Vanillin Production from Lignin and Its Use as a Renewable

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Vanillin production from lignin and its use as a renewable chemical Maxence Fache, Bernard Boutevin, and Sylvain Caillol ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01344 • Publication Date (Web): 10 Dec 2015 Downloaded from http://pubs.acs.org on December 14, 2015

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Vanillin production from lignin and its use as a renewable chemical Maxence Fache a, Bernard Boutevin a, Sylvain Caillol *a

a

Institut Charles Gerhardt Montpellier UMR 5253 - CNRS, Université Montpellier, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier, France * Corresponding author: Sylvain Caillol, [email protected], +33 4 67144327

ABSTRACT The use of vanillin as building block for the chemical industry is discussed in this article. Vanillin is currently one of the only molecular phenolic compounds manufactured on an industrial scale from biomass. It has thus the potential to become a key-intermediate for the synthesis of bio-based polymers, for which aromatic monomers are needed to reach good thermo-mechanical properties. After a first part dedicated to the current sourcing of vanillin, this article focuses on the alkaline oxidation lignin-tovanillin process, reporting advantages and limits, discusses the various post-depolymerization methods for product isolation and finally examines the outlook for the wider use of vanillin as a key-building block for the chemical industry.

KEYWORDS Vanillin; lignin; biobased; phenolic; process

INTRODUCTION Currently, petro-based resources are raw materials for the vast majority of organic chemicals and polymers. However, the unavoidable shortage of non-renewable resources due to an ever-rising demand will be accompanied by major supply problems and price increases, leading to social repercussions difficult to predict. Finding alternative solutions from the biomass feedstock is thus a top priority. In this context, the concept of bio-refinery started gaining importance.1 Similar to the crude oil refinery, the idea is to (bio)chemically turn each component of the feedstock into a variety of useful products. Bio-based chemicals are already industrially available but most of them are aliphatic or cycloaliphatic, for instance derived from cellulose2, starch2 or triglycerides3. However, many key chemicals are aromatic compounds, ultimately derived from petroleum. The production of shale gas increased massively in recent years. Shale gas is lighter than petroleum, which forced many petro-chemical majors to adapt to this influx of lighter raw material.4 This change drastically decreased the yield of heavier co-products such as aromatics (as well as propylene and butadiene). Consequently, aromatics availability have decreased and their price have increased.5 Thus, there is a challenge for finding aromatic building-blocks derived from biomass. In Nature, aromatics are found as (poly)phenolics. The three major classes of phenolics from renewable resources are lignin, tannins, both extracted from wood, and Cashew NutShell Liquid (CNSL).6 Their relative commercial availability is displayed in Figure 1, along with other general sourcing information.

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Figure 1: Commercial availability of lignin7, 8, tannins9 and CNSL10, 11 There is a striking paradox between the enormous amount of (poly)phenolic materials available from biomass – especially lignin and tannins – and their relatively under-developed industrial use compared to aliphatic resources like vegetable oil. This might be explained by the fact that each of these resources has drawbacks limiting their use. More precisely, the volume of CNSL available is 125.000 tons per year.10 It makes it an undoubtedly interesting resource for niche applications, but this potential volume might not be sufficient to make it a reliable renewable resource. Also, CNSL is a mixture of phenolics all bearing a C15 alkyl chain on the metaposition, which gives CNSL-based polymers with low Tg. Lignin and tannins, on the other hand, are available in potentially enormous amounts from biomass. However they are highly complex poly(phenols). Their polymeric nature, difficulty of process, and variability depending on the plant, method of extraction etc. prevent them from being an industrial reality as raw materials. For the time being, handling molecular compounds isolated from the depolymerization of these feedstocks seems a more viable option. The depolymerization of lignin especially has been intensively studied recently.12, 13 Indeed, the large volumes of lignin produced annually prompted a number of works dealing with its catalytic depolymerization to smaller species. Of specific interest are the molecular compounds ferulic acid and vanillin. Ferulic acid can be obtained by alkaline or enzymatic hydrolysis of lignins.14 The main raw materials for its production are rice bran, sugarcane bagasse, and sugar beet pulp. Around 318 tons are produced yearly,15 which makes it the second most available molecular phenolic compound from lignin, far behind vanillin. Indeed, vanillin is the most available pure mono-aromatic phenol currently produced at an industrial scale from lignin.16 Around 20,000 tons of vanillin are produced per year17, 15% of which coming from lignin18 (around 3,000 tons/y). Thus, vanillin has the potential to be a key renewable aromatic building-block. The advantages of vanillin are numerous: it is a safe compound, aromatic, and it bears two reactive functions that can be chemically modified (the methoxy group being less reactive than the aldehyde and phenol functions). Vanillin can thus be considered as a difunctional compound, which is useful to prepare

