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Jul 8, 2015 - Sorbic Acid as a Renewable Resource for Atom-Economic and. Selective Production of p‑Toluic Acid and Alkyl‑p‑Toluates: Intermediat...
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Research Note

Sorbic Acid as a Renewable Resource for Atom-Economic and Selective Production of p‑Toluic Acid and Alkyl‑p‑Toluates: Intermediates to Bioterephthalic Acid and Esters Sandrine Bérard,† Christophe Vallée,† and Damien Delcroix*,† †

IFP Energies Nouvelles, Rond-Point de l’Echangeur de Solaize, BP3, 69360 Solaize, France S Supporting Information *

ABSTRACT: A new pathway for the production of p-toluic acid and alkyl-p-toluates is described here, starting from industrially relevant but underexploited renewable sorbic acid. A simple optimized Diels−Alder reaction between sorbic acid esters and ethylene, followed by aromatization, afford a full selectivity to para-substituted toluates. p-Toluates and hydrolyzed p-toluic acid are industrial terephthalic and terephthalates precursors, which are major commodity chemicals in the packaging and fiber industries after copolymerization with diols.

INTRODUCTION Since the end of the 20th century, the awareness of the increasing depletion of fossil resources and environmental concerns has prompted academia and industry to imagine durable alternative raw materials and transformations. Valorization of renewable carbon contained in abundant biomasses such as vegetable oils,1 lignocellulosic biomass,2 or food residues3,4 has attracted growing interest to produce fuels, commodities, and specialty chemicals. Conjugated to the revolution of shale gas inducing retrofitting of naphtha crackers to gas crackers, aromatics belong to one of the most strongly impacted hydrocarbon cuts, and biomass could be all the more valuable in this context.5 Alternatives to oil-sourced aromatics, especially selective routes leading to terephthalic acid (TPA), the biggest market involving aromatics produced at ∼40 Mt/y, are highly desirable. The growth is moreover forecasted to be 5%/y until 2020.6 Since TPA/esters are exclusively synthesized industrially via the oxidation of p-xylene7 via toluic acid/esters,8 the latter also constitute first-class targets to be produced from renewable sources. Biobased routes are furthermore intensively desired by the packaging industry, such as The Coca-Cola Company, driving the research to durable solutions.9,10 Two main strategies have been elaborated to afford terephthalic acid or p-xylene via biobased pathways. First, biop-xylene can be extracted from a mixture of hydrocarbons resulting either from aqueous phase reforming of sugars and subsequent acid-catalyzed condensations (branded by Virent11 as BioFormate) or from the catalytic fast pyrolysis of lignocellulosic biomass11,12 (currently in development by Anellotech). Second, dedicated routes to TPA or precursors are also intensively studied. Colonna et al. elaborated a simple three-step valorization of biolimonene to p-xylene via pcymene,3 which implies carbon loss after oxidation. Gevo has developed a route from glucose to p-xylene via yeast-based fermentation to isobutanol, dehydration to isobutene, and dimerization/dehydrocyclization to xylenes.13 Selectivity to pxylene is higher than the two previous processes but separation of p-xylene from other aromatics is still needed. © 2015 American Chemical Society

