Heterogeneous
catalysts with precise surface and interface structures
are of great interest to decipher the structure–property relationships
and maintain remarkable stability while achieving high activity. Here,
we report the design and fabrication of the new sandwich composites
ZIF-8@Au25@ZIF-67[tkn] and ZIF-8@Au25@ZIF-8[tkn]
[tkn = thickness of shell] by coordination-assisted self-assembly
with well-defined structures and interfaces. The composites ZIF-8@Au25@ZIF-67 efficiently catalyzed both 4-nitrophenol reduction
and terminal alkyne carboxylation with CO2 under ambient
conditions with remarkably improved activity and stability, compared
to the simple components Au25/ZIF-8 and Au25@ZIF-8, highlighting the highly useful function of the ultrathin
shell. In addition, the performances of these composite sandwich catalysts
are conveniently regulated by the shell thickness. This concept and
achievements should open a new avenue to the targeted design of well-defined
nanocatalysts with enhanced activities and stabilities for challenging
reactions.
The classic Fokin mechanism of the CuAAC reaction of terminal alkynes using a variety of Cu(I) catalysts is wellknown to include alkyne deprotonation involving a bimetallic σ,π-alkynyl intermediate. In this study, we have designed a CNTsupported atomically precise nanocluster Au 4 Cu 4 (noted Au 4 Cu 4 /CNT) that heterogeneously catalyzes the CuAAC reaction of terminal alkynes without alkyne deprotonation to a σ,π-alkynyl intermediate. Therefore, three nanocluster−π-alkyne intermediates [Au 4 Cu 4 (π-CHC-p-C 6 H 4 R)], R = H, Cl, and CH 3 , have been captured and characterized by MALDI-MS. This Au 4 Cu 4 /CNT system efficiently catalyzed the CuAAC reaction of terminal alkynes, and internal alkynes also undergo this reaction. DFT results further confirmed that HCCPh was activated by π-complexation with Au 4 Cu 4 , unlike the classic dehydrogenation mechanism involving the bimetallic σ,π-alkynyl intermediate. On the other hand, a Cu 11 /CNT catalyst was shown to catalyze the reaction of terminal alkynes following the classic deprotonation mechanism, and both Au 11 /CNT and Cu 11 /CNT catalysts were inactive for the AAC reaction of internal alkynes under the same conditions, which shows the specificity of Au 4 Cu 4 involving synergy between Cu and Au in this precise nanocluster. This will offer important guidance for subsequent catalyst design.
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