Ethylene conversion process

This invention relates to a process for the converting ethylene to a mixture of 1-olefins by (a) feeding ethylene through an oligomerization catalyst bed to form an oligomer predominating in C2-C6 olefins and (b) feeding the oligomer, optionally admixed with fresh ethylene, containing at least 50% by volume ethylene, through a metathesis catalyst bed to form a product predominating in C2-C6 1-olefins. Steps (a) and (b) can be carried out sequentially and in a single reactor under the same conditions of reaction temperature and pressure. This mitigates the perceived disadvantages of low activity and low conversions of relatively inexpensive oligomerization catalysts and forms oligomer having the desired distribution of 1-olefin components for use as comonomer in a polyolefin process.

The present invention relates to a process for the conversion of ethylene 
to a mixture of lower olefins by subjecting ethylene to a sequential 
oligomerisation and metathesis using a separate catalyst system for each 
stage in order to produce a mixture of products which is suitable for use 
as a comonomer during the polymerisation of olefins. 
It is well known to use comonomers such as eg butene-1 or hexene-1 during 
the production of some grades of polyethylene such as linear low density 
polyethylene (hereafter "LLDPE"). However, the cost of eg hexene-1 
comonomer is significantly more than the cost of ethylene primarily 
because the comonomer is produced by a dedicated process. Thus, the 
potential for improving the economics of the process by producing the 
comonomers cheaply is significant. Hitherto, such comonomers have been 
synthesised on the polyethylene plant for instance by passing part or all 
of the ethylene feed over an oligomerization catalyst to selectively 
produce 1-olefins in situ. The oligomerization process is usually operated 
to maximise the yield of the oligomer and hence relatively higher reaction 
temperatures and pressures as well as stronger and relatively expensive 
oligomerization catalysts are used. Such catalysts are highly reactive and 
are therefore air sensitive and deactivate readily. Also, such an 
oligomerization process produces a mixture of linear olefins but the 
distributions of the various oligomeric 1-olefins in the mixture is not 
always constant. Moreover, the amount of non-polymerizable components 
(such as eg butene-2) of such a mixture is significantly above the 
preferred maximum tolerance level of 1 to 1.5 mole % and therefore cannot 
be fed directly to the ethylene polymerization process without further 
purification to minimise the concentration of the non-polymerizable 
components therein. In addition, the reaction pressure and temperature 
used for the oligomerization process to obtain the right proportion of 
components in the mixture of oligomers is often at variance with the 
process used for producing polyethylene from ethylene, especially in the 
conventional gas phase process. Furthermore, the oligomerization products 
thus produced is a complex mixture which has to be separated/purified in 
order to ensure that the higher 1-olefins therein are not fed to the 
polymerization stage. Such higher olefins are usually separated and sold 
to the detergent alkylate industry. 
It has now been found that such conversion of ethylene to the desirable 
mixture of lower olefins can be increased above 10% and up to 30% by 
subjecting ethylene to a combined oligomerization and metathesis process. 
Accordingly, the present invention is a process for the conversion of 
ethylene to a mixture of olefins predominating in 1-olefins said process 
comprising 
a. feeding ethylene through a bed of an oligomerization catalyst to form an 
oligomer predominating in C2-C6 olefins and 
b. feeding the oligomer so formed either as such or after admixture thereof 
with further aliquots of ethylene so as to maintain the concentration of 
ethylene in the oligomer to at least 50% by volume through a catalyst bed 
to metathesise the oligomer feed to a mixture of olefinic products 
predominating in C2-C6 1-olefins. 
The oligomerization catalyst used in step (a) suitably comprises at least 
one metal or at least one oxide of a metal selected from Groups VIA, VIIA 
and VIIIA according to the Periodic Table (IU) deposited or impregnated 
on a support either as such or by ion-exchange with a solution of the 
metal salt which may for example be a nitrate, acetate or oxalate, 
followed by calcination. The catalyst suitably has 0.1-50% w/w of the 
metal as such or in the form of its oxide, preferably from 1 to 20% w/w. 
