Enhancement of cyclic ether formation from polyalcohol compounds in high temperature liquid water by high pressure carbon dioxide

Article abstract of DOI:10.1039/b812318g

Cyclic ethers were produced by a dehydration reaction of polyalcohol compounds in high temperature liquid water, which was accelerated by the presence of carbon dioxide dissolved in the water. 3-hydroxytetrahydrofuran was produced by the dehydration of 1,2,4-butanetriol. Both tetrahydrofurfuryl alcohol and 3-hydroxytetrahydropyran were produced by the dehydration of 1,2,5-pentanetriol. Five-membered cyclic ethers were formed faster than six-membered cyclic ethers and the formation rates of the cyclic ethers depended strongly on the structure of the polyalcohol compounds. The position of the hydroxyl groups is crucial for the efficient intramolecular dehydration.

Full text of DOI:10.1039/b812318g

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PAPER
www.rsc.org/greenchem | Green Chemistry
Enhancement of cyclic ether formation from polyalcohol compounds
in high temperature liquid water by high pressure carbon dioxide
Aritomo Yamaguchi, Norihito Hiyoshi, Osamu Sato, Kyoko K. Bando and Masayuki Shirai*
Received 24th July 2008, Accepted 10th October 2008
First published as an Advance Article on the web 7th November 2008
DOI: 10.1039/b812318g
Cyclic ethers were produced by a dehydration reaction of polyalcohol compounds in high
temperature liquid water, which was accelerated by the presence of carbon dioxide dissolved in the
water. 3-hydroxytetrahydrofuran was produced by the dehydration of 1,2,4-butanetriol. Both
tetrahydrofurfuryl alcohol and 3-hydroxytetrahydropyran were produced by the dehydration of
1,2,5-pentanetriol. Five-membered cyclic ethers were formed faster than six-membered cyclic
ethers and the formation rates of the cyclic ethers depended strongly on the structure of the
polyalcohol compounds. The position of the hydroxyl groups is crucial for the efficient
intramolecular dehydration.
this manuscript, we investigated the dehydration mechanism of
Introduction
polyalcohol compounds having two or three hydroxyl groups,
Biomass has a large amount of oxygen atoms because plants
as simple model compounds of biomass-derived carbohydrates,
combine carbon dioxide with water using solar energy to store
to corresponding five- or six-membered cyclic ethers in water
oxygen as sugar building blocks, (CH2O)n. The selective removal
under high pressure carbon dioxide at 573 K.
of oxygen atoms from biomass-derived carbohydrates, which
are polyalcohol compounds like fructose, sorbitol, and glycerol
in most cases, by dehydration or hydrogenolysis is important
Experimental
to obtain valuable products with desired boiling points, water
solubilities, octane numbers and viscosities.1 The chemistry of
intramolecular dehydration of polyalcohol compounds provides
a key technology for developing an efficient conversion process
of biomass derivatives to useful materials;2 however, biomass-
derived carbohydrates, such as fructose and sorbitol, have five
or six hydroxyl groups in a molecule and their intramolecular
dehydration mechanisms are complicated.
High temperature liquid water has attracted much attention
as an alternative to harmful organic solvents because of its
high proton concentration, which enhances the rates of acid-
catalyzed reactions, such as dehydration in water, without
adding any hazardous acid.3,4 Cyclic ethers are essential ma-
terials for the chemical industry and they are produced from
diols by intramolecular dehydration over strong mineral acids,
aluminium silicates, and ion-exchange resins.5 Savage’s group
reported that the addition of carbon dioxide to water (473–
623 K) enhanced the production of tetrahydrofuran (THF) from
1,4-butanediol (1,4-BDO).6–8 The added carbon dioxide was
dissolved in water to form carbonic acid, accelerating the acid
catalysis of high temperature water. An acid solvent composed
of water and carbon dioxide is environmentally-benign not
only because both water and carbon dioxide are non-toxic,
but also separation and recycling of these two components
are easily performed by depressurization after reaction. In
1,2,4-Butanetriol (1,2,4-BTO, Wako Pure Chemical Industries),
1,2,5-pentanetriol (1,2,5-PTO, Tokyo Chemical Industry), 1,4-
butanediol (1,4-BDO, Wako Pure Chemical Industries), 1,4-
pentanediol (1,4-PDO, Aldrich) and 1,5-pentanediol (1,5-PDO,
Wako Pure Chemical Industries) were purchased and used
without any further purification.
