Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei

Thermal reactions of guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol) were compared in a closed ampoule reactor (N₂/400-600 °C/40-600 s) to obtain information on the thermal reactivities of lignin aromatic nuclei, guaiacyl and syringyl types. For both compounds, the O-CH₃ bond homolysis, which was observed at >400 °C, initiated their decomposition. This homolysis was followed by several temperature-dependent reactions; radical-induced rearrangement to convert the aromatic OCH₃ to aromatic CH₃ structures and condensation into high molecular weight (MW) products were the next steps (≈400 °C); then, coke formation became extensive (≈450 °C); effective gas formation required higher temperature such as >550-600 °C. The syringol- and guaiacol-characteristic GC/MS-detectable low MW products were explained with the above mentioned reactions. As for the difference between guaiacol and syringol, coke and gas (especially CH₄ and CO₂) formation was more extensive in syringol. This effective coking can be explained by the influence of the additional OCH₃ group in syringol, which doubles the opportunity for coke formation. This, in turn, reduces the yields of GC/MS-detectable low MW products. Demethoxylation to form guaiacol was also observed in syringol, even though the reactivity was not so high. These reactions are discussed at the molecular level.     

Thermal reactivities of catechols/pyrogallols and cresols/xylenols as lignin pyrolysis intermediates

Thermal reactivities of lignin pyrolysis intermediates, catechols/pyrogallols (O-CH₃ homolysis products) and cresols/xylenols (OCH₃ rearrangement products), were studied in a closed ampoule reactor (N₂/600 °C/40-600 s) to understand their roles in the secondary reactions step. Reactivity tends to be enhanced by increasing the number of substituent groups on phenol and this effect was greater for -OH than for -CH₃. Thus, catechols/pyrogallols were more reactive than cresols/xylenols and syringol-derived products were more reactive than corresponding guaiacol-derived products. Catechols/pyrogallols were effectively converted into CO (additionally CO₂ in the case of pyrogallols) in the early stage of pyrolysis. In contrast, cresols/xylenols were comparatively stable and produced H₂, CH₄ and demethylation products (cresols and phenol) after prolonged heating. All intermediates except phenol and 2-ethylphenol formed coke during a long heating time of 600 s (second stage coking). Based on the present results, the roles of intermediates in tar, coke and gas formation from guaiacol and syringol are discussed at the molecular level, focusing on their differences. Molecular mechanisms of gas formation from pyrogallols and demethylation of cresols/xylenols are also discussed.     

HCN and NH3 (NOx precursors) released under rapid pyrolysis of biomass/coal blends

Rapid pyrolysis of 6 biomass/coal blends (1:4, wt) including rice straw + bituminous (RS + B), rice straw + anthracite (RS + A), chinar leaves + bituminous (CL + B), chinar leaves + anthracite (CL + A), pine sawdust + bituminous (PS + B), and pine sawdust + anthracite (PS + A) was carried out in a high-frequency magnetic field based furnace at 600-1200 °C. The reactor could not only achieve high heating rates of fuel samples but also make biomass and coal particles contact well; secondary reactions of primary products during rapid pyrolysis can also be efficiently reduced. By comparing nitrogen distributions in products of blends (experimental values) with those of the sums of individual biomass and coal (weighted values), nitrogen conversion characteristics under rapid pyrolysis of biomass/coal blends were investigated. Results show that, biomass particles in blends lead to higher experimental char-N yields than the weighted values during rapid pyrolysis of biomass/anthracite blends. The decreased heating rates of both biomass and coal particles caused by the low packing densities of biomass may be the reason. For blends of CL + B in which packing density of chinar leaves is high, and for PS + B during pyrolysis of which melting and shrinkage happen to pine sawdust, both biomass and coal particles can obtain high heating rates, synergies can be found to promote nitrogen release from fuel samples and decrease char-N yields under all the conditions. But the low fluidity and not easily collapsed carbon skeletons of rice straw make the heating rates of rice straw and bituminous particles in RS + B lower than those of CL + B and PS + B, and weaker synergies can be found from char-N yields of RS + B. The synergies can obviously be found to decrease the (NH₃ + HCN)-N yields and make more nitrogen convert to N₂ except for those of several low-temperature conditions (600-700 °C). Under the low-temperature (600-700 °C) condition, synergies make molar ratios of HCN-N/NH₃-N higher than those of the weighted values.     

