The development of economic, environmental-friendly and energy-saving process for selective depolymerization of lignin is an outstanding challenge. Herein, a novel and efficient visible-light-induced photocatalytic process for the selective depolymerization of...
The proximity of different active sites in multicatalytic systems is crucial in influencing the catalytic reactions, that is, to occur or to be accelerated. Here, we reported a heterogeneous metallaphotocatalyst prepared by embedding Ni(II) species in a photosensitive covalent organic framework (COF). A concerted triad catalytic behavior executed by electrons, holes, and Ni species triggered a dramatic catalytic enhancement in the activation of aryl chlorides with water to phenols. It demonstrated a 50-fold increment in its activity when compared to the homogeneous analogy, for example, Ni(BPDA)Br 2 . Also, it was able to efficiently activate a variety of aryl chlorides to the corresponding phenols with moderate to high yields. Interestingly, it was effective even for base-sensitive substituents or chlorobenzene bearing NH or NH 2 groups. On the basis of the detailed experiments, the proximity of the photoactive COF and Ni(II) sites facilitates rapid electron transfer to produce Ni(I) active sites for the oxidative addition. Meanwhile, the photogenerated holes oxidized water to the hydroxyl radical, which then attacked Ni(III) intermediates to complete the catalytic cycle.
Metal nanoparticles have been recognized and widely explored as unique catalysts for carbon−carbon coupling reactions. However, due to their extreme tendency to agglomeration, the generation and stabilization of metal nanoparticles in a porous matrix is an important research field. Herein, novel mesoporous phenolic resin-supported palladium nanoparticles (Pd@NH 2 -MPRNs) were prepared via direct anionic exchange followed by gentle reduction by using primary amine-functionalized ordered mesoporous phenolic resin as the support. The obtained Pd@NH 2 -MPRN material still possessed large surface area and ordered two-dimensional hexagonal mesoporous structure. Meanwhile, uniform and well-dispersed palladium nanoparticles were formed in the mesoporous channels, which could be attributed to an efficient complexation and stabilization effect derived from the primary amine groups. As a result, it can promote Suzuki coupling of less activated aromatic bromides to various biaryls in water with high conversion and selectivity. This excellent performance was attributed to small particle sizes, ordered mesopores, and a hydrophobic pore surface, which resulted in the decreased diffusion limitation and the increased active site accessibility. It is noted that it is competitive with the best palladium catalysts known for water-medium Suzuki coupling reaction, and it can be reused at least seven times without significant reduction in the catalytic efficiency, showing a good recyclability. Therefore, this work provides a new potential platform for designing and fabricating robust ordered mesoporouspolymer-supported metal nanoparticles for various catalytic applications.
Conformational dynamics of active
sites in enzymes enable great
control over the catalytic process. Herein, we constructed a metal–organic
framework with conformationally dynamic active sites (Rh2-ZIF-8). The active sites in Rh2-ZIF-8 were composed of
the imidazolate-bridged bimetallic center with a catalytic dirhodium
moiety and structural zinc site. Even though the coordination sphere
of the dirhodium species was saturated with two circularly arranged
esp groups and two axial 2-MeIm ligands, it could still effectively
catalyze the direct synthesis of N–H aziridines from olefins
with high activity. We found that such a self-adaptive catalytic process
was based on the dynamic breakage and reformation of the rhodium–zinc
imidazolate bridges. Interestingly, the in situ generated
dirhodium site with a unique Rh2(esp)2(2-MeIm)1 configuration was able to exhibit obviously enhanced selectivity
compared to homogeneous catalyst Rh2(esp)2.
Furthermore, the surrounding zinc imidazolate groups could effectively
protect the dirhodium moieties from harsh environments, and this ultimately
endowed it with high stability.
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