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The unique decomposition pathways of hydrolytic lignin (HL) dissolved in acetone/water mixture (9:1) and dispersed by droplet evaporation technique under nitrogen gas flow has been investigated in conventional reactor at atmospheric condition, temperature region of 400 – 550°C, and residence time of 0.12 sec. The results validate the fact that disp...
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... LTMI-EPR allows accumulation and detection of trace quantities of radicals during the gas-phase thermal degradation of many classes of organics. An electrically heated reactor interfaced to a liquid nitrogen-cooled dewar located in the cavity of an EPR spectrometer was used for the pyrolysis of lignin ( Figure S1). 10 mg of HL was plugged from both sides by quartz wool and placed in the center of the reactor. ...
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... Radicals Trapped from Lignin Vacuum Pyrolysis. To investigate the radical characteristic of depolymerization of lignin in solid state, pyrolysis of lignin was performed in a simple tubular quartz reactor located near the EPR cavity under vacuum, Figure S1. A detailed description of the cryogenic trapping method, the LTMI EPR technique, is described elsewhere. ...
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... reaction was carried out at different temperatures ranging from 425 to 525 °C to investigate the effect of temperature on the nature and intensity of the EPR signal. The effluent of the reactor was pumped at vacuum of ∼10 −3 Torr; the time- of-flight of released molecules from the lignin matrix to the coldfinger at 77 K, placed within the microwave cavity of the EPR spectrometer ( Figure S1), was in the range of milliseconds. 51 2.1.3. ...
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... Gas-Phase Continuous Delivery: Droplets Evapo- ration. The continuous gas-phase lignin pyrolysis was performed using a syringe pump to inject the solution of lignin dissolved in an acetone/ water solvent (9:1) in a preferably high temperature area (≈250 °C) in a quartz reactor with i.d. of 12 mm and length of 14 cm (see Figure 1). ...
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... pyrolysis products were trapped by dichloromethane (DCM) in an impinger (Figure 1) at iced water temperature. The need for a second impinger was ruled out due to the fact that only trace amounts of compounds were detected. ...
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... Depolymerization in the Gas Phase. By droplet evaporation presented above, Figure 1, the HL was almost quantitatively transported through the hot zone at a high flow rate of the carrier gas, 1000 mL/min and a short residence time (0.12 s). Around 85−92% of HL was recovered at the end of the reaction zone almost in all temperature regions investigated. ...
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... was transported through the hot zone and collected on quartz wool, Figure 1. EPR analyses were subjected to fresh as well as aged samples. ...
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... LDI-TOF mass spectrometry results are demonstrated in Figure 10. Those data shown with green color arrows (m/z 358.39; 568.72; 596.76; 624.79; 668.80; 836.03) are the ions after lignin pyrolysis. ...
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... intriguing fact is the dominant formation of the peak at m/z 284 ( Figure 10). The GC-MS results also show the major peak at 26.42 min, Figure 6, with molecular weight of 284 Da. ...
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... literature data are similarly controversial: it was suggested that the peak at m/z 284 is C 18 fatty acid 75 from wood decay by white-rot and brown-rot fungi or derivatives of methoxyhydroxyphenanthrene. 37 How- ever, we are biased to an explanation developed in refs 76 and 77 stating a transformation of the protonated phenylcoumaran derivative 4 [C 19 The intensification of another lignin substructure at m/z 358 after pyrolysis, Figure 10, might be by dimerization of coniferyl alcohol 73 or dehydrogenation of coniferyl alcohol via radical intermediates suggested in ref 78, Figure S3. This is excellent evidence for existence of the radicals and self-condensation of them 78 by formation of such dimers at m/z 358 and prevailing of reactions of condensation (radical−radical reactions) over the formation of phenolics due to radical-molecule reactions (no coniferyl alcohol was detected by GC-MS). ...
