Hydroconversion of model Fischer‑Tropsch wax over noble metal/silica-alumina catalysts

Sammanfattning: Synthetic fuels produced using the Fischer-Tropsch technology will play an important role in the future of the transportation sector. The Fischer-Tropsch synthesis (FTS) allows converting synthesis gas (CO + H2) into fuels of outstanding quality. The synthesis gas can be obtained from different carbon sources: natural gas, coal and biomass. In order to maximize the yield of middle distillates, the process is carried out in two main steps: the FT-synthesis that produces long-chain hydrocarbons (waxes) and a hydrocracking step, to selectively convert the waxes into fuels. Diesel produced by this process is characterized by excellent combustion properties and reduced harmful tailpipe emissions compared to conventional diesel.Due to the growing interest in synthetic fuel production, from the industry and the academia, and to the peculiar characteristics of the Fischer-Tropsch products, research in hydrocracking has received renewed attention. Catalysts for the hydrocracking of long-chain paraffins have been the subject of the present work, which is the summary of four scientific publications.Noble metals supported on acid carriers have been compared, especially for what regards the mechanisms through which hydrocracking proceeds. The catalysts were synthesized and characterized by various techniques, including N2 physisorption, H2 chemisorption, TEM, pyridine adsorption FTIR, ammonia TPD, etc. It was shown that catalytic activity is mainly dependent on the acid support used; that selectivity is strongly dependent on conversion, high conversion favoring highly branched cracking products. Two main reaction routes were observed: bifunctional hydrocracking and hydrogenolytic cracking. Platinum-containing catalysts showed high selectivity towards the latter, while palladium/silica-alumina behaved as pure bifunctional catalysts. Catalyst deactivation was investigated and initial sintering of metal particles was observed. Coking was also a cause of deactivation. Formation of coke deposits was highly dependent on the metal loading of the catalysts. Metal loading also influenced catalyst selectivity, especially in the case of platinum/silica-alumina catalysts. Monofunctional hydrogenolysis on the platinum particles, superimposed to the bifunctional mechanism was observed. This route increased selectivity towards linear hydrocarbons and methane, with increasing amounts of platinum. The specific rate of hydrogenolysis was constant for different loadings of platinum on the same acid silica-alumina support. Using a different, less acid, support negatively affected the hydrogenolytic activity of the platinum catalytic sites. It was concluded that metal-support interactions might play an important role in the catalytic properties of platinum surfaces.This work has contributed to increasing the knowledge about hydrocracking of long-chain alkanes and pointed out some features that might have practical interest in the application of this technology to synthetic-fuel production.