, it can be concluded that the oxygen of DMF is protonated in Brønsted acid condition. 3.2.1. Diels−Alder Cycloaddition. For the Brønsted acid catalyzed reaction, the oxygen of DMF is protonated by H3O+ which is named H-D−A. In a quest to better understand the difference between H-D−A and D−A, the computed relative energies of H-D−A and D−A are listed in Table S4. In the D− A reaction, the energy barrier of H-D−Aendo is 38.0 kcal/mol, which is 11.2 kcal/mol higher than that of H-D−Aexo. It means that H-D−Aexo is much easier to occur than H-D−Aendo. Both the H-D−Aexo and H-D−Aendo reactions are exothermic by about 10−20 kcal/mol. Furthermore, to compare the energy changes when the oxygen of DMF is protonated, the energy barriers of H-D−Aendo and D−Aendo are 38.0 and 24.0 kcal/ mol, respectively. The energy barriers of H-D−A (endo and exo) are higher than those of D−A (endo and exo). It can be concluded that the energy barrier of D−A reaction will increase when the oxygen of DMF is protonated, and the energy barrier of route endo is also higher than that of route exo. 3.2.2. Ring-Opening Protonation. After the D−A reaction in which DMF is protonated by H3O+, the next step is the ring opening. The geometries of route endo and exo in POD pathways can be seen in Figure 6 (Rb−Pc) and (Rb′−Pc′), respectively. The protonated cycloadducts have been formed

Figure 4. Potential Gibbs free energy (kcal/mol) profiles with the structures of reactants, transition states, and products for the dehydration to PX and DMBA catalyzed by HSO4− in route endo (R3−P3) and route exo (R4′−P4′), respectively (bond distances in angstroms).

Thus, the catalyzed effects order for the dehydration step is HSO4− > H2PO4− > H2O. These results can explain that the HSO4− based ILs possess a superior ability to convert furanics and AA into aromatics. 3.1.4. Whole Process of the UPOD Pathways. On the basis of the above calculations, the reaction routes and associated energy surface for the conversion of DMF/AA to PX and DMBA are shown in Figure 5, marked as solid and dotted

Figure 5. Reaction routes (endo and exo) and potential Gibbs free energy surface for the formation of PX and DMBA from DMF and AA catalyzed by [Bmim]HSO4.

lines, respectively. In the reaction, there are two main products which are PX (route endo) and DMBA (route exo). DMF and AA will undergo D−A cycloaddition and produce the intermediateP1/P1′ which is derived from the asymmetry of AA at the beginning of the reaction. Comparing the energy barrier of the D−A reaction, the endo form (24.0 kcal/mol) is higher than the exo form (23.4 kcal/mol). In the ring-opening step, the energy barriers of endo and exo are 41.9 and 35.4 kcal/mol, respectively. The ring-opening step shows the highest energy barrier, which appears to determine the overall reaction rate. In route exo, the most interesting thing is that the carboxyl group can be removed during the ring-opening step. Moreover, the energy barrier of the exo form (27.1 kcal/mol) to produce DMBA is also lower than that of the endo form 11115


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Industrial & Engineering Chemistry Research Scheme 4. Overall UPOD Reaction Pathway for the Conversion of DMF and AA into PX and DMBA

Figure 6. Potential Gibbs free energy (kcal/mol) profiles with the structures of reactants, transition states, and products for the ringopening step in the POD pathway (bond distances in angstroms). Figure 7. Reaction routes and potential Gibbs free energy surface for the ring-opening protonation in the POD and UPOD pathways.

when the oxygen of DMF is protonated through D−A reaction. In route endo, a transition state (TSb) will be formed by about 7.0 kcal/mol energy barrier along with the C−O bond being cleaved. The ring-opening step is an exothermic reaction by about 5.5 kcal/mol. Then, the second step is that C−H dissociation by mediating the hydrogen and transferring to water by forming H3O+ with about 10.2 kcal/mol energy barrier. This process is also exothermic by about 11.1 kcal/ mol. Similarly, the parallel process will happen in route exo. The value of the energy barrier is approximately 2.8 kcal/mol lower than that of the endo route. Furthermore, the Gibbs free energy surfaces of the ring-opening step in the POD and UPOD pathways are plotted in Figure 7. Comparing the energy barrier of route endo and route exo in POD pathways, the energy barrier of route endo is higher than that of route exo in the ring-opening step. When the oxygen of DMF is protonated, the energy of ring opening will be much lower compared with the UPOD pathways. Thus, the ring opening is no longer the rate-determining step in POD pathways.

