**4. The UiO-66 MOF**

As one of the most important MOFs, the Zr-BDC (Zr6O4(OH)4(CO2)12) MOF, 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 triangular windows close to 6 Å [45].

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

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 BET surface area below 1000 m<sup>2</sup> g<sup>−</sup><sup>1</sup> .

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 the BET surface area of up to 1400 m2 g<sup>−</sup><sup>1</sup> . Schaate et al. suggested that the addition 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.

**Figure 7.** *Schematic illustration of the UiO-66 structure [45].*

#### **Figure 8.**

*(a) XRD patterns and (b–d) SEM images of the UiO-66 crystals synthesized with different amounts of benzoic acid additive [46].*

#### **4.2 Modulated synthesis of UiO-66 crystals using HF**

We have been working on precisely controlled synthesis of the UiO-66 and other MOFs with the aim to optimize their properties and extend their application potentials. We were the first to report a hydrofluoric acid (HF) modulated synthesis of the UiO-66 crystals from reactants ZrCl4 and H2BDC [47]. In the study, the amount of HF was varied from 1 eq. to 3 eq., where "eq." refers to the molar ratio of HF to ZrCl4 in a synthesis batch. A control sample was also synthesized without the addition of HF but under otherwise identical conditions. **Figure 9** shows the SEM images of the UiO-66 crystals synthesized. As seen, the addition of the HF facilitates growth of the UiO-66 crystals with the mean size of the crystals increasing from ~150 nm to 7 μm with increased HF addition. By controlling the concentration of the reactants, the morphology of crystals changed from truncated cube to cuboctahedron.

**67**

**Figure 9.**

*(h) HF = 2 eq., and (i) HF = 3 eq. [47].*

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

analysis of the resulted UiO-66 crystals were conducted.

reactant) are "needed" to coordinate with the defective sites.

defective sites instead of chlorine ions in the SBUs of the UiO-66 crystals. This mechanism was further supported by the 19F NMR spectra shown in **Figure 11**. As seen, the spectrum of 3F-UiO-66 heat-treated at 150°C (curve a) contains a strong signal of two partially overlapped peaks cantered at −155 ppm and −156 ppm. These peaks are assigned to the F bonded directly to Zr in the SBUs and the F physisorbed, respectively. When the 3F-UiO-66 is heat-treated at 300°C, as seen in curve (b), the peak at −155 ppm becomes stronger and narrower, whereas the peak at −156 ppm disappears. It indicates that F remains bonded to the Zr in the

3F-UiO-66 framework even after being heat-treated up to 300°C.

To understand the impact of the HF additive, elemental mapping and 19F NMR

**Figure 10** shows the EDS mapping of the synthesized UiO-66 crystals. As seen, without the addition of HF, a small amount of Cl was detected in the crystals even after heat treatment up to 300°C. This somewhat unexpected observation of the Cl is believed to be resulted from structure defects in the synthesized UiO-66 crystals. It is known that the UiO-66 crystal structure contains defects in the form of missing linkers, which would result in an unsaturated framework. To compensate for this charge imbalance of the framework, negatively charged Cl ions (from the ZrCl4

For the crystals synthesized with the addition of HF, EDS analysis of the crystals revealed noticeable F in the crystals, with no Cl observed. This indicates that, in the presence of stronger electronegative F, the F ions now coordinate to the linker

The thermostability of the UiO-66 crystals was enhanced after the introduction

of fluorine in the framework structure. **Figure 12** shows the TGA curves of the UiO-66 crystals synthesized with different amounts of HF additive. The significant weight loss event observed at ~500°C represents the complete decomposition of the

*SEM images of the UiO-66 crystals synthesized with different amounts of HF additive and changing concentrations of the reactants. At CZr = BDC = 13.6 mM, (a) HF = 1 eq., (b) HF = 2 eq., and (c) HF = 3 eq. At CZr = BDC = 18.2 mM, (d) HF = 1 eq., (e) HF = 2 eq., and (f) HF = 3 eq. At CZr = BDC = 27.2 mM, (g) HF = 1 eq.,* 