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thermoplastic polymers. Indeed, raw resources like lignin usually are multifunctional. Preparing thermoplastics from them is thus more challenging than preparing thermosets. Vanillin is also already produced from lignin which means that it is renewable and does not compete with food sources. It is available on an industrial scale from well-described, ever improving processes. The aim of this article is to examine in detail the pieces of information available in the literature on the methods of vanillin production, especially on the lignin-to-vanillin processes. From this review, it is possible to highlight the potential of vanillin as a renewable chemical, especially in the field of polymer science, in which large volumes of raw material are necessary.

DESCRIPTION AND SOURCING Brief history Vanillin (4-hydroxy-3-methoxybenzaldehyde, m.p. 81-83°C) is the highest volume aroma chemical produced worldwide.19 It is produced from a variety of sources, namely oil (85%), woody biomass (15%), orchid pods ( 12) allow to reduce vanillin degradation whereas at lower pH values (< 11.5), vanillin losses by oxidation become more significant and a sharp decrease in vanillin yield is observed.18, 53 High concentrations of alkaline species are thus necessary. Decrease of pH leads to almost complete suppression of vanillin formation. This phenomenon was attributed to the protonation of reaction intermediates, more basic than the phenolics produced, (vanillin pKa = 7.4).54 Finally, the lignin itself is of course of importance: low molecular weight lignin tends to give better results52, the presence of residual sugars is highly unfavorable55, and the fewer transformations or chemical treatments lignin suffers, the better the phenolic aldehydes yield.51, 56 The latter point is of importance as the isolation procedure can modify the chemical structure of lignin. More specifically, if lignin separation is performed by acid precipitation, condensation reactions can occur, especially at the

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position 5 of the aromatic cycle in G units, which leads to bi-aryl structures56 (Figure 2), and decreases the yield of vanillin.18, 51

Figure 2: 5-5’ linkage arising from the condensation of two G lignin fragments GS lignins tend to give better depolymerization yields since they comprise more S units, more stable to condensation because of the substitution of the position 5 by a methoxy group. Alternative methods to obtain vanillin are still actively investigated in the literature.57-61 Depolymerization products The alkaline oxidative depolymerization of lignin gives a complex mixture of products23, as illustrated by Tables 1 and 2. These products can be classified into oligomeric products, phenolic, and non-phenolic compounds. Oligomeric products18 are either unreacted lignin fragments or come from condensation reactions as discussed above. The amount of oligomeric residue varies a lot depending on processing conditions and lignin type. In most studies, authors focus their efforts in detecting specific phenolic compounds of interest. These compounds are usually di-functional and include of course vanillin but also other aldehydes like syringaldehyde, acids, or ketones. The most studied di-functional molecular phenolic compounds obtained for G and GS lignins are detailed in Table 2:

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Table 2: Products of the alkaline oxidative lignin depolymerization usually studied in the cases of softwoods and hardwoods Compound

aldehydes

vanillin

syringaldehyde

carboxylic acids

p-hydroxybenzaldehyde

Formula

Typical yields in softwoods

Typical yields in hardwoods

6-12%

1-5%

52, 53, 62-64

50, 55, 62, 64-70

0-0.7%

4-16%

62

50, 55, 62, 64-70

0-0.5%

0-0.5%

62, 64, 71

50, 62, 64-66, 69

vanillic acid

0.5-1.5% 64

55, 64, 67, 68, 70

syringic acid

N.D.

acetovanillone

0.6-6.4%

0.3-2.6%

62, 71

50, 62, 65, 66, 68

acetosyringone

N.D.

0.2-2.4%

0.5-3.9% 55, 64, 67, 68, 70

ketones

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1.5-4.2% 50, 62, 65, 66, 68

A non-exhaustive list of other compounds found in lignin alkaline oxidative depolymerization mixtures and less often studied in detail is given in Table 3. Interestingly mono-phenolics such as guaiacol or syringol are often mentioned as depolymerization products but their amount is usually not given.