The dehydration of polyalcohol compounds was carried out
in a batch reactor (inner volume: 6 cm3) made of a SUS316
tube.9 After 3 cm3 of polyalcohol aqueous solution (1.0 and
0.3 mol dm-3) was loaded in the reactor, the gas phase was
purged with argon gas to remove air. Carbon dioxide (10 and
15 MPa) was then loaded in the reactor at 323 K.10 The reactor
was submerged into a molten-salt bath at 573 K for a given
reaction time and then submerged into a water bath for cooling
to ambient temperature quickly after the reaction. The water
in the reactor maintained vapor-liquid equilibrium at 573 K
under 8.6 MPa of partial pressure. The partial pressure of carbon
dioxide at 573 K was estimated to be 17.7 and 26.6 MPa, based
on the equation of Charles’s law, corresponding to the initial
pressure of 10 and 15 MPa at 323 K, respectively. A mixture of
a reactant and liquid products was taken out from the reactor
with distilled water.
The quantitative analysis of liquid products was conducted
by gas chromatography with a flame ionization detector (GC-
FID) equipped with a DB-WAX capillary column (Agilent Tech-
nologies) using 1-propanol (Wako Pure Chemical Industries)
as an internal standard material. The products were identified
by their retention times of the GC-FID analysis, compared
with those for known materials; 3-hydroxytetrahydrofuran
(3-HTHF, Wako Pure Chemical Industries), tetrahydrofur-
Research Center for Compact Chemical Process, National Institute of
Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake,
Miyagino, Sendai, 983-8551, Japan. E-mail: m.shirai@aist.go.jp;
Fax: +81 22 237 5224; Tel: +81 22 237 5219
48 | Green Chem., 2009, 11, 48–52
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The Royal Society of Chemistry 2009
©

furyl alcohol (THFA, Sigma-Aldrich), tetrahydrofuran (THF,
Wako Pure Chemical Industries), 2-methyltetrahydrofuran (2-
MTHF, Sigma-Aldrich) and tetrahydropyran (THP, Wako Pure
Chemical Industries). 4-hydroxytetrahydropyran (Aldrich) was
used for the quantitative analysis for 3-hydroxytetrahydropyran
(3-HTHP). The material balance of all reactions was more than
95%. Initial formation rates of cyclic ethers were estimated from
initial slopes of the fitting curves of their profiles.
3-hydroxytetrahydropyran (3-HTHP) (Scheme 2, Fig. 2). The
yields of THFA (25.6, 35.7, and 45.6%) were about 6 times
larger than those of 3-HTHP (3.9, 5.2 and 7.6%) for 10 minutes
of reaction (under 0, 17.7, and 26.6 MPa of carbon dioxide,
respectively). The final yields of THFA and 3-HTHP for the
dehydration of 1,2,5-PTO were 70% and 10%, respectively, and
here again the pressure of carbon dioxide did not affect the
equilibrium yields of the products.
Results and discussion
Dehydration of 1,2,4-butanetriol (1,2,4-BTO) and
1,2,5-pentanetriol (1,2,5-PTO) in high temperature liquid
water at 573 K
Dehydration of 1.0 mol dm-3 of 1,2,4-butanetriol (1,2,4-BTO)
proceeded in water at 573 K (Fig. 1). 3-Hydroxytetrahydrofuran
(3-HTHF) was the only product of this reaction (Scheme 1).
The 3-HTHF yield for 10 minutes in liquid water at 573 K
was 2.9% (as 1,2,4-BTO conversion) and increased to 10.7 and
22.4% by the addition of 17.7 and 26.6 MPa of carbon dioxide,
respectively. On the other hand, the final yield of 3-HTHF after
3 hours of reaction was almost the same (70%) for the three
conditions (under 0, 17.7 and 26.6 MPa of carbon dioxide),
indicating that the added carbon dioxide enhanced the 3-HTHF
formation rates to reach the equilibrium yield (70%).