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... the existence of the intrinsic dimer, probably trimer lignin substructures (refer to Figure 10, dimer peaks at m/z 284 and 368, and the peak at m/z 522.79, 550.81 which might be some trimers), provides evidence that the oligomers may have a potential role in the formation of phenolics in corresponding experimental conditions. However, the possi- bility of parallel formation of these compounds, phenolics and oligomers in the processes of lignin pyrolysis, remains open for further discussion. ...
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... The synergic effect for formation of new products in mixtures is attractively shown in ref 85. The pyrolysis of single model compounds (hydroxyacetone/cyclopentanone for cellulose and hemicellulose and vinyl-guaiacol for lignin) did Figure 10. LDI-TOF spectra for fresh initial HL and the residues (collected on quartz wool in close proximity to the hot zone at ∼300 °C) after gas-phase pyrolysis of lignin. ...
Citations
... The 3-methoxy-1,2-benzenediol was also observed, which was due to the demethylation or demethoxylation reaction between 2-methoxy-4-vinylphenol and 2-methoxy-phenol [34]. Barekati-Goudarzi et al. [36] have investigated the pyrolysis products of hydrolytic lignin at 400-500 • C and the 2,6-dimethoxy-phenol, 2-methoxy-4-vinylphenol, 2,3-dihydro-benzofuran, 2-methoxy-phenol, 2,5-hexanedione, and benzene, 1,3,5-trimethyl were found as the major products at retention times of 12.81, 12.42, 11.52, 10.07, 8.09 and 7.83 min, respectively. This was consistent with the present work. ...
In this study, the pyrolysis behavior and reaction kinetics of bio-based polyurethane (BPU) was investigated in comparison to the individual components enzymatic lignin (EL) and polyurethane (PU). The produced gas composition during their pyrolysis were investigated using thermogravimetric (TG) analysis combined with FTIR and TG coupled with TG-GC/MS. TG analysis indicated that the decomposition of BPU was comparable with that of PU. However, the larger peak temperatures in DTA curves and residue mass for BPU than that for PU indicated that the incorporation of EL improved its thermal resistance. Most of the absorbance bands in FTIR for both samples were similar, except of the absorbance at 1500-1750 cm-1 for PU and 1000-1150 cm-1 for BPU. The dominant evolved products were N-containing compounds for PU and BPU, however, the phenols and furans were detected during BPU pyrolysis. Based on the Flynn-Wall-Ozawa model, the average activation energy was determined as 176.1 kJ mol-1 for BPU, which was larger than 159.5 kJ mol-1 for PU and smaller than that of 298.5 kJ mol-1 for EL. For PU and BPU, the experimental curves were comparable with F1.5 model at the conversion rate in the α range of 0.1-0.5 and F3 model beyond that range.
... 5 Some initial results involving our recent findings on the formation of resonantly stabilized radicals from homogeneous pyrolysis of lignin and its monomers and precursors transferred into the gas phase by dispersion have also been reported. 49 acetone/water (v/v of 9:1). The HL solution was introduced to the reactor using a commercial TSI 3076 Constant Output Atomizer; the reactor was denoted as continuous atomization (CA) reactor, 49,50 Figure 2. A flow of ultra-high-purity nitrogen gas was used to operate the atomizer and resulted in a 2 s residence time inside the quartz reactor (I.D. = 45 mm and L = 50 cm) situated in an electrical furnace. ...
... 49 acetone/water (v/v of 9:1). The HL solution was introduced to the reactor using a commercial TSI 3076 Constant Output Atomizer; the reactor was denoted as continuous atomization (CA) reactor, 49,50 Figure 2. A flow of ultra-high-purity nitrogen gas was used to operate the atomizer and resulted in a 2 s residence time inside the quartz reactor (I.D. = 45 mm and L = 50 cm) situated in an electrical furnace. The pyrolysis products/radicals were trapped by deactivated quartz wool located at the end of the reactor, Figure 2. The quartz wool was transferred into electron paramagnetic resonance (EPR) tubes (o.d. ...
