(329g) Catalytic Hydrodeoxygenation of Propionic Acid Over Supported Monometallic Palladium: A Study of the Support Effects

The conversion of low-cost biomass feedstock and other raw bio-renewable materials into liquid hydrocarbon fuels is a goal that is being significantly pursued. However, biomass contains a large amount of oxygen in their chemical structures that need to be removed for liquid fuels applications. Our project consists on exploring heterogeneous catalytic gas-phase hydrodeoxygenation (HDO) of biomass derived carboxylic acid (i.e. propionic Acid) to produce hydrocarbons. The reaction screening of propionic acid (PAc) has been studied over Pd supported on carbon, SiO₂, γ-Al₂O₃ and TiO₂. The catalysts were synthesized by conventional impregnation methods and Strong Electrostatic Adsorption (SEA), and characterized utilizing Fourier transform infrared spectroscopy (FTIR), Temperature Programmed Oxidation and Reduction (TPO/TPR), Atomic Absorption Spectroscopy and H₂ titration of oxygen pre-covered catalyst. The reaction screenings were carried out in a single-pass plug flow reactor where PAc is co-fed with hydrogen at temperatures ranges of 473- 673 K and atmospheric pressure. Palladium over carbon showed high activity and selectivity towards ethane, indicating that the reaction favored the C-C bond cleavage based on decarboxylation (DCX) and decarbonylation (DCN) reactions.  Interestingly, the screening results over Pd/SiO₂ and Pd/γ-Al₂O₃, showed high selectivity towards diethyl-ketone, suggesting that the acidity of the support influenced on the reaction with propionic acid.  Additionally, the palladium over titanium dioxide showed high selectivity toward esterification products (i.e. n-propyl propionate) at a temperature range of 473-550 K and high selectivity toward hydrocarbons (ethane and propane) at 573 K. The latter might indicate the presence of strong metal-support interaction (SMSI) which creates a bifunctional effect between the metal and the support. It is therefore proposed that treatment of the catalyst at a low temperature reduction (LTR) generates oxygen vacancies that can promote C=O bond cleavage of propionic acid.  Furthermore, kinetic studies to explore mechanistic details of the reaction order for PAc and H₂ for the different screened catalysts were investigated.