Syringaldehyde differs from vanillin by a second methoxyl group at C5 position of aromatic ring as depicted in Fig. 12.6. The oxidation of softwood lignins produces exclusively vanillin whereas the oxidation of hardwood lignins leads to syring — aldehyde plus vanillin, in a proportion that depends of the original syringyl:gua — iacyl ratio in the wood.

Syringaldehyde is a valuable starting chemical for the pharmaceutical industry. For example, as vanillin, this compound is the precursor of 3,4,5-trimethoxy — benzaldehyde, which is a building block of the antibacterial agents ormetoprim and trimethoprim, with the advantage of containing already two methoxyl groups [103-105].

In the past, between 1930s and 1950s, the separation technologies to recover vanillin and syringaldehyde produced by lignin oxidation were not readily accomplished [106]. Syringaldehyde has been produced by different chemical routes and starting materials as gallic acid, vanillin, [106] and pyrogallol [107]. New alternatives for synthesis of syringaldehyde are being investigated in order to find environmental friendly and efficient processes to obtain higher yields [108]. However, many of these synthetic pathways involve complex procedures and/or include expensive materials becoming not economically feasible a large scale production.

By end of 1970s, the production process of syringaldehyde by oxidation of hardwood spent liquor was reported [109] including a step of fractional distil­lation for the separation of the two aldehydes. The oxidation of hardwood lignin to produce a syringaldehyde-rich mixture seems to be very attractive. In fact, the production of this phenolic compound from lignin, in alkaline medium with O2, has been emerging as research topic [20, 110-113]. However, the sustainability of this process must be assured by the yield of products and economical advantageous purification processes. Considering its potential applications, it is expected an increased demand for this chemical already cited in Top Value — Added Chemicals from Biomass [38]. As an example, syringaldehyde was recently considered as a promising building block to dendrimers design with high antioxidant potential: the antioxidant activity of syringaldehyde-based dendrimer showed to be two and ten times higher than that one of quercetin and trolox, respectively [114].

Several species of fungi like Aspergillus niger, Penicillium simplicissimum are used for bioleaching. This form of leaching does not rely on microbial oxidation of metal, but rather uses microbial metabolism as source of acids which directly dissolve the metal.

Microfungi are heterotrophic organisms. They exist in all ecological niches, e. g. supporting the weathering of rocks as well as the mineralization of materials containing metals. Their development is encouraged by the acidic reaction, the presence of sugars, and the appropriate humidity. These microorganisms can produce large amounts of organic acids, such as citric, glycolic, oxalic, and other acids which work as chemical solvents, can be used on an industrial scale in bioleaching processes and impact the change of the environment’s reaction. The microfungi, due to their biochemistry and relatively high immunity to hostile factors (pH, temperature, etc.), provide an excellent alternative in the bioleaching of metals, since the classical chemical methods of acidic bioleaching cannot be used for environmental reasons. The extraction through microfungi consists mainly of producing metabolites like organic acids, amino acids, and peptides that serve as leaching agents for the dissolution of metals [11].

The metabolic process of fungi is similar to a great extent to those of higher plants with the exception of carbohydrate synthesis. The glycolytic pathway converts the glucose into variety of products including organic acids. So, these biomining processes are mediated due to the chemical attack by the extracted organic acids on the ores. The acids usually have dual effect of increasing metal dissolution by lowering the pH and increasing the load of soluble metals by complexion/chelating into soluble organic-metallic complexes [12].

Using lactose hydrolyzing yeast under anaerobic conditions, whey can be con­verted into alcohol [137]. The system though made primarily for SCP production from whey, can also be employed for production of alcohol. Prehydrolyzed whey with b-galactosidase enzyme in which most of the lactose is hydrolyzed has been used as a substrate for alcohol production. Since the alcohol produced is taxed in a similar manner as the potable alcohol for use in the beverage industry, this proposition also becomes expensive [117]. Such alcohol for use as industrial alcohol or alcohol as a chemical should be taxed at different rates than used for potable beverage production.

9.7.11 Cassava Roots

Cassava roots are used as raw materials for the production of ethanol in some countries like Brazil. The alcohol produced from cassava roots was used as motor fuel, mixed with gasoline (upto 20% alcohol) for which no motor modification is required. It is also used as pure anhydrous ethanol, in which there is need to modify the carburetor and some other parts. Both result in less atmospheric pol­lution than the use of 100% gasoline. Commercial production of ethanol from cassava is obviously not new in some parts of Asia like India and China. In China, several factories are now using solid waste (bagasse) of the cassava starch industry for the production of ethanol [59].

