No abstract

With dwindling reserves of fossil feedstock as a resource for chemicals production, the fraction of chemicals and energy supplied by alternative, renewable resources, such as lignin, can be expected to increase in the foreseeable future. Here, we demonstrate a catalytic process to valorize lignin (exemplified with kraft, organosolv, and sugarcane bagasse lignin) using a mixture of cheap, bio-renewable ethanol and water as solvent. Ethanol/water mixtures readily solubilize lignin under moderate temperatures and pressures with little residual solids. The molecular weight of the dissolved lignins was shown to be reduced by gel permeation chromatography and quantitative HSQC NMR methods. The use of liquid-phase reforming of the solubilized lignin over a Pt/Al(2)O(3) catalyst at 498 K and 58 bar is introduced to yield up to 17 % combined yield of monomeric aromatic oxygenates such as guaiacol and substituted guaiacols generating hydrogen as a useful by-product. Reduction of the lignin dissolved in ethanol/water using a supported transition metal catalyst at 473 K and 30 bar hydrogen yields up to 6 % of cyclic hydrocarbons and aromatics.

No abstract

Lignin is a component of lignocellulosic biomass from which important aromatic compounds can potentially be obtained. In the present work, Alcell and soda lignin were dissolved in the ionic liquid 1-ethyl-3-methylimidazolium diethylphosphate (EMIM DEP) and subsequently oxidized using several transition metal catalysts and molecular oxygen under mild conditions. CoCl 2 •6H 2 O in EMIM DEP proved particularly effective for the oxidation. The catalyst rapidly oxidized benzyl and other alcohol functionalities in lignin, but left phenolic functionality and 5-5¢, b-O-4 and phenylcoumaran linkages intact, as determined by analysis of various lignin model compounds and ATR-IR spectroscopy. The catalyst system oxidized the alcohol functionality contained in cinnamyl alcohol to form cinnamaldehyde or cinnamic acid or disrupted the double bond to form benzoic acid or an epoxide. The benzyl functionality in veratryl alcohol, a simple non-phenolic lignin model compound, was selectively oxidized to form veratraldehyde at a maximum turnover frequency of 1440 h -1 , compared to 10-15 h -1 reported for earlier systems. Phenolic functional groups contained in guaiacol, syringol, and vanillyl alcohol remained intact, although the benzyl alcohol group in the latter was oxidized to form vanillin. Incorporation of strongly bound tetradentate ligands to the catalyst yielded reduced activity relative to those derived from simple metal salts in EMIM DEP. The influence of reaction conditions, such as temperature, oxygen pressure and NaOH loading, were also investigated. The system represents a potential method in a biorefinery scheme to increase the oxygen functionality in lignin prior to depolymerization or additional functionalization of already depolymerized lignin.

The solubilization and aqueous phase reforming of lignin, including kraft, soda, and alcell lignin along with sugarcane bagasse, at low temperatures (T≤498 K) and pressures (P≤29 bar) is reported for the first time for the production of aromatic chemicals and hydrogen. Analysis of lignin model compounds and the distribution of products obtained during the lignin aqueous phase reforming revealed that lignin was depolymerized through disruption of the abundant β-O-4 linkages and, to a lesser extent, the 5-5' carbon-carbon linkages to form monomeric aromatic compounds. The alkyl chains contained on these monomeric compounds were readily reformed to produce hydrogen and simple aromatic platform chemicals, particularly guaiacol and syringol, with the distribution of each depending on the lignin source. The methoxy groups present on the aromatic rings were subject to hydrolysis to form methanol, which was also readily reformed to produce hydrogen and carbon dioxide. The composition of the isolated yields of monomeric aromatic compounds and overall lignin conversion based on these isolated yields varied from 10-15% depending on the lignin sample, with the balance consisting of gaseous products and residual solid material. Furthermore, we introduce the use of a high-pressure autoclave with optical windows and an autoclave with ATR-IR sentinel for on-line in situ spectroscopic monitoring of biomass conversion processes, which provides direct insight into, for example, the solubilization process and aqueous phase reforming reaction of lignin.