Chemical structure evolution and mechanism during pre-carbonization of PAN-based stabilized fiber in the temperature range of 350-600 °C

A series of PAN-based pre-carbonized fibers were prepared in pilot carbonization line at gradient temperature of 350-600 °C. DSC, FTIR, X-ray diffraction, EPR and elemental analysis were used to study the evolution of chemical structures in these pre-carbonized fibers. At the same time, the reaction mechanism during the pre-carbonization was also explored. According to the chemical structural changes, the process of pre-carbonization below 600 °C can be divided into three stages. Below 450 °C, cross-linking and aromatization are the main reactions accompanied by heat liberation and rearrangement of oxygen-containing groups. But around 500 °C the pyrolysis reactions acquire the priority and the aromatization structure is dominant in the fibers. Until 600 °C the ladder polymers cross-link to form the carbon basal planes, resulting in the growth of aromatization structures. Based on the dependence of fiber structure on temperature, the pre-carbonization technology was optimized.     

Investigation of physicochemical properties of biooils produced from yellow poplar wood (Liriodendron tulipifera) at various temperatures and residence times

Fast pyrolysis of yellow poplar wood (Liriodendron tulipifera) was performed under different temperature ranges and residence times in a fluidized bed reactor to maximize the yield of biooil. In this study, the pyrolysis temperature ranged from 400 °C to 550 °C, and the residence time of pyrolysis products was controlled between 1.2 and 7.7 s by inert nitrogen gas flow. The results revealed that the distribution of thermal degradation products (biooil, biochar, and gas) from the woody biomass was heavily influenced by pyrolysis temperature, as well as residence time. The highest yield of biooil was approximately 68.5 wt% (wet basis), with pyrolysis conditions of 500 °C and 1.9 s of residence time. Water content of the biooils produced at different temperatures was 25-30 wt%, and their higher heating values were estimated to be between 15 MJ/kg and 17 MJ/kg. Using GC/MS analysis, 30 chemical components were identified from the biooil, which were classified into 5 main groups: organic acids, aldehydes, ketones, alcohols, and phenols. In addition, biochar was produced as a co-product of fast pyrolysis of woody biomass, approximately 10 wt%, at temperatures between 450 °C and 550 °C. The physicochemical features of the biochar, including elemental analysis, higher heating values, and morphological properties by SEM, were also determined.     

Study of the pyrolysis of municipal solid waste for the production of valuable products

To obtain information on the potential of thermal conversion (pyrolysis) of municipal solid waste (MSW), a thermogravimetric study (TGA) is performed in a stream of nitrogen. Based on TGA results, pyrolysis experiments are carried out in a semi-batch reactor under inert nitrogen atmosphere. Slow pyrolysis is performed up to 550 °C (heating rate of 4 °C/min). Fast pyrolysis is performed at 450, 480, 510 and 550 °C and different input transfer rates (12 or 24 g material/min). The pyrolysis products are studied on composition and yield/distribution and investigated for their use as valuable product.The liquid obtained by slow pyrolysis separates spontaneously in a water rich product and an oily product. For all fast pyrolysis conditions, a viscous, brown oil which contains a poly(ethylene-co-propylene) wax is obtained. Composition analyses by GC/MS of the oil products (slow/fast pyrolysis) show that aliphatic hydrocarbons are the major compounds. The pyrolysis oils have high calorific value (between 35 and 44 MJ/kg), low wt% of water (around 6 wt%) and a low O/C value (between 0.2 and 0.3). The presence of waxy material is probably due to incomplete breakdown of poly(ethylene-co-propylene) present in MSW under study. The optimal pyrolysis conditions, regarding to oil yield, fuel properties, and wax yield is fast pyrolysis at 510 °C with 24 g material/min input transfer rate. The fast pyrolysis gases contain mainly hydrocarbons and have an averaged LHV around 20 MJ/Nm³. ICP-AES analyses of pyrolysis products reveal that almost none of the metals present in MSW are distributed within the liquid fractions.     

