The influence of water on the isomerization of α-pinene in a supercritical aqueous-alcoholic solvent

Article abstract of DOI:10.1134/S0036024408010081

The influence of water as a cosolvent and catalyst of the isomerization of α-pinene in a supercritical aqueous-alcoholic (ethanol) solvent was studied experimentally. At T = 657 K and p = 230 atm, an increase in the concentration of water in the reaction mixture was found to increase the rate of the reaction and its selectivity with respect to the desired product, limonene. Water exhibited the properties of an acid catalyst because of its ionization. Mathematical experimental data processing was performed to evaluate and separate the contributions of the radical and ionic paths to the total rate of the reactions that occurred during the thermal isomerization of α-pinene.

Full text of DOI:10.1134/S0036024408010081

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 1, pp. 62–67. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A. Ermakova, A.M. Chibiryaev, P.E. Mikenin, O.I. Sal’nikova, V.I. Anikeev, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 1, pp. 71–76.
CHEMICAL KINETICS
AND CATALYSIS
The Influence of Water on the Isomerization of a-Pinene
in a Supercritical Aqueous–Alcoholic Solvent
a
b, c
a
b
a
A. Ermakova , A. M. Chibiryaev , P. E. Mikenin , O. I. Sal’nikova , and V. I. Anikeev
a
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 5, Novosibirsk, 630090 Russia
b
Vorozhtsov Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences, Novosibirsk, Russia
c
Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia
e-mail: anik@catalysis.nsk.su
Received October 20, 2006
Abstract—The influence of water as a cosolvent and catalyst of the isomerization of α-pinene in a supercritical
aqueous–alcoholic (ethanol) solvent was studied experimentally. At T = 657 K and p = 230 atm, an increase in
the concentration of water in the reaction mixture was found to increase the rate of the reaction and its selec-
tivity with respect to the desired product, limonene. Water exhibited the properties of an acid catalyst because
of its ionization. Mathematical experimental data processing was performed to evaluate and separate the con-
tributions of the radical and ionic paths to the total rate of the reactions that occurred during the thermal isomer-
ization of α-pinene.
DOI: 10.1134/S0036024408010081
INTRODUCTION
EXPERIMENTAL
The selection of new reaction media (solvents) and
conditions for the synthesis of the desired organic com-
pounds at high reaction rates and with the required
selectivity is an important problem of physical and
organic chemistry. In view of this, we studied the ther-
mal isomerization of α-pinene in supercritical solvents
The thermal isomerization of α-pinene in supercrit-
ical solvents containing ethanol and water in various
proportions was performed using a laboratory unit with
a tube flow reactor 1.75 mm in diameter of length 6 m.
3
The volume of the reactor was ~7 cm . The temperature
and pressure were maintained constant, 657 K and
[
1, 2], such as the lower alcohols (methanol, ethanol,
2
30 atm, respectively. The reaction mixture was pre-
and propanol-1). In all these solvents, virtually 100%
conversion of α-pinene was obtained. The products
formed were limonene, (neo + allo)ocimene, and
pared by mixing two independent liquid flows directly
at the entrance of the reactor. Flow 1 consisted of a 5 wt %
solution of α-pinene in rectified ethanol with 4 wt %
water (the molar composition of flow 1 was α-pinene,
(
α + β)pyronene. The yield of limonene, which was
the main reaction product, reached 50–55% per mole of
α-pinene transformed. It was also shown in [1, 2] that
the experimental results did not contradict the supposed
radical reaction mechanism [3].
1
.7%; ethanol, 88.8%; and water, 9.5%). The second
flow was distilled water. The ratio between these flows
was varied from one experiment to another, whereas the
overall flow rate (6 ml/min) remained constant. The
concentration of water in the solvent increased from 2.5
to 68 vol % (from 9 to 89 mol %). The contact time
determined as the ratio between the volume of the reac-
tor and reaction mixture flow rate τ = V /Q was 70 s.
It is at the same time known that the isomerization
of α-pinene under acid catalysis conditions mainly fol-
lows the ionic mechanism (e.g., [4]). The role of such
an acid catalyst can be played by water [5, 6], which is
in the strongly dissociated state in the critical region of
its parameters. It was therefore of interest to study the
influence of water on this reaction as a catalyst on the
one hand and as a cosolvent (which is also attractive for
practical purposes) on the other.
r
mix
The critical parameters of the initial reaction mix-
ture calculated following the procedure developed in
[
7] varied over the temperature and pressure ranges
T = 520.8–630.6 K and p = 67.1–201.1 atm depend-
cr
cr
The purpose of this work was, first, to perform an ing on the content of water. The reduced critical param-
experimental study of the special features of the ther- eters T and p calculated as the ratio between the work-
R
R
mal isomerization of α-pinene in a supercritical solvent ing parameters T and p and the corresponding critical
containing ethanol and water depending on the concen- mixture parameters varied over the ranges T = 1.26–
R
tration of the latter and, secondly, evaluate the contribu- 1.04 and p = 3.43–1.14. The reaction mixture with
R
tions of the radical and ionic paths to the overall process such reduced parameters is usually considered to be in
of limonene formation.
the supercritical state [8].
6
2

