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Vanillin (4‐hydroxy‐3‐methoxybenzaldehyde) is an organic compound consisting of a benzene ring substituted with three functional groups: aldehyde –CHO, hydroxyl –OH, and methoxy –O–CH₃ (Figure 1.18a).

It is a naturally occurring compound (in the form of its β‐D‐glucoside, Figure 1.18b) that can be directly obtained in the extraction process from the bean or seed pods of Vanilla planifolia, the tropical orchid presently cultivated in a number of tropical countries. Although this method has been known for centuries and it is still used, actually less than 1% of vanilla produced in the world is obtained in such a way [79]. Almost all vanillin is now synthesized much more cheaply through chemical processes. Synthetic vanillin is commercially available and is commonly used in both food and nonfood applications, in fragrances, as a flavoring in pharmaceutical preparations, as an intermediate in the chemical and pharmaceutical industries for the production of herbicides, antifoaming agents or drugs, and in household products, such as air fresheners and floor polishes. Synthetic or semisynthetic vanillin can be derived from two compounds: guaiacol and eugenol, both available from petrochemical sources or of natural origin.

Figure 1.16 Route of the synthesis epoxy monomers from selectively hydrodeoxygenated lignin.

Figure 1.17 Lignin modification and cross‐linking: (a) ozone oxidation of Kraft lignin and (b) synthesis of multiple carboxylic acid derivatives.

Figure 1.18 Chemical structures of (a) vanillin and its naturally occurring precursors: (b) vanillin glucoside, (c) guaiacol, (d) eugenol, and (e) coniferyl alcohol.

Figure 1.19 Synthesis of vanillin from guaiacol.

The first one, guaiacol (2‐methoxyphenol) (Figure 1.18c), is a naturally occurring organic compound present in an aromatic oil from flowering plants Guaiacum. Guaiacol can also be gained from creosotes formed by distillation of various tars and pyrolysis of plant‐derived material, such as wood. Semisynthetic vanillin can be obtained from guaiacol through the Reimer–Tiemann reaction of phenols formylation (Figure 1.19) [80].

The reaction is carried out using chloroform deprotonated by a strong base (hydroxide typically) to form the chloroform carbanion and finally the dichlorocarbene, which is the principal reactive specie in nucleophilic substitution also occurred in deprotonated phenol. Another method of vanillin synthesis from guaiacol is its reaction with glyoxylic acid (Figure 1.20a), leading to the formation of 2‐hydroxy‐2‐(4‐hydroxy‐3‐methoxyphenyl)‐acetic acid (Figure 1.20b) [81]. The obtained vanillylmandelic acid is converted via 2‐(4‐hydroxy‐3‐methoxyphenyl)‐2‐oxoacetic acid (Figure 1.20c) to vanillin by the oxidative decarboxylation [82].

Eugenol (2‐methoxy‐4‐(prop‐2‐en‐1‐yl)phenol) (Figure 1.18d) present in an essential oil extracted from the clove plant Syzygium aromaticum is the next important natural raw material for the vanillin synthesis (Figure 1.21).

The synthesis consists of two steps: the basic isomerization of the double bond in eugenol leading to the formation of isoeugenol and oxidation of the rearranged double bond to vanillin [83, 84]. The process can be carried out with or without isolation of the intermediate product which is isoeugenol [85].

Figure 1.20 Synthesis of vanillin from guaiacol using glyoxylic acid.

Figure 1.21 Synthesis of vanillin from eugenol.

Lignin from softwood is still one of the most important sources of raw materials for the synthesis of vanillin. Three‐dimensional network structures of lignin are composed of three types of monolignols: p‐coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Coniferyl alcohol (Figure 1.18e) is the main intermediate in the pathways to vanillin from a softwood (coniferous) lignin, as well as the precursor for eugenol in its biosynthesis. Vanillin can be produced from the lignin‐containing waste manufactured by the sulfite pulping process for preparing wood pulp for the paper industry [86]. This first developed method of vanillin synthesis from lignin lost its relevance primarily for environmental reasons (the need to safely get rid of alkaline‐based liquid waste). However, thanks to the results of work on the process optimization [87], it was possible to achieve an increase in the yield of vanillin and a reduction in waste stream volume. Therefore, the volume of vanillin production from lignin is still estimated at around 15% of the total world vanillin production.

There are also known for years [88] and still developed biotechnological processes of vanillin synthesis [89]. They are promising for the industrial‐scale production of vanillin due to the use of natural raw materials, renewable and readily available in large quantities, such as rice bran [90] or corn sugar [91]. However, actually bio‐synthesized vanillin is still very expensive [92] and its high cost of production are justified only for specific applications in the food, cosmetics, and pharmaceutical industries. Nowadays, the biotechnological production is not suitable and profitable source of vanillin for the synthesis of polymeric materials.

Figure 1.22 Schematic illustration of possible synthesis pathways (a) and (b) for vanillin‐based epoxy resins.

