C O M M U N I C A T I O N S
In this case, UV-vis spectra cannot be used to monitor the
reaction speed because both the reactant trans-stilbene and the
product 1,2-diphenylethane absorb in the UV range that overlapped
1
with the surfactants. Therefore, H NMR spectroscopy was used
for identification of the product. Two minutes after the injection
of trans-stilbene and hydrogen in the microemulsion system, the
product was collected into acetone using the RESS technique. The
NMR spectrum of this solution indicates that almost all trans-
stilbene (>95%) in the reactor was converted to 1,2-diphenylethane.
Washing of the reactor with acetone after depressurizing showed
no detectable amount of trans-stilbene. In the absence of hydrogen
gas or PdCl2, NMR peaks of 1,2-diphenylethane were not observed.
Figure 2. (a) Variation of UV-vis spectra of 4-methoxycinnamic acid
with time during the hydrogenation process in CO2 at 20 °C and 200 atm.
Each spectrum was taken at 20-s intervals starting from zero time, spectrum
1. (b) Variation of the 4-methoxycinnamic acid absorption with time at
300 nm and 200 atm at (0) 20, (4) 35, and (]) 50 °C.
2a were taken at 20-second intervals after the injection of hydrogen
and 4-methoxycinnamic acid into the water-in-CO2 microemulsion
with PdCl2 in the water core (W ) 20). The first spectrum obtained
immediately after the injection (spectrum 1) was identical to that
of 4-methoxycinnamic acid dissolved in CO2. The broad absorption
peak centered around 300 nm decreased gradually and a new
absorption peak centered around 270 nm appeared. After about 2
min, the absorbance at 300 nm dropped to the baseline level. The
absorption peak (270 nm) in spectrum 2 (Figure 2a) was consistent
with that of 4-methoxyhydrocinnamic acid. In the absence of PdCl2
in the microemulsion, the absorption peak of 4-methoxycinnamic
acid did not show a measurable decrease after the injection of the
olefin and hydrogen into the fiber-optic reactor. The results indicate
that the Pd nanoparticles formed in the water core of the CO2 micro-
emulsion are capable of catalyzing hydrogenation of 4-methoxy-
cinnamic acid to 4-methoxyhydrocinnamic acid as shown in eq 1.
The water-in-CO2 microemulsion system can dissolve ionic spe-
cies and hydrophilic organic compounds in the water core. This
property can be used to perform hydrogenation of water-soluble
compounds in CO2. To demonstrate this, maleic acid was chosen
as a water-soluble starting material for this experiment (eq 3). After
formation of the microemulsion (W ) 20) with the water core con-
taining 0.04 M PdCl2 (total amount 2.2 × 10-3 mmol in the system)
and 1.8 M maleic acid (9.6 × 10-2 mmol in the system), 10 atm of
hydrogen in CO2 (total pressure ) 200 atm at 20 °C) was injected
into the system. Two minutes after injection, the product was col-
lected into acetone using the RESS technique. After evaporation of
acetone, the product was dissolved in D2O. The NMR spectra clear-
ly indicated that only succinic acid was detected (>95% conversion
based on the detection limit of the maleic acid peak). Washing of
the reactor with acetone after depressurizing showed no detectable
amount of maleic acid in NMR. In the absence of hydrogen gas or
PdCl2, only maleic acid NMR peaks were observed in this system.
Our results indicate that the Pd nanoparticles in the water-in-
CO2 microemulsion can also catalyze other hydrogenation processes
such as the conversion of the nitro group (NO2) to amine (NH2).
For example, the hydrogenation of nitrobenzene to aniline was
completed (>99%) within 30 min in supercritical CO2 at 50 °C
and 200 atm. These and other hydrogenation examples will be
detailed in a separate report.
Figure 2b shows the decrease in the absorbance at 300 nm with
time for the hydrogenation of 4-methoxycinnamic acid in liquid
CO2 at 20 °C and in supercritical CO2 at 35 and 50 °C. The speed
of the hydrogenation process is much faster in the supercritical CO2
phase (35 and 50 °C) compared with that in the liquid CO2 phase
(20 °C). The hydrogenation process at 50 °C under the specific
conditions was virtually completed in 20 s. The absorbance in
logarithmic scale for both the liquid and supercritical CO2 experi-
ments decreases linearly with time, suggesting the hydrogenation
process follows first-order kinetics. The apparent rate constants
obtained from the slopes are about 1.1 × 10-2, 6.9 × 10-2, and
9.4 × 10-2 sec-1 at 20, 35, and 50 °C, respectively. The difference
in the reaction rate between 20 and 35 °C was much larger than
that between 35 and 50 °C. The hydrogenation reaction probably
is controlled by the diffusion of the reactant from bulk CO2 to the
palladium nanoparticle surface. The diffusion coefficient of CO2
dramatically changes at the critical point (31 °C).
Acknowledgment. This research was supported by a DEPSCoR
grant (DAAD19-01-1-0458) from the Army Research Office
(ARO). H.K. thanks Kyung Hee University, Korea, for a one-year
sabbatical leave at the University of Idaho.
Supporting Information Available: NMR spectra (PDF). This
Immediately after completion of the UV-vis spectrum change
(after 20 s at 50 °C), the product was collected in acetone-d6 via
the RESS method for 1H NMR spectroscopy (BRUKER, AMX 300).
The NMR results are consistent with UV-vis spectra shown in
Figure 2, indicating that hydrogenation of 4-methoxycinnamic acid
to 4-methoxyhydrocinnamic acid at 50 °C was completed (>99%)
within 20 s. The reactor after depressurizing was also washed with
acetone-d6 for NMR analysis. No detectable amount of 4-meth-
oxycinnamic acid was observed in the spectrum. In the absence of
hydrogen gas or PdCl2, NMR spectra of the control systems did
not show any detectable 4-methoxyhydrocinnamic acid peaks.
Hydrogenation of another CO2-soluble olefin trans-stilbene uti-
lizing the Pd nanoparticles in the water-in-CO2 microemulsion (eq
2) was also investigated in liquid CO2 (20 °C and 200 atm).
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