Oxidative Depolymerization of Lignin Using a Novel Polyoxometalate

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Oxidative depolymerization of lignin using a novel polyoxometalate-protic ionic liquid system Gilbert Francesco De Gregorio, Raquel Prado, Charles Vriamont, Xabier Erdocia, Jalel Labidi, Jason P. Hallett, and Tom Welton ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01339 • Publication Date (Web): 21 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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ACS Sustainable Chemistry & Engineering

Oxidative depolymerization of lignin using a novel Polyoxometalate-protic ionic liquid system.

Gilbert F. De Gregorio,a Raquel Prado,a Charles Vriamont,b Xabier Erdocia,c Jalel Labidi,c Jason P. Hallettb and Tom Welton*a a

b

Department of Chemistry, Imperial College London, Exhibition Road, London, UK SW7 2AZ

Department of Chemical Engineering, Imperial College London, Exhibition Road, London, UK SW7 2AZ

c

Department of Chemical Engineering and Environment University of Basque Country, Plaza Europa 1, Donostia -San Sebastian, Spain, 20018 *corresponding author: [email protected]

Keywords: Vanillin, POM, Ionic Liquids, Catalysis, Lignin Valorization, Oxidation, Depolymerization, Platform Chemicals, Lignocellulosic Biomass,

Abstract: Oxidative depolymerization of lignin obtained from pine and willow can be achieved in a novel system encompassing the ionic liquid (IL) 1-butylimidazolium hydrogensulfate coupled with a vanadium based polyoxometalate (POM) under oxygen rich conditions. Along with an array of phenols and functionalized aromatics, vanillin and syringaldehyde were the main products extracted from the IL. The overall yield of aldehyde products were shown to be higher on lignins obtained with shorter pretreatment times, with vanillin being the exclusive aldehyde product obtained from pine. In the presence of molecular oxygen, the highest yield of aldehyde products was obtained when 5 wt % of the POM relative to the IL was employed and constituted the major product in the extracted oils. This system succeeds in exploiting the ability of ILs to depolymerize lignin and the remarkable properties of the POM to oxidize the lignin fragments into useful platform chemicals.

Introduction

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Lignin is a unique and highly abundant natural source of aromatic compounds, representing roughly 20% of the total mass of the planet’s biosphere1 and could potentially be a suitable alternative to petroleum feedstocks for the production of plastics, materials and synthetic intermediates. Due to its heterogeneity, high energy density and high chemical stability, most lignin is currently used as a feedstock for combustion; however, few markets currently exist where lignin is valorized beyond its application as a combustible fuel. A number of reviews have explored the different catalytic processes and the applications of lignin in obtaining high value, aromatic chemicals.2,3,4,5 Amongst all the catalytic transformations, oxidative technologies offer a large variety of valuable, functionalized aromatics, particularly benzaldehyde derivatives such as vanillin and syringaldehyde.

In most oxidative lignin depolymerization strategies, vanillin has been identified as one of the major bio-based aromatic products that can be obtained. Along with syringaldehyde and phydroxybenzaldehyde, vanillin originates from a single sub-unit in lignin and is more abundant in softwoods due to the low Syringyl:Guaiacyl (S:G) unit ratio. The functionality around the aromatic ring allows it to be a versatile platform chemical where through simple synthetic transformations, a number of useful polymers can be prepared.5

Polyoxometalates in delignification chemistry The high flexibility in the modification of the redox properties of POMs, in addition to their ease of handling, preparation, low cost and low toxicity have allowed POMs to flourish as oxidation catalysts. A further subclass of the Keggin structures are the mixed vanadium (V) substituted POMs and a series of studies have been reported, using molybdovanadophosphate POMs, for the oxidation of starch and lignin.6,7,8,9

One study which aimed to simultaneously delignify pine and oxidize lignin was achieved in a one-pot process, as reported by Rogers et al.10 [C2C1im][OAc], typically used in the ionic liquid biomass dissolution process,11,12 was coupled with the POM H5PV2Mo10O40 in an oxygen rich environment. Results showed that this system facilitated the dissolution of the biomass substrate, delignification, separation of the hemicellulose component and the oxidation of lignin into a range of small aromatics. Extraction with benzene and THF yielded methyl vanillate as the major lignin oxidation product, as


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well as smaller quantities of other oxidized aromatics. The POM has been hypothesized to undergo reduction during lignin oxidation, with vanadium reducing from VV to VIV with no reported structural change as presented in equations 1 and 2:10

Lignin + [PV(V)Mo10 O40]5- Ligninox + [PV2(IV) Mo10 O40 ]7-

Equation: 1

[PV2 (IV)Mo10 O40]7- + O2 + 4H+ [PV2(V) Mo10 O40]8- + 2H2O

Equation 2

This study presents the novel and attractive properties of heteropolyanionic POMs in a protic ionic liquid for the oxidative depolymerization of Ionosolv lignin samples.11,13,14,15 The POM, H5PV2Mo10O40 was selected to be the catalyst of choice and [HC4im][HSO4] as the IL. The preparation of the POM and all experimental procedures can be found in the SI.

