We chose the hydrogenation of DMO-a highly exothermic and heat-sensitive system 23- as a probe reaction to evaluate the effect of spatial confinement of copper NPs in functionalized nanotubes and surface chemistry of the nanoreactor on the catalytic performance. We report here a simple and efficient approach for the fabrication of a core-sheath nanoreactor with a copper-phyllosilicate nanotube (CuPSNT) sheath, selectively immobilizing copper NPs inside or outside the cavity with stable and balanced Cu 0 and Cu + active species. The structures with a strong metal-oxide and metal-support interaction make the copper species (particularly for Cu + species) very stable when the hydrogenation reaction temperature is below 673 K. We have demonstrated that copper phyllosilicates with a lamellar structure could produce stable Cu + species on its surface in the hydrogenation reaction 5. The rich porous structures and tunable composition of silicates make them rather appealing for encapsulation of metallic NPs in catalysis 21. Silicates-thermally stable mesoporous materials-are promising candidates for various applications including selective catalysis, adsorption and separation 20, 21, 22. Therefore, it is essential to fabricate catalysts in a controlled manner with stable structures and coexisting Cu 0 and Cu + active species for a better understanding of properties of active sites and the structure–activity relationship. However, the strongly reducing H 2 and oxidizing C-O make the Cu 0 and Cu + species unstable in the reaction. Moreover, the balanced surface Cu 0 and Cu + species can greatly improve the catalytic performance of the catalysts 17, 18, 19. It has been suggested that Cu + acts as the stabilizer of the methoxy and acyl species and could also function as electrophilic or Lewis acidic sites to polarize the C-O bond via the electron lone pair in oxygen. Earlier work on understanding the active sites of copper catalysts for the hydrogenation of dimethyl oxalate (DMO) 5, furfural 4 and CO 2 17 indicated that both Cu 0 and Cu + species were crucial to the activity of copper-based catalysts. However, other researchers were not able to establish a linear correlation between activity and copper-surface area relationship and suggested that a synergy between copper and the oxide components also influenced activity. Metallic copper is believed to be an active phase because the activity was frequently found to be proportional to copper-surface area 3, 5, 16, 17. Recently, the encapsulation of individual NPs in morphologically well-defined inorganic shells or tubes has received much attention because of the thermal and chemical stabilities in catalysis 12, 13, 14, 15.ĭespite extensive studies on C-O hydrogenolysis reactions, there is still debate regarding the nature of active sites of copper-based catalysts. However, these approaches are not generally applicable because they restrict chemical composition and functionality. Strategies to mitigate particle growth comprise alloying with a higher-melting point metal 8, tuning the properties of individual nanoparticles 9, 10 and increasing the metal-support interaction energy by using specific oxides as carriers 11.
Notably, the loss of active surface area by metal-particle growth is a major cause of deactivation for many supported catalysts. However, the tendency of copper nanoparticles (NPs) to grow into larger crystallites through migration and coalescence of particles, or through the transport of monoatomic or molecular species between individual particles, is an impediment to stable performance 6, 7. Copper-based catalysts have been intensively explored for hydrogenation reactions, as the copper sites account for the selective hydrogenation of carbon–oxygen bonds and are relatively inactive for the hydrogenolysis of carbon–carbon bonds 4, 5, 6. Hydrogenolysis of carbon–oxygen (C–O) bonds (for example, esters, ethers, furfural and CO 2) has emerged as a versatile synthetic tool in organic methodology 1, 2, 3, as it can produce a variety of products (for example, chemicals, fuels and polymers).
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