The suitability of extractive fermentation as a technique for improving the production of ethanol from lactose by Candida pseudotropicalis over the con­ventional technique has also been examined [81, 82]. Using Adol 85 NF, extractive solvent, biocompatible with microorganisms, extractive fed-batch and conventional fed-batch systems were operated for 160 h and the extractive system showed a 60% improvement in lactose consumption and ethanol production with 75% volumetric productivity.

In the syngas platform, the biomass is subjected through a process called gasification. In this process, the biomass is heated with no oxygen or only about one-third the oxygen normally required for complete combustion. It subsequently converts into a gaseous product, which contains mostly carbon monoxide and hydrogen. The gas, which is called synthesis gas or syngas, can be fermented by specific microorganisms or converted catalytically into ethanol. In the sugar platform, only the carbohydrate fractions are utilized for ethanol production, whereas in the syngas platform, all the three components of the biomass are converted into ethanol [41].

The extracted lignin with ethanol fractionation is rich in phenolic aromatic rings, suggesting that it is a potential feedstock for preparing phenolic resins in the replacement of phenol presenting an environmental and economical process [66]. The synthesis of lignin-formaldehyde resins involves primarily a hydroxymethy — lation step. Lignin extracted from sugarcane bagasse had a large amount of active centers toward formaldehyde as compared to that from wood due to its higher proportion of H unit which was easily attacked by electrophilic groups [67].

Lignin extracted from white pine with ethanol/water fractionation was used to synthesize phenol-formaldehyde resol resins [68]. Under the optimal conditions, i. e., ethanol concentration 50%, reaction temperature 180°C, reaction time 4 h, lignin was extracted with a yield of 26% and a purity of around 83%. The obtained lignin showed a wide molecular weight distribution: Mw 1150, Mn 537 and polydispersity 2.14. The lignin fraction was used to replace phenol for the synthesis of bio-based phenol-formaldehyde resol resins. By substitution of phe­nol with the pine lignins at various ratios ranging from 25 to 75%, a series of dark — brown viscous resol-type phenolic resins were prepared. The solid concentrations and viscosities of these bio-based resins could be adjusted readily by controlling their water contents. The obtained lignin-phenol-formaldehyde resols solidified upon heating with main exothermic peaks at 150-175°C, and secondary peaks at 135-145°C, depending on the lignin content in the resin formula. When the phenol substitution ratio was lower than 50%, the thermal cure of lignin-phenol-formaldehyde resols proceeded at lower temperatures than that of the corresponding phenol — formaldehyde resol. The introduction of lignin in the resin formula decreased the thermal stability, leading to a lowered decomposition temperature and a reduced amount of carbon residue at elevated temperatures. However, the thermal stability was improved by purifying the lignin feedstock (to remove aliphatic sugars and increase aromatic structures) before the resin synthesis.

The ethanol lignin extracted from bagasse was subjected to purification including cyclohexane/ethanol extraction and acid precipitation. Then the lignin fraction was further hydroxymethylated and used to prepare lignin-phenol-formaldehyde resins [69]. With increased lignin content from 10 to 40%, the Tg of the resins increased from 120 to 150°C, and the rate of cure and the heat of reaction also increased. The negative surface charges resulting from the interaction between the substrate and the lignin-PF resins can reduce the contact angle; therefore, the film prepared from lignin-PF resins was good water-barrier coatings and used as cardboard substrates.

Sugarcane lignin released from Dehini rapid hydrolysis (using ethanol cata­lyzed with diluted sulfuric acid) was used to prepare lignin-formaldehyde resins and lignocellulosic fiber-reinforced composites [70]. The presence of lignin in both fiber and matrix greatly improved the adhesion at the fiber-matrix interface. The increased affinity improved the load transference performance from the matrix to the fiber, leading to good impact strength of the bio-based composites.