Results and Discussion: Pretreatment of Willow and Pine Two different lignocellulosic feedstocks were selected for oxidative depolymerization studies; pinus sylvestris pine as the substrate for softwood lignin and salix viminalis willow for hardwood lignin. This assessed the importance of the lignin starting material on the array of products potentially released upon oxidative depolymerization. The pretreatment experiments were performed following established Ionosolv protocols using hydrogensulfate ILs14,15 and the feedstocks obtained are summarized in Table 1. Table 1 – Lignin substrates obtained from pretreatment of lignocellulosic biomass at 100 °C following protocol described in reference [11] using triethylammonium hydrogensulfate IL. Entry 1 2 3 4

Lignin Source Pine Willow Willow Willow

Pretreatment time 30 minutes 4 hours 8 hours 12 hours

Lignin Code 30MP 4HW 8HW 12HW

Lignin Characterization via 31P NMR These four lignin samples were characterized using 31P NMR spectroscopy using the phosphitylating agent TMDP used as a probe with the peaks assigned to the different hydroxyl groups present in lignin according to literature.16 The peaks were assigned as follows: the carboxylic acid OH at 134.75 ppm , p-hydroxyphenyl at 137.6 ppm, guaiacyl at 139.22 ppm, syringyl at 141.73 ppm, the internal standard



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at 144.70 ppm and the aliphatic OH at 147.23 ppm. This was carried out to determine the relative syringyl and guaiacyl ratios between the lignin samples obtained from pine and willow, to assess whether the concentrations of the phenolic hydroxyl groups vary in lignin obtained from willow at different pretreatment times and to also track the change in concentration of the aliphatic hydroxyl groups. An example of the 31P NMR spectra is given in Figure 1 with the results summarized in Figure 2.

Figure 1: 31P NMR of phosphitylated Ionosolv lignin pretreated from pine.


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20 18 Hydroxyl group content (mmol/g)

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16 14

Syringyl

12

Guaiacyl p-hydroxyphenyl

10

Carboxylic Acids Total Phenolics

8

Total Aliphatic OHs

6

Total Hydroxyls 4 2 0 pine 30 mins

willow 4 hr

willow 8 hr

willow 12 hr

Figure 2. Concentration of different hydroxyl groups in pretreated biomass. Error bars originate from the standard deviation from triplicate integrations of signals in 31P NMR spectra.

A low S:G ratio was identified in the lignin sample obtained from 30MP, with an overall ratio of circa 1:5. Equally, the p-hydroxyphenyl hydroxyl group was comparatively low in abundance, accounting for only 6 % of the total amount of phenolic hydroxyl groups. In all willow samples pretreated at different times, the S:G ratio was higher, with the maximum ratio identified after a 4 hour pretreatment time where the overall ratio was calculated to be circa 3:1. The S:G ratio was observed to decrease with time to circa 2:1 after a 12 hour pretreatment time. In all cases, the overall abundance of phenolic hydroxyl groups exceeded the abundance of aliphatic hydroxyl groups. In the case of Willow, with an increase in pretreatment time, the abundance of aliphatic hydroxyl groups decreased with the simultaneous increase in phenolic hydroxyl groups. This has been suggested by Brandt et al to be due to the increased cleavage of the β-O-4 linkages,13 yielding phenolic alcohols which remain inert post cleavage. The decrease in the abundance of aliphatic hydroxyl groups was assumed to be due to the dehydration reactions occurring prior to the hydrolysis of the β-O-4 linkages and the removal of hemicelluloses attached to the lignin. The small excess in abundance of the total phenolic alcohols compared to aliphatic alcohols seen in the pine sample, suggests that after a 30 minute pretreatment,



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most of the β-O-4 linkages are intact and that the lignin is still able to undergo further depolymerization. A comparison between the pine and willow lignin samples shows an overall higher hydroxyl group content with the willow samples.