More selective reconstructions of the required disubstituted aromatic ring thus often rely on a Diels−Alder cycloaddition in numerous studies: methyl coumalate reaction with long-chain terminal olefins (nC > 7) described by Kraus et al.14 surprisingly selectively affords p-alkyl-methylbenzoates without meta isomers. Nevertheless, the oxidation conditions of the long alkyl chain into a simple carboxylic group are not proven and would involve an important loss of carbon. Fully selective obtentions of biobased p-xylene, p-toluic acid/esters, or TPA/ esters consequently lean on suitable biosourced C6 dienes cycloaddition with ethylene as a dienophile. For instance, using ethylene and 2,4-hexadiene derived from ethylene, favorably biosourced by the dehydration of bioethanol, Brookhart et al. proposed an elegant multistep pathway to p-xylene.15 Disproportionation of 1-hexene into hexadiene still demands a parallel valorization for the coproduced hexane. A typical example that has attracted much interest is the cycloaddition between sugar-derived 2,5-dimethylfuran (2,5-DMF) and ethylene, leading selectively to p-xylene in high yields after in situ dehydration (see Figure 1, Route (1)).16−20 Acrolein has also been used as a dienophile with 2,5-DMF, but a subsequent step of decarboxylation is needed.21 Oxidized versions of 2,5DMF have also been screened by Davis et al. with ethylene, with interesting selectivities, but global yields still need improvement.22 The production of 5-hydroxymethylfurfural from hexoses, a precursor of 2,5-DMF, remains problematic for isolation and purification reasons.23 The more commercially available furfural has been challenged by Kasuya et al.24 as a raw material for production of bioterephthalic acid, but limited yield after ∼10 steps makes this process hardly affordable. Another bio-based alternative has been described by Frost et al.:25 an original yeast-based fermentation converts glucose to muconic acid (Figure 1, Route (2)). Reaction between alkyl muconates and Received: Revised: Accepted: Published: 7164

May 29, 2015 July 8, 2015 July 8, 2015 July 8, 2015 DOI: 10.1021/acs.iecr.5b01972 Ind. Eng. Chem. Res. 2015, 54, 7164−7168

Research Note

Industrial & Engineering Chemistry Research

were purified using a MBraun Solvent Purification System (SPS-MBraun). Product Characterization. All reactants and products were characterized by gas chromatography (GC) analysis on an Agilent Model 6890 GC apparatus equipped with a Model CPWAX 57-B column. Structure of new products were validated by gas chromatography−mass spectrometry (GC-MS). Quantitative analyses of esters and aldehyde reactants and products were performed using GC, thanks to calibration curves of the commercial products ethyl sorbate, ethyl p-toluate, and 2,4hexadienal. Quantitative analyses of carboxylic acids reactants and products were performed via NMR after evaporation of the solvent and dilution with a deuterated solvent. NMR spectra were recorded at room temperature on a Bruker Model AV 300 spectrometer (1H: 300 MHz; 13C: 75 MHz). Chemical shifts are reported in δ (parts per million), relative to tetramethylsilane, and referenced to the residual 1H/13C of the deuterated solvent. Diels−Alder Reaction. Diels−Alder reactions were performed in a 250 mL Parr reactor that was equipped with a pressure and temperature sensor. Diene (18.5 mmol) was solubilized in 120 mL of toluene. Dodecane was added as an internal standard. Reactor was then pressurized under 5 bar ethylene at ambient temperature and heated up to 150 or 180 °C. When the targeted temperature is reached, the pressure is adjusted to 20 or 40 bar. After sufficient reaction time, reactor is cooled to 20 °C and depressurized, and products are characterized by GC in the bulk. NMR spectra are recorded after evaporation of the solvent under reduced pressure and dilution in deuterated solvents. The conversion of diene is calculated from GC data analysis using the relationship

Figure 1. Biobased selective routes to terephthalic acid (TPA)/esters.