Scheme 2 Dehydration reaction of 1,2,5-pentanetriol.
Fig. 2 Yields of tetrahydrofurfuryl alcohol (THFA, closed symbols),
3-hydroxytetrahydropyran (3-HTHP, checked symbols), and 1,2,5-
pentanetriol (1,2,5-PTO, open symbols) as a function of elapsed time for
the 1,2,5-PTO dehydration reaction at 573 K in water (initial 1,2,5-PTO
concentration: 1.0 mol dm-3, carbon dioxide partial pressure: 0 (circles),
17.7 (triangles) and 26.6 MPa (squares)).
Fig. 1 Yields of 3-hydroxytetrahydrofuran (3-HTHF, closed symbols)
and 1,2,4-butanetriol (1,2,4-BTO, open symbols) as a function of elapsed
time for the 1,2,4-BTO dehydration reaction at 573 K in water (initial
1,2,4-BTO concentration: 1.0 mol dm-3, carbon dioxide partial pressure:
0 (circles), 17.7 (triangles) and 26.6 MPa (squares)).
Dehydration of 0.3 mol dm-3 1,2,4-BTO and 0.3 mol dm-3
1,2,5-PTO also proceeded in water at 573 K (Fig. 3). 3-HTHF
was produced from 1,2,4-BTO and the 3-HTHF yield increased
by the addition of 17.7 MPa of carbon dioxide (Fig. 3 (a)).
THFA and 3-HTHP were produced from 1,2,5-PTO and the
yield of THFA and 3-HTHP increased by the addition of
carbon dioxide (Fig. 3 (b)). The product yields from different
initial reactant concentrations (0.3 and 1.0 mol dm-3) were quite
similar to each other in both cases of 1,2,4-BTO and 1,2,5-
PTO. The initial formation rates of cyclic ethers are summarized
in Table 1. Formation rates of 3-HTHF, THFA and 3-HTHP
increased with increasing carbon dioxide pressure or the initial
concentrations of 1,2,4-BTO and 1,2,5-PTO. On the other hand,
the formation rates divided by the initial reactant concentrations
were unaffected by the initial reactant concentrations between
Scheme 1 Dehydration reaction of 1,2,4-butanetriol.
The dehydration of 1.0 mol dm-3 of 1,2,5-pentanetriol
(1,2,5-PTO) also proceeded in water at 573 K and pro-
vided two products; tetrahydrofurfuryl alcohol (THFA) and
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Green Chem., 2009, 11, 48–52 | 49
©

Table 1 Initial formation rates of cyclic ethers for dehydration reactions of 1,2,4-butanetriol (1,2,4-BTO) and 1,2,5-pentanetriol (1,2,5-PTO) in
water at 573 K
Reactant
Product
CO2 pressure/MPa
Cia/mol dm-3
rib,c/10-4 mol dm-3 s-1
ri/Cic/10-4 s-1
1,2,4-BTO
3-HTHF
0
0.3
1.0
0.3
1.0
0.3
1.0
0.3
1.0
0.3
1.0
0.3
1.0
0.21
0.61
0.61
1.7
1.4
4.3
1.9
5.9
0.12
0.47
0.24
0.87
0.69
0.61
2.0
1.7
4.7
4.3
6.3
5.9
0.41
0.47
0.81
0.87
17.7
0
1,2,5-PTO
THFA
17.7
0
3-HTHP
17.7
a Initial reactant concentration. b Initial formation rate. c The experimental errors were within 5%.