Oxidative Depolymerization of using POMs in [HC4im][HSO4] The lignin samples were submitted to catalytic oxidation reactions using the POM dissolved in [HC4im][HSO4], following a similar protocol to that described by Prado et al17 with molecular oxygen and hydrogen peroxide selected as oxidants. In all cases, once the reaction went to completion, water was added to precipitate the catalyst and any unreacted lignin. The suspensions were centrifuged to remove the aqueous phase and ethyl acetate was used to extract the aromatic products from the reaction mixture. Once the ethyl acetate was removed, an oil was obtained which was then analyzed by GC-MS. Through the use of a calibrated library of compounds, the abundance of 17 aromatic compounds, previously identified as common oxidation products from lignin, could be determined including a range of phenols and functionalized aromatics. The effects of catalyst loading and oxidant type were first assessed on lignin obtained from 4HW. A series of GC-MS chromatograms is presented in Figure 3, showing the effect of POM loading on the oxidation of 4HW lignin with molecular oxygen.


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Figure 3: GC-MS chromatograms of lignin obtained from 4HW, oxidized by molecular oxygen for 5 hours with different loadings of the POM catalyst at 100 °C

The absence of detectable compounds when no catalyst is present indicates the requirement of the POM catalyst to allow the reaction to proceed. However, the relatively low yield of products when 20.6 wt% catalyst is used, leads to a decrease in the abundance of detectable compounds. This indicates that catalyst loading must be finely tuned in order to optimise the oxidative degradation activity of the lignin.

All oils were composed of a complex mixture of compounds that originated from lignin, carbohydrates and the IL. In these studies, selectivity towards aldehyde products was observed, with very small quantities of the carboxylic acids detected. The array of calibrated compounds identified in the oil extracted from oxidized lignin obtained from 4HW at different catalyst loadings is presented in Table 2. Apart from the calibrated compounds, other substances were detected including N-butylformamide, from the oxidation of the IL at 6.342 min, contamination of the oil by the IL cation as 1-butylimizadole was detected at 9.984 min and finally sort chain lipids were also observed within the oil composition as linoleic acid ethyl ester was detected at 24.699 and octadecamide 26.455 min.

Table 2 – Calibrated aromatic products found in obtained oils post oxidative depolymerization of lignin obtained from 4HW oxidized by molecular oxygen for 5 hours with 5 wt % of the POM catalyst to the IL at 100 °C Compound

Retention time

1 wt % POM to IL (ppm)

5 wt % POM to IL (ppm)

Phenol Guaiacol Syringol Vanillin Acetovanillone Syringaldehyde Acetosyringone

5.57 7.26 13.7 14.9 16.7 19.4 20.3

12.6 4.06 7.00 24.5 1.18 56.1 1.60

69.3 15.4 52.2 65.3 3.04 201.0 4.40

10 wt % POM to IL (ppm) 141.0 30.7 91.4 44.3 4.39 141.0 5.58

20 wt % POM to IL (ppm) 37.8 29.2 58.5 59.0 3.85 153.0 3.82

Syringaldehyde was observed to be the compound present in highest abundance at 201 ppm at 5 wt % POM loading, constituting 3.7 wt % of the detected products in the oil. This confirmed the successful activity of the reaction to oxidize the syringyl units in hardwood. In all reactions, syringaldehyde was



identified as being the most abundant aromatic product present in the oils. 0.25

Yield with respect to lignin (%)

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0.2

Vanillin yield (O2 oxidant) Vanillin yield (H2O2 oxidant)

0.15 Syringaldehyde yield (O2 oxidant) 0.1

Syringaldehyde yield (H2O2 oxidant) Guaiacol yield (O2 oxidant)

0.05 Guaiacol yield (H2O2 oxidant) 0 0

5 10 20 Catalyst screening (wt % to IL)

Figure 4: Oxidative depolymerization of 4HW with different POM loadings in IL, stirring at 500 rpm, at 100 °C left for 5 hours, using molecular oxygen and hydrogen peroxide as oxidants. Error bars originate from the error in calibration of GC-MS for each compound.

Figure 4 presents a comparison of the vanillin and syringaldehyde yields when molecular oxygen and hydrogen peroxide were employed as oxidants. The highest yields of both vanillin and syringaldehyde were obtained with a 5 wt % loading of the catalyst to the IL using molecular oxygen as the oxidant. In all reactions, the calculated yield of syringaldehyde greatly exceeded the yield of vanillin. This supports the results obtained from the

31

P NMR data for hydroxyl group characterization in lignin,

whereby a higher S:G ratio is observed. Furthermore, when the catalyst is present in the reaction medium, molecular oxygen generally showed improved performance compared to hydrogen peroxide as an oxidant.


