One-Step Conversion of Biomass-Derived Furanics into Aromatics by

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One-step Conversion of Biomass-derived Furanics into Aromatics by Brønsted Acid Ionic Liquids at Room temperature Lingli Ni, Jiayu Xin, Kun Jiang, Lu Chen, Dongxia Yan, Xingmei Lu, and Suojiang Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04017 • Publication Date (Web): 01 Jan 2018 Downloaded from http://pubs.acs.org on January 1, 2018

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ACS Sustainable Chemistry & Engineering

One-step Conversion of Biomass-derived Furanics into Aromatics by Brønsted Acid Ionic Liquids at Room temperature Lingli Nia,b, Jiayu Xin*,a, Kun Jianga, Lu Chena,b, Dongxia Yana, Xingmei Lua, and Suojiang Zhang*,a aBeijing

Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process

Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, 1 North 2nd Street, Zhongguancun, Haidian District, Beijing 100190, China. bSino

Danish College, University of Chinese Academy of Sciences, 380 Huaibeizhuang, Huairou

district, Beijing 101408, China

*Corresponding authors: Jiayu Xin ([email protected]), Suojiang Zhang ([email protected]), Tel./ Fax.: (+86) 010-62558174

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Abstract: Aromatics play essential and unique roles in the areas ranging from synthetic chemistry to manufacturing industry. Production of aromatics from biomass is of great fundamental interest and practical importance to ease the burden of fossil resources. This work delineates a one-step route for the synthesis of renewable aromatics from various bio-based furanics and dienophiles by acidic ionic liquids at mild conditions. [Bmim]HSO 4 was used as a catalyst and solvent for the direct conversion of 2,5-dimethylfuran and acrylic acid into p-xylene and 2,5-dimethylbenzoic acid, up to 89% aromatic selectivity was achieved at 87% conversion of 2,5-dimethylfuran at room temperature and atmospheric pressure, and totally 84% selectivity of p-xylene can be obtained with a subsequent decarboxylation reaction. The reaction mechanism study supplemented with isotopic tracing and DFT calculations revealed the lowest-energy pathway for the two main products. Various starting materials were studied for further extensions of the method and it turned out that electron-donating methyl groups on the furan ring played crucial roles on the activation of dehydration and decarboxylation processes. This work provided convenient access to industrial commodity aromatics from fully biomass-derived feedstocks and thus can be regarded more economically and environmentally feasible. Keywords: biomass, cycloaddition, acidic ionic liquid, aromatics

INTRODUCTION Aromatics are elementary commodity products from petroleum resources. For example, p-xylene (PX) is a fundamental aromatic hydrocarbon and serves as the feedstock for the production of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), coatings, dyes and so on.1-3 With a research octane number of 127 and low toxicity, PX is regarded as an excellent octane booster while the price has limited 2 ACS Paragon Plus Environment

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its application.4 Since the last century, fossil resources have constituted the main feedstocks for the production of most fuels, chemicals and materials, but the environmental concerns together with diminishing fossil reserves result in a global challenge. Efficient processes that enable the production of valuable products from renewable feedstocks with high yields must be developed in order to reduce global warming whilst satisfy the growing energy demands. There is a growing interest in deriving aromatic chemicals from biomass in recent years.5-8 Cycloaddition with subsequent aromatization of biomass-derived furanics and dienophiles provides a promising strategy to synthetize renewable aromatic structures.

9

Furfural and 5-hydroxymethylfurfural (5-HMF) are primary sugar-derived chemicals from cellulosic biomass,10-13 which contain the conjugated diene structure and have the potential to react with dienophiles via tandem Diels-Alder and dehydration reactions to form the aromatic skeleton structures. The most considered approach focused on the reactions between furanic derivatives and high pressure ethylene to produce PX or its derivatives.14 Intrinsically, the catalytic conversion of oxidized derivatives of 5-HMF and ethylene into aromatics is difficult,15, 16 thus the widely studied case is the reaction of 2,5dimethylfuran (DMF) and ethylene over acidic zeolites (e.g. H-Y, H-Beta, Sn-Beta, ZrBeta, NSP-BEA) and other acidic catalysts.17-26 Recently, a breakthrough was achieved by using ethanol as the source of dienophile to generate ethylene in situ to react with DMF over HUSY-12 zeolite (Scheme 1a). 27 In principle, the involvement of ethylene in these reactions is a reasonable choice to exactly introduce two carbon atoms to the furan ring to form benzene ring, whereas, high temperature or pressure conditions (typically



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250 ˚C, 6.2 MPa)28 were inevitable in order to activate ethylene in the Diels-Alder reaction. To date, several strategies have been explored using other dienophiles. Toste and coworkers reported the reaction between DMF and acrolein by a four-step process using multiple reagents and catalysts to produce PX. 29 Lobo and co-workers used furan and acrylic acid for the synthesis of benzoic acid by a two-step reaction protocol, the methanesulfonic acid in combination with acetic anhydride was proved to efficiently dehydrate the DA adducts.30 As a variation, a solid-phase reaction was firstly applied to the aromatization of a hydrogenated DA adducts of (methylated) furans and maleic anhydride. The thermally stable intermediates which are shown in Scheme 1b eliminate the possibility of retro Diels-Alder reaction and the aromatization proceeds smoothly over H-Y zeolite.31-33 Nevertheless, these routes with the absence of ethylene still revealed challenges that have yet to be addressed. Specifically, complicated routes, severe side reactions and moderate product yields impose restrictions on further implementation. Scheme 1. Routes for the production of aromatics from furanics: (a)(b) previous works, 27, 31 (c) this work. (a)

OH O +

O

(b)

