Ind. Eng. Chem. Res. 1995,34, 1149-1155
1149
Catalytic Dimerization of Propene by Nickel-Phosphine Complexes in 1-Butyl-3-methylimidazolium Chloride/AlEt,Cls-, (x = 0, 1) Ionic Liquids Yves Chauvin,* Sandra Einloft? and H6lene Olivier Institut Franqais du Pktrole, BP 31 1, 92506 Rueil-Malmaison, France
The dimerization of propene was catalyzed by cationic nickel complexes in a two-phase solvent system using organochloroaluminate ionic liquids as the solvent for the catalyst. In ionic liquids containing a n excess of strongly coordinating chloride ions, Le., basic, no activity was observed. In contrast, melts containing a n excess of alkylchloroaluminum species, Le., acidic, stabilized the active cationic nickel species. The reaction products separate as a n organic layer. Molecular organochloroaluminum species were extracted, and the composition of the salt was strongly modified. This was circumvented using a salt which contains a n excess of aluminum chloride. The propene dimers obtained by this way can be transformed either into ethers or into alkanes to produce high octane number additives for gasoline. The effects of phosphine ligands coordinated on nickel and operating variables were investigated in order to maximize the octane number of the corresponding alkanes and ethers.
Introduction A two-phase solvent system for catalysis is an attractive alternative to homogeneous or heterogeneous catalysis. Several reactions have tentatively been studied using this technique (Knifton, 19881, and a t least two of them have industrial application: uiz., propene hydroformylation catalyzed by rhodium complexes dissolved in an aqueous phase (the RhBne-Poulenc-Ruhrchemie process) (Kuntz, 1987), and ethene oligomerization catalyzed by nickel complexes dissolved in a butanediol polar phase (the Shell-SHOP process) (Freitas and Gum, 1979). However, some organometallic catalytic systems, which are highly sensitive to protons or bases, cannot be used in these media. For these systems, only a nonprotic liquid, not containing coordinating species, could only be considered. Room-temperature molten salts based on aluminum(II1) chloride and 1,3-dialkylimidazolium chloride proved t o be particularly suitable. They are good solvents for inorganic salts and do not dissolve simple hydrocarbons. These systems are liquid a t room temperature and below over a wide range of composition (Hussey, 1983). They are all the more interesting as their Lewis acidity can be adjusted by varying their composition. When the molar fraction of aluminum(II1) chloride (noted X)is less than 0.5, the salt contains an excess of free C1- anions and can be considered as basic. When Xis higher than 0.5, the salt can be considered as acidic. This type of molten salt has been extended to mixtures of 1-butyl-3-methylimidazolium chloride (BMIC) and ethylaluminum derivatives such as dichloroethylaluminum (Gilbert et al., 1990). In 1966, G. Wilke et al. (1966) developed a family of cationic n-allyl nickel (11)complexes containing a phosphine ligand. When dissolved in chlorinated or aromatic hydrocarbons, these complexes proved to be efficient catalysts for the dimerization of propene. The reaction pathway for the formation of the isomeric hexenes is given in Figure 1. The regioselectivity of the dimeriza-
* Address correspondence to this author. E-mail: CHAUVINCZIRVAX1.1FP.FR. ' Present address: Universidade Federal do Rio Grande do Sul, Port0 Alegre, Brazil.
