Pulp properties and their influence on enzymatic degradability

Selecting appropriate cellulose morphology to enhance the nitrogen content of nitrocellulose

This study aims to determine the effect of cellulose morphology on enhancing the nitration of cellulose to achieve nitrocellulose with a high nitrogen content. Cotton linter was employed as a point of reference, and Luffa cylindrica and coffee pulp cellulose were used for comparison. Luffa cylindrica and coffee pulp cellulose were used considering their distinctive morphological characteristics compared to cotton linter. They were nitrated at room temperature for 60 minutes using a mixture of technical grade nitric acid and sulfuric acid with a nitric acid : sulfuric acid ratio of 1 : 3 and cellulose : nitric acid ratio of 1 : 45. The results showed that luffa cellulose is similar in characteristics to cotton linter, which exhibits short, thin-walled tubular ribbon-like microfibrils with 75.09% crystallinity and a specific surface area of 0.70 m2 g-1. Nitration of luffa cellulose resulted in nitrocellulose with a nitrogen content as high as 13.67%, which is higher than that of cotton linter with a value of 13.49%. This value of nitrogen content was high enough to be applied even in military-grade applications. Morphology was revealed as the most influential characteristic of nitration, which allows for the preparation of nitrocellulose with qualities similar to cotton linter-based nitrocellulose. Thus, it was demonstrated that nitrocellulose with a high nitrogen content could be achieved despite using a technical grade nitration reagent by selecting cellulose with an appropriate morphology.

Alkalization of Kraft Pulps from Pine and Eucalyptus and Its Effect on Enzymatic Saccharification and Viscosity Control of Cellulose

Bleached kraft pulps from eucalyptus and pine were subjected to cold caustic extraction (CCE) with NaOH (5, 10, 17.5, and 35%) for hemicelluloses removal and to increase cellulose accessibility. The effect of these changes was evaluated in enzymatic saccharification with the multicomponent Cellic CTec3 enzyme cocktail, and in viscosity reduction of pulps with the monocomponent Trichoderma reesei endoglucanase (EG). After CCE with 10% NaOH (CCE10) and 17.5% NaOH (CCE17.5), hemicellulose content lower than 1% was achieved in eucalyptus and pine pulps, respectively. At these concentrations, cellulose I started to be converted into cellulose II. NaOH concentrations higher than 17.5% decreased the intrinsic viscosity (from 730 to 420 mL/g in eucalyptus and from 510 to 410 mL/g in pine). Cellulose crystallinity was reduced from 60% to 44% in eucalyptus and from 71% to 44% in pine, as the NaOH concentration increased. Enzymatic multicomponent saccharification showed higher glucose yields in all CCE-treated eucalyptus samples (up to 93%) while only CCE17.5 and CCE35 pine pulps achieved 90% after 40 h of incubation. Untreated bleached pulps of both species presented saccharification yields lower than 70%. When monocomponent EG was used to treat the same pulps, depending on enzyme charge and incubation time, a wide range of intrinsic viscosity reduction was obtained (up to 74%). Results showed that eucalyptus pulps are more accessible and easier to hydrolyze by enzymes than pine pulps and that the conversion of cellulose I to cellulose II hydrate only has the effect of increasing saccharification of CCE pine samples. Viscosity reduction of CCE pulps and EG treated pulps were obtained in a wide range indicating that pulps presented characteristics suitable for cellulose derivatives production.

Effect of Enzymatic Depolymerization of Cellulose and Hemicelluloses on the Direct Dissolution of Prehydrolysis Kraft Dissolving Pulp

Prehydrolysis kraft (PHK) pulps account for more than half of the global market of dissolving pulp. Characterized by high reactivity toward dissolution, their performances can still be improved by activation treatments. This study compares the dissolution kinetics in cupriethylenediamine of a hardwood and a softwood PHK pulps before and after their activation by high-solid-content mechano-enzymatic treatments. Three enzyme combinations were tested: endoglucanase (E), xylanase and mannanase (XM), and endoglucanase, xylanase, and mannanase (EXM). Xylanase and mannanase reduced the hemicellulose content of only hardwood (by max. 2.4%). Mixing and carbohydrate depolymerization decreased the dissolution time of hardwood and softwood pulps by a maximum of 63 and 30% with E, 37 and 16% with XM, and 44 and 30% with EXM, respectively. The shortening of the dissolution time was partially hindered by hornification, which increased with hemicellulose degradation. Interestingly, XM accelerated the dissolution while preserving a high weight-average molecular mass.

