Soluble porous organic cage-encapsulated metal nanoclusters
The paper in Nature Catalysis is here: http://go.nature.com/2G7Q2XZ
Considerable interest has been focused on the fabrication of metal nanoclusters (MNCs) with sizes ranging from a few atoms to ～2 nm. One important reason is the smaller the size, the more the available surface atoms, which, in most cases, leads to an enhanced catalytic performance. Unfortunately, tiny MNCs easily aggregate and grow driven by the high surface energy, resulting in the loss of catalytic activity and stability.
A popular approach involving the excessive use of capping agents to stabilize MNCs. However, a low catalytic activity is often obtained owing to a high fraction of blocked active sites. On the other hand, clean MNCs can be anchored on high-surface-area materials to avoid primary aggregation. Our group has synthesized a series of multifunctional metal nanoparticles (MNPs). Of particular note is the use of a double solvents method (DSM) to confine highly catalytically-active MNPs within the pores of metal-organic frameworks (MOFs) (J. Am. Chem. Soc., 2012, 134, 13926; 2013, 135, 10210). Considering that solid supports, such as porous carbons, metal oxides and MOFs, cannot achieve a truly uniform distribution in liquid-phase catalysis, the dispersibility of MNPs still needs to be improved.
Porous organic cages (POCs) with intrinsic and guest accessible cavities are promising for gas storage and separation. Considering the ultrafine pores and high solubility in solvents of POCs, it is a fascinating idea of encapsulating MNCs inside the cavities of POCs. Obviously, since being stabilized inside the open cavities of discrete molecular cages, the resulted soluble MNCs can achieve extremely high dispersibiltiy in solution with more accessible active sites for liquid-phase catalysis, and thus an enhanced catalytic activity can be expected. But, how to encapsulate the MNCs into the ultrafine cavities of POCs？
POC-encapsulated tiny Pd NCs
Opportunity coexists with challenge. We have tried but failed several times through searching appropriate metal precursors, preparing appropriate cages as well as adjusting process parameters. Finally, inspired by the previous work for MNP@MOF hybrids, we have performed a reverse double solvents approach (RDSA) for MNC@POC hybrids considering that POCs possess the hydrophobic pores. Surprisingly, the ultrafine Pd NCs with a size of ～0.7 nm have been successfully encapsulated inside the cavities of POCs, which exhibit extremely high catalytic activities toward a series of liquid-phase catalytic reactions.
POC-encapsulated MNCs are promising for many applications. We are interested in the synthesis and application of MNC@POC hybrids, and hope our work may give inspiration to those who work in the related areas if possible.