tion reaction can be directed by the nature of the tertiary phosphine ligand. A sterically demanding phosphine favors the formation of 2,3-dimethylbutenes (tail-to-tail dimers). 2,3-Dimethylbutenes have found some applications as starting alkenes for the production of fine chemicals (Sato et al., 1993). The dimerization of propene with a catalyst that does not contain any phosphine gives oligomers with uncontrolled regioselectivity, which are used for high-octanenumber olefinic gasoline blending (the Dimersol process) (Chauvin et al., 1974). However, due to new regulations, in the near future the alkene content of gasoline must be lowered. On the other hand, branched alkanes and ethers are particularly suitable for use as high octane additives. It is therefore of interest to convert olefinic propene dimers, for example, either into alkanes by hydrogenation or into ethers by alcohol addition. Table 1gives the research octane number (RON) and the motor octane number (MON) of the three alkanes resulting from the hydrogenation of olefinic propene dimers. Therefore, in a first approach, the only way to produce high-octane-number alkanes from olefinic propene dimers is t o minimize the n-hexene content and to maximize the 2,3-dimethylbutene content. In a second approach, 2,3-dimethylbutenes and 2-methylpentenes, contained in propene dimers, can readily be transformed into ethers by reaction with methanol, under conventional conditions, in the presence of acidic ion-exchange resins. However, under these conditions, n-hexenes, 4-methyl-l-pentene, and 4-methyl-2-pentene cannot be either transformed into ethers or isomerized (no double bond shift). Thus, high-octane-number values can be obtained only by maximizing the (2-methylpentene 2,3-&methylbutene) content of the dimers. In both approaches, the selectivity of the dimers in converted propene has t o be maximized. We describe here the use of ionic liquids as regioselective catalyst solvents for the dimerization of propene in order to maximize the octane number of either hydrogenated dimers or ethers. This two-phase solvent system greatly improves the economics of the conventional dimerization processes and compares well with known processes (Sato et al., 1993). Preliminary results
+
0888-5885/95/2634-1149$09.00/0 0 1995 American Chemical Society
1150 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 C Ni-C-C-C-C-C
-
Ni-C-C-C-C-C
-
1 -and 2-hexene
-
3-Hexene
2-Methyl-1-pentene
-
2-Methyl-2-pentene
Ni-C-C-C
Ni-H t
cc
c=c-c
,
I
Ni-C-6-6-C
\
$
-
2,3-Dimethyl-l -butene
-
4-Methyl-1-pentene
-
2,3'-Dimethyl-2-butene
/ Ni-C-C
\
c
y Ni-(h-C-C
4-Methyl-2-pentene
-
2-Methyl-2-pentene
Figure 1. Reaction pathways of propene dimerization catalyzed by cationic nickel complexes. Table 1. Octane Number Values of Hydrogenated Propene Dimers RON MON
n-hexane
2-methylpentane
24.8 26
73.4 73.5
2,3-dimethylbutane 103.5 94.3
have been summarized in earlier papers (Chauvin et al., 1990; Olivier et al., 1992)
Experimental Section Air-sensitive compounds were manipulated in an inert-atmosphere box under argon. Aluminum(II1)chloride was first heated to 150 "C with a mixture of aluminum powder and sodium chloride and then sublimed under atmospheric pressure at 180 "C. Molten salts were prepared as described previously (Gilbert et al., 1991). AlEtCl2 (Witco) was distilled under vacuum before use. NiC12 was prepared by heating NiC1243H20 in SOCL under reflux, then dissolved in a basic BMIC/AlEtC12 molten salt (X = 0.45) forming a blue solution containing [NiC14I2-. NiCly2L complexes (L = pyridine, triisopropylphosphine, tri-n-butylphosphine, tribenzylphosphine, or tricyclohexylphosphine) were prepared by reacting commercial NiCly6HO dissolved in pure ethanol with 2 equiv of the ligand. The precipitates were filtered, washed with ethanol, and then dried. Propene (Air Liquide, 99.4%) was stored over 3A molecular sieves. Oligomerization reactions were carried out under an atmospheric pressure of propene in a 100 mL doublewalled glass reactor containing a magnetic bar and a temperature indicator. A schematic diagram of the system is given in Figure 2. It is obvious that, in systems consisting of two liquid layers, mass transfer is of great importance. Thus reaction rates and even yields could be limited by physical parameters rather than by chemical ones. Typical Example. (Table 5, entry 10). At room temperature [NiC12(P(i-Pr)3)2](45 mg; 0.1 mmol) was placed into the reactor, which was then evacuated and filled with gaseous propene t o 1 atm. Then, the temperature was adjusted to -15 "C with a circulating cooling bath. BMIC/AlCldAlEtC12 (1:1.2:0.1) ionic liquid (4mL) and heptane (4 mL) were injected into the reactor
and stirring was started. The ionic liquid coloration turned in a few seconds from red to yellow. The reactor was continuously fed with gaseous propene, and a t the same time it gradually filled up with the liquid dimers as a separated liquid phase. When the reactor was full of liquid, stirring was stopped, and most of the upper hydrocarbon layer (55-60 g) was withdrawn. Then the reaction was continued. This was done seven times (438 g of propene was converted). Analytical determinations were performed for each sample. Under an atmospheric pressure of propene and at -15 "C, the equilibrium concentration of propene dissolved in its oligomers is 15 wt %. At +5 "C this equilibrium concentration is 8%. Thus, by using such a semibatch technique, propene conversion is respectively 85% and 92%. Samples were analyzed by gas chromatography (flame ionization detector, PONA, 50 m capillary column). Temperature programming was 30 min at 0 "C and then 10 "C min-l up to 270 "C.
