Insight into the Effect of Copper Substitution on the Catalytic Performance of LaCoO₃-Based Catalysts for Direct Epoxidation of Propylene with Molecular Oxygen

Abstract

Direct epoxidation of propylene to propylene oxide (PO) with molecular oxygen as the most atomically economical route remains challenging due to the low PO yield. In this study, CuO supported on perovskites (LaCoO 3 ) modified with NaCl was found to be active for direct epoxidation of propylene with oxygen. LaCo x Cu 1−x O 3− modified with alkali metal salts were further fabricated to improve catalytic performance with the assistance of citric acid. Among the investigated catalysts, LaCo 0.8 Cu 0.2 O 3−δ exhibited the best catalytic performance, with a PO formation rate of 60.4 g PO •kg cat −1•h −1 (PO selectivity of 10.2% and propylene conversion of 12.0%) at a lower temperature (250 °C). The characterization results indicate that Cu-doped LaCoO 3 catalysts with enhanced oxygen vacancies and abundant electrophilic oxygen species are crucial to selectively attack on CC bond instead of C−H bond. Besides, LaCo x Cu 1−x O 3−δ catalysts demonstrated higher propylene conversion and more stable catalytic performance. This work exemplifies the direct propylene epoxidation with molecular oxygen employing perovskite materials.

Activation of molecular oxygen over Mn-doped La2CuO4perovskite for direct epoxidation of propylene

“…Hence, the atomic-scale doping of Mn promoted the distribution of Cu atoms in the perovskite framework and inhibited the formation of the separate phase CuO, which was consistent with XRD characterization results. 19 Meanwhile, the SEM and TEM images of the used catalysts are displayed in Fig. S4 and S5,† respectively.…”

“…Copper oxide catalysts usually encounter oxidation and agglomeration problems during the reaction. 19,35 The catalyst stability is an important parameter over continuous operation. As shown in Fig.…”

“…This is attributed to the Mn-O bond stretching vibration and enhanced Mn-O bond covalence, which might be caused by the formation of the Cu-O-Mn bond by electrons shared by Mn and Cu via oxygen atoms along with the change of the overlapped electron cloud of Cu 3d and O 2p orbitals. 19,27 This is related to the oxygen vacancy formation energy and the electron density of the cation.…”

“…14 However, excellent PO selectivity can be obtained only on modified Cu catalysts. 8 Several strategies such as regulation of exposed crystal planes, 15 metal modification, [16][17][18] and halide ion modification 8,19 have been proposed, among which metal modification performed better in governing catalytic stability. For example, Teržan et al 20,21 investigated alkali and earth alkali modification of CuO x /SiO 2 catalysts.…”

Selective formation of propan-1-ol from propylene via a chemical looping approach

“…22 Catalysts for chemical looping epoxidation of ethylene have been found to give stable performance over 33 cycles, 23 producing EO at 67% selectivity, with 19% conversion of ethylene. Perovskite supports have also been investigated 26,27 for propylene epoxidation and found to improve modestly the selectivity towards PO, by facilitating the dissociation of O 2 molecules, but under conventional operating conditions with oxygen in the gas phase, rather than chemical looping.…”