Ê
Á
ˆ
˜
KW
Ka1
+
È
˘
=
È ˘
CO2 aq
(2)
H
+
(
)
Î
˚
Î
˚
+
+
Á
Ë
˜
¯
È
˘
È
˘
H
H
Î
˚
Î
˚
where [H+], [CO2(aq)], KW, and Ka1 represent the concentrations
of proton and dissolved carbon dioxide, ionization constant
of water, and the first ionization constant of carbonic acid,
respectively. They claimed that proton concentration depends
approximately on the first dissociation of carbonic acid essen-
tially because its second dissociation is negligible. Equation (2)
can be written as,
Fig. 3 (a) Yields of 3-hydroxytetrahydrofuran (3-HTHF, closed sym-
bols) and 1,2,4-butanetriol (1,2,4-BTO, open symbols) as a function of
elapsed time for the 1,2,4-BTO dehydration reaction at 573 K in water
(initial 1,2,4-BTO concentration: 0.3 mol dm-3, carbon dioxide partial
pressure: 0 (circles) and 17.7 MPa (triangles)). (b) Yields of tetrahy-
drofurfuryl alcohol (THFA, closed symbols), 3-hydroxytetrahydropyran
(3-HTHP, checked symbols), and 1,2,5-pentanetriol (1,2,5-PTO, open
symbols) as a function of elapsed time for the 1,2,5-PTO dehydration
reaction at 573 K in water (initial 1,2,5-PTO concentration: 0.3 mol dm-3,
carbon dioxide partial pressure: 0 (circles) and 17.7 MPa (triangles)).
1/ 2
Ê
ˆ1/ 2
K
+
H
= K + Ka1 CO2 aq
= KW
+
a1 P
CO2
È
˘
(3)
È
˘
(
)
(
)
W
Î
˚
Á
Ë
˜
Î
˚
KH
¯
where KH and PCO2 represent the Henry’s law constant of carbon
dioxide dissolved in water and the pressure of carbon dioxide,
respectively.
The formation of cyclic ethers has a first-order dependence
on polyalcohol concentrations and is an acid-catalyzed reaction.
The formation rates could be represented as follows,
0.3 and 1.0 mol dm-3 (Table 1). This result indicates that
the dehydration of 1,2,4-BTO and 1,2,5-PTO to the cyclic
ether compounds has a first-order dependence on polyalcohol
concentrations.
r = k[triol][H+]a
(4)
where r, k, [triol], [H+], a represent a formation rate of the
cyclic ether, an apparent reaction constant, a concentration of
triol and proton, and an apparent reaction order of proton,
respectively. We fitted Equation (4) to the formation rates of
the cyclic ethers from 1,2,4-BTO and 1,2,5-PTO versus proton
concentrations at 573 K with 0, 17.7 and 26.6 MPa carbon
dioxide. The apparent reaction orders of proton (a) are 0.6, 0.2
and 0.3 for 3-HTHF from 1,2,4-BTO and THFA and 3-HTHP
from 1,2,5-PTO, respectively. The dehydration of 1,2,4-BTO was
enhanced strongly by proton concentration, compared with the
dehydration of 1,2,5-PTO because a of 1,2,4-BTO dehydration
(0.6) was larger than those of 1,2,5-PTO dehydration (0.2 and
0.3). It should be noted that the apparent reaction orders of
proton for the dehydration reactions are not unity, indicating
that the dehydration of 1,2,4-BTO and 1,2,5-PTO to the cyclic
ether compounds does not have a first-order dependence on
proton concentration and the reaction is not simply initiated by
a collision between the triol molecule and proton. We will discuss
Effect of carbon dioxide on cyclic ethers formation from triol
compounds in high temperature liquid water
The intramolecular dehydration reaction of 1,4-butanediol (1,4-
BDO) to tetrahydrofuran (THF) is reported to be catalyzed
by acid agents, such as ion exchange resins,11 zeolites12,13 and
heteropolyacids.14,15 The intramolecular dehydration reactions
of triols to the cyclic ether compounds would be an acid-
catalyzed reaction.
The increase of the proton concentration by the addition of
carbon dioxide could be explained by the dissolution of carbon
dioxide in high temperature liquid water, as shown below.
