Titanates and titania coated titanates as supports in the Fischer-Tropsch synthesis

Abstract

A titanate nanotube (TNT) material with a high surface area of 231 m2/g was prepared by reacting TiO2 with KOH in an autoclave for 24 hours. This surface area was about five times higher than that of the commercial TiO2 nanoparticles (TN) (or P25 Degussa) with a BET surface area of 47 m2/g. TEM images revealed that the TNT material consisted of clean tubular materials made up of coiled TiO2 sheets. The diameters of the tubes ranged from tens to hundreds of nanometers, while the lengths of these tubes varied from 60 to about 200 nm. Prolonged washing of this TNT material for about 3 weeks with deionised water was required to obtain pure tubular materials. Cobalt catalysts with loadings of 10 and 20% were prepared by impregnating a cobalt nitrate solution onto the TNT support using a pH controlled precipitation methodology. A range of techniques was used to characterize these catalysts and their catalytic performance was investigated in a plug flow reactor in the Fischer-Tropsch (FT) synthesis at 230, 250 and 280 oC with a syn-gas (H2/CO = 2) flowrate of 12 ml/min under a total pressure of 8 bar. The obtained FT results were compared with those obtained for cobalt loaded on TN. A low CO conversion of 2.2% was achieved when 10%Co/TNT was used at 230 oC while a higher CO conversion of 22.4% was achieved when 10%Co/TN was used at the same temperature. The reason for the low FT activity obtained with the tubular TNT based catalyst was partly due to the presence of potassium ions which poisoned cobalt active sites. TPR studies also revealed that it was more difficult to reduce 10%Co/TNT than it was for 10%Co/TN since its cobalt reduction peaks occurred at temperatures higher than those for the TN based catalysts. A titania coated titanate nanotube (TC-TNT) material was prepared by a sol-gel method so that a titania “protective” layer was created over the potassium TNT surface. This material was achieved by dispersing a TNT material in the solution of Ti[O(CH2)3CH3]4. The ratio of TNT to Ti[O(CH2)3CH3]4 was 1:5 and the reaction was hydrolyzed with HNO3. The surface area for this material was measured as 222 m2/g when the sol-gel preparation solution was heated at 35 oC. TEM revealed that the preparation of TC-TNT was successful and this was confirmed by the appearance of a TiO2 coating and the 1 Abstract Thabiso Terence Phadi University of vi the Witwatersrand formation of small black particles representing the sol-gel derived TiO2 on the surface of the TNT. The evaluation of 10%Co/TC-TNT in the FT reaction at 230 oC improved the CO conversion to 9.4%. However, this conversion was still lower than that obtained for 10%Co/TN. This low conversion was due to the formation of SMSI between cobalt and the TC-TNT support.