الفهرس 
Lignin in NatureOnly 14 pages are availabe for public view
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Abstract
Lignocellulosic materials such as agricultural residues have been identified as potential sustainable sources that can replace petroleum-based polymers used in many industrial applications. Clean technologies can also be implemented to replace organic wastes by recycling. from the green chemistry viewpoint, we are attempting to explore regenerative materials for carbon-based products as carbon fibers (CFs). Sustainable raw materials made up of biopolymers or biogenic polymers are an especially fascinating source for handling CFs. Given lignin (L) low price and carbon building chemical structure, it is reputable as a potential for CF precursor. The presenting thesis examines the synthesis of CFs from lignin derived from agricultural residues (bagasse (B), palm frond (PF) and banana bunch (BB)) by means of various processing techniques, as follows:
1- The first step involves the extraction of lignin from aforementioned residues used for the treatment of delignification. Among delignification techniques, alkali and organosolv treatments can produce high yields of free-sulfur lignin with better physicochemical properties including high levels of carbon, low ash and low levels of humidity. Alkali hydrolysis technique optimized alkali solution concentration (NaOH), processing time and temperature for high yield (%) of alkali extracted lignins (alkali bagasse lignin (ABL), alkali palm frond lignin (APFL), and alkali banana bunch lignin (ABBL). We concluded that 10% and 100°C are the optimum NaOH concentration and processing temperature for the three biomasses respectively, but the efficacy of alkaline pretreatment was assessed with 12, 6 and 2h run-time, adequate for B, PF and BB biomass.
2- For organosolv hydrolysis technique, the ratio of formic acid (85%) : acetic acid (85%) was (70 : 30) (v/v), boiling for 2 hours in the presence of H2SO4 as a catalyst (0.2% (v/v)). This condition increases the yield (%) of organosolv extracted lignins (organosolv bagasse lignin (OBL), organosolv palm frond lignin (OPFL) and organosolv banana bunch lignin (OBBL). Maintained the solid to liquid ratio of 1:10 (wt/v) .
3- Cellulosic pulp resulted from alkali treatment of abovementioned biomasses was used in producing of cellulose crystals (CCs) after bleaching with hydrogen peroxide and acidic hydrolysis with (64%,) H2SO4 with ratio of 1:10 (wt/v) (cellulose : dilute H2SO4) at 50°C, followed by ice quenching and dialysis processes. Stabilized cellulose crystals (SCCs) of three biomass residues (SCCs-B, SCCs-PF and SCCs-BB) were fabricated through thermal treatment at 230°C in order to improve CCs thermal stability.
4- Alkali extracted lignin samples (ABL, APFL and ABBL) were blended with polyvinyl alcohol (PVA) with total polymer concentration 11% (wt/v). However, organosolv derived lignin samples (OBL, OPFL and OBBL) were therefore blended with cellulose acetate (CA), polylactide copolymerized with poly(hydroxybutyrate) (PHB) and plasticized with acetyl tributyl citrate (PLA-PHB-ATBC) abbreviated as PLA, and polyethylene terephethalate (PET), respectively. The optimum total concentrations of polymers to produce as spun organosolv lignin-based fibers are 13.8% (wt/v( (OBL/CA), 36.7% (wt/v( (OPFL/PLA), and (13.5 and 24.5%) (wt/v( for OBBL/PET (9% and 20%), respectively.
5- The next step is oxidative thermal stabilization with the aim of converting thermoplastic lignin-based fibers into thermoset fiber mats to prevent their fusion during carbonization at high temperatures up to 1000°C. Alkali lignin/PVA-based fibers were successfully stabilized in air at temperature 250°C with heating rate (1°C/min) for 60 min, followed by carbonization with uniform fiber morphology in an inert atmosphere of N2 gas at 850°C for 15 min. The use of an oxidizing agent (iodine crystals) for the thermal stabilization process is main issue in an effort to maintain the fiber morphology of organosolv lignin-based fibers during carbonization treatment. Iodine handling of nanofibers was correlated for ameliorating and accelerating the thermostabilization stage. Thus, iodination of as-spun organosolv lignin-based fiber mats was tested, showing poor temperatures exceptional for OBL/CA fiber morphology. The oxidatively stabilized non-iodinated OBL/CA fiber mat generated at 220°C for 60 min was efficiently carbonized at 600°C. However, iodinated (I) fiber mats (I-OPFL/PLA/36.7% and I-OBBL/PET(13.5 and 24.5%)) were stabilized at 140 and 250°C, respectively, followed by carbonization at 500° C.
