SALT
SALT PROMOTED
PROMOTED HYDROTHERMAL
HYDROTHERMAL ACTIVATION
ACTIVATION OF
OF ALCOHOLS
ALCOHOLS
a
b
c
a
Sabine
SabineRaith
Raith a,, Frédéric
FrédéricGoettmann
Goettmann b,, Markus
MarkusAntonietti
Antonietti c,,and
and Werner
WernerKunz
Kunz a
Institute
of
Physical
and
Theoretical
Chemistry,
University
of
Regensburg,
D-93040
Regensburg,
Institute of Physical and Theoretical Chemistry, University of Regensburg, D-93040 Regensburg, Germany
Germany
bbInstitut de Chimie Séparative de Marcoule, UMR 5257, Site de Marcoule, BP 17171, 30207 Bagnols sur Cèze, France
Institut de Chimie Séparative de Marcoule, UMR 5257, Site de Marcoule, BP 17171, 30207 Bagnols sur Cèze, France
CCMax-Planck-Institute of Colloids and Interfaces Scientific Campus Golm, 14476 Potsdam, Germany
Max-Planck-Institute of Colloids and Interfaces Scientific Campus Golm, 14476 Potsdam, Germany
aa
Alcohol
INTRODUCTION
INTRODUCTION
Supercritical water has recently made its way into organic synthesis as
environmentally friendly solvent, due to its low dielectric constant acting
similar to conventional organic solvents. In addition, the high dissociation
constant stands for an increased proton concentration which provides a
potential medium for acid-based catalyzed reactions. [1-3]
However, supercritical condition means working at temperatures above 374°C
and pressures over 218 atm. This work, thus, attempted to lower
temperatures into regions of 150-200°C, where the inherent pressure can be
handled more easily, beside the benefit of saving energy.
In order to reach lower reaction temperatures, we investigate on the addition
of inorganic salts as potential catalysts.
For a model reaction, alcohols have been elected, in an attempt to shed light
into the reaction pathway of hydrothermal carbonization of biomass [4,5]. In
this area, the focus lies in defunctionalization of these complex molecules
(mainly the removal of hydroxyl groups), to get to basic hydrocarbons needed
in industry. The results can open up alternative pathways for sustainable
organic synthesis in hot water. [6]
Below some of the results of our studies of the influence of salt addition on
hydrothermal reactions are shown. As model reaction we chose the
dehydration of 1-phenyl-1-propanol. Whereas cation variation has apparently
only little impact on the 1-phenylpropene yield in the shown examples, a
change of the salt anion has drastic consequences on the reaction yield.
Arranging the anions due to their basicity, there we found agreement to some
extent. Also, the yield of trans-phenylpropene with increasing salt
concentrations mimicks somewhat the behaviour of the water dissociation
constant at the present temperatures and NaCl loadings [compare ref. 7]. This
leads us again to the conclusion that the amount of protons is the main reason
for changes in the trans-phenylpropene yield.
B
conversion rate of trans-phenylpropene [%]
1.0
0.8
0.6
x (cis-phenylpropene)
x (trans-phenylpropene)
x (1-phenyl-1-propanol)
0.4
0.2
0.0
Me4NCl
CsCl
KCl
NaCl
LiCl
1.0
NaClO4
92%
(92%)
7%
(6%)
OH
3%
(4%)
80%
(58%)
1%
(1%)
OH
85%
(88%)
1%
(0%)
1%
(0%)
0%
(1%)
72%
(95%)
exo-Norborneol
OH
HO
OH
5%
(0%)
Pinacol
HO
HO
O
2%
(0%)
O
OH
1-Phenyl-1,2ethanediol
OH
O
O
2%
(1%)
OH
65%
(84%)
α-Terpineol
OH
HO
OH
9%
(16%)
32%
(33%)
27%
(9%)
O
O
meso-Hydrobenzoin
(-)-Isopulegol
RESULTS
RESULTS AND
AND DISCUSSION
DISCUSSION
1%
(2%)
1-Methylcyclohexanol
99%
(99%)
Side product
0%
(0%)
OH
1,2,3,4-Tetrahydro-1naphthol
Main product
1%
(1%)
OH
1-Phenyl-1-propanol
(-)-Carveol
A
Structure of substrate
1-Heptanol
0%
(0%)
96%
(83%)
4%
(17%)
1%
(3%)
31%
(29%)
23%
(24%)
0%
(0%)
35%
(38%)
16%
(3%)
17%
(6%)
OH
2%
(2%)
11%
(1%)
Table 1: Overview of the reactions of different alcohols at 200°C in 1M NaCl solution, compared to pure
HTW. The figures next to the molecular formula refer to the relative abundance of the substance in the
extract, compared to 5mmol substance of starting material before the reaction. The top figures belong to a
reaction in 1 M salt solution, whereas the figures in brackets correspond to a reaction in pure water.
