Skeletal isomerization of n-pentane in the presence of an AlCl 3-based ionic liquid

Article abstract of DOI:10.1134/S0965544110050105

The skeletal isomerization of n-pentane in the presence of a trimethylamine hydrochloride-aluminum chloride ionic liquid has been studied at 20°C. It has been found that the catalytic activity of the ionic liquid in n-pentane isomerization increases with an increase in the trimethylamine hydrochloride: aluminum chloride molar ratio. The optimum ratio of the catalyst complex components at the highest yields of the desired isoparaffins attained has been determined. The catalytic activity of the ionic liquid increased when a copper salt was added. The maximum total yield of isoparaffins is achieved with copper(II) chloride as an activating additive for the ionic liquid, whereas the maximum yield of isopentane is attained when copper(II) sulfate is introduced.

Full text of DOI:10.1134/S0965544110050105

ISSN 0965ꢀ5441, Petroleum Chemistry, 2010, Vol. 50, No. 5, pp. 376–380. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © D.R. Zinurov, R.R. Zinurov, R.A. Akhmed’yanova, A.G. Liakumovich, 2010, published in Neftekhimiya, 2010, Vol. 50, No. 5, pp. 386–390.
Skeletal Isomerization of nꢀPentane in the Presence
of an AlCl3ꢀBased Ionic Liquid
D. R. Zinurov, R. R. Zinurov, R. A. Akhmed’yanova, and A. G. Liakumovich
Kazan State Technological University, ul. Karla Marksa 68, Kazan, 420015 Tatarstan, Russia
eꢀmail: rodopi@bk.ru; achra@kstu.ru
Received February 8, 2010
Abstract—The skeletal isomerization of
minum chloride ionic liquid has been studied at 20°C. It has been found that the catalytic activity of the ionic
liquid in ꢀpentane isomerization increases with an increase in the trimethylamine hydrochloride : aluminum
nꢀpentane in the presence of a trimethylamine hydrochloride–aluꢀ
n
chloride molar ratio. The optimum ratio of the catalyst complex components at the highest yields of the
desired isoparaffins attained has been determined. The catalytic activity of the ionic liquid increased when a
copper salt was added. The maximum total yield of isoparaffins is achieved with copper(II) chloride as an
activating additive for the ionic liquid, whereas the maximum yield of isopentane is attained when copper(II)
sulfate is introduced.
DOI: 10.1134/S0965544110050105
Isoꢀbranched hydrocarbons form the basis of the rosiveness, and regeneration problems, and when
contemporary petrochemical industry. They are necꢀ superacids are deposited on solid supports, their activꢀ
essary raw materials for the manufacture of alkylation ity in an isomerization reaction is extremely small.
gasoline, methyl tertꢀbutyl ether, and synthetic rubbers
(isobutene–isobutylene–butyl rubber, isopentane–
Ionic liquids are of particular value for the skeletal
isomerization of paraffin hydrocarbons in this context,
isoprene–isoprene rubber). The catalytic systems
and they are currently considered to be an alternative
based on AlCl3 allow for the isomerization process to
to traditional heterogeneous and homogeneous cataꢀ
lysts [6, 7]. Unlike superacid systems, ionic liquids are
be run under thermodynamically favorable condiꢀ
tions, since they make it possible to accomplish
recyclable, although they partially lose their activity.
isomerization at low temperatures, leading to an
Kustov et al. [8] have successfully carried out nꢀhepꢀ
increase in the equilibrium yield of isoparaffins.
tane isomerization using ionic liquids applied on a
solid support. These features show that ionic liquids
hold promise as catalytic media for the skeletal
isomerization of paraffin hydrocarbons.