6- Modification by Polyaniline (PANI) can elucidate the mechanical properties of the produced fiber mats, coating as-spun nanofibers by m-toluidine polymer (aniline derivative) was developed through in-situ oxidative chemical polymerization. Oxidative chemical polymerization was employed in previous literature, using the optimized conditions (0.4M) ammonium peroxydisulfate (APS, oxidant), (0.1M) m-Toluidine (monomer) and (0.1M) HCl (dopant) concentrations. The weight ratio of fibers : monomer (0.5:1) was kept constant in 50 ml total reaction solution. The obtained hybrid composites are labeled as(ABL/PVA/P-mTol, APFL/PVA/P-mTol, ABBL/PVA/P-mTol, OBL/CA/P-mTol, OPFL/PLA/36.7%/P-mTol, OBBL/PET/13.5%/P-mTol and OBBL/PET/24.5%/P-mTol), respectively.
7- Structural and morphological features of the tested samples were characterized using a number of complementary techniques including Scanning Electron Microscope (SEM), Elemental Analysis, Fourier Transform Infrared (FTIR), thermal characteristics (TGA), evaluating of texture parameters by determination of the N2/77K adsorption which are derived by application of the BET, and tensile properties. Testing of the as-integrated samples and their hybrid composites was achieved by adsorption technique applied by the uptake of a standard probe molecule of a bulky dye ”methylene blue”(MB).
8- Scanning electron microscopy (SEM) describes subsequent morphology studies of all the collected lignin samples from the three biomass treatments through alkali and organosolv hydrolysis techniques showing interconnected spherical particles ranging from micrometre to nanometer. SEM considered that B has more ordered lignin particles than PF and BB biomass by both pre-treatments. The micrographs of SCCs derived from B, PF and BB after processing treatment reflect curled soft flat shape of SCCs with thin porous large sheets and quietly normal rod-shape surfaces indicating that the intra-fibrillar structure was decomposed into individual cells with nano-porous and micro-scale diameter of the cellulosic sheets. The produced electrospun fiber mats appear as entangled uniform fibers with diameter range from micro to nanometer scale. The beaded fibers (beads on-a-string structure) and fiber irregularity substantially decreased by rising the polymer concentration. The interfacial adhesion between lignins and polyolefines is extremely low and insufficient for complete miscibility due to the deficiency of functional groups in polyolefines, so that they can reach only weak dispersion interactions in contrast to the adhesion between lignin and aromatic polymers (PET). The SEM micrographs demonstrated the effect of the use of ultrasonic device to enhance the fiber morphology of as-spun lignin/polymer fibers using water bath and finger sonicator equipment.
9- There is a smooth surface with uniform circular fibers and typical structure of CFs derived from alkali lignin/PVA blends. from these observations, we can infer that the compatibility between alkali lignins and PVA not only improves the efficiency of electrospinning, but also improves the morphology of the resulted fibers.
10- Comparing alkali lignin-based electrospun fiber mats with organosolv-based electrospun fibers, the last fibers required longer thermostabilization times to prevent fusion. Through the usage of iodine crystal as an oxidizing agent could solve the problem of maintaining fiber morphology when converting electrospun fibers into CFs through the process of carbonization. Iodine acts as an electron withdrawal from electron-rich molecules, especially aromatic rings, and polyiodides are formed. Through the heating process, the iodine and absorbent form the charging transfer complexes (CTCs) that leave behind a free radical through the heating process. The CFs produced from iodinated fiber mats appear as randomly oriented entangled fibers. A comparative study for produced fiber mats (as-spun fibers and CFs) represents the lowest fiber diameter for that derived from bagasse l lignin.
11- As-spun fiber mats modified with poly (m-Toluidine) into hybrid composites through in-situ oxidative chemical polymerization show that m-toluidine polymer appears as granular particles of average diameter (314 nm) sticked to each other as clumps on the enclosed fibers and all organosolv lignins/polymer-based hybrid composites have no significant improvement in the fiber diameter. PVA is, moreover, hydrophilic in nature, leading to the instability of alkali lignins/PVA fibers during oxidative chemical polymerization (acid/water media) treatment. These properties restrict broader PVA applications.
12- The physicochemical characteristics of extracted lignin samples and produced fiber mats declare that carbon content of bagasse lignin whether extracted by alkali (59.5%) and organosolv (59%) treatments exhibits higher content than that of the other manufactured samples. In accordance with this conceptualization, lignin extracted from bagasse has better carbon content to be applied for this utility. This was affirmed among carbonization process. In a comparative survey for carbon contents of CFs produced from lignins extracted by both delignification techniques, it is asserted that higher carbonization temperature yielded fibers with higher carbon and lower oxygen percentage. According to this conceptualization, CFs derived from alkali lignin/PVA carbonized at 850°C produce higher percentages of carbon contents.
