By using beamline I12 at Diamond Light Source, scientists have been able to observe the real time crystallisation of new hybrid materials known as metal-organic frameworks (MOFs). The fascinating observations, published in Angewandte Chemie, provide the most comprehensive understanding of the energetics of MOF formation to date.
MOFs are a new family of materials that are comprised of metals with organic linkers. The linkers are a key component of the structure, because their length can influence the properties of the resulting framework. Long linkers lead to low density structures with large empty spaces that are ideal for capturing gases or catalysing reactions. In fact, this remarkable new type of material has a wealth of applications from trapping carbon dioxide to storing hydrogen in fuel cells.
Fig. 1: In situ XRD measured during the transformation of lithium tartrate materials with their structures superimposed
In this study, with MOFs composed of lithium tartrate, the team observed the sequential crystallisation of three competing phases and discovered that they could track the effect of reaction conditions on the resulting crystals (Fig. 1).This enabled the scientists to fully quantify the reaction energy landscape, potentially allowing them to choose the end product of the reactions by selecting specific reaction temperatures and times. The formation energies of different crystal arrangements were predicted using a mathematical model and the results were verified at Diamond. “By using in situ X-ray diffraction, we could map the process of crystallisation and found that we had an efficient way of screening the reactions as they happened,” said Prof Walton.
Monochromatic high-energy X-ray diffraction at I12, the Joint Engineering, Environmental, and Processing beamline (JEEP), enabled the team to measure data quickly and to capture snapshots of chemical processes in-situ. The beamline offered flexibility with the space afforded to accommodate their apparatus. In fact, a parallel study conducted by Prof Walton at Diamond involved the application of a bespoke furnace, known as the ODISC (Oxford-Diamond In Situ Cell), to monitor the real time crystallisation of another type of MOF, comprising ytterbium (Fig. 2)1.
Fig. 2: 3D contour map of evolving diffraction patterns during the crystallisation of an Yb MOF at 120 oC in organic solvent.
The infrared furnace enabled the reaction to reach high temperatures rapidly and allowed the team to witness an amazing phenomenon: that the solvent composition changed within the crystals during their formation. This was only made possible by the high resolution of the powder diffraction data recorded, which uniquely allowed refinement of the crystal structure as it evolved. Prof Walton stated: “This observation had never been seen before. Initially water was trapped within the crystal structure, but it was replaced by the organic solvent.” The details of this groundbreaking work, also published in Angewandte Chemie, can be found in Wu et al.2.
The team plan to apply their methodology to other materials to explore the formation of different shapes and sizes of crystals. In particular, they hope to look at more extreme reaction conditions such as molten fluxes. These conditions also yield innovative new materials and are a hot topic among scientists as they are an emerging branch of as-yet unchartered chemistry. In the future it is hoped that MOFs could be designed to fulfil roles in healthcare, energy storage and the environment, so the study of their formation is vital.
1. Moorhouse SJ et al. The Oxford-Diamond In Situ Cell for studying chemical reactions using time-resolved X-ray diffraction. Rev Sci Instrum. (2012) DOI: 10.1063/1.4746382.
Yeung HH et al. In situ observation of successive crystallizations and metastable intermediates in the formation of metal-organic frameworks. Angew Chem Int Ed. 2016. DOI: 10.1002/anie.201508763.
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