Numerous works concerning the use of aluminum
chloride in organic synthesis have been conducted in
Russia under the supervision of I.S. Akhrem, a leading
research associate of the laboratory of complex orgaꢀ
nometallic catalysts at the Institute of Organoelement
Compounds, Russian Academy of Sciences. It was
EXPERIMENTAL
shown that the following reactions can occur at 20°С
We attempted to examine whether aluminum chloꢀ
ride can be used for mediating the isomerization of
paraffin hydrocarbons. nꢀPentane was taken as a feedꢀ
stock, but it was meant that it would be natural gas liqꢀ
uids (NGL), a fraction that contains С4–С7 and higher
hydrocarbons. Regarding the separation of isoꢀ
branched products into individual isoparaffins, the
existing gas fractionation units (GFU) can be successꢀ
fully used for this purpose.
in the presence of superacid catalytic systems based on
AlCl3: the cracking and isomerization of alkanes; alkyꢀ
lation of arenes; synthesis of ketones from alkanes,
cycloalkanes, CO, and silanes; functionalization of
alkanes and cycloalkanes; etc. [1]. An undoubted
advantage of the catalytic systems based on aluminum
chloride from the point of view of power savings is low
temperatures.
The isomerization of paraffins at ambient temperꢀ
Reagentꢀgrade aluminum chloride does not cataꢀ
ature was successfully carried out on superacid comꢀ lyze the isomerization reactions of saturated hydrocarꢀ
plexes based on aluminum halides [2, 3]. However, the bons at ambient temperature. However, isomerization
main conversion routes under such conditions are can occur when AlCl3 contains impurities (moisture,
cracking and disproportionation. It has been noted oxygen) which promote catalyst activity [9]. A
[4, 5] that the application of liquid superacids as decrease in the reaction temperature, increasing the
isomerization catalysts causes serious technological amount of isoalkanes in the equilibrium mixture and
difficulties because of their low stability, toxicity, corꢀ decreasing the contribution of side processes, simultaꢀ
376

SKELETAL ISOMERIZATION OF
n
ꢀPENTANE
377
%
80
70
60
50
40
30
20
10
0
1 : 2.25
1 : 2
1 : 1.75
1 : 1.5
TMAHC : AlCl molar ratio
3
Conversion
Selectivity
Isopentane yield
Fig. 1. Characteristics of the
n
ꢀpentane skeletal isomerization process depending on the component ratio of the ionic liquid
[TMAHC] : [AlCl ] = 1 : 2 (mol/mol), [AlCl ] = 1.05 mol/l,
T
= 20°C, τ = 6 h.
3
3
neously leads to a sharp decline in the activity of the ter, and a reflux condenser connected to a liquid nitroꢀ
known homogeneous catalysts based on AlCl3, as was genꢀcooled trap with an exhaust tube.
shown in [10].
The reaction mixture was analyzed by gas chromaꢀ
tography using a Kristallꢀ4000M chromatograph
equipped with a Petrocol DH capillary column of
100 m in length and 0.25 mm in diameter.
We used the trimethylamine hydrochloride–aluꢀ
minum chloride ionic liquid as a catalyst for
tane skeletal isomerization. It was shown that
tane isomerization effectively proceeds in the presence
of this catalyst system at a temperature of 20 : the
conversion reaches 60–70% at a 95% selectivity for
isoparaffins. The catalyst complex was prepared by
mixing preliminarily dried trimethylamine hydrochloꢀ
ride (Acros Organics) (98.8%) and aluminum chloride
(TU (Technical Specifications) 6ꢀ01ꢀ2ꢀ88). The temꢀ
n
n
ꢀpenꢀ
ꢀpenꢀ
°
С
RESULTS AND DISCUSSION
It was found that the reaction mixture produced by
the isomerization of nꢀpentane in the presence of the
ionic liquid contained isobutane, isopentane, and С6–
С12 saturated hydrocarbons with isoꢀbranched and
normal chains. The catalytic activity of the ionic liquid
depends on the ratio between the components (Fig. 1).
The maximum yield of isopentane (18%) was reached
at a [TMAHC] : [AlCl3] molar ratio of 1 : 2, with the
pentane conversion being 25% in this case. The conꢀ
perature in the flask was maintained at about 60°С.
After the 2ꢀ to 3ꢀmin mixing of the components on a
water bath, the intense evolution of the HCl gas was
observed, which was caused by both its presence as a
constituent of trimethylamine hydrochloride and the
possible partial hydrolysis of the resulting chloroaluꢀ
minate complex. The gas was absorbed with a 20%
NaOH aqueous solution. The transparent brown ionic
liquid was produced as a result.
