Catalytic Pyrolysis of Aliphatic Carboxylic Acids into Symmetric Ketones over Ceria-Based Catalysts: Kinetics, Isotope Effect and Mechanism

• Main Authors: Tetiana Kulik, Borys Palianytsia, Mats Larsson
• Format: Article
• Language: English
• Published: MDPI AG 2020-02-01
• Series: Catalysts
• Subjects: carboxylic acids upgrading; ketonization; deuterated acetic acid; acetone d-isotopomers distribution; h/d exchange; inverse deuterium kinetic isotope effect; kinetic parameters; activation energy; catalytic pyrolysis of biomass; bio-oil
• Online Access: https://www.mdpi.com/2073-4344/10/2/179

Ketonization is a promising way for upgrading bio-derived carboxylic acids from pyrolysis bio-oils, waste oils, and fats to produce high value-added chemicals and biofuels. Therefore, an understanding of its mechanism can help to carry out the catalytic pyrolysis of biomass more efficiently. Here we show that temperature-programmed desorption mass spectrometry (TPD-MS) together with linear free energy relationships (LFERs) can be used to identify catalytic pyrolysis mechanisms. We report the kinetics of the catalytic pyrolysis of deuterated acetic acid and a reaction series of linear and branched fatty acids into symmetric ketones on the surfaces of ceria-based oxides. A structure−reactivity correlation between Taft’s steric substituent constants Es* and activation energies of ketonization indicates that this reaction is the sterically controlled reaction. Surface D_3-n-acetates transform into deuterated acetone isotopomers with different yield, rate, E^≠, and deuterium kinetic isotope effect (DKIE). The obtained values of inverse DKIE together with the structure−reactivity correlation support a concerted mechanism over ceria-based catalysts. These results demonstrate that analysis of Taft’s correlations and using simple equation for estimation of DKIE from TPD-MS data are promising approaches for the study of catalytic pyrolysis mechanisms on a semi-quantitative level.