Enzymatic Fibre Modification During Production of Dissolving Wood Pulp for Regenerated Cellulosic Materials

The production of regenerated cellulosic fibres, such as viscose, modal and lyocell, is based mainly on the use of dissolving wood pulp as raw material. Enzymatic processes are an excellent alternative to conventional chemical routes in the production of dissolving pulp, in terms of energy efficiency, reagent consumption and pulp yield. The two main characteristics of a dissolving pulp are the cellulose purity and the molecular weight, both of which can be controlled with the aid of enzymes. A purification process for paper-grade kraft pulp has been proposed, based on the use of xylanases in combination with hot and cold caustic extraction, without the conventional pre-hydrolysis step before kraft pulping. This enzyme aided purification allowed the production of a dissolving pulp that met the specifications for the manufacture of viscose, < 3% xylan, > 92% ISO brightness and 70% Fock's reactivity. Endoglucanases (EGs) can efficiently reduce the average molecular weight of the cellulose while simultaneously increasing the pulp reactivity for viscose production. It is shown in this study that lytic polysaccharide monooxygenases act synergistically with EGs in the modification of bleached dissolving pulp.

A study elucidating the relation between cellulose dissolution and crystallinity after cellulase treatment at different doses

Cellulose is the most abundant renewable resource which has found a diverse range of applications. Cellulose dissolution is a significant property for manufacturing man-made cellulosic fiber through viscose process. Crystalline microfibrillar structure and relatively high ordered packing of polymeric chains contribute to recalcitrance and poor reactivity of cellulose. One of the most common methods to improve cellulose dissolution is cellulase treatment. Herein, cellulase treatment at different doses was studied to explore the correlation of cellulose dissolution with crystallinity. Pulp showed improvement in Fock reactivity and other properties related to viscose application. But contrary to previous studies, cellulose crystallinity as determined by XRD and FTIR did not correlate with Fock reactivity at a higher dose of cellulase. The results indicated some complex mechanism to be involved between the cellulose dissolution and crystallinity than a simple negative correlation. Cellulase treatment at 150 HCU/g resulted in the upgraded pulp suitable for viscose application.

Bacterial cellulase treatment for enhancing reactivity of pre-hydrolysed kraft dissolving pulp for viscose

To improve the process economy of reactivity improvement, crude cellulase from Bacillus subtilis was employed for the treatment and significant dissolving pulp properties were analyzed. With increase in enzyme dose from 0.25 to 2 U/g o.d. pulp, improvement in Fock reactivity and alkali solubilities (S10 and S18) were observed with simultaneous reduction in viscosity and yield. Fourier transform infrared spectroscopy and scanning electron microscopy were used to observe the molecular level effects on dissolving grade pulp. The most suitable cellulase dose for reactivity improvement with lowering of viscosity was 0.25 U/g o.d. pulp. With increases in enzyme dose, alkali solubilities (S10 and S18) of dissolving pulp showed continuous increment, while alpha-cellulose of pulp showed reduction due to chain scission of long cellulose fiber fraction.

Heteropoly acid catalytic treatment for reactivity enhancement and viscosity control of dissolving pulp

The reactivity enhancement and viscosity control are of practical importance during the manufacture of high-quality cellulose (also known as dissolving pulp). In the study, the concept of using phosphotungstic acid (HPW) for this purpose was demonstrated. The Fock reactivity of resultant pulp increased from 49.1% to 74.1% after the HPW catalytic treatment at a dosage of 86.4 mg HPW/g odp. The improved results can be attributed to the increased fiber accessibility, thanks to the favorable fiber morphologic changes, such as increased pore volume/size, water retention value and specific surface area. HPW can be readily recycled/reused by evaporating method, where maintaining 87.1% catalytic activity after six recycle times. The HPW catalytic treatment concept may provide a green alternative for the manufacture of high-quality dissolving pulp.

Assessing the reactivity of cellulose by oxidation with 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxo-piperidinium cation under mild conditions

The accessibility and reactivity of cellulose are key parameters in its conversion into various products. Several indirect measures, such as water retention value (WRV), fiber saturation point (FSP) and specific surface area (SSA), are often used to characterize cellulosic samples for their reactivity. In this paper, we report on using oxidation with 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxo-piperidinium cation (4-AcNH-TEMPO⁺) as a probe reaction for the reactivity of cellulose in mild conditions (pH 9, room temperature). 4-AcNH-TEMPO⁺ is able to selectively convert hydroxymethyl groups into carboxylate groups. The time dependence of the conversion was monitored by iodometric quantification of the residual 4-AcNH-TEMPO⁺. Soluble substrates, such as 1-propanol and maltose, were quantitatively oxidized in ca. 1min while 3-16% of cellulose was oxidized in ca. 15min depending on its origin. Extrapolation of the slow residual oxidation to zero time allowed quantification of the easily reactive or accessible cellulose. The 4-AcNH-TEMPO⁺ reactivity was correlated with several pulp characteristics, including WRV, FSP, SSA, chemical composition, crystallinity, the pulping process and the drying history.