Results Catalysis was performed with NiC12.2L complexes (L = P(Bu13, P(i-Pr)s, P(cyclohexyl)s, P(benzyl)s, or pyri-
dine), which have the advantage of being easy to handle, contrary to the air-sensitive organometallic complexes described by Wilke. However, in both cases, the active species is supposed to be of the same type, i.e., [HNi(PRs)I+A-. The ionic liquids used were mixtures of BMIC and aluminum(II1) chloride and/or dichloroethylaluminum. This latter compound was used as an alkylating agent for nickel(11) chloride complexes by metathesis of C1with Et- anions. Preliminary experiments demonstrated that in basic ionic liquids (X< 0.5; excess of free chloride) no active species were formed. W spectroscopy showed that the red diamagnetic square planar nickel(I1) complexes (e.g., [NiC12(PR3)21) were transformed into the inactive blue paramagnetic tetrahedral complexes according to equilibrium 1:
+
[NiC12(PR3)21 2C1-
[NiC1,12-
+ 2PR3
(1)
The position of this equilibrium depends largely on the phosphine basicity.
Ind. Eng. Chem. Res., Vol. 34, No. 4,1995 1161
I
I
I
\ d
I
Figure 2. Schematic experimental apparatus for the dimerization of propene. Table 2. Composition of Organochloroaluminate Ionic Liquids before and after Extensive Hydrocarbon Extraction starting mixture (mole ratio) anions present in the melt afker anions present in the melt before BMIC” AlCh AlEtC12 contacting with a hydrocarbon 1ayel.b extensive extraction with a hydrocarbon 1ayel.b [AlEtC131-, [AlC1411 1.2 A [AlEtC131-, [Alzl$tzC151[AlCl&, [AlEtCl& 1 0.82 0.26 B [AlEtCl&, [AlC4]-, [Al&3&15]-, [Al2EtCls]-, [AlzC17I1 1.2 0.1 C [AlEtChI-, [AlCl~l-,[AlzEt2C151-,[&Etch$, [AlzC17][NCbI-, [Al2C171a
1-Butyl-3-methylimidazolium chloride. As established by Raman spectroscopy.
Catalytically-active species can be generated only in acidic melts. Three types of acidic ionic liquids were selected for study: (1)BMIC/AlEtC12 (1:1.2) salts (A type), which contain mainly [AlEtCl& and [Al2Et&l5Ianions; (2) BMIC/AlC13/AZEtC12 (1:0.82:0.26) salts (B type), which contain mainly [AlC141-, [AlEtCl&, and [Al2Et~C15]-anions; (3)BMIC/AlC13/AIEtC12(1:1.2:0.1) salts (C type), which contain mainly [AlClJ, [Al2C17]-, [&Et&]-, and [Al2Et&15]- anions. When a ionic liquid containing dichloroethylaluminum is contacted with a hydrocarbon layer, the dissociation equilibrium of polynuclear chloroethylaluminum anions is shifted to the formation of lower nuclear chloroaluminum anions and molecular chloroethylaluminum compounds. These latter dissolve into the hydrocarbon layer (HC) (Gilbert et al., 1991,1994), e.g.