THE INFLUENCE OF WATER ON THE ISOMERIZATION OF α-PINENE
63
Experiments were performed with (+)-α-pinene
>98%) from Aldrich. Reaction products were ana- 70
η, mol %
(
lyzed by chromato-mass spectrometry on a Hewlett-
Packard 5890/II gas chromatograph with an HP MSD
1
2
6
0
5
971 quadrupole mass spectrometer as a detector. We
used an HP-5 quartz column, inside diameter 0.25 mm,
of length 30 m. The stationary phase film thickness was
5
3
0
0
~
~
0
.25 µm (5% diphenyl–95% dimethylsiloxane copoly-
mer). The carrier gas was helium supplied at a constant
flow rate of 1 ml/min. The temperature of the vaporizer 20
was 280°ë. The temperature conditions inside the col-
3
umn were 50°ë (2 min), 50–200°ë (4 K/min), 200–
1
0
4
3
00°ë (20 K/min), and 300°ë (20 min). The ionizing
electron energy was 70 eV.
0
20
40
60
Qualitative analysis was performed by comparing
the retention indexes of the components and their com-
plete mass spectra with the corresponding data on the
pure compounds (if available), data of the Wiley7 mass
spectrometric base (375000 mass spectra), and cata-
logue [9]. The percent composition of mixtures was
calculated from the areas of chromatographic peaks
without the use of correcting coefficients.
cH2O, vol %
Fig. 1. Dependences of the yields η of organic products on the
concentration of water in the reaction mixture: (1) limonene,
(
(
2) sum of alloocimenes, (3) sum of pyronenes, and
4) other products. The concentrations are normalized by
the initial concentration of α-pinene in solution.
catalyst, which often causes an increase in reaction rate
RESULTS AND DISCUSSION
[5, 6]. The degree of water dissociation in supercritical
water–ethanol mixtures (and, accordingly, the concen-
The primary experimental data are shown in Fig. 1
symbols) in the form of the dependences of the mole
fractions of reaction products (limonene, the sum of
allo- and neo-alloocimenes, the sum of α- and
β-pyronenes, and the sum of all the other products) on
the content of water in the initial reaction mixture, all
other conditions being equal. Note that the complete
+
tration of H ions) can be evaluated after correctly cal-
(
culating the dissociation constants of sub- and super-
critical water depending on the temperature, pressure
(
density), and water fraction in the supercritical sol-
vent.
The ionic product of water K . The degree of water
w
conversion of α-pinene occurred under all experimental ionization is as a rule characterized by the ionic product
+
–
2
conditions.
value, K = [H ][OH ] (mol/kg) . Under normal condi-
w
It follows from Fig. 1 that the yield of limonene tions, logK
= –14. The ionic product, however,
w
increases from 57 to 69% as the concentration of water increases by six orders of magnitude as the temperature
in the reaction mixture grows. The yield of isomeric
alloocimenes in reaction products remains virtually
unchanged, and the concentration of pyronenes
decreases noticeably, possibly, because of an increase
in the yields of the other products. The composition of
α-pinene isomerization products becomes similar to
that obtained in pure supercritical ethanol [1, 2] as the
concentration of water decreases. These observations
are evidence that an increase in the concentration of
water in the supercritical solvent results in an obvious
increase in the yield of limonene in reaction products.
This can be related to an increase in the contribution of
the ionic mechanism of the isomerization of α-pinene.
3
grows to 800°ë at a density of 1 g/cm .Accordingly, the
concentration of H increases by three orders of magni-
tude [10]. We calculated K using the empirical equa-
tion from [11], which generalized numerous experi-
mental data obtained over wide temperature and pres-
sure (density) ranges,
+
w
logK = Q (T) + Q logρ ,
w
1
2
w
2
3
Q = A + B/T + C/T + D/T ,
(1)
1
2
Q = E + F/T + G/T .
2
2
Here, K is in (g-ion/kg) and the density of water ρ is
w
3
w
The mechanism of the influence of supercritical
water. The catalytic action of sub- and supercritical
water and the influence of water on the rate and selec-
tivity of organic reactions have been studied fairly thor-
in g/cm . According to [11], the coefficients of (1) are
A = –4.098, B = –3245.2 K,
5
2
C = 2.2362 × 10 K ,
+
oughly. It was proved that the concentration of H ions
7
3
increased substantially in the supercritical region of
water parameters as the density of the supercritical
medium grew because of water dissociation [10, 11].
For this reason, water exhibits the properties of an acid
D = –3.984 × 10 K ,
(2)
E = 13.957, F = –1262.3 K,
5
2
G = 8.5641 × 10 K .
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