Antioxidant is a potential application of lignin. Research on lignin model compounds indicates that ortho-disubstituted phenolic groups are essential for antioxidant activity [71, 72]. The radical scavenging ability of lignin is decided by the ability to form a phenoxyl radical (i. e., hydrogen atom abstraction) as well as the stability of the phenoxyl radical. In lignin, ortho substituents such as methoxyl groups can stabilize phenoxyl radicals by resonance as well as hindering them from propagation. Conjugated double bonds can provide additional stabilization of the phenoxyl radicals through extended delocalization. Lignin was extracted with ethanol/water from hybrid poplar under various conditions, and the yield of the extracted lignin and the antioxidant activity were evaluated [73]. In general, the lignin prepared at elevated temperature, extended reaction time, increased catalyst and diluted ethanol shows high antioxidation activity due to more phenolic hydroxyl groups, low molecular weight and narrow polydispersity of the lignin. Under the optimal conditions, i. e., 190°C, 70 min, 1.4% H2SO4 and 60% ethanol, lignin yield was achieved at 20.1% with a high radical scavenging index of 56.4. Ethanol/water lignin extracted from Miscanthus sinensis with specific molecular weight was separated by ultra-filtration, and its antioxidant capacity was investigated [74]. The data indicated that even though phenolic content was the major factor that determined the antioxidation activity, the molecular weight and purity of the lignin were also contributors. Compared to the crude lignin, the resulted ultra-filtrated lignin exhibited higher antioxidation capacity due to its narrow molecular weight distribution and lower carbohydrate contamination.

Ethanol lignin has potential to sorb metals due to the richness in metal-binding functional groups including carboxylic and phenolic groups [75]. Lignin extracted with ethanol/water catalyzed by dilute sulfuric acid was used as an adsorbent for removal of copper (II) from CuSO4 aqueous solution [76]. It was found that the maximum removal of Cu (II) ions was achieved to *41% by using the organosolv lignin in 10 min at 20°C when the initial concentration of CuSO4 was 3 x In addition, the absorbed lignin can be recovered using HCl in a contact time of 10 min. In a comparative study, the organosolv lignin and kraft lignin from both softwood and hardwood were used to sorb Cu and Cd [77]. The conditions covered a range of pH (2-6.5), ionic strength (0.0001-0.1 M) and initial metal concen­tration (1-25 mg Me (II)/l). The results indicated all sorbents exhibited a prefer­ence for Cu over Cd, and kraft lignins showed higher sorption capacity and faster uptake rate. However, the absorption capacities of the lignin-based sorbents were lower than those reported such as chitosan, green alga. Therefore, further modi­fication of the organosolv lignin is necessary to achieve a higher metal sorption capacity thus can be used commercially.

Ethanol lignin was used as filler in printing ink vehicles and paints [78]. The lignin extracted by Alcell process with a lower molecular weight (Mn 700, Mw 1,700) can significantly improve the properties of the viscous media used for offset inks and paints with respect to tack and misting reduction. The addition of the lignin resulted in a brown coloration in these liquids, but did not bring about fundamental modification of their other basic physical and chemical properties. Therefore, no negative effects were produced for their most applications.

Selective hydrogenolysis is one effective way that can decrease the degree of polymerization while increasing the H/C ratio and lowering the O/C ratio of lignin, thus can convert it from a low grade fuel into potential fuel precursors or other value-added chemicals [79]. In a typical reaction catalyzed with RuCl2(PPh3)3 [80], the solubility of ethanol lignin in DMSO increased from 59.1 to 96.4% with increase in temperature from 50 to 175°C. The hydrogenolysis mechanism was mainly selective cleavage of aryl-O-aryl and aryl-O-aliphatic linkages, which was demonstrated by 31P NMR spectroscopy.

Despite the technical details to be solved or improved, the emergent availability of lignin sources and the know-how developed in researcher centers creates an opportunity to evaluate the profitability of including an oxidation process in the chemical platform of a biorefinery. The aldehydes productivity of lignins with different characteristics (due to the specie, delignification process and further processing or special treatments) is an emergent topic of research. In the perspective of biomass-based industries and biorefinery sustainability, this is very welcome information. Improving the treatment methods to preserve the lignin fraction with higher productivity on aldehydes (for example, by membrane separation before the reaction step) could also provide important technological advances for the application of lignin. Vanillin and syringaldehyde are just the beginning of a long road running to the sustainable future of commodities from renewable sources.

