Using a fully recyclable dicarboxylic acid for producing dispersible and thermally stable cellulose nanomaterials from different cellulosic sources

We fabricated cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) from different cellulose materials (bleached eucalyptus pulp (BEP), spruce dissolving pulp (SDP) and cotton based qualitative filter paper (QFP) using concentrated oxalic acid hydrolysis and subsequent mechanical fibrillation (for CNFs). The process was green as acid can be easily recovered, and the prepared cellulose nanomaterials were carboxylated and thermally stable. In detail, the CNC yield from the different materials was similar. After hydrolysis, the DP of the cellulose materials decreased substantially, whereas the mechanical fibrillation of the cellulosic solid residues (CSRs) did not dramatically reduce the DP of cellulose. CNCs with different aspect ratios were produced from different starting materials by oxalic acid hydrolysis. The CNCs and CNFs obtained from BEP and QFP possessed more uniform dimensions than those from SDP. On the other hand, CNFs derived from SDP presented the best suspension stability. FTIR analyses verified esterification of cellulose by oxalic acid hydrolysis. The results from both XRD and Raman spectroscopy indicated that whereas XRD crystallinity of CNCs from BEP and QFP did not change significantly, there was some change in Raman crystallinity of these samples. Raman spectra of SDP CNCs indicated that the acid hydrolysis preferably removed cellulose I portion of the samples and therefore the CNCs became cellulose II enriched. TGA revealed that the CNCs obtained from QFP exhibited higher thermal stability compared to those from BEP and SDP, and all the CNCs possessed better thermal stability than that of CNCs from sulfuric acid hydrolysis. The excellent properties of prepared cellulose nanomaterials will be conducive to their application in different fields.     

Isolation of a novel,crystalline cellulose material from the spent liquor of cellulose nanocrystals (CNCs)

We have modified the standard sulphuric acid hydrolysis method for the production of cellulose nanocrystals (CNCs) to successfully isolate a novel, highly crystalline cellulose material from the spent liquor of CNCs. The novel material has a cellulose II crystal structure that is distinctly different from the cellulose I crystal structure of CNCs. The modified method uses a shorter time for the hydrolysis, followed by maintaining a high residual acid concentration for the separation of the spent liquor and CNCs, and by adding the spent liquor to water. The modified method offers an opportunity to concurrently produce CNCs in up to ~40 % yield and the novel, highly crystalline, sulphated cellulose II in ~15 % yield in separate and pure forms from sulphuric acid hydrolysis of a commercial northern bleached softwood kraft pulp. It can potentially reduce the production cost of CNCs, allow easier downstream processing of CNCs and recovery of sulphuric acid, and generate a new cellulose bio-material for product development.     

Cellulosic nanocomposite membranes from hydroxypropyl cellulose reinforced by cellulose nanocrystals

Cellulosic nanocomposite membranes were prepared by incorporation of cellulose nanocrystals (CNCs) into a hydroxypropyl cellulose (HPC) matrix using a mixing/evaporation technique. CNCs were obtained from filter paper using the sulfuric acid hydrolysis method with the aid of ultrasonication. The relationship between the microstructure and mechanical properties of the CNCs/HPC nanocomposite membranes was studied. Scanning electron microscopy showed that the CNCs were well dispersed in the HPC matrix, and the fracture surface demonstrated a fibrous characteristic. With increasing CNCs content, the tensile strength and Young’s modulus of the CNCs/HPC nanocomposite membranes gradually increased. At 5 wt% content of CNCs, the strength was increased by 525 % and the Young’s modulus by 124 % compared with pure HPC membrane. Moreover, the effect of the phase change of HPC on the mechanical properties of the CNCs_5wt%/HPC nanocomposite membranes and the corresponding mechanism were also studied.     

Indirect evidence of supramolecular changes within cellulose microfibrils of chemical pulp fibers upon drying

Dried and never-dried chemical pulps were subjected to strong sulfuric acid hydrolysis and the dimensions of the resulting cellulose nanocrystals (CNCs) were characterized by AFM image analysis. Although the average length of CNCs was fairly similar in all samples (55–65 nm), the length distribution histograms revealed that a higher number of longer crystals and a lower number of shorter crystals were present in the CNC suspensions prepared from never-dried pulps. The distinction was hypothetically ascribed to tensions building in individual cellulose microfibrils upon drying, resulting in irreversible supramolecular changes in the amorphous regions. The amorphous regions shaped by tensions were deemed as more susceptible to acid hydrolysis.     