6
4
ERMAKOVA et al.
2
ρw, g/cm3
The ionic mechanism of the isomerization of
α-pinene. Schemes of the radical (path (A)) and ionic
path (B)) mechanisms of the isomerization of
–
2
log K [(g-ion/kg) ]
w
4
0
6
2
(
1
0
0
0
0
.0
α-pinene are shown in Fig. 3. Path (A) includes the for-
mation of intermediate biradical 2 of the p-menthane
.8 structure at the first stage of α-pinene transformations.
Biradical 2 formally transforms directly into limonene
2
1
1
•
as a result of H atom migration. A key stage of the ionic
.6
mechanism (path (B)) is the formation of pinane-type
carbocation 3, which is easily formed from α-pinene in
.4 an acid medium as a result of the electrophilic attack by
+
H ions [13]. Cation 3 experiences skeletal rearrange-
ment into p-menthane-type carbocation 4, which then
.2
transforms into limonene.
The elementary events of the ionic mechanism can
be represented as follows:
0
1000
2
00
400
600
800
T, K
+
+
+
+
A + H
AH , AH
BH ,
(
4)
Fig. 2. Temperature dependences of the negative logarithm
of the ionic product and water density at p = 230 atm.
+
+
BH
C + H ,
+
+
where A is α-pinene, C is limonene, and AH and BH
are the corresponding intermediate cations (see scheme
in Fig. 3). Which of these three stages limits the rate of
the reaction depends to a substantial extent on reaction
conditions. As a rule, the first stage is rate-determining
in aqueous acid solutions at elevated temperatures.
Equation (1) is valid over the temperature and pressure
ranges 273–1273 K and 1–10000 atm, respectively.
By way of example, the temperature dependences of
the logarithm of the ionic product of water and water
density at 230 atm calculated by (1) are shown in Fig. 2.
The density of water was calculated following the NIST
standards [12]. We see that the ionic product of water
increases by three orders of magnitude as the tempera-
ture grows from 300 to 600 K. After the attainment of
the critical temperature of water between 600 and 700 K,
the ionic product decreases sharply because of a
decrease in the density of water in its transition into the
supercritical region as the temperature grows at a con-
stant pressure. Our experiments were performed in
water–ethanol mixtures (solutions). For this reason, we
used “the density of water in a mixture” ρw, mix rather than
Mathematical experimental data processing.
According to [1, 2], a simplified picture of α-pinene
transformations can be represented by the scheme
shown in Fig. 4. Taking into account the two reaction
paths specified above (see Fig. 3), the overall rate of
limonene formation can be written as
R1 = R1,1 + R1,2,
(5)
where R1,1 = k y is the rate of the reaction along
1,1
1
path (A) (the radical mechanism) and y is the mole
1
fraction of α-pinene in solution. The first order of this
reaction with respect to the initial substance was estab-
lished in [1]. As the rate-limiting stage of the reaction
along path (B) is the first stage of mechanism (4), the
rate of the reaction along this path can be written as
the density of pure water ρ in calculations of K by (1),
w
w
3
ρw, mix, g/cm mix = ρ c ,
(3)
w
w
3
where c is the content of water in the solution, cm
w
3
H O/cm mixture. Although the definition of the den-
2
+
R1,2 = k y [H ],
(6)
1,2
1
sity in a mixture ρw, mix by (3) is arbitrary to a degree, it
+
+
well reproduces the dynamics of changes in K as the where [H ] is the concentration of H ions in solution.
w
concentration of water in the solution alters.
The second and third stages of mechanism (4) are
(A)
(B)
H•
H+
–H+
H+
~
5
(1)
(3)
(5)
(2)
(4)
Fig. 3. Scheme of α-pinene (1) transformations into limonene (5) along two paths: (A) radical mechanism and (B) ionic mechanism.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