O Neat,RT

O+

O

O

O

2-3h,93%

O

HUSY-12

O

+ Alkyl aromatics

300oC,12h ~77%

O

O

Pd/C,THF H2,RT 3h,>95%

H-Y

O

O O

O+

2h,200 oC 80%

O COOH +

+

O

COOH O

O

(c) O +

COOH [Bmim]HSO 4

COOH +

o

25 C,1h 78%


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Therefore, the one-step synthetic route at mild conditions with high selectivity would be a significant advance in the conversion of furanics into aromatics. Herein, we report a novel process to efficiently obtain PX and 2,5-methylbenzoic acid (2,5-DMBA) by acidic ionic liquids from bio-based DMF and acrylic acid which can be produced by oxidative dehydration of the side-product from biodiesel production (i.e. glycerol).34, 35 The reaction including Diels-Alder cycloaddition, dehydration and decarboxylation

could be

accomplished conveniently at room temperature and atmospheric pressure as shown in Scheme 1c. The predominant success of this efficient conversion relies on th e unique properties of ILs used as solvents and acidic catalysts to suppress the retro Diels-Alder reactions and side reactions. In order to further understand the reaction mechanism, isotopic labelling and computational study were used. We also highlighted the generality of this catalytic system with a series of related furanic compounds and dienophiles, then moderate aromatic yields were obtained and the influence of different substituents of the reactants are also described. EXPERIMENTAL Reactions 2,5-dimethylfuran (99%, J&K Scientific Ltd.), 2-methylfuran (99%, J&K Scientific Ltd.), furan (99%, Aladdin Industrial Inc.), methyl 5-methyl-2-furoate (97%, Alfa Aesar), acrylic acid (99.5%, contains 200 ppm hydroquinone monomethyl ether as an inhibitor, J&K Scientific Ltd.), 3-butenoic acid (97%, Sigma-Aldrich) and maleic anhydride (99.5%, J&K Scientific Ltd.) were stored under an inert atmosphere and used as received.



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The reactions were conducted in a 40 ml glass vial with a magnetic stirring bar at 25 ˚C to 100 ˚C, ethanol bath was used when the reaction temperature was below 25 ˚C. All experiments were carried out under atmospheric pressure. The reactions of dienes with acrylic acid or 3-butenoic acid: 1 mmol diene, 6.9 mmol dienophile and 2 mmol IL (details shown in supporting information) were loaded into the reactor, then the reactor was sealed using a Parafilm M(R) (Bemis NA) and placed into an oil bath at the selected temperature which was monitored by a heating plate (C-MAG HP 10, IKA, Germany) for a specific time period with magnetic stirring at 550 rpm. The reactions of dienes with maleic anhydride: 1.2 mmol diene and 1.0 mmol maleic anhydride were added to the reactor and mixed by stirring at 250 rpm at room temperature for 120 min. The solid adduct was obtained and 3 mmol IL was subsequently subjected into the reactor and the reaction further proceeded at 25 ˚C for 24 h in oil bath with magnetic stirring at 250 rpm. At the end of the reaction, the reaction was quenched by using acetone (3×5 ml) to extract or dilute the products from ionic liquids. When using [Bmim]HSO4, the samples from the reaction of DMF and acrylic acid was extracted by benzene/acetone (v/v: 4/1, 1×5 ml) before using acetone (3×5ml) as the excess acrylic acid made [Bmim]HSO 4 and acetone miscible. Then the sample was filtered with a 0.22 µm filter and qualified by GC analysis. Analytical methods The identification of products was performed on an Agilent 6890N/5975B GC-MS equipped with a HP-5 MS capillary column (25 m × 0.25 mm, using Helium as carrier gas) and a SHIMADZU GCMS-QP2020 equipped with a SH-Rxi-5Sil MS column (25 m × 0.25 mm, using Helium as carrier gas). The samples were qualified by a SHIMADZU GC-2014 gas chromatograph with a Rtx-5 (Restek) column (30 m × 0.25 mm) equipped with a flame 6 ACS Paragon Plus Environment

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ionization detector. The yield and selectivity were determined by the external standards and the peaks of the corresponding compounds in the sample. The GC parameters for the reaction of DMF and acrylic acid were: split ratio: 30 : 1, injection volume: 0.4 μL, initial temperature 40 °C, hold for 3 min; temperature ramp 10 °C/min; final temperature 280 °C; detector temperature 300 °C, injector port temperature 300 °C (330 °C for the reactions using maleic anhydride). The conversion of DMF, yield and selectivity of products are defined as follows:

Where t is the reaction time and ni is the molar mass of the reactant and the products, i. The total aromatics yield is the sum of the yields of PX and 2,5-DMBA, and the total aromatics selectivity is the sum of the selectivities of PX and 2,5-DMBA. UV-vis acidity evaluation The Brønsted acidity of the ILs used was evaluated by Hammett method in aqueous solution as water is one of the few solvents which have considerable solubility for all tested ILs and there was also a little amount of water in the reaction media during reactions. 36, 37 4-nitroaniline was used as indicator and the blank solution was aqueous solution of 4-nitroaniline (0.01 mmol/L) and other solutions containing 4-nitroaniline (0.01 mmol/L) and ILs (50 mmol/L) were subsequently prepared for every IL. Then all the solutions were analyzed by UV-vis spectrophotometer and the maximum absorbance of unprotonated form of 4-nitroaniline in these solutions were used to calculate the Hammett function (Figure S17, Table S1). All spectra were recorded with an SHIMADZU UV-2550 spectrophotometer.



