Fatty acid capped, metal oxo clusters as the smallest conceivable nanocrystal prototypes

Metal oxo clusters of the type M6O4(OH)4(OOCR)12 (M = Zr or Hf) are valuable building blocks for materials science. Here, we synthesize a series of zirconium and hafnium oxo clusters with ligands that are typically used to stabilize oxide nanocrystals (fatty acids with long and/or branched chains). The fatty acid capped oxo clusters have a high solubility but do not crystallize, precluding traditional purification and single-crystal XRD analysis. We thus develop alternative purification strategies and we use X-ray total scattering and Pair Distribution Function (PDF) analysis as our main method to elucidate the structure of the cluster core. We identify the correct structure from a series of possible clusters (Zr3, Zr4, Zr6, Zr12, Zr10, and Zr26). Excellent refinements are only obtained when the ligands are part of the structure model. Further evidence for the cluster composition is provided by nuclear magnetic resonance (NMR), infrared spectroscopy (FTIR), thermogravimetry analysis (TGA), and mass spectrometry (MS). We find that hydrogen bonded carboxylic acid is an intrinsic part of the oxo cluster. Using our analytical tools, we elucidate the conversion from a Zr6 monomer to a Zr12 dimer (and vice versa), induced by carboxylate ligand exchange. Finally, we compare the catalytic performance of Zr12-oleate clusters with oleate capped, 5.5 nm zirconium oxide nanocrystals in the esterification of oleic acid with ethanol. The oxo clusters present a five times higher reaction rate, due to their higher surface area. Since the oxo clusters are the lower limit of downscaling oxide nanocrystals, we present them as appealing catalytic materials, and as atomically precise model systems. In addition, the lessons learned regarding PDF analysis are applicable to other areas of cluster science as well, from semiconductor and metal clusters, to polyoxometalates

Fatty acid capped, metal oxo clusters as smallest conceivable nanocrystal prototypes

Metal oxo clusters of the type \ce{M6O4(OH)4(RCOO)12} (M = Zr of Hf) are valuable building blocks for material science. Here, we develop them as smallest conceivable nanocrystal prototypes. We synthesize a series of zirconium and hafnium oxo clusters with ligands that are typically used to stabilize oxide nanocrystals (fatty acids with long and/or branched chains). In contrast to previously reported discrete oxo clusters with short/rigid carboxylates (e.g., acetate, benzoate), the fatty acid capped oxo clusters have a high solubility but do not crystallize, precluding traditional purification and single-crystal XRD analysis of clusters. We thus develop alternative purification strategies and structural analysis tools. We use X-ray total scattering and Pair Distribution Function (PDF) analysis as our main tool to elucidate the structure of the cluster core. In contrast to traditional PDF analysis of larger clusters and nanocrystals, we show that the structure models need to include the carboxylate binding groups to obtain excellent refinements. Our methodology is able to pick up the correct structure from a series of possible structure models (\textbf{Zr4}, \textbf{Zr6}, \textbf{Zr12}). Further supporting evidence for the cluster composition (including their ligand shell) is provided by nuclear magnetic resonance (NMR), infrared spectroscopy (FTIR), thermogravimetry analysis (TGA) and mass spectrometry (MS). We find that the ligands have multiple binding modes and that hydrogen bonded carboxylic acid is an intrinsic part of the oxo cluster. Using our analytical tools, we elucidate the conversion from \textbf{Zr6} monomer to \textbf{Zr12} dimer (and vice versa), induced by carboxylate ligand exchange. Finally, we compared the catalytic performance of \textbf{Zr12}-oleate clusters with oleate capped, 3-5 nm zirconium oxide nanocrystals in the esterification of oleic acid with ethanol. The oxo clusters are much more catalytically active, due to their higher surface area. Since the oxo clusters are the limit of downscaling oxide nanocrystals, we thus propose them here as appealing catalytic materials, or at least as atomically precise model systems. In addition, our analytical (PDF) methodology is generally applicable and expected to find use in other areas of clusters as well, and will be especially valuable for clusters with weakly scattering core atoms

The central role of oxo clusters in zirconium- and hafnium-based esterification catalysis.

Oxo clusters are a unique link between oxide nanocrystals and MOFs, representing the limit of downscaling each of the respective crystals. Here, we show the superior catalytic activity of Zr12O8(OH)8(OOCR)24 clusters, compared to zirconium MOF UiO-66 and ZrO2 nanocrystals. We focus on esterification reactions given their general importance in consumer products and the challenge of converting large substrates. Oxo clusters have a higher surface-to-volume ratio than nanocrystals, rendering them more active. For large substrates, e.g., oleic acid, MOF UiO-66 has negligible catalytic activity while clusters provide almost quantitative conversion, a fact we ascribe to limited diffusion of large substrates through the MOF pores. Cluster do not suffer from limited mass transfer and we also obtain high conversion in solvent-free reactions with sterically hindered alcohols (hexanol, 2-ethylhexanol, benzyl alcohol and neopentyl alcohol). The cluster catalyst can be recovered and shows identical activity. The structural integrity of the cluster is confirmed using X-ray total scattering and Pair Distribution Function analysis. Even more, when homogeneous zirconium (or hafnium) alkoxides are used as catalyst, the same oxo cluster is retrieved, showing that oxo clusters are the active catalytic species, even in previously assumed homogeneously catalyzed reactions

An Amorphous Phase Precedes Crystallization: Unraveling the Colloidal Synthesis of Zirconium Oxide Nanocrystals

One can nowadays readily generate monodisperse colloidal nanocrystals, but the underlying mechanism of nucleation and growth is still a matter of intense debate. Here, we combine X-ray pair distribution function (PDF) analysis, small-angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) to investigate the nucleation and growth of zirconia nanocrystals from zirconium chloride and zirconium isopropoxide at 340 °C, in the presence of surfactant (tri-n-octylphosphine oxide). Through E1 elimination, precursor conversion leads to the formation of small amorphous particles (less than 2 nm in diameter). Over the course of the reaction, the total particle concentration decreases while the concentration of nanocrystals stays constant after a sudden increase (nucleation). Kinetic modeling suggests that amorphous particles nucleate into nanocrystals through a second order process and they are also the source of nanocrystal growth. There is no evidence for a soluble monomer. The nonclassical nucleation is related to a precursor decomposition rate that is an order of magnitude higher than the observed crystallization rate. Using different zirconium precursors (e.g., ZrBr4 or Zr(OtBu)4), we can tune the precursor decomposition rate and thus control the nanocrystal size. We expect these findings to help researchers in the further development of colloidal syntheses.</p

An amorphous phase precedes crystallization; unraveling the colloidal synthesis of zirconium oxide nanocrystals

One can nowadays readily generate monodisperse colloidal nanocrystals, but the underlying mechanism of nucleation and growth is still a matter of intense debate. Here, we combine X-ray pair distribution function (PDF) analysis, small angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) to investigate the nucleation and growth of zirconia nanocrystals from zirconium chloride and zirconium isopropoxide at 340 °C, in the presence of surfactant. We find that initially, many amorphous particles are formed. Over time, the total particle concentration decreases while the amorphous particles recrystallize into ZrO2 nanocrystals. After a sudden increase, the concentration of nanocrystals stays constant over the course of the reaction. Both findings stand in contrast to reports of continuous nucleation in other surfactant-assisted nanocrystal syntheses. The non-classical nucleation is likely related to the precursor decomposition rate that is an order of magnitude higher than the observed crystallization rate. Comparing different zirconium precursors, we observe higher smaller particles for reactions with ZrBr4 or with Zr(OtBu)4, which we could correlate with a higher precursor decomposition rate