Pretreatment with Sodium Methyl Mercaptide Increases Carbohydrate Yield during Kraft Pulping

Abstract

Kraft pulping is the predominant technology in the pulp and paper industry for removing lignin from wood carbohydrates to produce paper, board, packaging, tissue, and specialty cellulose. However, the kraft process is energy intensive and expensive, and its yield is limited by the degradation of carbohydrates. Pretreatment can increase carbohydrate yield by limiting degradation via primary peeling of reducing end groups. However, protection of galactoglucomannan (GGM), the primary hemicellulose component of softwood, is minimal when conventional pretreatments are used. Here, we investigated the effectiveness of sodium methyl mercaptide pretreatment on southern pine wood chips under a range of experimental conditions. We found that pretreatment of biomass with 4.38% sodium methyl mercaptide at pH 12 and 105 °C for 60 min provided small but significant increases in xylan and cellulose yields relative to control conditions, but preservation of GGM was minimal. To provide insight into molecular-scale details of primary peeling, pretreatment, and alkaline hydrolysis, we performed classical molecular dynamics (MD) simulations under selected process conditions and quantum mechanical (QM) calculations of selected reactions. MD simulations showed that C1 of the GGM reducing end is more readily accessible by HO– and CH3S– ions than in cellulose. The free energy barrier for peeling calculated with QM is lower for GGM than for cellulose, indicating increased susceptibility to peeling. In addition, we found that GGM may be more susceptible to internal chain cleavage than cellulose. Thus, even though reducing end groups may be protected initially through pretreatment, new unprotected reducing end groups may be generated through alkaline hydrolysis. Taken together, these findings show the promise of methyl mercaptide as a pretreatment technology for cellulose retention and also provide molecular insight for improving its effectiveness toward GGM.

Evolution of Methyl Mercaptan in Its-Assisted Kraft Pulping Process

“…Although the rst application of MeSH is realized by accumulating the MeSH-containing exhaust gas in the blowing stage of kraft pulping (Tirado et al 1969), after that the large-scale application is rarely reported. It should be emphasized that the MeSH has a stronger nucleophilicity than hydroxide (OH − ) and hydrosul de (HS − ) (Patil and van Heiningen 2022a; Cooper et al 2021). Thus, it is reasonable to suspect that the demethylation and other deligni cation reactions of OH − and HS − could be greatly replaced by MeSH to generate dimethyl sul de (MeSMe) and other thioethers, to create a signi cant consumption on the reactive additive of MeSH.…”