*
2[A12Et2C15]2[AlC14]-
=
2[A12EtClJ 2[AlC14]-
+ Al2Et4Cl2 (soluble in HC) (2) + Al2Et2Cl4 (soluble in HC) (3)
Thus, in a continuous dimerization operation in which the hydrocarbon layer is continuously renewed, such an extraction can considerably modify the composition of the molten layer. Table 2 gives the composition of molten salts (A, B, and C) before and after extensive hydrocarbon extrac-
tion. An excess of aluminum(II1)chloride in the starting ionic liquid (C type) ensures the presence of [Al2C171anions after extraction by the hydrocarbon layer. Catalysis in A Type and B Type Ionic Liquids. Typical catalysis results obtained with AlEtC12 based ionic liquids (A type) and using various nickel(I1) salts are given in Table 3. Identical product distributions were obtained using either pure NiC12 (entry 1) or [NiCl2(pyridine)a] complex (entry 2). However, this latter complex was not active at -15 “C (green solution) and the reaction started only at +5 “C. It is reasonable to suggest that at +5 “C the “hard” pyridine ligand preferentially coordinates with the “hard” dichloroethylaluminum acid. In B type ionic liquids in which the molar fraction of aluminum(II1) chloride is lower than 0.5, quite comparable results were obtained (Table 4). In these ionic liquids, the “phosphine effect” (entries 3 and 4, Table 3; entries 6-9, Table 4) was quite similar to that observed by Wilke (Wilke et al., 1966) using cationic n-allylphosphine nickel complexes dissolved in chlorinated hydrocarbons, or by us with the [NiClz(P(iPI-)&] based catalyst, without any solvent (Uchino et al., 1967). Increasing the temperature had no significant effect on the product distribution, but did result in the ionic liquid turning black (entry 5, Table 3). For all these experiments (Tables 3 and 4) the product and dimer distributions were constant throughout the run, but deactivation of the catalyst was observed (Figure 3). Under the best conditions the total amount
1152 Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 Table 3. Dimerization of Propene in BMIC/AlEtC12 (1:1.2) Ionic Liquids (A Type) (4 mL). Catalyst NiC12.2L (0.1 mmol) entry ligand (L)
1 2 3 4 5 no L pyridine P(i-Pr)a P(Bu)3 P(Bu)3
reaction conditions temperature(%) reaction time (h) productivity (kg/g Nila product distribution (%) dimers trimers higher dimer distribution (%) 2,3-dimethylbutenes 2-methylpentenes 4-methylpentenes n-hexenes product quality ethers (%Ib alkanesC RONc MON
-15 6 15
$5 4 9
-15 3.5 9
-15 6 20
+15 2.3 7
91 8
93 6
97 2
93 6
1
1
85 13 2
1
1
5 45 29 21
5 41 33 21
74 10 14 2
28 42 21 9
28 42 19 10
45.5
43
71
68
66
65 65
65 65
95 88
78 75
76 74
Table 4. Dimerization of Propene in BMIC/AlCls/A1EtClZ (1:0.82:0.26) Ionic Liquids (B Type) (4 mL). Catalyst [NiC12(PR.&] (0.1 mmol) entry 6 7 8 9 ligand (PR3) P(i-Pr)s P(Cy)sa P(benzy1)a P(Bu)3 reaction conditions temperature ("C) -15 +5 +5 +5 reaction time (h) 5.75 7 7 6.5 22 11 24 17 productivity (kglg Nilb product distribution (%) 83 82 77 94 dimers 15 17 21 6 trimers 2 1 2 traces higher dimer distribution (8) 2,3-dimethylbutenes 77 76 48 33 2-methylpentenes 9 11 22 44 4-methylpentenes 13 11 26 17 n-hexenes 1 2 4 6 product quality 54 72 ethers (%F 71 71 alkanes RONd 96 95.3 86 80.4 MON 81.5 77.5 89 88.4
kg of propene convertedg of nickel. % propene converted which can be transformed to cg ethers, Le., olefinic tertiary carbon atoms contained in the dimer fraction. RON and MON calculated for the hydrogenated dimers.
a Cy = cyclohexyl. * kg of propene convertedg of nickel. % propene converted which can be transformed to cg ethers, i.e., olefinic tertiary carbon atoms contained in the dimer fraction. RON and MON calculated for the hydrogenated dimers.
of oligomers produced before the catalyst deactivated was 45 kg (g N9-l. Catalysis in C Type Ionic Liquids. By using the BMiCIAlCldAlEtC12 (1:1.2:0.1) ionic liquid (C type), X(AlC13)> 0.5, catalyst activities were enhanced (Table 5, entries 10 and 11). Furthermore, the reaction rate increased after the first product withdrawals (Figure 3). Using [NiC12(P(Bu)&]complex as catalyst precursor, a constant dimer distribution was obtained throughout the run (entry 14, columns 1and 2). However, contrary t o the observation in "A type" liquids, the reaction proceeds very slowly at -15 "C and only really starts above 0 "C.
