and Fuels Changzhi Li,† Xiaochen Zhao,† Aiqin Wang,† George W. Huber,†,‡ and Tao Zhang*,† †State Key Laborotary of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States

Catalytic Transformation of Lignin for the Production of Chemicals and Fuels

The growing demand for transportation fuels, along with concerns about the harmful effects of greenhouse gas emissions from the burning of fossil fuels, has assured a viable future for the development of alternative fuels from renewable resources, such as lignocellulosic biomass. The effi cient utilization of these biomass resources is critically dependant on the in-depth knowledge of their chemical constituents. This, together with the desired fuel properties, helps tailor the chemical and/or enzymatic processes involved in converting biomass to biofuels. Hybrid poplars are among the fastest growing temperate trees in the world and a very promising feedstock for biofuels and other value-added products. Sequencing of the poplar genome has paved the way for tailoring new cultivars and clones optimized for biofuels production. Our objective is to review published research on the composition of the key chemical constituents of hybrid poplar species used for biofuels. Biomass yields, elemental composition, carbohy- drate and lignin content and composition are some of the characteristics reviewed, with emphasis on lignin struc- ture. Genetic modifi cations used to alter lignin content and composition, with the aim of improving biofuels yields, are also examined. © 2010 Society of Chemical Industry and John Wiley & Sons, Ltd

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Poplar as a feedstock for biofuels: A review of compositional characteristics

The last decade has seen tremendous growth and interest in renewable energy and fuels aimed primarily at addressing issues of climate change, energy security, and rising energy costs. These efforts coupled with the demand for efficient utilization of biomass place a premium on the detailed analysis of the fundamental chemical structures of biomass, especially in light of the ever-increasing efforts to generate transgenic plants with reduced recalcitrance and altered lignin structure. This review examines the growing application of phosphitylation followed by 31P NMR to quantitatively analyze biomass lignin structures including guaiacyl, syringyl, guaiacyl with carbon substituents at the C5 position, catechol, p–hydroxyphenyl, aliphatic and carboxylic hydroxyl groups. The application of this methodology to provide a rapid analytical tool for lignin/biomass derived bio-oils and biodiesel precursors is also discussed. Utilizing lignin isolated from native and transgenic plants as well as from pretreatment and biological/thermal deconstruction processes, researchers have demonstrated that this technique has unique characterization capabilities which have broad applicability in the biofuels research community.

Application of quantitative 31P NMR in biomass lignin and biofuel precursors characterization

The recently developed protocol for isolating enzymatic mild acidolysis lignins (EMAL) coupled with the novel combination of derivatization followed by reductive cleavage (DFRC) and quantitative 31P NMR spectroscopy were used to better understand the lignin isolation process from wood. The EMAL protocol is shown to offer access at lignin samples that are more representative of the overall lignin present in milled wood. The combination of DFRC/31P NMR provided a detailed picture on the effects of the isolation conditions on the lignin structure. More specifically, we have used vibratory and ball milling as the two methods of wood pulverization and have compared their effects on the lignin structures and molecular weights. Vibratory-milling conditions cause substantial lignin depolymerization. Lignin depolymerization occurs via the cleavage of uncondensed β-aryl ether linkages, while condensed β-aryl ethers and dibenzodioxocins were found to be resistant to such mechanical action. Condensation and side chai...

Toward a better understanding of the lignin isolation process from wood.

Milled wood lignin (MWL), cellulolytic enzyme lignin (CEL), and enzymatic mild acidolysis lignin (EMAL) were isolated from different wood species and characterized by various techniques. The EMAL protocol offered gravimetric lignin yields 2-5 times greater than those of the corresponding MWL and CEL. The purities of the EMALs were 3.75-10.6% higher than those of their corresponding CELs, depending upon the wood species from which they were isolated. Molecular weight analyses showed that the EMAL protocol isolates lignin fractions that are not accessed by the other procedures evaluated, while 31P NMR spectroscopy revealed that MWL is more condensed and bears more phenolic hydroxyl groups than EMAL and CEL. The yields and purities of EMAL, MWL, and CEL from hardwood were greater than those obtained for the examined softwoods. Structural details obtained by DFRC (derivatization followed by reductive cleavage)/31P NMR revealed different contents of condensed and uncondensed beta-O-aryl ether structures, dibenzodioxocins, and condensed and uncondensed phenolic hydroxyl and carboxylic acid groups within lignins isolated from different wood species.

