(363d) The Activation of CO2 By UiO-66 MOFs in CO2 Hydrogenation

As a material with well-ordered tunable porous structures, metal–organic frameworks (MOFs) have advantages such as structural adjustability, large surface area, and excellent gas adsorption capacity. Owing to these properties, the development of MOF materials for CO₂ catalytic conversion has attracted tremendous attention.

In this study, UiO-66 and -NH₂ modified UiO-66 (named UiO-66-NH₂) were synthesized via a hydrothermal method and their activation capacity and hydrogenation catalytic ability of CO₂ were investigated. Noticeably, the products of the CO₂ hydrogenation reaction over both catalysts were only CO, implying that both MOFs could efficiently activate CO₂, but could not hydrogenate the activated CO₂ molecule further. More interestingly, although the -NH₂ modification brought a new strong base site to UiO-66, which is normally considered an efficient activation site for CO₂, UiO-66-NH₂ showed a lower activation capacity of CO₂ compared to the unmodified UiO-66 (as shown in Table 1). Furthermore, in order to provide dissociated H for the activated CO₂ molecules, Pd NPs were selected as the active metal. After the loading of Pd NPs, the Pd@UiO-66 catalyst showed a much higher hydrogenation activity compared to the unloaded UiO-66. The CH₄ selectivity, CO₂ conversion, and STY of Pd@UiO-66-6% were as high as 97.3%, 56.0%, and 856 g·h^-1·kg_cat^-1, respectively. The good catalytic activity was attributed to the combined effect of high activation capacity of UiO-66 towards CO₂ and hydrogen dissociation ability of Pd.

Table 1. The result of CO₂ hydrogenation over UiO-66 and UiO-66-NH₂

Sample	CO2 Conversion	Products	Selectivity
UiO-66	10.3%	CO	100%
UiO-66-NH2	3.7%	CO	100%

Reaction conditions: 340°C, CO₂: H₂=1:4 mol, GHSV=15000 h^-1, 4 MPa.