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A. Dhakshinamoorthy and H. Garcia
MOF pore system. This adsorption can be carried out in the gas, liquid or solid
phases. Scheme 7.4 summarizes some of the most widely employed encapsulation
methods.
In some of the initial reports on the encapsulation of metal NPs inside MOF,
Fischer and co-workers reported the gas-phase adsorption of volatile organometallic
and metal carbonyl complexes such as Au(CO)Cl. Initially, the MOFs selected as
hosts were, however, not the most stable ones, but the adsorption procedure was developed with the objective to ensure the internal location of the metal NP. For this reason,
the MOF was initially submitted to a pre-treatment to eliminate adsorbed solvent
molecules and water. After desorption of weakly adsorbed species, the pre-treated
material was exposed to the presence of volatile organometallic complexes. Due
to the large proportion between internal and external surface, adsorption in the gas
phase in porous materials results preferentially in the internal location of the adsorbate. Subsequently, metal NPs are formed by decomposition of the organometallic
complex by physical and chemical means, hydrogenation of the metal precursor
being a simple procedure, compatible with preservation of the MOF structure. In
that way, Pd NPs were incorporated inside MOF-5 [31].
Besides gas phase, adsorption can also be performed in liquid media. In one of
the examples reported, Au NPs were prepared as colloidal suspension using PVP as
capping and protecting agent and these small Au NPs were adsorbed inside the pores
of MIL-101 without the need of additional treatment [32]. The mechanism of this
incorporation of performed Au NPs inside MOF is unknown since the particle size
of Au NPs is similar to the dimensions of the pores in MIL-101 and the presence of
PVP as a shell stabilizing the colloidal solution should in principle disfavor diffusion
of preformed Au NPs inside the pores.
One of the most common and probably general preparation procedures of metal
NPs inside MOFs by liquid phase adsorption is the method known as double-solvent
method [33]. This method is based on the adsorption of the minimum amount of a
polar solvent, typically water containing a metallic salt precursor, in a large excess of
highly apolar solvent in where the MOF is suspended. The rationale for the doublesolvent method is the high polarity contrast between an apolar solvent in large
excess and a polar solvent containing the metal salt precursor in small volume that
can become incorporated completely inside the polar pores of the MOF if the volume
Scheme 7.4 Most widely used procedures for encapsulation of metal NPs inside MOFs
A. Dhakshinamoorthy and H. Garcia
MOF pore system. This adsorption can be carried out in the gas, liquid or solid
phases. Scheme 7.4 summarizes some of the most widely employed encapsulation
methods.
In some of the initial reports on the encapsulation of metal NPs inside MOF,
Fischer and co-workers reported the gas-phase adsorption of volatile organometallic
and metal carbonyl complexes such as Au(CO)Cl. Initially, the MOFs selected as
hosts were, however, not the most stable ones, but the adsorption procedure was developed with the objective to ensure the internal location of the metal NP. For this reason,
the MOF was initially submitted to a pre-treatment to eliminate adsorbed solvent
molecules and water. After desorption of weakly adsorbed species, the pre-treated
material was exposed to the presence of volatile organometallic complexes. Due
to the large proportion between internal and external surface, adsorption in the gas
phase in porous materials results preferentially in the internal location of the adsorbate. Subsequently, metal NPs are formed by decomposition of the organometallic
complex by physical and chemical means, hydrogenation of the metal precursor
being a simple procedure, compatible with preservation of the MOF structure. In
that way, Pd NPs were incorporated inside MOF-5 [31].
Besides gas phase, adsorption can also be performed in liquid media. In one of
the examples reported, Au NPs were prepared as colloidal suspension using PVP as
capping and protecting agent and these small Au NPs were adsorbed inside the pores
of MIL-101 without the need of additional treatment [32]. The mechanism of this
incorporation of performed Au NPs inside MOF is unknown since the particle size
of Au NPs is similar to the dimensions of the pores in MIL-101 and the presence of
PVP as a shell stabilizing the colloidal solution should in principle disfavor diffusion
of preformed Au NPs inside the pores.
One of the most common and probably general preparation procedures of metal
NPs inside MOFs by liquid phase adsorption is the method known as double-solvent
method [33]. This method is based on the adsorption of the minimum amount of a
polar solvent, typically water containing a metallic salt precursor, in a large excess of
highly apolar solvent in where the MOF is suspended. The rationale for the doublesolvent method is the high polarity contrast between an apolar solvent in large
excess and a polar solvent containing the metal salt precursor in small volume that
can become incorporated completely inside the polar pores of the MOF if the volume
Scheme 7.4 Most widely used procedures for encapsulation of metal NPs inside MOFs