Hydrogen production by dark fermentation is achieved by strictly anaerobic or facultative anaerobic bacteria under anaerobic conditions. Hydrogen is an important compound for the metabolism of many anaerobic and a few aerobic microorganisms. Oxidation of hydrogen can be used by many organisms to drive energy generation. When external electron acceptors are absent, some organisms dispose of excess electrons generated during metabolism by reducing protons to hydrogen. Hydrogenase is the key enzyme for both situations. Ni-Fe hydrogenases and [FeFe] hydrogenases are the two main types of hydrogenases. They are phylogenetically distinct and contain different active sites. With some exceptions NiFe hydrogenases are known to catalyze hydrogen oxidation and [FeFe] hydrogenases are active in proton reduction catalysis of hydrogen evolution depends upon the organism. For example for Clostridia types hydrogen evolution

is catalyzed by soluble [FeFe] hydrogenase and for Escherichia coli is catalyzed by membrane bound NiFe hydrogenase [6].

Although in principle a variety of organic compounds; carbohydrates, proteins and lipids can be used for hydrogen production by dark fermentation, in reality hydrogen can only be obtained in practical yields by fermentation of carbohy­drates. Amino acids, obtained by protein hydrolysis, are fermented by Strickland reactions where one amino acid serves as the electron acceptor for the oxidation of the second amino acid. These reactions generate energy for the microorganisms carrying them out, but they do not yield hydrogen. Lipids can be hydrolyzed to glycerol and long chain fatty acids which in turn can be further degraded to acetate and hydrogen by synthropic bacteria. However, these reactions only occur at extremely low partial pressures of hydrogen maintained by associated methano — genic or sulfate-reducing bacteria [6].

Thus, a practical process for producing hydrogen must rely on fermentation of sugars derived from carbohydrates. This process can be modeled by considering hydrogen production from glucose, a typical hexose derived from various residues and wastes. The degradation of glucose to acetate (Fig. 10.1) is generally taken into account to estimate the theoretical yields, or to describe the reaction steps. By using the conversion of glucose to produce hydrogen the reaction is (Eq. 10.1) [7]:

C6H12O6 + 4H2O! 2 CH3COO— + 2HCO— + 4H+ + 4H2 AG0 = -206.3kJ/mol ( ‘ )

While in principle 4 mol of hydrogen can also be produced from glucose in two steps (Eqs. 10.2, 10.3) [7]:

Acetate reaction:

C6H12O6 + 2H2O! 2 CH3COO— + 2HCOO — + 4H+ + 2H2 AG0 = —209.1kJ/mol

2HCOOH! 2CO2 + 2H2 AG0 =-6kJ/mol

most organisms converting formate to hydrogen, giving 2 H2, are not capable of making hydrogen from NADH and thus are restricted to 2 H2/glucose.

Butyrate can also be an end product in anaerobic fermentation (Eq. 10.4) [7]:

C6H12O6 + 2H2O! CH3CH2CH2COO— + 2HCO— + 3H+ + 2H2 AG0 = —254.8kJ/mol ( ‘ )

To metabolize glucose to pyruvate the Embden-Meyerhoff-Parnas (i. e. Gly­colysis) or the Entner-Doudoroff pathways can be used [8]:

C6H12O6 + 2NAD+ ! 2 CH3COCOO— + 4H+ + 2NADH AG0 = -112.1 kJ/mol

As seen in reaction (10.4) 1 mol glucose can produce 2 mol pyruvate and 2 mol NADH. The NADH produced during glucose metabolism can in principle be used to provide electrons to reduce H+ to H2, but this reaction is thermodynamically unfavorable and hence cannot go to completion at high hydrogen partial pressures. A low NADH concentration, brought about by its oxidation during the production of other products; ethanol, lactate, butyrate, etc. is assumed by many researchers to result in low hydrogen yields.

The anaerobic metabolism of pyruvate formed during the catabolism of various substrates is the main reaction of hydrogen production. Two enzyme systems can catalyze the breakdown of pyruvate reactions (Eqs. 10.6, 10.7) [3]:

Pyruvate formate lyase:

Pyruvate + CoA $ acetylCoA + formate AG0 = — 16.3kJ/mol (10.6)

This reaction is a typical example of an enteric type fermentation, the metab­olism of Enterobacter species and Escherchia coli. Hydrogen production can become an advantage for the bacterium when the pH drops due to active metab­olism which causes induction of the FHL and the conversion of formic acid to hydrogen to prevent further acidification [6]. Thus, 2 mol of hydrogen can be produced by one mole of glucose by facultative anaerobic microorganisms [9].

