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.