Oligomerization of low molecular weight olefins in ambient temperature melts

An improved process is described for the catalytic oligomerization of light olefins, such as ethylene or propylene. Higher hydrocarbons, such as gasoline grade hydrocarbons, are produced from the light olefins using a liquid catalyst which comprises a Lewis acid and a Lewis base component which forms with the Lewis acid a melt which is liquid at room temperature. The Lewis acid is a metal halide, such as aluminum trichloride and the Lewis base is an organic salt, such as an organic halide salt containing an N-heterocyclic ring or salts containing fully substituted onium ions.

BACKGROUND OF THE INVENTION 
This invention relates to an improved process for the catalytic 
oligomerization of light olefins to higher hydrocarbons and particularly 
to a novel oligomerization catalyst used in the process. 
DESCRIPTION OF THE PRIOR ART 
Recently, light hydrocarbons have become increasingly attractive feedstocks 
for petrochemical and fuel application. Considerable effort has been 
placed on the development of light hydrocarbon and refinery by-product 
recovery and processing to high-quality gasoline and chemicals. The 
optimum use of light products is even more critical in view of the 
progressive decrease in the use of lead compounds in fuels. 
The use of heterogeneous catalysts for dimerization and oligomerization of 
olefins has been known for some time. Nickel oxide supported on silica or 
silica-alumina and various nickel salts (nitrate, chloride, sulphate) 
deposited on silica, alumina and silica-alumina have all been successfully 
used as catalysts. Recently, nickel-exchanged zeolites have been employed 
for the oligomerization of alkenes. The process conditions, conversions 
and selectivities to higher hydrocarbons depend on the nature of nickel 
catalysts. For example, Chauvin et al., Appl. Catal., 42 (1988) 205 
obtained 97% conversion of propylene with 66% dimerization selectivity at 
40.degree. C. and a pressure of 4 bar on NiSO.sub.4 supported on alumina 
as the catalyst. Elev et al., J. Catal. 89 (1984) 470 obtained a 15-20% 
conversion during ethylene dimerization with a selectivity of 62% to 
butene at room temperature and a pressure of 9.3 kPa on the NiCaY zeolite 
catalyst which was reduced by a photo-assisted means. Nickel-exchanged NaY 
zeolite has also been used for the oligomerization of ethylene into a 
diesel-range product at a pressure of 3500 kPa and a temperature of 
100.degree. C.-300.degree. C. 
Oligomerization of alkenes is catalyzed by strong acids. A commercial 
catalyst for the oligomerization of alkenes contains concentrated (90%) 
orthophosphoric acid in a mixture with Kiselguhr and is called 
"silicophosphoric acid". Ethylene and propylene oligomerize over this 
catalyst at +300.degree. C. and at an initial pressure of 5.1-6.1 kPa 
giving alkanes, alkenes, cycloalkanes and aromatic hydrocarbons. Ethylene 
also oligomerizes and polymerizes in the presence of AlCl.sub.3 and HCl as 
co-catalyst at super atmospheric pressure, producing liquid paraffin 
hydrocarbons and addition compounds of AlCl.sub.3 with highly unsaturated 
cyclic compounds corresponding to the formula C.sub.n H.sub.2n-x 
AlCl.sub.3. 
Zeolites from the pentasil family have been successfully used for 
oligomerization of olefins. Mobil's olefin to gasoline (MOGD) process 
serves as a good example. This process has two major concerns: controlling 
the heat of reaction and maximizing the yield of either gasoline- or 
distillate-range products. 
A recent process, Alphabutol, involving the dimerization of ethylene to 
butene-1 is described in Commereuc et al., Hydrocarbon Processing, 
November 1984, p. 118. The Alphabutol process uses a homogeneous catalyst. 
Ethylene is converted at 50.degree.-60.degree. C. and low pressure in the 
liquid phase which contains a titanium-based catalyst. Butene-1 is 
produced as the main product (65 wt %) along with higher molecular weight 
by-products (pentene, hexene). 
