Separation, Modification And Thermochemical Conversion Of The Components Of Lignocellulose

Since fossil fuels are nonrenewable, excessive consumption of fossil fuels has caused serious energy and environmental issues. As a large agricultural country, the annual agricultural and forestry waste of China is up to 7 hundred million tons. Effectively converting these lignocellulosic resources into valuable energy, chemicals and materials has very important significance for realizing the sustainable development. Therefore, using lignocellulosic biomass to supplement and replace fossil fuels applied in energy, chemical and material fields has drawn worldwide attention. In order to utilize these lignocellulosic resources, a series of researches were carried out to study the separation, characterization, modification and thermochemical conversion of the major components of lignocellulose in this study.(1) Progressive enhancement treatment was applied to sequentially treat the dewaxed Calamagrostis angustifolia Kom with hot water, 70% ethanol, and 70% ethanol containing 0.2%, 1.0%, 2.0%, 4.0%, and 8.0% Na OH. The effects of progressive enhancement treatment on the yield and structure of isolated hemicellulose and lignin fractions were comparatively studied. Results showed that in the sequential treatment above 99% of hemicelluloses and 64% of lignins were separated from the dewaxed sample. For hemicelluloses, water-soluble hemicelluloses contained noticeable amounts of β-D-glucan, as well as some pectic substances. Ethanol-soluble hemicelluloses were mainly arabinogalactan, while alkali-soluble hemicelluloses were mainly galactoarabinoxylans. The branched degree of the alkali-soluble hemicelluloses decreased with the increment of Na OH concentration, but the molecular weight showed a trend of first increased and then decreased. For lignin, water-soluble lignin only accounted for 3.5% of the total extracted lignin, and which had relatively small molecular weights(890 and 1090 g/mol) and high residual sugars. Ethanol-soluble lignin showed highest yield and largest molecular weights(2580 g/mol), while the molecular weights of alkali-soluble lignins showed a trend of first decreased and then increased with the increment of Na OH concentration. The lignin in Calamagrostis angustifolia Kom could be classified as “GSH” lignin and the interunit linkages of the lignin were mainly β-O-4′ together with small amounts of β-5′ and β-β′. Additionally, results from calculational chemistry indicated that the theoretical NMR values of various lignin model compounds were significantly correlated to their experimental values. Applying calculational chemistry in guiding the synthesis of novel lignin model compounds and the analysis of lignin structure has certain valuable feasibility and applicability.(2) Herein, a very efficient method for the preparation of cellulose esters(CEs) was developed. This method involved the transesterification of cellulose with various carboxylic acid vinyl esters in DMSO under the catalysis of aqueous Na OH. In this reaction system, the reaction time for preparing CEs by transesterification could be significantly reduced from dozens of hours to only several minutes. The reaction condition is mild and the depolymerization of cellulose is very slow during the reaction. Water content and the amount of Na OH in the reaction system had great influences on the degree of substitution(DS) of the prepared CEs. The DS of cellulose acetate in the range of 1.41-2.51 could be prepared by varying the reaction conditions. These prepared CEs had higher thermal stability than cellulose. Additionally, results from calculational chemistry indicated that the esterification activity of different esterifying reagents with alcohol hydroxyl was in the order of acyl chloride > acid anhydride > vinyl ester > carboxylic acid, and the esterification activity of hydroxyl groups on the C-2, C-3 and C-6 position of cellulose was not much different.(3) A novel type of hemicellulosic derivative, cyanoethyl hemicelluloses(CEH), derived from xylan-rich hemicelluloses and acrylonitrile was successfully prepared in aqueous Na OH via the Michael addition reaction, which can be selectively hydrated into carbamoylethyl hemicelluloses. The reaction was performed under various reaction conditions such as different temperature, time, the amount of sodium hydroxide, and the molar ratio of acrylonitrile to anhydroxylose units in hemicelluloses, and the relationship between reaction condition and DS of the CEH was investigated in detail. Results showed that the reaction temperature, time, and the amount of acrylonitrile had great influences on the degree of substitution(DS) of the prepared CEHs. A series of CEH with degree of substitution of cyanoethyl(DSC≡N) ranging from 0.23 to 1.64 were obtained. The trace amount of carbonyl group in the prepared CEH was originated from the hydrolysis of acrylonitrile instead of the cyanoethyl group of CEH. Additionally, CEH can be selectively hydrated into carbamoylethyl hemicelluloses in 30%H2O2/K2CO3/DMSO at room temperature. FT-IR, 1H and 13 C NMR spectra confirmed the introduction of cyanoethyl groups into the hemicelluloses backbone and the presence of carbamoylethyl groups in the hydrated product.