Reaction Acceleration Using Supercrital Water
J. Am. Chem. Soc., Vol. 122, No. 9, 2000 1909
a reactant for organic synthesis.18-23 Friedel-Crafts alkylation
reactions have been investigated using superheated H2O,18,23
which was found to serve as both the medium and the catalyst.
However, few research works have been reported on the use of
scH2O (e.g., above 648 K) for such “organic synthetic reac-
tions”,24,25 although scH2O has been used mainly for “break-
down” of organic reactants such as destruction of waste and
toxic organic compounds,26-28 production of liquid and gaseous
fuels from coal and biomass,29,30 and geochemical reactions.31
Therefore, the present work has been undertaken to explore the
further possibility of performing organic synthesis in scH2O.
The ion product (Kw) for water increases with increasing
temperature and pressure, reaches a maximum at a certain
temperature, and then remarkably decreases beyond the critical
point (or the critical temperature).32 For example, as the
temperature is increased at a fixed pressure of 25 MPa, log Kw,
is -11.16 at 473 K, has a maximum value of -11.01 around
523 K, and then decreases to -19.43 at 673 K. The Kw for
scH2O is about 5 orders of magnitude lower than that for
ambient liquid water, whereas the Kw for superheated H2O at
523 K is about 3 orders of magnitude higher than the value for
ambient water. Thus, it has been believed that superheated H2O
may promote acid-catalyzed reactions of organic compounds
because of a sufficiently higher H+ ion (proton) concentration
than in liquid water. Several acid-catalyzed organic synthetic
reactions were confirmed to actually proceed in superheated H2O
without any acid catalysts.18,19,22,23
in the near-critical region. Hence, we expect that acid-catalyzed
organic syntheses can proceed under a scH2O atmosphere even
in the absence of any acid catalysts.
In this paper, we first report interesting examples of non-
catalytic pinacol rearrangement with very excellent performance
and of noncatalytic Beckmann rearrangement in the near-critical
region in scH2O, which are both well-known to be catalyzed
by strong acids in conventional solutions. Beckmann rearrange-
ment of cyclohexanone-oxime into ꢀ-caprolactam is a com-
mercially important reaction for the production of synthetic
fibers, and the pinacol rearrangement is also important as a
fundamental way of producing aldehydes and ketones. However,
both reactions have disadvantages that highly concentrated
monobasic acids or solid acids of short lifetime should be used
as catalysts, and byproducts of low commercial value are formed
frequently.38,39 It would be significant if the use of scH2O could
promote these pinacol (eq 1) and Beckmann (eq 2) rearrange-
ments. The reactions in scH2O have been studied with an in
situ high-pressure and high-temperature FTIR spectroscopy.
Using an in situ Raman spectroscopy, we have recently found
that the extent and strength of hydrogen bonding in scH2O are
reducing peculiarly near the critical point.33 Raman studies can
clarify microscopically intra- or intermolecular forces, and imply
that the strength of hydrogen bonding weakens uniquely in the
near-critical region on a short time scale, suggesting that the
evolution of protons is significantly promoted, where dimers
and monomers are predominant.34-36 Some monomers could
be further broken into protons near the critical point, and this
dynamic change in the “local” water structure would induce
the evolution of protons.33,37 If the proton cannot escape, the
“local” proton concentration would be high and might have a
significant influence on reactivities in scH2O region, especially
II. Results
H2O Absorption. Although many of reactions in scH2O were
reported to be strongly affected by pressure and temperature
changes,24 these interesting effects have not been examined well.