In the presence of sterically demanding phosphines, the 2,3-dimethylbutene content is increased by comparison to experiments made with A or B type liquids. However, the high yields in 2,3-dimethylbutenes found after 1h of reaction time (84%;Table 5; entries 10 and 11,columns 11, decreased rapidly and after 8 h dropped t o 10% (columns 2). With P(benzy1)s as the ligand, the 2,3-dimethylbutene content was 45% at the beginning of the run but decreased more rapidly at +5 "C than at -15 "C (Table 5; entries 12 and 13, columns 1 and 2). This suggests that the phosphine effect gradually disappeared, mainly for the basic phosphines. Then, it might be assumed that there is competition, for the phosphine, between "soft" nickel active species and
600
0
3
0
500
9 400 0
2
z
r
2
EtAIC12:AIC13:BMIC (0.26:1.2:1
300
Q
E Q n
2
200
L .
Et ..312:BMIC (1.2:l)
-on
100
0
50
100
150
200
250
Time (min)
Figure 3. Propene converted us reaction time.
300
350
400
450
500
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1163 Table 5. Dimerization of Propene in BMIC/AlCl&UEtClz(1:1.20.1)Ionic Liquid (C Type) (4 mL). Catalyst NiC12.2L (0.1 "01) entry ligand
10 P(i-Pr)a
(L)
reaction conditions temperature ("C) reaction time (h) productivity (kg/g Ni)b product distribution (%) dimers trimers higher dimer distribution (%) 2,3-dimethylbutenes 2-methylpentenes 4-methylpentenes n-hexenes product quality ethers (%Ic alkanes RONd MON
12 P(benzy1)s
11
P(Cy13Q
-15
8
1
100
3
65
14 P(Bu)3
-15
+5
-15
8
1
13 P(benzy1)s
10
4
48
+5 4.5
10
34
79 19 2
84 14 2
80 17 3
86 13 1
77 19 4
82 17
83 7 8 2
12 38 34 16
84 8 7 1
8 41 36 16
71
42
74
97 90
69 68
98 91
15 pyridine
+5 13
3
100 87 12
1
78 17 4
44 26 26 4
16 36 29 18
42
53
69 68
85 81
9 52
1
91 7 2
91 7 2
77 19 4
a2 17 1
46 18 33 3
40 15 40 5
33 47 17 3
32 47 18 3
7 47 28 18
7 40 34 19
43
50
48
73
72
42
39
69 68
86 82
83 79
82 79
81.5 79
67 66
66 66
Cy = cyclohexyl. kg of propene converteag of nickel. % propene converted which can be transformed to c6 ethers, i.e., % olefinic tertiary carbon atoms contained in the dimer fraction. RON and MON calculated for the hydrogenated dimers.
Table 6. Dimerization of Propene in BMIC/AlCls/AlEtC12(1:1.2:0.1) Ionic Liquid (C Type) (4 mL). Catalyst NiC12(PR& (0.1 "01); Aromatic Hydrocarbons Added (2 "01); Temperature -15 "C entry ligand (PR3) aromatic hydrocarbon reaction conditions reaction time (h) productivity (kg/g NiY product distribution (%) dimers trimers higher dimer distribution (%) 2,3-dimethylbutenes 2-methylpentenes 4-methylpentenes n-hexenes product quality ethers alkanes RONg MON
(%r
16 P(i-Pr)3 TMBb
17 P(Cy13" TMBb
8
1
7
1
73
19 P(i-Pr)s MNd
18 P(i-Pr)3 PMBc
7
1
34
3
1
46
29
78 19 3
88 10 2
78 18 4
77 19 4
79 18 3
84 14 2
78 18 4
88 11 1
84 7 8 1
53 10 29 8
74 8 7 1
74 7 7 1
83 7 8 2
75 6 16 2
84 8 7 1
65 10 21 4
71
55
72
70
71
68
72
48
98.2 90.4
85.5 80.7
98.2 90.4
98.2 90.4
97.4 89.8
94 88
98.2 90.4
91.1 85.1
a Cy = cyclohexyl. TMP = 1,2,3,5-tetramethylbenzene. PMB = pentamethylbenzene. MN = a-methylnaphthalene. e kg of propene convertedlg of nickel. f % propene converted which can be transformed to C6 ethers, Le., % olefinic tertiary carbon atoms contained in the dimer fraction. RON and MON calculated for the hydrogenated dimers.
