Oxidation of chlorinated alkanes over Co₃O₄/SBA-15 catalysts. Structural characterization and reaction mechanism

Abstract

Cobalt oxide-based catalysts have been synthesised via a hard template of mesoporous silica SBA-15 in the form of massive Co3O4 nanoparticles, whose dimensions are controlled and limited by the support mesoporosity. Over this family of catalysts, both weak Brønsted and Lewis acidities have been detected, with the relative abundance a function of the cobalt content. The mesoporosity of the support leads to the growth of oxide nanoparticles, mainly in the small pores, thus improving their redox properties. Catalysts possessing cobalt oxide loadings higher than 30% present an adequate activity for the deep DCE oxidation toward carbon dioxide, hydrogen chloride and chlorine. It is believed that the reaction is markedly accelerated due to the simultaneous participation of the acid sites (where the chlorinated feed is efficiently adsorbed) and the redox sites (oxidation of the adsorbed feed with lattice oxygen anions). FTIR data on dichloroalkane oxidation evidence that lattice oxygen species are mainly involved in the Cl-VOC combustion, likely through a Mars-van Krevelen mechanism

Surface Characterization of Mesoporous CoOx/SBA-15 Catalyst upon 1,2-Dichloropropane Oxidation

“…Indeed, addition of cobalt oxide is fundamental to reach total conversion to CO 2 : The oxidation activity starts to be significant above 250 °C (T 50 = 320 °C) and is almost total above 350 °C (T 90 = 350 °C). These temperature levels were slightly lower than those noticed for the oxidation of 1,2-dichloroethane [20]. This increased reactivity for 1,2DCP can be related to the structure of the organic substrate, having a secondary C–Cl group.…”

“…The corresponding isotherms and textural properties of the samples are included in Figure S1 and Table S1. Cobalt oxide nanocrystallites are highly dispersed with an average size of 14 nm, as determined by XRD (Figure S2) [20]. As discussed before, the dispersion of the active phase due to the interaction with the silica matrix improved the redox properties of the catalyst, increasing the reducibility as long as the particle size decreased.…”

“…Nevertheless, we have been able to detect some weak, newly formed bands in the subtraction spectra together with a multiplicity of bands in the CH deformation and stretching spectral region (spectra not reported). Under these conditions no reaction products are detected in the gas phase and structural bands due to Co 3 O 4 , whose lattice oxygen is involved in the oxidation process through the Mars van Krevelen mechanism [20,23], are almost unperturbed, as deduced from the spectrum reported in Figure 4b. In this subtraction spectrum (i.e., (spectrum after treatment with 12DCP at 180 °C and adsorbed py)—(spectrum of the oxidized surface)), the main band related to oxidized CoO x centred near 660 cm −1 does not appear, pointing out that no perturbation of this features occurs below 200 °C.…”

“…Consequently, components at 1608 and 1450 cm −1 , became more evident. These features are due to ring stretching modes of pyridine molecules coordinated over Lewis sites of medium-weak intensity, for instance, exposed Co ions [20,23]. These features are detected up to 250 °C (Figure 2, inset).…”

“…Namely, at the highest cobalt oxide contents, large CoO x particles are located on the external surface, while the smallest particles are encapsulated within the mesoporous channels, enhancing the dispersion of the active phase and therefore improving redox properties. Acidic silanol groups of the mesoporous silica support and Lewis acidic sites arising from Co introduction are exposed at the surface in all the composition range studied (10–50 wt % Co 3 O 4 ) [20]. As a result, both mesoporosity and an optimal acidic sites distribution appear to be key parameters in order to, on one hand, provide good morphological properties, and, on the other hand, strongly adsorb chlorinated reactants, which become more prone to oxidative attack, and avoid deactivation phenomena, such as heavy coking.…”