(4) A novel multi-responsive hydrogel was prepared by free radical copolymerization of xylan-type hemicelluloses methacrylate with 4-[(4-acryloyloxyphenyl)azo]benzoic acid(AOPAB). The structure and properties of the prepared hydrogel was investigated. Results showed that the prepared hydrogel possessed multi-responsive behaviors to p H, alternating solutions, and light. Both the pore volume and the swelling ratios of the prepared hydrogels in distilled water decreased with the AOPAB content increase. The in vitro drug release test results suggested that the hydrogel loaded with Vitamin B12(VB12) showed a higher drug release rate and higher cumulative release amount in higher p H suffer solution or under UV irradiation.(5) Bamboo was hydrothermally treated at 180-240 °C for 3-60 min. The degradation behaviors of the major components of bamboo, the major degradation products and their distributions were comparatively studied. Results showed that the solid residue(SR) gradually declined from 89.98% to 37.11% as the hydrothermal temperature and reaction time increase. Hemicellulosic component was the most readily degradable component in lignocellulose. After hydrothermally treated at 220 °C for 10 min, the hemicellulose content in bamboo decreased dramatically from initial 19.7% to less than 1%. Lignin component gradually decomposed as the hydrothermal temperature and reaction time increase. Cellulose component decomposed slowly at 180-220 °C, while the degradation of cellulose speeded up at 240 °C, and the cellulose content in bamboo decreased dramatically from initial 36.6% to 15.5%. Except the SR product, the hydrothermal products of bamboo also contained 4.51-20.41% of water-soluble fractions(WS) and 4.23-16.17% of acetone-soluble fractions(AS). The compounds in the WS fractions mainly consisted of furfural(FF), 5-hydroxymethylfurfural(HMF), and phenolic compounds, while the compounds in the AS fractions were mainly complex aromatic compounds with multi-benzene rings.(6) The catalytic performances of various transition metal sulfates(Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) and transition metal oxides nanoparticles(Cr2O3, Mn2O3, Fe2O3, Co3O4, and Ni O) in the hydrothermal liquefaction of xylose, glucose, and cellulose under different conditions were explored. The degradation behaviors of xylose, glucose and cellulose, the major degradation products and their distributions were comparatively studied, which is helpful for screening appropriate non-noble metal catalysts for lignocellulose upgradation. Results showed that most transition metal sulfates, especially Zn2+ and Ni2+, could accelerate the degradation of xylose and glucose and enhance the yields of dehydration products(FF or HMF) at lower temperature and retro-aldol condensation products(lactic acid) at higher temperature. As compared with no catalyst, the highest yields of lactic acid converted from xylose, glucose and cellulose dramatically increased from 0.8,0.3 and 0.0 mg/m L to 7.0, 3.4 and 5.6 mg/m L, respectively, under the catalysis of Zn2+ and Ni2+. While Cu2+ and Fe3+ could significantly accelerate the hydrolysis of cellulose into glucose at 200 °C and displayed high efficiency on converting glucose and cellulose into levulinic acid and formic acid at 240 °C. For transition metal oxides nanoparticles, Cr2O3, Fe2O3 and Ni O almost had no effect on the hydrothermal liquefaction of xylose, glucose and cellulose. However, Mn2O3 and Co3O4 could significantly also promote the conversion of xylose, glucose and cellulose into lactic acid, and the highest yields of lactic acid converted from xylose, glucose and cellulose could reach 8.7, 4.9 and 5.2 mg/m L, respectively. The correlation analysis results showed that the hydrothermal liquefaction of glucose was closely correlated with the hydrothermal liquefaction of cellulose, suggesting that catalysts which could affect the hydrothermal liquefaction of glucose would also have similar effect on the hydrothermal liquefaction of cellulose. Additionally, possible hydrothermal liquefaction pathways of xylose and glucose and their thermochemical data were systematically calculated via calculational chemistry, results suggested calculational chemistry can be used to reasonably interpret the hydrothermal liquefaction of lignocellulose to some degree.(7) Py-GC/MS and TGA-FTIR were applied to investigate the pyrolysis behaviors of lignocellulose, holocellulose, lignin, hemicellulose and cellulose(900 °C), and the pyrolysis products(biochars and volatiles) and their distributions were comparatively studied. The pyrolysis of hemicellulose and cellulose components mainly occurred at 200-400 °C, while the pyrolysis of lignin happened at 300-700 °C. Except the biochar yield difference, no obvious elemental difference among these biochars was found. Results showed that carbonyl compounds were the major volatiles of polysaccharide component pyrolysis, while aromatic compounds were the major volatiles of lignin pyrolysis. Additionally, calculational chemistry was applied to systematically calculate the bond dissociation energies(BDEs) of the chemical bonds in the model compounds of hemicelluloses, cellulose and lignin. Results showed that the glucosidic bonds and C-C bonds in polysaccharides had lower BDEs, indicating that the pyrolysis of cellulose and hemicelluloses were apt to produce low carbon molecules containing oxygen atoms. For lignin, the methyl on the methoxy group and the Cα–Cβ on the side chain in the lignin monomer and β–O–4′ether bonds and the Cα–Cβ bonds in the lignin dimers had lower BDEs, suggesting that the pyrolysis of lignin was apt to produce phenolic compounds with short side chains. Therefore, calculational chemistry can be used to reasonably interpret the pyrolysis of lignocellulose and explore the reaction mechanism to some degree.