UC Berkeley

The conversion of lignocellulosic biomass to biofuel precursors and specialty chemicals was studied with the intention of developing a chemical understanding of and strategy for selectively converting cellulose, hemicellulose, and lignin to furanic compounds and phenolic monomers.

Beginning work with the cellulosic components, the kinetics of the Brønsted acid-catalyzed hydrolysis of hemicellulose in ionic liquid were studied and compared to those found for similar reactions involving cellulose. In 1-ethyl-3-methylimidazolium chloride ([Emim][Cl]) at 80 ºC, we found that hemicellulose could be hydrolyzed to xylose in a 90% yield after 2 h (with 5 wt% furfural and 4 wt% humins) when water was added in a stepwise fashion. The addition of water was found to be integral in curtailing the conversion of xylose and formation of humins under these conditions. This chemical process presents a viable pathway for producing both xylose and glucose from their respective parent polymers.

The Brønsted acid-catalyzed dehydration of xylose to furfural in water was investigated next. Water was chosen as the reaction solvent given that the strategy for producing xylose previously discussed results in a solution that is higher in water content than in [Emim][Cl]. Reaction temperatures were adjusted to account for the previously observed negligible xylose reactivity at or below 100 ºC in aqueous solutions. At these higher temperatures, xylose dehydration is accompanied by the significant formation of humins via complex side processes that ultimately result in a loss in the yield of furfural. The use of biphasic systems has been observed to curtail this loss by extracting furfural as it is produced, with metal halide salts added to the aqueous phase to enhance extraction. While the thermodynamics of using metal halides to improve liquid-liquid extraction are well studied, their impact on the kinetics of xylose dehydration catalyzed by a Brønsted acid are not. The aim of our investigation was to understand how metal halides affect the mechanism and kinetics of xylose dehydration in aqueous solution. We found that the rate of xylose conversion is affected by both the nature of the salt cation and anion, increasing in the order no salt < K+ < Na+ < Li+ and no salt < Cl- < Br- < I-. Furfural selectivity increases similarly with respect to metal cations, but in the order no salt < I- < Br- < Cl- for halide anions. Multinuclear NMR experiments identified that halide anions principally interact with the hydroxyl groups of xylose, while metal cations interact with both water molecules and xylose hydroxyl groups. These interactions permit halide anions to stabilize critical carbocation intermediates formed during the hydroxyl group removal stage of dehydration. Increasing the strength of halide nucleophilicity was found to promote substitution over elimination reactions at the carbocation site. Therefore, chloride salts were found to be selective to dehydration products, namely furfural, and iodide salts were found to enhance the kinetics of humins formation. The results of these experiments coupled with 18O-labeling experiments indicated that xylose dehydration is initiated by protonation at the C1OH and C2OH sites, with the addition of metal halides increasing the proportion of C1OH initiated dehydrations. Careful selection of a metal halide was found to improve the kinetics of xylose dehydration and selectivity to furfural in water strictly for the Brønsted acid-catalyzed case.

Understanding that the isomers of xylose and glucose dehydrate more readily and selectively to furanics in water than xylose and glucose, we investigated the use of various isomerization catalysts with the strategy to first isomerize xylose and glucose to xylulose and fructose, respectively, and then dehydrate the isomers to furfural and 5-hydroxymethyl furfural (5-HMF), respectively, using a Brønsted acid. A number of Lewis acid catalysts were screened for their effectiveness in converting both xylose and glucose in aqueous media. SnCl4 was identified as the most selective Lewis acid. Under the conditions of use, SnCl4 hydrolyzed to such an extent that the addition of an external Brønsted acid was unnecessary to convert carbohydrates to furanics. SnCl4-catalyzed isomerization/dehydration was performed with xylose and glucose, and found to produce selectivities toward furfural and 5-HMF of up to 58% and 27%, respectively, after 2 h at 140 ºC in water. The addition of a secondary organic layer (n-butanol) increased these selectivities to 85% and 69% at the same temperature after 5 h. Minor improvements to these selectivities occurred when small amounts of LiCl were added to the aqueous phase, showing that metal halides have less of an impact on the conversion of carbohydrates to furanics when catalyzed by Lewis acids rather than when strictly catalyzed by Brønsted acids.

The conversion of lignin, the third most abundant biomass component after cellulose and hemicellulose, was also explored. Using the ionic liquid [Emim][Cl] as solvent and a Brønsted acid as catalyst, we investigated the cleavage of β-O-4 ether linkages in both lignin and lignin model compounds. However, in neither case did we observe any evidence of ether bond cleavage. Such an incompatibility between lignin and the ionic liquid solvent was determined to be due to unfavorable solvent-solute interactions. The lack of reactivity of lignin and lignin model compounds in [Emim][Cl] makes this ionic liquid a suitable solvent for the hydrolysis of cellulosic biomass in the presence of and without interference from lignin. Lignin model compounds containing glycerol β-O-4 ether linkages could be hydrolyzed successfully in a variety of organic solvents, most notably a combination of 1,4-dioxane and water. The linkage of the guaiacylglycerol β-guaiacyl ether [G,G] model compound was found to be cleaved via a two-step dehydration/hydrolysis leading to guaiacol and Hibbert ketones. The Brønsted acid-catalyzed reactions of an array of lignin model compounds containing glycerol β-O-4 ether linkages revealed that the nature of phenyl group substitution does not influence the rate of ether cleavage.

The results of this study show that Brønsted acids can be used to promote the controlled hydrolysis of the hemicellulose and cellulose fractions of biomass in an ionic liquid such as [Emim][Cl] to their respective sugars, xylose and glucose. On the other hand, the cleavage of glycerol β-O-4 ether linkages in the lignin fraction does not occur due to inhibition of this process by interactions of the cations of the solvent with aromatic groups of the lignin. Dehydration of xylose and glucose dissolved in water can be achieved using a combination of Lewis and Brønsted acids. Notably, SnCl4 is found to effectively catalyze the aldose to ketose isomerization of xylose and glucose, and that the resulting products, xylulose and fructose, can undergo Brønsted acid-catalyzed dehydration to furfural and 5-HMF strictly via the Brønsted acidity provided by Sn4+ hydrolysis. The desired furanic products, furfural and 5-HMF, can undergo secondary reactions to produce humins and other byproducts, and thus it is necessary to separate the furanics as they are produced. Our work has shown that this can be done effectively using n-butanol as an extracting agent. The addition of metal halide salts to the aqueous phase promotes the extraction of furanics into the organic phase by enhancing the thermodynamic activity of these products in the aqueous phase. It is also found that specific interactions of the anions with the hydroxyl groups of xylose enhance the rate of xylose dehydration and that the interactions of metal cations with water enhance the activity of the protons that catalyze the dehydration of xylose.