Lignin Sulfonation and SO2 Addition Enhance the Hydrolyzability of Deacetylated and Then Steam-Pretreated Poplar with Reduced Inhibitor Formation

Abstract

The merit of deacetylation of corn stover prior to pretreatment is decreasing the formation of inhibitors and improving enzyme hydrolysis, proved in dilute acid pretreatment. However, few studies are done on how deacetylation would affect bioconversion process containing steam explosion. In this study, the effect of deacetylation on steam explosion was conducted using poplar as substrate. About 57 to 90% of acetyl group in poplar, depending on alkaline types and concentration, was removed by dilute alkaline deacetylation in 6 h. Deacetylation eliminated over 85% of inhibitor formation during downstream steam explosion. However, deacetylation prior to steam explosion decreased the dissolution of hemicellulose, thus reducing the cellulose accessibility of pretreated poplar, finally resulting in 5-20% decrease in glucose yield and 20-35% decrease in xylose yield. The addition of 5% SO during steam explosion significantly improved the hydrolysis of deacetylated and pretreated poplar without significantly increasing the concentration of inhibitors. Incorporating 45 mmol/kg sulfoacid group in lignin fraction of deacetylated and then pretreated poplar dramatically improved the xylose yield to about 100% and increased the glucose yield by 30%.

Improving Enzymatic Saccharification and Ethanol Production from Hardwood by Deacetylation and Steam Pretreatment: Insight into Mitigating Lignin Inhibition

“…Besides, the AIL/ASL ratio of DEA-SP-treated substrates was slightly higher than the DEA-treated substrates, but significantly lower than that of SP and LHW-SP substrates (Figure 1), owing to the reduced formation of acid catalyst in steam explosion pretreatment. 13 It was suggested that the formation of carbocation intermediates under acidic conditions was the root cause for lignin repolymerization, so that shifting the pH condition of steam treatment toward more alkaline side and the addition of a proper carbocation scavenger in acidic pretreatments were considered as two effective methods to reduce the formation of carbocation. Results in this work showed that a deacetylation step prior to steam pretreatment could largely reduce the formation of acid during steam pretreatment, which shifted the pH condition of steam treatment toward more neutral or alkaline side (Table 1), thus had the potential to reduce the generation of lignin carbocation intermediates.…”

“…Moreover, it has been reported that the addition of Na 2 S and Na 2 SO 3 in alkali media could diminish the formation of toxic byproducts such as acetic acid, furfural, and 5hydroxymethylfurfural, which would benefit subsequent saccharification and fermentation steps. 33,46,47 Additionally, the sodium carbonate, sulfide, and sulfite could be recovered in existing industrial practices of pulp mills, 13,32,33 in which the washing operation following pretreatment to separate spent liquor from pretreated solid for chemicals recovery (Figure 6) could remove inhibitory compounds to a greater extent. 54 These factors could account for the efficient enzymatic hydrolysis and fermentation of deacetylation−steam-pretreated biomass.…”

“…(3) the partial removal of lignin and ash in deacetylation plays a role in increasing the biomass-specific surface area or introducing the substantial nanoscale porosity to biomass, 13,17,19 thus promoting the cellulose accessibility; 20,21 (4) deacetylation decreases the formation of acetic acid in pretreatment, 14 which reduces the generation of pseudolignin and lignin droplets on the fiber surface. 17,20 Based on these advantages, significantly improved biomass saccharification could be anticipated after deacetylation−steam pretreatment and subsequent enzymatic hydrolysis for efficient bioethanol production from lignocellulosic biomass.…”

“…Furthermore, it has been proposed that the residual lignin in pretreated biomass plays two primary roles in affecting enzymatic hydrolysis: (1) physical blockage effect in restricting the fiber swelling or the cellulose accessibility to enzymes 31 and (2) unproductive binding of enzymes, thereby reducing the amount of enzymes available to cellulose. 10,13,32 For deeper interpretations, in this work, the physical blockage effect of lignin was isolated from unproductive binding effect, and the relative contributions of lignin toward enzyme binding and physical blockage were quantified. Based on the isolation and quantification, the lignin inhibition mitigating effect of deacetylation toward enzyme binding and physical blockage would be figured out.…”

Recent Trends in the Pretreatment of Lignocellulosic Biomass for Value-Added Products

“…Recently, a study on deacetylation of corn stover prior to steam explosion showed that deacetlyation eliminated over 85% inhibitor formation. However, an incorporation of a catalyst was required for better hydrolysis of the deacetylated feedstock (Tang et al, 2018).…”