Pre-equilibrium species in MOF crystallization

We’re very pleased to announce our paper on the crystallization of ZIF-8 has just been accepted! It’s been a challenging piece of work, not least because it all began when we made the surprising observation that crystallization got SLOWER when we increased the concentration of our reactants…

There is an increasingly large amount of interest in metal-organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. Their exciting properties arise from their modular architectures, which self-assemble from different combinations of metal-based and organic building units. However, the exact mechanisms by which they crystallize remain poorly understood, thus limiting any realisation of real “structure by design”. We report important new insight into MOF formation, gained using in situ X-ray diffraction, pH and turbidity measurements to uncover for the first time the evolution of metastable intermediate species in the canonical zeolitic imidazolate framework system, ZIF-8. We reveal that the intermediate species exist in a dynamic pre-equilibrium prior to network assembly and, depending on the reactant concentrations and the progress of reaction, the pre-equilibrium can be made to favour under- or over-coordinated species, thus accelerating or inhibiting crystallization, respectively. We thereby find that concentration can be effectively used as a synthetic handle to control particle size, with great implications for industrial scale-up and gas sorption applications. This finding enables us to rationalise the apparent contradictions between previous studies and, importantly, opens up new opportunities for the control of crystallization of network solids more generally, from the design of local structure to assembly of particles with precise dimensions.

The paper is published with Angewandte Chemie, International Edition and can be found here. A previous version can also be downloaded for free on ChemRxiv.

Many thanks to all co-authors, Diamond for beamtime, SCG Innovation for funding and everyone else that helped out along the way!

Thermodynamics and Kinetics of MOF formation

We’re excited to announce that we’ve got a new paper out– what a start to 2018!

It’s a review of research over the last 20 years on the fundamental factors affecting why certain metal-organic frameworks (MOFs) form the structures that they do. It’s entitled “Thermodynamic and Kinetic Effects in the Crystallization of Metal–Organic Frameworks” and it’s published in the journal Accounts of Chemical Research.

The paper describes how our understanding has grown over the years from simple qualitative observations in the 90’s to detailed, quantitative results provided by computational models and powerful in situ experiments today. The authors, my former supervisor Tony Cheetham, Gregor Kieslich at Munich and Hamish delve into how we’re able to understand not just the structures, but the enthalpic, entropic and kinetic factors that shape each material too.

Take a look at the paper here.

Summer project on MOF formation 2018

We have guaranteed funding for at least one 6-8 week summer project in 2018 here in Oxford and the possibility to apply for more! Prospective students should be a current undergraduate in Chemistry or a related discipline and show interest in one or more of these topics: chemical kinetics, crystallisation, metal-organic frameworks, X-ray diffraction and in-situ measurements. Deadlines for other funding sources occur early in 2018 so please contact Hamish if interested.