Dimerization of n-Butenes for High Octane Gasoline Components Michael Golombok* and Jacques de Bruijn Shell International Oil Products, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands Dimerization of linear olefins represents an attractive route for the production of high octane number blending components. The oligomerization needs not only to be high conversion and to produce mainly dimers but also to be selective within the dimer range, as only certain isomers have advantageous blending octane numbers. Moreover, the octane numbers of these dimers blend in a highly nonlinear fashion. Normally the focus for such gasoline polymerization processes is on maximizing branching; however, in this study we seek to maximize the fraction of allylic hydrogens, because we have shown that these are directly linked to the blending octane number of the species. 1-Butene and 2-butene produce different quality dimer gasoline. This unexpected result can be explained from a consideration of the isomerization kinetics between the butene monomers. 1. Introduction The manufacture of high octane gasoline blending components is particularly attractive when it is coupled to the upgrading of a relatively low value component such as one of the subsidiary products of a refinery or chemical plant process stream. In addition, gas to liquid conversions are desirable for reasons of transport and storage. One way of adding value is to convert a gas sidestream into a high octane number liquid. Typical feeds are the butene (so-called “BB”) streams emerging from steam or catalytic crackers. These have become particularly attractive because very recently methyl tert- butyl ether (MTBE)sthe standard high octane oxygen- ate blending componentshas been outlawed in Califor- nia, and the trend is expected to continue the usual expansion eastward. This is resulting in a return to previously evaluated processes for manufacturing high octane gasoline componentssand one of the most fa- vored of these are butene dimers in the form of branched octenes. One of the oldest commercial processes for making “polygasoline” is the CATPOLY process of UOP using phosphoric acid on kieselguhr. 1 Temperatures are suf- ficiently high (200 °C) that the oligomerization is clearly not selective to the particular isomers associated with high octane value (see below). The high acid conditions and the use of phosphoric acid were superseded by the OCTOL and CATCON processes which convert a mix- ture of isobutene and n-butene to C8 olefins with a relatively high degree of branching. 2 The process has a high conversion (up to 90%) yet maintains a high selectivity to dimers of 85%. Work on various zeolites has been directed at opti- mizing catalyst preparation to get the best gasoline yield, 3,4 and recent work has successfully modeled the oligomer distribution that is obtained. 5 However, there has been little focus on optimizing the product mixture for best product end use in an engine. The main issues that emerge from a survey of previous work is that much of it is chemicals directed. 6-8 The octane properties do not emerge as important parameterssat least in explicit descriptions of this work. The OCTOL process is geared toward high isoindex. 2 Isoindex is a parameter devel- oped by UOP-Huls to describe the average properties of mixtures of gasoline molecules. It measures the branching of species be weighting it by the mole content x i of species i where the average branching is defined by the presence of primary, secondary, or tertiary carbons on a molecule. Thus, for n-octane η ) 1, for methylheptane (and ethylhexane) η ) 2, and for trimethylpentane η ) 3. A 1:1:1 mixture would have η ) 2, thus representing the average branching of the molecules. However, we have previously shown that, whereas branching is a good measure of octane number for paraffinic components, it fails for olefins. 9 This is because the best value of the octane boost provided by an olefin comes from its blending octane number, which describes the nonlinear blending as shown in Figure 1. From this, we obtain the blending octane number as where ON ref is the octane number of the base fuel and ON bl is the octane number of the blend containing fraction f of the component whose pure octane number is ON. (There are two ways of measuring octane * Corresponding author currently with Shell International Chemicals at the same address. Tel: 31 20 630 2794. Fax: 31 20 630 8004. E-mail: mike.m.golombok@opc.shell.com. Figure 1. Relationship between octane number of a pure com- ponent, blend, and base fuel. η ) ∑ η i x i (1) BON ) ON ref + 1 f (ON bl - ON ref ) (2) 267 Ind. Eng. Chem. Res. 2000, 39, 267-271 10.1021/ie9906060 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000