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Combining chemi-mechanical pulping with autohydrolysis pretreatment is an efficient and value-added utilization approach for lignocellulosic biomass in paper industry. To further promote the utilization of autohydrolyzed biomass in chemi-mechanical pulping, the color evolution of poplar wood chips in autohydrolysis pretreatment and its chromogenic mechanism were investigated by using CIELab color system, FT-IR, NMR and GPC. The results showed that the total color difference ΔE* increased obviously, which were remarkable as the combined hydrolysis factor (CHF) increased. The lignin content led to a more significant influence on the color of poplar wood chips than the extractives. The autohydrolysis pretreatment with a higher CHF accelerated the degradation and subsequent condensation of lignin, resulting in the formation of chromophoric groups, such as Hibbert ketone, quinones and quinone methides. It is of great significance for biomass refinery and paper industry to reveal the color evolution of poplar wood chips caused by autohydrolysis pretreatment from the point of view of chemical components' structure. Unlabelled Image • The color evolution of autohydrolyzed wood chips was investigated. • Chromogenic mechanism of the wood chips in autohydrolysis process was revealed. • The color of poplar wood chips became deep after autohydrolysis with a higher CHF. • Lignin has a greater impact on the coloring of poplar wood chips than extractives. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Poplar sapwood chips were autohydrolyzed followed by an alkali impregnation. • Other chemical components had less impact on alkali impregnation than surface lignin. • Surface lignin had no effect on alkali impregnation compared to the volume porosity. • Autohydrolysis can facilitate chemi-mechanical pulping. Abstract Chemi-mechanical pulping, a typical high-yield pulping method, combined with autohydrolysis pretreatment prior to pulping is an efficient and value-added utilization method for biomass in pulp and paper industry. This study investigated the surface lignin changes of poplar sapwood chips in autohydrolysis pretreatment and their effect on the subsequent alkali impregnation for chemi-mechanical pulping. The results showed that the surface lignin content went up with the increase of autohydrolysis intensity, and that the existence of the surface lignin had nearly no impact on the subsequent alkali impregnation in making chemi-mechanical pulps (CMPs) compared to the volume porosity, which was validated by using the stepwise regression analysis. It can be further concluded that autohydrolysis can facilitate the subsequent alkali impregnation of the autohydrolyzed sapwood chips in making CMP, which would be of significance for the combination of biomass refinery and pulp and paper industry. [ABSTRACT FROM AUTHOR]

Poplar sapwood chips were autohydrolyzed at three severity levels and subsequently treated with alkali for the production of chemi-mechanical pulp (CMP). The aim was to study the variation of the principal pathways for sodium hydroxide solution and the alkali concentration and distribution inside the autohydrolyzed chips as a function of autohydrolysis (AH) severity. The tyloses in the vessels and some pit membranes were partly dissolved after AH, while the skeletal structure of the fiber and vessel cell walls remained intact. AH had a positive influence on the diffusion and the uniform distribution of NaOH solution both in the axial and radial direction of the chips, but the axial direction was more favored than the radial one. [ABSTRACT FROM AUTHOR]

This work is aimed at investigating the influence of changes in the content of hemicelluloses of the autohydrolyzed poplar sapwood on the subsequent alkali impregnation for chemi-mechanical pulping (CMP). An alkali impregnation process was conducted using the autohydrolyzed poplar sapwood with different content of hemicelluloses as raw materials. The results showed that both the amount of NaOH consumption and swelling degree of poplar sapwood increased with the removal of hemicelluloses, thus enhancing the alkali impregnation effectiveness. The hemicelluloses removal can also shorten the alkali impregnation time for the autohydrolyzed poplar sapwood to achieve the similar impregnation effectiveness of unautohydrolyzed poplar sapwood. All of these can be attributed to the fact that the hemicelluloses removal would result in the exposure of more free hydroxyl groups on the cellulose and an increase in the porosity of the fiber cell walls. [ABSTRACT FROM AUTHOR]

