The over-arching goal of this research is to develop a detailed, molecular level understanding of an emerging class of ultrahigh performance sorbents named Microporous Coordination Polymers (MCPs). MCP sorbent performance derives from their high fractional volume (porosity) of tiny, nanometer-sized pores resulting in exceedingly high specific surface areas. An understanding of how these materials can adsorb such enormous amounts of gases and the chemical/physical basis for selective uptake are the major thrusts of the proposed research. In particular, the fundamental role of pore size, pore architecture, and pore interconnectedness will be observed for the first time during the actual processes of gas adsorption and solvent removal. Enabling this unique viewpoint is the first use in MCPs of positron annihilation lifetime spectroscopy, an antimatter probe of porous materials recently developed to characterize microporous thin film dielectric insulators in microelectronic devices. Adoption of this technique has the potential to transform how researchers probe porosity in sorbents. Only by understanding how changes on the microscale and nanoscale exert an effect on apparent porosity can the best modes of exploiting existing MCPs be realized. Ultimately these results will be of practical use to guide the synthesis of new materials. MCP?s are expected to find broad application in energy research (gas storage) and environmental research (gas purification) and since MCPs are now being commercialized they are at the point where direct impact on society can and will be felt.
NON-TECHNICAL SUMMARY
This research brings together chemists and physicists in an effort to transform the study of ultrahigh performance sorbents?the nanomaterials that are themselves transforming the field of chemical separations. This unique collaboration seeks to use powerful antimatter probe techniques recently developed to study newly engineered porous insulators in microchips to provide unprecedented structural characterization. These highly porous sorbents are expected to find broad application in alternative energy (gas storage) and industrial processes (gas purification) and since they are now beginning to be commercialized they are at the point where direct impact on society will be felt. The impact of this research is totally dependent on the successful interaction of chemists and physicists. Graduate and undergraduate students will cross discipline boundaries to learn in a broadly diverse and collaborative environment involving frequent interaction with industry. For these reasons the potential for obtaining transformative research results is high. However, this will only be possible if the properties of the materials are sufficiently well understood to allow widespread deployment in new more efficient processes. It is clear that sorbent technology has not yet achieved this point, but the proposed research will do much to enable reaching this goal.
While fossil fuels remain the dominant source of energy in the United States, the environmental hazards associated with such fuels, along with their eventual and inevitable depletion are driving scientists to explore alternative options for sustainable, cleaner energy sources. A recently discovered class of materials called "microporous coordination polymers" (MCPs) may play an important role in the future energy landscape of the United States. These materials contain very large amounts of empty space in which different molecular guests can be captured, stored, and released – ideal for storing next-generation fuels such as natural gas or hydrogen, fuels which would otherwise require exceedingly high pressures to store efficiently. These materials are also being examined for their use in energy-efficient industrial chemical separations, the capture and sequestration of CO2 and toxic gases, and environmental chemical remediation. While many of these materials have been made in the laboratory, their properties often do not meet those expected based on molecular-level structural models. As such, a great deal of research has focused on the fabrication of different kinds of MCPs, but little is known about why underwhelming behavior occurs in many cases. We have focused on understanding defects, degradation, and unexpectedly poor performance in MCPs that would otherwise serve as promising new materials for the 21st century. Our foremost tool to examine MCPs is termed "positron annihilation lifetime spectroscopy" (PALS). PALS is a technique based on using antimatter to provide a measure of the empty space in an MCP. Since antimatter will annihilate with matter (converting both to high-energy packets of light), the amount of time antimatter is able to exist in a material can provide a measure of the size of empty (matter-free) spaces in that material. Longer-lived antimatter indicates larger pores, and vice versa. PALS holds a few key advantages over other techniques used to characterize porous materials: first of all, the depth of antimatter implantation into a material can be controlled, so the pore characteristics of that material can be characterized down to the level of a few nanometers (one million times smaller than edge of a dime!). Secondly, the technique allows detection of empty spaces that are physically closed off from the outside of the material, akin to being able to look into a box without having to open it. With this technique, we were able to determine why a number of MCPs behave as they do, and discovered a number of previously unknown physical phenomena along the way. Our first project involved the examination of a well-known MCP, referred to commonly as "MOF-5." While MOF-5 often serves as a benchmark material due to its typically predictable and well-understood performance, we discovered that CO2 uptake in the material isn’t quite as efficient as previously thought due to an inability to pack CO2 molecules into every bit of empty space available. In an MCP denoted "Zn-HKUST-1," we discovered that an inability to fill empty pores comes from a thin barrier layer present just at the material’s surface. In another MCP known as "IRMOF-8," we found that low porosity (deviating from what has been expected from very promising molecular models) is due to two crystals becoming interlocked on the molecular level, thereby filling up half of the space that would otherwise be open and accessible. We also investigated the factors influencing water stability of MCPs with cubic structures. During our examination of MCPs, we also discovered that positronium, the antimatter particle we use to probe MCPs, can act like a wave and spread across hundreds of pores simultaneously. Our work has broad implications for researchers studying MCPs, and has revealed new physical phenomena that will both enable physicists to explore theoretical models and expand the potential for using PALS as a method to study porous materials. Our work spans the fields of chemistry, materials science, and physics, and we have presented it at international conferences in San Francisco (CA), New Orleans (LA), and Waterville Valley (NH). We have also been able to collaborate internationally with researchers in China and most recently, Switzerland. We have published four scientific papers in chemistry and physics journals, and are currently working on further manuscripts to communicate our results to the broader scientific community.
