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

Tetiana Kulik,Mats Larsson,Borys Palianytsia

doi:10.3390/catal10020179

Abstract

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 D3-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.

Highlights

Catalytic pyrolysis can effectively convert second-generation feedstocks into bio-oil or pyrolysis oil [1,2,3,4,5,6,7]
We suggest that optimal temperature conditions for the efficient catalytic ketonization of carboxylic acids over CeO2 /SiO2 catalyst could be in this range
The structure–reactivity correlation between Taft’s steric substituent constants Es* and activation energies of catalytic ketonization for the reaction series of fatty acids on the surface of nanosized CeO2 /SiO2 was obtained. This correlation shows that ketonization is a sterically sensitive reaction

Summary

Introduction

Catalytic pyrolysis can effectively convert second-generation feedstocks (lignocellulose, waste oils, fats, algae, agricultural and forest residues, etc.) into bio-oil or pyrolysis oil [1,2,3,4,5,6,7]. The potential for the use of biomass resources of second-generation feedstocks in Europe is very high [8]. The cost of second-generation raw materials is low [8]. Some of them, including acetic, valeric, levulinic (γ-ketovaleric acid), 2,5 furan dicarboxylic acids, etc., are considered as key-building platforms in biomass conversion technologies [1,2,3,4,5,6,7,12,14]. The upgrading of bio-derived carboxylic acids has great economic, social and environmental advantages compared with the traditional use of fossil hydrocarbon resources

Methods

Results

Conclusion