Structure-activity relationships of ruthenium Fischer-Tropsch catalysts (metal particle size effects)

In the group VIII transition metal catalytic conversion of hydrogen/carbon monoxide mixtures to hydrocarbons (Fischer-Tropsch catalysis), it is known that certain catalysts catalyze the production of a narrow boiling range (C{dollar}sb6{dollar}-C{dollar}sb{lcub}12{rcub}{dollar}) product which does not fit the traditional Anderson-Schulz-Flory (ASF) chain growth model. Among the proposed explanations for this selectivity is one based on control of hydrocarbon chain propagation by metal particle size. The focus of this work was to study the effect of metal particle size on catalytic activity for the F-T synthesis.; Nonaqueous Soxhlet extractions of Ru{dollar}sb3{dollar}(CO){dollar}sb{lcub}12{rcub}{dollar} were used to prepare silica-supported ruthenium catalysts with surface area weighted average metal particle sizes (d{dollar}sb{lcub}rm s{rcub}{dollar}) of {dollar}sim{dollar}5nm. Aqueous impregnations of silica with RuCl{dollar}sb3cdot{dollar}3H{dollar}sb2{dollar}O were used to prepare catalysts. It was found that average Ru particle sizes are more sensitive to percent metal loading than to catalyst preparation technique and that Ru particle size distributions followed a skewed Gaussian or log-normal curve.; The silica-supported and unsupported Ru catalysts catalyzed the production of a hydrocarbon product which followed the ASF chain growth model and which consisted primarily of n-alkanes and linear 1-alkenes. It is postulated that primary products from silica-supported and unsupported Ru catalysts are alkenes and that alkanes in the reactor effluent are produced by hydrogenation of the alkenes. An equation was derived relating the weight fraction of alkenes and alkanes to the residence times of the alkenes in the reactor and this equation produced a reasonable fit to the experimental data.; It was observed that hydrocarbon, CO{dollar}sb2{dollar} and CH{dollar}sb4{dollar} production increased with time apparently reaching steady state after {dollar}sim{dollar}200h. It was also found that increasing reactant gas space velocities (SHSV's) increased the steady state turnover numbers for hydrocarbon, CO{dollar}sb2{dollar} and CH{dollar}sb4{dollar} production, while at the same time, the ASF probabilities of chain growth and alkene/alkane ratios remained effectively constant. From these observations, it is postulated that the number of active catalytic sites increase with time and increasing SHSV's. (Abstract shortened with permission of author.)...