Jansons et al. U.S. Pat. No. 4,879,366, issued Nov. 7, 1989 describes the 
production of aromatic oligomers by reacting an appropriate monomer system 
in the presence of a co-catalyst in the form of a complex between a Lewis 
acid and a Lewis base. The Lewis acid is typically aluminum trichloride, 
while the Lewis base may include a wide range of organic halide salts. 
Lane et al. U.S. Pat. No. 5,012,030, issued Apr. 30, 1991, describes a 
process for producing isobutylene polymers utilizing a co-catalyst 
comprising aluminum trichloride and an organic nitro compound. 
Pratt, U.S. Pat. No. 3,842,134 issued Oct. 15, 1974 relates to the 
polymerization of olefins using a catalyst comprising a mixture of 
anhydrous aluminum chloride and a mononitroalkane, e.g. mononitromethane. 
Mandai et al, U.S. Pat. No. 4,198,534, issued Apr. 15, 1980 describes 
subjecting olefins to cationic polymerization using Lewis acid catalyst or 
Lewis acid/Lewis base complex catalyst. 
It is the object of the present invention to provide a process for 
converting low molecular weight hydrocarbons into gasoline grade higher 
hydrocarbons. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has been discovered that 
higher hydrocarbons, and particularly gasoline grade hydrocarbons, can be 
efficiently produced from low molecular weight olefins by using as 
catalyst a molten salt mixture which is liquid at ambient temperatures. 
The molten salt mixture is typically a mixture of a Lewis acid, e.g. an 
inorganic halide such as aluminum trichloride and a Lewis base component, 
e.g. an organic halide salt capable of forming with the Lewis acid a melt 
which is liquid at room temperature. It has been found that high yields of 
the higher hydrocarbons are obtained and the ratios of the individual 
higher hydrocarbons can be varied depending upon the composition of the 
melt being used as catalyst. 
Room temperature melts or molten salt systems are known in the literature 
and are described for instance in Jones et al., J. Electrochem. Soc., Vol. 
136, No. 2 (1989) 424. These melts have been primarily of interest as 
electrolytes for battery applications. The room temperature melts are 
typically prepared by combining inorganic salts, such as aluminum 
chloride, and certain organic salts, such as the alkylpyridinium 
chlorides. The molten salt mixture is prepared by simply combining the two 
solids. In some cases, some external heat may be needed but usually the 
melt begins to form spontaneously upon mixing the two solids. 
When used as a catalyst according to the present invention, the Lewis acid 
portion of the melt can be any of the well-known Lewis acids. A Lewis acid 
is a substance which can accept an unshared electron pair from another 
molecule and may include aluminum trichloride, aluminum tribromide, 
antimony pentachloride, indium trichloride, gallium trichloride, boron 
trichloride, zinc chloride, ferric chloride, stannic chloride, titanium 
tetrachloride, etc. However, an aluminum halide, such as aluminum 
trichloride is preferred. The Lewis base portion is a substance capable of 
donating an unshared electron pair to a Lewis acid an can be selected from 
a wide range of organic salts capable of forming a room temperature melt 
with the Lewis acid portion. However, particularly preferred as the Lewis 
base portion are organic halide salts containing an N-heterocyclic ring 
and organic salts containing fully substituted onium ions. The central 
atom of the onium ion can be nitrogen, phosphorus (quaternary ammonium or 
phosphonium) or sulphur (ternary sulphonium). Preferably, at least one of 
the substituents should be an aromatic radical such as phenyl or benzyl. 
The other substituents are preferably small aliphatic groups such as 
methyl or ethyl. The side chain can contain ethereal oxygen. For use as an 
oligomerization catalyst, the Lewis acid and Lewis base portions are 
preferably in the molar ratio of about 1:2 to 2:1. 
The low molecular weight feedstocks are typically light ends from petroleum 
processing containing 2 to 4 carbon atoms, e.g. low molecular weight 
olefins such as ethylene and propylene. The oligomerization is carried out 
by passing these low molecular weight olefins through the liquid catalyst 
at a temperature between 0.degree. C. and 150.degree. C. and a pressure 
between 10 kPa and 1,000 kPa. 
