Isoparaffin alkylation

The enhanced diffusivity of polynuclear compounds in sc C02 has been utilized to enhance catalyst lifetimes in both 1-butene/isoparaffin alkylations (Clark and Subramaniam, 1998 Gao et al., 1996). The former may be catalyzed using a number of solid acid catalysts (zeolites, sulfated zeolites, etc.), and the use of sc C02 as a solvent/diluent permits the alkylations to be carried out at relatively mild temperatures, leading to the increased production of valuable trimethylpentanes (which are used as high-octane gasoline blending components). The enhancement of product selectivity in the latter process is believed to result from rapid diffusion of ethylbenzene product away from the Y-type zeolite catalysts, thus preventing product isomerization to xylenes. 

Alkylation was first practiced for gasoline production about 60 yr ago. At that time, most of the alkylate was used as fuel for the airplanes used in World War II. Four quite distinct reactors were developed in which isobutane and olefins were introduced as liquids to the reactor. In the reactor, the hydrocarbon liquids are contacted with either liquid sulfuric acid or liquid hydrofluoric acid (HF), which acts as a catalyst. Dispersions of these two relatively immiscible liquids are formed. The alkylate product formed is a mixture of mainly C5-C16 isoparaffins. Alkylate products often have research octane numbers (RONs) varying from 93 to 98 (the motor octane numbers tend to be two to three units lower). 

The basic reactions occurring during isoparaffin alkylation are shown in figure 1. Alkylation of isobutane with light (C3 -Cs) olefins in die presence of a strong acid catalyst involves a series of consecutive and simultaneous reactions occurring through carbocation intermediates (2). The protonation of an olefin. 

Successful design of an isoparaffin alkylation catalyst must overcome the following two challenges ... 

The key challenge in isoparaffin alkylation is to create a catalyst/reactor system that minimizes the rate of deactivation of the catalyst. To maintain economic viability, the system should not be overly complex or expensive. 

Using grants from the US Department of Energy and the National Science Foundation, Exelus started developing a viable solid-acid catalyst alternative for isoparaffin alkylation. In order to produce a commercially viable process, the goal was to develop a catalyst that was useable in a simple fixed-bed reactor process using a benign solid-acid catalyst. Our desire to use a fixed-bed reactor... 

Isoparaffin alkylation reactions are very fast and they suffer from severe pore diffusion limitations. As a result, when catalyst particle size is increased from 100 pm (for slurry reactors) to 1.6 mm for fixed-bed reactors, the catalyst activity reduces by 10-fold according to basic mass transfer models using experimental values of the intrinsic rate constant, as shown in figure 4. To match the catalyst productivity of a slurry reactor, one would need to build a fixed-bed reactor with ten times the volume - not practical for a commercial scale system. In addition to using a fixed-bed reactor, we wanted to ensure that the solid-acid catalyst was both robust as well as benign (i.e. environmentally fiiendly). 

Nafion is a perfluorinated polymer with sulfonic acid groups grafted to side chains, yielding acidity similar to that of sulfuric acid [5]. Nafion has not been extensively studied as a catalyst for isoparaffin alkylation, although it has shown good activity for a number of acid catalyzed reactions [6-9]. Nafion is available in both unsupported and supported forms. In the supported form, the polymer is impregnated on high surface area silica supports, which has been shown to improve accessibility to acid sites [10,11]. 

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