Synthesis of highly active unsupported molybdenum sulfide catalysts for hydrosulfurization and hydrodeoxygenation

Abstract

The rigorous regulation on sulfur emission calls for newly designed catalysts with deep hydrodesulfurization capability. This has triggered a significant increase in research activities to seek suitable materials and synthesis techniques. Unsupported MoS2 becomes a good candidate as it possesses high catalytic activity and thermo stability. However, so far it is still a challenge to synthesize a dispersed MoS2 catalyst with low- cost precursors and simple methods. In this thesis, a novel hydrothermal method was developed using MoO3, Na2S, and HCl as precursors for the synthesis of unsupported MoS2. Several key factors that influence the catalyst synthesis and performance were carefully investigated, including the ratio of precursors, synthesis temperature, initial temperature, solvents, and promoters. Various characterization techniques, including XRD, TEM, SEM, XANES, EXAFS, Auto-sorb, TPR, microprobe analyzer, XPS, EDX, etc., were used to identify the crystalline structures of synthesized catalysts and to establish the relationship between their structures and hydrotreating activities. The hydrothermal synthesis method was successfully established. Results show that the weakly acidic medium is beneficial to the formation of MoS2. Proper MoO3/Na2S ratios lead to characteristics, such as nanocrystalline structure and large surface areas and pore volumes. Both the synthesis temperature and the initial temperature are key factors for the nucleation and growth of MoS2. A minimum synthesis temperature exists for the formation of crystalline structure, and high synthesis temperatures results in curved and shortened slabs. A high initial temperature benefits fast nucleation, thus leading to shorter slabs. Organic solvent (decalin) and supercritical heptane can dramatically change the aqueous environment. MoS2 slabs could be bent with the addition of decalin, and the curvature degree increases with the increase of decalin amounts. Supercritical heptane aids the creation of highly curved catalysts with better HDS performance (92.95% S conversion). The synthesized MoS2 catalysts were applied for the HDS of light cycle oil and for deoxygenation of waste cooking oil and canola oil to investigate the catalysis mechanisms. A new insight is proposed to distinguish the roles of promoters on deoxygenation pathways. NiMoS creates abundant sulfur vacancies that improve the hydrodeoxygenation, while CoMoS shows saturated edge sites in hydrogen atmosphere and facilitates the hydrodecarbon(x)ylation. From a kinetic model of deoxygenation process over CoMoS, it is found that direct hydrodecarbonylation of fatty acids dominates the hydrodecarbon(x)ylation routes; in the hydrodeoxygenation route, the reduction of fatty acids to alcohols is the rate limiting step in the production of C18 hydrocarbons from fatty acids.