The products obtained are typically mixtures of C.sub.4 -C.sub.6 
hydrocarbons. Not only are these products obtained in good yield but it 
has been surprisingly discovered that the ratios of the different C.sub.4 
-C.sub.6 hydrocarbons may be varied by varying the composition of the 
liquid catalyst. For instance, when using as catalyst a melt of AlCl.sub.3 
and N-n-butyl-pyridinium chloride, overall ethylene conversion to C.sub.4 
-C.sub.6 hydrocarbons was as high as 80%, with selectivity for C.sub.4 
compounds of about 86%. 
There is a fundamental difference between the catalysts used in the process 
of this invention and the catalysts used in the prior patents of Jansons 
et al, Lane et al, Pratt and Mandai et al mentioned above. This 
fundamental difference exists even though components forming the catalysts 
of the prior patents may individually be similar to the components of the 
catalysts used in the present invention. 
Pratt and Lane et al. dissolve AlCl.sub.3 Lewis acid in basic, organic 
solvent, e.g. nitromethane in order to improve the yield of oligomers. The 
real catalyst is AlCl.sub.3 whereas nitromethane acts only as a solvent 
and controlling agent. Nitroparaffin in Lane et al. allows solubilization 
and fluidization of the AlCl.sub.3 catalyst which is not able to act as 
oligomerization catalyst while insoluble in the reaction mixture. Jansons 
et al. claim the action of Lewis acid/Lewis base complex as a solvent for 
the polymer/Lewis acid complex formed during reaction. Thus, the 
inventions of the prior art improve only the acid-catalyzed 
oligomerization of alkenes which basically takes place without basic 
co-catalyst according to the well known mechanism called cationic 
polymerization, as shown below: 
##STR1## 
where H.sup.+ ions are produced as a result of AlCl.sub.3 hydrolysis in 
the presence of small amount of water: 
EQU AlCl.sub.3 +H.sub.2 O.fwdarw.AlO.sub.2.sup.- +2H.sup.+ ( 2) 
The dissolution of AlCl.sub.3 catalyst in organic, basic solvent (e.g. 
nitromethane) may lead to the situation where AlCl.sub.3 itself or 
AlCl.sub.3 -base complex acts as an electron acceptor in the 
oligomerization process. 
In this well known, classic approach, the only active catalyst is 
AlCl.sub.3 (or H.sup.+ ions resulting from AlCl.sub.3 hydrolysis) and the 
presence of organic base (e.g. nitromethane) is not required for the 
oligomerization process to proceed. The acid induced oligomerization 
proceeds very well without Lewis base, as shown by Pratt's Examples 1 and 
2. The dissolution of the active catalyst, AlCl.sub.3, in a basic organic 
solvent (e.g. nitromethane) improves only the yield of oligomerization 
product. It is common knowledge that acidic salt (AlCl.sub.3) dissolves in 
basic solvent. Of course, as the result of this dissolution the acid-base 
complex may be formed of the formula: 
##STR2## 
which is being dissolved in the excess of nitroalkane. The role of 
organic, basic solvent is limited here to keep the AlCl.sub.3 catalyst as 
a fluid, as pointed out by Lane et al. The basic character of this solvent 
is obvious as the AlCl.sub.3 catalyst is a strong Lewis acid. 
The above shown AlCl.sub.3 -Lewis base complex represents a donor-acceptor 
character of bonding between an electron acceptor (AlCl.sub.3) and an 
electron donor (Lewis base, e.g. nitromethane). In these classes of 
compounds the chemical bond between two components is well defined and 
localized strictly between donor and acceptor atoms. Therefore, these 
compounds may be usually separated as a crystalline phase which is also 
well known in the prior art. 
