Kinetics of the Formation of Solketal in the Presence of Sulfuric Acid

Article abstract of DOI:10.1134/S002315841804002X

The kinetics of reversible solketal synthesis in the presence of sulfuric acid as a catalyst is studied. The parameters of kinetic equations describing direct and reverse reactions are found.

Full text of DOI:10.1134/S002315841804002X

ISSN 0023-1584, Kinetics and Catalysis, 2018, Vol. 59, No. 4, pp. 504–508. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © G.S. Dmitriev, A.V. Terekhov, L.N. Zanaveskin, A.L. Maksimov, S.N. Khadzhiev, 2018, published in Kinetika i Kataliz, 2018, Vol. 59, No. 4, pp. 488–492.
Kinetics of the Formation of Solketal in the Presence of Sulfuric Acid
a,
a
a
a
a, †
G. S. Dmitriev *, A. V. Terekhov , L. N. Zanaveskin , A. L. Maksimov , and S. N. Khadzhiev
a
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
*
e-mail: dmitriev.gs@mail.ru
Received September 5, 2017
Abstract—The kinetics of reversible solketal synthesis in the presence of sulfuric acid as a catalyst is studied.
The parameters of kinetic equations describing direct and reverse reactions are found.
Keywords: solketal, glycerol, acetone, sulfuric acid, kinetics
DOI: 10.1134/S002315841804002X
INTRODUCTION
,2-Dimethyl-4-hydroxymethyl-1,3-dioxolane
solketal) belongs to a new type of component for glycerin with acetone:
automotive fuels produced from natural renewable raw
materials [1]. Solketal is formed from a reaction of
2
(
O
OH + H2O
(I)
O
H+
H+
2
,2-Dimethyl-4-hydroxymethyl-1,3-dioxolane
HO
OH +
OH
OH
O
(II)
glycerol
acetone
(AC)
+ H2O
O
O
(GL)
2,2-Dimethyl-1,3-dioxan-5-ol
Simultaneously, a zolketal isomer, 2,2-dimethyl- parameters for the synthesis of solketal in the presence
,3-dioxan-5-ol, is formed.
of sulfuric acid.
1
In the presence of acid catalysts the reaction pro-
ceeds fairly quickly even at room temperature. Zeolites
2–8] or ion-exchange resins [2, 4–9] can be used as
heterogeneous catalysts. Acids, for example, H SO , pure), sulfuric acid (chemically pure), and a 0.1 M
EXPERIMENTAL
Glycerol (analytically pure), acetone (chemically
[
2
4
are used as homogeneous catalysts [8, 10]. Sulfuric solution of NaOH were used as the reagents in the
experiments.
acid is the most active catalyst and hence the techno-
logical scheme for the production of solketal in the
The experiments were carried out in batch and
presence of H SO is considered to be the most ratio- continuous mixing reactors at the initial molar ratio of
2
4
glycerol : acetone = 1 : 5. Previously, this component
ratio was shown to be optimal [8], as a relatively high
nal scheme [8].
The optimization of the reaction unit is an import- conversion of glycerol (~80%) can be achieved at this
ant stage in the development of this processing ratio with reasonable costs for the separation of reac-
scheme. For the solution of this problem, a kinetic tion products by rectification.
model of the reaction should be considered. However,
In the batchwise experiments, 25 g of glycerol (GL)
no information on this model was found in the litera- and 80 g of acetone (AC) were introduced into a 200-mL
ture. Thus, this paper aims to determine the kinetic glass reactor, equipped with a jacket, a reflux con-
†
Deceased.
504