... The importance of the radical mechanism of lignin pyrolysis as one of the major depolymerization pathways has been discussed in our recent publication. 49 The respective free radicals have been detected in a variety of systems associated with pyrolysis of lignin and lignin model compounds, such as cinnamyl (CnA), p-coumaryl (p-CMA), and recently reported coniferyl (CFA) alcohols, 50,52,53 using a low-temperature matrix isolation (LTMI) EPR technique. 54 Detailed information about cryogenic trapping of radicals is presented in the Supporting Information (e.g., Figures S3 and S4 for some models and for lignin, respectively). ...
To assess contribution of the radicals formed from biomass burning, our recent findings toward the formation of resonantly stabilized persistent radicals from hydrolytic lignin pyrolysis in a metal-free environment are presented in detail. Such radicals have particularly been identified during fast pyrolysis of lignin dispersed into the gas phase in a flow reactor. The trapped radicals were analyzed by X-band electron paramagnetic resonance (EPR) and high-frequency (HF) EPR spectroscopy. To conceptualize available data, the metal-free biogenic bulky stable radicals with extended conjugated backbones are suggested to categorize as a new type of metal-free environmentally persistent free radicals (EPFRs) (bio-EPFRs). They can be originated not only from lignin/biomass pyrolysis but also during various thermal processes in combustion reactors and media, including tobacco smoke, anthropogenic sources and wildfires (forest/bushfires), and so on. The persistency of bio-EPFRs from lignin gas-phase pyrolysis was outlined with the evaluated lifetime of two groups of radicals being 33 and 143 h, respectively. The experimental results from pyrolysis of coniferyl alcohol as a model compound of lignin in the same fast flow reactor, along with our detailed potential energy surface analyses using high-level DFT and ab initio methods toward decomposition of a few other model compounds reported earlier, provide a mechanistic view on the formation of C- and O-centered radicals during lignin gas-phase pyrolysis. The preliminary measurements using HF-EPR spectroscopy also support the existence of O-centered radicals in the radical mixtures from pyrolysis of lignin possessing a high g value (2.0048).
... The formation of oxygen-centered phenoxy type PhO• radicals during lignin pyrolysis has been recently confirmed in a number of publications (Barekati-Goudarzi et al., 2017;Khachatryan et al., 2016;Kibet et al., 2012;Kim et al., 2014). A reaction pathway similar to that of reaction (3) could serve as an alternative route to remove more reactive PhO• radicals in process of lignin pyrolysis by replacing them with less active thiyl radicals, RS•. ...
Softwood Kraft lignin (s-KL) and methanol-fractionated extracted Kraft lignin (ex-KL) samples were thermally depolymerized via fractional (step-wise) pyrolysis at temperatures from 175 °C to 700 °C in an isothermal System for Thermal Diagnostic Studies (STDS) reactor. Major pyrolysis products include guaiacols, vanillins, phenols, syringols, and sulfur-containing compounds. Sulfur-containing compounds, as intrinsic contaminants (both adsorbed and covalently bound to KL matrixes), strongly inhibited the formation of pyrolysis products. The lesser the sulfur content in the KL matrix, the higher the yields of major pyrolysis products. The high yields and early release of tar components from pyrolysis of ex-KL were attributed to the initial pretreatment of s-KL (Soxhlet extraction in methanol) causing a decrease in sulfur content from 3.61% (s-KL) to 2.91% (ex-KL). Mechanistic explanations for the absence of sulfur-containing bio-oil products, inhibitory effect of sulfur-containing compounds, and the relatively high char content in depolymerization of both lignin substrates, are developed.
... During pyrolysis, lignin is heated to temperatures between 160 and 900°C where cleavage of the ether (CeO) and CeC linkages takes place (Yang et al., 2007). Lignin pyrolysis produces a range of pyrolytic aromatic compounds in oil form in addition to gas products and residual char (Barekati-Goudarzi et al., 2017;Khachatryan et al., 2018). The yield and composition of pyrolytic oil are influenced by many factors, including lignin type and operation conditions (Mullen et al., 2010). ...
