284695 Role of Metal Halides in Enhancing the Dehydration of Xylose to Furfural: A 1H and 13C NMR Spectroscopy Study

Monday, October 29, 2012: 3:15 PM
322 (Convention Center )
Kristopher Enslow, Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA and Alexis T. Bell, Department of Chemical and Biomolecular Engineering, University of California - Berkeley, Berkeley, CA

Role of Metal Halides in Enhancing the Dehydration of Xylose to Furfural: a 1H and 13C NMR Spectroscopy Study

Kristopher R. Enslow1,2 and Alexis T. Bell1,2*

1University of California, Berkeley, California 94720-1462

2Energy Biosciences Institute, Calvin Laboratory,

Berkeley, California, USA  94720-1462

*bell@cchem.berkeley.edu

Introduction

       High value fuel molecules (aromatics, alcohols, and alkanes) can be formed in a renewable and carbon-neutral manner from biomass [1]. These chemicals represent a wide range of fuel molecules that can be blended with gasoline and diesel or combined to create stand-alone fuels. A major component of biomass, hemicellulose (20 – 40 wt%) [1], is a complex amorphous polymer consisting of a backbone of b-1,4 glycosidically linked xylose sugars branched with arabinose, glucose, galactose, and mannose sugars. Previous work has shown that hemicellulose can be hydrolyzed to its monosaccharides [2] and those sugars can be either dehydrated to furanic chemicals [3] or hydrogenated to their respective alcohols [4]. The only large volume chemical produced industrially from biomass-derived carbohydrates is furfural, which is typically obtained through the concentrated acid-hydrolysis of agricultural waste, where selectivities never exceed 70% [5]. Other dehydration reaction systems have shown improvements over this conventional process, increasing furfural selectivity to 74% using β-zeolites (Si/Al = 12) [6], to 85% when employing a biphasic reactor (water/MIBK) under Brznsted-acidic conditions [7], and to 88% using metal halides with sulfuric acid in a biphasic reactor (water/toluene) [8]. While there have been proposed xylose dehydration mechanisms in the literature [9] suggesting the formation of 1,2-enediol (in the presence of metal halides), there has been little empirical evidence to support these mechanisms.

The objective of this study is to develop an understanding of the role metal halides play in the dehydration of xylose to furfural under acidic conditions and to elucidate a reaction mechanism supported by empirical NMR data. The kinetics of xylose dehydration in the presence of metal halides were also investigated.

Materials and Methods

       Xylose (99%, from Sigma-Aldrich) was used as a reagant and standard (for quantification), and 2-furaldehyde (furfural, 99%, from Sigma-Aldrich) was used as a standard (for quantification). All salts were used as purchased from Fisher-Scientific. Typical reaction: 5 wt% of xylose was dissolved in water in an HEL high pressure Chem-SCAN autoclave at 180 C while stirred at 1000 rpm. Toluene was added in 4:1 v/v ratio with water, and 0.5 M of salt and 50 mM H2SO4 were added to start the reaction.

Discussion

While adding metal halides to aqueous/organic biphasic systems are known and have been shown (see Table 1 below) to enhance the dehydration of xylose to furfural under acidic conditions, the role that the salt plays is not entirely known.

Time, min

No Salt

NaCl

KCl

NaBr

KBr

CaCl2

15

25%

61%

69%

68%

67%

77%

30

32%

72%

76%

75%

80%

87%

60

25%

68%

73%

70%

78%

85%

Table 1. Yields of furural from xylose in a biphasic water/toluene system (1:4 v/v) at 180C with 50 mM H2SO4 and 500 mM salt.

The presence of these metal halides in solution may lead to an interaction, or complexation, with xylose that promotes the stablization of xylose transition states that exist in the dehydration pathway to furfural. These salts could also be salting out furfural from the aqueous phase by making that phase less hydrophillic and altering the partition coefficient between the aqueous and organic phases in favor of toluene (keeping the xylose separate from the furfural has been shown to increase furfural yields by barring the degradative coupling reactions that lead to humins formation [10]). With these two fields of thought on the role of metal halides, NMR experiments have been performed looking specifically at how changes in the concentration of metal halides effect the chemical shift of the protons of xylose and the water proton. Comparing these changes in chemical shift provides a qualitative picture of how and to what degree these salts interact with xylose and how this compares to the interaction of these salts with water. NMR labelling has been used to determine whether proton transfer occurs to initiate xylose dehydration to furfural. With this information, a reaction mechanism has been proposed, the role of metal halides has been discovered, and the reaction kinetics associated with xylose dehydration have been determined.

References

1.      Huber, G.W., Iborra, S., and Corma. A. Chem. Rev. 106, 4044 (2006).

2.      Wyman, C.E., et al. in "Hydrolysis of Cellulose and Hemicellulose (Polysaccharides: Structural Diversity and Functional Versatility)" (S. Dumitriu, Ed.) Vol. 2 p. 995. Marcel Dekker, New York, 2005.

3.      Chheda, J.N, Roman-Leshkov, Y., and Dumesic, J.A. Green Chemistry 9, 342 (2007).

4.      Wisniak, J., et al. Ind. End. Chem. Prod. Rd. 13, 75 (1974).

5.      Moreau, C., et al. Industrial Crops and Products. 7 (2-3), 95 (1998).

6.      Lima, S., et al. Appl. Catal., A. 388, 141 (2010).

7.      Weingarten, R., et al. Green Chem. 12, 1423 (2010).

8.      Marcotullio, G., and Jong, W. D. Green Chem. 12, 1739 (2010).

9.      Marcotullio, G., and Jong, W. D. Carb. Res. 346, 1291 (2011).

10.    Sievers, C., et al. ChemSusChem 2 (7): 665-671 (2009).


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