Catalytic oxidation of mercaptans by bifunctional catalysts composed of cobalt phthalocyanine supported on Mg–Al hydrotalcite-derived solid bases: effects of basicity

Abstract

The oxidation of 1-octanethiol to corresponding disulfide as a model reaction for mercaptan sweetening was examined on the bifunctional catalysts prepared by supporting cobalt phthalocyanine tetrasulfonate (CoPcTs) on mixed-oxide solid bases (designated as Mg(Al)O) derived from thermal decomposition of Mg–Al hydrotalcites with five Mg/Al molar ratios of 3.0, 4.9, 6.5, 9.0 and 13.2, and for comparison on Al₂O₃, MgO and K⁺-modified Mg(Al)O (Mg/Al = 3.0). For the five Mg(Al)O samples, at a similar CoPcTs dispersion, the oxidation rates increased in parallel with the number of the basic sites provided by the support surfaces, but their rates were greater than those of the corresponding MgO and K⁺-modified Mg(Al)O samples although MgO and K⁺-modified Mg(Al)O possessed more basic sites with stronger base strength compared to Mg(Al)O, showing the dependence of the oxidation rates not only on the numbers of the basic sites, but also on their nature and strength. In combination of the in situ infrared results for 1-hexanethiol adsorption on Mg(Al)O, effects of titration of the basic sites with CO₂, BF₃ and H₂O on the reaction rates for CoPcTs/Mg(Al)O (Mg/Al = 3.0) led to the identification of the basic OH⁻ rather than O²⁻ ions as the active basic sites for the mercaptan oxidation. 1-propanethiol temperature-programmed desorption showed the effects of the base strength on the strength of the mercaptan adsorption on the Co²⁺ site of CoPcTs as a result of the effects on the electron density of the Co²⁺ site. Such effects led to the poor activities for the MgO and K⁺-modified Mg(Al)O samples, due to their strong base strength and consequently weaker mercaptan adsorption and activation on the Co²⁺ sites. It is thus clear that the OH⁻ basic sites and medium base strength are required for the mercaptan oxidation, which provides the basis for the rational design of more efficient solid bases for the sweetening process.