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COMPUTATIONAL METHOD All computational calculations were performed to provide mechanistic understanding of the chemistry of the desired reaction catalyzed by [Bmim]HSO 4. Geometry optimizations and frequency calculations were performed using Gaussian 09 at the DFT B3LYP/6-31++G(d,p) theory level with Grimme’s D3 dispersion corrections.38, 39 All the structures were fully relaxed, and vibrational frequencies were calculated. The initial geometries of transition structures were manually guessed and further verified by the presence of a single imaginary frequency and IRC (intrinsic reaction coordinate) calculations. The kinetically uncompetitive pathways were discarded, thus the computed potential energies and reaction pathway data herein were based on the lowest energy pathway. RESULTS AND DISCUSSION Conversion of DMF and acrylic acid with ILs We have reported that the reaction of DMF and acrylic acid was preferred to be carried out with excess acrylic acid.40 The acidic ionic liquids have advantages of general ionic liquids and their acidic properties allow them to act as efficient catalysts to integrate several processes in one-step, particularly for the fractionation and conversion of lignocellulosic biomass, 12, 41-46 thus they were potential catalysts for such kinds of reactions. 47-49 The screening of a series of ILs was carried out for the reaction of DMF and acrylic acid, the results are summarized in Table 1. It was found that 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4) exhibited the best performance for the reaction. PX and 2,5-DMBA were identified as two main products by GCMS and HPLC (Figure S2. S4), 45% yield of PX and 78% yield of total aromatics were obtained at 87% conversion of DMF at 25 ˚C in 60 min over [Bmim]HSO4. In Table 1, the reactivity and selectivity of the reaction were found to highly depend on the properties of ILs. The neutral IL, 8 ACS Paragon Plus Environment

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[Bmim]OTf was unable to catalyze the reaction while the acidic ILs investigated here were all demonstrated to be effective. Notably, most related studies were carried out either with multiple steps or at extremely low temperature in order to avoid the retro Diels-Alder reaction which could even occur at room temperature,29, 33, 50 whereas such problems were successfully avoided with the addition of ILs. A controlled reaction was carried out at 25 ˚C without ILs and did not give any products. By increasing the temperature to 100 ˚C, trace amount of the corresponding product was detected, while the decomposition of DMF occurred severely. Therefore, the acidic ILs are considerably effective to catalyze this reaction. On the other hand, if the amount of [Bmim]HSO4 was reduced to a catalytic amount (0.1 g) and the reaction was carried out without solvent, the yields of PX and 2,5-DMBA were 11% and 13% respectively with a DMF conversion of 29% at the identical condition. If [Bmim]OTf (0.5 g) was added as a solvent, the yields of PX and 2,5-DMBA were 11% and 26% with 46% DMF conversion at 1 h. With prolonging the reaction time to 3 h, the conversion increased to 76% and the yields of PX and 2,5-DMBA increased to 15% and 28%. It can be found that the reaction rate and yield to aromatics decreased when using only catalytic amount of [Bmim]HSO 4. Therefore, [Bmim]HSO4 was proved to be an efficient catalyst for the reaction while it also has a positive solvent effect to reduce side reactions and improved the aromatics yield. To further understand the catalytic performance of ILs, the relative acidity of Brønsted acid ILs was determined b y Hammett method, the acid ity decreased in the order of [BSO 3HMIm]HSO4 > [BSO3HMIm]OTf > [Bmim]HSO 4 > [Emim]HSO4 > [Hmim]HSO4 ≈ [Bmim]H2PO4 (Figure S17, Table S1).36 Therefore, it is clear that the activity of ILs was closely related to the acidity as the weakly acidic [Bmim]H 2PO4 did not show significant catalytic activity. Considering the lower yields using [BSO 3HMIm]HSO 4 and [BSO3HMIm]OTf, the 9 ACS Paragon Plus Environment


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Table 1. Conversion of DMF and acrylic acid into aromatics in ILs.a COOH

COOH 2mmol IL

O +

+

25oC,1h

1

2

entry

catalystc

conversion yield (mol%)b (mol%) 1 2

total aromatics total aromatics yield (mol%) selectivity (mol%)

1

[Bmim]HSO4

87±2

45±3

33±1

78±3

89±1

2

[Emim]HSO4

86±3

34±1

22±2

56±3

65±5

3

[Hmim]HSO4

4±1

2±1

1±1

2±0

54±14

4

[Bmim]H2PO4

7±1

2±1

3±1

4±0

59±9

5

[BSO3HMIm]HSO4

92±3

18±2

22±1

40±3

43±4

6

[BSO3HMIm]OTf

88±4

28±2

22±1

48±2

56±2

7

[Bmim]OTf

2±2

0±0

0±0

0±0

0±0

8

[Emim]Cl/AlCl3

94±2

12±1

9±1

20±1

22±2

9

_d

3±1

0±0

0±0

0±0

0±0

10

_e

15±3

0±0

1±0

1±0

4±3

aReaction

conditions: 1 mmol DMF, 6.9 mmol acrylic acid, 2 mmol IL, 25 ˚C, 60 min. bThe yield of aromatics was determined by GC analysis. c[Emim]HSO4: 1-ethyl-3-methylimidazolium hydrogen sulfate, [Hmim]HSO4: 1-hexyl-3-methylimidazolium hydrogen sulfate, [Bmim]H2PO4: 1-butyl-3-methylimidazolium dihydrogen phosphate, [BSO 3HMIm]HSO4: 1-butylsulfonate-3methylimidazolium hydrogen sulfate, [BSO 3HMIm]OTf: 1-butylsulfonate-3-methylimidazolium trifluoromethansulfonate, [Bmim]OTf: 1-butyl-3-methylimidazolium trifluoromethansulfonate, [Emim]Cl/AlCl3: 1-ethyl-3-methylimidazolium chloroaluminate, [Emim]Cl:AlCl3= 1:2 mol/mol. dBlank: 1 mmol DMF, 6.9 mmol acrylic acid, 25 ˚C, 60 min. eBlank: 1 mmol DMF, 6.9 mmol acrylic acid, 100 ˚C, 60 min.