The hydrogen chloride dissolved in the ionic liquid
participates during the formation of reactive protons
to impart superacid properties to ionic liquids owing to
the reactions [11, 12]:
version of nꢀpentane and the yield of isopentane both
decrease considerably with an increase in the molar
ratio of the components (1 : 1.75; 1 : 1.5). A decrease
in the molar ratio to 1: 2.25 leads to a slight growth in
the conversion to 27%; however, the isopentane yield
drops to 15%. Thus, [TMAHC] : [AlCl3] = 1 : 2 is the
optimum molar ratio.
When the components of the catalytic system are
mixed, the following ion pair is formed:
HCl + AlCl–4 = [AlCl4
HCl]–,
HCl + Al2Cl– = [Al2Cl7
HCl]–,
HCl + Al2 Cl– = 2AlCl–4 + H+.
⋅
+
CH3
H3C NH
CH3
CH3
H3C N
CH3
⋅
7
[AlCl4]–
⋅ HCl + AlCl3
,
7
The skeletal isomerization of nꢀpentane (TU 0272ꢀ
029ꢀ00151638ꢀ99) in the presence of the ionic liquid which is presumably difficult to dissociate into free
was carried out in a glass reactor equipped with a ions and insufficiently effective to catalyze the isomerꢀ
mechanical stirrer with a hydraulic seal, a thermomeꢀ ization process.
PETROLEUM CHEMISTRY Vol. 50
No. 5
2010

378
ZINUROV et al.
9
8
7
6
5
4
3
2
1
1
2
3
4
0
1
2
3
4
5
Time, h
Fig. 2. Time dependence for the concentration of
liquid and different activating additives: ( ) additiveꢀfree; (
[TMAHC] : [AlCl ] = 1 : 2 (by mole); [AlCl ] : [Activating agent] = 15 : 1 (by weight).
n
ꢀpentane during the isomerization reaction in the presence of the ionic
1
2
) CuSO , ( ) CuCl, ( ) CuCl ; = 20°C; [AlCl ] = 1.05 mol/l;
3
4
T
2
4
3
3
3
If 1 mole of AlCl3 is additionally introduced into increases during the course of the reaction and
the system to give the molar ratio of [TMAHC] : reaches 15.5% within 5 h. The process selectivity for
AlCl3] = 1 : 2, the isomerization becomes much more isopentane is 70% at a 22% conversion (table). When
intense, likely because of the easier dissociation of the copper(II) sulfate is added to the ionic liquid at a
[
resulting ion pair into free ions:
[CuSO4] : [AlCl3] mass ratio of 1 : 15, the nꢀpentane
conversion during the skeletal isomerization is
2.5 times higher and reaches 56%; however, the selecꢀ
tivity for isopentane decreases to 39% (table). This fact
can be explained by enhancement of the competing
cracking (which yields isobutane as the main product
(24%)) and disproportionation (which presumably
leads to C6–C10 isoalkanes with a yield of 10%, of
which 7% is made up of isohexanes) processes. The
isopentane yield reaches 20% within 3 h (Fig. 3b), and
then slightly changes (to 22%). This finding suggests
that the cracking process begins to prevail with time.
+
CH3
[AlCl4]–
+ AlCl3
H3C NH
CH3
+
CH3
CH3
[Al2Cl7]–
+ Al2Cl7–
H3C NH+
.
H3C NH
CH3
CH3
The introduction of a copper salt into an ionic liqꢀ
uid reportedly [9, 13] leads to the enhancement of the
The conversion of nꢀpentane reaches the highest
activity of the catalytic complex. In this work, we used value (72%) when copper(II) chloride is used as an
the copper salts CuCl2, CuCl, and CuSO4. The addiꢀ activating agent ([CuCl2] : [AlCl3] = 1 : 15 by weight).
tion of a copper salt to the catalyst system indeed The selectivity for isopentane and the yield of isopenꢀ
resulted in an increase in the conversion of
with CuCl2 exhibiting the highest activating power and 18%, respectively). The yields of isobutane and
(Fig. 2), which led to a decrease in the ꢀpentane conꢀ disproportionation products are 35.5 and 18.5%,
nꢀpentane, tane are less than those in the case of copper sulfate (25
n
centration in the reaction mixture from 8.5 to respectively (table), thereby explaining the high conꢀ
2.4 mol/l within 5 h. Admixed СuSO4 and CuCl2 were version at a low isopentane selectivity.
almost identical in activity.