Comparative evaluation of three lignin isolation protocols for various wood species.

Softwood kraft pulp was subjected to a laboratory one- and three-stage oxygen delignification process. Pulp and liquor samples were collected at different stages of the process with particular attention being paid to the early and late stages. A novel residual lignin isolation method extracted about 65% of the oxidized residual lignins at a purity exceeding 90%. Using this methodology coupled to quantitative 31 P NMR, 13 C NMR, 2D heteronuclear (HMQC) NMR spectroscopic and analytical pyrolysis techniques allowed a thorough characterization of the residual and solubilized lignin fractions at the various stages of the process. Our conclusions do not point to a single factor as being responsi - ble for the limits confronting oxygen delignification. Amongst the major factors impeding the effectiveness of oxygen delignification are: (i) the accumulation of relatively inert 5,5 ' biphenyl structures originally present in kraft lignin from dibenzodioxocin ring opening reactions; and (ii) the accumulation of considerably less reactive p-hydroxyphenyl struc- tures. Detailed quantitative information was obtained and discussed in relation to the presence and role of these unreactive phenolic fractions on the residual oxidized lignins. As such we arrived at important conclusions as to why and how these structures remain and accumulate on the fiber. In addition, issues related to the profiles of the total phenolic hydroxyl content of the residual lignins and the remaining arylglycerol β-O-4 structures are discussed.

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Factors limiting oxygen delignification of kraft pulp

Milled wood lignins isolated from genetically modified poplar trees were studied by quantitative 31 P NMR spectroscopy, in combination with thioacidolysis and mild alkaline hydrolysis. The genetic transformation of the trees included down regulation of CAD and COMT enzyme activities. These analyses confirmed that moderate CAD down-regulation does not substantially alter lignin structure. In contrast, severe CAD deficiency alters the lignin structure more profoundly by decreasing the syringyl/guaiacyl ratio and increasing the degree of condensation of the lignin. The most severe alterations were observed in the lignins from the COMT-transformed lines.

Quantitative 31P NMR Spectroscopy of Lignins from Transgenic Poplars

In an effort to augment the efficiency of an oxygen delignification stage, we examined the effectiveness of various heterocyclic aromatic nitrogen compounds as possible activators of such a stage. It was shown that a variety of such compounds may act as activators in the presence of hydrogen peroxide, improving the delignification efficiency of oxygen delignification. Among the group of activators examined, 1,10-phenanthroline was found to be the most potent. Injecting the activator in the middle of the delignification stage (simulating a two-stage process) was shown to provide most of the benefits. The optimum injection time was found to at 30 min from the onset of the delignification. At the optimal charge of 1,10-phenanthroline (0.3%), the delignification was improved by 18%. While the presence of copper ions was found to further improve the efficiency of the activating system, this was accompanied by an extensive carbohydrate degradation.

Nitrogen-Centered Activators of Peroxide-Reinforced Oxygen Delignification

Surface morphologies of softwood and hardwood lignin-carbohydrate complexes. and of humic and fulvic acids (the products of biochemical degradation of plant matter in soil) were examined under identical conditions using atomic force microscopy coupled with transformation of digital images into virtual reality. The structural organization of the observed surfaces did not always coincide with that found previously using electron microscopy, suggesting that sample preparation played a significant role in the formation of aggregates. Morphological differences were observed in the samples that belonged to the same class of materials and to different classes. The morphologies of lignin-carbohydrate complexes and humic and fulvic acids followed a similar pattern.,however, suggesting similarities in the chemical structure of both classes of compounds.

Leonid G. Akim

Papers

Factors limiting oxygen delignification of kraft pulp

Quantitative 31P NMR Spectroscopy of Lignins from Transgenic Poplars

Nitrogen-Centered Activators of Peroxide-Reinforced Oxygen Delignification

Comparing Surface Morphology of Lignin-Carbohydrate Complex and Humic Substances: AFM/VR Approach