Thermal decomposition products of copoly(arylene ether sulfone)s characterized by direct pyrolysis mass spectrometry

Pyrolysis products with mass of up to 850 Da were detected by direct pyrolysis mass spectrometric (DPMS) analysis of a series of copoly(arylene ether sulfone)s (PES-PPO) synthesized by nucleophilic condensation of either 4,4′-dichlorodiphenylsulfone (CDPS) or 4,4′-bis-(4-chlorophenyl sulfonyl) biphenyl (long chain dichloride, LCDC) with different molar ratios of hydroquinone (HQ) or dihydroxydiphenylsulfone (HDPS). Pyrolysis products retaining the repeating units of the initial copolymers were formed at temperatures ranging from 420 °C to 470 °C (near the initial decomposition temperature). At temperatures higher than 450 °C were observed products containing biphenyl units, formed by the elimination process of SO₂ from diphenyl sulfone bridges. Products having biphenyl and dibenzofuran moieties were detected in the mass spectra recorded at temperatures above 550 °C. These units were formed by loss of hydrogen atom from diphenyl ether bridges. Although the EI (18 eV) mass spectra of the pyrolysis products of the samples investigated were very similar, it was found that the relative intensity of some ions reflects the molar composition of the copolymers analysed. Cyclic and linear oligomers with very low molecular mass, present in the crude copolymers, were also detected by DPMS. Thermogravimetric analysis also showed their excellent thermal stability below 400 °C. It indicates that the copolymers yield a char residue of 40-45% at 800 °C, which increases with the PPO mole fraction in the samples.     

Pyrolysis of eicosane in supercritical water

Pyrolysis of eicosane and redox reactions of the pyrolysis products in supercritical water (SCW) were studied in a batch reactor at 30 MPa, in the temperature range from 450 to 750 °C and with reaction times ranging from 75 to 600 s. The rate constants for eicosane pyrolysis (k" = 10^16.5±0.5exp[–(32000±2000)/T] s^–1) and for the formation of H₂ (k = 10^25±0.8exp[–(64000±4000)/T] s^–1) were determined. The time and temperature dependences of the heat of reaction were elucidated. Water accelerates pyrolysis and participates in the subsequent transformations of the pyrolysis products. The yield of H₂ sharply increases for T > 700 °C.     

Thermal behaviour and degradation of a liquid crystalline alkylene-aromatic polyester: Poly(decamethylene-fumaroyl-bis-4-oxybenzoate)

The thermal behaviour and degradation of an alkylene-aromatic liquid crystalline polyester, poly(decamethylene-fumaroyl-bis-4-oxybenzoate), were studied by thermogravimetric analysis under dynamic conditions and by pyrolysis-gas chromatography and pyrolysis-gas chromatography/mass spectrometry in the temperature range 450-650 °C. Among the degradation products detected, maleic anhydride, phenol, 4-hydroxybenzoic acid and the corresponding decenyl ester were the most abundant. The type and the composition of the pyrolysis products gave useful information about the mechanism of thermal degradation. The polyester decomposition starts with a free-radical scission in the mesogenic fragment and continues with electrocyclic reactions in the spacer. The influence of fullerene C₆₀ addition and of the deuteration of the decamethylene spacer on thermal behaviour and degradation were investigated.     

