Haoxi Ben

Lignin Pyrolysis Components and Upgrading—Technology Review

Biomass pyrolysis oil has been reported as a potential renewable biofuel precursor. Although several review articles focusing on lignocellulose pyrolysis can be found, the one that particularly focus on lignin pyrolysis is still not available in literature. Lignin is the second most abundant biomass component and the primary renewable aromatic resource in nature. The pyrolysis chemistry and mechanism of lignin are significantly different from pyrolysis of cellulose or entire biomass. Therefore, different from other review articles in the field, this review particularly focuses on the recent developments in lignin pyrolysis chemistry, mechanism, catalysts, and the upgrading of the bio-oil from lignin pyrolysis. Although bio-oil production from pyrolysis of biomass has been proven on commercial scale and is a very promising option for production of renewable chemicals and fuels, there are still several drawbacks that have not been solved. The components of biomass pyrolysis oils are very complicated and related to the properties of bio-oil. In this review article, the details about pyrolysis oil components particularly those from lignin pyrolysis processes will be discussed first. Due to the poor physical and chemical property, the lignin pyrolysis oil has to be upgraded before usage. The most common method of upgrading bio-oil is hydrotreating. Catalysts have been widely used in petroleum industry for pyrolysis bio-oil upgrading. In this review paper, the mechanism of the hydrodeoxygenation reaction between the model compounds and catalysts will be discussed and the effects of the reaction condition will be summarized.

Torrefaction of Loblolly pine

The torrefaction of Loblolly pine (Pinus taeda) was examined at 250 and 300 °C, to determine the effects of treatment temperatures on the chemical structure of the torrefied Loblolly pine. Solid-state cross-polarization/magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy was used to characterize the torrefied and native Loblolly pine. The NMR results indicate that aryl-ether bonds in lignin were cleaved during the torrefaction. The methyl carbons in hemicellulose acetyl groups were no longer present after the torrefaction at 250 °C for 4 h, which is consistent with HPLC carbohydrate analysis of the torrefied wood which indicated that the hemicellulose fraction of pine was completely absent, whereas the cellulose and lignin remained largely intact. Under these conditions the torrefied wood has a relatively high energy yield of 81.29% and a HHV of 24.06 MJ kg−1. After torrefaction at 300 °C for 4 h, the cellulose and hemicellulose in the wood were completely eliminated, the residue contains enriched amounts of carbonyl groups, aromatic carbons and methoxyl groups, which represent complex condensed aromatics, these aromatics units were linked with aliphatic C–O and C–C bonds and the product has a very high HHV of 32.34 MJ kg−1.

Comparison for the compositions of fast and slow pyrolysis oils by NMR characterization

The pyrolysis of softwood (SW) kraft lignin and pine wood in different pyrolysis systems were examined at 400, 500 and 600 °C. NMR including quantitative (13)C and Heteronuclear Single-Quantum Correlation (HSQC)-NMR, and Gel Permeation Chromatography (GPC) were used to characterize various pyrolysis oils. The content of methoxyl groups decreased by 76% for pine wood and 70% for lignin when using fast pyrolysis system. The carbonyl groups also decreased by 76% and nearly completely eliminated in 600 °C pine wood fast pyrolysis oil. Compared to the slow pyrolysis process, fast pyrolysis process was found to improve the cleavage of methoxyl groups, aliphatic CC bonds and carbonyl groups and produce more polyaromatic hydrocarbons (PAH) from lignin and aliphatic CO bonds from carbohydrates. Another remarkable difference between fast and slow pyrolysis oils was the molecular weight of fast pyrolysis oils increased by 85-112% for pine wood and 104-112% for lignin.

One step thermal conversion of lignin to the gasoline range liquid products by using zeolites as additives

One step thermal conversion of lignin to gasoline range liquid products was accomplished by pyrolyzing softwood (SW) kraft lignin with select zeolites at 600 °C. Gel Permeation Chromatography (GPC) and Nuclear Magnetic Resonance (NMR) including quantitative 13C, 31P-NMR and Heteronuclear Single-Quantum Correlation (HSQC)-NMR were used to characterize various pyrolysis oils. By employing a zeolite catalyst aliphatic hydroxyl groups decreased by 70–100% in the resulting bio-oil and the content of carboxylic acid groups also decreased by 44–85% in comparison to a control pyrolysis oil generated with no additive. Of the additives studied MFI and MOR zeolites provided the best in situ decarboxylation of bio-oils. The results of 13C-NMR indicated after the use of FAU and BEA zeolites, the pyrolysis oils contained ∼80% less methoxy groups than the native pyrolysis oil, and almost all the oxygen (up to ∼87%) belonged to phenolic hydroxyl groups. In addition, the average molecular weight of these two upgraded pyrolysis oils decreased by ∼60% with respect to the control pyrolysis oil and they had a molecular weight profile in the gasoline range (80–120 g mol−1). By adding MFI, FAU and BEA zeolites, the pyrolysis oils contained some polyaromatic hydrocarbons (PAH). In contrast, there were very limited amount of PAH in FER and MOR upgraded pyrolysis oils and almost no PAH in the native pyrolysis oil.

In Situ NMR Characterization of Pyrolysis Oil during Accelerated Aging

COMING OF AGE: A method for investigating the accelerated aging of biomass pyrolysis oils is reported. The in situ NMR investigation, done by using quantitative ¹H, ¹³C NMR and heteronuclear single-quantum correlation (HSQC)-NMR techniques, reveals the chemical structural changes of pyrolysis oil during the aging process, providing insight into the mechanism of aging process.

