MOF-Stabilized Pd Single Sites for CO Esterification to Dimethyl Carbonate

Pd single site catalysts (Pd SSCs) are recognized to be efficient for CO esterification to a valuable product, dimethyl carbonate (DMC), but the tendency of reduction to Pd nanoparticles in CO atmosphere under reaction process leads to a reduced selectivity and limits the industrial application. Hence, the development of appropriate supports to stabilize Pd single sites is of great importance. Metal-organic frameworks (MOFs) are expected to become one of the most ideal choices for supporting SSCs due to their large specific surface area, good stability, and ease of modification. Herein, a stable and highly modifiable MOF, UiO-66-NH2, has been selected as the support. The pyridine-2-formaldehyde is grafted to UiO-66-NH2 by post-synthetic modification, connecting to the skeleton of UiO-66-NH2 via amine aldehyde condensation. In this way a pair of adjacent N atom sites are created to jointly chelate Pd(II) species, yielding PdII-UiO-66-X%. The Pd SSCs exhibit high DMC selectivity (>98%) in the CO esterification reaction. Moreover, PdII-UiO-66-X% shows excellent stability and more than 85% DMC selectivity can be maintained in 70 h continuous test, thanks to the good dispersion and strong interaction of Pd(II) species and the MOF support. As a control, the Pd(II) species is supported on ZrO2 to give PdII/ZrO2. Obviously, it is difficult for ZrO2 to disperse Pd(II) species and to strongly interact with them due to the absence of binding groups. As a result, PdII/ZrO2 is rapidly reduced by CO during the reaction, resulting in decreased DMC selectivity. In addition, the signals of key intermediates are detected and the reaction mechanism on PdII-UiO-66-X% is proposed based on in-situ diffuse reflectance infrared Fourier transform (in-situ DRIFT) spectra. The successful fabrication of PdII-UiO-66-X% with high selectivity and stability provides great opportunity for CO esterification to DMC.