negative roles of these strong acidic ILs should come from their impact on promoting the hydrolytic decomposition of DMF to 2,5-hexanedione hence hindering the cycloaddition step. As 10 ACS Paragon Plus Environment

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previously reported, the desired pathway to PX and 2,5-DMBA generates an equivalent of water, thus the hydrolytic reaction of DMF is unlikely to avoid. 50 A predominant amount of 2,5hexanedione was formed using [BSO3HMIm]HSO4 and [BSO3HMIm]OTf as catalysts, whereas a comparative trial carried out using DMF in water at 25 ˚C for 300 min produced no 2,5hexanedione, suggesting that the strong acidic ILs catalyzed the hydrolytic decomposition of DMF. Moreover, the acidity has been considered as the key factor for the performance of various acidic ionic liquids on catalysis.41, 42, 51 [Bmim]HSO4 with the addition of different loading of H2SO4 was further used as the catalysts to assist in the understanding of the influence of acidity. We can see an increase of PX yield and a decrease of 2,5-DMBA yield with the increasing loading of H2SO4 (Table S4). Thus the dehydration was promoted while the decarboxylation was weakened at more acidic condition. There should be an optimal acidity range for the catalysts to obtain good aromatic yield. Notably, [Emim]HSO 4 has shorter substituent on the cation associated with more favorable steric interaction with reactants while lower conversion and yield were obtained. This could presumably be explained by its lower acidity and higher polarity compared with [Bmim]HSO4. The reactants were more soluble in [Bmim]HSO 4 affording more possibilities for the interactions between reactants and [Bmim]HSO 4. As for the low activity of [Hmim]HSO4, the acidity and steric hindrance could both be the reasons. Unlike Brønsted acid ILs, a Lewis acidic chloroaluminate ionic liquid, [Emim]Cl/AlCl3 was also employed as catalyst, and moderate aromatic yield was obtained revealing the potential of Lewis acidic ILs as catalysts for this reaction. Due to the good performance, [Bmim]HSO 4 was used for further investigation. Condition optimization for the reaction of DMF and acrylic acid In Figure 1a, DMF conversion, yield and selectivity of aromatics were monitored as a function of time. High selectivity (60% to PX and 35% to 2,5-DMBA) was achieved in the initial state of 11 ACS Paragon Plus Environment

(a) Conversion or yield (mol%)

Conversion Aromatics 100

Aromatic selectivity PX 2,5-DMBA 100

80

80

60

60

40

40

20

20

0

20

40

60

80

100

Time (min)

Conversion or yield (mol%)

(b)

Conversion

PX

2,5-DMBA

Aromatics

100 80 60 40 20 0

10

25

40

55

70

Temperature (°C)

(c)

100

Selectivity (mol%)

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

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Aromatic selectivity (mol%)


80

PX 2,5-DMBA Aromatics

60 40 20 0

10

25

40

55

70

Temperature (C)

Figure 1. (a) Conversion of DMF, yield and selectivity of aromatics as a function of time at 25 ˚C. (b) Effect of temperature on DMF conversion and aromatic yield. (c) Effect of temperature on aromatic selectivity. Reaction conditions: 1mmol DMF, 6.9 mmol acrylic acid, 2 mmol [Bmim]HSO4, (b)(c) 60 min (10 ˚C, 25 ˚C), 30 min (40 ˚C, 55 ˚C, 70 ˚C). 12 ACS Paragon Plus Environment

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the reaction and it decreased with time due to the formation of side products as detected by GCMS. Particularly, the side reactions occurred fiercely at high DMF conversion thus resulting in a highest yield of aromatics at 87% DMF conversion in 60 min. Furthermore, the reaction temperature was found to exert a great influence on the product selectivity. In Figure 1c, 10 ˚C was the optimal reaction temperature with 61% selectivity to PX and total 98% aromatic selectivity even though the reaction time needed to be prolonged. As acrylic acid has been reported to polymerize easily, 52-54 the controlled experiments using acrylic acid in [Bmim]HSO 4 at 25 ˚C and 70 ˚C were conducted in order to find whether the polymerization of acrylic acid was competitive to the main reaction. The results showed that undetectable acrylic acid was consumed at 25 ˚C and 11% acrylic acid was converted at 70 ˚C without any side products detected by GC-FID implying that the formed side products were high boiling point polymers. Therefore, room temperature is beneficial for the reaction considering the atom economy. Interestingly, the variation of temperature also revealed the opposite changing tendency of the selectivities to PX and 2,5-DMBA with the increase of temperature. Theoretically, the temperature condition affects the reactivity, selectivity and equilibrium of every step, and the observed total aromatic yield associated with multiple factors. The Diels-Alder cycloaddition was the starting point of this reaction and it is preferred to proceed at lower temperature because of its reversibility and exothermicity. It has been reported that at least 35% of the related oxanorbornene adduct decomposed via retro Diels-Alder reaction by Brønsted acid zeolites (H-beta) at 150 ˚C and negligible aromatic compound was obtained. 30 Moreover, the furanics and acrylic acid also decomposed to a series of side products, making the dehydration of cycloadduct unselective at higher temperature. Whereas, the yield of 2,513 ACS Paragon Plus Environment