The catalytic complexes based on AlCl3 catalyze ([CuCl] : [AlCl3] = 1 : 15 by weight) in
the cracking and disproportionation processes, along etal isomerization with the ionic liquid has shown that
with isomerization, even at low temperatures (20 ). the conversion is close to that when copper(II) sulfate
The use of copper(I) chloride as an activating agent
ꢀpentane skelꢀ
n
°С
The trimethylamine hydrochloride–aluminum chloꢀ is used (59%). However, the selectivity for isopentane
ride ionic liquid is no exception. Thus, the contribuꢀ and the yield of isopentane appear to have the lowest
tions of isomerization, cracking, and disproportionꢀ values (24 and 14%, respectively), and the selectivity
ation reactions to
nꢀpentane conversion differ for isobutane has the highest value (63%).
depending on the presence and the nature of the copꢀ
per salt as an activating agent (Figs. 3a–3d).
The total yield of С4–С6 isoparaffins (63%) and the
ꢀpentane conversion (72%) have maximal values
n
In the absence of activating agents, the contribuꢀ when copper(II) chloride is used as an activating
tion of the competing cracking and disproportionation agent. The maximum selectivity for С4–С6 isoparafꢀ
reactions is minimal (Fig. 3a). The yield of isopentane fins (95%) is reached with CuCl introduced into the
PETROLEUM CHEMISTRY Vol. 50
No. 5
2010

SKELETAL ISOMERIZATION OF
n
ꢀPENTANE
379
Yield, %
18
(а)
Additiveꢀfree
Yield, %
26
(b)
16
14
12
10
8
1
24
22
20
18
16
14
12
10
8
2
1
CuSO
4
6
3
4
4
6
3
4
2
2
4
2
0
1
2
3
4
5
0
1
2
3
4
5
Time, h
Time, h
Yield, %
(c)
Yield, %
(d)
2
35
2
35
30
25
20
15
10
5
CuСl
2
CuСl
30
25
20
15
10
5
1
1
3
4
3
4
0
1
2
3
4
5
0
1
2
3
4
5
Time, h
Time, h
Fig. 3. 20. Dependence of the yields of
n
ꢀpentane skeletal isomerization products upon the activating agent: (
1
) isopentane,
3
3
(2
) isobutane, ( ) isohexanes, ( ) disproportionation products;
3
4
T
= 20°C; [AlCl ] = 1.05 mol/l; [TMAHC] : [AlCl ] = 1 : 2 (by
mole); [AlCl ] : [Activating agent] = 15 : 1 (by weight);
3
catalytic system: the proportion of isobutane is 63%
(table). The formation of isopentane is facilitated
(maximum yield 22%) by the admixture of CuSO4. In
addition, it should be borne in mind that disproporꢀ
tionation products mainly contain branched saturated
С6–С12 hydrocarbons.
+
CH3
H3C NH+
CH3
CH3
H3C NH
CH3
[Al2Cl7]–
+ Al2Cl7–
,
CuCl2
Cu2+ + 2Cl–,
The role of the activating agent is obviously to
accelerate the dissociation of the catalytic complex
into free ions and to generate the carbocation.
2+
CH3
CH3
4
++ Cu2+
+ 4H+,
H3C N
Cu
H3C NH
Consider a possible mechanism of activation by
activating agents using copper(II) chloride as an
example. Copper salts form complexes with amines
and are capable of catalyzing some ionic reactions,
like the salts of neighboring Group IIB elements [14].
It is possible to suppose that when copper salt is added
to the ionic liquid, the complex of trimethylamine
with copper ions forms by the following mechanism:
CH3
4
CH3
H+ + Cl–
HCl,
H+ + Al2Cl–
HAl2Cl7.
