C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

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Revealing the structure of porous metal-organic framework glasses through X-ray absorption spectroscopy

Carboxylate-linked metal-organic framework glasses with high porosity were synthesised from titanium- oxo (Ti-oxo) clusters and organic linkers. X-ray absorption near-edge structure and extended X-ray absorption fine structure data helped to reveal the structural integrity of the discrete Ti-oxo clusters inside the glasses.

The principles of reticular chemistry allow the synthesis of metal-organic framework (MOF) glasses [1]. This is achieved by integrating metal clusters with organic ligands, resulting in transparent, amorphous and monolithic solids. The strong and directional coordination bonds between the building blocks allow interconnected micropores that are accessible to guest molecules to be created inside the monolith, making them useful for gas storage, energy storage and catalysis. While prior studies demonstrated MOF glasses with permanent porosity, their nitrogen Brunauer Emmett Teller (BET) surface areas have typically been of the order of 100 m2 g-1 [2]. This value is considerably lower, by at least an order of magnitude, when compared to their crystalline counterparts. For MOF glasses to truly reach their maximal potential, an enhancement in porosity is vital. However, this objective is hindered by the weak connectivity, which leads to a vulnerable pore architecture.

In this work, carboxylate linkage was employed to link Ti-oxo clusters and organic linkers into MOF glasses with significantly improved BET surface areas of greater than 900 m2 g-1. Compared to previously utilised phenolate linkage, carboxylate linkage is anticipated to have stronger binding affinity towards the Ti species. This allows the full substitution of the bulky modulating and structure- directing agents within the pores, without compromising the structural integrity.

One example of the carboxylate-linked MOF glass, termed Ti-Fum, was synthesised from Ti-oxo cluster Ti16O16(OEt)32, fumaric acid, and m-cresol (Figure 96a). The formation of coordination bonds between the Ti-oxo cluster and the fumaric acid linker was confirmed by Fourier-transform infrared (FT-IR) spectroscopy (Figure 96b). Following the methanol exchange and subsequent vacuum activation, both m-cresol and free methanol were removed from the pore, revealing a readily accessible internal surface. The porosity of the activated Ti-Fum was measured via nitrogen sorption analysis at 77 K (Figure 96c). The carboxylate-linked MOF glass displayed a Type I isotherm with a BET surface area of 923 m2 g-1, approximately triple that of its phenolate- linked counterpart Ti-BPA. Powder X-ray diffraction (PXRD) analysis of the activated Ti-Fum revealed a broad peak at 2θ = 10.4°. This correlates to a d-spacing of 8.5 Å, indicating the existence of a certain level of localised order within the predominantly amorphous structure of the MOF glass (Figure 96d).

Fig. 96: a) Proposed mechanism of synthesis and activation of Ti-Fum. Some structural elements are indicated by dashed lines for clarity. b) FT-IR spectra of as-synthesised and activated Ti-Fum, fumaric acid, as well as Ti16O16(OEt)32. c) N2 sorption isotherm and

pore-size distribution (inset) of Ti-Fum at 77K. d) PXRD patterns of Ti-Fum and Ti16O16(OEt)32.