Pyruvate: ferredoxin oxido reductase (PFOR)

Pyruvate + CoA + Fdox $ acetylCoA + CO2 + Fdred AG0 = — 19.2kJ/mol

(10.7)

This reaction is an example of H2 production by strict anaerobes. Clostridia can convert pyruvate to acetyl-CoA and CO2 producing reduced ferredoxin in a reaction catalyzed by the enzyme pyruvate:ferrodoxin oxidoreductase with ferre — doxin as electron acceptor. This enzyme can be found in many strictly anaerobic bacteria as well as facultative bacteria and some cyanobacteria. The acetyl-CoA which is produced can be metabolized to produce acetate and butyrate. Reoxi­dation of ferredoxin results in the formation of hydrogen by hydrogenase (One H2 per pyruvate). If acetate is the final product, one additional mole of hydrogen can be produced from the oxidation of each mole of NADH (NADH+H+?NAD++H2) that was produced during glycolysis, making the total hydrogen yield 4 mol H2/ mol glucose. If the final product is butyrate, NADH produced from glycolysis will be used for oxidation of acetyl-CoA to butyrate, giving a hydrogen yield of 2 mol H2/mol glucose. These reactions are typical examples for Clostridia species [6, 7]. As a result of glucose fermentation, the generation of other products; propionate, succinate and lactate, can also occur besides acetate, butyrate and formate. Since their production is at the expense of hydrogen production, these metabolites are undesired by-products of dark fermentation.

Paula C. Rodrigues Pinto, Eduardo A. Borges da Silva and Alirio E. Rodrigues

12.1 Lignin, a Fascinating Complex Polymer

Lignin is a three-dimensional phenolic macromolecule that constitutes roughly 15-25% of vegetal biomass acting as structural and cohesion components of the cell walls in vascular plants [1]. Following cellulose, lignin is the most abundant natural biopolymer and contains about 30% of non-fossil organic carbon on Earth [2].

The principle of lignin biosyntheses is the polymerization by dehydrogenation of the hydroxycinnamyl alcohols, the monolignols p-coumaryl, coniferyl, and sinapyl alcohols [2-6]. Each of these monolignols gives rise to one subunit type in lignin structure, p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively, differing between them in the methoxylation of the aromatic nuclei as depicted in Fig. 12.1.

The general structural unit of lignin is commonly called a phenylpropane unit or, briefly, ppu. Although with some exceptions, softwood lignins primarily con­tain G units and a small proportion of H units: G lignin. Pinus and spruce are some examples of trees containing this lignin. Hardwood lignins contain both S and G units, with a very small proportion of H units: GS lignin. Some examples of hardwoods are birch, eucalyptus, beech, and aspen. The lignin of some crop plants, palm trees, and banana plants are composed by all the three subunits, although with the predominance of H type: HGS lignin.

Lignin ppus are linked by ether and carbon-carbon bonds either in aliphatic and/or aromatic moiety [1, 7]. Types and frequencies of the most abundant dilignols in softwood and hardwood lignins are summarized in Table 12.1.

P. C. Rodrigues Pinto • E. A. Borges da Silva • A. E. Rodrigues (H)

Laboratory of Separation and Reaction Engineering—LSRE, Associate Laboratory LSRE/ LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal e-mail: arodrig@fe. up. pt

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_12, © Springer-Verlag Berlin Heidelberg 2012

The respective letters are shown in Fig. 12.2, representing one fragment of hardwood lignin. The numbering system for the ppu is also shown in Fig. 12.2.

The dilignol b-O-4 (A) (arylglycerol-b-aryl ether) is, by far, the most frequent dilignol, accounting for more than 50% of the structures. It is also the one most easily cleaved, providing a basis for industrial processes, such as chemical pul­ping, and several methods in lignin chemical analysis. The other linkages are all more resistant to chemical degradation [1, 3]. The proportion of each linkage depends on the relative contribution of a particular monomer to the polymerization process. For example, G-type lignins (softwood lignins), contain more resistant linkages as those involving the C5 of aromatic nuclei (b-5 (C), 5-5′(E) and 4-O — 5′(F)) than SG lignins (hardwood lignins) due to the availability of the C5 position for coupling. This is the reason for the higher condensation degree (frequency of C-C linkages between aromatic rings) for softwood than for hardwood lignins. This fact has implications on lignin reactivity. In spite of these common features, chemical structure of lignin cannot be described by a simple structural formula due to the enormous and apparently random possibilities of combination between units in the macromolecule. Monolignols can also form bonds to other cell wall poly­mers as polysaccharides in a complex three-dimensional network [3, 8].