Lignin holds tremendous potential as a renewable feedstock for upgrading to a number of high-value chemicals and products that are derived from the petroleum industry at present. Since lignin makes up a significant fraction of lignocellulosic biomass, co-utilization of lignin in addition to cellulose and hemicelluloses is vital to the economic viability of cellulosic biorefineries. The recalcitrant nature of lignin, originated from the molecule's compositional and structural heterogeneity, however, poses great challenges toward effective and selective lignin depolymerization and valorization. Ionic liquid (IL) is a powerful solvent that has demonstrated high efficiency in fractionating lignocellulosic biomass into sugar streams and a lignin stream of reduced molecular weight. Compared to thermochemical methods, biological lignin deconstruction takes place at mild temperature and pressure while product selectivity can be potentially improved via the specificity of biocatalysts (lignin degrading enzymes, LDEs). This review focuses on a lignin valorization strategy by harnessing the biomass fractionating capabilities of ILs and the substrate and product selectivity of LDEs. Recent advances in elucidating enzyme-IL interactions as well as strategies for improving enzyme activity in IL are discussed, with specific emphases on biocompatible ILs, thermostable and IL-tolerant enzymes, enzyme immobilization, and surface charge engineering. Also reviewed is the protein engineering toolsets (directed evolution and rational design) to improve the biocatalysts' activity, stability and product selectivity in IL systems. The alliance between IL and LDEs offers a great opportunity for developing a biocatalytic route for lignin valorization.
... However, there is no published data on the composition and structures of HL sulfur compounds in the literature. No sulfur compounds in the pyrolysis products of HL were discovered (Barekati-Goudarzi et al. 2017). The aim of this work is the desulfurization of HL. ...
The hydrolysis (or hydrolytic) lignin as a byproduct of ethanol production is made by the treatment of wood with a sulfuric acid solution. It contains up to 2% total sulfur. This work proposes the hydrolysis lignin desulfurization by acid–base methods. These methods are based on data on the presence of residual sulfuric acid and its reaction products (organic sulfates and sulfonates) with wood components in the hydrolytic lignin. It was found that the sodium hydroxide treatment has the best desulfurizing effect, which allowed reducing the total sulfur content to 25 ppm.
Coniferyl alcohol (CFA) is one of lignin’s main building blocks and a relevant model compound. Two aspects of CFA pyrolysis are studied in the current work to gain insight into the molecular mechanism of lignin thermolysis: (i) the nature of intermediate radicals generated from vacuum pyrolysis of CFA, and (ii) the product distribution from pyrolysis of CFA dispersed into the gas phase. Low temperature matrix isolation electron paramagnetic resonance (LTMI-EPR) spectroscopy is employed to investigate the intermediate radicals associated with gas phase low-pressure pyrolysis of CFA. An anisotropic EPR spectrum of radicals trapped at liquid nitrogen temperature was registered throughout the studied temperature range (400-700 oC) characterized by high g-value (≤ 2.0100), and broad EPR line-width (~13.5-13.0 G) prescribed to a mixture of O-centered radicals produced from cleavage of O−CH3 and O−H terminal bonds. Thermal annealing of frozen radicals suggested a major contribution of methylperoxyl (CH3O2) radicals produced by the liberated CH3 and trace amounts of oxygen. The pyrolysis of CFA, for the first time, was conducted in a dispersed, gas-phase conditions to minimize the heterogeneous secondary reactions mediated by particle surfaces typical for condensed-phase pyrolysis in conventional reactors. The CFA, dissolved in acetonitrile and pulverized homogeneously into nitrogen carrier gas, was pyrolyzed at 400 oC. Several unique products including polyhydroxylated derivatives of mandelic acid and some carboxylic compounds were identified in addition to those normally produced during the conventional pyrolysis - coniferyl aldehyde, vanillin (and its oxidized form homovanilic acid), eugenol and its isomers. A comprehensive analysis of product formation mechanisms, particularly the pathways for generating abundant hydroxylated compounds is performed and the role of trace oxygen is studied at the wB97XD and M06-2X hybrid density functional theory levels.