Synthesis of multi-walled carbon nanotubes catalyzed by substituted ferrocenes

A range of substituted ferrocenes were used as catalysts for the synthesis of multi-walled carbon nanotubes (MWCNTs) and carbon fibers (CFs). These products were obtained in the temperature range 800-1000 °C, in a reducing atmosphere of 5% H₂ by pyrolysis of (CpR)(CpR′)Fe (R and R′ = H, Me, Et and COMe) in toluene solution. The effect of pyrolysis temperature (800-1000 °C), catalyst concentration (5 and 10 wt.% in toluene) and solution injection rate (0.2 and 0.8 ml/min) on the type and yield of carbonaceous product synthesized was investigated. Carbonaceous products formed include graphite film (mostly at high temperature; 900-1000 °C), carbon nanotubes and carbon fibers. The carbonaceous materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The ferrocene ring substituents influenced both the CNT diameter and the carbon product formed.     

Metastability of atomic ordering in lead-strontium nitrate solid solutions

A decrease in the anomalous birefringence of ostensibly cubic crystals of (Pb,Sr)(NO₃)₂ during annealing between 280-450 °C shows first-order reaction kinetics with Arrhenius-like temperature dependence. The activation energies associated with this process were 111(5) and 359(17) kJ/mol below 370 °C and above 400 °C, respectively. Such behavior agrees with theoretical predictions and confirms that the ordering of cations is the primary cause of the anomalous birefringence.     

Degradation of benzoic acid and its derivatives in subcritical water

In this research, the stability of benzoic acid and three of its derivatives (anthranilic acid, salicylic acid, and syringic acid) under subcritical water conditions was investigated. The stability studies were carried out at temperatures ranging from 50 to 350 °C with heating times of 10–630 min. The degradation of the benzoic acid derivatives increased with rising temperature and the acids became less stable with longer heating time. The three benzoic acid derivatives showed very mild degradation at 150 °C. Severe degradation of benzoic acid derivatives was observed at 200 °C while their complete degradation occurred at 250 °C. However, benzoic acid remained stable at temperatures up to 300 °C. The degradation products of benzoic acid and the three derivatives were identified and quantified by HPLC and confirmed by GC/MS. Anthranilic acid, salicylic acid, syringic acid, and benzoic acid in high-temperature water underwent decarboxylation to form aniline, phenol, syringol, and benzene, respectively.     

Kinetics of reduction of iron oxides by H2: Part II. Low temperature reduction of magnetite

This study deals with the reduction of Fe₃O₄ by H₂ in the temperature range of 210-950 °C. Two samples of Fe₃O₄ produced at 600 and 1200 °C, designated as Fe₃O₄₍₆₀₀₎ and Fe₃O_4(1200), have been used as starting material.Reduction of Fe₃O₄₍₆₀₀₎ by H₂ is characterized by an apparent activation energy ‘E_a’ of 200, 71 and 44 kJ/mol at T < 250 °C, 250 °C < T < 390 °C and T > 390 °C, respectively. The important change of E_a at 250 °C could be attributed to the removal of hydroxyl group and/or point defects of magnetite. This is confirmed during the reduction of Fe₃O_4(1200). While transition at T ≈ 390 °C is probably due to sintering of the reaction products as revealed by SEM.In situ X-rays diffraction reduction experiments confirm the formation of stoichiometric FeO between 390 and 570 °C. At higher temperatures, non-stoichiometric wüstite is the intermediate product of the reduction of Fe₃O₄ to Fe.The physical and chemical modifications of the reduction products at about 400 °C, had been confirmed by the reduction of Fe₃O₄₍₆₀₀₎ by CO and that of Fe₃O_4(1200) by H₂. A minimum reaction rate had been observed during the reduction of Fe₃O_4(1200) at about 760 °C. Mathematical modeling of experimental data suggests that the reaction rate is controlled by diffusion and SEM observations confirm the sintering of the reaction products.Finally, one may underline that the rate of reduction of Fe₃O₄ with H₂ is systematically higher than that obtained by CO in the explored temperature range.     

