Alkoxide-Decorated Copper Nanoparticles on Amidine-Modified Polymers as Hydrogenation Catalysts for Enabling H₂ Heterolysis

Abstract

Direct formation of Cu nanoparticles was accomplished on an amidine- and guanidine-functionalized polymer (polystyrene-bound 1,8-diazabicyclo[5.4.0]undec-7-ene (PS-DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (PS-TBD), respectively) by thermal treatment of Cu(acac)2 (acac = acetylacetonato) in methanol under pressurized H2. The immobilized structure was characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), elemental analysis, and microscopic analysis. X-ray photoelectron spectroscopy (XPS) data indicated that the nanoparticles consisted of Cu(0) and oxygenated Cu(I). The Cu(0)/Cu(I) ratio was determined by X-ray absorption spectroscopy. In particular, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies indicated the inclusion of alkoxides bound to Cu nanoparticles. Further 13C-labeled experiments allowed us to identify the Cu methoxide species by solid-state 13C nuclear magnetic resonance (NMR). A series of alkoxide-incorporated Cu nanoparticles were proven to facilitate H2 heterolysis and were applied in the catalytic hydrogenation of CO2 to formate salts in the presence of a nitrogen base. The catalytic activity increased with decreased mean particle size and increased Cu(I) content, indicating that the Cu(I) alkoxide species played an indispensable role in hydrogenation. The combination of Cu/PS-DBU and 2-tert-butyl-1,1,3,3-tetramethylguanidine (BTMG) achieved a maximum turnover number of 2450 at 100 °C without leaching or aggregation of the nanosized catalysts.

Nanoarchitectonics of Non-Noble-Metal-Based Heterogeneous Catalysts for Transfer Hydrogenation Reactions: Detailed Insights on Different Hydrogen Sources

“…In the case of direct hydrogen transfer, the solid catalysts having electron-deficient Lewis acidic site and basic site play a crucial role in catalyzing CTH reactions by forming a six-membered ring intermediate. This mechanism is commonly known as the Meerwein–Ponndorf–Verley (MPV) mechanism. ,− While in the case of metal hydride transfer pathway, hydrogenation is generally catalyzed by a transition-metal catalyst by forming metal hydride bonds. − When molecular hydrogen is used as a hydrogen source, it is absorbed on the surface of the catalyst and considered as chemically equivalent hydrogen species. When liquid hydrogen is used as a hydrogen source, the acidic site of the metal forms a metal hydride bond with the liquid hydrogen and catalyzes hydrogenation reactions. − For example, Ma et al utilized Cu/ZrOCO 3 for the transfer hydrogenation of methyl levulinate (ML) to γ-valerolactone (GVL) and 1,4-pentanediol (1,4-PDO) by using isopropanol (IPA) as a hydrogen source.…”