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DMBA increased from 33% to 45% in [Bmim]HSO 4 when the temperature increased from 25 ˚C to 70 ˚C (Figure 1b). This results proved the high efficiency of dehydration by [Bmim]HSO4. However, the yield of PX decreased significantly with increasing temperature, thus the production of PX and 2,5-DMBA may possibly be two competitive reactions and the pathway to 2,5-DMBA was dominant at elevated temperature. Otherwise, the side reactions consuming the PX may occurred severely when increasing temperature. Meanwhile, an increasing amount of 2,5-hexanedione was observed at elevated temperature, which also led to the decrease of the total aromatic yield at higher temperature (Figure 1b, 1c). Reaction kinetics of the conversion of DMF over [Bmim]HSO4 The conversion of DMF in the initial state of reactions was monitored over a range of temperature (10-70 °C), while the data processing revealed that the overall decay of DMF perfectly fit the first-order reaction model as shown in Figure 2a. Meanwhile, the corresponding rate constant (k) determined from Figure 2a was used to generate Arrhenius plot in Figure 2b, giving an estimated apparent activation energy ( Ea) for the overall conversion of DMF to be 15.5 kcal/mol. However, the retro Diels-Alder reaction and a number of side reactions occurred simultaneously with the main reaction, thus the measured Ea was not the real value for the desired reaction and it corresponded to multiple terms in the reaction regime. Notably, this result still revealed the efficient catalytic performance of [Bmim]HSO 4 as the apparent energy barrier (15.5 kcal/mol) was relatively low. Actually, such side reactions and retro reaction were reported to result in a low Ea value for related reactions.19 For example, the Ea of the conversion of DMF was about 7.9 kcal/mol when Sc(OTf) 3 was used as catalyst for the same reaction in 14 ACS Paragon Plus Environment

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[Emim]NTf 2.40 The selectivities of target products were lower in the metal triflates catalyzed system, thus the side reactions had greater influence on the Ea value. Temperature (C) 55 40 25

(b) 70

2.0 70 C 55 oC 40 oC 25 oC 10 oC

1.6 1.2

10

0

o

Ea=15.5 kcal/mol -1

ln k(min-1)

(a) -ln(nDMF/mmol)

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


0.8 0.4

-2 -3 -4 -5

0

20

40

60

80

100

120

0.36

Time (min)

0.38

0.40

0.42

3

1/[RT(K)]*10

Figure 2. (a) The kinetics of the reaction between DMF and acrylic acid at different temperatures. Reaction conditions: 1 mmol DMF, 6.9 mmol acrylic acid, 2 mmol [Bmim]HSO 4. (b) Arrhenius plot for the conversion of DMF catalyzed by [Bmim]HSO 4 from 10 °C to 70 °C. Reaction mechanism of the conversion of DMF and acrylic acid over [Bmim]HSO4 As seen in Figure 3, totally more than 20 compounds were detected in the reaction mixture by GC-FID and 9 of them were identified by GC-MS (Figure S1-S6) as listed in Table 2. DMF (1), acrylic acid (2), PX (4), 2,5-hexanedione (5) and 2,5-DMBA (7) were further identified by comparisons of their retention times to the commercial standards using GC-FID. Compound 3 has the same molecular skeleton as PX while the benzene ring was not completely formed, suggesting that it was an isomer formed from the precursor of PX. Compounds 6, 8, 9 were unstable reaction intermediates. 6 has a parent ion of 124 m/z which is consistent with the well-known oxanorbornene precursor of PX or its isomer from the reaction of DMF and ethylene identified by other groups.17-22 Taken 15 ACS Paragon Plus Environment


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the reaction mechanism discussed latter and the instability of the oxanorbornene precursor into consideration, the structure of 6 was determined as the isomer in Scheme 2. For compounds 8 and 9, GC-MS revealed a patent ion of 168 m/z with slightly different fragmentation patterns for two compounds, the 168 m/z is exactly consistent with the oxanorbornene intermediate of 2,5-DMBA and its isomer.50 Moreover, a markedly amount of 8 and 9 was observed at 10 ˚C while only minor amount was detected at higher temperatures, revealing that they could properly be the intermediates during dehydration and decarboxylation since these two processes were less efficient at lower temperature. Therefore, we tentatively assigned them as the oxanorbornene precursors of 2,5-DMBA and the isomeric alcohol respectively based on the properties of GC column, their retention times and the mechanism analysis. uV(x10,000) uV(x1,000) 7.0 6.5

9.0

6.0

2

4

5.5 5.0 4.5

8.0

4.0

6

3

3.5 3.0

7.0

7

2.5 2.0 1.5

6.0

1.0 6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

min

5.0 4.0

1 89

5

3.0 2.0 1.0

5.0

7.5

10.0

12.5

15.0

17.5

min

Figure 3. Gas Chromatogram of liquid products from the reaction of DMF and acrylic acid. Peak numbers correspond to compounds listed in Table 2. Reaction conditions: 1 mmol DMF, 6.9 mmol acrylic acid, 2 mmol [Bmim]HSO4, 25 ˚C, 60 min.


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Table 2. Products distribution of reaction of DMF and acrylic acid over [Bmim]HSO 4 detected by GC-MS. compound

chemical formula

1

C6H8O

2

C3H4O2

3

C8H12

4

C8H10

chemical structure O

COOH

O

5

C6H10O2

6

C8H12O

O

OH

COOH

7

C9H10O2 COOH

8

C9H12O3

O

HOOC

9

C9H12O3

OH

Furthermore, after taking a sample for GC-FID analysis from the reaction mixture at 10 ˚C for 300 min at which the DMF conversion was up to 95%, the reaction was continuously conducted with further addition of 0.25 mmol H 2SO4 and stored at -10 ˚C to enhance the dehydration of intermediates for 180 min, then the reaction mixture was analyzed by GC-FID in the identical conditions again. As expected, the yield of 2,5DMBA was significantly improved and the peak areas of 8 and 9 were detected to be much smaller compared with the result at 300 min, which implied that these two compounds were converted into 2,5-DMBA by H2SO4 in the 180 min process. While only a small amount of 6 was detected at 300 min and it disappeared after the extra 180 min reaction. All three compounds were confirmed to be the precursors of the final products. For other compounds, they are relatively minor side products which are difficult to be 17 ACS Paragon Plus Environment