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

*Synthesis Methods and Crystallization*

*Schematic illustration of the UiO-66 structure [45].*

**Figure 7.**

**66**

**Figure 8.**

*acid additive [46].*

**4.2 Modulated synthesis of UiO-66 crystals using HF**

We have been working on precisely controlled synthesis of the UiO-66 and other MOFs with the aim to optimize their properties and extend their application potentials. We were the first to report a hydrofluoric acid (HF) modulated synthesis of the UiO-66 crystals from reactants ZrCl4 and H2BDC [47]. In the study, the amount of HF was varied from 1 eq. to 3 eq., where "eq." refers to the molar ratio of HF to ZrCl4 in a synthesis batch. A control sample was also synthesized without the addition of HF but under otherwise identical conditions. **Figure 9** shows the SEM images of the UiO-66 crystals synthesized. As seen, the addition of the HF facilitates growth of the UiO-66 crystals with the mean size of the crystals increasing from ~150 nm to 7 μm with increased HF addition. By controlling the concentration of the reactants, the

*(a) XRD patterns and (b–d) SEM images of the UiO-66 crystals synthesized with different amounts of benzoic* 

morphology of crystals changed from truncated cube to cuboctahedron.

To understand the impact of the HF additive, elemental mapping and 19F NMR analysis of the resulted UiO-66 crystals were conducted.

**Figure 10** shows the EDS mapping of the synthesized UiO-66 crystals. As seen, without the addition of HF, a small amount of Cl was detected in the crystals even after heat treatment up to 300°C. This somewhat unexpected observation of the Cl is believed to be resulted from structure defects in the synthesized UiO-66 crystals. It is known that the UiO-66 crystal structure contains defects in the form of missing linkers, which would result in an unsaturated framework. To compensate for this charge imbalance of the framework, negatively charged Cl ions (from the ZrCl4 reactant) are "needed" to coordinate with the defective sites.

For the crystals synthesized with the addition of HF, EDS analysis of the crystals revealed noticeable F in the crystals, with no Cl observed. This indicates that, in the presence of stronger electronegative F, the F ions now coordinate to the linker defective sites instead of chlorine ions in the SBUs of the UiO-66 crystals.

This mechanism was further supported by the 19F NMR spectra shown in **Figure 11**. As seen, the spectrum of 3F-UiO-66 heat-treated at 150°C (curve a) contains a strong signal of two partially overlapped peaks cantered at −155 ppm and −156 ppm. These peaks are assigned to the F bonded directly to Zr in the SBUs and the F physisorbed, respectively. When the 3F-UiO-66 is heat-treated at 300°C, as seen in curve (b), the peak at −155 ppm becomes stronger and narrower, whereas the peak at −156 ppm disappears. It indicates that F remains bonded to the Zr in the 3F-UiO-66 framework even after being heat-treated up to 300°C.

The thermostability of the UiO-66 crystals was enhanced after the introduction of fluorine in the framework structure. **Figure 12** shows the TGA curves of the UiO-66 crystals synthesized with different amounts of HF additive. The significant weight loss event observed at ~500°C represents the complete decomposition of the

#### **Figure 9.**

*SEM images of the UiO-66 crystals synthesized with different amounts of HF additive and changing concentrations of the reactants. At CZr = BDC = 13.6 mM, (a) HF = 1 eq., (b) HF = 2 eq., and (c) HF = 3 eq. At CZr = BDC = 18.2 mM, (d) HF = 1 eq., (e) HF = 2 eq., and (f) HF = 3 eq. At CZr = BDC = 27.2 mM, (g) HF = 1 eq., (h) HF = 2 eq., and (i) HF = 3 eq. [47].*

#### **Figure 10.**

*EDS maps of the UiO-66 crystals synthesized (a) without HF additive, (b) with 3 eq. HF additive, and (c) with 3 eq. HF additive and then heat-treated at 150°C [47].*

**Figure 11.** *19F NMR spectra of the 3F-UiO-66 crystals heat-treated at (a) 150°C and (b) 300°C. The asterisks indicate the spinning sidebands [47].*

UiO-66 framework. It can be seen that the F introduction has stabilized the framework, as indicated by the progressively increased decomposition temperature. This is attributed to the higher coordination strength of Zr▬F in the fluorine-involved crystals than the Zr▬Cl or Zr▬O in the fluorine-free crystals, resulting in the stabilization of framework structure of the fluorine-involved UiO-66.

Furthermore, as the UiO-66 crystals were modulated from small cube-like morphology to micron-sized cuboctahedron morphology with the addition of the HF during synthesis, their porosity increases [47]. **Figure 13** displays the Ar sorption–desorption isotherms and pore size distributions of the parent and fluorineinvolved crystals. As seen, the 3 fluorine-involved crystals (samples c, f, and i in **Figure 9**) exhibit significantly higher Ar adsorption values than the parent crystals

**69**

g<sup>−</sup><sup>1</sup>

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

of the fluorine-involved crystals.

