Tuning the activity
and selectivity of metal nanoparticles (NPs)
is a long-term pursuit in the field of catalysis. Herein, we report
successfully improving both the activity and chemoselectivity of Pd
NPs (1.1 nm) with triphenylphosphine (PPh3) cross-linked
in the nanopore of FDU-12. The electron-donating effect of PPh3 increases the surface electronic density of Pd NPs and weakens
the Pd–H bond, as evidenced by the results of XPS, in situ
FT-IR adsorption of CO, and H2–D2 exchange
reactions. Consequently, Pd NPs modified with PPh3 obtain
>99% selectivity to 1-phenylethanol in acetophenone hydrogenation
and 94% selectivity to styrene in phenylacetylene hydrogenation. Furthermore,
the activity of Pd NPs is enhanced and suppressed by PPh3, respectively, in the hydrogenation of electrophilic nitro compounds and nucleophilic carbonyl
substrates. Our primary results shed some light on judiciously choosing
organic ligands for modifying the catalytic performance of metal NPs
toward specific chemical transformations.
Covalent
organic frameworks (COFs) emerging as a novel kind of
visible light-responsive organic semiconductor have attracted extensive
research attention in the field of photocatalytic organic transformations.
However, the key parameters affecting their photocatalytic properties
are still not clear. In this work, a series of [3 + 3] COFs with similar
two-dimensional hexagonal structure but different compositions are
synthesized and employed as model materials for investigating the
key factors affecting the photocatalytic properties in the visible-light-driven
reductive dehalogenation reaction and the aerobic cross-dehydrogenative
coupling reaction. In comparison with −H and −CF3, the −OH substituent in the aromatic ring could narrow
the band gap of the COFs. The COFs with a triazine skeleton in the
framework usually boost the photocatalytic activity, possibly because
of the enhanced charge separation efficiency by the formation of a
donor–acceptor domain. As a combined result of the narrow band
gap, efficient charge separation, and high conductivity, the COF possessing
both a −OH group and triazine skeleton shows the highest activity
in the photocatalytic reductive dehalogenation reaction. Notably,
COFs could be easily recovered and reused several times without the
loss of crystallinity. Our primary results may shed light on the design
of efficient COF-based semiconductors for photocatalytic organic transformations.
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