**4.4 In situ synthesis of Ti-doped hybrid UiO-66 crystals**

Introducing foreign metal centres into the framework structure to obtain hybrid (i.e. mixed metal) MOFs is a strategy for developing novel MOFs. We have explored the possibility of facile synthesis of Ti4+-doped hybrid UiO-66-*n*Ti MOFs via an in situ crystallization process [61]. In this work, a series of Ti4+-doped UiO-66-*n*Ti (*n* represents the mass fraction of Ti4+) crystals were synthesized following the synthesis procedure of the UiO-66, but with the addition of varying amounts of Ti4+ in the

**73**

**Figure 18.**

*125(Ti) crystals [61].*

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

clusters is observed at ~682 cm<sup>−</sup><sup>1</sup>

of the Ti4+ in the [Zr▬O] clusters.

(O▬Ti▬O) vibrations occur at ~642 cm<sup>−</sup><sup>1</sup>

form of titanium isopropoxide in the precursor solution. The ICP analysis showed that the concentration of Ti4+ increases gradually from around 0.7 to 4.0 wt% in the

**Figure 18** shows the FT-IR spectra of the parent UiO-66 and doped UiO-66 *n*Ti crystals. As seen, for the parent UiO-66, the Zr▬Oμ3-O stretch in the [Zr▬O]

Ti4+ doping amounts, indicating the structural change in the Ti4+-doped UiO-66 *n*Ti crystals. As reported in the literatures [32, 62], the MIL-125(Ti) and NH2-MIL-125(Ti) MOFs are built from coordination between Ti8 clusters and H2BDC-type organic linkers. Both structures display a FT-IR band at between 600 and 700 cm<sup>−</sup><sup>1</sup> which resulted from (O▬Ti▬O) vibrations, and for the NH2-MIL-125(Ti), the

Zr▬Oμ3-O stretch in the UiO-66-*n*Ti crystals could be caused by the incorporation

**Figure 19** shows the EXAFS spectra of the parent UiO-66 and doped UiO-66- 2.7Ti crystals. It further demonstrates the alternation in coordination state around the Zr4+ caused by the Ti4+ doping. As seen, the peak at ~1.7 Å that relates to the Zr▬O bonds shifts to 1.6 Å after Ti4+ doping. Additionally, the peak at around 3.0 Å

*FT-IR spectra of the parent UiO-66, doped UiO-66-*n*Ti (*n *= 0.7, 1.4, 2.1, 2.7, and 4.0), and NH2-MIL-*

, moving towards 664 cm<sup>−</sup><sup>1</sup>

with the increase in the

. We thus believe that the red shift of the

**Figure 17** shows the SEM images and EDS elemental mapping of the synthesized UiO-66-*n*Ti crystals. It is evident that there is a significant difference on the crystal morphology and size of the UiO-66 crystals as the result of Ti4+ doping. The parent UiO-66 contains octahedral crystals of a mean size ~265 nm. With increased Ti4+ doping concentration, the UiO-66-*n*Ti crystals change from octahedrons to spherelike crystals with rougher facets, and their mean crystal sizes decrease sharply from 265 to ~65 nm. This seems to suggest that the doping of Ti4+ restraints the growth of the UiO-66-*n*Ti crystals. It is noteworthy that when the Ti4+ concentration was further increased, the UiO-66-*n*Ti crystals maintain the sphere-like morphology,

resulted crystals with increased addition of Ti4+ in the synthesis.

but the mean size increases from 65 to 183 nm, unexpectedly.

**Figure 17.** *SEM images of the UiO-66-*n*Ti MOFs and EDS elemental mappings of UiO-66-4Ti [61].*

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

*Synthesis Methods and Crystallization*

**Table 1.**

**Sample Average crystal size of the synthesize nano** 

**UiO-66/nm**

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.

