November 20, 2020
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Structure-Directing Agents Mediate a Two-Step Pathway of Metal Organic Framework Formation

Direct interactions control final material structure

Reaction schematic for the formation of a metal-organic framework

The presence of an intermediate containing a structure-directing agent demonstrates its active role in the formation of the final sodalite (sod) structured material.

(Illustration by Michael Sinnwell |  Pacific Northwest National Laboratory)

The Science

When combined under precise conditions, simple carbon and metal-containing molecules can create complex functional solids. These materials, referred to as metal-organic frameworks (MOFs), show promise for applications as diverse as energy storage and gas purification. To tailor a MOF for an application, scientists must be able to rationally design and synthesize a target material. To do this, they often use structure-directing agents (SDAs), relatively simple molecules that somehow control the final form of the MOF without becoming part of it. Previously thought to be spectators, researchers determined that the SDA actually forms an important intermediate in an indium-based MOF system.

The Impact

To effectively design new MOFs, researchers need to know how to control their final structure. The new finding that SDAs directly interact with the MOF components alters previous scientific understanding of how they determine MOF structure. Learning how SDAs act during MOF synthesis will allow better design of new materials that possess specific, desired properties with less trial and error.

Summary

Understanding how various factors in synthesis control the final structure of MOFs is key to the continued development of the field. Researchers utilized an indium-based MOF system with a range of potential structures to probe the role of an SDA in the synthesis. Although previously thought to act simply as spectators, in situ X-ray diffraction and small-angle X-ray scattering experiments identified a molecular intermediate in the solid phase and solution conditions containing the SDA. The disappearance of this intermediate, identified as an indium-SDA complex, coincided with the formation of the In-MOF, an indication that it is involved in the reaction. This provides crucial information that significantly enhances scientific knowledge of how SDAs contribute to MOF structural control.

PNNL Contact

Dr. Praveen K. Thallapally, Hierarchical Materials Team, Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, praveen.thallapally@pnnl.gov

Dr. Michael A. Sinnwell, Nuclear and Radiochemistry, National Security Directorate, Pacific Northwest National Laboratory, michael.sinnwell@pnnl.gov

Funding

This work was supported by the U.S. Department of Energy (DOE) Office of Science (SC), Basic Energy Sciences (BES), Materials Sciences and Engineering Division under Awards KC020105- FWP12152 (Pacific Northwest National Laboratory [PNNL]). Small-angle X-ray scattering work was done at Oregon State University supported by DOE SC, BES, under Award DE SC0010802. Structure determination was carried out using resources of the Advanced Photon Source, a DOE SC User Facility operated for DOE SC by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 from an X-ray Powder Diffraction pattern collected at the beamline 28-ID-2 of the National Synchrotron Light Source II, a DOE SC User Facility operated for the DOE SC by Brookhaven National Laboratory under Contract No. DESC0012704. QRSM’s in situ X-ray Powder Diffraction efforts were supported by the DOE SC, BES, Chemical Sciences, Geosciences, and Biosciences Division through its Geosciences program at PNNL. The microscopy portion of this research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. 

Published: November 20, 2020

M. A. Sinnwell, Q. R. S. Miller, L. Palys, D. Barpaga, et al. “A Molecular Intermediate in the Directed Formation of a Zeolitic Metal-Organic Framework.” J. Am. Chem. Soc. 142, 41, 17598–17606 (2020).

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