Kinetics of Aqueous-Phase Hydrogenation of Organic Acids and Their Mixtures over Carbon Supported Ruthenium Catalyst

Abstract

Aqueous-phase hydrogenation of lactic acid (2-hydroxypropanoic acid) and propionic (propanoic) acid over 5 wt % Ru/C catalyst was performed in a three-phase stirred batch reactor. Kinetic data were collected for reactions at 343−423 K, 3.4−10.3 MPa hydrogen pressure, and 0.05−5 M acid feed concentrations. Adsorption and reaction of individual acids, acid mixtures, and combinations of acids with their alcohol products were investigated to characterize relative hydrogenation rates of the two acids and the extent to which the presence of one species influences the reactivity of another. Mass transfer analysis showed that acid conversion rates were not limited by mass transport resistances over the reaction conditions studied. A two-site Langmuir−Hinshelwood (L−H) kinetic model with a single set of rate and adsorption constants fits the conversion kinetics of both individual and mixed acid hydrogenations. Competitive adsorption of acids and their alcohol products strongly affects hydrogenation rates.

Theoretical investigation of the decarboxylation and decarbonylation mechanism of propanoic acid over a Ru(0 0 0 1) model surface

“…To gain more insights into the DCN and DCX mechanisms of organic acids (such as propanoic acid) on transition metal surfaces and to possibly design novel metal catalysts, it is necessary to study multiple metal surfaces. Ruthenium is more oxophilic than palladium and has experimentally been found to be an active catalyst for the conversion of organic acids to hydrocarbon molecules [2,[13][14][15]. Experiments on the HDO of propanoic acid over Ru/C by Chen et al [13] and the HDO of acetic acid over Ru/C by Olcay et al [14] show that at high H 2 partial pressure (>20 bar) and at temperatures of around 400 K, Ru/C has a high selectivity to propanol and ethanol, respectively.…”

“…Ruthenium is more oxophilic than palladium and has experimentally been found to be an active catalyst for the conversion of organic acids to hydrocarbon molecules [2,[13][14][15]. Experiments on the HDO of propanoic acid over Ru/C by Chen et al [13] and the HDO of acetic acid over Ru/C by Olcay et al [14] show that at high H 2 partial pressure (>20 bar) and at temperatures of around 400 K, Ru/C has a high selectivity to propanol and ethanol, respectively. On the other hand, it has been shown that when flowing acetic acid over Ru/C at a low H 2 partial pressure (<1 bar) [14], mostly methane ($70% selectivity) and only small amounts of ethanol are produced ($6% selectivity).…”

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

“…In an earlier study using a batch reactor, Chen et al. reported 30–60 % selectivity to 1‐propanol (the rest being C 1 −C 3 alkanes) from HDO of propanoic acid in aqueous phase, catalyzed by Ru/C (Ru dispersion of ∼9 %) at 403 K and 7 MPa H 2 . Initial rates of acid conversion (TOF ∼110 h −1 ) became no longer dependent on the acid concentration beyond 1 M and the apparent activation energy was measured to be 68 kJ mol −1 .…”

“…0.2 and 1.4, respectively, in hydrogen concentration (dissolved in water). Lower temperatures generally favor selectivity to alcohol products;,, at temperatures (∼573 K) typically used in hydro‐processing of bio‐oils, extensive C−C cleavage occurs on most metal catalysts . It should be noted that CO x products are often detected only at trace levels from C−C cleavage of an acid on Ru catalysts, as a result of efficient removal of CO* by chemisorbed H* as CH 4 ,,…”

“…It can be generally said that competitive adsorption of oxygenates (as well as solvent such as water) on the catalyst surface and the formation of poisonous species (e. g., CO) are two main causes for the inhibition of one oxygenate on the conversion rates of another oxygenate. For example, adding 1‐propanol or lactic acid to the initial aqueous solution decreased the conversion rate of propanoic acid on Ru/C, while the presence of 1,2‐propanediol had little effect on the hydrogenation rates of propanoic acid or lactic acid . In a recent work, Huber and coworkers performed experiments on hydrogenation over Ru catalysts of single‐ and multi‐component model compounds (hydroxyacetone, acetic acid and formic acid) and the aqueous fraction of bio‐oil collected through staged recovery of pyrolysis vapors .…”