(552d) Effect of Pre- and Post-Reaction Measurement of Cobalt Particle Size and Metal Fraction On the Turnover Frequency of Silica Supported Fischer-Tropsch Catalysts

Fischer-Tropsch synthesis (FTS) is receiving renewed attention, driven by the global need to convert non-petroleum-based energy resources into fuels and chemicals. Cobalt catalysts are known to favor higher molecular weight hydrocarbons in FTS. The influence of support pore size on the cobalt dispersion and intrinsic activity of the resultant catalysts has been examined by several groups recently.[1-4] In addition to influencing the catalyst dispersion, narrow pore size supports may also shift product distribution through the restriction of molecular diffusion. With the wide variety of supports available as well as new synthetic methods it is important to understand the relationships between support pore size and catalytic performance. Despite the recent advances, there remains much to learn regarding the effect of cobalt dispersion and support on FTS. In this work, we characterized cobalt catalysts impregnated onto silica supports with different pore sizes. Cobalt catalysts were prepared by wet impregnation of aqueous cobalt nitrate (10 wt% cobalt) onto four silica supports: CoSi1, wide pore commercial silica gel (d_pore=22.3nm; S_area=307m²/g); CoSi2, mesoporous beads (d_pore=13.0nm; S_area=464m²/g); CoSi3, ordered mesoporous, SBA-15 (d_pore=10.0nm; S_area=862m²/g); and CoSi4, ordered mesoporous MCM-41 (d_pore=3.2nm; S_area=978m²/g).  The nitrate precursor was thermally converted to the oxide by calcination in air. The catalyst was then reduced in hydrogen for activation followed by Fischer-Tropsch synthesis which was performed at 543 K and 10 bar in an Altamira AMI-200 R-HP characterization instrument. The catalyst samples were calcined in air at 723K for one hour and reduced in 10% H₂ in Ar at 773K for 5 hrs prior to the FT reactions. During the FT synthesis, two gases, 10% CO in He and 10 % H₂ in Ar (with a 1:2.1 mole ratio), were fed to the catalyst. The reactor was heated 543 K at 10 K/min and held at temperature for 10 hours. Reaction products were analyzed with a SRS RGA-300 Mass Spectrometer. Inert gases were used as internal standards. Conversion was calculated from the change in CO/He and H₂/Ar ratios. We characterized the catalyst properties, Co⁰ particle diameter and extent of reduction, at three different stages in catalyst history: (1) pre-reduction/pre-reaction; (2) post-reduction/pre-reaction; and (3) post-reaction. Catalyst properties were characterized by nitrogen porosimetry, temperature programmed reduction and x-ray diffraction (phase identification and particle size. Table 1 shows the Co⁰ particle size at all three stages measured by XRD. In stage one, Co⁰ size was determined from XRD data of Co₃O₄ particle size reduced by 75%. Turnover frequencies (TOF) calculated from specific reaction rates were significantly different depending on the measurements used to determine dispersion at shown in Figure 1. We also observed the oxidation of Co⁰ to Co(II)O during FTS of the catalyst supported on MCM-41, the silica support with the smallest pore size.

Table 1. Mole fraction (χ) Co⁰ as determined by TPR and XRD and the Co⁰ particle diameters (nm) in parentheses, determined by XRD.

Catalyst	Stage 1	Stage 2	Stage 3
CoSi1	(9.6)	0.78 (21.4)	0.77 (15.7)
CoSi2	(9.2)	0.52 (9.8)	0.66 (13.1)
CoSi3	(7.6)	0.88 (8.4)	0.89 (7.8)
CoSi4	(8.0)	1 (10.2)	0.52 (3.0)