This study provides molecular insights into the light absorption properties of biomass burning (BB) brown carbon (BrC) through the chemical characterization of tar condensates generated from heated wood pellets at oxidative and pyrolysis conditions. Both liquid tar condensates separated into "darker oily" and "lighter aqueous" immiscible phases. The molecular composition of these samples was investigated using reversed-phase liquid chromatography coupled with a photodiode array detector and a high-resolution mass spectrometer. The results revealed two sets of BrC chromophores: (1) common to all four samples and (2) specific to the "oily" fractions. The common BrC chromophores consist of polar, monoaromatic species. The oil-specific BrC chromophores include less-polar and nonpolar polyaromatic compounds. The most-light-absorbing pyrolysis oily phase (PO) was aerosolized and size-separated using a cascade impactor to compare the composition and optical properties of the bulk versus the aerosolized BrC. The mass absorption coefficient (MAC300-500 nm) of aerosolized PO increased compared to that of the bulk, due to gas-phase partitioning of more volatile and less absorbing chromophores. The optical properties of the aerosolized PO were consistent with previously reported ambient BB BrC measurements. These results suggest the darkening of atmospheric BrC following non-reactive evaporation that transforms the optical properties and composition of aged BrC aerosols.
Microwave assisted liquefaction (MAL) of alkali lignin in ethylene glycol (EG) in the presence of hydrazine was carried out using response surface methodology. Dielectric properties of mixtures of lignin/EG at different hydrazine concentrations were determined to enhance the understanding of the MAL process. MAL parameters were hydrazine concentration (1–3%), temperature (100-180 °C), time (5-45 min) with the maximum microwave heating power limited to 750 W. Lignin/EG ratio was fixed at 1 g:10 ml. Dielectric properties were significantly influenced by hydrazine concentration and temperature. Major products from the liquefaction process (as % of peak areas) were grouped into guaiacol and derivatives (17–42%), Nitrogen compounds (0–27%), and Hydroxylated products (8–22%), and their yields were modelled with polynomial equations. Product yields were significantly influenced by process parameters. Optimum parameters for maximizing yields of guaiacol and nitrogen containing compounds were 0.5%, 180 °C, 25 min and 2.5%, 130, 25 min for hydrazine concentration, temperature, and time, respectively. Low hydrazine concentration increases guaiacol yield at higher temperatures, indicating that at high temperatures hydrazine behaves as a hydrogen donor for lignin liquefaction. The yield of nitrogen containing compounds increases at relatively low temperatures and high concentration of hydrazine. Apart from playing the respective roles of liquefaction solvent and hydrogen donor, EG and hydrazine also take part in the reaction thereby significantly modifying the composition of reaction products. A mechanistic reaction mechanism was proposed to explain the formation of the liquefaction products.
Pyrolysis of hydrolytic lignin (HL) in the newly designed, gas phase continuous droplet evaporation (CDE) and continuous atomization (CA) reactors, was studied. The products distribution was strongly dependent on the heterogeneous character of either delivery of lignin solution into CDE reactor (in-situ formation of solid phase) or sampling conditions using quartz wool in both CDE and CA reactors. The effect of residence time, initial concentration of HL solution and injection temperature on products distribution in CDE reactor was investigated and discussed in terms of mass and heat transfer limitation. The experimental data confirm that at low initial mass delivery rates of lignin (micrograms per second) and by increasing initial lignin concentration (up to 40 times), the formation of phenolics is slightly intensified (6 times). However, the solid surface or any condense phase that forms in-situ during reaction in the gas phase may largely govern the pyrolysis processes. The detailed experimental examination of homogeneous pyrolysis of lignin in both gas-phase reactors by implication of diverse analytical techniques (GC, GPC, LDI, FTIR, EPR, NMR) revealed break down of HL macromolecules into oligomer-fragments after pyrolysis at negligible amounts of phenolics detected. A mechanistic interpretation of primary steps for formation of dominant intermediate products – oligomers and oligomer stable radicals, is represented.
