Malic acid represents a chemical platform to oxaloacetic acid (OAA) and pyruvic acid (PA) synthesis, which in turn are raw materials for essential aminoacids synthesis. The use of malic acid is a viable alternative because has the great potential to be produced from biomass.
1. Objectives
L-Malic acid, easily accessible from natural sources such as berries, apple and grape, represents an important biomass derived compound, used as precursor to essential aminoacids and biopolymers synthesis. Its transformation in other added values chemical such as OAA and PA is of interest for the scientific community, therefore to find the proper catalyst to do this transformation represents an important challenge.
Among different materials, iron-cobalt based mixed oxides are very interesting ones with high stability, magnetic properties and furthermore, are environmentally friendly and inexpensive compounds. They are used in many fields and especially in organics degradation, as materials for energy storage, for water electrocatalytic oxidation and for water photo-reduction1.
Our objective was to prepare iron-cobalt mixed oxides, to investigate their morphological and structural properties as well as their catalytic properties in malic acid oxidative dehydrogenation.
2. Results and discussion
Samples with different ratio between Co and Fe (0, 0.02, 0.03, 0.05), named Co0Fe, Co1Fe, Co2Fe and Co3Fe respectively, were synthesized by coprecipitation of cobalt and iron precursors with ammonium carbonate.
XRD patterns (Fig. 1) of synthesized samples present diffraction lines pertaining to α-Fe2O3 rhombohedral hematite structure. No lines corresponding to cobalt oxides were identified, meaning that the Co atoms are part of the hematite structure, they replacing the Fe atoms from the lattice.
The FTIR spectrum (Fig. 2) of Co0Fe sample presents a broad band located at 3450 cm-1 assigned to stretching vibration of water molecules physisorbed on the surface. This band almost disappears in the samples with cobalt, indicating that cobalt presence in the material reduces the adsorption of water on the surface.
The prepared catalysts were tested in malic acid oxidative dehydrogenation reaction. The solvents used were water and ethanol. The main products identified by gas-chromatography were oxaloacetic acid, pyruvic acid and esters of malic acid. It was observed that, oxaloacetic acid yield depends on the reaction temperature and time, being favored by low temperatures and short reaction times.
The FTIR spectra recorded for the tested samples provide clear evidence of oxaloacetic acid adsorption on the surface, characterized by the bands located in the 1220-1320 cm-1 region. Also, the pyruvic acid production is evidenced through the presence of bands located in the 1700-1800 cm-1 region, while unreacted malic acid is evidenced by the presence of bands in the 2850-3000 cm-1 region.
Figure 1. XRD patterns of iron-cobalt oxides
Figure 2. FTIR spectra of Co-Fe oxides,
before (a) and after (b) catalytic reaction
Conclusions
Iron-cobalt mixed oxides catalyze malic acid oxidative dehydrogenation with O2 using water or ethanol as solvent, at low temperature (25-70°C). The most favorable conditions for oxaloacetic acid production are low temperatures and short reaction times.
References
1. N. Helaili, G. Mitran, I. Popescu, K. Bachari, I.C. Marcu, A. Boudjemaa, J. Electroanalytical Chem. 742, 2015, 47–53
Acknowledgements:
This work was supported by a grant from the Romanian Ministry of Education and Research, CNCS–UEFISCDI, project number PNIII-P4-ID-PCE-2020-0580, within PNCDI III.
Contact information:
geta.mitran@chimie.unibuc.ro; florentina.neatu@infim.ro
