Elemental Depth Profiling of Intact Metal–Organic Framework Single Crystals by Scanning Nuclear Microprobe

Abstract

The growing field of MOF–catalyst composites often relies on postsynthetic modifications for the installation of active sites. In the resulting MOFs, the spatial distribution of the inserted catalysts has far-reaching ramifications for the performance of the system and thus needs to be precisely determined. Herein, we report the application of a scanning nuclear microprobe for accurate and nondestructive depth profiling of individual UiO-66 and UiO-67 (UiO = Universitetet i Oslo) single crystals. Initial optimization work using native UiO-66 crystals yielded a microbeam method which avoided beam damage, while subsequent analysis of Zr/Hf mixed-metal UiO-66 crystals demonstrated the potential of the method to obtain high-resolution depth profiles. The microbeam method was further used to analyze the depth distribution of postsynthetically introduced organic moieties, revealing either core–shell or uniform incorporation can be obtained depending on the size of the introduced molecule, as well as the number of carboxylate binding groups. Finally, the spatial distribution of platinum centers that were postsynthetically installed in the bpy binding pockets of UiO-67-bpy (bpy = 5,5′-dicarboxyy-2,2′-bipyridine) was analyzed by microbeam and contextualized. We expect that the method presented herein will be applicable for characterizing a wide variety of MOFs subjected to postsynthetic modifications and provide information crucial for their optimization as functional materials.

An X-ray Photoelectron Spectroscopy Study of Postsynthetic Exchange in UiO-66

“…To date, four experimental methods have been employed to probe the distribution of ligands following PSE or other synthesis methods of mixed linker MOFs: (1) confocal laser scanning microscopy (CLSM), , (2) confocal scanning Raman spectroscopy (CSRS), , (3) Rutherford backscattering spectrometry (RBS), , and (4) nuclear magnetic resonance (NMR). ,− Each method has its own advantages and disadvantages. In CLSM, individual MOF crystals can be imaged at varying depths in a nondestructive process, but a fluorophore is required.…”

“…Furthermore, defects present in the parent crystals, known to vary depending on synthetic conditions, may also influence the amount, distribution, and coordination environment of PSE ligands in the final product. 22,23 To date, four experimental methods have been employed to probe the distribution of ligands following PSE or other synthesis methods of mixed linker MOFs: (1) confocal laser scanning microscopy (CLSM), 24,25 (2) confocal scanning Raman spectroscopy (CSRS), 26,27 (3) Rutherford backscattering spectrometry (RBS), 28,29 and (4) nuclear magnetic resonance (NMR). 24,30−32 Each method has its own advantages and disadvantages.…”

“…In a later study utilizing a microbeam RBS method, a larger UiO-66 single crystal (∼15 μm in size), and the same PSE ligand, a gradient distribution was observed with an enhanced amount of PSE ligand at the surface. 29 The different outcomes in these two studies were attributed to the relative ease of diffusion of the incoming ligand into smaller particles in comparison to the larger single crystal.…”

Exploring cation distribution in ion-exchanged Al,Ga-containing metal–organic frameworks using 17O NMR spectroscopy

“…It is not uncommon for post-synthetic exchange reactions of MOFs to produce materials with a core-shell structure, through either metal or ligand exchange processes, although detailed information on the framework composition and distribution of components is not always easily available and sometimes has to be inferred from bulk measurements. [55][56][57][58] There is growing interest in frameworks with core-shell structures, enabling additional functionally within a material which in turn leads to enhanced performance, for example, increasing gas adsorption capacity and the number of accessible active sties. 59,60 This work shows the utility of 17 O solid-state NMR spectroscopy to follow such reactions by controlling how, when and where 17 O enrichment occurs, providing detailed information on the local structure.…”

A Method for Determining Incorporation Depth in Core–Shell UiO-66 Nanoparticles Synthesized Via Postsynthetic Exchange

“…Factors influencing PSE include a choice of solvent, − reaction time, ,− temperature, ,,, and the functional groups ,, on the linker. The particle size of the original MOF used in the PSE process is also crucial and typically remains unchanged after PSE. − ,,,, Nair and co-workers demonstrated that, under identical reaction conditions, the incorporation rate for nanoparticles significantly increases compared to the micrometer-sized crystals . Additionally, particle size can change substantially after PSE, often due to a dissolution and recrystallization process that leads to a significant increase in the particle size. , Furthermore, defects in the MOF structure significantly affect the exchange process. ,,− The incorporation of linkers at defect sites, referred to as postsynthetic linker insertion, competes with the process of linker exchange.…”