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Another compound of interest is the dehydrodivanillin, a by-product of vanillin production formed by aromatic oxidative coupling of two vanillin molecules. Being an aromatic di-functional phenol, this compound might have many applications, especially in the field of bio-based polymers.72 Given the high pH of the crude mixture, all phenolics are present as phenolates, difficult to extract from water and/or isolate. The reaction mixture also contains non-phenolics such as lactones23 or the other compounds shown in Table 3: Table 3: Other products of the alkaline oxidative lignin depolymerization found in literature Compound

Phenolics

Formula

Reference

Phenol

73

Benzoic acid

65

Hydroxybenzoic acid

65

Guaiacol

23, 50, 62, 73

Catechol

50, 73

3-Methoxy catechol

73

Syringol

23, 50, 62, 73

Pyrogallol

73

4-Methyl catechol

73

4-Methyl syringol

73

4-Ethyl catechol

73

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Other molecules

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2-(4-Hydroxy-3methoxyphenyl)acetaldehyde

55

2-(4-Hydroxy-3,5dimethoxyphenyl)acetaldehyde

55

Dehydrodivanillin

52

1,2-Bis(4-hydroxy-3methoxyphenyl)ethane-1,2-dione

52

1,2,3-Trimethoxy benzene

73

2-Hydroxy-3-methyl-2cyclopentenone

62

3-Ethyl-2-hydroxy-2-cyclopentenone

62

3,4-Dimethyl maleic anhydride

62

Butyrolactone

62

Maleic acid

65

Fumaric acid

65

Succinic acid

68

Malonic acid

68

Propionic acid

65

Oxalic acid

65, 68

Acetic acid

65, 68

Formic acid

65, 68

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Given the complexity of this mixture, the downstream treatment of the alkaline oxidative lignin depolymerization reaction is itself a complex topic that was and is still extensively discussed in both patent and academic literature. Downstream treatment Finding a way to isolate each of these compounds is currently not possible and even less economically viable. Thus, efforts have been directed to harvest the molecules of interest from this mixture, i.e. vanillin and recently syringaldehyde23. The difficulty of isolating pure vanillin from such a mixture is illustrated by the number and variety of processes available in the literature. The two major problems are the acidification of the mixture and the residual lignin removal. One of the oldest process25 performs these steps by lignin precipitation in acidic conditions and phenolics are then extracted (Scheme 6). This process requires huge amounts of acid and solvents and the mixture obtained is still difficult to purify.

Scheme 6: Lignin-to-vanillin process involving acidification and an extraction step by organic solvents A process developed at about the same time towards aldehydes.

24

(Scheme 7) improves the selectivity of the products

Scheme 7: Lignin-to-vanillin process involving bisulfitation and an extraction step by organic solvents The key step is the bisulfitation of the mixture. Briefly it consists in mixing the crude lignin depolymerization mixture with a solution of NaHSO3 (sodium hydrogen sulfite, or sodium bisulfite) to prepare from vanillin a “vanillin bisulfite complex”23, 74, i.e. sodium vanillyl-α-hydroxysulfonate as depicted in Scheme 8:

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O

H

HO HSO3

SO 3

Na

Na

O OH

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O OH

Scheme 8: Reaction of vanillin with sodium bisulfite to form the “vanillin bisulfite complex” The derivatives produced have a good solubility in water, as opposed to the other products from the crude mixture.24 The hydrosulfite anion reacts selectively with the aldehyde moiety, which means that phydroxybenzaldehyde and syringaldehyde75 are extracted along with vanillin. Products of high molecular weight precipitate during this step, due to the pH increase of the medium.24 Once this precipitate has been removed, the aqueous phase must be further acidified to recover aldehydes and SO2.24 One major drawback of these two methods is the use of large amounts of organic solvents such as benzene or toluene. Methods have been proposed to circumvent this problem. The first one is based on the replacement of these solvents by supercritical CO2 (Scheme 9).76, 77

Scheme 9: Lignin-to-vanillin processes involving an extraction step by supercritical CO2 Another way to extracts low molecular weight phenolics from the aqueous phase is by adsorbing them. Zeolites78 as well as a macroporous resin79 have been proposed as adsorbents (Scheme 10).