Insight into the extraction and characterization of cellulose nanocrystals from date pits

This study aims to extract and characterize cellulose nanocrystals (CNCs) from date pits (DP), an agricultural solid waste. Two methods were used and optimized for the cellulose nanocrystals (CNCs) extraction, namely the mechanical stirrer method (CNCs₁) and the Soxhlet apparatus method (CNCs₂) in terms of chemical used, cost, and energy consumption. The results showed that scanning electron microscopy revealed the difference in the morphology as they exhibit rough surfaces with irregular morphologies due to the strong chemical treatments during the delignification and bleaching process. Moreover, transmission electron microscopy analysis for CNCs reveals the true modification that was made through sulfuric acid hydrolysis as it presents cellulose microfibrils with a packed structure. Fourier transform infrared proved that the CNCs were successfully extracted using the two methods since most of the lignin and hemicellulose components were removed. The crystallinity index of CNCs₁ and CNCs₂ was 69.99%, and 67.79%, respectively, and both presented a high yield of CNCs (≥10%). Ultimately, both techniques were successful at extracting CNCs. Based on their cost-effectiveness and time consumption, it was concluded that method 1 was less expensive than method 2 based on the breakdown of the cost of each step for CNCs production.     

The structural changes in crystalline cellulose and effects on enzymatic digestibility

The enzymatic hydrolysis of cellulose I achieves almost complete digestion when sufficient enzyme loading as much as 20 mg/g-substrate is applied. However, the yield of digestion reaches the limit when the enzyme dosage is decreased to 2 mg/g-substrate. Therefore, we have performed three pretreatments such as mercerization, dissolution into phosphoric acid and EDA treatment. Transformation into cellulose II hydrate by mercerization and dissolution into phosphoric acid were not sufficient because substrate changed to highly crystalline structure during saccharification. On the other hand, in the case of crystalline conversion of cellulose I to III_I by EDA, almost perfect digestion was achieved even in enzyme loading as small as 0.5 mg/g-substrate, furthermore, hydrolyzed residue was typical cellulose I. The structural analysis of substrate after saccharification provides an insight into relationships between cellulose crystalline property and cellulase toward better enzymatic digestion.     

Preparation of Cellulose Nanocrystals from Jujube Cores by Fractional Purification

Jujube cores are fiber-rich industrial waste. Dewaxing, alkali treatment, bleaching, and sulfuric acid hydrolysis were used to generate cellulose nanocrystals (CNCs) from the jujube cores in this study. The morphological, structural, crystallinity, and thermal properties of the fibers were investigated using FE-SEM, TEM, AFM, FT-IR, XRD, and TGA under various processes. CNCs’ zeta (ζ) potential and water contact angle (WAC) were also investigated. The findings demonstrate that non-fibrous components were effectively removed, and the fiber particles shrunk over time because of many activities. CNCs had a rod-like shape, with a length of 205.7 ± 52.4 nm and a 20.5 aspect ratio. The crystal structure of cellulose Iβ was preserved by the CNCs, and the crystallinity was 72.36%. The temperature of the fibers’ thermal degradation lowered during the operations, although CNCs still had outstanding thermal stability (>200 °C). Aside from the CNCs, the aqueous suspension of CNCs was slightly agglomerated; thus, the zeta (ζ) potential of the CNCs’ suspension was −23.72 ± 1.7 mV, and the powder had high hydrophilicity. This research will be valuable to individuals who want to explore the possibility for CNCs made of jujube cores.     