Recently the Raman spectra of H2O in supercritical and
superheated conditions have indicated a feature of the OH
symmetric stretching at high pressures and temperatures.33 Only
the OH symmetric stretching (ν1) around 3400-3600 cm-1 is
active,40 and detailed analysis of the ν1 frequency can help us
to better understand the features of H2O molecules. Changes in
temperature and pressure in the near-critical region were found
to result in a uniquely weakening of the strength of the hydrogen
bonding.33 The present FTIR measurements show that liquid
water has two main absorption bands in the mid-IR region which
are attributed to intramolecular displacements, one having a very
intense and broad band centered around 3400 cm-1 and
corresponding to the OH symmetric and asymmetric stretching
(ν3) mode, and the other a maximum at 1645 cm-1 correspond-
ing to the bending (ν2) mode. The typical representative spectra
of scH2O in the mid-IR region are presented in Figure 1, where
the temperature was raised from 603 to 708 K at a fixed pressure
of 22.6 MPa just above the critical pressure (Figure 1a), and
also the pressure was varied from 22.6 to 28.0 MPa at a fixed
temperature of 650 K (Figure 1b). The integrated intensity of
the bending mode decreases significantly in the near-critical
region with decreasing pressure and increasing temperature,
while the peak position (frequency) is insensitive to pressure
and temperature. In the frequency region of stretching vibration,
the absorption band weakens and blue shifts as the temperature
is raised above the critical temperature. The large intensity of
the stretching band points out the importance of intermolecular
(19) Kuhlmann, B.; Arnett, E. W.; Siskin, M. J. Org. Chem. 1994, 59,
3098.
(20) Katritzky, A. R.; Balasubramanian; Siskin, M. J. Chem. Soc., Chem.
Commun. 1992, 1233.
(21) Kuhlmann, B.; Lapucha, A. R.; Murugan, R.; Luxem, F. J.; Siskin,
M.; Brons, G. Energy Fuels 1990, 4, 493.
(22) An, J.; Bagnell, L.; Cablewski, T.; Strauss, C. R.; Trainor, R. W. J.
Org. Chem. 1997, 62, 2505.
(23) Chandler, K.; Deng, F.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A.
Ind. Eng. Chem. Res. 1997, 36, 2634.
(24) Savage, P. E. Chem. ReV. 1999, 99, 603.
(25) Sato, O.; Ikushima, Y.; Yokoyama, T. J. Org. Chem. 1998, 63, 9100.
(26) Hatakeda, K.; Ikushima, Y.; Ito. S.; Saito, N.; Sato, O. Chem. Lett.
1997, 245.
(27) Lee, D.-S.; Gloyna, E. F. J. Supercrit. Fluids 1990, 3, 249.
(28) Yang, H. H.; Eckert, C. A. Ind. Eng. Chem. Res. 1988, 27, 2009.
(29) Sealock, L. J., Jr.; Elliott, D. C.; Baker, E. G.; Butner, R. S. Ind.
Eng. Chem. Res. 1993, 32, 1535.
(30) Adschiri, T.; Shibata, R.; Sato, T.; Watanabe, M.; Arai, K. Ind. Eng.
Chem. Res. 1998, 37, 2634.
(31) Siskin, M.; Katritzky, A. R. Science 1991, 254, 231.
(32) Marshall, W. L.; Franck, E. U. J. Phys. Chem. Ref. Data 1981, 10,
295.
(33) Ikushima, Y.; Hatakeda, K.; Saito, N.; Arai, M. J. Chem. Phys. 1998,
108, 5855.
(34) Hoffmann, M. M.; Conradi, S. J. Am. Chem. Soc. 1997, 119, 3811.
(35) Matsubayashi, M.; Wakui, C.; M. Nakahara, M. Phys. ReV. Lett.
1997, 78, 2573.
(38) Burguet, M. C.; Aucejo, A.; Corma, A. Can. J. Chem. Eng. 1987,
65, 944.
(36) Matsubayashi, M.; Wakui, C.; Nakahara, M. J. Chem. Phys. 1997,
107, 9133.
(37) Gorbaty, Yu. E.; Kalinichev, A. G. J. Phys. Chem. 1995, 99, 5336.
(39) Holderich, W. F.; Roseler, J.; Heitmann, G.; Liebens, A. T. Catal.
Today 1997, 37, 353.
(40) Walrafen, G. E.; Chu, Y. C. J. Phys. Chem. 1995, 99, 8557.