*3.2.2 Coordination modulation synthesis*

Besides changing synthesis conditions, introducing additives during crystallization is another strategy of controlling the morphology and size of MOF crystals. The research lead by Kitagawa first reported the coordination modulation of carboxylic acid additives on the growth of [{Cu2(ndc)2(dabco)}*n*] MOF crystals [34]. In this work, a specific amount of acetic acid is added in the crystallization mother liquor of the [{Cu2(ndc)2(dabco)}*n*]. Acetic acid hinders the coordination

**Figure 2.**

*SEM images of NH2-MIL-125(Ti) crystals synthesized with different total solvent volumes of (a) 40 mL, (b) 30 mL, (c) 20 mL, (d) 15 mL, (e) 14 mL, and (f) 13.5 mL [32].*

**63**

**Figure 3.**

*Synthesis Methods and Crystallization of MOFs DOI: http://dx.doi.org/10.5772/intechopen.90435*

sively [35, 36].

and size.

between metal clusters and organic linkers through selective coordination with the metal clusters. This influences the expansion of lattice structure and alters the growth of the [{Cu2(ndc)2(dabco)}*n*] crystals. **Figure 3** shows the nanocrystals of [{Cu2(ndc)2(dabco)}*n*] formed under the capping effect of acetic acid [34].

three kinds of monocarboxylic acid additives, the acetic acid, dodecanoic acid, and lauric acid under ultrasonic condition. **Figure 4** shows the effect of additive quantity on the morphology of the resulted HKUST-1 crystals. As seen, with the increased additive quantity, not only the size of the HKUST-1 crystals increases from tens of nanometres to several micrometres, the shape of the crystals also changes from cube to octahedron, truncated cube, and tetradecahedron, progres-

Subsequently, Kitagawa's group has also synthesized the HKUST-1 crystals using

It has since been established that the coordination modulation which resulted

Some surfactants, such as the cetyltrimethylammonium bromide (CTAB) [37] and polyvinylpyrrolidone (PVDF) [38, 39], can also be used for the modulated synthesis of MOFs. In the crystallization of MOF crystals, the surfactant molecules can be selectively absorbed on one or more specific facets of the MOF crystals, thus hinder or alter their growth, and result in the modification of crystal morphology

Take the hydrothermal synthesis of ZIF-67 MOF crystals as an example. As shown in **Figure 5**, by simply changing the amount of CTAB from 0.0025 to 0.025 wt%, a series of the ZIF-67 cubic or rhombic dodecahedron crystals with sizes ranging from ~150 nm to 1 μm can be produced [40]. Additionally, by carbonization of the resulted ZIF-67 crystals in N2 flow, a series of Co-based porous carbon catalysts can be obtained. These Co-based porous carbon catalysts retain the

*Coordination modulation method for fabricating [{Cu2(ndc)2(dabco)}*n*] nanocrystals [34].*

from using the carboxylic acids has certain universality. It is shown to effectively control the crystal morphology and size of a number of MOFs, including {Cu2(ndc)2(dabco)}*n*, HKUST-1, Zr-based MOFs, etc. The amount of the additive used ranges from 2 to 100 equimolar of the reactants. The carboxylic acid additives

do not affect the crystal structure of the resulted MOFs.

*3.2.3 Surfactant modulation synthesis*

#### *Synthesis Methods and Crystallization of MOFs DOI: http://dx.doi.org/10.5772/intechopen.90435*

*Synthesis Methods and Crystallization*

*3.2.2 Coordination modulation synthesis*

applications.

synthesis [33].

crystals can be modified from circular plates to tetragons and octahedrons [32]. As the crystal morphology changes, the light absorption band of the NH2-MIL-125(Ti) can be tuned from 480 to 533 nm, making it advantageous in photocatalytic

It has been found that the reactant concentration has a significant effect on the deprotonation rate of the organic linkers during the synthesis of NH2-MIL-125(Ti) crystals. The deprotonation rate plays a critical role in the nucleation and growth of the NH2-MIL-125(Ti) crystals. Modulating crystal morphology and size of MOFs by changing the rate of deprotonation is called the deprotonation regulation

Besides changing synthesis conditions, introducing additives during crystallization is another strategy of controlling the morphology and size of MOF crystals. The research lead by Kitagawa first reported the coordination modulation of carboxylic acid additives on the growth of [{Cu2(ndc)2(dabco)}*n*] MOF crystals [34]. In this work, a specific amount of acetic acid is added in the crystallization mother liquor of the [{Cu2(ndc)2(dabco)}*n*]. Acetic acid hinders the coordination