Thermal decomposition course of Eu(CH3COO)3·4H2O and the reactivity at the gas/solid interface thus established

The thermal decomposition course of europium acetate tetrahydrate (Eu(CH₃COO)₃·4H₂O) was probed on heating up to 1000 °C in a dynamic atmosphere of air by thermogravimetry and differential thermal analysis. The solid- and gas-phase decomposition products were identified by X-ray diffractometry, ex- and in situ infrared spectroscopy and mass spectrometry. Results obtained showed the acetate to dehydrate stepwise at 145-283 °C, and then decompose stepwise to yield eventually cubic-Eu₂O₃ at ≥663 °C encompassing the formation of intermediate oxycarbonate (Eu₂O(CO₃)₂/Eu₂O₂(CO₃) solid products (at 347-466 °C)) and H₂O, (CH₃)₂CO and CO₂ as primary gaseous products. A considerable enhancement of the production of the primary gas phase products at 400-450 °C and the emergence of (CH₃)₂CCH₂, CH₄ and CO molecules in the gas phase are ascribed to reactions occurring at the gas/solid interface at the expense of some of the primary products. These interfacial activities impart application-worthy adsorptive and catalytic functions for the associated solid products.     

Solid/liquid- and vapor-phase interactions between cellulose- and lignin-derived pyrolysis products

Solid/liquid- and vapor-phase interactions between cellulose- and lignin (Japanese cedar milled wood lignin)-derived pyrolysis products were studied under the conditions of N₂/600 °C/40–80 s. A dual-space closed ampoule reactor was used to eliminate the solid/liquid-phase interactions, and careful comparison of the resulting data with those of the pyrolysis of the mixed samples gave some insights into the solid/liquid- and vapor-phase interactions separately. With the solid/liquid-phase interactions, the tar yields from both cellulose and lignin increased with the decreasing yields of the char fractions in a short pyrolysis time of 40 s (primary pyrolysis stage). Most of the identified tar components from cellulose and lignin increased in their yields. The vapor-phase interactions were significant at a longer pyrolysis time of 80 s (secondary reaction stage) when the methoxyl groups of the lignin-derived volatiles were cleaved homolytically. The vapor-phase interactions accelerated the gas formation from the cellulose-derived volatiles with suppressing the vapor-phase char formation of the lignin-derived volatiles. The yields of methane and catechols from lignin also increased greatly instead of the formation of o-cresols. Most of these influences are explained with a proposed interaction mechanism, in which the cellulose-derived volatiles act as H-donors while the lignin-derived volatiles (radicals) act as H-acceptors.     

Bio-oil production from Onopordum acanthium L. by slow pyrolysis

In this study, the usability of the plant thistle, Onopordum acanthium L., belonging to the family Asteraceae (Compositae), in liquid fuel production has been investigated. The experiments were performed in a fixed-bed Heinze pyrolysis reactor to investigate the effects of heating rate, pyrolysis temperature and sepiolite percentage on the pyrolysis product yields and chemical compositions. Experiments were carried out in a static atmosphere with a heating rate of 7 °C/min and 40 °C/min, pyrolysis temperature of 350, 400, 500, 550 and 700 °C and particle size of 0.6 < D_p < 0.85 mm. Catalyst experiments were conducted in a static atmosphere with a heating rate of 40 °C/min, pyrolysis temperature of 550 °C and particle size of 0.6 < D_p < 0.85 mm. Bio-oil yield increased from 18.5% to 27.3% with the presence of 10% of sepiolite catalyst at pyrolysis temperature of 550 °C, with a heating rate of 40 °C/min, and particle size of 0.6 < D_p < 0.85 mm. It means that the yield of bio-oil was increased at around 48.0% after the catalyst added. Chromatographic and spectroscopic studies on the bio-oil showed that the oil obtained from O. acanthium L. could be used as a renewable fuels and chemical feedstock.     