*crystal sizes and morphologies [47].*

**Figure 12.**

**Figure 13.**

synthesized without HF, and the pore size distributions show the enhanced porosity

*(a) Ar sorption-desorption isotherms and (b) pore size distributions of the synthesized UiO-66 with various* 

*TGA curves of the UiO-66 crystals synthesized with different amounts of HF additive (C = 18.2 mM) [47].*

In more recent years, nanosized MOFs (also known as nMOFs or NMOFs) have attracted great attentions. nMOFs have short diffusion lengths and high specific surface areas, both are of critical importance in catalysis and sorption, especially in liquid-phase applications [34, 48]. In addition, nMOFs are highly desirable for porous membranes [49, 50], thin film devices [51], and medical applications [52]. We have reported a novel method for the modulated synthesis of nUiO-66 crystals, in which a new type of additive, solid Cu2O, was used to mediate the synthesis [53]. **Figure 14** shows the TEM images of the UiO-66 crystals synthesized with different amounts of Cu2O additive. As illustrated, by increasing the amount of Cu2O additive, the mean sizes of the UiO-66 crystals reduce progressively from ~265 to 40 nm. The crystals maintain an octahedral morphology until when the crystal size is too small to display distinguishable facets and thus appear as spherical particles. The synthesized nano UiO-66 crystals were found to be chemically pure without the contamination of Cu. They have high BET surface area of more than 1100 m<sup>2</sup>

 and contain rich porosity. **Figure 15** shows the Ar sorption isotherms and pore size distributions of the UiO-66 crystals. Although the amount of micropores

**4.3 Modulated synthesis of UiO-66 crystals using solid Cu2O**

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

#### **Figure 12.**

*Synthesis Methods and Crystallization*

**68**

*spinning sidebands [47].*

**Figure 10.**

UiO-66 framework. It can be seen that the F introduction has stabilized the framework, as indicated by the progressively increased decomposition temperature. This is attributed to the higher coordination strength of Zr▬F in the fluorine-involved crystals than the Zr▬Cl or Zr▬O in the fluorine-free crystals, resulting in the

**Figure 11.** *19F NMR spectra of the 3F-UiO-66 crystals heat-treated at (a) 150°C and (b) 300°C. The asterisks indicate the* 

*EDS maps of the UiO-66 crystals synthesized (a) without HF additive, (b) with 3 eq. HF additive, and* 

*(c) with 3 eq. HF additive and then heat-treated at 150°C [47].*

Furthermore, as the UiO-66 crystals were modulated from small cube-like morphology to micron-sized cuboctahedron morphology with the addition of the HF during synthesis, their porosity increases [47]. **Figure 13** displays the Ar sorption–desorption isotherms and pore size distributions of the parent and fluorineinvolved crystals. As seen, the 3 fluorine-involved crystals (samples c, f, and i in **Figure 9**) exhibit significantly higher Ar adsorption values than the parent crystals

stabilization of framework structure of the fluorine-involved UiO-66.

*TGA curves of the UiO-66 crystals synthesized with different amounts of HF additive (C = 18.2 mM) [47].*

#### **Figure 13.**

*(a) Ar sorption-desorption isotherms and (b) pore size distributions of the synthesized UiO-66 with various crystal sizes and morphologies [47].*

synthesized without HF, and the pore size distributions show the enhanced porosity of the fluorine-involved crystals.

### **4.3 Modulated synthesis of UiO-66 crystals using solid Cu2O**

In more recent years, nanosized MOFs (also known as nMOFs or NMOFs) have attracted great attentions. nMOFs have short diffusion lengths and high specific surface areas, both are of critical importance in catalysis and sorption, especially in liquid-phase applications [34, 48]. In addition, nMOFs are highly desirable for porous membranes [49, 50], thin film devices [51], and medical applications [52].

