Run-Cang Sun

Influence of alkaline pre-treatments on the cell wall components of wheat straw

Abstract The effects of alkaline pre-treatments of wheat straw have been examined in detail. In particular, treatments with increasing amounts of sodium hydroxide at a range of temperatures and exposure times have been investigated. A number of other alkaline or oxidizing agents have also been utilized and briefly assessed. Optimal conditions for delignification and dissolution of hemicellulosic polysaccharides were found to be pre-treatment with 1.5% sodium hydroxide for 144 h at 20 °C. This resulted in release of 60% and 80% of lignin and hemicellulose, respectively. The residues of the pre-treated wheat straw were sequentially extracted with 0.25% ammonium oxalate for the isolation of pectin, acidic sodium chlorite solution for delignification and 24% potassium hydroxide with 2% boric acid (2 h at 20 °C) for the determination of remaining hemicellulose and α-cellulose. The yields were determined by gravimetric analysis and expressed as a proportion of the starting material. It was found that xylose was the major sugar constituent in all the hemicellulose fractions and the hydrolysates obtained from pre-treatment, while glucose and galactose were present as minor components. The content of arabinose was higher in all the pre-treatment hydrolysates than in hemicellulose fractions isolated from the residues, whereas the relative amount of xylose in remaining hemicellulose was higher than in the hydrolysates material. The content of uronic acid and the range of molecular weights both in remaining hemicellulose and in hydrolysate fractions were also studied. The contents of phenolic acids and aldehydes remaining both in 0.5 h pre-treated (1.5% sodium hydroxide) wheat straw hemicellulose and cellulose were 0.15%.

Comparative study of hemicelluloses obtained by graded ethanol precipitation from sugarcane bagasse.

The sequential treatment of dewaxed sugarcane bagasse with H(2)O and 1 and 3% NaOH at a solid to liquid ratio of 1:25 (g mL(-1)) at 50 degrees C for 3 h yielded 74.9% of the original hemicelluloses. Each of the hemicellulosic fractions was successively subfractionated by graded precipitation at ethanol concentrations of 15, 30, and 60% (v/v). Chemical composition, physicochemical properties, and structures of eight precipitated hemicellulosic fractions were elucidated by a combination of sugar analysis, nitrobenzene oxidation of bound lignin, molecular determination, Fourier transform infrared (FT-IR), (1)H and (13)C nuclear magnetic spectroscopies, and thermal analysis. The results showed that the sequential treatments and graded precipitations were very effective on the fractionation of hemicelluloses from bagasse. Comparison of these hemicelluloses indicated that the smaller sized and more branched hemicelluloses were extracted by the hot water treatment; they are rich in glucose, probably originating from alpha-glucan and pectic polysaccharides. The larger molecular size and more linear hemicelluloses were dissolved by the alkali treatment; they are rich in xylose, principally resulting from l-arabino-(4-O-methylglucurono)-d-xylans. In addition, noticeable differences in the chemical composition and molecular weights were observed among the graded hemicellulosic subfractions from the water-soluble and alkali-soluble hemicelluloses. The Ara/Xyl ratio increased with the increment of ethanol concentration from 15 to 60%, and the arabinoxylans with higher Ara/Xyl ratios had higher molecular weights. There were no significant differences in the structural features of the precipitated hemicellulosic subfractions, which are mainly constituted of l-arabino-(4-O-methyl-d-glucurono)xylan, whereas the difference may occur in the distribution of branches along the xylan backbone.

Fractional and structural characterization of wheat straw hemicelluloses

Abstract Six hemicellulosic fractions were extracted successively from dewaxed wheat straw with sodium hydroxide at increasing strength from 0.25 to 2.00M, and the chemical composition are reported. The structure of the hemicellulosic fraction 2 was investigated using acid hydrolysis, methylation analysis and 13C-NMR experiments. The hemicelluloses were confirmed to be a (1→4)-linked β-D-xylan with D-glucopyranosyluronic acid (or 4-O-methyl-α-D-glucopyranosyluronic acid) group attached at position 2, and L-arabinofuranosyl and D-xylopyranosyl groups attached at position 3. For every 26 D-xylopyranosyl residues in the main chain, there was one uronic acid unit. For 13 such D-xylopyranosyl residues, there was one L-arabinofuranosyl group, and for 18 such D-xylopyranosyl residues, there was one D-xylopyranosyl group.