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0.25

Yield with respect to lignin (%)

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0.2

0.15

Vanillin yield (H2O2 oxidant)

0.1

Syringaldehyde yield (H2O2 oxidant) Guaiacol yield (H2O2 oxidant)

0.05

0 0

5 10 20 Catalyst loading (wt % to IL)

Figure 5: Oxidative depolymerization of 30MP with different POM loadings in IL, stirring at 500 rpm, at 100 °C left for 5 hours, using hydrogen peroxide as oxidants. Error bars originate from the error in calibration of GC-MS for each compound.

A similar study was then carried out on lignin obtained from 30MP to obtain higher quantities of vanillin using hydrogen peroxide as presented in Figure 5. Once again, oxidation is favored with the presence of the POM catalyst where more than a 20 fold increase in vanillin is observed when 20 wt % of the catalyst is employed compared to the system with no catalyst. Furthermore, when no catalyst is present, a considerable amount of vanillic acid is detected, therefore demonstrating the importance of the POM to also mitigate the potent oxidizing ability of hydrogen peroxide. The absence of syringaldehyde as a product supports the results obtained from the 31P NMR data, where a lower S:G ratio provided increased yields of vanillin.

Optimization of the reaction in order to obtain higher yields of vanillin was attempted. Lignin obtained from both 30MP and 4HW were used as substrates and neither showed any detectable amounts of aromatic products when the temperature was increased to 150 ° C. One possibility is that the increased temperature may have caused the aldehyde products to engage in further reactions after oxidation, potentially yielding involatile solids. The effect of shorter reaction time at 100 °C was subsequently



investigated. The reaction was repeated on all four lignin samples over the course of an hour at 100 °C to assess whether such conditions may favor higher yields of aldehyde products. This also compared the importance of lignin substrate and is presented in Figure 6.

0.04 Vanillin (5 wt % cat. loading to IL)

0.035 Yield with respect to lignin (%)

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0.03 Syringaldehyde (5 wt % cat. loading to IL)

0.025 0.02

Vanillin (20 wt % cat. loading to IL)

0.015 0.01

Syringaldehyde (20 wt % cat. loading to IL)

0.005 0 Pine (30 mins)

Willow (4 h) Willow (8 h) Willow (12 h)

Figure 6: Oxidative depolymerization of all lignin samples with different POM loadings in IL, stirring at 500 rpm, at 100 °C left for 1 hour, using hydrogen peroxide as the oxidant. Error bars originate from the error in GC-MS calibration for each compound.

Lignin from 30MP was shown to provide the highest yields of aldehyde products and amongst all the lignin samples obtained from willow; lignin from 12HW afforded the lowest yields of aldehydes. This was presumed to be due to the increased amount of condensation products precipitated from the liquor, where there is a decrease in abundance of β-O-4 linkages and an increase in the abundance of the more robust C-C bonds. In terms of the overall quantities of vanillin and syringaldehyde produced and extracted, the yields were calculated to be an order of magnitude lower than when the reaction is left over a 5 hour period. This shows that the reaction conditions must be finely tuned in order to obtain the optimal amount of aldehyde product, with 5% catalyst loading under oxygen left for 5 hours at 100 °C currently identified providing the highest yield of product.


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Conclusions A novel system encompassing a POM catalyst in the protic ionic liquid [HC4im][HSO4] was employed with molecular oxygen and hydrogen peroxide to depolymerize lignin samples extracted from pine and willow. Vanillin and syringaldehyde were identified as the most abundant aromatic products in all samples with no indication of over-oxidation to the carboxylic acid. The highest yields were obtained when molecular oxygen was used as an oxidant along with a 5 wt % loading of the catalyst. Characterization of these samples showed that the balance of S:G ratio was reflected in the distribution of syringaldehyde and vanillin yields and longer pretreatment times were shown to decrease the overall yield of the aldehyde products, with the highest yields obtained with pine obtained after a 30 minute pretreatment at 150 °C. These preliminary studies succeed in showing the novel synergistic properties of vanadium based POM catalysts in protic ionic liquids and will encourage further exploitation of these systems to obtain high quantities of useful platform chemicals from ionosolv lignin.

Supporting information: Full experimental for analysis of lignin fractions via 31P NMR, preparation of polyoxometalate catalyst, oxidative depolymerisation process of lignin and GC-MS analysis of oils.

Acknowledgements: The authors would like to thank the Engineering and Physical Sciences Research Council (EP/EP/K014676/1) for funding for RP, CV, JPH and TW.

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Varanasi, P.; Singh, P.; Auer, M.; Adams, P. D.; Simmons, B. A.; Singh, S. Survey of renewable chemicals produced from lignocellulosic biomass during ionic liquid pretreatment. Biotechnol.