We have reported a novel method for the modulated synthesis of nUiO-66 crystals, in which a new type of additive, solid Cu2O, was used to mediate the synthesis [53]. **Figure 14** shows the TEM images of the UiO-66 crystals synthesized with different amounts of Cu2O additive. As illustrated, by increasing the amount of Cu2O additive, the mean sizes of the UiO-66 crystals reduce progressively from ~265 to 40 nm. The crystals maintain an octahedral morphology until when the crystal size is too small to display distinguishable facets and thus appear as spherical particles. The synthesized nano UiO-66 crystals were found to be chemically pure without the contamination of Cu. They have high BET surface area of more than 1100 m<sup>2</sup> g<sup>−</sup><sup>1</sup> and contain rich porosity. **Figure 15** shows the Ar sorption isotherms and pore size distributions of the UiO-66 crystals. Although the amount of micropores

#### **Figure 14.**

*TEM images of the UiO-66 crystals synthesized with different amounts of Cu2O additive. Here the molar ratio R =* n*[Cu]/*n*[ZrCl4]: (1a) R = 0, (1b) R = 0.2, (1c) R = 0.5, (1d) R = 1.0, (1e) R = 1.2, and (1f) R = 1.5. The scale bar is 100 nm [53].*

#### **Figure 15.**

*(a) Ar sorption isotherms and (b) pore size distributions (measured between 0 and 50 nm) of the UiO-66 crystals synthesized with different amounts of Cu2O additive. Here the molar ratio R =* n*[Cu]/*n*[ZrCl4]: (1a) R = 0, (1b) R = 0.2, (1c) R = 0.5, (1d) R = 1.0, (1e) R = 1.2, and (1f) R = 1.5 [53].*

between 6 Å and 10 Å decreased gradually with the decreasing crystal size, noticeable quantity of mesopores of 10–30 nm became prevailed, demonstrating the dual micro- and mesoporosity of the nano UiO-66 crystals.

During the Cu2O-modulated synthesis, the added solid Cu2O was observed to dissolve gradually in the mother liquor, resulting in the formation of a pale orangecolored solution. **Figure 16** shows the far infrared (FIR) spectra of solid Cu2O and a number of mixed solutions relevant to the synthesis. As illustrated in the inset

**71**

material.

109 cm<sup>−</sup><sup>1</sup>

**Figure 16.**

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

of the figure, solid Cu2O shows the Cu▬O absorption band at 138 cm<sup>−</sup><sup>1</sup>

spectrum b. When the ZrCl4 was added to the suspension of the Cu2O and DMF, the FIR spectrum (c) shows dramatically decreased Cu▬O adsorption band accompa-

*Far-IR spectra of the solid Cu2O and a number of mixtures relevant to the UiO-66 synthesis, where the molar ratio of Cu/ZrCl4 is 1.2. (a) ZrCl4/DMF solution, (b) Cu2O/DMF suspension, and (c) ZrCl4 + Cu2O/DMF* 

be ascribed to a shifted stretching vibration of Cu▬Cl bonds, which are usually at

that the dissolution of Cu2O in the precursor solution of the UiO-66 is driven by the

It has also been found [53] that the amount of ZrCl4 in the Cu2O-modulated synthesis plays a crucial role in the growth of the UiO-66 crystals. The higher the concentration of the ZrCl4, the weaker the Cu2O's impact on the crystal growth and thus benefits the formation of large size UiO-66 crystals. In contrast, decreasing the concentration of the ZrCl4 favors the production of small UiO-66 crystals, which also accompanied with the loss of crystallinity. It is thus believed that the [Cu▬Cl] complex is capable of coordinating with the Zr4+, leading to a decreased coordination of the Zr6 clusters with BDC linkers, pre-

Additionally, under the modulation of Cu2O additive, the synthesized UiO-66 crystals not only have reduced sizes, but also enriched structure defects [53]. Both the XRD and TGA analyses of the nano UiO-66 crystals indicate the existence of linker defects and metal cluster defects. **Table 1** presents quantified amounts of the linker missing defects in the synthesized nano UiO-66 crystals. It is seen that the number of linker defects increases with the decreasing crystal size, revealing a more

In recent years, there have been many reports on the structural defects of the UiO-66 crystals [55–60]. These structural defects are found to relate strongly to the structure stability, adsorption, and catalysts performance of the UiO-66

the addition of solid Cu2O in DMF, the Cu▬O bond shifts to 143 cm<sup>−</sup><sup>1</sup>

nied by the arrival of an intense adsorption band at 113 cm<sup>−</sup><sup>1</sup>

for the anionic [CuCl2]

formation of the [Cu▬Cl] complexes.

*solution. The inset is for solid Cu2O [53].*

open framework of the nano UiO-66.

venting crystal growth.

