(129a) Role of Metal Halides in Enhancing the Dehydration of Xylose to Furfural: A 1H and 13C NMR Spectroscopy Study

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

Kristopher R. Enslow^1,2 and
Alexis T. Bell^1,2*

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.

Table 1. Yields of furural
from xylose in a biphasic water/toluene system (1:4 v/v) at 180¼C 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

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