3.2.3. Dehydration. Recently, the hydroxyl protonated by the Brønsted acid catalysts for the glucose/fructose dehydration has been investigated by DFT calculations.58,59 Through the ring-opening protonation pathway, the production is the same in route endo and route exo. Similarly, Brønsted acid catalyzed dehydration is initiated by the protonation of the hydroxyl group, and the geometries of dehydration (Rd−Pe) are shown in Figure 8. For Rd−Pd, the energy barrier for this hydration reaction is 1.1 kcal/mol and it is also exothermic by 6.5 kcal/mol. The dehydration of Pd produces an unstable carbocation at the C1 position, which undergoes deprotonation to a neighboring water molecule with only 0.7 kcal/mol energy barrier. The second step is an exothermic reaction by 9.5 kcal/mol. The energy barrier of the ring-opening step can be substantially decreased when the oxygen of DMF is protonated in the Brønsted acid ILs. 11116


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3.2.4. Whole Process of the POD. In summary, the energy surfaces of each elementary step of conversion DMF and AA to DMBA in POD pathways are shown in Figure 9. There are also two routes of D−A reaction just like the UPOD pathways. The route endo and route exo are plotted as solid and dotted lines, respectively. All the xyz coordinates of the TSs are depicted in Table S6. In route endo, the oxygen of DMF is first protonated in the Brønsted acid ILs. Then, the protonated DMF and AA could occur in D−A cycloaddition to produce the intermediate (Pa) with 38.0 kcal/mol barrier energy. The existence of hydrogen protonated in the oxygen of DMF has a big influence on facilitating the ring-opening step. The ring-opening step becomes easier to occur with 7.0 kcal/mol barrier energy and is exothermic by 5.5 kcal/mol. Hereafter, the hydrogen of C−H near the carboxyl will be dissociated with a relatively low barrier of about 10.2 kcal/mol. At last, the protonated dehydration catalyzed by H3O+ will produce DMBA with only 1.1 kcal/mol energy barrier. For route exo, the whole process is similar to route endo. However, the energy barrier of route endo is a little bit higher than that of route exo. The overall reaction pathways of POD are presented in Scheme 5. The decarboxylation reaction of DMBA is more difficult to occur. It can be concluded that the production is the same in the different π−π forms of DMF and AA. The barrier of D−A in POD will be higher than that of UPOD. Moreover, the limiting step is attributed to D−A cycloaddition, while the barrier of the ring-opening step is relatively lower than before. To account for the solvent effects, the SMD-GIL solvation model62 at the M06-2X-D3 level of theory with the same basis sets has also been performed in this work. As shown in Figure S12, the data in SMD is a little bit lower than that of vacuum. But the trends in SMD and vacuum are similar. The SMD-GIL solvation model provides a liquid environment in the DFT calculations and will not essentially impact the reaction pathways. On the basis of the recent work by Nikbin et al., reaction energies which are carried out in the vacuum also have a fairly good description of the DFT calculations of DMF and ethylene.32 For the novelty, we also have done the comparison with the normal route of DMF and ethylene, which is shown in Table 1. In the D−A reaction, the energy barrier of AA as the

Figure 8. Potential Gibbs free energy (kcal/mol) profiles with the structures of reactants, transition states, and products for the dehydration to DMBA catalyzed by H3O+ (bond distances in angstroms).

To yield PX, the DMBA needs to be decarboxylated. However, the decarboxylation step is hard to occur in the experiment. The Cu2O-catalyzed proto-decarboxylation of aromatic carboxylic acids was reported by Goossen et al.60 The reaction required the presence of Cu2O catalysts.61 The decarboxylation of DMBA catalyzed by H2O, H2PO4−, and HSO4− has also been calculated in this study. The computed relative Gibbs free energies of decarboxylation are shown in Table S5. For the decarboxylation of DMBA catalyzed by H2O, the hydroxyl group of water is added to the carboxyl group to produce carbonic acid with 83.8 kcal/mol energy barrier. For the decarboxylation of DMBA catalyzed by HSO4−, the hydrogen of the carboxyl transfers to HSO4− and the hydrogen of HSO4− transfers to the benzene ring to produce CO2. The energy barrier is 40.1 kcal/mol. The decarboxylation is an exothermic reaction by about 1.5 kcal/mol. It means that the decarboxylation of DMBA is really hard to happen, which is consistent with the experiment.31 The result reveals that the rate-determining step is decarboxylation, and PX will be made when the DMBA occurs in the decarboxylation reaction.