In the case of molten salt catalyst of this invention, Lewis acid 
(AlCl.sub.3) is not dissolved in the Lewis base and no acid-base complex 
is formed. As the result of mixing of AlCl.sub.3 (Lewis acid) with organic 
salt (RCl, Lewis base) as shown herein, a pure mixture of ions (not 
acid-base complex) is formed, according to the reactions: 
EQU RCl+AlCl.sub.3 =R.sup.+ +AlCl.sub.4.sup.- ( 3) 
EQU 2AlCl.sub.4.sup.- =Al.sub.2 Cl.sub.7.sup.- +Cl.sup.- ( 4) 
where R.sup.+ denotes an organic cation, e.g. imidazolium, 
N-n-butylpyridinium. In order to produce a melt under ambient conditions, 
as described herein, the molar ratio of AlCl.sub.3 and RCl is maintained 
approximately within the range of about 1:2 to 2:1, which is a simple 
consequence of the above equilibrium reactions (3) and (4). Reaction (4) 
is completely analogous to the Bronsted acid-base properties of water 
(2H.sub.2 O=H.sub.3 O.sup.+ +OH.sup.-) where a Lewis acid (AlCl.sub.3) 
transfer occurs between two amphoteric anions. The ionic composition of 
the melt is dependent only on AlCl.sub.3 /RCl ratio (within the range of 
about 1:2 to 2:1). There is no complex formed between Lewis acid and Lewis 
base and also no solution of Lewis acid in Lewis base is produced by 
mixing of AlCl.sub.3 with RCl. Instead, due to a spontaneous reaction 
between AlCl.sub.3 and RCl a mixture of ionic species, according to 
equilibrium reactions (3) and (4), is formed. The specific conductivities 
of these melts are comparable with conductivities of diluted aqueous 
solutions of inorganic salts. These conductivities are much higher than 
those of AlCl.sub.3 -organic base complexes in organic solvent. 
One of the main reasons contributing to the low melting temperature 
behaviours of AlCl.sub.3 /RCl mixtures is the delocalization of the 
interaction between acid and base due to large size of pi electron cloud 
in aromatic organic salt. The delocalization of interaction means that the 
entropy of the melt is considerably enhanced over separate AlCl.sub.3 and 
RCl solids so that a lower melting temperature will result. In fact, X-ray 
crystallographic studies have shown that at least the acidic melts (with 
AlCl.sub.3 /RCl ratio&gt;1) are unlikely to possess any degree of ordering in 
the liquid phase that arise from a very weak (if any) hydrogen bond to 
chloroaluminate anions. This supports the fact that the formation of Lewis 
acid/Lewis base complex is not possible in the case of molten salt 
systems. If the interaction between Lewis acid and Lewis base were more 
localized as with an unshared pair of electrons or an unfilled orbital, a 
strong interaction may occur and crystalline adduct formation rather than 
low melting behaviour will result. Such adducts, with strongly localized 
acid-base interaction are formed when AlCl.sub.3 is dissolved in basic 
electrolytes, e.g. nitromethane, as described in the prior patents. This 
kind of crystalline adducts between inorganic salt and organic solvent is 
well known in the scientific literature. 
The simple structural considerations, as presented above, show the 
principal difference between molten salt catalytic system of this 
invention and Lewis acid-Lewis base complex of the prior art. As pointed 
out above the active catalyst in the prior patents is AlCl.sub.3 or 
AlCl.sub.3 -basic solvent complex. In the case of AlCl.sub.3 /RCl melts 
the catalytic system is composed of mixture of Al.sub.2 Cl.sub.7.sup.- ; 
AlCl.sub.4 .sup.- ; Cl.sup.- and R.sup.+ ions. The only entity which may 
be considered as an acceptor of olefinic electron (and the acting 
catalyst) is organic cation, R.sup.+. Therefore the mechanism of olefin 
oligomerization in the presence of ambient temperature melts is clearly 
different from that catalyzed by AlCl.sub.3 or AlCl.sub.3 -organic solvent 
complex. 
This difference in the mechanism of olefin oligomerization according to 
this invention also has important advantages as stated above. Thus, not 
only are mixtures of C.sub.4 -C.sub.6 hydrocarbons obtained in good 
yields, but with the molten salt catalyst of this invention it has very 
surprisingly been discovered that the ratios of the different C.sub.4 
-C.sub.6 hydrocarbons in the product may be varied by varying the 
composition of the catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following examples illustrate the process and compositions of the 
instant invention but are not to be construed as limiting the scope of the 
invention. 
