Silica-Immobilized Sulfur Compounds as Solid Calibrants for Temperature-Programmed Reduction and Probes for the Thermal Behavior of Organic Sulfur Forms in Fossil Fuels

For the well-swept fixed-bed reactors used in temperature-programmed reduction (TPR) to specify the organic sulfur forms present in coals and kerogens, calibrants must neither melt nor evaporate before the onset of thermal decomposition. In this respect, nonmelting silica-immobilized substrates are suitable with the Si-O-C linkage being stable up to ca. 500 degrees C. Silica-immobilized samples of dibenzothiophene, diphenyl sulfide, phenyl benzyl sulfide, and thioanisole have been synthesized and noncatalytic tests have been conducted in atmospheric and high-pressure TPR reactors. The characteristic reduction temperatures of the non-thiophenic compounds investigated are well resolved from that of dibenzothiophene for both techniques and the results have validated previous findings by TPR on coals. The use of high hydrogen pressure (150 bar) lowered the reduction temperatures substantially. The H2S recoveries from the atmospheric experiments are low suggesting that, for the non-thiophenic compounds, secondary reactions occur yielding refractory thiophenes which are not detected. Although sulfur recoveries are greatly improved, such reactions are still evident with 150 bar hydrogen pressure especially in the case of the phenyl benzyl sulfide, possibly as a consequence of the high surface coverages used. Insights into the retrogressive chemistry occurring for the immobilized phenyl benzyl sulfide have been provided from GC-MS analysis of hydrolyzed TPR residues obtained at different temperatures, and from vacuum pyrolysis experiments conducted as a function of surface coverage.