Exploration of Carbon-Supported Molybdenum Carbides as Fatty Acid Hydrodeoxygenation Catalyst

Abstract

Oils and fats were used historically mainly for candle and soap production. Nowadays, they have become essential components of the chemical industry. Fatty acids, derived predominantly from vegetable oils, find extensive applications in industrial processes, notably in the synthesis of fatty acid derivatives such as alcohols and aldehydes. This study explores the potential of molybdenum carbide as a catalyst for the hydrodeoxygenation (HDO) of fatty acids, focusing on oleic acid—a prominent component in vegetable oils. While noble metals like palladium are commonly employed for HDO, issues such as cost and susceptibility to poisoning, especially by sulphur, make necessary to find an alternative catalyst. Molybdenum carbide emerges as a promising substitute for the HDO reaction due to its resemblance to noble metals in electronic structure, low cost, and resistance to sulphur poisoning. Following Bitter et al.[1]'s investigation, where molybdenum carbides were employed as catalysts for the HDO reaction of fatty acids at high temperatures (350 °C). This work inquiries into the catalytic behaviour of molybdenum carbide at lower temperatures where activity was not reported. Temperatures under 250 °C can help to improve the selectivity of the catalyst towards hydrodeoxygenation products avoiding the formation of cracking and hydrodeoxygenation products. This study on the activity of molybdenum carbide as catalyst for HDO reaction in the presence of sulphur, further explores methods to enhance molybdenum carbide effectiveness, including increasing molybdenum loading and reducing nanoparticle size. The molybdenum carbide catalysts were prepared through the carbothermal hydrogen reduction (CHR) method using a molybdenum precursor impregnated on a carbonaceous support, which in this case were graphene nanoplatelets (GNP). By manipulating the carburization conditions of the catalyst, the research demonstrates the tunability of particle size towards smaller sizes with higher surface area, which led to higher activity. The catalysts were characterized by X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) , physisorption, and transmission electron microscopy (TEM). Obtaining nanoparticle size in the range of 2 nm what improved the activity of the catalyst, making it active at sub-250 °C for the HDO reaction. To prove that molybdenum carbide is an active catalyst even with the presence of sulphur, this catalyst was also tested at two different concentrations of sulphur: 4 ppm and 80 ppm. Products like stearic acid, octadecanal and octadecanol were obtained even in presence of sulphur showing the resistance of molybdenum carbide to be poisoned by sulphur. Of special importance is the production of the unsaturated counterpart of octadecanal, product that have never been reported on literature for the HDO reaction. Additionally, these molybdenum carbides catalysts were evaluated in comparison to the conventionally used palladium catalysts for hydrodeoxygenation. Palladium catalyst showed high activity and high selectivity towards hydrogenation products, but its activity was drastically reduced by the presence of sulphur, unlike molybdenum carbide that was less affected by sulphur.