CHAIN-LENGTH DEPENDENCE OF ALPHA-OLEFIN READSORPTION IN FISCHER-TROPSCH SYNTHESIS

The total product concentration and the paraffin/olefin ratio have bee n measured up to C-14 for Fischer-Tropsch synthesis on polycrystalline Co foils. The influences due to surface area, a wax coating, the H-2/ CO ratio and flow velocity on concentration and selectivity have been determined. The paraffin/olefin ratio increases exponentially with cha in length which is attributed to a chain-length-dependent olefin reads orption mechanism. The probability of readsorption depends on the heat of physisorption of the olefins on the catalyst as well as on their h eat of dissolution in and their diffusivity through the product wax. A ll three factors predict an increase of the paraffin/olefin ratio with carbon number. Physisorption and dissolution are shown to cause a muc h stronger chain-length dependence than diffusion and will usually dom inate. (C) 1995 Academic Press, Inc.