Blanc SIMI 8 - Sciences de l'information, de la matière et de l'ingénierie : Chimie du solide, colloïdes, physicochimie

Hydrothermal and Mechanical Stability of Soft Porous Crystals – SOFT-CRYSTAB

Ageing/degradation and regeneration mechanisms of porous materials at the atomic level.

Hydrothermal and mechanical stability of soft porous crystal in view of industrial applications.

The Soft-Crystab project aims at characterizing the hydrothermal and mechanical stability of soft porous crystals

Soft Porous Crystals (SPC) are a fascinating sub-class of metal-organic frameworks (MOFs) exhibiting stimuli-responsive behavior, i.e. reversible changes of large amplitude under external stimulation. An important question, from the perspective of practical industrial-scale applications, is that of their stability in working conditions. While their faculty to respond to external physical or chemical stimuli give them high potential for applications (e.g. in sensing and detection), such large-amplitude changes can also be detrimental to their long-term stability in the case of temperature and pressure variations.<br />This project aims at providing a better understanding and predicting the evolution, degradation and regeneration of Soft Porous Crystals and their properties, when they are exposed to thermal, mechanical and chemical constraints. To this aim, we investigate at a fundamental level and at the microscopic scale the behavior of these materials, in order to rationalize it and help guide the design of novel materials with targeted properties.

The Soft-Crystab project capitalizes on the synergy between groundbreaking experimental and theoretical approaches, to synthesize and characterize novel materials, study the links between their structure, morphology, responsive properties and stability at all levels between macroscopic and nanoscopic scales.
A variety of materials have been synthesized, and we have focused on the influence of morphological and textural properties (especially crystal size) on their hydrothermal stability.
Systematic experimental characterizations were performed in a large range of working conditions, including variations of temperature and activation/evacuation protocols; under vacuum or controlled atmosphere; in presence of water vapor (controlled pressure) or liquid water. In particular, we investigated the response of these materials over very long cycles of stimulation, with full control of kinetic factors, all the while following their behavior by structural (X-ray diffraction) and spectroscopic (IR, NMR) methods, both in situ and ex situ.
Finally, we developed novel theoretical methods for the understanding of these systems, both in terms of theoretical chemistry techniques and statistical thermodynamics modeling. There again, the problem was addressed on a large range of time and length scales, through quantum chemistry calculations, ab initio dynamics, classical molecular simulations and analytical thermodynamic models.

We have vastly improved the understanding of the temperature/temperature/adsorption phase diagram of flexible materials of the MIL-53 family, and developed a systematic methodology of study for this type of system.
We proposed and validated a computational method for the prediction of flexibility (and the characterization of its extent) in nanoporous materials. We showed that the high anisotropy of elastic properties and the existence of low-stiffness deformation modes are intrinsically linked to structural flexibility.
Studying the large family of ZIF materials, we shone light on the interplay between porous topology, particle size, activation protocol and intrinsic porosity on the hydrothermal stability of the crystals.

Degradation of soft porous crystal.

This project yielded 22 papers in international peer-reviewed journals, and 25 oral presentations in conferences, including one keynote lecture and two plenary lectures in international conferences. The project consortium also organized two international workshops (under the auspices of CNRS), related to the project: International workshop on Adsorption in Compliant Solids (2011 and 2013, and now continuously held biannualy).

A new field of research has emerged in the past decade in the context of solid-state chemistry and physical chemistry. It is the Science of Metal-Organic-Frameworks (MOF's). Metal-Organic-Frameworks, also called "Porous Coordination Polymers" (PCP's) are hybrid crystalline porous materials consisting of metallic species connected to one another with organic linkers. They display an extremely large range of crystal structures and host-guest properties, which makes them an important class of materials with potentially major impact in adsorption/separation technologies of strategic gas linked with energy supply and environmental problems. The combination of tunable porosity, the functionalization of the internal surface together with the structural flexibility of the host opens the way to an extremely rich host-guest chemistry, putting this class of materials in a unique position.

New MOF's with exciting new properties are being synthesized today at a furious pace. There is little doubt that some of these new materials will be used in the near future in real applications such as sensing, catalysis, molecular recognition, storage/memory effects, storage and transport of energy and matter, and of course selective separation/purification of fluid mixtures. This latter topic is considered as one the most promising applications of MOF's today. But before one can consider a given MOF for a specific industrial process, one has to ascertain the stability of the framework with respect to constraints such a temperature, moisture (hydrolysis), and repeated cycling of the framework (mechanical stability).

The present project aims at disclosing the mechanical and hydrothermal stability of Soft Porous Crystals (SPC's), a sub-class of MOF's that behave in a remarkable guest-responsive fashion. They exhibit a variety of large amplitude dynamic behaviors of their frameworks in response to external stimuli of weak intensity (light, electric field, gas exposure…), maintaining their crystalline character. The issue of the stability of such materials under the external constraints in a given process is particularly important. The large amplitude (reversible) structural change of the framework is most certainly the property that one will want to exploit. In a real process, the reversible change of structure might take place at a very important rate, and this poses the question of the long term mechanical stability of the framework subjected to numerous back and forth "breathing" of its framework. In addition, one also has to take into account such effects as temperature and the presence of traces of water in the system, as it is known that such effects may have an important impact on the materials' ageing. Very few studies exist along these lines in the literature, especially when it comes to investigating the SPC's ageing under the conditions that mimic a true process.

This project aims at understanding and predicting the evolution, degradation and regeneration, of the specific flexibility properties of SPC's exposed to mechanical and hydrothermal constraints. This involves understanding the materials' degradation as well as the regeneration mechanisms at the atomistic level, in order to help rationalizing the search for the best porous material for a given application.
The originality of the present project is to address one of the main upscaling issues, namely the hydrothermal and mechanical stability, from a fundamental, atomistic point of view. The project makes use of newly developed experimental as well as theoretical methods that enable to tackle the complexity of the problem raised.

Project coordination


The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.



Help of the ANR 540,000 euros
Beginning and duration of the scientific project: - 36 Months

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