Metal-Organic Frameworks for solar energy utilization

The research strategy followed in this work was inspired by natural photosynthesis that, for simplicity, can be divided into three principal steps: (i) efficient light absorption at wavelengths abundant in the solar light; (ii) long-lived charge separation achieved by funnelling of the photogenerated electrons away from the light-absorbing antenna; (iii) chemical reaction taking place at the specific catalytic sites. The idea of the present research was to mimic the key-principles of photosynthesis and implement them in Metal-Organic Frameworks. The first principle is covered in Chapter 4 where the light absorption properties of a Ti-based framework were synthetically tuned as to allow for lowering the excitation energy and sensitizing the MOF to a larger fraction of the solar spectrum. Chapters 2 and 3 investigate the photoexcitation processes in MOFs with the aim of prolonging the lifetime of photoexcited states similarly to (ii). Finally, in Chapter 5 additional catalytic sites (iii) were introduced in NH2-MIL-125(Ti) improving the catalytic activity by one order of magnitude. Chapter 1 introduces Metal-Organic Frameworks (MOFs) as potential photocatalysts and their characteristics. At first, general synthetic strategies of improving photocatalytic activity are discussed and a brief historical overview of the TiO2-based catalysts development is given followed by recent advances in the field. Further, the pioneering examples of using MOFs as photocatalysts are reviewed. The development of MOF-based photocatalysts is compared against more common photocatalysts such as TiO2. Some of the strategies such as sensitization towards a desired wavelength and introduction of additional catalytic sites are similar in both cases. At the same time MOFs are characterized by the microporous and well-defined structure, excellent tuneability and the lack of photoconductivity. All these aspects have strong implications for the synthetic strategies of the catalyst development. In this chapter three different roles of MOFs are discussed: a) MOFs as mere containers for photocatalytically active species; b) MOFs as photocatalysts both absorbing light and carrying out catalytic reactions; c) MOF-based composites where the frameworks are multifunctional, they absorb light and carry additional catalytic sites. Chapter 2. Here fundamentals of photoexcitation in NH2-MIL-125(Ti) are investigated with the help of ultrafast spectroscopy. MIL-125(Ti)-type frameworks are known to undergo ligand-to-metal charge transfer (LMCT) yielding Ti3+ and the positively charged organic-defined counterparts. UV-pump Visible-probe spectroscopy has shown that the excited states of NH2-MIL-125(Ti) possess a substantially longer lifetime as compared to the amine-free analogous MOF, MIL-125(Ti). Moreover, the difference in spectral signatures of the excited MIL-125(Ti) and NH2-MIL-125(Ti) was documented and associated to the difference in the organic-defined positively charged holes. The assignment of the transient spectral features of NH2-MIL-125(Ti) was carried out with the help of spectroelectrochemistry. Vis-pump IR-probe spectroscopy demonstrated that the photogenerated holes within NH2-MIL-125(Ti) are likely localized on the NH2-group of aminoterephthalates as the N-H stretch vibrations are heavily involved in the photoexcitation process. Kinetics of the photogenerated charge carriers in different solvents is described: in contrast to what was expected we found that the lifetimes of the excited states do not change depending on the solvent polarity. Chapter 3. In this chapter the role of d0 metal ion constituting frameworks of NH2-MIL-125 and NH2-UiO-66 is unravelled. In their works Walsh et al. have shown for the case of the MIL-125(Ti) series MOFs that the highest occupied crystal orbital (HOCO) is localized at the organic linker whereas the lowest unoccupied crystal orbital (LUCO) localizes at the inorganic secondary building unit (SBU), titanium oxocluster. Thus the HOCO-LUCO optical gap can be tuned by introducing various substituents into the aromatic system of the linker for the cost of modifying the oxidative power of the resulting photocatalyst. In this chapter the target reaction was hydrogen evolution. In this case, the reductive power of a photocatalyst is of crucial importance. The main hypothesis was that by changing the metal in the same group from Ti to Zr or Hf one should be able to tune this reductive power for the frameworks bearing the same organic linker and a similar topology. However, this is only true if LMCT