"hard" aluminum chloride:
[HNiPR31++ [A12C17]-s [HNil' + AlC13(PR3)+ [AlCl,l- (4) Increasing the reaction temperature would favor the displacement of this equilibrium to the right. The formation of a phosphine-aluminum(II1) chloride complex was supported by the 31PNMFt spectrum of the [NiC12{P(cyclohexyl)3}2] complex dissolved in BMIC: AlC13 (1:1.2) ionic liquid. Spectra exhibit a pair of resonances at 30 ppm (sharp) and -10 ppm (broad) which can be attributed to the free phosphine and to the P(cyclohexyl)dAlCl3complex (Lunsford et al., 1985). The same 31Pchemical shifts have been observed in dichloromethane. On the other hand [NiC12(P(Bu)3)21 complex dissolved in the same salt showed only one sharp resonance a t 20 ppm, suggesting that no P(Bu)d AlCl3 adduct formed. Thus in catalytic experiments, equilibrium 4 was strongly displaced to the left.
We have anticipated that the best way to prevent a PR3/AICl3 interaction was to add a "soft" competitive base which does not interfere with the cationic nickel active species. In that way, we have observed that aromatic hydrocarbons (ArH) interact with acidic type C ionic liquid containing dichloroethylaluminum, and thus not containing any proton, giving colored species. We suggest that such an interaction could be ascribed to the following equilibrium: [A12C1,1-
+ ArH s AlC13*ArH+ [AlClJ
(5)
Thus, we performed the reaction with NiC12.2PR3 complexes (R = i-Pr and cyclohexyl) in ionic liquids to which different aromatic hydrocabons were added (1 molar equivalent per [Al2C171- anion present in the melt). Representative results are given in Table 6 and Figure 4. In the presence of tetramethylbenzene (entries 16 and 17,columns 1and 2) or pentamethylbenzene (entries 18, columns 1 and 2), the 2,3-dimethylbutene content decreased only very slowly, thus indi-
1154 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995 toluene 90 -
80 --
A
s
E
.p
-s
7 0 --
60 --
4 -
Entry 10, Table V
---u--
Entry 16, Table VI
-*-
Entry 18, Table VI
-
$ 5 0 -' C
Entry 19, Table VI
--A-
E
x
hexam6thylbenzene
Entry 21, Table VI1
30
--+-
Entry 23,Table VI1
1
210 o
0~ 50
I
1
1
I I
1
I
100
150
200
250
300
350
I
i
400
450
Propene converted (Q) Figure 4. Selectivity into 2,3-dimethylbutene us propene converted. Table 7. Dimerization of Propene in BMIC/AlCls/A1EtC12 (1:1.20.1)Ionic Liquid (C Type) (4 mL). Catalyst NiC12(P(i-PR)& (0.05 mmol) NiCln(Pyridine)z(0.05 mmol) in the Presence of Aromatic Hydrocarbons; Temperature -15 "C
+
entry aromatic hydrocarbon reaction conditions reaction time (h) productivity (kg/g Nild product distribution (%) dimers trimers higher dimer distribution (%) 2,3-dimethylbutenes 2-methylpentenes 4-methylpentenes n-hexenes product quality ethers alkanes RONf MON
TMB"
tolueneb 7
1
78 18
4
1
79 5 14 2
71 98.2 90.4
1
96.2 89
4.5
3.5
8
54
62
50
84 7 8
5
1
toluene"
78 20 2
88 11 1
79 19 1
83 7 9
83 6 9
1
26 24 36 14
69
65
70
97.9 90.3
74.4 72.1
97.9 90.3
1
79 19 2 83 6 9 1
83 6 9
81 5 13
1
1
68
97.9 90.3
97.9 90.3
97.3 89.9
a TMB = 1,2,3,5-tetramethylbenzene: 4 mmol at the startup. Toluene: 4 mmol at the startup. Toluene: 2 mmol added after each withdrawal. kg of propene converteag of nickel. e % propene converted which can be transformed to c6 ethers, i.e., 8 olefinic tertiary carbon atoms contained in the dimer fraction. f RON and MON calculated for the hydrogenated dimers.
cating that the interaction between the aromatic hydrocarbon and was strong enough to prevent the dissociation of the phosphine from the nickel(11). In contrast, the 2,3-dimethylbutene content decreased much more rapidly when methylnaphthalene (Table6, entry 19, columns 1 and 2) or toluene (Table 7, entry 21, columns 1 and 2) was added to the melt. This suggests that the interaction between the aromatic hydrocarbon and the dinuclear anion [AlzC17]- (equilibrium 5) is all the weaker as the aromatic hydrocarbon is less basic. The effect of hexamethylbenzene, the most basic of the hydrocarbons studied, is limited due to its insolubility in the melt. However, in the presence of a continuous flow of propene dimers, aromatic hydrocarbons tend t o be extracted, as observed by vapor phase chromatographic analysis of the organic layer.