THE INFLUENCE OF WATER ON THE ISOMERIZATION OF α-PINENE
65
assumed to be equilibrium and characterized by equi-
librium constants K and K , respectively,
A1
R1
2
3
+
+
+
+
K = [BH ]/[AH ], K = [C][H ]/[BH ].
(7)
2
3
A2
R2
Reactions of mechanism (4) should be augmented by
the solution electroneutrality equation,
+
+
+
–
[
H ] + [AH ] + [BH ] = [OH ],
(8)
–
+
where [OH ] = K /[H ]. Determining the concentra-
A3
w
+
+
tions of the [AH ] and [BH ] cations according to (7)
R3
R4
and substituting them into (8), we can solve this equa-
+
tion with respect to [H ] as
A4
A5
+
1/2
[
H ] = (K /D') ,
(9)
w
Fig. 4. Simplified scheme of α-pinene thermal isomeriza-
tion paths; A1 is α-pinene, A2 is limonene, A3 is the sum of
alloocimenes, A is the sum of pyronenes, and A is other
[
K
C] 1
⎛
⎞
D' = 1 + ------ -- ----- + 1 .
(10)
⎝
⎠
K
3
2
4
5
products; R is the reaction rate along the jth direction.
j
Using (9) in (6), we obtain the rate of the reaction in the
form
The solution of the inverse problem. To check the
validity of the assumptions made above, we calculated
the parameters of model (12) using the experimental
data. The problem was solved by the minimization of
the objective function
1
/2
R1,2 = k y (K /D') .
(11)
1,2
1
w
An analysis of denominator (10) in (11) shows that the
accumulation of limonene in reaction products can
decelerate the reaction of its formation. At the same
time, as the absolute limonene concentrations in the
supercritical solution are low (≤0.05 wt %), the second
term in (10) is much smaller than one, and the equation
Nexp NS
exp
calc
2
S =
(yij – yij
)
min.
(15)
∑∑
j = 1 i = 1
+
for the rate of the reaction is first-order in H ions.
Here, N = 5 is the number of components and N = 8
S
exp
A mathematical description of the experiment. We
assume that the rates of all the other reactions of the
scheme shown in Fig. 4 are proportional to the concen-
is the number of experiments performed with different
water concentrations. The parameters of each experi-
ment are the constant temperature, pressure, and con-
+
tration of H ions. A mathematical model of the exper-
+
tact time values and the concentrations of H calculated
iment can then be represented by the system of ordinary
differential equations
as described above for water concentrations in each
experiment. The minimization was performed by the
combined “direct/indirect” gradient method developed
in [14]. According to this method, the calculated con-
dy /dτ = –(k + k )y , dy /dτ = k y ,
1
1
2
1
2
1 1
dy /dτ = k y – (k + k )y ,
(12)
3
2
1
3
4
3
calc
centration values yij are found by the integration of
dy /dτ = k y , dy /dτ = k y ,
the right-hand sides of Eqs. (12),
4
3
3
5
4 4
τ
0
1
τ = 0: y = y ,
calc
ij = y
0
i
+
1
(13)
y
+ Φ
i
(y, k, [H ])dτ.
(16)
∫
y = y = y = y = 0.
2
3
4
5
0
+
Subscripts i in y (mole fractions) correspond to the Here, Φ (y, k, [H ]) are the corresponding right-hand
i
i
numbers of components in the scheme (Fig. 4).
sides of differential equations (12). The integrals in (16)
Since reaction rates along all the paths (see scheme were calculated by the method described in [14]. The
in Fig. 4) are not zero in the absence of water in super- method that we used proved to be more reliable than the
critical ethanol (that is, under the conditions of the pre- corresponding indirect method [15] based on the calcu-
calc
dominantly radical mechanism of A-pinene transfor-
lation of yij by the integration of system (11), because
mations), the overall rate constants for reactions k (j =
j
the objective function formulated with the use of inte-
1
–4) in model (12) can be written as
grals (16) was a quadratic function of unknown param-
(14) eters. The confidence intervals of the parameters there-
fore sharply shrank and, accordingly, their statistical
+
k = k + k [H ], j = 1, 2, 3, 4.
j
j,1
j,2
Here, k is the contribution of the radical mechanism
j,1
reliability increased. At the same time, the objective
and k is the contribution of the ionic mechanism to the
j,2
calc
function of the indirect method with yij obtained by
overall reaction rate along all paths shown in the
scheme (Fig. 4).
numerically or analytically solving system (12) was
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