Due to its chemical structure as a phenolic compound, vanillin is the promising raw material that could replace bisphenol A (or other commonly used bisphenols) providing epoxy resins with adequate mechanical strength and thermal stability. However, as the trifunctional compound, but also only a monoalcohol, vanillin must be modified in order to serve as a substitute for bisphenols in the synthesis of epoxy resins. Figure 1.22 shows schematically the possible pathways of vanillin modification described in the literature that lead to obtaining epoxy resins.

Generally, two strategies for the synthesis of vanillin‐based epoxy resins are mainly being investigated. The first one (Figure 1.22a) assumes converting vanillin into derivatives also containing, in addition to the phenol group already present in the vanillin molecule, a second functional group through which the epoxy functionality could be introduced. The second strategy (Figure 1.22b) involves coupling of two vanillin (or its derivative) molecules using another chemical compound, resulting in a product containing at least two phenolic or other groups through which the epoxy group can also be introduced. In both strategies, commonly used methods for introducing epoxy functionality have been applied: the oxidation of double bonds and the reaction with epichlorohydrin.

Figure 1.23 Synthesis of 2‐methoxyhydroquinone and its epoxy derivatives ‐ strategies (a) and (b).

According to the first strategy, the Dakin oxidation can be applied to convert aldehyde group in vanillin to hydroxyl group [93] (Figure 1.23).

Synthesized 2‐methoxyhydroquinone can be reacted with the large excess (10‐fold) of epichlorohydrin under the typical phase‐transfer catalysis conditions in the presence of triethylbenzylammonium chloride (TEBAC). The resulting product mainly contains diglycidyl ether of 2‐methoxyhydroquinone (Figure 1.23a), which can be used together with 2‐methoxyhydroquinone to obtain an epoxy resin (with an epoxy value of 0.060–0.340 mol/100 g) via the fusion process [94] in the presence of triphenylbutylphosphonium bromide (TPBPB) (Figure 1.23b). Such epoxy resin with an epoxy value of 0.404 mol/100 g [95] could be successfully cross‐linked with the cycloaliphatic amine curing agent (commercial product Epikure F205), preferably in the presence of calcium nitrate as an accelerator. The vanillin‐based epoxy resin cured using 2 wt% of the inorganic accelerator exhibits the tensile strength and the Izod impact strength higher than those for liquid diglycidyl ether of bisphenol A (epoxy resin Epon 828 with an epoxy value of 0.541 mol/100 g) used for comparison. As an aldehyde, vanillin can be easily oxidated to vanillic acid, as well as reduced to vanillic alcohol (Figure 1.22a). Under analogous conditions as in the case of 2‐methoxyhydroquinone, the diglycidyl monomers (Figure 1.22a) can be obtained from both vanillic acid and alcohol [96]. After cross‐linking with isophorone diamine bio‐based epoxy resins derived from them are characterized by the high glass transition temperature (132 and 152 °C, respectively) and the storage modulus comparable with the value determined for diglycidyl ether of bisphenol A. They also exhibit high thermal stability, typical for epoxy resins based on bisphenol A. 2‐Methoxyhydroquinone as well as vanillic alcohol and acid could be reacted with allyl bromide giving derivatives (Figures 1.22a and 1.24a) with terminal unsaturated bond [97], which can be e.g. enzymatically oxidized to oxirane rings using percaprylic acid as an oxygen carrier and immobilized lipase B from Candida antarctica (Novozym 435) as a biocatalyst [98] (Figure 1.24b).

Figure 1.24 The O‐alkylation of vanillin derivatives (a), followed by the epoxidation of the resulting double bonds (b).

This is another interesting reaction pathway for the synthesis of above‐mentioned diglycidyl monomers without using bisphenol A and epichlorohydrin, and under mild conditions. Moreover, the other interesting epoxy compound derived from two coupled vanillic acid molecules (Figure 1.25) could also be prepared throughout this way.

However, obtaining the completely epoxidized products and the formation of various regioisomers still remain challenging.

According to the second strategy, the dimerization of vanillin is possible [99] by the selective enzymatic oxidative coupling (Figure 1.26a). After the reduction of aldehyde groups, a divanillin alcohol is obtained, which can be then reacted with epichlorohydrin (Figure 1.26b) [100].

The vanillin‐based epoxy compounds are obtained as a mixture of glycidyl derivatives at different ratios, which can be fractionated by flash chromatography. The content of individual glycidyl derivatives in the product mixture can be controlled primarily by the sodium hydroxide content, as well as the duration of the second step reaction with epichlorohydrin (adding a base at room temperature in order to perform the ring closure of intermediate halohydrin species). For example, with a NaOH/OH ratio equal to 10, the tetraglycidyl compound is mainly obtained with about 90% yield. In contrast, the diglycidyl derivative is mainly created (80% yield) at lower NaOH/OH ratios. Separated vanillin‐based epoxy compounds cross‐linked with isophorone diamine, characterized with the glass transition temperature in terms of 138–198 °C, exhibit similar Young modulus and thermal stability values to the bisphenol A‐based epoxy thermoset, but lower elongation at break.

