Catalysis: A Potential Alternative to Kraft Pulping. A Synthesis of the Literature, 2014 PEERS Conference

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A thorough analysis of the kraft pulping process makes it obvious why it has dominated for over a century as an industrial process with no replacement in sight. It uses low cost raw materials, collects and regenerates over 90% of the chemicals needed in the process, is indifferent to wood raw material and good at preserving the cellulose portion of the wood which is the part that provides strong fibers. Although an odiferous process, extremely capital intensive, and very poor at preserving hemicellulose yield, no alternatives have been able to replace it for process cost and product quality. There is a misconception that there have been no new pulping processes discovered since the discovery of kraft pulping. Besides the minor adjustments like antrhraquinone or polysulfide, chlorine, chlorite and peracetic acid holopulping were discovered and evaluated decades ago. Various solvent pulping methods were discovered and evaluated in the 1980’s and 1990’s and this litany continues with ionic solvents, deep eutectic solvents and most recently protic ionic liquids. Where all of these alternative processes fail is process cost. The chemicals are too expensive and too hard to recover to use for commercial production of wood pulp. The premise of this review is that the only way to achieve better performance and a lower cost than the existing kraft pulping process will be a process using a catalyst to control and direct the reactions. With a high enough reaction rate and a sufficiently high number of turnovers, even an expensive catalyst can still be low cost. This effort has reviewed the literature of existing pulping and delignification catalysts and proposes research areas of interest for more intensive experimental efforts.