7
Less stable catalytically active copper chloride (sulꢀ
fate) complexes decompose, and it is necessary to have
PETROLEUM CHEMISTRY Vol. 50
No. 5
2010

380
ZINUROV et al.
Characteristics of nꢀpentane skeletal isomerization process in the presence of the [TMAHC] : [AlCl3] = 1 : 2 (mol/mol) ionic
liquid, [AlCl3] = 1.05 mol/l, [AlCl3] : [Activating agent] = 1 : 15 (w/w), T = 20°C, τ = 5 h
Selectivity, %
Yield, %
disproꢀ
isoꢀC4– portionꢀ
disproꢀ
(isoꢀC4– portionꢀ
Converꢀ
sion, %
Additive
S
(
Σ
isoꢀC4 isoꢀC5 isoꢀC6
isoꢀC4 isoꢀC5 isoꢀC6
isoꢀC6)
ation
isoꢀC6)
ation
products
products
Additiveꢀfree
+ CuSO4
+ CuCl2
22
56
72
59
5
43
49
63
70
39
25
24
16
12
13.5
8
91
9
5
1.5
24
15
22
18
14
3.5
7
20
53
63
56
2
3
9
3
94
87.5
95
12.5
5
35.5
37
9.5
5
+ CuCl
a large excess of the Cl– (SO42–
) ligands to prepare
such complexes.
3. I. S. Akhrem, et al., Dokl. Akad. Nauk SSSR 298, 107
(1988).
4. I. R. Bursian, D. S. Orlov, and A. I. Shakun, Catalysis
on Superacids (TsNIITENeftekhim, Moscow, 1979) [in
Russian].
Because the isomerization process follows the acid
mechanism, the resulting HCl and HAl2Cl7 raise the
acidity of the medium and, thus, promote an increase
in the rate of formation of the carbenium ion, which
then participates in the abstraction of the hydride ion
from pentane to form isopentane and the secondary
carbocation that transforms into the more stable terꢀ
tiary form and participates again in the hydrideꢀion
abstraction from paraffins.
5. G. A. Ola, Usp. Khim. 44, 793 (1975).
6. A. P. Bolton, Zeolite Chemistry and Catalysis, Ed. by
J. A. Rabo (Americal Chemical Society, Washington,
1976; Mir, Moscow, 1980), p. 714.
7. B. N. Dolgov, Catalysis in Organic Chemistry (Goskꢀ
himizdat, Leningrad, 1959) [in Russian].
8. L. M. Kustov, T. V. Vasina, and V. A. Ksenofontov, Ros.
Khim. Zh. 48, 13 (2004).
Thus, the experimental data obtained in this study
show the efficacy of the given ionic liquid as an
isomerization catalyst for both normal hydrocarbons
and hydrocarbon fractions. The latter option is of parꢀ
9. Yu. M. Zhorov, Isomerization of Hydrocarbons: Chemisꢀ
try and Technology (Khimiya, Moscow, 1983) [in Rusꢀ
sian].
ticular importance in relation to the potential use of 10. A. L. Glasebrook, N. Phillips, and W. Lowell, J. Am.
Chem. Soc. 58, 1944 (1936).
ionic liquids in the process of the isomerization of
NGL for the manufacturing of motor fuel compoꢀ 11. J. L. E. Campbell and K. E. Johnson, J. Am. Chem.
Soc. 117, 7791 (1995).
nents.
12. P. C. Trulove and D. K. Sukumaran, Inorg. Chem. 32
,
4396 (1993).
REFERENCES
13. K. Herbst, J. Houzviska, T. V. Jesrersen, and J. Zavilla,
EP Patent No. 1346768, 2003.
14. T. T. Bakumenko et al., Handbook of Catalytic Properties
of Substances Ed. by V. A. Roiter (Naukova Dumka,
Kiev, 1968) [in Russian].
1. I. S. Akhrem, A. V. Orlinkov, and M. E. Vol’pin, Izv.
Akad. Nauk, Ser. Khim., No. 5, 771 (1998).
2. A. V. Orlinkov et al., Izv. Akad. Nauk SSSR, Ser.
Khim., No. 6, 1416 (1986).
PETROLEUM CHEMISTRY Vol. 50
No. 5
2010