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Chen, X.; Souvanhthong, B.; Wang, H.; Zheng, H.; Wang, X.; Huo, M. Polyoxometalate-based Ionic liquid as thermoregulated and environmentally friendly catalyst for starch oxidation. Appl. Catal. B, Environ. 2013, 138-139, 161–166.

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Evtuguin, D. V.; Neto, C. P. Catalytic Oxidative Delignification with Keggin-Type Molybdovanadophosphate Heteropolyanions. Oxidative delignification Chem. 2001, 21, 342–355.

10. Cheng, F.; Wang, H.; Rogers, R. D. Oxygen Enhances Polyoxometalate-based Catalytic Dissolution and Delignification of Woody Biomass in Ionic Liquids. ACS Sustain. Chem. Eng. 2014, 2, 2859–2865. 11. Brandt, A.; Grasvik, J; Hallett, J.P; Welton, T.; Deconstruction of lignocellulosic biomass with ionic liquids. Green Chem. 2013, 15, 550-583 12. Li, W.; Sun, N.; Stoner, B.; Jiang, X.; Lu, X.; Rogers, R.D.; Rapid dissolution of lignocellulosic biomass in ionic liquids using temperatures above the glass transition of lignin. Green Chem. 2011, 13, 2038-2047 13. Brandt, A.; Chen, L.; van Dongen, B. E.; Welton, T.; Hallett, J. P. Structural changes in lignins isolated using an acidic ionic liquid water mixture. Green Chem. 2015, 17, 5019–5034. 14. Verdía, P.; Brandt, A.; Hallett, J. P.; Ray, M. J.; Welton. Fractionation of lignocellulosic biomass with the ionic liquid 1-butylimidazolium hydrogen sulfate. T. Green Chem. 2014, 16, 1617-1627. 15. Brandt, A.; Ray, M. J.; To, T. Q.; Leak, D. J.; Murphy, R. J.; Welton, T. Ionic liquid pretreatment of lignocellulosic biomass with ionic liquid–water mixtures. Green Chem. 2011, 13, 2489-2499. 16. Pu, Y.; Cao, S.; Ragauskas. A. J. Application of quantitative 31P NMR in biomass lignin and biofuel precursors characterization. Energy Environ. Sci., 2011, 4, 3154-3166 17. Prado, R.; Brandt, A.; Erdocia, X.; Hallet, J.; Welton, T.; Labidi, J. Lignin oxidation and depolymerisation in ionic liquids. Green Chem. 2016, 18, 834–841.


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Table of Contents graphic page:

Oxidative depolymerisation of lignin using a novel Polyoxometalate-protic ionic liquid system. Gilbert F. De Gregorio,a Raquel Prado,a Charles Vriamont,b Xabier Erdocia,c Jalel Labidi,c Jason P. Hallettb and Tom Welton*a

For Table of Contents Use Only:

Synopsis: Lignin can be valorized, yielding a range of functionalized platform chemicals using a non-toxic polyoxometalate catalyst in ionic media under oxygen rich conditions.



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20 18 Hydroxyl group content (mmol/g)

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16 14

Syringyl

12

Guaiacyl p-hydroxyphenyl

10

Carboxylic Acids

8

Total Phenolics

Total Aliphatic OHs

6

Total Hydroxyls 4 2 0 pine 30 mins

willow 4 hr

willow 8 hr

willow 12 hr


ACS Sustainable Chemistry & Page Engineering 16 of 19

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0.25


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Vanillin yield (O2 oxidant) Vanillin yield (H2O2 oxidant)

0.15 Syringaldehyde yield (O2 oxidant) 0.1

Syringaldehyde yield (H2O2 oxidant) Guaiacol yield (O2 oxidant)

0.05 Guaiacol yield (H2O2 oxidant) 0 0

5

10

20

Catalyst screening (wt % to IL)



0.25


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0.15

Vanillin yield (H2O2 oxidant)

0.1

Syringaldehyde yield (H2O2 oxidant) Guaiacol yield (H2O2 oxidant)

0.05

0 0

5

10

20

Catalyst loading (wt % to IL)


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0.04 Vanillin (5 wt % cat. loading to IL)


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0.03 Syringaldehyde (5 wt % cat. loading to IL)

0.025 0.02

Vanillin (20 wt % cat. loading to IL)

0.015 0.01

Syringaldehyde (20 wt % cat. loading to IL)

0.005 0 Pine (30 mins) Willow (4 h)

Willow (8 h) Willow (12 h)


Oxidative Depolymerization of Lignin Using a Novel Polyoxometalate

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