Understanding negative gas adsorption in highly porous networks for the design of pressure amplifying materials

Porous materials play a key role for air purification, separation of gases and energy storage. Metal-Organic Frameworks (MOFs) stand out as porous materials offering the highest specific surface areas and porosity ever achieved. Engineering the building blocks of MOFs leads to novel dynamic porous materials with the ability to adapt their pore size in response to a guest molecule. AMPLIPORE realizes novel highly porous materials responding by pressure amplification to a molecular stimulus. This counterintuitive phenomenon, also termed "negative gas adsorption" (NGA) is rarely observed among porous solids. AMPLIPORE provides the fundamental understanding and identifies the structural building blocks required to achieve a high degree of pressure amplification. The main objective of the project is to gain a fundamental understanding of pressure amplification phenomena in porous solids, development and design of new scalable NGA materials and exploration of the technology in applications. The fundamental knowledge will open new horizons for designing pressure amplifying porous materials for applications in pneumatic systems or autonomous robots.

At the project outset, only one singular MOF (DUT-49, DUT = Dresden University of Technology) was recognized as pressure amplifying dynamic material for limited gases such as methane and butane. Within the project, conditions were identified to achieve pressure amplification with DUT-49 using a wide variety of gases, temperatures (67 – 308 K) and pressure range, establishing NGA as a general phenomenon. Significant progress towards the fundamental understanding of NGA in DUT-49 was achieved regarding the critical impact of metal content, crystallite size, pore filling mechanism. Systematic study of flexibility and NGA using inert gases and hydrocarbons in the broad pressure and temperature range allows to derive the empirical equations, allowing to predict the NGA and flexibility for a variety of fluids. A crucial methodology developed is in situ adsorption experimentation in parallel to advanced diffraction and spectroscopic techniques, but also sophisticated theoretical calculations allow assessing the conditions, required for NGA. Subsequently, a complete energy surface has been charted indicating the possible pathways and energy barriers. Based on this knowledge, new NGA materials for example DUT-50, DUT-147, DUT-148, DUT-160 and DUT-161 were identified as pressure amplifying porous solids. An exciting fluorescence and optical properties could be observed in DUT-140(Cu). The study on influence of metal ion in the paddle-wheel on the stability of the framework allowed to identify DUT-49(Ni) as potential candidate for further studies of NGA properties. The study of the desolvation mechanism by in situ PXRD confirmed this finding. Significant efforts have been made in terms of identifying the existing and new MOFs and COFs, showing NGA phenomenon, however, the desired stress/strain conditions could not be reached and only adsorption-induced breathing could be reached in DUT-13 and DUT-190 frameworks. In turns, interesting acidochromic effects have been observed in the new covalent-organic frameworks DUT-175 and DUT-176. Due to the softness of this class of materials the NGA phenomenon is not expected. Implementation of photoactive azo-group in the ligand leads to DUT-163 solid showing NGA. At defined pressure and temperature, pressure and gas loading conditions the irradiation of the MOF by UV light triggered the transition in the cp phase. In depth studies of CO2 induced transitions in DUT-49 and DUT-50 proved the concept of pressure amplification devices, based on this solid.

At the project start, one NGA material, denoted as DUT-49, was discovered. The mechanism and origin of this phenomenon was also understood from experimental and theoretical point of view. In the frame of AMPLIPORE project we aim to apply different synthesis strategies, including variation of the ligand length and stiffness, metal nodes, crystal size and defect implementation in order to gain the further understanding and control over NGA. Additionally, we aim to extend our theoretical studies in order to understand the general criteria, required for triggering of NGA. Base on the calculations, we will search for new and existing MOFs and COFs, which may show this phenomenon, involving stable zirconium and aluminum based frameworks. Finally, we aim to build the demonstrator showing repeating NGA ideally at high pressures and room temperature conditions.
Pneumatic systems are ubiquitous in automotive, industrial production control systems, microsystems, pumping technologies and even lab-on chip technologies. An intrinsic characteristic of NGA is the specific pressure and temperature at which a pressure jump (amplification) is observed, a feature that can in principle be used to control pneumatic instrumentation and regulate systems, especially when used as a safety control when this pressure value is surpassed or for signaling. Due to the massive micro- and macroscopic volume changes of NGA materials, a direct read-out of the mechanoresponse and integration into pneumatic actuator technologies can be anticipated. Rational development of such technologies is economically only reasonable based on understanding fundamental NGA principles. AMPLIPORE will provide this understanding and deliver a group of NGA materials to comply with industrially relevant environments in future.

Periodic Reporting for period 4 - AMPLIPORE (Understanding negative gas adsorption in highly porous networks for the design of pressure amplifying materials)

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