Navigating Copper-Atom-Pair Structural Effect inside a Porous Organic Polymer Cavity for Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural

In recent times, selective hydrogenation of biomass-derived 5-hydroxymethylfurfural (5-HMF) to produce the novel difuranic polyol scaffold 2,5-dihydroxymethylfuran (DHMF) has attracted the interest of the many researchers due to its peculiar symmetrical structure and its widespread application as a monomer for the preparation of cross-linked polyesters and polyurethane. Copper-based catalysts have been explored for selective catalytic hydrogenation; however, hurdles are still associated with the strongly reducing H 2 atmosphere and oxidizing C–O bond that make the Cu 0 and Cu x + surface active species unstable, limiting the rational design of highly efficient integrated catalyst systems. To address this, herein, we built catalytic systems for 5-HMF hydrogenation with stable and balanced Cu 0 and Cu x + active surface species inside the nanocage of a catechol-based porous organic polymer (POP) endowed with large surface areas, impressive stabilities, and spatial restriction inhibiting nanoparticle aggregation. Batch reactor screening identified that a superior catalytic performance (DHMF selectivity of 98%) has been achieved with our newly designed Cu@C-POP at 150 °C temperature and 20 bar H 2 pressure, which was also higher than that of other reported copper catalysts. Comprehensive characterization understanding with H 2 -TPR and X-ray photoelectron spectroscopy (XPS) study revealed that substantially boosted activity is induced by the presence of the bulk CuO x phase and atomically dispersed Cu species incorporating isolated Cu ions, which are further confirmed through the positive binding energy shift of Cu 2p 3/2 XPS spectra (∼0.4 eV). The Cu environment in our catalytic systems comprises a predominantly square planar geometry (probably Jahn–Teller distorted OH), which we gleaned from the extended X-ray absorption for fine structure (EXAFS) analysis featuring two adjacent copper atoms with the valence state in between of 0 and +2, as validated by XANES absorption edge positions. EXAFS studies further revealed a lowering of the Cu coordination number for the most active Cu@C-POP- B catalyst, suggesting the presence of metal vacancies. Density functional theory calculations showed that the presence of Cu metal vacancies stabilized the reaction intermediates formed during 5-HMF hydrogenation and decreased the hydrogenation barriers, resulting in an enhanced catalytic activity of the Cu@C-POP- B catalyst.