Page 18 of 36

isolated sufficiently and their structures could not be identified from the mass spectral fragmentation patterns, thus they were not discussed here. In this reaction, it is clear that [Bmim]HSO 4 serves as a Brønsted acid to catalyze the dehydration of oxanorbornene intermediates to aromatics. However, the appearance of PX in the product mixture was unexpected, and the envisioned transformation of DMF into PX normally requires the addition of a building block of exactly two carbon atoms, thus the use of acrylic acid as dienophile will inevitably introduce an unwanted carboxyl group to the benzene ring. It has been reported that PX can be obtained via an decarboxylation process over Cu 2O based catalyst from 2,5-DMBA at 210 ˚C in the study of Toste et al.29 Whereas, van Es et al. also observed the dehydration of dimethylphthalic anhydride accompanied by full decarboxylation giving PX with 11% yield over H-Y zeolite at 200 ˚C and a lactone was proved to be a primary intermediate. 31 Here, the precursors of PX and 2,5-DMBA (compound 6, 8, 9) were all detected by GC-MS suggesting that the production of 2,5-DMBA follows the known Diels-Alder and dehydration reaction pathway and an extra decarboxylation process may occur from 8 to 6 in ILs and finally leading to the formation of PX. Indeed, subjection of 2,5-DMBA to [Bmim]HSO4 did not produce PX, this clearly demonstrated that 2,5-DMBA was a stable final product and the decarboxylation occurred prior to the dehydration of 8. Considering the conversional decarboxylation processes, it is normally carried out at high temperature and accompanied by the release of CO 2,55-57 small amount of gas bubbles were observed in the reaction and the analysis of liquid samples by GC-MS confirmed the existence of CO2 indicating that the solubility of CO 2 in the reaction mixture was considerable (Figure


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S15, S16). Therefore, it can be concluded that the removal of –COOH as CO2 is one of the decarboxylation pathways to produce PX. Based on the above results, a potential reaction mechanism was depicted in Scheme 2. The synthesis of 2,5-DMBA starts from the formation of the oxanorbornene intermediates 8 via Diels-Alder cycloadditon, then the protonation of bridging ether 8 leads to TS1 and further ring-opening gives an allylic cation, the positive charged TS2 could be deprotonated to the species with conjugated double bonds, shown as TS3 (9).58 Latter the Brønsted acid catalyzed dehydration produces the 2,5-DMBA. Considering the formation of PX, the decarboxylation pathway via acid catalyzed ring protonation with hydration of the carboxyl to produce carbonic acid was preferred as the removal of -COOH as HCO2is energetically prohibited, 59,

60

and the solubility of CO 2 in the crude mixture was

considerable which may properly due to the exist of CO 2 as more stable carbonic acid in the liquid phase. Therefore, the decarboxylation follows the procedure including covalent Scheme 2. Overall Reaction pathway for the conversion of DMF and acrylic acid into PX and 2,5-DMBA. COOH

COOH O +

O

COOH [Bmim]HSO4

COOH H O

O

H O 8

3

1

S

COOH O

O O

O

TS1

S

HO

H O

O O

O

TS2

H O

S

O O

TS3 ( 9 )

[Bmim]HSO4

+H2O

O O

O O H O S O OH O

+H2O

O

O 5

H O OH O S O O

OH OH

OH OH

O

O H O S O O H O

TS6

TS5

TS7

COOH H O H O H O O S O H TS4 -H2O

H

O O

4

S

O

H H HO O O S H O O

-H2O

O TS9

-CO2 -H 2O

O

H O

COOH

O

H O S

TS8 ( 6 )

O O H O S O

O HO

OH

O 7



Page 20 of 36

hydration of the –COOH on 8, ring-opening protonation of bridging ether, carbon–carbon bond cleavage of the hydrate along with proton transfer to form carbonic acid and 6, shown from TS5 to TS8 (6) in Scheme 2. Then PX was formed following Brønsted acid catalyzed dehydration from TS8 to 4. Isotope tracing is helpful for providing further information regarding the reaction mechanism, here we designed a 13

13

C tracing experiment that acrylic acid with labelled -

COOH was used as dienophile in [Bmim]HSO4 catalyzed system to verify the sources

of the departing CO2 and the carboxyl group of 2,5-DMBA (Figure S18). Gratifyingly, GC-MS gave the results that the detected CO2 had parent ions of 45 m/z, 29 m/z and 16 m/z (Figure S19), which fully demonstrated that the CO 2 was labelled with

13

C from

acrylic acid. Moreover, the direct addition of catalyst into acrylic acid did not produce any CO2, thus the decarboxylation exactly occurred on the PX precursor rather than acrylic acid. The mass spectrum of 2,5-DMBA also revealed a parent ion of 151 m/z which further confirmed the formation of 2,5-DMBA from DMF and the labelled acrylic acid (Figure S20). Additionally, the 169 m/z parent ion detected from the mass spectrums of 8 and 9 confirmed their structures with carboxyl group (Figure S21), and the mass spectrums of compound 4 and 6 maintained unchanged, which was consistent with the deduction of their structures in the proposed reaction mechanism. In the previously reported reaction of DMF and acrolein, 4Å molecular sieve was used to remove water formed in the reaction, and the dehydration of 8 was improved a lot.29 Further evidence for the hydrolytic decarboxylation was given by a controlled reaction using 4Å molecular sieve as an additional drying agent in an identical [Bmim]HSO 4 catalyzed reaction at 25 ˚C. Interestingly, 35% conversion of DMF, 29% selectivity of 20 ACS Paragon Plus Environment