6
6
ERMAKOVA et al.
Reaction rate constants
in a supercritical solvent noticeably influences the rate
of the reaction at concentrations higher than 50 mol %.
The concentration of H ions increases sharply in the
2
–1
–7
j
kj,1 × 10 , s
kj,2 × 10 , kg/(g-ion s)
+
same region. The conclusion can therefore be drawn
that the ionic reaction mechanism, along with the radi-
cal mechanism, makes a noticeable contribution to the
rate of the thermal isomerization of α-pinene in a super-
critical aqueous–alcoholic solvent at water concentra-
tions higher than 50 mol %. This influence is most
1
2
3
4
8.971 ± 1.509
6.775 ± 1.139
1.698 ± 0.323
0.3917 ± 0.779
58.91 ± 29.59
11.44 ± 5.07
4.030 ± 1.610
0.3827 ± 0.279
noticeable for the rate R (see scheme in Fig. 4), that is,
the rate of limonene formation. It also follows from
1
Note: Root-mean-square deviation of the experimental data from
calculation results is 1.74%.
Fig. 6 that the k /(k + k ) ratio between the overall con-
1
1
2
stants, which characterizes reaction selectivity with
strongly nonlinear with respect to the unknown param- respect to limonene, substantially increases as the con-
eters. This expanded their confidence intervals and, as
a result, decreased their statistical reliability.
centration of water grows from 50 mol % up. At y
<
H O
2
5
0 mol %, the reaction mainly occurs by the radical
The rate constants obtained are listed in the table.
We see that all the constants are well conditioned and
characterized by acceptable confidence intervals.
mechanism.
To summarize, the use of water as a supercritical
The root-mean-square absolute deviation of the cosolvent in the isomerization of α-pinene results in a
experimental data from the calculation results is less substantial increase in the fraction of the desired prod-
than 2%.
uct (limonene) in reaction products at water concen-
trations of 50 mol % and higher. The suggestion was
Agreement between the experimental data and cal-
culation results is illustrated by the plots shown in Fig. 5, made that the presence of water in a supercritical
which correspond to the dependences of the yields of medium substantially increased the concentration of
+
+
the main reaction products on the concentration of ç
ç ions because of its dissociation. As a result,
ions in a supercritical aqueous–alcoholic solvent. Lines limonene was formed not only by the radical but also
are the calculation results obtained by the integration of by the ionic (catalyzed by acids) mechanism. The pro-
system (12), and symbols are the experimental data. It
follows from Fig. 5 that the model suggested with the
constant values found well describes the experimental
data.
cessing of the experimental data on the thermal
isomerization of α-pinene in a supercritical aqueous–
alcoholic solvent at various water concentrations with
the use of the mathematical model that took into
account the degree of dissociation of supercritical
water as a function of temperature and pressure sub-
The dependences of the overall rate constants k
and the concentration of H ions on the concentration of
1–4
+
water in solution are shown in Fig. 6. We see that water stantiated this suggestion.
η, mol %
0
0
.16
.12
1
0
.2
.8
7
0
1
1
5
0
~
0.08
~
2
5
2
0
0.4
2
0
.04
0
3
3
4
4
10 20 30 40 50 60 70 80 90
, mol %
y
H O
2
0
0.4
0.8
1.2
+
–10
[
H ] × 10 , g-ion/kg
+
Fig. 6. Dependences of the observed rate constants and H ion
Fig. 5. Experimental data (symbols) and calculation results
lines): (1) limonene, (2) sum of alloocimenes, (3) sum of
pyronenes, and (4) other products.
concentrations on the concentration of water in solution at
(
657 K and 230 atm. Lines 1–4 correspond to constants k –k
1 4
+
+
10
and line 5 is the concentration of H , [H ] × 10 g-ion/kg.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

THE INFLUENCE OF WATER ON THE ISOMERIZATION OF α-PINENE
67
ACKNOWLEDGMENTS
8. Ph. E. Savage, S. Gopalan, T. I. Mizan, et al., AIChE J.
1 (7), 1723 (1995).
4
This work was financially supported by the Russian
Foundation for Basic Research, project nos. 05-08-
5437 and 06-08-00024.
9
. A. V. Tkachev, Library of Chromato-Mass Spectrometry
Data on Volatile Materials of Plant Origin (Novosib.
Inst. Org. Khim. im. N.N. Vorozhtsova, Sib. Otd. Ross.
Akad. Nauk, Novosibirsk, 2006) [in Russian].
6
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