Modification of Different Pulps by Homologous Overexpression Alkali-Tolerant Endoglucanase in Bacillus subtilis Y106

Cellulase (mainly endoglucanase, EG) has been used in pulp modification for improving paper quality through environmentally friendly process. But low activity in alkaline pH and high filter paper activity (FPA) were still obstacles for extending the cellulase application in papermaking industry. In the study, an alkali-tolerant EG gene of Bacillus subtilis Y106 was homologous over-expressed for obtaining suitable enzyme used in pulp modification. The engineering strain could produce the crude enzyme with more alkali-tolerant EG and little PFA. Potential of the crude enzyme in modification of different pulps were investigated. It was found that the enzyme could be used for improving drainage and strength properties of pulps from softwood, hardwood and non-wood materials, especially non-wood pulp such as wheat straw pulp. The underlying mechanisms of pulp modification and different effects on various types of pulps by the EG treatment were also discussed by studying the change in fibers characteristics and fiber bonding.

Kinetics and mechanism of hemicelluloses removal from cellulosic fibers during the cold caustic extraction process

The effective separation of hemicelluloses and cellulose is desirable for the production of high-purity cellulose, which is a sustainable raw material for many value-added applications. For this purpose, the kinetics and mechanism of hemicelluloses removal from the cold caustic extraction (CCE) were investigated in the present study. The hemicelluloses removal process consists of: 1) the bulk phase, characteristic of significant hemicelluloses removal; 2) the transition phase, hemicelluloses transferring from the inner to the outer region of the fiber wall, with negligible overall hemicelluloses removal; 3) the residual phase, presenting a weak but continuing hemicelluloses removal. Furthermore, the enzymatic peeling method was adopted to study the fundamentals of hemicelluloses removal. The results showed that the molecular weight of hemicelluloses is the main parameter governing their diffusion/dissolution processes, and that the low molecular weight hemicelluloses are preferentially removed.

A biorefinery scheme to fractionate bamboo into high-grade dissolving pulp and ethanol

BACKGROUND

Bamboo is a highly abundant source of biomass which is underutilized despite having a chemical composition and fiber structure similar as wood. The main challenge for the industrial processing of bamboo is the high level of silica, which forms water-insoluble precipitates negetively affecting the process systems. A cost-competitive and eco-friendly scheme for the production of high-purity dissolving grade pulp from bamboo not only requires a process for silica removal, but also needs to fully utilize all of the materials dissolved in the process which includes lignin, and cellulosic and hemicellulosic sugars as well as the silica. Many investigations have been carried out to resolve the silica issue, but none of them has led to a commercial process. In this work, alkaline pretreatment of bamboo was conducted to extract silica prior to pulping process. The silica-free substrate was used to produce high-grade dissolving pulp. The dissolved silica, lignin, hemicellulosic sugars, and degraded cellulose in the spent liquors obtained from alkaline pretreatment and pulping process were recovered for providing high-value bio-based chemicals and fuel.

RESULTS

An integrated process which combines dissolving pulp production with the recovery of excellent sustainable biofuel and biochemical feedstocks is presented in this work. Pretreatment at 95 °C with 12% NaOH charge for 150 min extracted all the silica and about 30% of the hemicellulose from bamboo. After kraft pulping, xylanase treatment and cold caustic extraction, pulp with hemicellulose content of about 3.5% was obtained. This pulp, after bleaching, provided a cellulose acetate grade dissolving pulp with α-cellulose content higher than 97% and hemicellulose content less than 2%. The amount of silica and lignin that could be recovered from the process corresponded to 95 and 77.86% of the two components in the original chips, respectively. Enzymatic hydrolysis and fermentation of the concentrated and detoxified sugar mixture liquor showed that an ethanol recovery of 0.46 g/g sugar was achieved with 93.2% of hydrolyzed sugars being consumed. A mass balance of the overall process showed that 76.59 g of solids was recovered from 100 g (o.d.) of green bamboo.

CONCLUSIONS

The present work proposes an integrated biorefinery process that contains alkaline pre-extraction, kraft pulping, enzyme treatment and cold caustic extraction for the production of high-grade dissolving pulp and recovery of silica, lignin, and hemicellulose from bamboo. This process could alleviate the silica-associated challenges and provide feedstocks for bio-based products, thereby allowing the improvement and expansion of bamboo utilization in industrial processes.