. Upon

as seen in

. This new band can

<sup>−</sup> complex [54]. The FIR analysis seems to suggest

#### **Figure 16.**

*Synthesis Methods and Crystallization*

**70**

**Figure 15.**

**Figure 14.**

*scale bar is 100 nm [53].*

between 6 Å and 10 Å decreased gradually with the decreasing crystal size, noticeable quantity of mesopores of 10–30 nm became prevailed, demonstrating the dual

*(a) Ar sorption isotherms and (b) pore size distributions (measured between 0 and 50 nm) of the UiO-66 crystals synthesized with different amounts of Cu2O additive. Here the molar ratio R =* n*[Cu]/*n*[ZrCl4]:* 

*TEM images of the UiO-66 crystals synthesized with different amounts of Cu2O additive. Here the molar ratio R =* n*[Cu]/*n*[ZrCl4]: (1a) R = 0, (1b) R = 0.2, (1c) R = 0.5, (1d) R = 1.0, (1e) R = 1.2, and (1f) R = 1.5. The* 

During the Cu2O-modulated synthesis, the added solid Cu2O was observed to dissolve gradually in the mother liquor, resulting in the formation of a pale orangecolored solution. **Figure 16** shows the far infrared (FIR) spectra of solid Cu2O and a number of mixed solutions relevant to the synthesis. As illustrated in the inset

micro- and mesoporosity of the nano UiO-66 crystals.

*(1a) R = 0, (1b) R = 0.2, (1c) R = 0.5, (1d) R = 1.0, (1e) R = 1.2, and (1f) R = 1.5 [53].*

*Far-IR spectra of the solid Cu2O and a number of mixtures relevant to the UiO-66 synthesis, where the molar ratio of Cu/ZrCl4 is 1.2. (a) ZrCl4/DMF solution, (b) Cu2O/DMF suspension, and (c) ZrCl4 + Cu2O/DMF solution. The inset is for solid Cu2O [53].*

of the figure, solid Cu2O shows the Cu▬O absorption band at 138 cm<sup>−</sup><sup>1</sup> . Upon the addition of solid Cu2O in DMF, the Cu▬O bond shifts to 143 cm<sup>−</sup><sup>1</sup> as seen in spectrum b. When the ZrCl4 was added to the suspension of the Cu2O and DMF, the FIR spectrum (c) shows dramatically decreased Cu▬O adsorption band accompanied by the arrival of an intense adsorption band at 113 cm<sup>−</sup><sup>1</sup> . This new band can be ascribed to a shifted stretching vibration of Cu▬Cl bonds, which are usually at 109 cm<sup>−</sup><sup>1</sup> for the anionic [CuCl2] <sup>−</sup> complex [54]. The FIR analysis seems to suggest that the dissolution of Cu2O in the precursor solution of the UiO-66 is driven by the formation of the [Cu▬Cl] complexes.

It has also been found [53] that the amount of ZrCl4 in the Cu2O-modulated synthesis plays a crucial role in the growth of the UiO-66 crystals. The higher the concentration of the ZrCl4, the weaker the Cu2O's impact on the crystal growth and thus benefits the formation of large size UiO-66 crystals. In contrast, decreasing the concentration of the ZrCl4 favors the production of small UiO-66 crystals, which also accompanied with the loss of crystallinity. It is thus believed that the [Cu▬Cl] complex is capable of coordinating with the Zr4+, leading to a decreased coordination of the Zr6 clusters with BDC linkers, preventing crystal growth.

Additionally, under the modulation of Cu2O additive, the synthesized UiO-66 crystals not only have reduced sizes, but also enriched structure defects [53]. Both the XRD and TGA analyses of the nano UiO-66 crystals indicate the existence of linker defects and metal cluster defects. **Table 1** presents quantified amounts of the linker missing defects in the synthesized nano UiO-66 crystals. It is seen that the number of linker defects increases with the decreasing crystal size, revealing a more open framework of the nano UiO-66.

In recent years, there have been many reports on the structural defects of the UiO-66 crystals [55–60]. These structural defects are found to relate strongly to the structure stability, adsorption, and catalysts performance of the UiO-66 material.


**Table 1.**

*Amounts of linker missing defects in the UiO-66 crystals synthesized with the addition of Cu2O [53].*

Here R represents the molar ratio of Cu/ZrCl4, and x is the number of linker missing defects. The formula represents the defective molecular formula of the UiO-66 crystals, and Zr6O6(BDC)6 is the formula of a defect-free dehydroxylated UiO-66.
