Exploring the Role of Cluster Formation in UiO Family Hf Metal-Organic Frameworks with in Situ X-ray Pair Distribution Function Analysis.

Francesca C N Firth,Michael W Gaultois,Matthew J Cliffe,Clare P Grey,Yue Wu,Dean S Keeble,Joshua M Stratford

doi:10.1021/jacs.1c06990

Abstract

The structures of Zr and Hf metal-organic frameworks (MOFs) are very sensitive to small changes in synthetic conditions. One key difference affecting the structure of UiO MOF phases is the shape and nuclearity of Zr or Hf metal clusters acting as nodes in the framework; although these clusters are crucial, their evolution during MOF synthesis is not fully understood. In this paper, we explore the nature of Hf metal clusters that form in different reaction solutions, including in a mixture of DMF, formic acid, and water. We show that the choice of solvent and reaction temperature in UiO MOF syntheses determines the cluster identity and hence the MOF structure. Using in situ X-ray pair distribution function measurements, we demonstrate that the evolution of different Hf cluster species can be tracked during UiO MOF synthesis, from solution stages to the full crystalline framework, and use our understanding to propose a formation mechanism for the hcp UiO-66(Hf) MOF, in which first the metal clusters aggregate from the M6 cluster (as in fcu UiO-66) to the hcp-characteristic M12 double cluster and, following this, the crystalline hcp framework forms. These insights pave the way toward rationally designing syntheses of as-yet unknown MOF structures, via tuning the synthesis conditions to select different cluster species.

Highlights

Metal−organic frameworks (MOFs) are of great interest for a wide variety of applications, including energy storage and carbon capture,[1,2] and have outstanding chemical tunability.[3]
Zirconium and hafnium are favorable for the design of new MOF structures: in contrast to copper, for example, there is a rich variety of known zirconium and hafnium clusters, with a wide range of nuclearities, geometries, and coordination denticities.[7−11] There are over 1300 structures containing between 3 and 21 Zr or Hf ions in their molecular formula forming either molecular clusters or being
In order to study the in situ formation of UiO-66 family MOFs, we designed a series of experiments based on the conditions used in our lab syntheses of these materials, as summarized in DFW 65:25:10a no aThe notation “DFW 65:25:10” gives the volume ratio of DMF/

Summary

Introduction

Metal−organic frameworks (MOFs) are of great interest for a wide variety of applications, including energy storage and carbon capture,[1,2] and have outstanding chemical tunability.[3] In order to fully exploit the potential of MOFs for real-world applications, we must work toward designing syntheses to obtain MOFs with new sorption or catalytic properties, enabled by new or previously inaccessible structures. Zirconium and hafnium are favorable for the design of new MOF structures: in contrast to copper, for example (which often forms dinuclear “paddlewheel” units6), there is a rich variety of known zirconium and hafnium clusters, with a wide range of nuclearities, geometries, and coordination denticities.[7−11] There are over 1300 structures containing between 3 and 21 Zr or Hf ions in their molecular formula forming either molecular clusters or being.

Methods

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Discussion

Conclusion