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

1a 265 0 0.42 Zr6O6.42(BDC)5.58 1b 155 0.2 0.50 Zr6O6.50(BDC)5.50 1c 96 0.5 0.84 Zr6O6.84(BDC)5.16 1d 75 1.0 0.84 Zr6O6.84(BDC)5.16 1e 48 1.2 0.84 Zr6O6.84(BDC)5.16 1f 40 1.5 1.05 Zr6O7.05(BDC)4.95

**R Cu2O addition**

**x Formula**

Introducing foreign metal centres into the framework structure to obtain hybrid (i.e. mixed metal) MOFs is a strategy for developing novel MOFs. We have explored the possibility of facile synthesis of Ti4+-doped hybrid UiO-66-*n*Ti MOFs via an in situ crystallization process [61]. In this work, a series of Ti4+-doped UiO-66-*n*Ti (*n* represents the mass fraction of Ti4+) crystals were synthesized following the synthesis procedure of the UiO-66, but with the addition of varying amounts of Ti4+ in the

**4.4 In situ synthesis of Ti-doped hybrid UiO-66 crystals**

*SEM images of the UiO-66-*n*Ti MOFs and EDS elemental mappings of UiO-66-4Ti [61].*

**72**

**Figure 17.**

form of titanium isopropoxide in the precursor solution. The ICP analysis showed that the concentration of Ti4+ increases gradually from around 0.7 to 4.0 wt% in the resulted crystals with increased addition of Ti4+ in the synthesis.

**Figure 17** shows the SEM images and EDS elemental mapping of the synthesized UiO-66-*n*Ti crystals. It is evident that there is a significant difference on the crystal morphology and size of the UiO-66 crystals as the result of Ti4+ doping. The parent UiO-66 contains octahedral crystals of a mean size ~265 nm. With increased Ti4+ doping concentration, the UiO-66-*n*Ti crystals change from octahedrons to spherelike crystals with rougher facets, and their mean crystal sizes decrease sharply from 265 to ~65 nm. This seems to suggest that the doping of Ti4+ restraints the growth of the UiO-66-*n*Ti crystals. It is noteworthy that when the Ti4+ concentration was further increased, the UiO-66-*n*Ti crystals maintain the sphere-like morphology, but the mean size increases from 65 to 183 nm, unexpectedly.

**Figure 18** shows the FT-IR spectra of the parent UiO-66 and doped UiO-66 *n*Ti crystals. As seen, for the parent UiO-66, the Zr▬Oμ3-O stretch in the [Zr▬O] clusters is observed at ~682 cm<sup>−</sup><sup>1</sup> , moving towards 664 cm<sup>−</sup><sup>1</sup> with the increase in the Ti4+ doping amounts, indicating the structural change in the Ti4+-doped UiO-66 *n*Ti crystals. As reported in the literatures [32, 62], the MIL-125(Ti) and NH2-MIL-125(Ti) MOFs are built from coordination between Ti8 clusters and H2BDC-type organic linkers. Both structures display a FT-IR band at between 600 and 700 cm<sup>−</sup><sup>1</sup> which resulted from (O▬Ti▬O) vibrations, and for the NH2-MIL-125(Ti), the (O▬Ti▬O) vibrations occur at ~642 cm<sup>−</sup><sup>1</sup> . We thus believe that the red shift of the Zr▬Oμ3-O stretch in the UiO-66-*n*Ti crystals could be caused by the incorporation of the Ti4+ in the [Zr▬O] clusters.

**Figure 19** shows the EXAFS spectra of the parent UiO-66 and doped UiO-66- 2.7Ti crystals. It further demonstrates the alternation in coordination state around the Zr4+ caused by the Ti4+ doping. As seen, the peak at ~1.7 Å that relates to the Zr▬O bonds shifts to 1.6 Å after Ti4+ doping. Additionally, the peak at around 3.0 Å

#### **Figure 18.**

*FT-IR spectra of the parent UiO-66, doped UiO-66-*n*Ti (*n *= 0.7, 1.4, 2.1, 2.7, and 4.0), and NH2-MIL-125(Ti) crystals [61].*

**Figure 19.** *EXAFS spectra of the parent UiO-66 and the UiO-66-2.7Ti crystals [61].*

that represents the Zr⋯Zr bonds connecting through the Oμ3-O or Oμ3-OH decreases noticeably after doping, likely due to the formation of Zr⋯Ti coordination.

