Olefin dimerization method

Olefins with between 4 and 24 and preferably between 10 and 20 carbon atoms are selectively dimerized by using zeolite catalysts in acid form which are partially exchanged with salts of metals having a valency of two or more.

This invention to an olefin dimerization method. More particularly, the 
invention relates to a method for the dimerization of linear olefins with 
between 4 and 24 and preferably between 10 and 20 carbon atoms. 
The patent and scientific literature describes various methods for the 
oligomerization of olefinic compounds. The catalysts used in the known 
art, which can be of vary varied types and be either homogeneous or 
heterogeneous, are mainly active with lower olefins. 
Moreover, the obtained reaction products, including dimers, generally have 
a rather wide molecular weight distribution. If the requirement is to 
selectively obtain the dimer in an efficient manner, the known methods and 
catalysts demonstrate their limits, particularly if a long-chain olefin is 
to be dimerized. In this respect, known catalysts are sufficiently active 
only with the lower olefins. 
Said known catalysts can be organometallic complexes, which are difficult 
to synthesize and are sensitive to atmospheric agents, or can be very 
strong acids such as BF.sub.3, which have to be recovered on termination 
of the reaction and present the problems associated with homogeneous acid 
catalysis. 
Among the known catalytic systems there are some of zeolite type, but the 
products obtained with these latter catalysts are a rather complex mixture 
deriving from both cracking and oligomerization reactions. 
An example of this latter type of catalyst is described in European patent 
application 150105. 
Zeolite catalysts of the known art are therefore particularly useful in 
converting lower olefins into a more easily usable product having a higher 
boiling point, but are unsuitable for the selective dimerization of 
olefins. 
It has been surprisingly found possible, with high conversion and 
selectivity, to dimerize olefins with between 4 and 24 and preferably 
between 10 and 20 carbon atoms by using acid zeolite catalysts exchanged 
with suitable salts. 
The present invention provides a method for the dimerization of olefins, in 
particular linear olefins, with between 4 and 24 and preferably between 10 
and 20 carbon atoms, codmprising bringing a stream containing one or more 
olefins having a number of carbon atoms lying within said range into 
contact with a catalyst consisting of a zeolite in acid form, 
characterised in that the zeolite is partially in acid form and partially 
exchanged with cations of one or more bivalent or trivalent metals, the 
remainder being exchanged with alkaline metals, the dimerization reaction 
being conducted at a temperature not exceeding 280.degree. C. 
The catalysts used in the method according to the present invention are 
obtained from natural or synthetic zeolites in a form exchanged with 
alkaline metals, of the faujasite X and Y type, by firstly exchanging the 
natural or synthetic zeolite with an ammonium salt in order to replace at 
least 50% of the alkaline metal cations present on the zeolite with 
ammonium, then calcining at a temperature of between 400.degree. and 
600.degree. C., preferably 500.degree. C., to convert the zeolite into 
partially acid form, then subjecting it to partial ion exchange with the 
salt of a bivalent or trivalent metal, and finally drying at moderate 
temperature. The catalyst obtained by the aforesaid procedure is one which 
is exchanged to the extent of 10-50% with the originally present alkaline 
metal and to the extent of 1-30% with the bivalent or trivalent metal, the 
remainder being acid sites. The aforesaid preparation procedure relates to 
commercial starting materials, ie natural or synthetic zeolites exchanged 
with alkaline cations. 
If a completely acid zeolite is to be used, this must be exchanged to the 
extent of 10-50% with an alkaline cation and to the extent of 1-30% with a 
bivalent or trivalent metal cation, the rest remaining acid. 
The bivalent or trivalent metal cations derive from water-soluble salts of 
bivalent or trivalent metals, preferably Fe, Co, Ni, La, Ca, Ba, Sr, Cu, 
Nd or Zn, and preferably from sulphates, nitrates or acetates of the 
aforesaid metals, and more preferably from halides, particularly 
chlorides. 
The catalyst pdrepared in this manner demonstrates strong activity in the 
dimerization of C.sub.4 -C.sub.24 olefins with high selectivity and 
conversion. The reaction is preferably conducted in the liquid phase at a 
temperature of between 50.degree. C. and 270.degree. C., and preferably 
between 120.degree. C. and 200.degree. C., in the presence or absence of 
an inert solvent at atmospheric or greater than atmospheric pressure. The 
reaction can be conducted batchwise or continuously, the reaction products 
being separated from the effluent and the unconverted reagents being 
recycled. High conversions are generally obtained, with a dimer 
selectivity exceeding 90%. The remaining reaction product is the trimer 
containing negligible quantities of higher oligomers. Cracking reactions 
are negligible under the described conditions. 
With the method according to the present invention it is therefore possible 
to obtain a reaction product which by virtue of the fact that its chemical 
and physical properties are better defined within a narrow range of values 
possesses superior attributes for particular applications. 
In this respect, by choosing a suitable olefin or olefin cut, for example 
C.sub.12 -C.sub.14 n-olefins, a product can be obtained which after 
hydrogenation is particularly suitable in the formulation of lubricants 
because of its viscosity characteristics and narrow boiling range.