In addition to the studies on 1-phenyl-1-propanol, we screened different
alcohols for their reactivity. Simple primary alcohols present themselves stable
under these conditions, although traces of isomerisation can be found. In
contrast, good yields are achieved for secondary and tertiary alcohols in general,
where already simple polyols allow complicated decomposition cascades. It is
difficult to establish a general rule about the influence of NaCl on these
reactions, both in term of conversion and selectivity. There is no obvious
relation between the conversion rate and the water solubility of the alcohol, or
the boiling point, or the polarity of the transition state.
NaNO3
NaCl
0.8
NaI
NaBr
0.6
NaPhSO3
NaHSO4
O
OH
dehydration 2x
pinacol rearrangement
0.4
linear fit: y = -0.054x + 0.353
Diels-Alder reaction
OH
0.2
Na2HPO4
0.0
1/2 CaCl2 1/2 BaCl2
-12
-10
-8
-6
-4
-2
0
Figure 3: reactions pathways of pinacol according to table 1.
NaF
NaTFA
2
4
6
8
pKa of the corresponding acid
C
D
0.9
1.0
CONCLUSION
CONCLUSION
0.8
x (trans-phenylpropene)
0.7
0.6
x (cis-phenylpropene)
x (trans-phenylpropene)
x (1-phenyl-1-propanol)
0.5
0.4
0.3
0.2
0.5
200°C
180°C
160°C
140°C
0.1
0.0
0
1
2
3
4
5
0.0
0.00
NaCl concentration [ M]
0.25
0.50
0.75
1.00
NaCl concentration [ M]
Figure 1: Yield of trans-Phenylpropene from 1-Phenyl-1-propanol versus different variations of the reaction
conditions. (A): Variation of the salt cation. (B): Variation of the salt anion. (C): Variation of the NaCl
concentration. (D): Variation of the reaction temperature within the marked area in graph C. Standard
reaction conditions equal to 5 mmol 1-Phenyl-1-propanol and 10 mmol NaCl in 10 mL H2O in a stainless steel
autoclave with PTFE inlet, at 180°C and for 16h in the oven.
[1] A. Katritzky, D. Nichols, M. Siskin, R. Murugan, M. Balasubramanian, Chem. Rev. 2001, 101,
837-892
We showed that it is possible to promote the dehydration of 1-phenyl-1propanol in HTW by the addition of salt. The efficiency of salt addition
decreases with increasing temperature, although the overall yield of transphenylpropene decreases. It is assumed that this behaviour correlates with the
intrinsic pH value at the corresponding temperature, pressure, salt
concentration, as well as the salt type.
Furthermore, a screening of various alcohols shows that the prediction of
reactivity, salt catalysis, and the variety of products is difficult. Nevertheless, we
observe that also pinacol rearrangements, aldol condensations, Diels-Alder
reactions and Friedel-Crafts reactions can take place under these conditions.
[4] M. Titirici, A. Thomas, M. Antonietti, New J. Chem. 2007, 31, 787-789
[2] N. Akiya, P. Savage, Chem. Rev. 2002, 102, 2725-2750
[5] T. Werpy, G. Petersen, Top Value added Chemicals from Biomass I, august 2004,
available online at www.osti.gov/bridge
[3] Y. Ikushima, K. Hatakeda, O. Sato, T. Yokohama, M. Arai, J. Am. Chem. Soc. 2000, 122,
1908-1918
[6] S. Raith, F. Goettmann, M. Antonietti, W. Kunz, submitted to ChemSusChem
[7] R. Busey, R. Mesmer, J. Chem. Eng. Data 1978, 23, 175-176