Cellulose nanocrystals as promising adsorbents for the removal of cationic dyes

Cellulose nanocrystals (CNCs) prepared from cellulose fibre via sulfuric acid hydrolysis was used as an adsorbent for the removal of methylene blue (MB) from aqueous solution. The effects of pH, adsorbent dosage, temperature, ionic strength, initial dye concentration were studied to optimize the conditions for the maximum adsorption of dye. Adsorption equilibrium data was fitted to both Langmuir and Freundlich isotherm models, where the Langmuir model better described the adsorption process. The maximum adsorption capacity was 118 mg dye/g CNC at 25 °C and pH 9. Calculated thermodynamic parameters, such as free energy change (ΔG = ?20.8 kJ/mol), enthalpy change (ΔH = ?3.45 kJ/mol), and entropy change (ΔS = 0.58 kJ/mol K) indicates that MB adsorption on CNCs is a spontaneous exothermic process. Tunability of the adsorption capacity by surface modification of CNCs was shown by oxidizing the primary hydroxyl groups on the CNC surface with TEMPO reagent and the adsorption capacity was increased from 118 to 769 mg dye/g CNC.     

Comparison of changes in cellulose ultrastructure during different pretreatments of poplar

One commonly cited factor that contributes to the recalcitrance of biomass is cellulose crystallinity. The present study aims to establish the effect of several pretreatment technologies on cellulose crystallinity, crystalline allomorph distribution, and cellulose ultrastructure. The observed changes in the cellulose ultrastructure of poplar were also related to changes in enzymatic hydrolysis, a measure of biomass recalcitrance. Hot-water, organo-solv, lime, lime-oxidant, dilute acid, and dilute acid-oxidant pretreatments were compared in terms of changes in enzymatic sugar release and then changes in cellulose ultrastructure measured by ¹³C cross polarization magic angle spinning nuclear magnetic resonance and wide-angle X-ray diffraction. Pretreatment severity and relative chemical depolymerization/degradation were assessed through compositional analysis and high-performance anion-exchange chromatography with pulsed amperometric detection. Results showed minimal cellulose ultrastructural changes occurred due to lime and lime-oxidant pretreatments, which at short residence time displayed relatively high enzymatic glucose yield. Hot water pretreatment moderately changed cellulose crystallinity and crystalline allomorph distribution, yet produced the lowest enzymatic glucose yield. Dilute acid and dilute acid-oxidant pretreatments resulted in the largest increase in cellulose crystallinity, para-crystalline, and cellulose-I_β allomorph content as well as the largest increase in cellulose microfibril or crystallite size. Perhaps related, compositional analysis and Klason lignin contents for samples that underwent dilute acid and dilute acid-oxidant pretreatments indicated the most significant polysaccharide depolymerization/degradation also ensued. Organo-solv pretreatment generated the highest glucose yield, which was accompanied by the most significant increase in cellulose microfibril or crystallite size and decrease in relatively lignin contents. Hot-water, dilute acid, dilute acid-oxidant, and organo-solv pretreatments all showed evidence of cellulose microfibril coalescence.     

Oxidation of wood cellulose using 2-azaadamantane N-oxyl (AZADO) or 1-methyl-AZADO catalyst in NaBr/NaClO system

A wood cellulose was oxidized with catalytic amounts of 2-azaadamantane N-oxyl (AZADO) or 1-methyl-AZADO, in an NaBr/NaClO system, in water at pH 10. The oxidation efficiency, carboxylate/aldehyde contents, and degree of polymerization (DP_v) of the oxidized celluloses thus obtained were evaluated in terms of the amount of AZADO or 1-methyl-AZADO catalyst added, in comparison with those prepared using the TEMPO/NaBr/NaClO system. When the AZADO/NaBr/NaClO and 1-methyl-AZADO/NaBr/NaClO oxidation systems were applied to wood cellulose using the same molar amount of TEMPO, the oxidation time needed for the preparation of oxidized celluloses with carboxylate contents of 1.2–1.3 mmol/g was reduced from ≈80 to 10–15 min. Moreover, the molar amounts of AZADO and 1-methyl-AZADO that had to be added for the preparation of oxidized celluloses with carboxylate contents of 1.2–1.3 mmol/g were reduced to 1/32 and 1/16 of the amount of TEMPO added, respectively. The DP_v values for the AZADO- and 1-methyl-AZADO-oxidized celluloses after NaBH₄ treatment were in the range of 600–800. This indicated that not only C6-carboxylate groups but also C2/C3 ketones were formed to some extent on the crystalline cellulose microfibril surfaces during the AZADO- and 1-methyl-AZADO-mediated oxidation. When the AZADO-oxidized wood cellulose, which had a carboxylate content of 1.2 mmol/g, was mechanically disintegrated in water, an almost transparent dispersion consisting of individually nano-dispersed oxidized cellulose nanofibrils was obtained, with a nanofibrillation yield of 89 %.     