Dilute acid pre-extraction enhanced the mechanically refined poplar pulp substrates’ enzymatic hydrolysis efficiency obviously. The results showed that the surface lignin distribution was changed significantly in residual wood chips and pulp substrates, and the surface lignin distribution showed important impact on the following enzymatic hydrolysis. Acid pre-extraction can lead to a redistribution of lignin in fiber cell walls, i.e., the lignin was degraded and migrated to fiber surface in the form of re-deposited lignin and pseudo-lignin. However, higher pre-extraction intensity was not desired due to the formation of redeposited lignin and pseudo-lignin. This study will help to reach a deeper understanding on the lignin distribution in the view of molecular and ultrastructure, and promote the development of a cost-efficient pretreatment strategy for biomass processing. [ABSTRACT FROM AUTHOR]

Given the current scarcity of wood fiber resources, it is imperative to investigate the potential of employing weak alkaline hydrogen peroxide for bleaching high-yield pulp (HYP) to meet the increasing demands for paper and paperboard. This work presents an investigation into the characteristics of triploid poplar chemi-thermomechanical pulp (CTMP) hydrogen peroxide bleaching, including the brightness, bleaching yield, bulk, physical strength, and contents of carbohydrates and lignin. The findings show that partial substitution of MgO for NaOH as an alkali source could improve the yield and bulk of the bleached CTMP. When the substitution ratio of MgO for NaOH reached 75 %, the brightness of 74.62 % ISO for the bleached pulp dropped by only 0.47 percent points but the bulk of 2.21 cm3/g increased by 5.74 % compared to those of the control, respectively. Meanwhile, the yield of the bleached CTMP could reach 93.45 %, 5.07 percent points higher than that of the control. The results would be of benefit to the HYP enterprises concerned. • Substitution of MgO for NaOH fulfilled the criteria for achieving good brightness. • Substitution of MgO for NaOH improved the yield of bleached CTMP and bulk. • Adding MgO increased the content of carbohydrate and lignin in the bleached pulp. • The surface lignin content would increase with raising the ratio of MgO-to-NaOH. [ABSTRACT FROM AUTHOR]

• Mixed triploid poplar and acacia woodchips P-RC APMP was made. • The obtained pulp with the substitution of 50 wt% NaOH by MgO had an excellent bulk. • The bleaching effluent of the obtained pulp had a low COD. • The bleaching reaction rate depended on the concentration of bleaching reagents. • The magnitude of the dependent extent is [H 2 O 2 ] > [NaOH] > [MgO] > [Na 2 SiO 3 ]. In the present work, the H 2 O 2 bleaching with the substitution of 50 wt% NaOH by MgO as the alkali source in the high-consistency (HC) retention stage of mixed triploid poplar (Populus tomentosa Carr.) and acacia (Acacia confusa Merr.) woodchips P-RC APMP (preconditioning followed by refiner chemical treatment, alkaline peroxide mechanical pulp) was conducted. Meanwhile, the related bleaching reaction kinetics was well established. The obtained high-yield pulp presented a higher bleaching yield, improved bulk, favorable brightness and physical properties in comparison with the obtained pulp using 100 % NaOH as the alkali source. Most importantly, the chemical oxygen demand (COD) of the bleaching effluent was greatly decreased. The established bleaching kinetic equation is as following: dw dt = K [ H 2 O 2 ] 3.63 [ NaOH ] 1.08 [ MgO ] 0.57 [ Na 2 SiO 3 ] 0.05 w − 1.89 K = A·e –E/RT where the activation energy E =57.2 kJ/mol. It demonstrates that the bleaching reaction rate greatly depends on the concentration of primary bleaching reagents. The magnitude of the dependent extent is [H 2 O 2 ] > [NaOH] > [MgO] > [Na 2 SiO 3 ]. The rapid rise of the pulp brightness increment occurred in the first 35 min in the bleaching reaction. [ABSTRACT FROM AUTHOR]

