Alkyl lead compounds, which have been conventionally used as anti-knock additives to motor fuels, exhibit undesirable properties such as the toxicity of the lead and their poisoning effect on catalytic converters. As a result, a need has arisen worldwide for other high octane compounds to be blended into the pool of fuel components available from refineries and synfuel plants.
Because of certain limits to the extent to which oil refineries and synfuel plants can increase the octane number of available fuel pool components, the demand for isoalkenes, especially in the C.sub.4 to C.sub.5 range, has risen considerably in recent years. One of the products which can be synthesized from isobutene is methyl tert-butyl ether (MTBE), which is a valuable octane boosting fuel additive. As a consequence, there is an increased demand for isobutene and for isomerization processes for the conversion of linear butenes to isobutene. Similarly, linear pentenes can be converted to isopentenes which could then be used to produce tert-amyl methyl ether (TAME).
lsomerization processes can be directed towards either skeletal isomerization or double bond isomerization. Skeletal isomerization is concerned with the reorientation of the molecular structure of a hydrocarbon, so as to increase the number of side chains. Double bond isomerization is concerned with the relocation of a double bond between carbon atoms forming part of a chain, and is not of great interest in the context of increasing octane values.
A large number of isomerization processes has been described in the literature, which has been summarized by Butler and Nicolaides in Catalysis Today, Vol. 18, 1993, 443 -471. Some of these processes rely on the use of metal oxides or crystalline aluminosilicates, some of them halogenated and others not, some of them containing metals and others not, some of them requiring a diluent in the isomerization reaction and others not. Most of the zeolite and halogenated alumina catalysts for the isomerization of linear butenes have a short life due to the formation of coke during the isomerization reaction, resulting from the occurrence of parasitic reactions such as cracking, polymerization and oligomerization, and a lack of stability frequently resulting in a rapid decrease of the conversion rate.
RSA Patent Application No 92/1318 and European Patent Application No 92200516.0 disclose a process for the conversion of a feedstock comprising linear olefins into a product enriched in branched olefins, which process comprises contacting the feedstock with a tectometallosilicate having a ferrierite crystal structure at a temperature between 150.degree. C. and 450.degree. C., and olefin partial pressure of more than 0.5 bar and a total pressure of between 0.5 and 25 bar. As will be appreciated by those skilled in the art, a tectometallosilicate which has a ferrierite crystal structure, is a fully or at least substantially crystalline material.
The use of the ferrierite zeolite, as a catalyst for the skeletal isomerization reaction of linear buteries, at an olefin partial pressure of Jess than or equal to 0.5 bar is described in European Patent Application 9320 1677.7. As expected, the lower partial pressures result in higher yields of the branched isomer.
European Patent Application No 92305090.0 discloses a process for structurally isomerizing a linear olefin of at least 4 carbon atoms to its corresponding methyl branched iso-olefin, which process comprises contacting a hydrocarbon feedstream containing at least one said linear olefin with an isomerizing catalyst at a temperature of from 340.degree. C. to 650.degree. C., said isomerizing catalyst comprising at least one zeolite with one or more one-dimensional pore structure having a pore size small enough to retard by-product and coke formation within the pore structure and large enough to permit entry of the linear olefin and allow formation and diffusion of the methyl branched iso-olefin. Likewise, it will be appreciated that a zeolite is considered by those skilled in the art, as a fully crystalline or at least substantially crystalline material. Examples of zeolites which can be used according to the aforementioned publication, include ferricrite, FU-9, NU-23, NU-10, ZSM-12, ZSM-22 etc, and molecular sieves such as SAPO-11, SAPO-31, SAPO-41, etc.