KINETICS OF THE FORMATION OF SOLKETAL
505
denser, and a thermocouple sleeve. The reaction mix- equipped with a column (3 mm × 2 m) packed with a
ture was stirred with a magnetic stirrer. The tempera- Polysorb-1 phase.
ture in the reactor was regulated by feeding a heat car-
rier to the jacket. After the predetermined temperature
RESULTS AND DISCUSSION
At the initial instant of time the reaction proceeds
was reached, 0.0036–0.1215 g of the catalyst (sulfuric
acid) was introduced to the reactor under continuous
stirring of the reaction mixture. The catalyst concen- in a two-phase system due to the limited miscibility of
–
3
tration in the reaction mixture ranged from 3.3 × 10
glycerol with acetone. As the products (solketal and
water) accumulate, the reaction mixture becomes
homogeneous. The kinetics of the reaction were stud-
ied, taking into consideration only the data obtained
after the complete homogenization of the system. A
typical view of the accumulation curves for organic
–1
to 1.2 × 10 wt %. The reaction time was started from
the moment the catalyst was introduced. After
homogenizing the reaction mixture, the reaction mix-
ture was sampled and analyzed.
In experiments in the continuous mode, a 130-mL products (2,2-dimethyl-4-hydroxy-methyl-1,3-diox-
electrically heated glass reactor equipped with a stirrer, olane and 2,2-dimethyl-1,3-dioxan-5-ol) is shown in
a reflux condenser, a thermocouple sleeve, and a side Fig. 1.
outlet was used. The rotation speed of the stirrer was
It can be seen that 2,2-dimethyl-4-hydroxy-
methyl-1,3-dioxolane is the main product and the
concentration of 2,2-dimethyl-1,3-dioxane-5-ol
decreases with time. The formation of 2,2-dimethyl-
1
200 rpm, providing the efficient mixing of the reac-
tion mass. The temperature in the reactor was moni-
tored using a set consisting of a single-channel micro-
processor programmable temperature meter TRM-
4
-hydroxymethyl-1,3-dioxolane is likely to be ther-
1
01, an external relay, and a transformer. The standard
modynamically more advantageous and the initially
formed 2,2-dimethyl-1,3-dioxan-5-ol isomerizes in
the synthesis until equilibrium is reached.
reaction temperature was 35°C. Glycerol containing
0
.066 wt % of sulfuric acid was continuously fed into
the reactor with a pump at a flow rate of 16.5 g/h.
Independently, acetone was introduced into the reac-
tor at a flow rate of 52 g/h. The contact time was
The fraction of 2,2-dimethyl-1,3-dioxane-5-ol in
the equilibrium mixture is less than 2–3%. Besides,
both isomers can be used as a fuel component. Thus,
the kinetics of glycerol ketalisation were studied taking
into account the total number of isomers. Further, the
isomer mixture is referred to as solketal.
1.73 h. The reaction products were removed from the
reactor top through the side outlet. Only the experi-
ments with temperature fluctuations of less than 1°C
and a material balance accuracy better than 5% were
taken into account.
According to the existing concepts on the forma-
tion mechanisms for semiketals (hemiacetals) and
The reaction products were analyzed using a Tsvet- ketals (acetals) [13, 14], the preparation of solketal can
00 gas-liquid chromatograph (OAO Tsvet, Russia) be described by the following sequence of reactions:
8
+
OH
H
δ−
H+
H+
+
O
C O + O CH2
C
O
CH2
C
CH + H O
(III)
2
2
+
δ
HO CH
CH OH
HO CH
Acetone HO CH
CH2OH
CH OH
2
2
Glycerol
O
CH2
O
O
CH2
CH
+ H+
C
C
(IV)
(V)
.
O+ CH
+
O
C
CH2
H
CH OH
CH OH
2
2
2
,2-Dimethyl-4-hydroxymethyl-1,3-dioxolane
HO CH
O
CH2
CH HO
O+ CH2
O
CH2
CH HO + H+.
CH2
HO CH2
C
C
O
2,2-Dimethyl-1,3-dioxan-5-ol
H
In the first stage, the nucleophilic addition of glycerol the second stage, a cyclic ketal is formed through the inter-
to the carbonyl group takes place to form a semiketal. In mediate formation of a carbenium ion. The formation of a
KINETICS AND CATALYSIS Vol. 59 No. 4 2018

