Better sulphide catalysts through optimized active phase-support interaction

Abstract

In this project, improved hydrotreating catalysts have been synthesized by varying the active phase-support interaction. This interaction has been changed by differing the calcining procedures and choice of catalyst precursors and carriers. The active phase consisted basically of molybdenum or tungsten disulphide, and Ni, Co, Cr or Fe as the promoter. Also phosphorus has been added as secondary promoter. As for the preparation of the catalysts, a new type of carrier, filamentary carbon supported by alumina, has been synthesized. To improve the interaction between Co and Mo in the active catalyst, nitrilotriacetic acid (NTA) has been added as a complexing agent to a solution containing both metal salts. Also, a new type of active phase precursor (acetylacetonate) has been used with a number of solvents (water, ethanol, toluene). The final consequences of these variations have been investigated from a physico-chemical point of view in relation to the actual performance of the newly prepared catalysts in the hydrodesulphurization of thiophene. In the physico-chemical characterization of the catalysts, Mössbauer emission and absorption spectroscopy (MES, MAS), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure (XANES) have been used. As a result, a detailed model of the so-called ‘Co-Mo-S’ phase could be proposed. Mössbauer experiments on the sulphidation of carbon-supported Co and Co-Mo catalysts indicate the formation of Co species (consisting of Co and Co-Mo, respectively), which have the same spectral features in Mössbauer spectroscopy. Hence, care should be taken into account when referring to the so called ‘Co-Mo-S’ phase. In addition, fingerprinting techniques such as TPR (temperature programmed reduction) and TPS (temperature programmed sulphidation) have been applied extensively. The sulphided catalysts contain different sulphur species. The existence of a very reactive sulphur species called ‘excess sulfur’ is correlated with the hydrodesulphurization activity. A model has been developed describing the coke deposition on hydrotreating catalysts under commercial reaction conditions. As a result, improved hydrotreating catalysts have been developed and a better understanding of the fundamental characteristics of the catalysts has been acquired. Novel equipment has been developed. Two of its major features are a simple design and the possibility for the in situ measurement of physical parameters of the autoclave contents. For future development of the catalysts, a detailed kinetic analysis of their performance, and the application of in situ characterization techniques is of paramount importance.