Example 1 
A melt catalyst was prepared by combining together aluminum trichloride and 
pyridinium hydrochloride in the molar ratio 2:1. The two solids readily 
formed into a room temperature melt. This melt catalyst was used for the 
oligomerization of ethylene and propylene by passing the ethylene or 
propylene through the melt. The identification of the hydrocarbon products 
was confirmed by gas chromatographic (GC) and mass spectroscopic (GC/MS) 
methods. 
(a) Using .about.160 g of the melt at 80.degree. C. and an ethylene flow 
rate of 20 mL/min, the overall conversion reached 90% during the first 19 
hours of operation. Thereafter, the conversion decreased almost linearly 
and then reached .about.15% after 32 hours of operation. An average 
selectivity to C.sub.2 -C.sub.6 fraction was .about.36%. The selectivity 
of each hydrocarbon product within this fraction changed with time, but 
was relatively stable during the first 20 hours of operation. The 
selectivities are shown in Table 1 below: 
TABLE 1 
______________________________________ 
Selectivities (%) at 
time of operation 
Hydrocarbon 10 h 32 h 
______________________________________ 
ethane 2.5 0.05 
C.sub.3 (propane) 7.5 0.1 
C.sub.4 unsaturated 
30.0 19.5 
C.sub.4 saturated 40.0 27.0 
C.sub.5 unsaturated 
14.0 24.0 
C.sub.6 unsaturated 
5.5 29.0 
______________________________________ 
Also, a small amount of methane (maximum selectively 0.01%) was monitored 
during the first few hours of operation. 
(b) When propylene was passed through the melt (.about.160 g) at 80.degree. 
C. and a flow ratio of 20 mL/min, the overall conversion reached 100% 
during the first 28 hours of operation. Thereafter, the conversion 
decreased and reaches .about.10% after 51 hours. An average selectively 
to C.sub.1 -C.sub.6 fraction was .about.65%. The selectivity to each 
hydrocarbon product within this fraction was relatively stable during the 
first 28 hours and then changes as shown in Table 2 below: 
TABLE 2 
______________________________________ 
Selectivities (%) at 
time of operation 
Hydrocarbon 10 h 51 h 
______________________________________ 
C.sub.3 (propane) 8.5 0.0 
C.sub.4 unsaturated 
60.0 29.5 
C.sub.4 saturated 11.0 3.0 
C.sub.5 unsaturated 
18.5 37.0 
C.sub.6 unsaturated 
1.5 30.5 
______________________________________ 
Also, a small amount of methane and ethane (maximum selectivity 0.004 and 
0.13%, respectively) was monitored, although methane appeared only during 
the first hours of operation. 
The solids products, which accumulated in the melt, were analyzed by GG/MS 
method. The results showed a mixture of high molecular weight compounds 
(MW&lt;262) containing saturated and unsaturated hydrocarbons and/or 
chlorinated unsaturated hydrocarbons. The difference between each mass 
fragment was typically 14 corresponding to CH.sub.2 fragments. 
Example 2 
A melt catalyst was prepared in the same manner as described in Example 1 
using aluminum trichloride and N-n-butylpyridinium chloride in the molar 
ratio 2:1. Ethylene and propylene were again passed through melt at 
oligomerization conditions. 
Using .about.120 g of the melt at 40.degree. C. and an ethylene flow rate 
of 20 mL/min, the overall conversion at the beginning of the process was 
.about.80%, and achieved a steady-state value of .about.30% after 5 hours 
which was maintained for the next 5 hours. An average selectivity to 
C.sub.1 -C.sub.6 fraction was 100%. The steady-state selectivities to each 
hydrocarbon product were as follows: saturated C.sub.4 -74%, unsaturated 
C.sub.4 -12%, saturated C.sub.5 -7.5%, unsaturated C.sub.6 -6%, 
unsaturated C.sub.5 -0.1%, propane 0.05%, ethane--0.01%. The overall 
steady-state conversion decreased to 42% when a higher flow rate of 
ethylene (40 mL/min) was utilized at 40.degree. C., while the 
selectivities remained practically unchanged. 
