Effects of pH and vanadium concentration during the impregnation of Na-SiO2 supported catalysts for the oxidation of propane

SiO2 supported NaVOx catalysts have raised interest due to reports showing an increase in the selectivity to propene in propane oxidation; a process that may become an alternative to conventional petrochemical routes for producing this valuable olefin. In this work, the effects of pH, the concentration of vanadium in solution, [V]Sol, and of their interaction; i.e., non-additive effect, over the properties of Na-SiO2 used for the oxidation of propane under oxygen rich conditions were studied. In general, the studied experimental factors had no net effects over the difference between the nominal and final loadings of vanadium in the catalysts, the surface area, and porosity. However, all catalysts presented a ~50% decrease in surface area due to partial mesopore blocking reflected by a ~30% increase in the average pore diameter. On the other hand, NaVO3 microcrystals and particles constituted -NaVO3 and -NaVO3 were formed at acidic pH, whereas Na metavanadate nanoparticles were formed at pH=9.0. These nanoparticles transformed into an -NaVO3 type phase upon dehydration. The reducibility of the catalysts was a function of the impregnation pH, where the catalysts synthesized at pH=3.8 and 9.0 displayed similar reduction patterns but lower reducibility than those synthesized at pH=1.5. The produced patterns were correlated to the existence of a mixture of V5+ and V4+ species among which V4+ was prevalent for the catalysts synthesized at pH=3.8 and 9.0. Consequently, the basicity of the catalysts decreased with the increase in pH. The surface concentration of vanadium increased with the increase in the concentration of vanadium in solution, while the pH had a weak negative effect over the former. On the other hand, it was established that chemical surface state of oxygen in the synthesized catalysts was influenced weakly by the synthesis pH and, more importantly, by non-additive effects between the pH and the concentration of vanadium in solution. The latter was coherent with the detection of surface oxygen species related to the partial dissolution of the Na-SiO2 support; a phenomenon that seemed to be favored at acidic pH. The latter properties were correlated to changes in basicity. Concerning the catalytic performance, the catalysts synthesized at pH=9.0 displayed the best steady state performance in terms of the selectivity to propene and oxygen consumption. The collected evidence allowed corroborating that both the oxidative dehydrogenation of propane to propene and its combustion to CO and CO2 are kinetically controlled by the initial activation of one of the C-H bonds of propane. But, the production of CO2 depended on the level of consumption of O2; where, a surplus of the latter compared to the conversion of propane combusted the hydrocarbon into CO2. Such a finding agreed with what has already been established for supported vanadium oxides for hydrocarbon oxidation. An analysis of the Raman spectra of the spent catalysts suggested that under the applied reaction conditions, all catalysts were provided of an active phase mostly constituted by -NaVO3 type structures. Therefore, the observed differences in catalytic behavior were rather associated to changes in particle size, reducibility, and acidity.