(113d) The Role of Adsorption Processes in Lignin Biorefinery to Make Vanillin and Syringaldehyde

Lignin is an undervalued byproduct of the pulp and paper industry, contained in the black liquors. Concentrated black liquors are burned in boilers that usually have a fixed capacity and are very costly to upgrade. The application of lignin for other uses may debottleneck the production of cellulose by allowing larger quantities of black liquor to be burned since it would have a lower heating value after partial lignin precipitation [1]. Lignin is the second most abundant biopolymer and it is produced annually in the order of 1.2 Mt/y for lignin precipitated from different pulping processes, with 10% coming from Kraft processes [2]. There are commercially available technologies to remove lignin from the black liquor, for instance LignoBoost (Valmet) and LignoForce (FPInnovations) [3, 4].

Lignin has also the potential to become an alternative source for aromatic compounds due to its phenylpropane structure . The oxidative depolymerization of lignin produces a range of phenolic compounds that are already highly functionalized but they need a very laborious process of separation. Functional groups can be removed/reduced and the ratio of oxygen/hydrogen decreased all the way to benzene like structures. The functionalized molecules like vanillin and syringaldehyde can be used in the production of pharmaceuticals avoiding several steps of synthesis [5] while vanillic and syringic acids can be directed to the production of polymers [6].

Over the years, an integrated process for the production of monomers compounds including vanillin and syringaldehyde has been put forward (Figure 1). It encompasses several unit operations that include: oxidation of the lignin solution, filtration (ultra- and nanofiltration), adsorption, extraction and crystallization. The integrated process also allows the removal of lignin fractions for other applications from the membrane steps that can be used in the polyurethane resins/foams benefiting from the oxidation step, which increases the number of OH groups in lignin [7].

Figure 1 – Integrated process for the production of value added products like vanillin, syringaldehyde and lignin-based polyurethanes from Kraft lignin.

Alkaline wet oxidation is the most widely used method for the depolymerization of lignin, having the best yields. The yields also vary according to the lignin isolation method. The lignin solution is oxidized in a continuous reactor, a structured packed bubble column reactor [8]. The lignin is oxidized by an oxygen rich stream at 10 bar of total pressure, at an initial temperature of 140 C, in the continuous reactor.

The lignin that is not depolymerized is removed by ultra-/ nanofiltration in a flatsheet membrane (140 cm²) in a SEPA CFII filtration system. After removal of part of the undepolymerized lignin, the permeate is then fed to the chromatographic column. The membranes are able to reduce the total organic solids in solution. The ultrafiltration membrane has a cut‑off of 1000 Da and the nanofiltration membrane 600Da. The last membrane plays an important role since the lignin in solution will hinder the chromatographic process and the crystallization step due to contamination.

The chromatographic column is packed with SP700 resin and then the feed is fed at very alkaline pH. Annual plant (Tobbaco) and a softwood (Indulin AT) depolymerized lignin solutions were tested. The chromatographic step is able to fractionate the compounds in the oxidized solution into families of phenolic acids, phenolic aldehydes and phenolic ketones in a single column (Figure 2). The separation is achieved by using a two-eluent desorption scheme where water and ethanol are used [9, 10]. The solution eluting during the feed phase is rich in phenolic acids, (vanillic acid and syringic acid), the solution eluted with water is rich in the aldehydes (vanillin and syringaldehyde) and the solution eluted with ethanol is rich in ketones, (acetovanillone and acetosyringone). The fractionation scheme can be adjusted to collect the most concentrated fractions while reducing the amounts of desorbent used. This opens up great potential, since it is easier to work with solutions rich in vanillin/vanillic acid than with a mixture of several phenolic compounds. The fraction enriched in vanillin is then used to perform crystallization. The remaining contaminants are left in the crystallization broth and can be recycled back to the feed phase of the column since it may still contain phenolic acids and phenolic ketones. The same can be done with the other enriched fractions, achieving higher purities by crystallization.

Figure 2 – Fractionation of phenolic aldehydes, phenolic acids and phenolic ketones from a lignin (from tobacco stalks) oxidation mixture in a chromatographic column packed with SP700 resin. H – p‑hydroxybenzaldehyde; VA – vanillic acid; V – vanillin; SA – syringic acid; S – syringaldehyde; VO – acetovanillone; SO – acetosyringone. A – feed phase, B – water desorption phase, C – ethanol desorption phase. Feed pH = 10. Feed concentrations: [H]=0.014g/L, [VA]=0.152 g/L, [V]=0.138g/L, [SA]=0.170 g/L, [S]=0.103 g/L, [VO]=0.038 g/L, [SO]=0.037 g/L.

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