Scheme 10: Lignin-to-vanillin process involving an extraction step by adsorption A major disadvantage of these processes is the requirement of large amounts of acids for neutralization and/or acidification prior to the extraction of vanillin.71 In order to reduce this amount, a method of neutralization of the crude lignin depolymerization mixture on a cation exchange resin prior to any other step has been proposed.80 Another strategy consists of directly isolating the molecular phenolates from the crude mixture and then performing the acidification. The amount of acid necessary and the dilution of the crude liquor is thus greatly reduced.81 A first method described in the literature is the separation of phenolates from higher molecular weight fragments by elution of the crude lignin depolymerization mixture on a strong cationexchange resin in the Na+ form (Scheme 11, top path). Indeed, these two types of species do not have

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the same retention times and can thus be separated.71 The second method is rather old and consists of extracting the phenolates with an appropriate alcoholic solvent81, 82, which can then be distilled off (Scheme 11, middle path). The last method (Scheme 11, bottom path) is more recent and is based on an ultrafiltration step to remove high molecular weight lignin fragments83 followed by an acidification of the phenolates in the permeate on an ion-exchange resin.[84] This last method has been proposed in an integrated process that can potentially be adapted in a biorefinery.18 It is worthy to note that the ultrafiltration technology has also been applied to concentrate lignosulfonates in spent sulfite liquor prior to the depolymerization reaction.85

Scheme 11: Lignin-to-vanillin processes involving a direct extraction step of phenolates The stream of product from all these methods is essentially composed of crude vanillin with varying amounts other phenolics, depending on the process used. Further purification of vanillin to technical grades or food grades is a difficult task. Indeed, the remaining other compounds like acetovanillone or syringaldehyde have very close structures and properties. Separating vanillin from these compounds is thus a difficult task, as pointed out by numerous works.23, 75, 86 The methods proposed in these works, multistage crystallization for instance, are all tedious and quite difficult to implement on an industrial scale. This is the reason why food-grade vanillin prepared from lignin still displays high amounts of many different impurities, such as syringaldehyde, syringic acid and acetovanillone.87 Recently, an original method of purification has been proposed. Briefly, it consists of preparing a vanillin-molecularly imprinted polymer and using it to adsorb with a high selectivity vanillin in the stream to be purified.88

OUTLOOK FOR VANILLIN UTILIZATION Vanillin is currently one of the only molecular phenolic compounds manufactured on an industrial scale from biomass. It has thus the potential to become a key-intermediate for the synthesis of bio-based polymers, for which aromatic monomers are needed to reach good thermo-mechanical properties. The amount of research continuously generated on the topic of vanillin production from lignin proves the industrial interest of this technique. The fact that the raw material is bio-based, that petroleum price is

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on the rise, and the improvements achieved in terms of mechanism understanding, productivity, and waste reduction make this vanillin production path more and more attractive from both a green chemistry and economic point of view. However, the current vanillin market as aroma and fragrance is mature and dominated by the low price (10 $/kg), petro-based vanillin. Only new markets can stimulate the production on a large scale of the currently more expensive bio-based vanillin (20 $/kg). These emerging markets already exist. Indeed, many recent works deal with the use of vanillin for the synthesis of renewable polymers. These works have been summarized in a review article.89 Vanillin, taken as a bio-based building-block90, presents many advantages as mentioned in the introduction. Therefore, it is no surprise that the use of vanillin for the preparation of bio-based polymers attracted that much attention recently. In fact, vanillin could provide a solution to the long-standing problem of finding bio-based, aromatic monomers able to replace traditional petro-based, aromatic monomers. The potential of vanillin has been demonstrated in epoxy polymers91-94, as a substitute of bisphenol A. This reprotoxic compound is also used for the synthesis of polycarbonates, and vanillin-derived monomers might also be good substitutes in these polymers.95 This potential could be extended to any polymer in which aromatic monomers are needed. Vanillin was used as a base material to prepare polyesters with good thermal properties72, 96, 97, without employing any petro-based terephthalic acid. It was also used to prepare reactive diluents or cross-linkers for vinyl ester formulations.98, 99 These vanillin-derived compound advantageously replaced styrene, usually used in such formulations. Styrene needs substitutes as it is a petro-based Volatile Organic Compound (VOC) as well as a Hazardous Air Pollutant (HAP). Vanillin is not only able to provide substitutes to problematic petro-based aromatics, it can also be the basis of entirely new polymers possessing interesting properties in areas as diverse as metal ions chelating100, 101, antimicrobial polymer films102, liquid crystalline polymers103, or high performance polybenzoxazines thermosets.104 In turn, large-scale production of vanillin from biorefineries could lead to a price decrease. The biorefinery concept has gained more and more importance in recent years. Vanillin production can be integrated in this concept but cannot be the only aim of a given plant, and the process has to be carefully optimized to be viable. Indeed, the shutting down of the lignin-to-vanillin plants in the 1990’s should serve as an example: producing diverse chemicals and carefully controlling effluents to be economically and ecologically viable is necessary to modern biorefineries. Vanillin is a high added-value product but its yield from raw biomass is low because of the difficult purification steps needed to obtain a food-grade compound. This is a first limitation of this process. Thus, vanillin production must be combined with the production of large volume, low added-value products such as cellulose, pulp, paper, lignin, etc. In the case of such complementary productions, vanillin is viable in a bio-refinery context. Another limitation of the lignin-to-vanillin process is the various environmentally-damaging steps of the process. These problematic steps are mostly needed to purify the mixture obtained after lignin depolymerization. Early processes used large amounts of acids, organic solvents, or energy. As summarized in this work, a great deal of effort has been and is still made to improve these processes or find new ones, more environmentally-friendly. These efforts need to be pursued as they are what will make the lignin biorefinery an industrial reality. Lignins are characterized by their composition in hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units. There is thus a maximum obtainable vanillin yield from a specific type of lignin. The formation of vanillin