ethylene affords esters of cyclohex-2-ene-1,4-dicarboxylic acid, which can be aromatized to dialkyl terephthalate. Despite a short number of steps from sugar to TPA, muconic acid can only be produced by genetically modified strains. Genetic modifications are still currently demanding much efforts to optimize the productivity and sensitivity of the micro-organisms to muconic acid.26 Inspired by these transformations and willing to overcome the limitations enounced in the different approaches, here we propose an original, atom-economic, and rapid transformation route from biosourced sorbic acid via Diels−Alder cycloaddition of alkyl sorbate with ethylene to yield alkyl p-toluate. Sorbic acid can indeed be directly extracted from nonedible berries of Sorbus aucuparia. Its inhibitory effects against a wide spectrum of yeasts, molds, and bacteria, including most foodborne pathogens, were recognized in the early 1950s. Its alkaline potassium salt has identical properties and both are industrially comanufactured. Sorbic acid and sorbates production is growing very fast and has been multiplied 10-fold in the last 15 years to reach 30 kt/y, with the production being located primarily in China.27 Their current commercial success is related to their use as preservatives (E200 and E202 for sorbic acid and potassium sorbate, respectively) for substitution to benzoates, the latter being progressively withdrawn from the market for safety reasons. Since quantities extracted from the Sorbus aucuparia are limited, biosourced sorbic acid can also be synthesized from bioethanol after oxidation into acetaldehyde, trimerization to 2,4-hexadienal (sorbaldehyde), and final oxidation to sorbic acid. This industrial pathway was developed and has been exploited for decades by Union Carbide to overcome safety issues connected with the ketene−crotonaldehyde route.28 Our goal is to take advantage of the sorbic acid route to integrate p-toluic acid or alkyl p-toluates as intermediates for production of TPA and dimethylterephthalate (DMT), respectively, by the Amoco and Dynamit-Nobel processes. (See Figure S1 in the electronic Supporting Information (ESI).) Alkyl p-toluates that have been isolated, thanks to our new proposition, can also be valorized as plasticizers29 in poly(vinyl chloride) as substitutes to alkyl phthalates and benzoates.

Conv(diene) (%) =

[diene]0 − [diene]t × 100 [diene]0

The selectivity in Diels−Alder cycloaddition product is estimated from GC areas and is calculated as the GC area of desired cycloadduct divided by the sum of the areas of all compounds produced: Selec(adduct) (%) =

AreaGC(adduct) × 100 SumAreasGC(all products)

The yield in Diels−Alder cycloaddition product is calculated by multiplication of the conversion and the selectivity: Yield(adduct) (%) = Conv × Selec × 100

Aromatization Reaction. Aromatization reactions were performed in a 250 mL Parr reactor that was equipped with a pressure sensor and a temperature sensor. A quantity of cycloadduct (12.0 mmol) is dissolved in 120 mL of cyclohexane. Five hundred milligrams (500 mg) of 5 wt % Pt/C are added. An air pressure of up to 16 bar is applied, and the reactor is heated at 150 °C during 24 h. Reactor is then cooled to 20 °C and depressurized, and the products are characterized by GC. Conversion of the adduct is calculated from GC areas by the relationship

EXPERIMENTAL SECTION Materials. Sorbic acid (99%), 2,4-hexadienal (95%), and ethyl sorbate (98%) were supplied by Sigma−Aldrich and used as received. The majority of all dienes are composed of the trans,trans isomer, as illustrated in all schemes. Quantities of 5 wt % Pt/C, ZnCl2, Yb(OTf)3, and Sc(OTf)3 were supplied by Sigma−Aldrich and used as received. Toluene and cyclohexane

Conv(adduct) (%) =

[adduct]0 − [adduct]t × 100 [adduct]0

The yield in desired aromatic product is calculated from GC area determination by the relationship 7165

DOI: 10.1021/acs.iecr.5b01972 Ind. Eng. Chem. Res. 2015, 54, 7164−7168

Research Note

Industrial & Engineering Chemistry Research Yield(aromatic) (%) =

n(aromatic) × 100 n(adduct)0

RESULTS AND DISCUSSION A prescreening of diene substrates was first performed, comprised of sorbic acid, sorbaldehyde, and ethyl sorbate, in reaction with ethylene. Ethyl sorbate was chosen as a benchmark ester for commercial availability reasons. Reactivity is expected to be identical with industrially relevant methylsorbate. Since ethylene is a nonactivated dienophile and sorbic derivatives contain both moderate electron-withdrawing and electron-donating groups, high temperature and pressure were first applied to promote the Diels−Alder cycloaddition. In solvents in which substrates were unsoluble (such as water), only poor conversions could be obtained. First attempts were thus performed in toluene at 150 °C without a catalyst under a total pressure of 20 bar. Conversion and selectivity in corresponding cycloadduct is first determined after 18 h. Results are summarized in Table 1 (see the Experimental Section for calculation details).