*SEM images of NH2-MIL-125(Ti) crystals synthesized with different total solvent volumes of (a) 40 mL,* 

*(b) 30 mL, (c) 20 mL, (d) 15 mL, (e) 14 mL, and (f) 13.5 mL [32].*

**62**

**Figure 2.**

between metal clusters and organic linkers through selective coordination with the metal clusters. This influences the expansion of lattice structure and alters the growth of the [{Cu2(ndc)2(dabco)}*n*] crystals. **Figure 3** shows the nanocrystals of [{Cu2(ndc)2(dabco)}*n*] formed under the capping effect of acetic acid [34].

Subsequently, Kitagawa's group has also synthesized the HKUST-1 crystals using three kinds of monocarboxylic acid additives, the acetic acid, dodecanoic acid, and lauric acid under ultrasonic condition. **Figure 4** shows the effect of additive quantity on the morphology of the resulted HKUST-1 crystals. As seen, with the increased additive quantity, not only the size of the HKUST-1 crystals increases from tens of nanometres to several micrometres, the shape of the crystals also changes from cube to octahedron, truncated cube, and tetradecahedron, progressively [35, 36].

It has since been established that the coordination modulation which resulted from using the carboxylic acids has certain universality. It is shown to effectively control the crystal morphology and size of a number of MOFs, including {Cu2(ndc)2(dabco)}*n*, HKUST-1, Zr-based MOFs, etc. The amount of the additive used ranges from 2 to 100 equimolar of the reactants. The carboxylic acid additives do not affect the crystal structure of the resulted MOFs.

## *3.2.3 Surfactant modulation synthesis*

Some surfactants, such as the cetyltrimethylammonium bromide (CTAB) [37] and polyvinylpyrrolidone (PVDF) [38, 39], can also be used for the modulated synthesis of MOFs. In the crystallization of MOF crystals, the surfactant molecules can be selectively absorbed on one or more specific facets of the MOF crystals, thus hinder or alter their growth, and result in the modification of crystal morphology and size.

Take the hydrothermal synthesis of ZIF-67 MOF crystals as an example. As shown in **Figure 5**, by simply changing the amount of CTAB from 0.0025 to 0.025 wt%, a series of the ZIF-67 cubic or rhombic dodecahedron crystals with sizes ranging from ~150 nm to 1 μm can be produced [40]. Additionally, by carbonization of the resulted ZIF-67 crystals in N2 flow, a series of Co-based porous carbon catalysts can be obtained. These Co-based porous carbon catalysts retain the

#### **Figure 4.**

*(a) TEM images of the HKUST-1 crystals obtained with various concentrations of dodecanoic acid and H3BTC; here* C *is the concentration of the H3BTC and* r *is the molar ratio of dodecanoic acid to H3BTC [35]. (b) SEM images of the HKUST-1 crystals obtained with different amounts of the lauric acid in mmol: (a) 0, (b) 2.34, (c) 4.75, (d) 7.13, (e) 9.5, and (f) 11.88 [36].*

#### **Figure 5.**

*ZIF-67 crystals synthesized with different amounts of the CTAB additive (1: 0, 2: 0.0025 wt%, 3: 0.01 wt%, and 4: 0.025 wt%). SEM and TEM images of (a) as-synthesized samples and (b–d) carbonized samples [40].*

original shape of the ZIF-67 crystals and display an outstanding catalytic performance towards the CO2 methanation at low temperatures.

It has also been reported that the addition of CTAB can modulate the crystal morphology and size of some other MOFs. **Figure 6** shows a number of such examples including IRMOFs [41, 42], HKUST-1 [43], and ZIF-8 [44].

**65**

*Synthesis Methods and Crystallization of MOFs DOI: http://dx.doi.org/10.5772/intechopen.90435*

**4. The UiO-66 MOF**

**Figure 6.**

triangular windows close to 6 Å [45].