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or other compounds is strictly related to the available percentage of its precursor in the lignin structure18, i.e. in the case of vanillin to the amount of G units. Therefore, in practice, only G lignins from softwoods, containing high amounts of G units, are used in the lignin-to-vanillin process. This is a third limitation of this approach. Importing softwood from the other side of the globe to improve the yield in bio-based vanillin is not viable in terms of carbon footprint. These limitations can be dealt with if one considers not only the selective production of vanillin, but the valorization of mixtures of phenolics obtained from lignin depolymerization, either non- or partially purified. Indeed, an advantage of the polymer market is that it does not necessarily require pure molecules as raw materials, even if it has been the norm for quite some time with the petro-based refinery. Some polymers can be manufactured from biomass-derived mixtures of compounds. It has been recently proven with the synthesis of epoxy thermosets with good thermo-mechanical properties from mixtures of phenolics found as products of the lignin-to-vanillin process.105 This approach of using mixtures improves the yield in high value-added products in two ways. Firstly, vanillin is not the sole product, vanillic acid and acetovanillone for instance can also be considered as valuable products. Secondly, some of the difficult purification steps that impact the yield are not needed in this approach. In terms of yield improvement, lignins having a structure as close as possible to their native one found in wood seem to be the best. Therefore, lignins from new types of processes such as organosolv lignins might be better choices as source of mixtures of bio-based aromatics than the ones from older processes, such as lignosulfonates or Kraft lignins, which structure is heavily altered. Another important positive point of this approach is that all types of wood can then be used, as all types of lignin units, H, G, and S, lead to valuable phenolics. For instance, syringaldehyde and syringic acid, obtained from S units of GS lignins from hardwoods, do not have to be considered as impurities and removed. More biomass can thus be processed, and the sourcing of the wood can be local, which is less environmentally damaging. Finally, partially or totally removing the purification steps makes the process less demanding in acids, solvents, or energy. The valorization and use of mixtures of products, as would arise from lignin and maybe more generally from bio-refineries, will be a great challenge.16 Simply put, “there is a vast difference between a valuable mixture of chemicals and a mixture of valuable chemicals”.43 However, it will be worth it. The advantages of using unrefined mixtures are numerous: more biomass can be used, which means less waste, and the difficult purification steps are no longer necessary, improving the overall economic and environmental viability of the bio-refinery concept.

REFERENCES The authors declare no fundings.

REFERENCES (1) Kamm B, Gruber PR, Kamm M. Biorefineries – Industrial Processes and Products. Ullmann's Encyclopedia of Industrial Chemistry: Wiley-VCH Verlag GmbH & Co. KGaA; 2000. (2) Belgacem MN, Gandini A. Monomers, polymers and composites from renewable resources: Elsevier; 2011. (3) Meier MAR, Metzger JO, Schubert US. Plant oil renewable resources as green alternatives in polymer science. Chem. Soc. Rev., 2007, 36, 1788-1802.

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For Table of Contents Use Only VANILLIN PRODUCTION FROM LIGNIN AND ITS USE AS A RENEWABLE CHEMICAL Maxence Fache, Bernard Boutevin, Sylvain Caillol* This manuscript proposes a perspective article on lignin-derived vanillin for further use as an aromatic building block for chemistry

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