Vanillin as a molecular building block

Sustainable alternatives to fossil oil are a prerequisite for climate change. Finding cost competitive and scalable renewable sources of energy is challenging by itself. What can be even more challenging is to find replacements for the 10% of the oil barrel that is converted into a wide array of chemicals and materials. Both aliphatic and aromatic molecules need to be replaced, and they pose different challenges. The German nova institute reported in 2020 that about 8% of the carbon used in manufacturing of chemicals and materials was bio-based. In the same study they find that 4% of the carbon is recycled from fossil sources and 0.03% originates from captured CO₂. Together these three sources comprise what nova institute refer to as renewable carbon.

Figure from nova institute

While plant-based fatty acids and essential oils are convenient and ubiquitous sources of aliphatic carbon chains, most vegetable oils do not contain aromatic building blocks. However, there are some exceptions.

• Cashew nut shell oil contains various cardanols which are phenolic lipids comprising a phenol with long aliphatic side chains.
• Cloves and clove trees are a source of eugenol.
• Ferulic acid is isolated from many crops, including rice and wheat straw.
• Caffeic acid is found in coffee as well as many other plants.
• Phloroglucinol (1,3,5-trihydroxybenzene) which can be isolated from macroalgae.
• Wood tar contains hundreds of low molecular weight phenolic compounds, including phenol, diphenol, guaiacol, methyl guaiacol, methoxy phenol, vanillin, and syringaldehyde to mention a few.

Despite the apparent abundance of sources, there is still a long way to go. The volumes of some biomass sources, such as cashew nuts and cloves, are limited. Lignin however is abundantly available as it constitutes about 1/3 of all lignocellulosic biomass. The aromatic molecules found in wood tar largely originate from lignin. Yet, while there are abundant volumes available, the complex mixture limits commercial use. It is therefore not surprising to see an increasing number of publications and patents searching for other ways to utilise lignin as a raw material for bio-based aromatic building blocks.

Lignin is a complex organic polymer found in the cell walls of plants, providing structural support. It is composed of three main monomers: coniferyl alcohol, sinapyl alcohol, and coumaryl alcohol. In softwood, the predominant lignin monomer is coniferyl alcohol, hardwood has a higher content of sinapyl alcohol, whereas lignin from annual crops typically is dominated by coumaryl alcohol. Lignin is a randomly branched polymer, displaying a handful of different bonding patterns, some of which are stronger and more difficult to break than others. This makes lignin depolymerisation challenging, especially if the aim is to obtain a simple mixture of monomers.

Technical lignins are a practical source of lignin. They result from various industrial processes such as pulp and paper manufacturing, as well as biorefinery processes. Sulfite lignin is an example of a technical lignin, originating from the sulfite pulping process. Manufacturing of vanillin from so called spent sulfite liquor was pioneered about 80 years ago. Today Borregaard is the only manufacturer of vanillin from lignin.

Vanillin is well known and loved for its delicious flavor and pleasant fragrance. Its use however extends well beyond typical flavor and fragrance applications. Vanillin exhibits antimicrobial properties and shows synergistic effects in combination with conventional preservatives. Adding to that, the vanillin molecule has two functional groups, the phenolic OH and the aldehyde, which may act as “handles” for chemicals reactions. These functional groups allow vanillin to participate in polymerisation reactions to yield bio-based polymers and resins.

A common challenge when turning to biomass to find replacement for fossil-based chemicals is the high oxygen content and the many functional groups. Vanillin is a good example of a molecule where this plays out as an advantage compared to a fossil route that would typically go via benzene before functional groups are added.

Besides the cured vanilla bean which contains about 2% of vanillin, several other feedstocks are used for manufacturing bio-based vanillin. These include bio-based eugenol, ferulic acid, sugar (glucose), guaiacol, and lignin. A range of chemical and biochemical processes are used to convert these starting materials to vanillin.

At Borregaard, production of vanillin from lignin derived from Norway spruce (Picea abies) commenced in 1962. Borregaard was not the first, nor the only manufacturer of bio-based vanillin. However, today Borregaard remains as the only manufacturer of vanillin from spent sulfite liquor, and by far the largest manufacturer of bio-based vanillin. Key to the success has been the production of vanillin in an integrated biorefinery where side streams not converted to vanillin are further processed for valorisation.

To meet the demand for a bio-based vanillin in technical applications, Borregaard has made a technical grade bio-based vanillin available: Conifera technical (see documentation available for download). This vanillin has been tailored for non-food applications and meets Borregaard’s strict quality requirements. Conifera technical is 100% bio-based, non-GMO, and made from Norway spruce sourced from certified forests. This makes Conifera technical different from vanillin where a chain of custody model is used to allocated “bio-based” as a feature from a remote bio-based feedstock to a vanillin molecule that is actually fossil-based. There are different ways to do this: In a mass balance approach, some of the bio-based raw material may end up in the product that is claimed to be “bio-based”, whereas in the book and claim model nothing from the bio-based feedstock needs to end up in the product. Chain of custody models are typically done by an independent 3^rd party according to certification schemes such as ISCC PLUS, REDcert, and RSB. A mass balance approach is also described in the European standard CSN EN 16785-2.