Synthesis, stoichiometry and thermal stability of Zn3N2 powders prepared by ammonolysis reactions

Zn₃N₂ powders were prepared by ammonolysis reactions at 600 °C and examined by thermogravimetric analysis, powder X-ray and neutron diffraction. The powders obtained in this way are unstable in an oxygen atmosphere above 450 °C. In an argon atmosphere, the powders are stable up to their decomposition point at around 700 °C. Structural models obtained from Rietveld refinements against the powder neutron diffraction data indicate that the Zn₃N₂ powders so-prepared have the anti-bixbyite structure and are almost certainly stoichiometric with no compelling evidence of nitrogen vacancies. Further, no evidence was found for aliovalent oxygen substitution at the nitrogen sites. The calculated bond valence sums imply that Zn₃N₂ cannot be described as a 100% ionic compound. The structural findings are supported by photoluminescence measurements that reveal a band gap of approximately 0.9 eV.     

Large-scale synthesis, characterization and microwave absorption properties of carbon nanotubes of different helicities

Carbon nanotubes of high helicity (H-HCNTs, Sample A) have been synthesized in large-scale by pyrolysis of acetylene at 450 °C over Fe nanoparticles derived from coprecipitation/hydrogen reduction method. With controlled introduction of hydrogen during acetylene pyrolysis, CNTs of low helicity (L-HCNTs, Sample B) and worm-like CNTs (Sample C) were obtained in large quantities. The yields of the CNTs products are high, especially that of H-HCNTs (ca. 7474%). The complex permittivity and permeability of Composites A, B, and C that contain Samples A, B and C (30 wt%) were measured in the 2-18 GHz frequency range. Good absorption of electromagnetic wave (reflection loss<−20 dB) was observed in the 7.18-10.68 and 7.5-10.7 GHz range over Composites B and C (2.0-3.0 mm thickness), respectively. Thus, through the suggested route, CNTs can be produced easily and selectively in large quantities. The lightweight materials can be utilized for microwave absorption.     

A thermal degradation mechanism of polyvinyl alcohol/silica nanocomposites

The thermal degradation mechanism of a novel polyvinyl alcohol/silica (PVA/SiO₂) nanocomposite prepared with self-assembly and solution-compounding techniques is presented. Due to the presence of SiO₂ nanoparticles, the thermal degradation of the nanocomposite, compared to that of pure PVA, occurs at higher temperatures, requires more reaction activation energy (E), and possesses higher reaction order (n). The PVA/SiO₂ nanocomposite, similar to the pure PVA, thermally degrades as a two-step-degradation in the temperature ranges of 300-450 °C and 450-550 °C, respectively. However, the introduction of SiO₂ nanoparticles leads to a remarkable change in the degradation mechanism. The degradation products identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA) and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) suggests that the first degradation step of the nanocomposite mainly involves the elimination reactions of H₂O and residual acetate groups as well as quite a few chain-scission reactions. The second degradation step is dominated by chain-scission reactions and cyclization reactions, and continual elimination of residual acetate groups is also found in this step.     

Thermal degradation behaviour of aromatic poly(ester-amide) with pendant phosphorus groups investigated by pyrolysis-GC/MS

The thermal stability of a novel phosphorus-containing aromatic poly(ester-amide) ODOP-PEA was investigated by thermogravimetric analysis (TGA). The weight of ODOP-PEA fell slightly at the temperature range of 300-400 °C in the TGA analysis, and the major weight loss occurred at 500 °C. The structural identification of the volatile products resulted from the ODOP-PEA pyrolysis at different temperatures was performed by pyrolysis-gas chromatography/mass spectrometry (pyrolysis-GC/MS). The P-C bond linked between the pendant DOPO group and the polymer chain disconnected first at approximately 275 °C, indicating that it is the weakest bond in the ODOP-PEA. The P-O bond in the pendant DOPO group was stable up to 300 °C. The cleavage of the ester linkage within the polymer main chain initiated at 400 °C, and the amide bond scission occurred at greater than 400 °C. The structures of the decomposition products were used to propose the degradation processes happening during the pyrolysis of the polymer.