UBC Theses and Dissertations
Catalytic hydroconversion of diphenylmethane with unsupported MoS2 Kukard, Ross S.
The mechanism by which hydroconversion catalysts promote residue conversion and coke suppression is unclear. Several theories are proposed in the literature but these have all been opposed, usually due to their lack of controlled mechanistic studies. A promising catalyst for residue hydroconversion is unsupported MoS₂. This catalyst is effective but expensive and deactivates during the reaction. Model compound studies were needed to elucidate the mechanism of MoS₂ catalysis in hydroconversion reactions, how this relates to residue hydroconversion and hence propose deactivation mechanisms and regeneration methodologies. Model compound screening in a commercially available stirred slurry-phase batch reactor identified diphenylmethane (DPM) as a suitable model reagent. Experiments were conducted at industrially applicable conditions of 445°C, 13.8 MPa H₂ and catalyst loadings of 0 - 1800 ppm Mo (introduced as Mo octoate which formed the MoS₂ active phase in-situ). Slow heat-up rates and wall catalysis, however, made this reactor unsuitable for detailed mechanistic studies. A novel mixed slurry-phase micro-reactor system was designed using externally applied vortex mixing and removable glass-inserts to allow for greater analytical resolution and determination of the thermocatalytic mechanism. Deactivated MoS₂ catalysts, as coke-catalyst agglomerates recovered from residue hydroconversion studies (Rezaei and Smith, 2013), were evaluated using the DPM testing methodology and a deactivation mechanism proposed. It was determined that the unsupported MoS₂ crystallites hydrogenate the DPM feed to cyclohexylmethylbenzene (CHMB) which undergoes thermolysis to short chain hydrocarbon radicals. These short chain radicals stabilise, by radical addition or radical disproportionation, other radicals in the system by a chain stabilisation reaction, itself promoted by catalytic hydrogenation (for instance of olefins formed during disproportionation). Deactivation of unsupported MoS₂ in residue hydroconversion was proposed to be due to the formation of an unreactive, porous carbonaceous structure upon which the otherwise unaltered catalyst particles become supported. The pores physically exclude larger species, such as asphaltenes, from reaching the active sites. Inter-recycle solvent extraction to remove coke precursors was proposed to inhibit deactivation in residue hydroconversion whilst mechanical and chemical size reduction were suggested for breaking the porous structure and re-exposing the MoS₂ crystallites.
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