13- FT-IR spectroscopy has proved to be a highly effective means of studying the interpolymer miscibility through the formation of H-bonding within lignin particles. It was discovered that the intensities of all prominent characteristic bands of lignins have significantly increased and shifted to some extent throughout blending with polymer solutions to produce as-spun fiber mats. Therefore, it can confirm the physical blending of lignin/polymers interaction. Carbonization process can diminish O-H, C-H, C-O and phenyl groups, cause closer structure of benzene rings, produce cross links and change the structure to form the network of carbon-six rings. As evidenced, the stretching vibrational frequency at about 3400 cm-1 originated from hydrogen-bonded OH groups was responsible for the strong hydrogen bonds among the OH groups of blended polymers and lignins representing lignins/polymer interaction. And this band is weakened significantly in the produced CFs due to dehydration, dehydroxylation mechanisms and evolution of water contents during carbonization process. Throughout polymerization with m-Toluidine, a complex interaction arose resulted in a decrease in hydrogen bond formation between PVA and alkali lignins, consequently PVA has a tendency to be less hydrophilic, thus its intensity decreased. In addition, sharp absorption peaks from 1600 to 1580 cm-1, 1513 cm-1 and 1250 cm-1 correspond to (quinoid (Q) C=C), (benzenoid (B) C=C) and C-N vibration of aromatic amines, respectively.
14- Thermal stability mapping observation of extracted lignin specimens showed that ABBL has lower thermal stability with char yield 3% relative to the other lignin samples. Consideration must be given to the fact that the temperature of degradation increases by using chemical treatment. Removal of hemicellulose and lignin during delignification, bleaching pretreatment and removal of sulfate groups after acid treatment effects on thermal stability of SCCs. A low thermal stability is anticipated for SCCs containing sulfate groups, since these groups may raise the rate of cellulose dehydration reactions. This is because the incorporation of sulfate groups on the surfaces of cellulose after acid hydrolysis has a catalytic effect on their thermal degradation. Moreover, oxidative thermal stabilization treatment affects positively on thermal stability of extracted cellulose crystals. CFs, APFL/PVA and CFs, ABBL/PVA manifested good thermal stability having 61.4 and 77% residual char yield, respectively as compared to CFs, ABL/PVA having 5% residual char yield. Generally, the thermal stability of lignin can be explained by the amount of aromatic rings, different functional groups and chemical bonds. OBL-based CFs have higher thermal stability with char yield of (47.5%) corresponding to the alternative of both samples OPFL and OBBL-based CFs individually.
15- The textural characteristics of the lignin surface are significantly impacted by the source of biomass and the pretreatment process conditions. Organosolv extracted lignins have a significant higher surface area beside mesoporous nature in comparison to alkali lignins. This can be ascribed to the increase in the pretreatment ferocity of the used process conditions for the organosolv lignins. As demonstrated by the results of SEM and textural characteristics, particularly the OBL-based materials give a stronger candidate towards adsorption of water pollutants. Within this scope, the actual fact that organosolv lignin has higher surface area and pore size permit easier access of adsorbates (pollutants), if applied to the organosolv extracted lignins-based adsorbents. This disparity in textural properties. could be associated to the topological defects of lignin which are induced by the biomass source. It was observed that SCCs-BB have a relatively smaller surface area (122 m2/g) and total pore volume compared to most of the SCCs-B and SCCs-PF. This was manifested to the inhibitory effect of compacting process during freeze drying method. During bleaching of cellulosic pulp, further removal of amorphous materials (lignin and hemicellulose) from the inner matrix of the biomass substrate occurred through de-polymerization and delignification, after hydrolysis and freeze drying, the samples tend to become self-assembled into micro-fibrillated fibers separated from each other via hydrogen bonding interaction. The intermolecular hydrogen bonding is responsible for producing stable, strong and rigid structure of produced cellulose. The fibers were overlapping together due to evaporation of water during oxidative thermal stabilization at temperature 230°C forming interconnected mesopores in the host lattice. Hydrolysis, freeze drying and stabilization treatment resulted in mesoporous hydrogen-bonded organic framework of stable cellulose.
16- It was observed that as-spun fiber mats derived from bagasse and palm frond by both alkali and organosolv treatments (ABL/PVA, APFL/PVA, OBL/CA and OPFL/PLA/36.7%) have a considerably smaller surface area (87, 64, 37 and 71 m2/g) and pore volume compared to most other spun fiber mats. This can be attributed to the connection of the fibers during electrospinning, and the obstruction of gas adsorption and desorption by organic salt crystals incorporated on the rough surface of fibers. In addition, CA and PLA are hydrophobic in nature and this boundaries their dispersion in polymer matrix and hydrophilic solvents, in contrast lignin is hydrophilic due to the presence of hydroxyl groups on its surface. So, CA and PLA are not soluble in water while lignin as a consequence of its strong H-bonding cannot be used in system without water. Therefore, a weak filler-matrix interaction is established as there is poor interfacial compatibility among the non-polar matrix and the highly polar surface of lignin that leads to decrease in surface area.
