Abstract
This manuscript summarizes observations made during the conversion of propanoic acid over Pt/SiO2 under H2-rich environments. Under these conditions, Pt is active for hydrogenation and hydrodeoxygenation, which leads to the formation of propionaldehyde, 1-propanol, and propane. Pt also facilitates decarbonylation of propanoic acid and propionaldehyde, which forms ethane and CO. The accumulation of CO with increasing residence time poisons the Pt catalyst and makes it difficult to achieve high propanoic acid conversions on practical time scales. During the conversion of propanoic acid on Pt/SiO2, sequential reactions play a critical role in determining product distributions. We resolve their contributions through analysis of rates and selectivities during the conversion of propanoic acid, propionaldehyde, 1-propanol, CO, and CO2 in various environments. We conclude that the main challenge facing the selective production of partial hydrodeoxygenation products─namely propionaldehyde and 1-propanol─is that one must facilitate dehydroxylation of propanoic acid while avoiding thermodynamically favorable decarbonylation and alcohol hydrogenolysis pathways. Because these side reactions are effectively irreversible under hydrodeoxygenation conditions, this can only be accomplished by suppressing rates of decarbonylation and 1-propanol hydrogenolysis. We reconcile macroscopic trends with a reaction mechanism that includes parallel and sequential reactions occurring during the hydrodeoxygenation of propanoic acid over Pt/SiO2.
Highlights
Carboxylic acids are an accessible class of biobased platform molecules.[1]
Decarboxylation and decarbonylation are hydrogen-free strategies for removing oxygen from carboxylic acids via C−C scission, respectively producing alkanes and 1-alkenes; this requires sacrificing one carbon atom to the formation of CO or CO2.26−28 Decarbonylation and decarboxylation of carboxylic acids result in total oxygen removal; as such, they disallow the production of intermediate aldehydes and primary alcohols
To correct for dispersion effects, production rates are reported as site time yields, which are based on CO chemisorption
Summary
Carboxylic acids are an accessible class of biobased platform molecules.[1]. Long-chain “fatty acids” (C10−C22) can be sourced from lipid-producing plants (soybeans, canola, palm),[2] microalgae,[3,4] yeast,[5] and diacids are produced buascintegrifae;r6mCen2−taCtio[6] ncabrabsoexdylmicetahcoiddss;7a−n1d0 hemicellulose hydrolysis provides a well-known pathway to acetic acid;[11,12] and short-chain carboxylic acids are a major component of bio-oils prepared through pyrolysis[13] or hydrothermal liquefaction[14,15] of lignocellulose. Hydrodeoxygenation broadly entails the use of metal or metal−acid catalysts with a hydrogen source to facilitate C−O bond scission (i.e., dehydroxylation or hydrogenolysis) and hydrocarbon saturation.[20] During strict hydrodeoxygenation of carboxylic acids,, one avoids C−C scission such that the carbon chain of the parent molecule remains intact.[21,22] Selective hydrodeoxygenation strategies facilitate partial oxygen removal, which allows one to convert carboxylic acids into their mono-oxygenated analogs namely aldehydes[23] and primary alcohols24,25―which are attractive as commodity chemicals. Decarboxylation and decarbonylation are hydrogen-free strategies for removing oxygen from carboxylic acids via C−C scission, respectively producing alkanes and 1-alkenes; this requires sacrificing one carbon atom to the formation of CO or CO2.26−28 Decarbonylation and decarboxylation of carboxylic acids result in total oxygen removal; as such, they disallow the production of intermediate aldehydes and primary alcohols
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