Development of biopolymer/cellulose/silica nanostructured hybrid materials and their characterization by NMR relaxometry

The objective of this study was to investigate the preparation and properties of hybrid materials composed of poly(lactic acid) (PLA) and poly(lactic acid)/poly(lactic-co-glycolic acid) (PLA/PLGA) blends employing cellulose nanocrystals (CNCs) and/or organophilic silica (R972) as nanoparticles. The CNCs were obtained by acid hydrolysis of commercially available microcrystalline cellulose (MCC). The materials were produced in film form by solution casting. Organophilic silica was incorporated at a ratio of 3 wt.%, and CNCs were added at ratios of 3 wt.% and 5 wt.% in relation to the weight of the polymer matrix. Two series of films were obtained. The first was prepared using only PLA as the matrix, and the second was obtained using blends of PLA and PLGA. The properties of the films were evaluated by X-ray diffractometry, nuclear magnetic resonance, Fourier-transform infrared spectroscopy and measurement of mechanical properties. The results revealed that each nanoparticle, whether added individually or combined with the other type of nanoparticle, induced different final material properties. Cellulose nanocrystals can act as nucleating agents for the crystallization of PLA. There was an improvement in the mechanical performance of films with the addition of CNCs. Further, the incorporation of silica combined with CNCs resulted in the generation of films with the strongest mechanical properties. The results of this study indicate that silica decreases the surface tension between PLA-cellulose and PLA/PLGA-cellulose.     

Phase-equilibrium and cellulose-coagulation kinetics for cellulose solutions in <Emphasis Type="Italic">N</Emphasis>-methylmorpholine-<Emphasis Type="Italic">N</Emphasis>-oxide

The dissolution of cellulose in N-methylmorpholine-N-oxide monohydrate and the dissolution of N-methylmorpholine-N-oxide monohydrate in water have been studied via optical interferometry. A part of the phase diagram for the cellulose/N-methylmorpholine-N-oxide system has been constructed. The phase diagram is characterized by crystalline equilibrium, hysteresis of the melting temperatures of the solvents, and a region of anisotropy. Optical interferometry has been used for the first time to study the kinetics of cellulose coagulation during the interaction of cellulose solutions in N-methylmorpholine-N-oxide with water and water solutions of N-methylmorpholine-N-oxide. Information on the values of interdiffusion coefficients and the morphologies of the resulting cellulose films has been obtained. The possibility to use optical interferometry to analyze the interaction of a solution with the coagulating agent in the case of cellulose fiber and film formation has been demonstrated. The influences of temperature, the nature of the coagulating agent, and the cellulose content on the kinetics of the process and morphologies of the formed films have been shown. The use of N-methylmorpholine-N-oxide as a part of the coagulation system decreases the rate of interdiffusion of solutions, thereby resulting in a more uniform and dense morphology of cellulose films. Increased temperature causes diffusion acceleration, thereby leading to the formation of an anisotropic morphology of the cellulose films.     

Effect of oligosaccharide deposition on the surface of cellulose nanocrystals as a function of acid hydrolysis temperature