H2O + CO2 ꢀ H2CO3 ꢀ H+ + HCO3 ꢀ 2H+ + CO3
(1)
-
2-
Hunter and Savage estimated the proton concentration of high
temperature liquid water with an addition of carbon dioxide
(Equation (2)).6–8
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Table 2 Initial formation rates of cyclic ethers for dehydration reac-
the results that the dehydration rate of 1,4-BDO to THF (0.68 ¥
10-4 mol dm-3 s-1) was larger than that of 1,5-PDO to THP
(0.063 ¥ 10-4 mol dm-3 s-1); and, (2) the dehydration of 1,4-PDO
proceeds through the formation of a secondary carbocation,
and/or the basicity of the secondary hydroxyl group in the 4-
position in 1,4-PDO is higher than that of the primary hydroxyl
group, as shown by the result that the dehydration rate of 1,4-
PDO to 2-MTHF (0.86 ¥ 10-4 mol dm-3 s-1) was higher than
that of 1,4-BDO to THF (0.68 ¥ 10-4 mol dm-3 s-1). Therefore,
in the case of the dehydration of 1,2,5-PTO, the formation
rate of THFA, which is a five membered heterocyclic ether
and formed by dehydration between primary and secondary
hydroxyl groups, is much faster than that of 3-HTHP (a
six membered heterocyclic ether and formed by dehydration
between two primary hydroxyl groups). Table 2 also shows that
the promotion effect by carbon dioxide on the dehydration
rates of alkanediols (1,4-BDO and 1,5-PDO) was larger than
that of corresponding alkanetriols (1,2,4-BTO and 1,2,5-PTO).
These results may be explained by the intramolecular hydrogen
bond between the two hydroxyl groups in the 1- and 2-position
in triols, which was investigated by density functional theory
(DFT) methods.16–18 The DFT studies show that the hydroxyl
group in the 2-position was preferentially protonated by the
intramolecular hydrogen bond. The intramolecular interaction
between the two hydroxyl groups in the 1- and 2-position of
triols would make the effect of protons from external molecules
smaller, which results in less promotion effect of carbon dioxide
for triols.
tions in water at 573 K
Reactanta
1,4-BDO
Product
THF
CO2 pressure/MPa rib,c/10-4 mol dm-3 s-1
0
17.7
0
17.7
0
0.68
4.7
0.61
1.7
1,2,4-BTOd 3-HTHF
1,4-PDO
1,5-PDO
2-MTHF
THP
0.86
17.7
0
17.7
0
10.0
0.063
0.42
4.3
1,2,5-PTOc THFA
3-HTHP
17.7
0
17.7
5.9
0.47
0.87
a Initial reactant concentration: 1.0 mol dm-3. b Initial formation rate.
c The experimental errors were within 5%. d The same data as shown in
Table 1.
the promotion effect by carbon dioxide in terms of basicities
of the hydroxyl groups and intramolecular hydrogen bond in
triol compounds along with the dehydration results of various
polyalcohol compounds in the following section.
Formation rates of cyclic ethers from various polyalcohol
compounds
We investigated initial dehydration rates concerning three more
polyalcohol compounds to understand the dehydration mech-
anism of polyalcohol compounds to five- or six-membered
cyclic ethers, and the results are summarized in Table 2. The
dehydration of 1,4-pentanediol (1,4-PDO) and 1,5-pentanediol
(1,5-PDO) proceeded to produce a five-membered ether, 2-
methyltetrahydrofuran (2-MTHF) and a six-membered ether,
tetrahydropyran (THP), respectively (Scheme 3).
Conclusions
Cyclic ethers were produced by the dehydration reactions of
polyalcohol compounds in high temperature liquid water, which
was accelerated by the presence of carbon dioxide dissolved in
the water. Five-membered cyclic ethers were formed faster than
six-membered cyclic ethers and the formation rates of the cyclic
ethers depended strongly on the structure of the polyalcohol
compounds. The position of the hydroxyl groups is crucial
for the efficient intramolecular dehydration, which leads to a
new technology for conversion of biomass derivatives to useful
materials.
Notes and references
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Renewable Energy Laboratory (NREL), Top value added chemicals
from biomass, Report, 2004.
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