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