Noble metal catalyzed aqueous phase hydrogenation and hydrodeoxygenation of lignin-derived pyrolysis oil and related model compounds

Aqueous phase hydrodeoxygenation of lignin pyrolysis oil and related model compounds were investigated using four noble metals supported on activated carbon. The hydrodeoxygenation of guaiacol has three major reaction pathways and the demethylation reaction, mainly catalyzed by Pd, Pt and Rh, produces catechol as the products. The presence of catechol and guaiacol in the reaction is responsible for the coke formation and the catalysts deactivation. As expected, there was a significant decrease in the specific surface area of Pd, Pt and Rh catalysts during the catalytic reaction because of the coke deposition. In contrast, no catechol was produced from guaiacol when Ru was used so a completely hydrogenation was accomplished. The lignin pyrolysis oil upgrading with Pt and Ru catalysts further validated the reaction mechanism deduced from model compounds. Fully hydrogenated bio-oil was produced with Ru catalyst.

Chemical characterization and water content determination of bio-oils obtained from various biomass species using 31P NMR spectroscopy

Background: Pyrolysis is a promising approach to utilize biomass for biofuels. One of the key challenges for this conversion is how to analyze complicated components in the pyrolysis oils. Water contents of pyrolysis oils are normally analyzed by Karl Fischer titration. The use of 2-chloro-4,4,5,5,-tetramethyl-1,3,2-dioxaphospholane followed by 31P NMR analysis has been used to quantitatively analyze the structure of hydroxyl groups in lignin and whole biomass. Results:31P NMR analysis of pyrolysis oils is a novel technique to simultaneously characterize components and analyze water contents in pyrolysis oils produced from various biomasses. The water contents of various pyrolysis oils range from 16 to 40 wt%. The pyrolysis oils obtained from Loblolly pine had higher guaiacyl content, while that from oak had a higher syringyl content. Conclusion: The comparison with Karl Fischer titration shows that 31P NMR could also reliably be used to measure the water content of pyrolysis oils. Simultaneously with analysis of water content, quantitative characterization of hydroxyl groups, including aliphatic, C-5 substituted/syringyl, guaiacyl, p-hydroxyl phenyl and carboxylic hydroxyl groups, could also be provided by 31P NMR analysis.

In situ upgrading of whole biomass to biofuel precursors with low average molecular weight and acidity by the use of zeolite mixture

The pyrolysis of whole biomass—pine wood and bark—with mordenite (M), beta (β) and Y zeolites has been examined at 600 °C. The GPC results indicated that the pyrolysis oils upgraded by Y and β zeolites have a very low average molecular weight range (70–170 g mol−1). Several NMR methods have been employed to characterize the whole portion of pyrolysis products. After the use of these two zeolites (Y and β), the two main products from the pyrolysis of cellulose—levoglucosan and HMF—were eliminated; this indicates a significant deoxygenation process. When a mixture of zeolites (Y and M) was used, the upgraded pyrolysis oil exhibited advantages provided by both zeolites; this pyrolysis oil represents a biofuel precursor that has a very low average molecular weight and a relatively low acidity. This study opens up a new way to upgrade pyrolysis oils by employing mixtures of different functional zeolites to produce biofuel/biochemical precursors from whole biomass.

19 F NMR spectroscopy for the quantitative analysis of carbonyl groups in bio-oils

The carbonyl groups in pyrolysis oil have been reported to be responsible for the two most challenging properties with regard to the usage of pyrolysis oil – corrosion and aging problems; indeed, the carbonyl groups also bring huge difficulties for any required upgrading process. Therefore, a comprehensive and quantitative understanding of the structural information on these carbonyl groups is a challenging but crucial topic. However, owing to the highly complex nature of pyrolysis oil, how to quantitatively determine carbonyl groups appears to be very important. To the best of our knowledge, this is the first study that has used the 4-(trifluoromethyl)phenylhydrazine derivatization 19F NMR spectroscopy method for the quantitative analysis of carbonyl groups in various bio-oils. Different pyrolysis oils produced from various sources were analyzed after treatment with 4-(trifluoromethyl)phenylhydrazine followed by 19F NMR spectroscopy, and semiquantitative FT-IR spectroscopy, and were also quantitatively determined by the wet chemistry oximation method. The results indicated that the 19F NMR method can be regarded as more efficient (24 h vs. 48 h; single step vs. multiple steps) while being as reliable as the traditional oximation method.

The use of combination of zeolites to pursue integrated refined pyrolysis oil from kraft lignin

A mixture of Y and M type zeolites were applied to pyrolyze kraft softwood (SW) lignin with the objective of studying the combination effect of different types of zeolite on pyrolysis. The chemical structures of the subsequent pyrolysis oils were examined. Nuclear Magnetic Resonance (NMR) spectroscopy including 13C, 31P of phosphitylated bio-oils, Heteronuclear Single-Quantum Correlation (HSQC)-NMR, and Gel Permeation Chromatography (GPC) were used to characterize the pyrolysis oils. The yields of pyrolysis products (light oil, heavy oil and char) from the zeolites combination ‘Y + M’ catalyzed pyrolysis ranged between the pyrolysis oil yields from zeolite Y or M catalyzed pyrolysis. 31P NMR analysis of the phosphitylated bio-oils revealed that the mixture of ‘Y + M’ during pyrolysis could decrease the carboxyl groups by 84%, which is close to the effect of the M zeolite. The yields of hydroxyl groups and other functional groups in the ‘Y + M’ generated bio-oil was between the individual Y and M generated oils. The molecular weight of the pyrolysis oil using a zeolite mixture of ‘Y + M’ was similar to the individual zeolite Y assisted pyrolysis. These results show that the zeolite mixture of ‘Y + M’ manifests additive characteristics for pyrolysis.