Hydrotreating of Gas Oils Using Ni-Mo Catalyst Supported on Carbon Nanohorns and Associated Carbon Materials

Abstract

For many decades, crude oil refinery companies and catalysis research establishments worldwide are being challenged to develop new hydrotreating processes aimed at producing cleaner fuel. Use of an active and stable catalyst in hydrotreating is one way of improving catalytic activity of hydrotreating reactions such as hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). Hence, this research primary goal is to develop a NiMo catalyst supported on carbon nanohorns (CNH) and associated carbon materials for hydrotreating. CNH is the support of choice used in this research due to its inert nature and excellent textural properties (mesoporosity, high surface area and desirable pore volume) which are known to enhance hydrotreating activities. The research plan is divided into five phases, and the research goals, experimental methods and outcomes pertaining to each of the phases are summarized in this thesis. For phase one, the goal was to maximize the production of CNH in the lab for hydrotreating application by investigating the effects of, current settings, processing time and equipment design on CNH production. A current setting of 90 A and 30 minutes processing time were determined to be the best conditions for laboratory scale maximization of CNH production. In phase two, the primary aim was to incorporate functionality in an otherwise inert CNH material using, 30 wt% HNO3 under reflux for 15 minutes to 4 hours at 110°C. Production of CNH concurrently generates other carbon particles (OCP) as by-products that can be differentiated into fine (OCPf) and chunk (OCPc) forms. Due to the superior quality of the CNH material in phase three, the goal was to test the influence of four different supports (i.e. gamma-alumina (γ-Al2O3), CNH, OCPf, and carbon nanotubes (CNT)) on hydrotreating performance, and to determine how the physico-chemical properties of each support has on the hydrotreating performance. In comparison to the remaining carbon-supported catalysts, NiMo/CNH catalyst exhibited higher HDS (89%) and HDN (42%) activities for light gas oil containing 3 wt% sulfur and 0.18 wt% nitrogen and this is ascribed to, the high textural properties of the NiMo/CNH catalyst as well as its ability to induce, high metal dispersion plus reduction of metal oxides at a lower temperature in comparison to the remaining carbon-supported catalysts. The yield of OCP material from the laboratory synthesis of CNH was more than 45% of the CNH yield. Moreover, from phase three, OCPf support was also found to be a viable support in hydrotreating hence, the main focus of phase four work was to improve the hydrotreating performance of NiMo/OCPf catalyst by varying Ni and Mo compositions on the functionalized OCPf support. To this effect, a series of hydrotreating experimental runs with varying Ni (2.5, 3.5, 5.0 wt%) and Mo (13, 19, 26 wt%) compositions with light gas oil indicated that 5.0wt%Ni19wt%Mo/OCPf catalysts exhibited the highest HDS and HDN activity. The last phase (phase 5) of the research plan focused on the development of the best NiMo/CNH catalyst for hydrotreating by varying Ni loadings (2.5 wt%, - 5 wt%) at constant Mo loading of 19 wt%. Hydrotreating results from these catalysts showed that, the 3.5wt%Ni19wt%Mo/CNH catalyst exhibited the highest HDS and HDN activity. This catalyst was further modified by adding 2 wt% P to the 3.5 wt% Ni and 19 wt% Mo concentrations via a co-impregnation method to study the effects of phosphorus as a secondary promoter on HDS and HDN activity. Optimization of the operating parameters, kinetic and stabilization studies were further carried out using the 3.5wt%Ni19wt%Mo/CNH catalyst. The results from this phase are also presented in this thesis. The overall results from all phases demonstrated that CNH is not only a material that have superior physicochemical properties as compared to CNT and OCP material but, also a potential catalyst support with great benefit in hydrotreating.