Herein, we report on a molecular catalyst embedding metal–organic framework (MOF) that enables enhanced photocatalytic CO2 reduction activity. A benchmark photocatalyst fac-ReBr­(CO)3(4,4′-dcbpy) (dcbpy = dicarboxy-2,2′-bipyridine) and photosensitizer Ru­(bpy)2(5,5′-dcbpy)­Cl2 (bpy = 2,2′-bipyridine) were synergistically entrapped inside the cages of the nontoxic and inexpensive MIL-101-NH2(Al) through noncovalent host–guest interactions. The heterogeneous material improved Re catalyst stabilization under photocatalytic CO2 reduction conditions as selective CO evolution was prolonged from 1.5 to 40 h compared to the MOF-free photosystem upon reactivation with additional photosensitizer. By varying ratios of immobilized catalyst to photosensitizer, we demonstrated and evaluated the effect of reaction environment modulation in defined MOF cages acting as a nanoreactor. This illustrated the optimal efficiency for two photosensitizers and one catalyst per cage and further led to the determination of ad hoc relationships between molecular complex size, MOF pore windows, and number of hostable molecules per cage. Differing from typical homogeneous systems, photosensitizerand not catalystdegradation was identified as a major performance-limiting factor, providing a future route to higher turnover numbers via a rational choice of parameters.

A strategy to improve homogeneous molecular catalyst stability, efficiency, and selectivity is the immobilization on supporting surfaces or within host matrices. Herein, we examine the co‐immobilization of a CO2 reduction catalyst [ReBr(CO)3(4,4′‐dcbpy)] and a photosensitizer [Ru(bpy)2(5,5′‐dcbpy)]Cl2 using the isoreticular series of metal–organic frameworks (MOFs) UiO‐66, ‐67, and ‐68. Specific host pore size choice enables distinct catalyst and photosensitizer spatial location—either at the outer MOF particle surface or inside the MOF cavities—affecting catalyst stability, electronic communication between reaction center and photosensitizer, and consequently the apparent catalytic rates. These results allow for a rational understanding of an optimized supramolecular layout of catalyst, photosensitizer, and host matrix.

We report on the preparation of surface-anchored nanoparticles of the metal–organic framework (MOF) UiO-66 (Universitet i Oslo; Zr6O4(OH)4(bdc)6; bdc2– = 1,4-benzene dicarboxylate). The surface-anchored nano-MOFs (SA-NMOFs) were prepared by covalent anchoring of a presynthesized, functionalized UiO-66 nano-MOF (NMOF) on surface-modified poly­(dimethylsiloxane). The SA-NMOFs exhibit discrete NMOFs (<30 nm) which do not aggregate. The SA-NMOFs retain a high surface area, rendering them interesting catalysts. We compared the catalytic activities of SA-NMOFs in the cyanosilylation of benzaldehyde with those of the bulk UiO-66 and colloidal-dispersed UiO-66 NMOFs (size: 22 ± 3 nm). The SA-NMOFs exhibit a boost in activity by a factor of 100,000–1,000,000 owing to (a) the generally larger surface area of NMOFs and (b) the suppressed aggregation of the nanoparticles by surface immobilization. In contrast, colloidal NMOFs rapidly aggregate, as shown by dynamic light scattering. The general applicability of our approach for other Lewis acid-catalyzed reactions is demonstrated by comparing the activities of the three catalyst systems for the cycloaddition of CO2 and propylene oxide to propylene carbonate, where SA-NMOFs by far outperform the bulk MOFs and defect-engineered MOFs, respectively. This discovery paves the way for application of SA-NMOFs as efficient catalyst materials.

Since the development of solid palladium catalysts for the Heck coupling reaction, many studies focused on the differentiation between homogeneous and heterogeneous reaction pathways. It is generally accepted now that also in heterogeneous catalytic systems, the reaction is catalyzed by molecular, dissolved Pd(0) complexes. In recent time, the mechanism of leaching and, in particular, the dynamics of dissolution and redeposition of metal species came into the focus of research reports. In this context, the specific approach of this study is the comparison of different solid palladium precatalysts regarding their activity in the Heck reaction and the corresponding relation to the dissolution process of palladium in particular with demanding substrates. The potential correlation of leaching and reaction kinetics has been studied for bromobenzene and chlorobenzene. The contemporaneous determination of the conversion and concentration of palladium in solution and separate temperature-programmed Pd dissolution experiments revealed differing leaching mechanisms comparing supported palladium oxide catalysts with isolated palladium surface complexes.

No abstract