Recent findings indicate there is only a small window of sulfuric acid concentration (60–65 %) and temperature (45–65 °C) which allows efficient extraction of cellulose nanocrystals in significant quantities from bleached chemical pulp. In the present report, we develop a systematic explanation for how hydrolysis temperature, at a specific acid concentration, governs CNC surface properties. We demonstrate that CNCs with different suspension viscosity, stability in electrolyte-containing solutions, and optical properties can be produced, based on the presence or not of a precipitated oligosaccharide layer (OSL) on the surface of the nanocrystals. At hydrolysis temperatures below 65 °C, the degree of polymerization (DP) distribution of cellulose chains in CNC samples exhibits a bimodal distribution, indicating an accumulation of oligosaccharides on the CNC surface which increases as the hydrolysis temperature is decreased. At low hydrolysis temperature (45 °C), the oligosaccharides dissolved in the strong acid phase have a DP between 7 and 20 and precipitate onto CNCs when the reaction is quenched by diluting with water. As the temperature of hydrolysis is increased (50–60 °C), the dissolved oligosaccharides are hydrolyzed faster and their DP decreases such that they remain soluble after quenching. At 65 °C, no precipitated oligosaccharides can be detected on the CNC surface. Based on these results, we propose possible explanations to account for the effects of the OSL on the CNC suspension viscosity and stability and on optical properties of CNC films.     

Chiral nematic phase formation by aqueous suspensions of cellulose nanocrystals prepared by oxidation with ammonium persulfate

There is continuing interest in the growing family of nanocellulosic materials prepared from plant cell wall material. While most of the research on cellulose nanocrystals has focused on the product of sulfuric acid hydrolysis stabilized by surface sulfate half-esters, cellulose nanocrystals with surface carboxyl groups have also been prepared by oxidation of lignocellulosic materials with ammonium persulfate. The major difference is that the persulfate oxidation leads to nanocrystals stabilized by surface carboxyl groups. Some properties of cellulose nanocrystals from cotton and wood, prepared by persulfate oxidation, are compared with those observed for nanocrystals prepared by sulfuric acid hydrolysis. Evidence from polarized light microscopy showed that the nanocrystal suspensions prepared by persulfate oxidation also form chiral nematic ordered phases in water.     

Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis

The conditions required for the accurate measurement of the sulfur content of cellulose nanocrystals (CNCs) by conductometric titration are discussed. CNCs from sulfuric acid hydrolysis are electrostatically stabilized in aqueous suspension due to the introduction of charged sulfate ester groups onto the surface of the crystallites during reaction. The sulfur content thus largely reflects the surface charge of the crystals, and is crucial to the characterization and understanding of material properties. Conductometric titration is commonly used to quantify the sulfur content of CNCs, however, the exhaustive removal of free acid by dialysis and the necessity, type, quantity and duration of ion-exchange resin treatments are not always consistent. Here we explore the standard conditions of dialysis, ion-exchange, and the reproducibility of titration results. Extensive dialysis is found to be effective in the removal of free acid, but similar results are also achieved in shorter times and with less water using membrane ultrafiltration. It is also shown that the conditions of ion-exchange most commonly employed in the literature can lead to inaccurate sulfur contents. Finally, good agreement is obtained between the sulfur contents of different CNC batches prepared using the same hydrolysis conditions, and from titration and elemental analysis when thoroughly purified, well-dispersed samples, and appropriate resin conditions are used.     

A study of the mechanical, thermal and morphological properties of microcrystalline cellulose particles prepared from cotton slivers using different acid concentrations

Microcrystalline cellulose (MCC) particles are mostly prepared by acid hydrolysis of various agro sources. Acid hydrolysis is usually carried out with high concentration (64 wt%) of sulfuric acid. Here, an attempt has been made to optimize lower acid concentrations which can effectively produce MCC particles. In this work, different concentrations of sulfuric acid (20, 30, 35, 40, 47 and 64 wt%) have been used to prepare MCC particles, which have been characterized by XRD, particle size analysis, scanning electron microscopy, transmission electron microscopy, nanoindentation and thermogravimetric analysis. MCC prepared with 35 and 47% sulfuric acid (MCC 35 and MCC 47) had finest particle size and fibrils were produced in the range of 15–25 nm. MCC 20 showed wide particle size distribution, indicating low breakdown of the cellulose chains. The energy absorption behavior and mechanical properties of the MCC pellets were determined by nanoindentation test for the first time. MCC 35 pellets exhibited lowest modulus and hardness.     