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PX, 57% selectivity of 2,5-DMBA was obtained for 180 min. The reaction rate was significantly decreased and a large amount of 8 and no trace of 6 were observed. The increased selectivity of 2,5-DMBA confirmed the promotion of dehydration with less amount of H2O in the reaction media, thus the unreacted 8 should be attributed to the unsuccessful decarboxylation which was also indicated by the absence of 6 and lower selectivity of PX, it can be concluded that H 2O plays an indispensable role in the decarboxylation process. Besides, the decarboxylation of benzoic acids has already been reported, and the Cu 2Obased catalyzed method was employed here to give 89% yield of PX from 2,5-DMBA (see supporting information). 29, 61, 62 As a result, PX could be obtained with up to 73% yield and 84% selectivity from DMF and acrylic acid in this work. Computational results of the potential reaction mechanism We have noted that the capture and identification of reaction transition states could be rather

difficult

particularly

with

the

complex

reaction

mechanism

involving

cycloaddition, dehydration and decarboxylation. As a result, computational simulation was employed to evaluate the reliability of the proposed mechanism. 63-66 All calculations herein were performed with Gaussian 09 at the DFT B3LYP/631++G(d,p) theory level with Grimme’s D3 dispersion corrections. 38, 39 In Figure 4, the energy profiles of the elementary steps of the formation of 2,5-DMBA and PX depicted in Scheme 2 was calculated, demonstrated as red and blue line, respectively. At the beginning of the reaction, the cycloaddition of DMF and acrylic acid could occur to produce 8 as the starting substrate for all the subsequent conversions. In the reaction, the endo/exo ratio of 8 is hard to be determined, the simulation reveals that endo form of 8 21 ACS Paragon Plus Environment


has a little bit higher energy barrier compared with the exo form as shown in Figure 4. Moreover, we observed that the direct dehydration of exo form of 8 to produce 2,5DMBA could have lower energy barrier than using the endo form, while the situation was exactly opposite in the pathway to PX, therefore the simulation was mainly concentrated on the potential lowest-energy pathways in the mechanism.

40

Total energy (kcalmol)

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

Page 22 of 36

20

TS0endo TS1+TS2

TS6+TS7

TS0exo

0

TS8+TS9

8endo 1+2

-20

8exo

TS5 8‘exo

TS3+TS4 6

-60

11

8'endo

-40 Diels-Alder reaction

11''

9 9'

ring-opening protonation + decarboxylation

dehydration 6'

PX

2,5-DMBA

Intrinsic Reaction Coordinate

Figure 4. Free energy profile of the formation of PX and 2,5-DMBA from DMF and acrylic acid in [Bmim]HSO4 corresponding to Scheme 2. From the depicted reaction barriers, the calculated free energy barrier for the DielsAlder reaction of DMF and acrylic acid is 17.9-19.0 kcal/mol, which is in good agreement with the experimental value of 15.5 kcal/mol for DMF conversion regardless of the errors in simulation and experimental calculations. The lower apparent free energy of DMF conversion in experiment may due to the retro Diels-Alder reaction and competitive side 22 ACS Paragon Plus Environment

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reactions proceeding along with the forward reaction. In the route to 2,5-DMBA, the ringopening of bridging ether in the dehydration which shows highest energy barrier (35.0 kcal/mol) appears to determine the overall reaction rate. Differing from the stepwise pathway depicted in Scheme 2, the proton transfers from catalyst to the ether oxygen (TS1) and from the carbon on –COOH to catalyst (TS2) are nearly completed at the same time, leaving 9 as the product to undergo the final step (TS3+TS4). Likewise, the ringopening protonation of bridging ether (TS6) occurs along with the carbon–carbon bond cleavage of the hydrate (TS7), which is the rate limiting step (40.4 kcal/mol) of the route to PX, shown in Figure 4. The hydrate (11) with requisite proton transfer among water, – COOH and catalyst is proved to be energetically accessible with a free energy barrier of 15.7 kcal/mol. Therefore, the acid catalyzed decarboxylation could be efficient with the release of neutral carbonic acid.60 Finally, PX was produced from the dehydration of 6, which is similar to the process from 9 to 2,5-DMBA. Comparing the two pathways, we can find that the free energy barrier of the limiting step of PX pathway (40.4 kcal/mol) is higher than that of 2,5-DMBA pathway (35.0 kcal/mol) in simulation. However, the production of 2,5-DMBA was enhanced more significantly at elevated temperature in the experiment, thus there should be some side reactions consuming PX and its precursors when increasing temperature. In addition, the rate of PX pathway may be faster than 2,5-DMBA pathway at 25 °C as the yield and selectivity of PX were higher than that of 2,5-DMBA. Scope of IL catalyzed reactions for the production of aromatics from furanics To get more insights into the application of this protocol, the reaction was further extended to other dienes (i.e. 2-methylfuran, furan, methyl 5-methyl-2-furoate) and 23 ACS Paragon Plus Environment


Page 24 of 36

dienophiles (i.e. maleic anhydride, 3-butenoic acid), shown in Table S3. By using less substituted furan, 2-methylfuran (MF), as the diene, 45% aromatic yield was obtained with toluene and 3-methylbenzoic acid being the main products (Figure S12). However, a minor amount of 2-methylbenzoic acid was also observed even though the steric bulk of the methyl group on furan ring prevents its formation. The yield of methylbenzoic acids was almost the same as 2,5-DMBA, whereas the yield of toluene was much lower than that of PX from DMF. The high selectivity observed for aromatics from DMF is presumably due to the electron-donating methyl groups on the furan ring which will force the furan ring toward the formation of DA cycloadduct and stabilize the positive charge developing on the carbon atom in the transition states of the tandem dehydration of the cycloadduct.26, 33 The electron rich DMF and electron deficient acrylic acid resulted in an active Diels-Alder reaction and the related cycloadducts were previously reported to be well stabilized in ILs at room temperature to block retro Diels-Alder reaction.47 However, very limited selectivity was achieved for the accumulated aromatics with other starting materials in [Bmim]HSO 4 (Table S2). For the reaction using maleic anhydride as dienophile, the dehydration and decarboxylation were found difficult to proceed at 25 ˚C while severe retro Diels-Alder reaction and DMF decomposition occurred at higher temperature in [Bmim]HSO 4. Considering the structure of maleic anhydride, the reasons may be the limited acidity of catalyst and the unfavorable steric interaction between the intermediates and IL. 67 As a result, [BSO 3HMIm]HSO4 and [Emim]HSO4 were selected to be catalysts for the reaction of DMF and maleic anhydride to explore the effects of steric hindrance and Brønsted acidity on the reactivity.


