Paired-Pd(II) centers embedded in HKUST-1 framework: Tuning the selectivity from dimethyl carbonate to dimethyl oxalate

Abstract

CO oxidative coupling to dimethyl oxalate (DMO) is the most crucial step in coal to ethylene glycol. Pd-based supported catalysts have been verified effective for generating DMO, but concomitant generation of dimethyl carbonate (DMC) is always unavoidable. It is generally accepted that Pd(0) is the active species for producing DMO, while Pd(II) for DMC. However, density functional theory calculations have proposed that the selectivity to DMO or DMC highly depends on the space state of Pd species rather than its oxidative state. It is thus urgently desired to develop high-efficient catalysts with well-defined structure, and further to elucidate the structure-performance relationship. In this work, HKUST-1 with unique structure of paired-Cu(II) centers was chosen as ideal support to construct the catalysts with respective paired-Pd(II) centers and isolated-Pd(II) centers via in situ Pd species doping. In despite of featuring Pdδ+ (δ≈2) oxidation state, the synthesized paired-Pd(II)/HKUST-1 catalyst still exhibited DMO as dominant product (90.8% of DMO selectivity). For isolated-Pd(II)/HKUST-1 catalyst, however, the main product was DMC (84.8% of DMC selectivity). Based on catalyst characterizations, the structures of paired-Pd(II) centers and isolated-Pd(II) centers were determined. DMO was generated from the coupling of adjacent *COOCH3 intermediates adsorbed on paired-Pd(II) centers, while DMC was produced from the reaction between methyl nitrite and the *COOCH3 intermediates formed on isolated-Pd(II) centers. Current work is the first MOFs-based catalyst with well-defined structure being applied in CO oxidative coupling reaction, which not only sheds light on the structure-performance relationship, but also inspires the potential of using MOFs as tunable platform to design high-efficient catalysts in heterogeneous catalysis.