As observed by Wilke and others (Sato et al., 19931, the best catalyst formulation (and the most economical) results from the interaction of only 1equiv of phosphine per nickel atom. However it is well-known that a mixed "hard-soft" phosphine-pyridine nickel complex cannot be obtained. This is the reason why 1 equiv of each complex [NiC12(P(i-Pr)&I and [NiClz(pyridine)zl was dissolved in ionic liquid in the presence of tetramethylbenzene (Table 7, entry 20). The selectivity was as good as that obtained using 2 equiv of phosphine. This suggested that, after pyridine dissociation, each nickel atom coordinated one phosphine ligand. To compensate for the extraction of the aromatic hydrocarbon by the reaction products, toluene (0.5 equiv per [Al2C17]- anion present in the ionic liquid) was added to the melt after each withdrawal of the hydro-
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1155 carbon layer. In that way, the 2,3-dimethylbutene content remained nearly constant as long as toluene was added but decreased when addition was suppressed. (Table 7, entry 23, columns 1-4, and Figure 4).
Discussion It seems obvious that as long as dinuclear anions are present in the ionic liquid, i.e., acidic liquid, the catalyst retains its activity. However for “A and B type” liquids (Tables 3 and 4) it is noted that catalysts deactivate progressively. This deactivation can be ascribed to the dissociation of the dinuclear anions into [AlCW- and AlzEt4Clz or AlzEtzC14 in the presence of a hydrocarbon layer (Table 2; equilibria 2 and 3). After extensive extraction, A and B type liquids contain only mononuclear anions and thus are “neutral”. Obviously, ”neutral” salts (X= 0.5) do not stabilize cationic nickel complexes. This can be ascribed t o the suspected equilibrium (6): 2[HNiLl+[AlEtCl31- t 2“NiHClL” A12Et2C1, (soluble in HC) (6) (inactive)
+
The only way t o stabilize the active species in the presence of a continuous flow of hydrocarbons is for the [AlC141- anion to be associated with the nickel cation. This is made possible only by using the “C type” molten salts, which after extensive extraction are composed of the [AlClJ and [Al2C171- anions. In this latter case, the increase of the reaction rate which was observed after the first product withdrawals (Figure 3) could be ascribed t o the existence of the equilibria 7 and 3:
+ [Al2Cl71-*
[HNiLI+[AlEtCl,l-
+
[HNiLl+[AlCl41- [A12EtC1,1- (7) 2[A12EtC1,1- * 2[AlCl,I-
+ A12Et2C14 (soluble in HC) (3)
In the presence of the [AlEtC131- and [AlClJ anions, the cationic [HNiLl+ species associates preferentially with the most basic [AlEtC131- anion affording the less catalytically active species. After extensive extraction of hydrocarbon-soluble AlzEt2C14, [HNiLI+ associates with the remaining less basic [AlCl& anion affording a more catalytically active species. Aromatic hydrocarbons could be considered as buffers in mixtures of “soft” ([RNil+) and “hard” (AlC13)acids thus stabilizing the “phosphine effect”: they displace equilibrium 5 to the right and equilibrium 4 to the left.
Conclusion Organochloroaluminate ionic liquids, which are interesting electrolytes for secondary batteries and electrochemistry, proved to be good alternatives for organic solvents. They favor the dissociation of organometallic ionic species and are readily separated from alkenes,
with the catalyst remaining in the ionic phase as long as it is active. From a practical point of view, an interesting catalytic formulation seems to be an equimolar mixture of [NiC12(PR3)2l-[NiClz(pyridine)zldissolved in an acidic aluminum chloride/BMIC ionic liquid to which small amounts of dichloroethylaluminum and aromatic hydrocarbons have been added. The alkanes and ethers, resulting from respectively the hydrogenation or the methanol addition to the propene dimers thus obtained, are good feedstocks for octane additives.
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* Abstract published in Advance ACS Abstracts, March 1, 1995.