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As expected, the reactivity was significantly improved by using [BSO 3HMIm]HSO4 and the yield of the aromatics was further improved by introducing a one-pot two-step reaction sequence to the reaction. Whereas [Emim]HSO 4 almost showed the same performance as [Bmim]HSO 4 which further proved that the steric hindrance was not the determinant for the reaction and it was the acidity that significantly affected the reaction. [BSO3HMIm]HSO4 had the strongest acidity so that it was the most active catalyst. Nevertheless, the adducts’ tendency for retro Diels-Alder reaction still remained to be a crucial problem in [BSO 3HMIm]HSO4 at 25 ˚C. It has been reported that the yield of the DA adduct of DMF and maleic anhydride can be up to 90% at room temperature for 120 min without catalysts,33 however, only moderate conversion was achieved at the end of the aromatization here demonstrating that the retro Diels-Alder reaction occurred during the aromatization. Unlike [Bmim]HSO 4, severe hydrolytic decomposition of DMF was observed in [BSO3HMIm]HSO4 at 25 ˚C and the high viscosity of [BSO 3HMIm]HSO4 made the mixing difficult, which further limited the mass transfer, and all these gave the less reactive and selective result of the reaction. Additionally, [BSO 3HMIm]HSO4 was also effective for other reactions investigated (Table S3, Figure S7-S14). It was interesting that the yield of methylbenzoic acids (2-methylbenzoic acid and 3methylbenzoic acid) was higher than that of methylphthalic anhydride in the reaction of MF and maleic anhydride (Table S3, entry 8). This result was different from that reported by van Es et al., in which the methylphthalic anhydride was the main product. 31 Also in entry 6 Table S3, the 2,5-DMBA was the dominant product in the reaction of DMF and maleic anhydride. Taken together, [BSO 3HMIm]HSO4 made it more available for decarboxylation process at 25 ˚C. 25 ACS Paragon Plus Environment


Page 26 of 36

The reactions were also greatly influenced by different substituents and temperature, the reactions using furan as the diene at room temperature all yielded negligible aromatics (Table S2). And surprisingly the attempt to conduct the reaction of furan and acrylic acid in [BSO3HMIm]HSO4 at 100 ˚C for 120 min afforded much better yield of benzoic acid (58%) while the reaction of furan and maleic anhydride still gave poor aromatic yield. As high yield of oxanorbornene dicarboxylic anhydride can be obtained from the Diels-Alder reaction of furan and maleic anhydride at the identical condition, 68 the low aromatic yield must be owing to the unreactive aromatization. Therefore, the appearance of electron-donating methyl group on the furan ring was much important for the dehydration and decarboxylation processes. The long substituent group on the furan ring also resulted in poor aromatic yield due to steric hindrance (Table S3, entry 3). Moreover, the removal of carboxyl group on the alkyl chain can also be achieved in ILs, which was verified by the 7% yield of 1,2,4-trimethyl benzene using 3-butenoic acid as the dienophile. However, undetectable decarboxylate was obtained when furan was used as the diene (Table S3, entry 10), further proving the crucial role of methyl group in the decarboxylation process. CONCLUSION In summary, the one-step conversion of bio-based furanics into aromatics via a synthetic route including Diels-Alder, dehydration and decarboxylation reactions can be efficiently catalyzed by acidic ILs at mild conditions. [Bmim]HSO 4 gave high yield of PX and 2,5-DMBA from DMF and acrylic acid with up to 89% aromatic selectivity in a single step at room temperature. The reaction mechanism supported by computational simulation and isotopic tracing was studied and the energy barriers of every elementary step were presented. With application of [BSO 3HMIm]HSO4 to the reactions using 26 ACS Paragon Plus Environment

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different dienes and dienophiles, moderate yields of various aromatics were obtained, which suggested the great potential to obtain excellent yield of renewable aromatics by tuning the structure and properties of ILs, particularly the acidity. It was also proved that the electron-donating methyl groups on the furan ring could significantly benefit the dehydration and decarboxylation processes. We anticipate that the catalytic method will find use on some other acid catalyzed cascade reactions in one-step and the economic attractiveness of such catalytic system will be greatly enhanced if IL recycling can be well demonstrated in the future. ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge on the ACS Publication website at DOI: XXX. Details on the materials, GC Chromatogram spectra, GC-MS mass spectrum, UV-vis absorption spectra, simulation results, decarboxylation reaction, calculation of apparent activation energy, ionic liquid recycling, isotopic tracing experiment and tables describing the additional experiments. AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] Notes



Page 28 of 36

The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program, No. 2015CB251401) and National Natural Science Foundation of China (No. 21576269, 21210006, 21476245, 21406230). We thank Prof. Anker Degn Jensen (Technical University of Denmark) for helpful discussion and Zhaoyang Ju (Institute of Process Engineering) for his help in computational simulation. REFERENCES 1.

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For Table of Content Use Only

A novel route to access aromatics from fully biomass derived feedstocks in a single step at room temperature and atmospheric pressure was developed by acidic ionic liquids.


One-Step Conversion of Biomass-Derived Furanics into Aromatics by

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