(77d) Hydrotreating Reaction Optimisation Using Carbon Nanotubes Supported NiMo and Mo Based Catalysts in Refining Crude Oil

As conventional crude oil reserves continually decline worldwide, much focus has been turned towards potential sources for producing synthetic crude oil. However, high concentrations of organic sulphur compounds within the synthetic crude feeds can. Another problem is the deactivation of noble metal catalysts within downstream refining processes due to high organonitrogen concentrations within the feedstock. The regulation of fuel quality has become a worldwide effort in recent years in the face of growing health and environmental concerns.

High concentrations of organic nitrogen compounds, are the most common source of catalyst poisoning due to their strong adsorption on catalyst active sites, can cause significant deactivation for reforming, cracking, hydrotreating, or any other type of hydroprocessing catalysts .

The main aim of the paper is to provide a comparative study of carbon nanotube supported NiMo and Mo based catalysts (for hydrotreating) against the traditional hydrotreating catalysts usually containing cobalt or nickel and molybdenum or tungsten supported on alumina. Although this type of hydrotreating catalysts has been used successfully and commercially, there are some defects in the traditional catalysts, such as difficulty in sulfurization and strong effect between support and active species, and so on. Recently, carbon supported catalysts have attracted more and more attention, behind which the driving forces are carbon supported hydrotreating catalysts that show quite high activity, excel in lower coking deposition and easily recoverable from the waste catalysts by burning off the carbonaceous support.

The impure organic compounds undergo hydrodesulphurization (HDS) and hydrodenitrogenation (HDN) reactions, resulting in the impurities being removed in the form of H2S and NH3 gases, respectively. It is common industrial practice to use a heterogeneous catalyst system to perform such a process. E.g. A bimetallic catalyst is commonly used with a metal sulfide, often MoS2 or WS2, accompanied by a promoter, often Ni or Co. The metal particles are usually deposited on a γ–Al2O3 catalyst support.

Of all the new and innovative ways of improving on the performance of an existing catalyst process, carbon nanotubes (CNTs) have garnered much interest due to the high specific surface areas they can possess while still maintaining large pore diameters. Multi-walled carbon nanotubes (MWCNTs) are a potential alternative to commonly used catalyst support structures in hydrotreating processes.

Combined with the high surface areas exhibited by CNTs, it can be theorized that less sintering of metal particles would occur on the surface of a NiMo/CNT catalyst compared to a commercial NiMo/γ–Al2O3 catalyst. This would further limit the deactivation effects of a NiMo catalyst from the use of a CNT support. Synthesis of MWCNTs with specific pore diameters can be achieved by chemical vapour deposition (CVD) of a carbon source onto an anodic aluminium oxide (AAO) template.

This paper provides a study of the application of CNT catalyst supports with varying pore diameters is examined. These CNTs were used for the preparation of NiMo/CNT catalysts. The performance of these catalysts, along with a commercial NiMo/γ–Al2O3 catalyst under equivalent mass and volume loadings, were compared based on the HDS and HDN activities displayed during the hydrotreating of LGO derived from Athabasca bitumen. After determining the most suitable pore diameter, the optimum catalyst metal loadings were found to be 2.5 wt.% for Ni and 19.5 wt.% for Mo. The optimum catalyst was found to offer HDS conversions of 90.5%, 84.4%, and 73.5% with HDN conversions of 75.9%, 65.8%, and 55.3% for temperatures of 370°C, 350°C, and 330°C, respectively. An equal mass loading of commercial NiMo/γ-Al2O3 catalyst offered HDS conversions of 91.2%, 77.9%, and 58.5% with HDN conversions of 71.4%, 53.2%, and 31.3% for temperatures of 370°C, 350°C, and 330°C, respectively.  

Also, in another study of the effect of the supported catalysts including oxide state Mo, Co–Mo and sulfide state Mo on carbon nanotube (CNT) were prepared, while the corresponding supported catalysts on γ-Al2O3 were also prepared for comparison. The HDS (Hydrodesulphurisation) of DBT (dibenzothiophene) showed that Co–Mo/CNT catalysts were more active than Co–Mo/γ-Al2O3 and the hydrogenolysis/hydrogenation selectivity of Co–Mo/CNT catalyst was also much higher than Co–Mo/γ-Al2O3. For the Co–Mo/CNT catalysis system, the catalyst with Co/Mo atomic ratio of 0.7 showed the highest activity, whereas, the catalyst with Co/Mo atomic ratio of 0.35 was of the highest selectivity.