Electrochemical Delignification of Wood Pulp Using Polyoxometalate Mediators

It has been found that polyoxometalates (POMs) can act as mediators in the electrochemical oxidation of lignin in pulps. An electrochemical cell, with a Nafion membrane separating the anode and cathode compartments, was used in the delignification experiments. A softwood kraft pulp was placed in the anode compartment with a buffered 0.01M solution of the polyoxometalate Na5[SiVW11O40]. During electrolysis the lignin content of the pulp was reduced from kappa number 26 to 6. The energy required for this delignification was equivalent to 120 kW·hr per ton of pulp. At $0.05/ kW·hr, the cost for this 20-point drop in kappa number would be $6 per ton of pulp. Based on this and other data, it may be both technically and economically feasible to delignify pulps electrochemically using POMs as mediators. The POMs are neither consumed nor modified during the reaction and can, therefore, be recovered and reused in this novel, closed-mill, delignification process. INTRODUCTION A significant amount of work has been concentrated on using enzymes, derived from woodrotting fungi, to remove lignin from wood. These enzymes work by using transition metals within organic ligands to shuttle electrons to oxygen resulting in the oxidation and removal of lignin from the wood while leaving the cellulose components intact. However, enzymatic technology is not generally used on an industrial scale because of their expense, slow reaction rates and susceptibility to oxidation and thermal degradation. POMs can function as inorganic analogs of these enzymes, facilitating control of complex transition-metal chemistry in water. POMs are early-transition-metal, oxygen-anion clusters and are a large, structurally diverse and rapidly growing class of inorganic compounds. [1,2] They are formed by the condensation of common, soluble oxoanions of vanadium, molybdenum and tungsten. Additionally, a variety of redox-active transition metals can be substituted into the POM structures. POMs exhibit a number of properties that make them useful in industrial applications. They are reversible oxidants that are water-soluble, thermally stable, oxidatively stable, minimally toxic, relatively inexpensive and easily synthesized. Additionally, POMs offer tremendous flexibility as many of their key physical properties, such as redox potential, acidity, charge, solubility, etc., can be controlled to a marked degree by choice of precursors and conditions. It is unlikely if another class of compounds exists whose properties can be so extensively and readily altered. POMs can range in size from 9Å to over 30Å. The most common and most thoroughly investigated structure is the Keggin anion (ca. 11 Å) shown in Figure 1. Figure 1. A substituted, α-Keggin [SiVW11O40] heteropolyanion displayed in polyhedral notation. Each polyhedron represents a maingroup or transition-metal atom at its center with oxygen atoms at each of its vertices. The black tetrahedron in the center represents the oxide of the Si atom, the eleven gray octahedra represent the oxides of the structural W atoms and the white octahedron represents the oxide of the substituted V atom. These novel POM systems are currently being studied and optimized for use in a closed-mill delignification process of wood pulp fibers. Their performance, in the context of pulp bleaching, is indicated by the capacity to reduce the kappa levels of softwood kraft pulps from 30 to below 5 while retaining viscosity above 20 mPa⋅s. It is now clear that POM delignification technology is applicable both toward achieving a totally-chlorine-free (TCF) closed-mill process in the context of pulp bleaching and toward the cost-effective expansion of capacity of recovery-boiler-limited pulp mills. [3] In the proposed electrochemical bleaching process, shown in Figure 2, POMs are utilized as recoverable and regenerable intermediaries in the electrochemical delignification of wood pulp. As currently envisioned, a closed bleaching mill will consist of five unit operations: electrochemical bleaching, pulp washing, concentration of wash water, removal of non-process elements (NPEs) and wet air oxidation (WAO) of dissolved organic compounds. During the bleaching step, dilute POM solutions are applied to pulp in an electrochemical cell. In this operation, the POMs are reduced while lignin is selectively oxidized and dissolved into the bleach liquor. The POMs will then be electrochemically regenerated within this same reactor. Thus, a relatively small amount of POM catalyst will be repeatedly used in multiple reduction and oxidation cycles within the same reactor. Equations 1 and 2, below, summarize these reactions. (In Equations 1 and 2, POMox and POMred represent the oxidized and reduced polyoxometalates respectively; and LigH2 and Ligox represents the reduced and oxidized lignin.) Depending upon the configuration of the electrochemical reactor, hydrogen and sodium hydroxide may be produced. The hydrogen could be used as a fuel or chemical agent anywhere in the pulp mill while any sodium hydroxide would be returned to the POM stream to maintain the chemical balance of the POM system. 2 POMox + LigH2 → 2 POMred + Ligox + 2 H (Eq. 1) 2 POMred + 2e + 2H → 2 POMox + H2 (Eq. 2) Next, the POM is separated from the bleached pulp with pressing and washing; recovery of the POMs in this step is important to the economic viability of the process. The POM liquor is then concentrated and the wash water recovered. At some point in the process, there must be a step to remove NPEs from the POM stream. These include undesirable elements from the pulp, e.g., calcium, iron, copper, etc., and chemicals carried over from the kraft pulping process. Finally, the dissolved organic compounds must be separated from the POM stream. Currently, we envision their complete mineralization to carbon dioxide and water. Under the aggressive aerobic conditions of this step, POMs initiate and catalyze the wet air oxidation (WAO) of dissolved lignin and polysaccharide fragments introduced during the bleaching step. [5] As a consequence, carbon dioxide and water are the only by-products of the POM delignification process. Kraft Pulp Bleached Pulp