Well defined thermostable cellulose nanocrystals via two-step ionic liquid swelling-hydrolysis extraction

Cellulose nanocrystals were prepared from cotton fibers by a two-stage method involving ionic liquid swelling treatment followed by hydrolysis under mild acid conditions. Controlled swelling of cellulosic fibers was achieved in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) at 80 °C, while avoiding extensive dissolution of crystalline regions. Since the accessibility of the substrate was considerably enhanced, the hydrolysis occurred even under mild conditions, using up to 60 times less sulfuric acid than the traditional extraction methods based on concentrated sulfuric acid. The effects of process parameters on nanoparticle morphology, composition and stability were investigated. The individual rod-like nanocrystals, observed under field emission gun scanning electron microscopy, exhibited an average diameter of around 20 nm and a length ranging from 150 to 350 nm. According to X-ray photoelectron spectroscopy and thermogravimetric analysis, the surface of the so-extracted nanoparticles proved to be deprived of contaminating sulfate groups leading to significantly higher thermal stability with respect to cellulose nanocrystals extracted by traditional method in concentrated sulfuric acid.     

Novel aspects of nanocellulose

Novel nanoscaled cellulose particles were prepared using high-pressure homogenization of aqueous media contenting treated cellulose samples in a Microfluidizer^® processor (MF). Here, we present the generation of spherical cellulose nanoparticles as an extension of previously published reports of nano fibrillated cellulose. Although MF treatment of unmodified cellulose yields nanofibrils which are reported in several publications, in the current work different kinds of pretreatments were proven to be necessary to obtain spherical structured cellulose nanoparticles. One such treatment may be the decrystallization of cellulose regenerating it from N-methylmorpholine-N-oxid-monohydrate (NMMNO*H₂O). Nanocellulose was then obtained by a subsequent high-pressure mechanical treatment of the precipitate in aqueous dispersion. Decrystallization was also realized by grinding cellulose in a planetary ball mill. The resulting amorphous intermediates were characterized by Raman spectroscopy. Another approach tested was hydrolysis and subsequent mechanical treatment using an Ultra-Turrax^® and MF. Another alternative was given by the mechanical treatment of aqueous dispersions of low substituted cellulose derivatives such as carboxymethyl cellulose and oxidized cellulose without any further hydrolysis.     

Controlled production of spruce cellulose gels using an environmentally “green” system

Stable spruce cellulose suspensions were generated in NaOH/urea aqueous solutions and used to make thermally induced gels with various swelling ratios and compressive strengths. Wood cellulose cannot be easily dissolved in water or any common organic solvent due to its high molecular weight, which largely limits its applications. Spruce cellulose was hydrolyzed by diluted sulfuric acid of various concentrations and hydrolysis times. The dissolution of these partially degraded samples was investigated in a NaOH/urea aqueous solution system considered environmentally “green.” The effects of acid hydrolysis on the structure and properties of subsequent thermally induced gels were examined using scanning electron microscopy, swelling and re-swelling experiments, and mechanical testing. The molecular weight of spruce cellulose was significantly reduced by acid hydrolysis, whereas its crystallinity slightly increased because of the removal of amorphous regions. All samples could be partially dissolved in the NaOH/urea aqueous solution and formed stable suspensions. Hydrolyzed cellulose samples with lower molecular weight exhibited a higher solubility. Rheological experiments showed these cellulose suspensions could form gels easily upon heating. A porous network structure was observed in which dissolved cellulose was physically crosslinked upon heating and then regenerated to form a three-dimensional network, where the dispersed swollen cellulose fibers filled spaces to reinforce the structure. The swelling behavior and mechanical properties of these ‘matrix-filler’ gels could be controlled by varying the mild acid hydrolysis conditions, which adjusts their degree of solubility. This research provides several opportunities for manufacturing wood cellulose based materials.     

Synthesis of a tetrasaccharide related to the triterpenoid saponin isolated from Schima noronhae

A concise synthesis of a tetrasaccharide related to the cell-growth inhibitory triterpenoid saponin isolated from Schima noronhae is reported. A late stage 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-mediated oxidation of a primary hydroxyl group to carboxylic acid has been achieved under phase-transfer conditions. Stereoselective glycosylations were carried out using thioglycoside or glycosyl trichloroacetimidate activation using sulfuric acid immobilized on silica (H₂SO₄–silica) in conjunction with N-iodosuccinimide and alone, respectively.