For Borregaard's Conifera technical vanillin no such Chain of custody model is used. Every single carbon atom in Conifera technical originates from the lignin molecule found in the wood. Furthermore, the entire vanillin production process at Borregaard has been assessed in a life cycle assessment (LCA). Borregaard has committed to targeted reductions in GHG emissions Scope 1 and 2 with 42% reduction by 2030 (base year = 2020) and a net-zero target in 2050, corresponding to a 90% absolute reduction by 2050.

As mentioned above, the two chemical “handles” in opposite positions on the aromatic ring ensure that vanillin is an attractive starting material for bio-based resins and materials. Below are some examples of polymers that incorporate vanillin or a vanillin moiety:

• Vanillin derived molecules may be used in epoxy resins and thus replace bisphenol A (BPA) and the corresponding diglycidyl ether (DGEBA, CAS: 1675-54-3). Examples of vanillin derived bisphenols and their corresponding glycidyl ethers that may serve as starting materials for epoxy resins include vanillin monoglycidyl ether (VGE, CAS: 22590-66-5), vanillin diglycidyl ether (DGEVA, CAS: 1584677-14-4), or a divanillin diglycidyl ether (DADE, CAS: 2505274-08-6).
• The 1,4-substitution pattern of polyamides fits well with the molecular structure of vanillin. For high performance aramide type polyamides such as Kevlar^® and Twaron^® type materials, bio-based vanillin may thus be an attractive building block.
• Bio-based vinyl ester resins may be made with vanillin methacrylate (CAS: 36195-34-3).
• Vanillin may serve as a starting material for synthesis of monomers benzoxazine resins.
• Oxidation of vanillin to vanillic acid (CAS: 121-34-6) makes it a relevant starting material for use in polyesters, either by direct condensation or co-polymerisation with other monomers.

The above gives a few examples of how bio-based vanillin could be a key enabler. Below are some relevant reviews showcasing many more examples of how vanillin may be utilised as a bio-based molecular building block in polymers, resins, and materials:

• Llevot, A., Grau, E., Carlotti, S., Grelier, S. & Cramail, H. From Lignin‐derived Aromatic Compounds to Novel Biobased Polymers. Macromol Rapid Comm 37, 9–28 (2016). DOI: 10.1002/marc.201500474
• Nikafshar, S. et al. A renewable bio-based epoxy resin with improved mechanical performance that can compete with DGEBA. RSC Adv. 7, 8694–8701 (2017). DOI: 10.1039/c6ra27283e
• Fache, Maxence, Boutevin, Bernard & Caillol, Sylvain. Vanillin, a key-intermediate of biobased polymers. Eur Polym J 68, 488–502 (2015). DOI: 10.1016/j.eurpolymj.2015.03.050
• Holmberg, A. L., Reno, K. H., Wool, R. P. & Epps, I. T. H. Biobased building blocks for the rational design of renewable block polymers. Soft Matter 10, 7405–7424 (2014). DOI: 10.1039/c4sm01220h
• Hayes, G. et al. Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers. Chem. Rev. 123, 2609–2734 (2023). DOI: 10.1021/acs.chemrev.2c00354
• Yang, W. et al. Bio‐renewable polymers based on lignin‐derived phenol monomers: Synthesis, applications, and perspectives. SusMat 2, 535–568 (2022). DOI: 10.1002/sus2.87

In addition to polymeric materials and resins, vanillin may also be an attractive starting molecule for the synthesis of specialised molecules. Some examples can be found in the table below (links point to CAS Common Chemistry).

• 2-Methoxy-1,4-benzoquinone, 2880-58-2
• 2-Methoxy-4-vinylphenol, 7786-61-0
• 2-Methoxyhydroquinone, 824-46-4
• Capsaicin, 404-86-4
• Entacapone (Comtan^®), 130929-57-6
• Piperonal (Heliotropine), 120-57-0
• Protocatechuic acid, 99-50-3
• Protocatechuic alcohol, 3897-89-0
• Protocatechuic aldehyde, 139-85-5
• Vanillyl butyl ether, 82654-98-6
• Veratraldehyde (3,4-Dimethoxybenzaldehyde), 120-14-9
• Veratric acid (3,4-Dimethoxybenzoic acid), 93-07-2
• Zingerone (vanillylacetone), 122-48-5