The aluminosilicate support in the catalyst suitably has a silica to 
alumina ratio of20:1 to 500:1, preferably from about 60:1 to 200:1. More 
specifically, the oligomerization catalyst used is preferably such that 
under the reaction conditions, the conversion of ethylene to the oligomer 
is suitably below 30% at steady state. An example of such a metal oxide is 
nickel oxide and an example of a suitable support is an aluminosilicate 
such as eg grade SP 2-8341 (ex Grace GmbH). The oligomerization catalyst 
is suitably activated prior to use. The activation is suitably carried out 
by calcining initially in air and then optionally in an inert atmosphere, 
eg nitrogen, at an elevated temperature, eg about 500.degree. C. The same 
process can be used to regenerate the used catalyst. 
The oligomerization reaction (a) is suitably carried out at pressures 
ranging from 100 to 10000 kPa, preferably 500-5000 kPa, and at a 
temperature ranging from ambient to 120.degree. C. in order to obtain an 
oligomer which predominates in C2-C6 olefins. In such a process, at 
relatively lower temperatures within this range, the conversion of 
ethylene is suitably maintained below 30% at steady state and the oligomer 
is rich in 1-olefins and hence the subsequent metathesis of such an 
oligomer results in a product mixture which is rich in ethylene. At 
relatively higher temperatures, the oligomer comprises a significant 
proportion of 2-olefins such as butene-2; metathesis of such an oligomer 
gives rise to a product mixture rich in propylene. 
The oligomer feed to the metathesis step (b) is suitably such that said 
feed is rich in ethylene in order to achieve the desired metathesis. The 
concentration of ethylene in this oligomer feed should be at least 50% 
v/v, suitably at least 70% v/v and preferably at or above 80% v/v. Using 
an olefinic feed to the metathesis step rich in ethylene ensures that 
self-metathesis amongst the higher olefin components of such a feed is 
minimised and the product mixture emergent from the metathesis step has 
the desired distribution of 1-olefins therein. The desired concentration 
of ethylene in the feed to the metathesis step (b) to at least 50% v/v can 
be achieved either 
i. by controlling the oligomerization reaction conditions in step (a) to 
achieve a low conversion of ethylene or 
ii. by carrying out step (a) to achieve a higher conversion of ethylene to 
the oligomers but admixing the oligomerization product with further 
aliquots of fresh ethylene in order to bring the concentration thereof in 
the feed to the metathesis step (b) to at least 50% v/v. 
The metathesis reaction (b) of the oligomer from step (a) is suitably 
carried out using a metathesis catalyst comprising at least one metal or 
at least one oxide of a metal from Group VIA or Group VIIA of the Periodic 
Table (IU). The metathesis catalyst is preferably used in the 
heterogeneous phase. If the catalyst is used in a heterogeneous phase, it 
is suitably in the form of a metal oxide deposited or impregnated on a 
support. The amount of metal oxide on the support in the metathesis 
catalyst is suitably in the range from 0.1 to 15% w/w, preferably 0.5-12% 
w/w based on the total weight of the metal oxide and the support. Examples 
of suitable metal oxides include oxides of rhenium, tungsten, cobalt or 
molybdenum. Examples of suitable supports which may be used include 
alumina, phosphated alumina, silica and aluminosilicates. Rhenium 
heptoxide on alumina is preferred. The metathesis catalyst is suitably a 
heterogeneous catalyst and is activated prior to use. The activation is 
suitably carried out by calcining initially in air and then optionally in 
an inert atmosphere, eg nitrogen, at an elevated temperature, eg about 
500.degree. C. The same process can be used to regenerate the used 
catalyst. A feature of the present invention is that the oligomerization 
and metathesis can be carried out under the same conditions of reaction 
temperature and pressure. Thus, by using a combined oligomerization and 
metathesis process carried out sequentially and in a single reactor, the 
perceived disadvantages of relatively inexpensive oligomerization 
catalysts such as low activity and low conversions can be mitigated to 
obtain the desired oligomer having the desired distribution of 1-olefin 
components therein thereby improving the economics of the polyolefin 
process.