Activation of n-octane using VOx/MgO catalysts prepared by a mixed fuel solution combustion synthesis method

Abstract

Oxidative dehydrogenation (ODH) is an important chemical process used to produce various industrial fuels and building blocks, such as butadiene, styrene, and propylene. In this process, a catalyst facilitates the oxidation of hydrocarbons, leading to the removal of two hydrogen atoms and the formation of an olefin. This reaction has attracted significant attention due to its ability to generate high yields and low by-product formation, making it an environmentally sustainable approach for olefin production. However, ODH reactions face certain challenges that must be addressed to make them more efficient and viable for large-scale production. The major challenge in this process is the low selectivity for the formation of olefins, as side reactions lead to the formation of unwanted by-products. In addition, the catalyst employed should exhibit stability and maintain high levels of activity and selectivity under the specified operating conditions. In recent years, the solution combustion synthesis method (SCS) has emerged as a promising technique for producing highly active and stable catalysts for oxidative dehydrogenation. In the SCS method, a precursor solution is heated in a furnace to produce a high-temperature exothermic reaction that drives the formation of nanoscale particles. SCS has several advantages over traditional synthesis methods such as sol-gel and solid-state reactions. This includes its simplicity in eliminating the need for an inert atmosphere or subsequent calcination procedures. Additionally, it allows for precise control of the particle size, morphology, and composition, which can significantly influence the catalyst's performance. It is also a cost-effective technique as it uses simple and inexpensive chemicals as precursors. The nanoparticles created through SCS are characterized by their ability to bind to high surface areas and tailor their specific morphology, such as a highly porous structure or a specific crystal facet. This structural control can significantly impact the catalyst's performance, as it influences the catalyst's active sites and the reactant's diffusion and adsorption properties. In this study, VOx/MgO catalysts were synthesized using a solution combustion synthesis method with three fuel mixtures as reducing agents. The fuel mixtures comprised of urea/hydrazine hydrate (UR/HH), glycine/urea (GLY/UR), and glycine/hydrazine hydrate (GLY/HH). The catalysts were characterized using BET, XRD, ICP-OES, HR-TEM, TPR and TPD-MS. Catalytic testing was performed in a continuous flow fixed-bed tubular stainless-steel reactor, operated in a down-flow mode, using air as the oxidant and nitrogen as the diluent gas. Reactions were carried out over a temperature range of 350-450 °C with 5% feed and 350-450 °C at 12% feed. The gas flow rates were varied to attain gas hourly space velocities of 8000 h-1. The carbon to oxygen ratio was 8:2. The catalyst bed was maintained at a volume of 1 cm3 with pellet sizes ranging between 600 – 1000 um. The reaction product stream was analyzed using gas chromatographs, one fitted with a thermal conductivity detector, and the other with a flame ionization detector. The results obtained from this study demonstrated varying morphologies and surface areas for each catalyst. The VOx/MgO catalysts showed the distribution of vanadium on the surface of the support with an aggregation of VOx clusters at certain locations. The %V2O5 loading varied from 14 wt%- 14.9 wt%, which correlated with the SEM-EDS results. XRD and Rietveld refinement results demonstrated that the vanadium supported on MgO catalysts was present as biphasic constituents, with the predominant phase being attributed to the MgO periclase phase. The second phase was identified as the magnesium orthovanadate (Mg3(VO4)2), while the crystalline Mg2V2O7 magnesium pyrovanadate phase was only identified only in the GLY/UR catalyst. Further characterization was performed using TPR for redox properties and TPD-MS for acid-base properties. The GLY/UR and GLY/HH catalysts displayed a high H2 consumption, with two distinct reduction peaks followed by a high concentration of Brønsted acid sites of >100 µmole/g for both catalysts. Catalytic testing revealed that all three catalysts exhibited activity for the ODH of n-octane. The selectivity towards the desired products was dependent on the vanadium concentration and the phase composition of the catalysts. Octene isomers, aromatic compounds, cracked products and carbon oxides were identified in the product stream. The GLY/HH catalyst displayed the best selectivity of 66% towards the formation of octenes. A decrease in feed concentration from 12% to 5% led to an increase in octene selectivity, specifically at 450 oC. This confirmed that the selectivity and conversion depend on the reaction temperature, phase composition and morphology.