Abstract
A new strategy for controlled synthesis of a MOF composite with a core–shell structure, ZIF-8@resorcinol–urea–formaldehyde resin (ZIF@RUF), is reported for the first time through in situ growth of RUF on the surface of ZIF-8 nanoparticles via an organic–organic self-assembly process by using hexamethylenetetramine as a formaldehyde-releasing source to effectively control the formation rate of RUF, providing the best opportunity for RUF to selectively grow around the nucleation seeds ZIF-8. Compared with the widely reported method for MOF composite synthesis, our strategy not only avoids the difficulty of incorporating MOF crystals into small pore sized materials because of pore limitation, but also effectively guarantees the formation of a MOF composite with a MOF as the core. After carbonization, a morphology-retaining N-doped hierarchical porous carbon characterized by its highly developed microporosity in conjunction with ordered mesoporosity was obtained. Thanks to this unique microporous core–mesoporous shell structure and significantly enhanced porosity, simultaneous improvements of CO2 adsorption capacity and kinetics were achieved. This strategy not only paves a way to the design of other core–shell structured MOF composites, but also provides a promising method to prepare capacity- and kinetics-increased carbon materials for CO2 capture.
Highlights
Metal–organic frameworks (MOFs) have attracted tremendous attention and have intriguing potential applications in gas storage,[1,2,3] catalysis,[4,5] optoelectronic devices,[6,7] and separation[8,9] because of their high surface area, high adsorption affinity, and diverse structures with various pore shapes, sizes, volumes, and surface chemistry owing to the diversity of metallic centers and organic ligands
It can be seen that the ZIF-8 spheres are composed of ZIF-8 nano-sheets, which can be evidenced from Transmission electron microscopy (TEM) images of the small fragments in Fig. S2.† scanning electron microscopy (SEM) images of RUF in Fig. 3B and S3† showed that RUF particles possess a uniform prism structure of several micrometers in size
Further investigations of ZIF-8@resorcinol– urea–formaldehyde resin (ZIF@RUF) SEM images in Fig. S4† revealed that ZIF-8 spheres cannot be found any more from the panoramic view, this demonstrates that all the ZIF-8 particles were totally embedded into RUF
Summary
A wide variety of methods for the preparation of metal nanoparticle/MOF composites have been developed, such as solution in ltration method, vapor deposition and solid grinding.29À31 as far as combining traditional porous materials with MOFs into one construct is concerned, there is almost only one way reported in the literatures, that is, immersion of pre-synthesized porous substrates into the MOF precursor solution, followed by procedures that promote MOFs growth in the cavity and/or on the surface of the porous supports.[21,22,23,24,25,26,27,28] the particle sizes of MOFs (generally from hundreds of nanometers to several microns) are far much larger than the pore sizes of most porous supports, MOFs generally tend to grow on the outer surface of porous. The successful and exclusive doping of MOF-5 into mesoporous SBA-15 without pre-functionalization has been recently reported,[21] unevenly incorporation of MOF-5 into SBA-15 are obtained, more seriously, the provided evidences for supporting the incorporation of MOF-5 into SBA15 pore channels are not sufficient because the observed Zn signal in the EDX scans of mesoporous region of SBA-15 is likely to originate from the adsorbed Zn2+ ions by hydroxyl groups of SBA-15 and/or the basic structural building units of Zn4O. Metallic Zn generated via reduction by carbons subsequently evaporates, which may play a potential role in the formation of additional porosity This is to say that if a “so ” material, such as polymers, can be well-designed to coat on Zn-based MOF particles to form MOF@polymer composites, the escaped Zn from inside will further pass through shell during their carbonization process, inevitably creating new pores in the shell material. A morphology-preserved hierarchically porous N-doped carbon material (ZIFC@RUFC) with microporous core and mesoporous shell and signi cantly enhanced CO2 capture performance than the individual counterparts can be directly derived from the obtained ZIF@RUF composite through a simple carbonization process under N2 atmosphere, which demonstrates a promising way to prepare porosity-enhanced porous carbon materials by employing the in situ self-activation of ZnO nanoparticles and the escape of Zn atoms
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