Abstract:
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 H2 atmosphere and oxidizing C−O bond that
make the Cu0 and Cux+ 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 Cu0 and Cux+ 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 H2 pressure, which was also higher than that of other reported copper
catalysts. Comprehensive characterization understanding with H2-TPR and X-ray photoelectron spectroscopy (XPS) study revealed
that substantially boosted activity is induced by the presence of the bulk CuOx phase and atomically dispersed Cu species
incorporating isolated Cu ions, which are further confirmed through the positive binding energy shift of Cu 2p3/2 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.
