Nucleation of MOFs

Falcaro et al. reported how functional ceramic particles can be used as a heterogeneous seed for the nucleation of MOFs [25]. The authors developed a seeding method using a number of different ceramic materials as heterogeneous seeds to spatially control the position as well as the growth rate of MOFs [25,81,82]. 

The concept of using seeds to induce controlled nucleation of MOFs has been successfully implemented to master MOF formation on a variety of substrates. The nucleation has been obtained using nano- or microparticles of the MOF itself ( homogeneous seeding) however, more recently, MOFs have been grown from nano- or micro-ceramic seeds ( heterogeneous seeding). 

The growth of MOF nanoparticles inside porous silica monolith is a promising approach for catalysis applications. Sachse et al. demonstrated the successful formation of HKUST-1 crystals inside silica monolith [79]. Macro/mesoporous silica monolith was synthesized using TEOS as a Si02 precursor and ammonia as a catalyst. For the subsequent MOF formation, the as formed monolith was immersed in HKUST-1 precursor solution using DMSO as a solvent. The employment of DMSO instead of classical solvents (ethanol and water mixture) for the synthesis of HKUST-1 crystals led to slower nucleation of smaller crystals in the mesopores of monolith rather than at the outer surface. 

Hermes et al. utilized p-perfluoromethylbenzene-carboxylate (pfmbc) as a modulator to block the growth of MOF-5. A growth habit where a fast nucleation step... 

The synthesis of OD and ID nanoMOF crystals chiefly depends on the spatial and/or temporal control of MOF crystal growth and nucleation. These processes can be partially controlled by adapting classical methodologies generally used to obtain bulk MOFs (e.g., solvothermal synthesis). This requires tuning of one or more of the reaction conditions, including the solvent (a poor solvent is often needed), the MOF precimsors, base (if needed), temperature and time, to promote nucleation and slow down the crystal growth. For instance, Horcajada et al reported that the careful selection of solvents, temperatures and reaction times was the key factor to synthesize nano-MIL-88A, nano-MIL-89, and nano-MIL-53 crystals smaller than 200 nm. 

Surface composition can influence the formation of homogeneous MOF thin Aims. Reboul et al recently reported that a metal-dense Al(III) oxide surface provides Al(III) ions to spontaneously interact with the organic ligands bdc, btc, and ndc, to form thin films of [Al(OH)bdc], [Al30(0H)(H20)2(btc)2l, and [AlOH(ndc)], respectively. Likewise, surfaces can be modified to promote the nucleation and growth of MOF crystals. This is typically done in two ways by introducing a self-assembled... 

External controls to drive the nucleation in confined areas. The external control can be any external driving force promoting the growth of MOFs (e.g., electrochemical approach) or controlling/depositing the amount of MOF precursors in precise locations (e.g., microcontact printing and evaporation-induced growth). 

It is challenging to develop MOF membranes with high gas-separation performance due to the heterogeneous nucleation and the very poor growth of MOF crystals on a porous surface. MMM with MOFs, which combines the advantages of polymers and MOFs, is a promising membrane for future. Table 12.5 shows the applications of MOFs for gas separation. 

Chronoamperometric data for Cu-MOF in contact with aqueous acetate buffer revealed the existence of different current/time responses depending on the applied potential (Domenech et al., 2007d). At increasingly negative applied potentials, such responses can be associated to diffusion-controlled ion insertion process, a three-dimensional nucleation/growth deposition, and a layer-by-layer deposition process, respectively, in agreement with AFM and TEM observations. From such data, the diffusion coefficient for electrons was calculated as = 2.0 x 10 " cmVsec, whereas the diffusion coefficients for Li+, Na, and K ions were )j (Li+) = 1.4 x 10" )j (Na+) =... 

Falcaro et al. precisely localized MOF-5 crystals using mineral microparticles as both nucleating seeds and carriers for embedding controlled functionality into PCP crystals. The authors showed that nanostructmed a-hopeite microparticles possess exceptional ability to nucleate PCP crystals. In a one-pot synthesis procedme, where a solution contains both precursors of the a-hopeite microparticles and of the MOF-5, the a-hopeite microparticles formed in the first few minutes of reaction act as nucleating agent on which the heterogeneous nucleation... 

Other studies have subsequently proposed the use of different ceramic seeds for heterogeneous nucleation. Although further studies are needed to fully understand the potential of this approach, the heterogeneous nucleation represents an effective and inexpensive method to induce controlled MOF growth. 

The concept of this widely used technique consists in the decoupling of (i) nucleation and seed formation at high supersaturation, and (ii) growth of the seed crystals to a continuous layer at low supersaturation. Usually, in the crystal growth step a new nucleation can be avoided and only the seeds grow to the molecular sieve layer. Therefore, the layers obtained by secondary growth are less polycrystalline. If the support surface was covered by a homogeneous and dense seed layer, relative thin zeolite and MOF layers can be obtained. 

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