Catalysis-a potential alternative to kraft pulping

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; and is indifferent to wood raw material and good at preserving the cellulose portion of the wood, the part that provides strong fibers. Although the process is odiferous, 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 no new pulping processes have been discovered since the discovery of kraft pulping. Besides the minor adjustments—such as anthraquinone or polysulfide— chlorine, chlorite, and peracetic acid holopulping were discovered and evaluated decades ago. Various solvent pulping methods were discovered and evaluated in the 1980s and 1990s. This work continues with ionic solvents, deep eutectic solvents, and most recently protic ionic liquids. Where all these alternative processes fail is process cost. The chemicals are too expensive and too difficult to recover for use in a commercial process to produce wood pulp. The premise of this review is that the only way to achieve better performance and 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. We reviewed the literature of existing pulping and delignification catalysts and propose research areas of interest for more intensive experimental efforts.