It is widely recognized that lignin content and composition differ between the major groups of higher plants (as evidenced by the data in Table 12.1) and also between species and even between trees and morphological parts of the tree [1]. This fact denotes the flexibility of the combinatorial polymerization reactions allowing significant variations in the final structure and high number of possible isomers.

This is a natural and powerful tool of higher plants to adaptive response to the various environmental conditions stresses, for example, the lignin formed in compression wood [3]. Simultaneously, it reveals an unexploited opportunity to engineer the lignin structures to modify their proprieties for required applications [11, 12].

Volatile SCM bio-oil products were characterized by GC-MS analysis and the chromatograms are shown in Fig. 13.4. The identified products and their yields are given in Table 13.2. The total yield of volatile components from spruce and birch were 19.8 and 27.1 mg/g (wood), respectively. Differences in volatile product profiles were mainly due to different lignin compositions between softwoods and hardwoods and therefore their derived products. Spruce yielded mainly guaiacyl derivatives (guaiacol, methyl guaiacol, eugenol, isoeugenol, etc.) from lignin while birch gave both guaiacyl and syringyl (syringol, 4-allyl syringol, 4-propenyl syringol, etc.) derivatives from lignin (Table 13.2, Fig. 13.4). In addition,

carbohydrate degradation products were also observed (such as 2,5-dimethylfuran and a series of organic acids as their methyl ester).

Reportedly, the process of lignocellulosic fragmentation begins with the cleavage of lignin b-O-4 linkages [21, 36]. The cleavage process results in olig­omeric and monomeric phenolic structures that are stabilized by methylating reactive sites (phenols and carboxylic acids). This process, which occurs under elevated temperatures and pressures of SCM conditions, results in first-order product distributions for lignin marked by methoxy functionalities (Table 13.2), as well as traditional pyrolysis products such as guaiacol, homoguiacol, eugenol, syringol and other guaiacyl structures. For example, the presence of 3,4-dimethoxy toluene and 3,4,5-trimethoxy toluene suggests that this originated from methyl — guaiacol and methyl-syringol, respectively after methylation.

Lignin phenolic monomers exhibiting both guaiacyl and syringyl nuclei structures readily decompose in SCM [37]. Hardwood lignins contain less con­densed inter-lignin linkages and more b-O-4 linkages due to the presence of syringyl units and therefore more readily cleaved during SCM than softwood lignins and thus a greater yield of volatile products [38]. This is evident from the GC-MS data listed in Table 13.2.

Studies by Soria et al. [24] have also shown that (i) lignin oligomers and polymers were present in the bio-oil as determined by gel permeation chroma­tography and (ii) oligosaccharides and methyl glycosides by high performance liquid chromatography. Furthermore, they showed that lignin solubilization occurs first followed by hemicellulose degradation and then cellulose. This process combines thermochemical breakdown of the carbohydrate crystalline and amor­phous regions into oligosaccharides. Lignin solubilization/degradation is similar to what occurs during organosolv pulping with ethanol as the solvent, but under milder thermal conditions around 180°C [39-41]. Under SCM processing, meth — anolysis reactions occur resulting in methyl glycosides and methyl ester derivatives (Table 13.2) [20, 24]. Further decomposition of these oligomers into furfural and other dehydration products also occurs [20], leading to the production of methyl a — and b-D glucosides, levoglucosan and 5-hydroxymethyl-furfural from polysaccharides [24].

The methylated biomass SCM products are known to be more stable than comparable pyrolysis products [21], and hence provide a better platform for upgrading. The resulting bio-oil composition shows a volatile species distribution that enhances the stability of the bio-oil, as well as creates a methylated platform which can enhance the catalytic upgrading of the bio-oil into a new generation biofuel. For an application where the engine, logistical and processing infra­structure are limited to hydrocarbon-based fuels, such as the one we currently have, the SCM processing platform presents a unique opportunity to produce consistent bio-oils from different biomass streams. This is further enhanced by the reaction conditions which are a third to half less thermal energy intensive as traditional pyrolysis.