operates in all MOFs considered. With the help of DFT it was found that the LUCO of NH2-MIL-125(Ti) is localized at the titanium oxocluster whereas the ones of NH2-UiO-66(Zr/Hf) are localized at the organic linker. The same conclusion was confirmed by EPR. This LMCT of NH2-MIL-125(Ti) possesses a substantially longer lifetime compared to the purely organic-based transitions found in the UiOs as revealed by transient absorption spectroscopy. This in turn results in a superior performance in photocatalytic H2 evolution in the case of Ti-based MOF. The positive role of structural defects in H2 evolution catalysed by the UiOs, although modest, was also documented. Chapter 4 introduces a framework with an extended conjugated aromatic system of the linker derived from NH2-MIL-125(Ti). The NH2-containing parent MOF was post-functionalized with dye-like molecular fragments resembling a well-known organic dye, methyl red. This was done by following a traditional synthetic pathway of obtaining organic dyes through the diazotization reaction. At the first stage the NH2-groups of NH2-MIL-125(Ti) react with nitrosonium ions, NO+, generated in situ by the HCl/NaNO2 pair while the second step is the reaction of the obtained N?N+-MIL-125(Ti) with diethylaniline. The resulting MOF absorbs a larger fraction of visible light extending to ca. 620 nm and thus has a superior performance in photocatalytic oxidation of benzyl alcohol. Furthermore, the photoconductance of MIL-125(Ti) was assessed by time-resolved microwave conductivity (TRMC). The experiments revealed that the material demonstrates very low levels of photoconductance that are temperature dependent. This type of behaviour is characteristic of the temperature-activated hopping as a type of charge carrier transport and is in contrast to the conductance of semiconductors that hardly changes with temperature. Based on these results it was recovered that MIL-125(Ti) and possibly many other MOFs should not be seen as true semiconductors but rather as arrays of coordination complexes arranged in a crystalline lattice. Chapter 5 illustrates an example of Active Site Engineering in Metal-Organic Frameworks. A well-known electrocatalyst, cobaloxime was encapsulated within the cavities of NH2-MIL-125(Ti). The direct sorption of the molecular complex into the pores of MOF did not proceed likely due to the size-selectivity of the pore windows. Therefore, a two-step synthetic strategy was developed where at the first stage the pores of the MOF were loaded with a flexible organic ligand while at the second CoBr2 was added under oxidative conditions. The organic ligand complexates with cobalt ions in the pores of NH2-MIL-125(Ti) forming a cobaloxime bearing Co(III) encapsulated the pores. The chapter contains an extensive characterization of the obtained composite and a discussion of possible differences between the molecular cobaloxime in solution vs. the one encapsulated in the MOF. The obtained Co@MOF catalyst evolves hydrogen at a rate 20 times greater than the one of the bare MOF. The external quantum efficiency for the composite was found to be 0.5%. This is a rather modest number if compared against the state of the art semiconductor-based systems. However, it should be emphasized that this catalyst outperforms the majority of reported MOF-based photocatalysts as explained in Chapter 5. Moreover, the composite purely consists of abundant elements (Ti, Co, C, O, N, Br, H) and, from that perspective, fulfils the global scale up requirements. Cobaloximes typically act as excellent catalytic sites yet they are not able to undergo charge separation upon illumination and require an external source of electrons. The electrons can be supplied by electrodes as well as by excited organic dye molecules. In this work with the help of EPR it was found that the MOF acts as a photosensitizer absorbing photons and injecting electrons in the cobalt complex. At the same time no Ti3+ was detected in irradiated Co@MOF composite in contrast to what was found in the bare NH2-MIL-125(Ti). Instead, cobalt reduction from Co3+ to Co2+ was detected. This suggests that the electron injection from the photoexcited MOF is fast while the resting state of the Co@MOF composite is Co(II). However, the turn-over frequency of the composite is still rather moderate and a more detailed investigation of the mechanism is needed to rationalize this performance. This is an elegant example of a multifunctional catalytic system for photocatalytic generation of hydrogen by water splitting, based on the abundant element Co.Chemical EngineeringApplied Science