Figure 2. Diels−Alder reaction between ethyl sorbate (1) and ethylene to yield ethyl 4-methylcyclohex-3-enoate (2).

and 10% extent, respectively, with Yb(OTf)3 and Sc(OTf)3 (see Figure S5 in the ESI). Isomerization of double bonds prevents a suitable 2,4 insaturation configuration for [4 + 2] cycloaddition with ethylene. The best results were achieved without a catalyst while temperature was increased to 180 °C. Higher temperatures lead to degradation of the substrate with the formation of oligomeric products. Positive effects were observed at 180 °C through improvement of ethyl sorbate conversion from 21% to 41% after 18 h, whereas the selectivity in 2 remained superior to 90%. To further improve the yield in cycloadduct and accelerate this slow reaction, the total pressure was increased from 20 bar to 40 bar at 180 °C, increasing the partial pressure of ethylene and, consequently, its solubility in toluene, corresponding, in our conditions, to a 10-fold excess of dienophile. This consequently benefited the reaction kinetics while improving the conversion and selectivity, to values of 66% and 97%, respectively, after 18 h. These improvements encouraged us to lengthen the reaction time under optimized temperature and pressure conditions. Kinetic monitoring within 120 h is presented in Scheme 1. Maximum conversion of ethyl sorbate can be reached after 40 h at 180 °C under 40 bar of ethylene, while maintaining a constant selectivity in 2 (as high as 97%). Therefore, here, we describe, for the first time, a Diels−Alder reaction between ethyl sorbate and ethylene without catalyst at 180 °C affording desired cycloadduct 2 in almost quantitative yields. A simple distillation of 2 allowed us to isolate it in a purity of >99% in 82% isolated yield (see the NMR spectrum shown in Figure S6 in the ESI). On our way to targeting the production of p-toluic acid and alkyl p-toluates from sorbic acid, aromatization (dehydrogenation) of 2 has been carried out to yield ethyl p-toluate (3), as depicted in Figure 3. Here, quantitative aromatization described by Brookhart et al.15 for conversion of 3,6-dimethylcyclohexene to p-xylene at 400 °C catalyzed by Pt/Al2O3 is inapplicable to ester-functionalized adducts without degradation of the reactant. Oxidative dehydrogenation conditions under air pressure were thus directly inspired from aromatization of cycloadduct between muconate dimethylester and ethylene.25 Distillated 2 was dissolved in cyclohexane in the presence of 5 wt % Pt/C. After temperature and air pressure optimization, the best results were obtained at 150 °C and pressurized under air at 16 bar. After 24 h of reaction, conversion of 2 reached 80% with a selectivity for 3 of 41%, giving access to ethyl ptoluate in a 30% isolated yield after column chromatography purification (see Figure S8 in the ESI). The major coproduct corresponds indeed to the hydrogenated ethyl 4-methylcyclohexanoate (4), as expected by hydrogen transfer between substrates (see Figure S7 in the ESI). Optimization of the aromatization step and separation of the products need improvement and are currently under active investigation with different dehydrogenation methods.31

Table 1. Dienes Screening for Diels−Alder Reaction with Ethylene and Yield in the Respective Cycloadducta entry


conversion (%)

selectivity (%)

yield GC (%)

1 2 3

sorbic acid sorbaldehyde ethyl sorbate

4 18 14

0 27 90

0 5 13


Conditions: [diene] = 0.15 M in toluene, 150 °C, 20 bar, 18 h.