5
06
DMITRIEV et al.
Concentration, wt %
0
Concentration of SK, mol/L
1.8
.7
1.6
.5
.4
.3
1.2
.1
3
2
2
1
1
5
2
3
1
0
1
1
5
1
1
0
1
5
0
2
1
0
100
200
300
400
500
100
200
300
400
500
Time, min
Time, min
Fig. 1. Time dependence of concentrations of 2,2-
dimethyl-4-hydroxymethyl-1,3-dioxolane (1) and 2,2-
dimethyl-1,3-dioxan-5-ol (2) in batch reactor at 30°C. Load-
ing: glycerol, 25 g; acetone, 80 g; sulfuric acid, 0.0036 g.
Fig. 2. Time dependence of solketal concentration in reac-
tion mixture at different concentrations of sulfuric acid,
–2
–3
–3
wt %: (1) 1.2 × 10 ; (2) 6.5 × 10 ; (3) 3.3 × 10 . Reaction
temperature, 30°C; loading: glycerol, 25 g; acetone, 80 g.
carbenium ion is assumed to be the limiting stage of the
Integrating Eq. (2) allows the time dependence of
process. At the same time, the carbenium ion is an inter- the glycerol concentration to be estimated:
mediate in the isomerization of 2,2-dimethyl-1,3-dioxan-
5-ol to 2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane.
1
(
K − K )τ =
1
2
(
1)
(2)
According to the mechanism mentioned above, the
formation of solketal should be described by the fol-
lowing kinetic equation:
(
С
− СGL)
GL
(3)
(
1)
(2)
С
GL,0
− С
− С
С
− СGL
GL GL
2)
×
ln
= Y.
(
(1)
С
GL,0
С
− СGL
GL GL
w = k С
C С − k С С СН2О,
H2SO4 SK
(1)
1
H2SO4 GL АC
2
where С are the molar concentrations of glycerol, ace- Here τ is the reaction time, CGL is the current concen-
i
tone, solketal (SK), water, and sulfuric acid in the tration of glycerol (mol/L), CGL0 is the initial concen-
reaction mixture (mol/L); k is the constant of the
1
(1)
(2)
tration of glycerol (mol/L), and С and С are the
GL
GL
direct reaction rate for the formation of solketal
L mol s ); and k is the constant of the reverse
reaction rate for the hydrolysis of solketal (L mol s ).
roots of the quadratic equation found from the expres-
sion for the equilibrium constant
2
–2 –1
(
2
2
–2 –1
After the catalyst concentration is included in the
rate constant, Eq. (1) takes the form
K1
K2
С С
SK Н2О
K
=
=
с
С С
GL АC
(4)
w = K C С − K С СН2О,
(2)
1
GL АC
2
SK
(СGL,0 − С )(С
+ СGL,0 − СGL)
GL
Н2О,0
=
,
where K and K are the rate constants of the direct and
1
2
С
GL(САC,0 − СGL,0 + СGL)
–1 –1
reverse reactions, respectively (L mol s ).
The adequacy of Eq. (1) was verified experimen- where С and С are the equilibrium and initial con-
i
i,0
tally by varying the concentration of the sulfuric acid.
In Fig. 2 kinetic curves describing the formation of
solketal at different concentrations of sulfuric acid
centrations of the ith substance (mol/L), respectively.
In the coordinates of Eq. (3) the dependence of Y
–2
–3
–3
(
1.2 × 10 , 6.5 × 10 , and 3.3 × 10 wt %) are given, on τ should be a straight line starting from zero, and
with the other conditions being equal. The maximum the tangent of the slope angle is equal to the difference
conversion of glycerol was 79.5%.
of the rate constants: tanα = (K − K ). The experi-
1 2
mental dependences obtained are given in Fig. 3.
The experimental data were treated according to
Eq. (2), as the concentration of sulfuric acid in all the
experiments was the same. According to the reaction
equation (II), the concentrations of all the compo-
nents can be expressed through the initial glycerol
concentration.
The values K and K are equal to the product of the
1
2
concentration of the catalyst (sulfuric acid) and the
rate constants k and k , respectively. Thus, the values
1
2
of k and k can be calculated from this formula.
1 2
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KINETICS OF THE FORMATION OF SOLKETAL
507
–3
In Table 1 it is seen that the deviation of the rate
constants for the direct and reverse reactions from the
mean values is less than 2.4%. This indicates that the
experimental data are adequately described by the
kinetic equation (1).
Time ×10 , s
25 30
0
5
10
15
20

0.02
The experiments were carried out in the tempera-
ture range from 0 to 50°C in order to determine the
activation energy. In this range the conversion of glyc-
erol under equilibrium conditions ranged from 74.9 to
87.2%. The temperature dependences of the rate con-
stants k and k in the Arrhenius coordinates are

0.04
0.06
3

2
1

0.08
1
2
shown in Fig. 4.