Comparative study of lignins isolated by alkali and ultrasound-assisted alkali extractions from wheat straw

Treatment of the dewaxed wheat straw with 0.5 M KOH at 35 degrees C for 2.5 h without ultrasonic irradiation and with ultrasound assistance for 5, 10, 15, 20, 25, 30, and 35 min resulted in a dissolution of 43.9%, 43.9%, 43.9%, 44.4%, 44.4%, 45.6%, 46.8%, and 49.1% of the original lignin, respectively. Much better results were achieved under the ultrasonic irradiation time for 35 min where nearly 50% of the original lignin was solubilized at 35 degrees C for 2.5 h. The purity of the lignin preparations isolated by alkali with ultrasound assistance was higher than that of the lignin fraction obtained by alkali without ultrasonic irradiation, and their purity increased with an increment of irradiation time between 5 and 35 min, in which the content of associated polysaccharides in the former lignin preparations (0.87-1.06%) was lower than that of the latter lignin fraction (1.16%). In addition, the lignins isolated by alkali with ultrasonic irradiation time between 5 and 30 min showed a slightly higher molecular weight and thermal stability than the lignin obtained by alkali without ultrasound assistance. No substantial differences in the main structure features between the lignin preparations isolated by alkali and ultrasound-assisted alkali extractions were found.

Highly effective adsorption of heavy metal ions from aqueous solutions by macroporous xylan-rich hemicelluloses-based hydrogel.

Xylan-rich hemicelluloses-based hydrogel was developed as a novel porous bioadsorbent by graft co-polymerization of acrylic acid (AA) and xylan-rich hemicelluloses for adsorption of heavy metal ions (Pd(2+), Cd(2+), and Zn(2+)) from aqueous solutions. The chemical structure, the interaction between the hydrogel and metal ions, and the porous structure of xylan-rich hemicelluloses-g-AA hydrogel were revealed by Fourier transform infrared spectroscopy and scanning electron microscopy. The effects of AA and cross-linker dosage, pH value, contacting time, and initial concentration of metal ion on the adsorption capacity were studied. The adsorption equilibrium time was about 60 min from the adsorption kinetics study. The maximum adsorption capacities of Pd(2+), Cd(2+), and Zn(2+) were 859, 495, and 274 mg/g, respectively. Furthermore, xylan-rich hemicelluloses-g-AA hydrogel also exhibited highly efficient regeneration and metal ion recovery efficiency and can be reused without noticeable loss of adsorption capacity for Pd(2+), Cd(2+), and Zn(2+) after quite a number of repeated adsorption/desorption cycles.

Comparative study of hemicelluloses from wheat straw by alkali and hydrogen peroxide extractions

Abstract As compared to traditional alkaline extractions, alkaline peroxide was used to isolate hemicelluloses from wheat straw. Yields of the solubilized hemicelluloses ranged from 18.9% (2% H2O2 extraction at 90°C for 2 h at pH 11.5) to 26.6% (2% H2O2 extraction at 50°C for 16 h at pH 12.5). The optimum hemicellulose yield (92% of the original hemicelluloses in water treated wheat straw) was obtained when the treatment was performed at 50°C for 16 h at pH 12.5 by use of 2% H2O2. All the hemicellulosic preparations were much lighter in color than those obtained using traditional alkaline extractions in the absence of bleaching. The results, obtained by the destructive method such as acid hydrolysis, showed that extraction of wheat straw with aqueous 10% KOH following alkaline bleaching released the hemicellulosic fractions, which were enriched in xylose, whereas extraction of the straw with aqueous 2% H2O2 under alkaline conditions (pH 11.5–12.5) solubilized the hemicellulosic fractions, which were relatively higher in arabinose and glucose. The nine isolated hemicellulosic samples were further characterized by non-destructive methods such as Fourier transform infrared (FT–IR), and carbon-13 magnetic resonance spectroscopy (13C-NMR) as well as gel permeation chromatography (GPC). It has been demonstrated that the alkaline peroxide treatments under the conditions used do not affect the overall structure of hemicelluloses. The only one major change of the hemicelluloses was found to be degradation during the beaching of alkali-soluble hemicelluloses with 2% H2O2 at 60°C for a period of 16 h at pH 11.5 or bleaching using a relatively higher concentration (5%) of hydrogen peroxide at 60°C for 8 h at pH 11.5.