Fractionation and cellulase treatment for enhancing the properties of kraft-based dissolving pulp

The aim of this study was to investigate a combined process involving pulp fractionation and cellulase treatment of each fraction for improving the molecular weight distribution (MWD) and reactivity of a kraft-based dissolving pulp. Three pulp fractions, namely long-fiber, mid-fiber and short-fiber fractions (LF, MF and SF, respectively), were used as the substrates. The results showed that the SF had the highest accessibility, lowest viscosity, and highest cellulase adsorption capacity, while the opposite was true for the LF. At a given viscosity, the combined process led to a lower polydispersity index (3.71 vs 4.98) and a higher Fock reactivity (85.6% vs 76.3%), in comparison to the conventional single-stage cellulase treatment.

Pretreatment with xylanase and its significance in hemicellulose removal from mixed hardwood kraft pulp as a process step for viscose

The upturn of viscose fiber market has triggered an augmented dissolving pulp usage over the last decade. Dissolving pulp is feasible to obtain from kraft pulp after two essential steps including hemicellulose removal and subsequent pulp activation. Prerequisite of conversion being hemicellulose reduction can be gently done by using xylanase treatment prior to alkali extraction. Herein, the significance of xylanase treatment and the optimum xylanase dose required in conjunction with subsequent alkali extraction was investigated. An increase in xylanase dose prior to alkali extraction had no significant effect on pentosans while the Fock reactivity and viscosity both improved at the dose of 50AXU/g. Also, alkali extraction without xylanase pretreatment resulted in decreased Fock reactivity, alpha cellulose, brightness and viscosity of paper grade pulp. A moderate dose of xylanase prior to alkali extraction can thus be used to facilitate the hemicellulose removal while simultaneously protecting the native structure of cellulose.

Combination of mechanical, alkaline and enzymatic treatments to upgrade paper-grade pulp to dissolving pulp with high reactivity

A modified process consisting of an initial mechanical refining (R) followed by a low-alkali (5.5% NaOH) cold caustic extraction (CCE) and finally an endoglucanase (EG) treatment (R-5.5%CCE-EG) was investigated for upgrading paper-grade pulp to dissolving pulp. Results showed that compared to the conventional process (9%CCE-EG), the modified process can decrease the alkali concentration (from 9% to 5.5%) to achieve a similar hemicelluloses removal while simultaneously enhancing the Fock reactivity (from 62.2% to 81.0%). The improved results were due to the fact that the mechanical refining resulted in favorable fiber morphological changes, including increased pore volume/size, water retention value and specific surface area. Consequently, the hemicelluloses removal was enhanced even under the subsequent low-alkali CCE condition. A synergic effect of refining, low alkali concentration and enzymatic activation was responsible for the higher reactivity of resulting dissolving pulp.

Mechanical pretreatment improving hemicelluloses removal from cellulosic fibers during cold caustic extraction

Hemicelluloses removal is a prerequisite for the production of high-quality cellulose (also known as dissolving pulp), and further recovery and utilization of hemicelluloses, which can be considered as a typical Integrated Forest Biorefinery concept. In this paper, a process of combined mechanical refining and cold caustic extraction (CCE), which was applied to a softwood sulfite sample, was investigated. The results showed that the hemicelluloses removal efficiency and selectivity were higher for the combined treatment than that for the CCE alone. The combined treatment can thus decrease the alkali concentration (from 8% to 4%) to achieve a similar hemicelluloses removal. The improved results were due to the fact that the mechanical refining resulted in increases in pore volume and diameter, water retention value (WRV) and specific surface area (SSA), all of which can make positive contributions to the hemicelluloses removal in the subsequent CCE process.

Enhancing hemicelluloses removal from a softwood sulfite pulp

Hemicelluloses removal is highly desirable in many biomass processes, including the pretreatment steps of the bioconversion for ethanol production, production of high-quality dissolving pulps. In this study, a sequential treatment consisting of pulp fractionation, followed by caustic treatment to remove hemicelluloses from a softwood sulfite pulp, was investigated. The long-fiber fraction obtained after pulp fractionation, had a lower hemicelluloses content and smaller specific surface area, but larger pore diameter than the short-fiber fraction. The fiber fractions were subsequently treated in a cold caustic extraction (CCE) or hot caustic extraction (HCE). Results showed that hemicelluloses removal in the long-fiber fraction was more pronounced than the short-fiber fraction in both CCE and HCE processes. Other parameters, such as hemicelluloses removal selectivity, yield were studied. The underlying explanations were given.