17- The capability of isolated lignin materials to adsorb dyes was stated. Bagasse lignin-based fiber mats (ABL/PVA; OBL/CA; CFs, ABL/PVA; CFs, OBL/CA; ABL/PVA/P-mTol and OBL/PVA/P-mTol) demonstrated higher adsorption capacities for methylene blue (25, 24.7, 16, 18, 26 and 29 mg/g, respectively) than that of PFL and BBL-based fibers at pH=6. This implies that MB dye is a basic (cationic) dye stuff with (Mwt=319 g/mol, cross sectional area=120Å and molecular size ~ 15 Å) and can’t easily penetrate material with pores smaller than 1.3 nm. The cationic dye is adsorbed as ionic micelles and expressed as the smallest enclosing rectangle. The mechanism responsible for MB removal through hydrogen bonding between –OH groups of lignin fibers and aromatic hetero-cyclic ring of MB is a major donor for MB capture. Van der Waals forces among the phenolic ring of lignin fiber and aromatic heterocyclic ring of MB may also have assisted significantly. At pH=6, MB removal efficacy is pronounced to the possibility of electrostatic interactions as the surface of adsorbents tend to be negatively charged due to deprotonated acid groups. The oxygen atoms have a large affinity of partially charged molecules and cations because of persistent electrostatic interactions. Thus, we can say that through Van der Waals forces, hydrogen bonding and electrostatic interactions, the MB molecules were captured from water. While for hybrid composites, poly (m-Toluidine) possesses a lot of π-π stacking, and hydrophobic interactions among poly (m-Toluidine) and MB dye through H-bonding.
18- SCCs-B exhibited the highest adsorption capacity (36 mg/g) towards MB dye. This can be assigned to various factors like as high surface area, presence of many aliphatic hydroxyl groups and mesoporous structure predominantly making SCCs-B primly an excellent candidate for MB dye adsorption via hydrogen bonding and electrostatic interaction. The competence of biomass bagasse-based materials (SCCs-B (36 mg/g); ABL/PVA (25 mg/g); ABL/PVA/P-mTol (26 mg/g); CFs, ABL/PVA (16 mg/g); OBL/CA (24.7 mg/g); OBL/CA/P-mTol (29 mg/g) and CFs, OBL/CA (18 mg/g)) provides a better candidate towards their adsorption valorization (water purification) than the other two biomass-based materials.
19- The interfacial adhesion between the fibers and polymer matrix plays a vital role in strengthening the mechanical behavior of composite reinforced with fiber mats. The tensile properties of lignin/polymer-based fibers differ according to the form of lignin samples. Accordingly, they rely upon the sort of biomass. In line with this result, the improved mechanical properties were obtained for banana bunch lignin-based fibers. Generally, the factors assist the reinforcement tendency are (i) good dispersion, and (ii) good interfacial interaction among polymer and nanofiller. By achieving each of those factors, it would enhance the tensile properties considerably due to the effective transmittance of the tensile loads across the polymer-filler interfaces. In our study, the presence of functional moieties such as PVA (hydroxyl groups), PET end groups (ester and ethylene groups) and the lignin (OH), leads to good interfacial interaction among them, strong fiber-fiber interface package and hydrogen bond formation. Thus, there is an increase in the tensile strength and decrease in Young’s modulus in all the systems due to the existence of physical interaction of the polymer chains with lignin particles that would limit the movement of the chains by placing them in their vicinity. It may take into consideration for excellent characteristics of poly esters such as high strength, good stretch-ability, and durability. The dispersion of one constituent into the other is important when producing composites, because this has a significant impact on the determination of mechanical properties of the composite material. If the matrix accomplishes a uniform dispersion of the reinforcement filler as well as good interfacial adhesion, a substantial improvement in the mechanical properties is feasible as in case of OBL/CA/P-mTol composite. Blending of OBL with an emeraldine base of aniline polymers exhibited a strong homogeneous blend enabling interaction between amine groups of (aniline) with carbonyl group and hydroxyl groups of lignin.
20- Finally, all the above results show that the characteristics of the obtained as-integrated manufactured materials are virtually affected by the biomass source and the pretreatment process conditions. The comparative study for produced SCCs and fiber mats (as-spun fibers and CFs) represents that products derived from lignin extracted from bagasse is the best carbon-precursor according to their uniform fiber diameter and high carbon contents of produced lignins. The competence of biomass bagasse-based materials provides a better candidate towards water purification than the other two biomass-based materials. However, the improved mechanical properties were obtained for banana bunch lignin-based fibers. Agricultural wastes seem to be an excellent carbon fibers precursor, not only in terms of ease of fabrication and high carbon yield, but also to disseminate utilizing the clean technology and bio-refinery sector in the future application prospects.