0.10
Y
According to the data obtained from the experi-
ments, the kinetic equations for the direct reaction of
the formation of solketal (6) and the reverse reaction
of hydrolysis (7) have the form:
Fig. 3. Linearization of kinetic curves of solketal synthesis
according to Eq. (3) at 30°C and different concentrations
–2
–3
of sulfuric acid, wt %: (1) 1.2 × 10 ; (2) 6.5 × 10
;
(
3) 3.3 × 10–3. Loading of glycerol, 25 g; acetone, 80 g.
−
87110
1
3
RT
w = 4.36 ×10 e
C
C С
1
H2SO4 GL АC
−1
(6)
(7)
The equilibrium constant for the formation of
−1
(
mol L s ),
solketal in the presence of sulfuric acid in the tempera-
ture range 0–50°C is described by the equation [8]
−
101670
1
6
RT
w = 1.8 ×10 e
CH2SO4C С
SK Н2О
2
1
T
lnK = 1745.8 − 6.0.
(5)
c
−1
−1
(mol L s ).
According to Eq. (2), it is a first-order reaction
For the industrial implementation of the process, it
with respect to all the reagents. Until the equilibrium is preferable to use a continuous flow mixing reactor.
is reached, the dependences shown in Fig. 3 are linear. The applicability of the suggested kinetic model for
This confirmed the adequacy of Eq. (2). In Fig. 3, a the calculation of such a reactor was estimated by
dependence of the slope of the straight lines on sulfu- comparing the experimental data with the calculation
ric acid concentration is observed. The reaction rate results from the kinetic equations (6) and (7). The
increases with the catalyst concentration, while the conditions for a continuous flow mixing reactor were
equilibrium time becomes shorter.
described in the experimental section. The experi-
mental and calculated glycerol conversions were 68.4
and 69.0%, respectively. Thus, the experimental data
in both batch and continuous reactors can be assumed
to be adequately described by Eqs. (6) and (7). These
In Eq. (2), K and K can be easily calculated by
1
2
Eq. (5) from the slope of the line and the equilibrium
constant at a given temperature. Having divided these
values by the concentration of sulfuric acid, we find equations can be used in the calculation of the instal-
the rate constants for the direct (k ) and reverse (k ) lation for the synthesis of solketal in the presence of
1
2
reactions in Eq. (1). The data are given in the table.
sulfuric acid as a catalyst.
Table 1. Results of processing experimental data obtained at different concentrations of sulfuric acid, with other conditions
being equal (30°C; glycerol, 25 g; acetone, 80 g)
Concentration of H SO4
k1
k2
2
tanα = K − K
Kc
1
2
L2 mol–2 s–1
mol/L
.0 × 10–3
wt %
1.2 × 10–2
6.5 × 10–3
3.3 × 10–3
−1.24 × 10–5
−6.98 × 10–6
−3.11 × 10–6
4.11 × 10–2
4.06 × 10–2
3.94 × 10–2
4.04 × 10–2
5.33 × 10–2
5.27 × 10–2
5.12 × 10–2
5.24 × 10–2
0
.77
.77
.77
1
.8 × 10–4
.9 × 10–4
0
0
5
2
Mean value
KINETICS AND CATALYSIS Vol. 59 No. 4 2018

5
08
DMITRIEV et al.
3
–1
the Federal Agency for Scientific Organizations
FASO) Russia.
1
/T ×10 , К
(
3
.0
3.1
3.2
3.3
3.4
3.5 3.6 3.7
0








1
2
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–2 –1
lnk [L mol s ]
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ACKNOWLEDGMENTS
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Arinicheva, Yu.A., Dzyubenko, A.A., and Khadzhiev, S.N.,
Pet. Chem., 2011, vol. 51, no. 1, p. 61.
The work was carried out as part of a state task of
the Institute of Petrochemical Synthesis of the Rus-
sian Academy of Sciences with the financial support of
Translated by D. Yakusheva
KINETICS AND CATALYSIS
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No. 4
2018