In this work, poplar wood chips (PWCs) were subjected to autohydrolysis pretreatment with different levels of intensity, and the autohydrolyzed PWCs were used to make chemi-thermomechanical pulp (CTMP). The changes of the lignin structure of PWCs and the bleachability of the CTMP with autohydrolyzed PWCs as raw materials were both investigated. The results showed that the lignin structure changed to some extent in both CTMP pulping and the following bleaching process. The contents of conjugated carbonyl and quinone groups in the lignin structure were negatively correlated with the CTMP bleachability. Compared to the quinone groups, the conjugated carbonyl groups had a much greater impact on the bleachability. The present work would play a vital role for the application of autohydrolysis to the poplar CTMP pulping and the subsequent bleaching processes. [Display omitted] • Poplar wood chips (PWCs) were autohydrolyzed with different intensity levels. • Autohydrolyzed PWCs were used to make bleached chemi-thermomechanical pulp (CTMP). • Changes in lignin structure of PWCs and the CTMP bleachability were both explored. • Autohydrolysis was beneficial to the bleachability as the CHF did not exceed 10.77. • Conjugated carbonyl had bigger impact on CTMP bleachability than quinone group. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrothermal & chemi-mechanical pretreatment greatly improved enzymatic hydrolysis. • Different combined pretreatments on enzymatic hydrolysis efficiency were compared. • HFM pretreatment showed the highest enzymatic hydrolysis performance. • Adsorption kinetics of enzymes on different substrates were investigated. • Adsorption mechanism of enzymes on different substrates was summarized. An integration of different pretreatments is important to overcome recalcitrance and realize efficient bioconversion of lignocellulosic biomass. This study aims at the effects of combination of hydrothermal pretreatment and different chemi-mechanical pretreatments on enzymatic hydrolysis, and understanding the enzymes adsorption mechanism. The combination of hydrothermal and chemi-mechanical pretreatments effectively improved the enzymatic hydrolysis of poplar substrates, in which the enzymatic hydrolysis of substrates pretreated by hydrothermal pretreatment + Fenton pretreatment + mechanical refining (HFM) was the highest (92.39% of glucose conversion yield, and 20.88 g/L of glucose concentration). The substrates' main characteristics were obviously changed after combined pretreatments, such as swelling ability and specific surface area of substrates were increased. The Langmuir adsorption model (R2 > 0.98) and pseudo second-order adsorption kinetic model (R2≈1) were well suitable to describe the adsorption of enzymes on substrates, meanwhile the adsorption mechanism was summarized. [ABSTRACT FROM AUTHOR]

• Hydrothermal and Fenton pretreatment synergistically promote enzymatic hydrolysis. • Fenton pretreatment can help to solve inhibition of pseudo lignin problem. • Pre-hydrolysates provide additional revenues such as XOS, GOS, and xylose. • Main substrates' characteristics were changed by combined pretreatments. The combination of different pretreatment methods can effectively overcome recalcitrance of lignocellulosic biomass to ensure its highly efficient conversion into bio-based products. In this study, the combined pretreatments of chemical methods (hydrothermal treatment and Fenton treatment) with mechanical refining were used to improve the enzymatic hydrolysis efficiency of poplar branches. The results indicated that hydrothermal pretreatment and Fenton pretreatment can effectively improve the enzymatic hydrolysis of poplar substrates, e.g., the maximum glucose conversion yield and glucose concentration reached 92.4% and 20.8 g/L, respectively. The pre-hydrolysates contained some valuable components such as monosaccharides, oligosaccharides, acetic acid, furfural, and hydroxymethylfurfural. The main characteristics (specific surface area, water retention value, fines content, and surface lignin concentration) of poplar substrates were obviously changed by the combined pretreatment, which benefit the enzymatic hydrolysis. [ABSTRACT FROM AUTHOR]