13098 J. Phys. Chem. B, Vol. 107, No. 47, 2003
Gao et al.
mixture. However, the effect of pressure on the Kx is not
significant outside the critical region. The sensitivity of Kx to
pressure is closely related with the KT of the reaction mixture.
The KT increases sharply as the pressure approaches the CP,
BP, and DP in the critical region, and the Kx is very sensitive
to pressure. The Kx increases significantly as pressure approaches
the DP and CP, whereas it decreases as pressure approaches
the BP; i.e., pressure has the opposite effect on the Kx in the
subcritical region and in the supercritical region. To tune the
conversion of the reaction by pressure effectively, it is necessary
to conduct the reaction in supercritical region. Peng-Robinson
equation of state can predict the Kx far from the critical region
very well. However, the difference of the predicted results and
the experimental data becomes larger as the reaction system
approaches CP, BP, and DP in the critical region.
Figure 7. Equilibrium constant (Kx) predicated by the PR equation of
state at 329.9 K.
To tune the reaction effectively by pressure, the reaction
system should be carried out in the critical region; i.e., the
critical parameters of the reaction mixture should be considered.
CO2 not only can be used as a clean medium for chemical
reactions but also can be used as green solvent to tune the critical
points and phase behaviors of reaction systems, and the reaction
may be optimized by pressure effectively. Reaction in the critical
region of the reaction mixtures is an interesting field where lots
of new concepts and new experimental findings are to be
investigated.
On the basis of the equation of state and mixing rules, the
fugacity coefficient of each component can be calculated by
the following equation.22
x a
∑
j
ij
x
V + (1 - 2)bmix
V
j
RT ln γi ) RT ln
+
ln
+
V - bmix
x
x
2bmix V + (1 + 2)bmix
x
amixbmixbi V + (1 + 2)bmix
biPV biRT
PV
ln
- RT ln
+
-
Acknowledgment. This work was financially supported by
National Key Basic Research Project (G2000048010) of China,
National Natural Science Foundation of China (20073056).
RT bmix
bmix
3
x
x
( 2bmix
)
V + (1 - 2)bmix
(18)
The values of Kx predicted are illustrated in Figure 7.
Comparing the experimental data in Figure 6 and the predicted
results in Figure 7, we can conclude that the predicted results
agree well with the experimental data as the reaction mixtures
are far from the critical point. However, the difference of the
predicted results and the experimental data becomes larger as
the reaction system approaches CP, BP, and DP in the critical
region. We think the clustering of the molecules in the critical
region may be one of the main reasons. The bulk composition
is used in the prediction process. This is reasonable as the
reaction mixture is far from the critical region because the
inhomogeneity in the mixture is not significant. In the critical
region, the local density and/or local composition are more
pronounced, and the local composition around the reaction
species may differ from that of the bulk significantly, and we
cannot consider this in the calculation process. Therefore, the
difference between the experimental and calculated results is
larger.
All the interesting phenomena are related with special
intermolecular interactions in the critical region of the mixtures.
Reaction in the critical region of the reaction mixtures is an
interesting field where lots of new concepts and new experi-
mental findings are to be investigated. Conducting reactions
under SC conditions have potential advantages to optimize many
reactions effectively and will solve more challenging problems
after our fundamental understanding of SCFs improves.
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4. Conclusion
The phase behavior, KT, and the Kx of the reaction system
for the transesterification between ethyl acetate and n-butanol
in compressed CO2 were studied systematically.
The Kx was very sensitive to pressure as the reaction mixture
approaches the CP, BP, and DP in the critical region where
both composition and pressure are close to the CP of the reaction