Abstract
Mechanism analysis and kinetic modeling of glycerol conversion into lactic acid in the alkaline media with and without heterogeneous catalyst Cu NPs are reported. The reaction pathways were determined in agreement with the experimental results and comprise several types of reactions, namely dehydrogenation, hydrogenolysis, dehydration and C–C cleavage. Experimental concentration-time profiles were obtained in a slurry batch reactor at different glycerol, NaOH and Cu NPs concentrations in a temperature range of 483–518 K. Power law, Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) models were chosen to fit the experimental data. The proposed reaction pathways and obtained kinetic model adequately describe the experimental data. The reaction over Cu NPs catalyst in the presence of NaOH proceeds with a significantly lower activation barrier (Ea = 81.4 kJ∙mol−1) compared with the only homogeneous catalytic conversion (Ea = 104.0 kJ∙mol−1). The activation energy for glycerol hydrogenolysis into 1,2-propanediol on the catalyst surface without adding hydrogen is estimated of 102.0 kJ∙mol−1. The model parameters obtained in this study would be used to scale an industrial unit in a reactor modeling.
Highlights
Nowadays, the use of renewable raw materials to produce various valuable chemicals is actively developing due to the decreasing reserves of petroleum resources and increasing environmental problems
One of the most promising chemicals, which can be obtained from glycerol, is lactic acid known as 2-hydroxypropanoic acid
There were 82 parameters to be estimated, namely: 50 kinetic constants (7 constants for homogeneous and 3 constants for surface reactions for five temperature values (483, 493, 503, 513, and 518 K)), 10 adsorption constants of glycerol and PG on catalyst surface bGLY, bPG, 10 activation energies Eaj, 2 adsorption enthalpies, 10 concentration exponents for Lactic acid (LA) and acetic acid (AA) in the reaction rate equation r4 optimized in the range from 0.1 to 1.5
Summary
The use of renewable raw materials to produce various valuable chemicals is actively developing due to the decreasing reserves of petroleum resources and increasing environmental problems. One of the most promising chemicals, which can be obtained from glycerol, is lactic acid known as 2-hydroxypropanoic acid. This product is widely used in the food [9], cosmetic and pharmaceutical industries [10]. Previous studies have shown that lactic acid can be produced from glycerol through various catalytic approaches, including hydrothermal conversion [18] and selective oxidation [19]. Oxidative conversion of glycerol to LA in an alkaline media in the presence of Pt and Au based catalysts supported on various surfaces proceeds under milder conditions compared to the hydrothermal treatment [25,26]. Based on the obtained experimental results, kinetic model is developed, aiming at a quantified understanding of the catalytic chemistry both for the main and the side products
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