(b) When propylene was passed through the melt (.about.100 g) at 80.degree. 
C. and a flow rate of 20 mL/min, the overall conversion was 100% during 
the first 8 hours, then decreased almost linearly and reached .about.20% 
after 28 hours. An average selectivity to C.sub.1 -C.sub.6 fraction was 
.about.35%. The selectivity to each hydrocarbon product changed with time, 
but eventually achieved the values in Table 3 below: 
TABLE 3 
______________________________________ 
Selectivities (%) at 
time of operation 
Hydrocarbon 9 h 28 h 
______________________________________ 
C.sub.3 (propane) 7.0 0.0 
C.sub.4 unsaturated 
52.5 22.0 
C.sub.4 saturated 7.0 0.1 
C.sub.5 unsaturated 
0.0 13.0 
C.sub.5 saturated 29.0 30.5 
C.sub.6 unsaturated 
4.5 34.0 
______________________________________ 
Also, small amounts of methane and ethylene (maximum selectivity 0.002 to 
0.09% respectively) were monitored during the first four hours of 
operation. At lower temperature, e.g., 40.degree. C., both the overall 
conversion and the selectivities were lower, although this effect was 
rather minor. 
The liquid products, which accumulated in the melt, were analyzed by GC/MS 
methods. The results showed that a mixture of high molecular weight 
compounds (MW&lt;206) containing saturated and unsaturated hydrocarbons were 
produced. The difference between each mass fragment was typically 14. 
Example 3 
Once again using the same procedure as in Example 1, a melt catalyst was 
prepared by combining aluminum trichloride and 
1-methyl-(3-ethyl)-imidazolium chloride in the molar ratio 3:2. Ethylene 
and propylene were separately passed through the melt at oligomerization 
conditions. 
(a) Using .about.120 g of the melt at 80.degree. C. and ethylene flow rate 
of 20 mL/min, the overall conversion reached 70% during the first 4 hours 
of operation. Thereafter, the conversion decreased and then reached 
.about.25% after 10 hours of operation. An average selectivity to C.sub.1 
-C.sub.6 fraction was 100%. The selectivity to each hydrocarbon product 
changed with time. These values were as shown in Table 4 below: 
TABLE 4 
______________________________________ 
Selectivities (%) at 
time of operation 
Hydrocarbon 2 h 10 h 
______________________________________ 
ethane 0.10 0.03 
C.sub.3 (propane) 0.4 0.01 
C.sub.4 unsaturated 
18.0 18.0 
C.sub.4 saturated 72.0 36.5 
C.sub.5 unsaturated 
7.0 16.5 
C.sub.6 unsaturated 
2.5 28.5 
______________________________________ 
(b) When propylene was passed through the melt (.about.120 g) at 60.degree. 
C. and 20 mL/min, the overall conversion reached 100% during the first 11 
hours. Thereafter, the conversion decreased to .about.50% after 31 hours. 
An average selectivity to C.sub.1 -C.sub.6 fraction was 44%. The 
selectivity to each hydrocarbon product changed gradually with time. These 
values are shown in Table 5 below: 
TABLE 5 
______________________________________ 
Selectivities (%) at 
time of operation 
Hydrocarbon 10 h 31 h 
______________________________________ 
ethane 0.05 0.35 
C.sub.3 (propane) 9.0 13.0 
C.sub.4 unsaturated 
67.0 53.0 
C.sub.4 saturated 5.0 2.0 
C.sub.5 unsaturated 
17.5 27.5 
C.sub.5 saturated 0.1 0.35 
C.sub.6 unsaturated 
1.0 3.5 
______________________________________ 
Also, a small amount of methane (maximum selectivity 0.001%) was monitored 
at all times. 
The liquid products, which accumulated in the melt, were analyzed by GC/MS 
methods. The results showed that a mixture of high molecular weight 
compounds (MW&lt;183) containing saturated and unsaturated hydrocarbons were 
produced. The difference between each mass fragment was typically 14.