Issues with scalability and solids transfer due to the elevated pressures continue to be ongoing areas of development, and are the greatest shortcomings of this novel thermochemical technique at this time. The GC-MS results are limited to volatile compounds, with boiling points lower than 300°C. Depolymerized com­pounds can undergo rapid re-polymerization as a result of the low pH of bio-oil, the presence of water and the formation of reactive sites, promoting oligomers and polymers to form with elevated boiling points and molecular weights in excess of

20,0 g mol-1 [24]. Evaluating the fractions of non-volatile compounds is a significant shortcoming of the current work, in particular the mass distribution of volatile versus non-volatile species.

13.4 Conclusion

Product consistency in the development of an alternative renewable biofuel is of paramount importance. Processing different biomass in a single step, often leads to inconsistent product streams. Supercritical methanol processing of both hardwoods and softwoods in a batch reactor show consistent product outputs and conversions greater than 92 wt%, surpassing traditional pyrolysis processing. The methylated products, generated by the SCM treatment process, show potential stability and great promise in catalytic upgrading into new generation biofuels. The results show a series of chemicals that have established markets and post new options for creating value added products, while providing fundamental knowledge on the chemical makeup of that biomass.

Acknowledgments This project was supported by USDA-CSREES Wood Utilization Research program grant #2008-34158-19486. The FTIR spectrometer was supported by a USDA-CSREES- NRI equipment grant #2005-35103-15243.

Hydrogen production with large-scale reactors is the main aim of many researchers. Before scale-up operations, it is very important to understand the process and the factors effecting the process clearly. The process parameters that can affect the photofermentative hydrogen production are discussed below.

10.3.1 C/N Ratio

Maximizing hydrogen production yields greatly depends on the carbon source used. Lactic acid and malic acid seem to be the most suitable organic acids. Since nitrogenase is very important for effective hydrogen production the ratio of carbon to nitrogen should be adjusted carefully. Different Rhodobacter species have different capacities to metabolize carbon and nitrogen sources. Photofermentative hydrogen production by Rhodobacter capsulatus from acetate as carbon source and glutamate as nitrogen source gave the best result with C/N ratio of 35 as 1.36 mg/l/h hydrogen productivity [119]. For Rhodobacter sphaeroides 15 mM malic acid and 2 mM glutamic acid (C/N ratio of 33) resulted with best hydrogen production rate of 10 ml/l/h [120].

Nitrogen source is a very important concern for photofermentative hydrogen production. Among tested 19 amino acids as N source for Rhodobacter capsulatus glutamate, serine and alanine gave the best results but glutamate is accepted as the more common nitrogen source and higher concentrations of NH4+ can inhibit the nitrogenase activity [105].

10.3.2 Inoculum Age

It is very important to use the bacteria at the early stationary phase for more hydrogen productivity [121]. After long retention times in the growth media the PNS bacteria are known to change the direction of metabolic pathway toward producing PHB [122]. At optimum conditions of growth medium microorganisms can produce more insoluble polymers that can be oxidized to generate ATP.

The production of phenolic compounds, mainly vanillin, from several sources of lignin in alkaline medium with O2 has been the subject of many publications in the last decades [20, 36, 92, 110-113, 115-125]. Some researches have been devel­oped also in acidic medium [126-128] or in ionic liquids [129, 130]. Biotechno­logical routes [100, 101, 117], electro-oxidation of lignin [56] and the use of microwaves [131] have also been considered. Some studies reported also the direct oxidation of wood to produce vanillin and syringaldehyde [ 132]. The separation of product from the reaction medium was also a subject of intense research [133­138], as will be pointed out in the next section.

The chemical oxidation of the spent liquors or lignin, from several sources, is focused in operating conditions, apparatus, and catalyst. The general aim is to achieve the maximum conversion of lignin to vanillin (in the case of softwoods) or also to syringaldehyde (in the case of hardwoods or annual plants). Besides the process yields on products, also the kinetics laws, and the chemical mechanisms have been achieved. The impact of a particular lignin on its performance toward the oxidative process have been considered more recently [20]. A summary of some of the representative studies concerning raw material, conditions, and results are gathered in Table 12.3.

Insights on reaction mechanism of lignin oxidation have been consistently developed by Tarabanko et al. [19, 132, 139-142], although some are not so recent papers [143-146] constitute also a valuable survey of information. In this chapter, the mechanism of lignin oxidation will be briefly described.