The synthesized UiO-66-*n*Ti crystals have been found to be highly selective and efficient in removing anionic dye, the Congo red, from water. The doped UiO-66-2.7Ti (i.e. with 2.72 wt% Ti4+ incorporation) exhibited the highest adsorption capacity of 979 mg g<sup>−</sup><sup>1</sup> , 3.6 times of the parent UiO-66 crystals.

#### **5. Conclusions**

MOFs, with their inherent high surface areas, uniform, versatile, and tuneable pores, and tailorable physicochemical properties, have been projected to have a broad range of potential applications in various areas. Since the discovery of the first MOF structure in the 1990s, hundreds of new MOFs have been synthesized successfully.

In the past two decades, much attention has been paid to develop methods to control the morphology and size of MOF crystals, in order to tailor the structure properties and performance of MOF materials. Such research efforts have resulted in the development of effective synthesis methods which are capable of modulation of morphology and size of MOF crystals through different mechanisms, including the deprotonation regulation, coordination modulation, and surfactant modulation synthesis. This review chapter has highlighted a number of successful synthesis routes using, for example, the carboxylic acids, HF, and solid Cu2O additives to modulate the MOF crystal formation and growth. It has also demonstrated the ability of doping to create hybrid MOFs with improved properties and functionalities.

It remains a significant challenge on how to best fully utilize the unique set of properties and functionalities of the chemically versatile MOFs and realize their application potentials in various identified areas. While there have been significant achievements on the structure optimization and performance development of MOFs to date, it is anticipated that further progress can be made with more fundamental research and industrial application of MOFs.

#### **Acknowledgements**

This work was supported by the National Natural Science Foundation of China (Grant no. 21236008).

**75**

**Author details**

China

Dalian, China

Yitong Han1,2, Hong Yang3

Australia, Perth, WA, Australia

\*Address all correspondence to: guoxw@dlut.edu.cn

provided the original work is properly cited.

and Xinwen Guo2

\*

1 Dalian Research Institute of Petroleum and Petrochemicals, SINOPEC, Dalian,

2 State Key Laboratory of Fine Chemicals, PSU-DUT Joint Centre for Energy Research, School of Chemical Engineering, Dalian University of Technology,

3 Department of Mechanical Engineering (M050), The University of Western

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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

*Synthesis Methods and Crystallization*

capacity of 979 mg g<sup>−</sup><sup>1</sup>

**5. Conclusions**

**Figure 19.**

that represents the Zr⋯Zr bonds connecting through the Oμ3-O or Oμ3-OH decreases noticeably after doping, likely due to the formation of Zr⋯Ti coordination.

efficient in removing anionic dye, the Congo red, from water. The doped UiO-66-2.7Ti (i.e. with 2.72 wt% Ti4+ incorporation) exhibited the highest adsorption

*EXAFS spectra of the parent UiO-66 and the UiO-66-2.7Ti crystals [61].*

The synthesized UiO-66-*n*Ti crystals have been found to be highly selective and

, 3.6 times of the parent UiO-66 crystals.

MOFs, with their inherent high surface areas, uniform, versatile, and tuneable pores, and tailorable physicochemical properties, have been projected to have a broad range of potential applications in various areas. Since the discovery of the first MOF structure in the 1990s, hundreds of new MOFs have been synthesized successfully. In the past two decades, much attention has been paid to develop methods to control the morphology and size of MOF crystals, in order to tailor the structure properties and performance of MOF materials. Such research efforts have resulted in the development of effective synthesis methods which are capable of modulation of morphology and size of MOF crystals through different mechanisms, including the deprotonation regulation, coordination modulation, and surfactant modulation synthesis. This review chapter has highlighted a number of successful synthesis routes using, for example, the carboxylic acids, HF, and solid Cu2O additives to modulate the MOF crystal formation and growth. It has also demonstrated the ability of doping to create hybrid MOFs with improved properties and functionalities. It remains a significant challenge on how to best fully utilize the unique set of properties and functionalities of the chemically versatile MOFs and realize their application potentials in various identified areas. While there have been significant achievements on the structure optimization and performance development of MOFs to date, it is anticipated that further progress can be made with more

This work was supported by the National Natural Science Foundation of China

fundamental research and industrial application of MOFs.

**74**

**Acknowledgements**

(Grant no. 21236008).
