Alkali and earth alkali modified CuOx/SiO2 catalysts for propylene partial oxidation: What determines the selectivity?

In this work, CuOx/SiO2 catalysts were investigated in the propylene partial oxidation reaction. Ordered mesoporous silica (KIT-6) was used to deposit 1-10 wt. % copper and subsequently modified with Na, K and Ca. The synthesized materials were characterized by N-2 physisorption, XRD, TEM-EDS, CO2-TPD, operando UV/Vis DRS, operand XANES and pyridine DRIFT spectroscopy. Regardless of the CuOx loading, catalyst deactivation was observed during propylene oxidation reaction in non-modified catalysts, which was related to sintering of oligomeric [Cu-O-Cu](n) species. Sintering of CuOx is strongly promoted under a reducing propylene atmosphere and related to the presence of Cu+1. The resulting bulk CuOx promotes acrolein selectivity. We produced modified catalysts with finely dispersed alkali metal cations, associated with the subnanometer CuOx phase, resulting in a greatly stabilized morphology and catalytic activity. Operando XANES analysis revealed that a substantial fraction of Cu2+ is transformed to Cu+ during the propylene oxidation reaction (52-68%, depending on the modifying atom). Also, the dynamics of reaching the quasi steady oxidation state differ strongly. The kinetics of oxygen abstraction and replenishment are substantially different, indicative of modified chemistry of the nucleophilic oxygen species, present in 5CuNa catalyst in contrast to others (5Cu and SCuCa). We propose that Cu+ is not crucial for PO formation. Instead the electropositive Na+ and K+ decrease the nucleophilic strength of oxygen in CuOx by attracting its electrons. Consequently, the catalytic action of oxygen changes from oxidative attack on the allylic hydrogen to oxygen insertion into the C=C bond of propylene. This results in a noticeable selectivity shift from acrolein to propylene oxide. The effect of calcium on decreasing the nucleophilic character of O species in CuOx is negated by charge compensation by strongly adsorbed hydroxyl groups and Ca modification for PO selectivity is inefficient. Additionally we found, that further oxidation of propylene oxide is, most likely, the main factor determining high selectivity for COx products. The alkali modification which increases the PO selectivity does not function via elimination of LAS, but exclusively through attenuation of nucleophilic character of oxygen species.