New Understandings of the Relationship and Initial Formation Mechanism for Pseudo-lignin, Humins, and Acid-Induced Hydrothermal Carbon

Abstract

The generation of pseudo-lignin as byproduct during the lignocellulose acidic pretreatment has been proposed for many years. However, the detailed formation mechanism is still unclear. Moreover, there is a lack of understanding in the initial reaction during the formation of humins (byproducts in furfural production) and acid-induced hydrothermal carbon (carbon material). In this work, the initial formation of these three substances were investigated. We first found the common feature of their formation process was that carbohydrate-hydrolyzed compounds could form black polymers by condensing in acidic media, but the difference was dependent on the reaction degree. Furthermore, the results revealed that oxidation was an accelerator for condensations during producing black polymers because oxidized compounds could enhance the acidity of the reaction system. However, condensations of oxidized compounds were more difficult to proceed. Meanwhile, during the initial stage, the dominating pathway was that furfural condensed with itself and isomerized xylose via aldol-condensation.

Insights into the effect of hydrolysis medium on the holocellulose decomposition, furfural, and pseudo‐lignin formation

“…4). The initial lignin can accelerate subsequent pseudo‐lignin formation through condensation between lignin fractions and the generated intermediates 24,32,39 . In this case, the degraded lignin fractions in the liquor probably act as a matrix, where other intermediates are constantly growing through complex reactions.…”

“…The sugar composition in the filtrate was detected by high‐performance liquid chromatography (HPLC) (Shimadzu, Japan) prior to the test. The sample was treated as discussed in the literature 24 . Acid‐insoluble lignin and acid‐soluble lignin was determined by the Tappi standard 23 .…”

A new approach to recycle oxalic acid during lignocellulose pretreatment for xylose production

“…5 e). These microspheres might be lignin microspheres or pseudo-lignin microspheres which were formed from carbohydrates [ 36 ].

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“…For the fifth pretreatment, the decreases of xylose contents were due to the losses of oxalic acid. Furthermore, byproduct such as humins from previous cycle would lead to decreases in the xylose contents [ 36 ]. After the fifth cycle, there are still some xylose, arabinose and glucose in residual Solid 3 (Fig.…”

Conversion of Xylose to Furfural over Lignin-Based Activated Carbon-Supported Iron Catalysts

“…This indicates that the catalyst was adsorbing some reaction products, such as humins. Humins are black carbonaceous side-products generated either by the cross-polymerization of furfural by itself, between the just-formed furfural and free xylose present in solution or between furfural and intermediate products [50][51][52]. Solid humins can plug the pores of the catalyst surface, block the access of xylose to metal surface sites or totally encapsulate a metal particle [53].…”