Generation of Value-Added Products from Glycerol Oxidation by Pd Based Catalysts in Alkaline Medium

Introduction

Glycerol is a nonflammable and volatile compound obtained from biodiesel production and has its main use in therapeutic, diagnostic and industry applications. However, these uses not absorb all glycerol produced opening up the opportunity of its use as an alternative to energy generation. The final product from glycerol oxidation is carbonate, which means a theoretical energy density of 5.0 kWh kg^-1. But releasing the 14 electrons from its molecule is not a simple task considering the complexity oxidation mechanism that involves the formation of several byproducts with important value-added and/or industrial applications [1]

Thus, in this work we aimed to investigate the glycerol oxidation in alkaline medium employing PdM (M= Sn, Fe, and Mn) catalysts supported on Carbon Vulcan prepared by the microwave-assisted heating.

Experimental

Catalysts with nominal composition Pd:M (50:50), M = Sn, Fe, and Mn, supported in carbon Vulcan XC-72 were prepared by the microwave-assisted heating (MW) in order to obtain catalysts with 40% wt. metal loading.

The catalysts were characterized by X-ray diffraction (DRX), Energy dispersive X-ray (EDX) and Transmission Electron Microscopy (TEM). Electrochemical tests were performed in presence of NaOH 0.1 mol L^-1, and glycerol 0.1 mol L^-1 by Cyclic voltammetry and chronoamperometry at 0.8 V vs RHE for four hours and the byproducts of glycerol oxidation were determined by high performance liquid chromatography (HPLC) and infrared reflectance spectroscopy.

Results and Discussion

Mn and Fe containing catalysts presented an experimental composition close to the nominal one (Pd₅₃Mn₄₇/C; Pd₅₄Fe₄₆/C) with a face-centered cubic (FCC) crystalline structure characteristic of the Pd. Pd₅₄Fe₄₆/C catalyst displayed peaks at 35^o, 57^o and 63^o, which attributed to the Fe₂O₃ phases [2]. Pd₆₃Sn₃₇/C catalyst showed a different composition from the nominal one, justified by the formation of Pd₂Sn orthorhombic phase [3] as showed in their respective phase diagram. Pd₆₃Sn₃₇/C catalysts showed large agglomerated distribution that did not allow obtaining the average particle size of this material, in contrast to Pd₅₃Mn₄₇/C and Pd₅₄Fe₄₆/C that exhibited particles well distributed on carbon support. Crystallite size and particle size obtained from DRX and TEM data respectively shows a good correlation for all compositions, except for Pd₆₃Sn₃₇/C.

Electrochemical characterization by cyclic voltammetry in the absence of glycerol showed that all of the catalysts display characteristic peaks of OH^- adsorption and PdO formation as described in [4]. Electrocatalytic activity towards glycerol oxidation of all catalysts in 0.8 V vs RHE potential is obtained from chronoamperometry and the current density dropped continually as a function of the time until the first 2 h and after the catalytic activity became relatively stable. Higher current density was obtained from Pd₅₃Mn₄₇/C catalysts showing the beneficial synergistic effect between Pd and Mn. On the other hand, Pd and Pd₆₃Sn₃₇/C showed an intermediated results and Pd₅₄Fe₄₆/C presented the worst catalytic activity after 2 hours of continuous polarization test.

The IR spectra showed several bands attributed to the different byproducts or intermediates of glycerol as hydroxypyruvate, mesooxalate/tartonate/formate, glyceraldehyde/glycerate, and glycolate. The presence of CO linear or bridge bonded was not observed. However, a band about 1680 - 1690 cm^-1 may be attributed to CO adsorbed in a ternary mode [1, 3, 4]. The byproducts determined by HPLC are in good agreement with those identified by in situ reflectance spectroscopy measurements under the same conditions (electrolytic solution and potential electrode). The Pd₅₄Fe₄₆/C material exhibited the best catalytic conversion. Furthermore, all the catalysts are capable of breaking the C-C bond of glycerol at ambient temperature. Glycerate, glycolate, formate were well detected and a few amounts of tartronate and oxalate are present in the electrolytic solution. As evidence, at the catalyst surface leads to a C-C bond cleavage resulting in the production of glycerate as main reaction byproduct

Acknowledge: This work was mainly conducted within the framework of a collaborative programme CAPES/COFECUB under Grant No Ch 747-12. The authors acknowledge FAPESP, Capes, and CNPq (Brazil) for financial support. L. M. Palma acknowledges FAPESP foundation under contract number 2011/10213-6 for PhD scholarship.

References

[1] M. Simões, S. Baranton, C. Coutanceau, Applied Catalysis B: Environmental, 93, 354, (2010).

[2] Pattern: 00-052-1449, 00-039-1346, 00-033-0664, 00-025-1402, in, International Centre for Diffraction Data (ICDD), 2005.

[3] D.Z. Jeffery, G.A. Camara, Electrochem. Commun., 12, 1129, (2010).

[4] J. Gomes, G. Tremiliosi-Filho, Electrocatal, 2, 96, (2011).