The other possibility of the vanillin dimerization is the electrochemical synthesis of meso‐hydrovanilloin (Figure 1.27) [101]. The symmetrical compound divanillin (two molecules of vanillin coupled by aromatic rings) can be easily prepared by the oxidative dimerization of vanillin catalyzed by FeCl₃ or heme iron enzymes [102]. However, vanillin can also be reductively dimerized at the aldehyde function using low‐valent titanium generated via TiCl₄‐Mn or by the electrochemical method [103]. The electrochemical coupling is a highly stereoselective reaction giving meso‐hydrovanilloin. The vanillin electrolytic reduction in alkaline solution at a metallic cathode gives interesting bisphenol compound, which can be used as a direct substitute for bisphenol A [104].

The obtained meso‐hydrovanilloin‐based epoxy resin cured using long‐chain aliphatic diamine (1,6‐diaminohexane) and cycloaliphatic amine (isophorone diamine) showed the glass transition temperature and Shore hardness (D‐type) values comparable with commercial diamine‐cured bisphenol A‐based epoxy resins.

Two molecules of vanillin can be coupled by the crossed aldol condensation (Figure 1.28) with cyclopentanone [105].

Figure 1.25 Esterification of vanillic acid, followed by the O‐alkylation and subsequently by the epoxidation of the allylic double bonds.

Figure 1.26 Synthesis of glycidyl derivatives (b) based on the product of vanillin dimerization (a).

Figure 1.27 Synthesis of hydrovanilloin and the epoxy resin based on this vanillin dimer.

Figure 1.28 Synthesis of 2,5‐bis(4‐hydroxy‐3‐methoxybenzylidene)cyclopentanone and its diglycidyl derivative.

The thermal and mechanical properties of the synthesized resin cured with bio‐based (quercetin and guaiacol novolac) hardeners and a petroleum‐based hardener (phenol novolac) are comparable with those of the bisphenol A‐based resins cross‐linked with the same hardeners.

A diamine can also be used to couple two vanillin molecules [106]. Vanillin coupled with aromatic diamines and diethyl phosphite, followed by the reaction with epichlorohydrin, yields high‐performance biorenewable and environment‐friendly flame‐retardant epoxy resins (Figure 1.29).

The coupling product with 4,4‐diaminodiphenylmethane (DDM) or p‐phenylenediamine (PDA) is synthesized (Figure 1.29a) through Schiff base condensation, and the generated Schiff base is further reacted with diethyl phosphite by the phosphorus–hydrogen addition reaction to yield phosphorus‐containing vanillin‐based bisphenols. The resulted bisphenol can be converted into diglycidyl derivative via the above‐described reaction with an excess of epichlorohydrin, preferable under PTC conditions. The reactivity of the epoxy resins synthesized in this way is similar to the bisphenol A‐based epoxy resin. After curing with a stoichiometric amount of 4,4‐diaminodiphenylmethane, both resins showed excellent flame retardancy with UL‐94 V0 rating and high LOI value of 31.4% (coupling with DDM) and 32.8% (coupling with PDA), due to their outstanding intumescent and dense char formation ability. They also exhibit high glass transition temperature value of 183 °C (DDM) and 214 °C (PDA), the tensile strength of 80.3 MPa (DDM) and 60.6 MPa (PDA), and the tensile modulus of 2114 MPa (DDM) and 2709 MPa (PDA), much higher than the cured bisphenol A‐based epoxy resin with a T_g of 166 °C, a tensile strength of 76.4 MPa, and a tensile modulus of 1893 MPa, respectively.

Figure 1.29 Synthesis of the vanillin coupling product (a) and the flame‐retardant epoxy resin based on it (b).

Figure 1.30 The coupling of vanillin with pentaerythritol and synthesis of the epoxy resins containing spiro‐ring structure.

Two molecules of vanillin can also be coupled through the dehydration condensation with pentaerythritol, leading to obtain the bisphenol with the specific spiro‐ring structure (Figure 1.30) [66], which can be further reacted with epichlorohydrin to give the epoxy resin.

This vanillin‐based resin exhibits very interesting properties [107]. This solid resin with an epoxy value of 0.355 mol/100 g, cross‐linked with diamine hardeners, DDM or 3,9‐bis(3‐aminopropyl)‐2,4,8,10‐tetroxaspiro(5,5)undecane, has several relaxations. The first is the β‐relaxation, caused by the micro‐Brownian motion of the aromatic methoxy group, observed from 50 to 100 °C for the spiro‐ring‐type resin systems in both mechanical and dielectric measurements. The peak height and the activation energy of this relaxation are independent of the degree of curing. The second one is the relaxation caused by the hydrogen bonding between the methoxy and the hydroxyl groups at around 0 °C [108]. This relaxation behavior is expected to have a positive effect on the damping characteristics. Moreover, the fracture toughness of the spiro‐ring‐type epoxide resin with methoxy branches is considerably greater above the temperature region of the β‐relaxation than that of the bisphenol A type resin [109].