The chemical modification of lignins with succinic anhydride in aqueous systems

Abstract Chemical modification of oil palm trunk fibre lignin, poplar lignin, maize stem lignin, and barley, wheat, and rye straw lignins was achieved by esterification with succinic anhydride in aqueous solutions. FT–IR spectroscopy clearly revealed the differences in the structure of the modified lignins as a result of this succinoylation. These changes were reflected by thermal analyses such as thermogravimetric analysis and differential scanning calorimetry. All the succinylated lignin preparations showed an increased thermal stability compared to the corresponding unmodified lignins.

Fractional characterization of ash-AQ lignin by successive extraction with organic solvents from oil palm EFB fibre

Abstract The ash-AQ lignin from the black liquor of oil palm empty fruit bunch was fractionated into four fractions by successive extraction with organic solvents of increasing hydrogen-bonding capacity. A comparison study of the lignin heterogeneity between the fractions was made in terms of fractional yield, content of associated polysaccharides, molecular weight, contents of carbonyl, phenolic hydroxyl, and aliphatic hydroxyl groups, and thermal stability. It was found that the content of carbonyl groups and the ratio of non-condensed syringyl/guaiacyl units decreased considerably with increasing molecular weight whereas the thermal stability increased.

Delignification of rye straw using hydrogen peroxide

Alkaline peroxide delignification of rye straw has been first investigated in this paper. The results showed that treatment of dewaxed and water-extracted rye straw with 2% H2O2 at pH 11.5 for 12 h at 20, 30, 40, 50, 60, and 70°C resulted in a dissolution of 52.7, 75.7, 81.8, 83.1, 85.8, and 87.8% of the original lignin, and 44.2, 52.5, 70.0, 70.0, 71.3, and 71.9% of the original hemicelluloses, respectively. The isolated pure lignin fractions contained rather low amounts of neutral sugars, 0.4–1.1%, and had weight-average molecular weights between 2420 and 3480 g mol−1. They contained almost equal amounts of noncondensed guaiacyl and syringyl units with fewer p-hydroxyphenyl units. The β–O–4 ether bonds together with β–β and β–5 carbon–carbon linkages were found to be present in the lignin structural units. Hydroxycinnamic acids such as p-coumaric and ferulic acids appeared to be strongly associated to lignin molecules. Comparison of these lignin samples indicated that the alkaline peroxide treatment of the straw under the conditions given did not affect the overall structure of lignin.

Understanding the chemical transformations of lignin during ionic liquid pretreatment

Unveiling the fundamental chemistry of lignin under ionic liquid (IL) pretreatment will facilitate the understanding of biomass recalcitrance involved in pretreatment processes. To examine in greater detail the chemical transformations of lignin under different IL pretreatment conditions without competing reactions from plant polysaccharides, the IL pretreatment of the isolated poplar alkaline lignin (hardwood lignin) under varying IL pretreatment conditions (i.e., 110–170 °C, 1–16 hours) was performed in an appropriate manner. The structural transformations of the lignin have been investigated by elemental analysis, 2D-HSQC spectra, quantitative 13C-NMR spectra, 31P NMR, and GPC analysis. Results revealed that a decrease of aliphatic OH and an increase in phenolic hydroxyl groups occurred in lignin as the pretreatment proceeded. The increased phenolic OH was mainly as a result of cleavage of β-O-4′ linkages, while the reduced aliphatic OH is probably attributed to the dehydration reaction. The cleavage of β-O-4′ linkages, degradation of β–β′ and β-5′ linkages obviously happened at high temperatures and resulted in the decrease of molecular weights. In addition, IL pretreatment selectively degraded the G-type lignin fractions and the condensation reaction took place more easily at S units than G units. Moreover, the demethoxylation preferentially occurred in G units, especially at higher temperatures. It is believed that investigating the fundamental chemistry of lignin during IL pretreatments would be beneficial to optimize and control the pretreatment process.