Secondary reactions of ethylene and propylene in the Fischer-Tropsch synthesis on cobalt-aluminum and cobalt-chromium catalysts

Abstract

The only secondary reaction of olefins in the Fischer-Tropsch synthesis on Co/SiO 2 ·Al(III) and Co/SiO 2 ·Cr(III) with 2.7·10 -6 and 4.0·10 -6 g-ion/m 2 cation concentration in the support matrix, respectively, is dimerization of ethylene to give a four-carbon intermediate, which participates in the growth of C-C bond chains according to the classical FT synthesis mechanism. When the chromium concentration is increased by a factor of 1.7, secondary ethylene reactions occurring on the acid sites of the catalyst are also observed. When the aluminum concentration is increased by a factor of 3.4, we observe the oligomerization of ethylene and propylene. The secondary reactions of ethylene mainly result in linear hydrocarbons, while oligomerization involving propylene gives a significant amount of branched hydrocarbons.The secondary reactions of olefins in the Fischer-Tropsch (FT) synthesis on cobalt catalysts have been studied in the past few decades [1]. The interest in these reactions has arisen primarily in attempts to control the selectivity of the hydrocarbon synthesis toward specific products. Thus, these reactions hold great theoretical and practical importance in the study of the FT synthesis. The secondary reactions of olefins may proceed not only on cobalt but also on the acid sites of the catalysts [2]. Such sites may be metal cations (Lewis sites (L sites)) or related Brönsted acid sites [3].In the present work, we studied the effect of the surface concentration of aluminum and chromium cations on the secondary reactions of olefins in the FT synthesis on Co/SiO 2 ·Al(III) and Co/SiO 2 ·Cr(III) catalysts. EXPERIMENTALThe experiments were carried out in a flow reactor at 5.0 MPa and 530 K. The volumetric rate for the introduction of synthesis gas (H 2 : CO = 2.5 ± 0.1) was 1.2 ± 0.1 h -1 .Silicon dioxide was obtained by adding a sodium silicate solution to neutralize aqueous sulfuric acid to pH 6.5-7.0. After syneresis, the silica hydrogel was ground and washed with distilled water to remove the sodium sulfate side-product (monitoring by reaction with 5% aqueous BaCl 2 ). The washed silica gel was subjected to steam treatment at water bath temperature over 7 h. The chromium and aluminum forms of silica gel for the preparation of samples 1 were obtained by direct ion exchange through treatment of the sodium form by chromium nitrate and aluminum sulfate at 373 K with subsequent 0040-5760/08/4402-0121