After 18 h, only clues of sorbic acid degradation were observed by 1H NMR analysis (see Table 1, Entry 1, as well as Figure S2 in the ESI). No traces of cycloaddition product were detected by NMR, indicating a lack of reactivity of the acidic reactant. Its low initial solubility in toluene at room temperature could be one of the reasons for such a low conversion. Reaction has been conducted in water without any improvement in conversion. The conversions of sorbaldehyde and ethyl sorbate are higher than their sorbic acid analogues (see Table 1, Entries 2 and 3, as well as Figures S3 and S4 in the ESI). Although their conversion extents are similar, the selectivity to their respective Diels−Alder cycloadduct is much better in the case of ethyl sorbate. This order of reactivity is in good agreement with prior literature dealing with Diels−Alder reactions between sorbic derivatives with quinones as dienophiles.30 Products corresponding to polycondensates of sorbaldehyde were detected, indicating the sensitive nature of the aldehyde at this temperature. For reactivity and selectivity reasons, ethyl sorbate (1) was consequently chosen for reaction optimization. In the route of biosourced sorbic acid transformation to p-toluic acid and p-toluates, esterification to alkyl sorbate seems to be an unavoidable step. The temperature, ethylene pressure, and influence of some Lewis acid catalysts were screened to maximize the yield in ethyl 4-methylcyclohex3-enoate (2) (Figure 2). Results are reported in Table 2. The use of Lewis acidic catalysts did not show any promotion activity upon conversion; the activation of substrates was insufficient (Table 2, Entries 2− 4). Their performances were unchanged in the presence of zinc chloride, whereas their selectivities to the desired cycloadduct were seriously damaged with rare-earth triflates. Analyses of gas chromatograms revealed isomerization of ethyl sorbate in a 5% 7166

DOI: 10.1021/acs.iecr.5b01972 Ind. Eng. Chem. Res. 2015, 54, 7164−7168

Research Note

Industrial & Engineering Chemistry Research Table 2. Screening Conditions for the Diels−Alder Reaction between Ethyl Sorbate and Ethylene after 18 ha temperature, T (°C) 1 2 3 4 5 6 a

150 150 150 150 180 180


total pressure (bar)

conversion (%)

selectivity (%)

yield GC (%)

20 20 20 20 20 40

14 13 16 21 41 66

90 85 68 51 91 97

13 11 11 11 37 64

ZnCl2 Yb(OTf)3 Sc(OTf)3

Conditions: [ethyl sorbate] = 0.15 M in toluene, 150 °C, 20 or 40 bar, 18 h. bIf present, the molar ratio of catalyst to substrate is 1%.

NMR spectra of reactants and products are also shown. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01972.

Scheme 1. Optimized Conditions for Diels−Alder Reaction between Ethyl Sorbate and Ethylenea


Corresponding Author

*Tel.: +33-4-37-70-24-25. E-mail: [email protected] Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank IFPEN for permission to publish this work.


Conditions: [ethyl sorbate] = 0.15 M, 180 °C, 40 bar, 120 h.

Figure 3. Dehydrogenation of Diels−Alder adduct (2) to ethyl ptoluate (3).

CONCLUSIONS In conclusion, here, we have presented an original, atomeconomic, and selective transformation of biosourced and industrially relevant sorbic acid into ethyl p-toluate via quantitative ethyl sorbate Diels−Alder addition with ethylene. Diels−Alder cyclization proceeds very selectively with quantitative yield without catalyst. The aromatization step must be improved concerning the selectivity to aromatics versus hydrogenated compounds, but ethyl toluate could be synthesized in a global 30% yield from ethyl sorbate in two steps. Isolated p-toluic acid and p-toluates could also be oxidized thereafter to afford 100% sorbic acid based bioterephthalic acid and biodimethyl terephthalate. Eventually, access to 100% biopoly(ethyleneterephthalate) would be reachable after copolymerization with biosourced ethylene glycol32 derived from bioethanol or from direct transformation of lignocellulose33 or glucose34 to ethylene glycol. This solution would participate in a global set of alternatives to substitute major commodity fossil chemicals, such as TPA or DMT, with biosourced equivalents.


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S Supporting Information *

TPA and DMT industrial process pathways are provided. Gas chromatography−flame ionization detection (GC-FID) and 1H 7167

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Semicontinuous Reaction System. Ind. Eng. Chem. Res. 2013, 52 (28), 9566−9572.

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DOI: 10.1021/acs.iecr.5b01972 Ind. Eng. Chem. Res. 2015, 54, 7164−7168