Vanillin, production from lignin

Fargues, C., Mathias, A., and Rodrigues, A., Kinetics of vanillin production from om Krafl lignin oxidation. Industrial Eng Chem Res 1996, 35 (1), 28-36. 

Preparation from Waste Sulfite Liquors. The starting material for vanillin production can also be the lignin (qv) present in sulfite wastes from the ceUulose industry. The concentrated mother Hquors are treated with alkaH at elevated temperature and pressure in the presence of oxidants. The vanillin formed is separated from the by-products, particularly acetovanillone, 4-hydroxy-3-methoxyacetophenone, by extraction, distillation, and crystallization. 

Guaiacylglycerol-/3-coniferyl ether is a dimeric decomposition product of coniferous lignin dehydrodivanillin may be also formed by dimerization of vanillin after formation from lignin. Among the lignin decomposition products are mono-, di- and triphenol derivatives which are derived from the different types of lignin of needle trees, deciduous trees, or graminees. 

Vanillin, syringaldehyde, and related phenolic products derived from lignin appear to be important in the desirable odor contributed to beverages by oak barrels (25, 26, 27). These substances are present in the wood, but more appears in the beverage than simple extraction seems to allow. Also the odor of vanillin seems to increase with time and mellowing of the beverage compared with a fresh extract of new wood (25). 

Products from the direct oxidation of lignin that give information about its structure include acetic, oxalic, and succinic acids, vanillin, vanillic acid, and dehydrodivanillin. Regarding lignin constitution, they are of only minor interest, but the yields in which they are obtained are significant. For vanillin and its derivatives the total yield is about 33% of spruce lignin (24, 36, 37, 38). Two extraordinary products of this direct oxidation deserve special attention—i.e., benzenepentacarboxylic (45) and tricarballylic acids (47), (XXIX) and (XXX). They will be mentioned later in connection with lignenolide (XXVII). 

The homolytic evidence is further strengthened by the product distribution obtained from lignin. Both copper(II) and nitrobenzene give substantial and similar yields of aldehyde products (19). Benzoic acids rather than benzaldehy-des, however, are the expected terminal oxidation products from the action of heterolytic oxidants. If vanillin is produced from lignin via heterolytic oxidation by nitrobenzene, then the high survivability of an aldehyde in the presence of an... 

Vanillin occurs naturally in many essential oils and particularly in the pods of Vanilla planifolia and Vanilla tahitensis. Industrially, vanillin is prepared from lignin, which is obtained from the sulfite wastes produced during paper manufacture. Lignin is treated with alkali at elevated temperature and pressure, in the presence of a catalyst, to form a complex mixture of products from which vanillin is isolated. Vanillin is then purified by successive recrystallizations. 

The vanillin market is in fact shared by two products the natural vanillin contained in vanilla beans, an extremely highly priced ( 4000 /kg) and scarce product, and the synthetic (from phenols) or hemisynthetic (from lignin) vanillins which are produced in large quantities and are relatively cheap (40 /kg). 

Unfortunately, the natural product is heavier than its synthetic counterpart and it is relatively easy and not prohibitively expensive to add a small quantity of C enriched vanillin to the synthetic product. From this point of view, vanillin exlignin is preferred to vanillin ex-guaiacol since the quantity of labelled material required is lower. Applying isotopic dilution equations, it is apparent that less than one hundred milligrams of 99% 13C-labelled vanillin are required to transform 1kg vanillin ex-lignin into 1kg natural vanillin ... 

The difference between calling a molecule artificial or natural is a legal definition, not a chemicfd or biological one. If a molecule of vanillin is isolated in a lab by extraction of a particular seed pod, the chemical is called natural vanilla. If that same molecule is made from lignin, which is a polymer found in naturally in wood, that substance is called artificial vanilla. The difference is that transforming lignin to vanillin requires chemical steps not covered by the legal definition of the word natural. To be clear, the natural and artificial versions of a molecule are exactly the same chemical species (same atoms, same bonds, same stereochemistry, etc.), but there may be other chemicals present that may differ in the natural and artificial versions of a product. 

The first indication that vanillin could be obtained out of the sulfite washings from paper production came in the year 1875, and was due to the vaniUin-like smell of these solutions. In order to obtain the vanillin, the sulfonated lignin fragments were oxidised under alkaline conditions often the oxidants used are atmospheric oxygen or persulfate in the presence of a copper salt. Ceric or cobalt salts are also efficient catalysts. The mechanism proceeds via free benzyl radicals. The yields of vanillin range between 4-6.2 %. 

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