BET surface area below 1000 m<sup>2</sup>

the BET surface area of up to 1400 m2

As one of the most important MOFs, the Zr-BDC (Zr6O4(OH)4(CO2)12) MOF,

The UiO-66 crystals are commonly synthesized via a solvothermal method at 110–130°C and allowed to crystallize for 24 h. Without modulation, the obtained UiO-66 crystals are usually agglomerates of small cube-like particles of 80–200 nm in size and with low crystallinity. These UiO-66 crystals show low porosity with the

To improve the crystallinity of the UiO-66 crystals, carboxylic acid additives have been applied in the synthesis. Schaate et al. first studied the influence of benzoic acid and acetic acid on the crystal growth of the UiO-66 and other Zr-based MOFs [46]. They have found the UiO-66 crystals synthesized are octahedrons of several hundred nanometres in size, as shown in **Figure 8**. These crystals have a high crystallinity with

of the carboxylic acid additives changes the original coordination equilibrium and thus the crystal growth rate during the crystallization of the UiO-66 crystals. There exists a competition between the coordination of the BDC linkers and carboxylic acid additives towards the Zr6 clusters. This becomes an obstacle for the connection of the BDC linkers and Zr6 clusters, shifting the original coordination equilibrium. This behaviour can be exploited as a way to modulate the morphology and size of the resulted MOF crystals.

. Schaate et al. suggested that the addition

commonly known as the UiO-66, has been extensively studied due to its high porosity and excellent structural stability at high temperatures and pressures, and excellent stability in chemical (acidic/basic) aggressive environments. The framework of the UiO-66 is built on the [Zr6O4(OH)4] SBUs, each coordinated with 12 1,4-benzene-dicarboxylate (H2BDC) linkers. **Figure 7** illustrates the crystal structure of the UiO-66. Its cubic structure is composed of octahedral cages close to 11 Å and tetrahedral cages close to 8 Å, and these cages are connected through narrow

*The influence of CTAB additive on the crystal morphology and size of several MOFs. (I) SEM images of IRMOF-3 synthesized with different amounts of CTAB in mg: (a, a1, a2) 0, (b, b1, b2) 5, (c, c1, c2) 8, (d, d1, d2) 12, and (e, e1, e2) 15 [42]. (II) SEM images of HKUST-1 synthesized with different amounts of CTAB in M: (a) 0, (b) 0.005, (c) 0.01, (d) 0.05, (e) 0.1, and (f) 0.5 [43]. (III) (a) XRD patterns of ZIF-8; (b) ZIF-8 synthesized without CTAB; ZIF-8 synthesized with different amounts of CTAB in wt%: (c) 0.0025, (d) 0.010, and* 

*(e) 0.025; (f) mean particle size of ZIF-8 crystals versus the concentrations of CTAB added [44].*

**4.1 Modulated synthesis of UiO-66 crystals using carboxylic acids**

 g<sup>−</sup><sup>1</sup> .

g<sup>−</sup><sup>1</sup>

#### **Figure 6.**

*Synthesis Methods and Crystallization*

*(b) 2.34, (c) 4.75, (d) 7.13, (e) 9.5, and (f) 11.88 [36].*

**64**

**Figure 5.**

**Figure 4.**

original shape of the ZIF-67 crystals and display an outstanding catalytic perfor-

*(a) TEM images of the HKUST-1 crystals obtained with various concentrations of dodecanoic acid and H3BTC; here* C *is the concentration of the H3BTC and* r *is the molar ratio of dodecanoic acid to H3BTC [35]. (b) SEM images of the HKUST-1 crystals obtained with different amounts of the lauric acid in mmol: (a) 0,* 

It has also been reported that the addition of CTAB can modulate the crystal morphology and size of some other MOFs. **Figure 6** shows a number of such

*ZIF-67 crystals synthesized with different amounts of the CTAB additive (1: 0, 2: 0.0025 wt%, 3: 0.01 wt%, and 4: 0.025 wt%). SEM and TEM images of (a) as-synthesized samples and (b–d) carbonized samples [40].*

mance towards the CO2 methanation at low temperatures.

examples including IRMOFs [41, 42], HKUST-1 [43], and ZIF-8 [44].

*The influence of CTAB additive on the crystal morphology and size of several MOFs. (I) SEM images of IRMOF-3 synthesized with different amounts of CTAB in mg: (a, a1, a2) 0, (b, b1, b2) 5, (c, c1, c2) 8, (d, d1, d2) 12, and (e, e1, e2) 15 [42]. (II) SEM images of HKUST-1 synthesized with different amounts of CTAB in M: (a) 0, (b) 0.005, (c) 0.01, (d) 0.05, (e) 0.1, and (f) 0.5 [43]. (III) (a) XRD patterns of ZIF-8; (b) ZIF-8 synthesized without CTAB; ZIF-8 synthesized with different amounts of CTAB in wt%: (c) 0.0025, (d) 0.010, and (e) 0.025; (f) mean particle size of ZIF-8 crystals versus the concentrations of CTAB added [44].*
