III International Conference

PP-II-146THE INFLUENCE OF THE ACIDITY CHARACTERISTICS OF A COBALTCATALYST ON THE FISCHER-TROPSCH SELECTIVITYViljoen, Elvera 1, 2 , Van Steen Eric 1 , Van de Loosdrecht Jan 1 , Claeys Michael 11 Sasol Technology (Pty) Ltd, P.O. Box 1, Sasolburg 1947, South Africa2 Department Chemical Engineering, University of Cape Town, Cape Town, Private Bag,Rondebosch, 7701, South AfricaE-mail: elvera.viljoen@sasol.comThe incorporation of strong acid sites by the addition of promoters has a large influenceon the Fischer-Tropsch selectivity of a cobalt catalyst. Strong acid sites increases the rate ofdouble bond isomerisation, decreasing the rate of re-adsorption of olefins and re-incorporationduring chain growth leading to a decrease in the chain growth probability.The Fischer-Tropsch synthesis is the catalytic conversion of synthesis gas, a mixture ofhydrogen and carbon monoxide, yielding long chain hydrocarbons. Desired products in theFischer-Tropsch synthesis can be low and intermediate molecular weight olefins, liquidhydrocarbons (C 5+ ) and/or linear C 20+ hydrocarbons. Methane is an undesired product of theFischer-Tropsch synthesis. Therefore, the methane selectivity should be decreased with anincrease in the Fischer-Tropsch chain growth probability.Some literature on CO hydrogenation indicates that molybdenum oxide and vanadiumoxide used as a promoter may be beneficial for the selectivity of a cobalt based catalyst bydecreasing the methane selectivity and increasing the chain growth probability. 1-7An alumina support was modified with molybdenum and vanadium in attempt to createmolybdenum and vanadium oxide layer covering the alumina surface. Cobalt was added tothe modified support by impregnating precipitated nano-size cobalt hydroxide particles.The incorporation of molybdenum oxide and vanadium oxide in the formulation of acobalt-based Fischer-Tropsch catalyst introduced strong acidic sites as determined by densityfunctional theory calculations (DFT) and NH 3 -TPD in combination with the iso-propanol testreaction.DFT calculations showed that an increase in the strength of the acid sites decreased theactivation energy of double bond isomerisation. The increase in the strength of the acid sitesin the catalysts due to the addition of molybdenum oxide and vanadium oxide thus led to an431