Figure 9. Reaction routes (endo and exo) and potential Gibbs free energy surface in the POD pathway. 11117


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Industrial & Engineering Chemistry Research Scheme 5. Overall POD Reaction Pathway for the Conversion of DMF and AA into DMBA

transferability than H2O. The experimental observation of these intermediates also provides strong evidence to support the theoretical calculation. This work provides a clear theoretical mechanism about the conversion reaction pathways of DMF/AA to aromatics and how Brønsted acid ILs promote the reactions.

Table 1. Calculated Gibbs Free Energies (kcal/mol) of TS Structures in Each Step of the POD Pathwaysa route endo (AA) D−A ring-opening dehydration

route exo (AA)

ethylene

Ga

Gr

Ga

Gr

Ga

Gr

37.9 10.6 1.1

−10.8 −14.6 −14.9

26.8 7.3 1.1

−19.5 −24.2 −14.9

30.2 6.1 0.8

−20.9 −3.8 −21.1

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ASSOCIATED CONTENT

S Supporting Information *

a

Acrylic acid (AA) and ethylene as the reactant, respectively (Ga, active energy; Gr, reaction energy).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01585. 13 C GC−MS experiment data, optimized structures of interaction between P1/P1′ and ILs, potential energy 3D surface scan for the ring-opening step, each step catalyzed by different ILs, SMD-GIL solvation model, and snapshot and coordinates of TS structures in UPOD and POD pathways (PDF)

reactant is higher, about 7.7 kcal/mol, than that of ethylene in route endo. However, in the route exo, the energy barrier of AA is lower, about 3.4 kcal/mol. In the ring-opening and dehydration steps, the energy barrier of ethylene as the reactant is lower than that of AA. Each of them has their advantages and disadvantages such as the relatively low energy barrier for ethylene, but the gas phase of ethylene is not easy to handle under the mild condition.

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AUTHOR INFORMATION

Corresponding Authors

4. CONCLUSION It is of practical importance and great fundamental interest to produce aromatics from biomass, as the transformation provides alternative routes to obtain renewable chemicals. Brønsted acid ionic liquids (ILs), such as [Bmim]HSO4, have shown great catalytic activity for the synthesis aromatics via the D−A reaction of DMF and AA. Using the M06-2X-D3/6311+G** method, two different mechanisms are explored: UPOD and POD. Because of the asymmetric structure of AA, there are also two kinds (endo and exo) of D−A reaction routes in each pathway. In the UPOD pathways, the endo route produces PX along with the C−C bond cleavage, and the ring-opening protonation occurs to release CO2. The exo route produces DMBA, and the carboxyl group will not be removed in the ring-opening step. That is the reason why there are mainly two aromatic products. Besides, the limiting step is attributed to the ring-opening protonation step, while in the POD pathways, the D−A cycloaddition is the rate-determining step, and the energy barrier of cycloaddition is higher than that of UPOD. In addition, the ring-opening step is with very low energy cost. Comparison of the active energy data of route endo and route exo indicates that the energy barrier of exo is lower than that of the endo irrespective of the UPOD and POD pathways. By the calculations of all elementary steps, the anions of the Brønsted acid ILs play a substantial role in functioning as a proton shuttle to promote the reaction. Furthermore, HSO4− and H2PO4− anions have better proton

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaomin Liu: 0000-0001-7320-1995 Jiayu Xin: 0000-0002-0728-294X Weihua Xiao: 0000-0003-0142-8829 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Key R&D Program of China (2016YFE0112800), National Natural Science Foundation of China (21776281), National Science Fund for Excellent Young Scholars (21722610), National Key R&D Program of China (2017YFB0307303), and Natural Science Foundation of Beijing, China (2182073).

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