Engineered Nanoporous Frameworks for Adsorption Cooling Applications.

The energy demand for traditional vapor-compressed technology for space cooling continues to soar year after year due to global warming and the increasing human population's need to improve living and working conditions. Thus, there is a growing demand for eco-friendly technologies that use sustainable or waste energy resources. This review discusses the properties of various refrigerants used for adsorption cooling applications followed by a brief discussion on the thermodynamic cycle. Next, sorbents traditionally used for cooling are reviewed to emphasize the need for advanced capture materials with superior properties to improve refrigerant sorption. The remainder of the review focus on studies using engineered nanoporous frameworks (ENFs) with various refrigerants for adsorption cooling applications. The effects of the various factors that play a role in ENF-refrigerant pair selection, including pore structure/dimension/shape, morphology, open-metal sites, pore chemistry and possible presence of defects, are reviewed. Next, in-depth insights into the sorbent-refrigerant interaction, and pore filling mechanism gained through a combination of characterization techniques and computational modeling are discussed. Finally, we outline the challenges and opportunities related to using ENFs for adsorption cooling applications and provide our views on the future of this technology.

Molecular-level insight into the chlorofluorocarbons adsorption by defective covalent organic polymers.

Halocarbons have important industrial applications, however they contribute to global warming and the fact that they can cause ozone depletion. Hence, the techniques that can capture and recover the used halocarbons with energy efficiency methods have recently received greater attention. In this contribution, we report the capture of dichlorodifluoromethane (R12), which has high global warming and ozone depletion potential, using covalent organic polymers (COPs). The defect-engineered COPs were synthesized and demonstrated outstanding sorption capacities, ~226 wt% of R12 combined with linear-shaped adsorption isotherms. We further identified the plausible microscopic adsorption mechanism of the investigated COPs via grand canonical Monte Carlo simulations applied to non-defective and a collection of atomistic models of the defective COPs. The modeling work suggests that significant R12 adsorption is attributed to a gradual increment of porosities due to isolated/interconnected micro-/meso-pore channels and the change of the long-range ordering of both COPs. The successive hierarchical-pore-filling mechanism promotes R12 molecular adsorption via moderate van der Waals adsorbate-adsorbent interactions in the micropores of both COPs at low pressure followed by adsorbate-adsorbate interactions in the extra-voids created at moderate to high pressure ranges. This continuous pore-filling mechanism makes defective COPs as promising sorbents for halocarbon adsorption.

In Situ Raman Methodology for Online Analysis of CO2 and H2O Loadings in a Water-Lean Solvent for CO2 Capture.

Carbon capture represents a key pathway to meeting climate change mitigation goals. Powerful next-generation solvent-based capture processes are under development by many researchers, but optimization and testing would be significantly aided by integrating in situ monitoring capability. Further, real-time water analysis in water-lean solvents offers the potential to maintain their water balance in operation. To explore data acquisition techniques in depth for this purpose, Raman spectra of CO2, H2O, and a single-component water-lean solvent, N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2-EEMPA) were collected at different CO2 and H2O concentrations using an in situ Raman cell. The quantification of CO2 and H2O loadings in 2-EEMPA was done by principal component regression and partial least squares methods with analysis of uncertainties. We conclude with discussions on how this simultaneous online analysis method to quantify CO2 and H2O loadings can be an important tool to enable the optimal efficiency of water-lean CO2 solvents while also maintaining the critical water balance under operating conditions relevant to post-combustion CO2 capture.

Manipulating Pore Topology and Functionality to Promote Fluorocarbon-Based Adsorption Cooling.

ConspectusWith the worldwide demand for refrigeration and cooling expected to triple, it is increasingly important to search for alternative energy resources to drive refrigeration cycles with reduced electricity consumption. Recently, adsorption cooling has gained increased attention since energy reallocation in such systems is based on gas adsorption/desorption, which can be driven by waste/natural heat sources. Eco-friendly sorption-based cooling relies on the cyclic transfer of refrigerant gas from a high to low energy state by the pseudocompression effect resulting from adsorption and desorption. The driving force for energy transfer relies on heat rather than electricity. The performance of a sorption chiller is primarily influenced by this cyclic sorption behavior, which is characterized as the working capacity of the porous sorbent. Thus, increases in this working capacity directly translate to a more compact and efficient cooling system. However, a lack of highly effective sorbent/refrigerant pairs lowers cooling performance and therefore has limited applicability. To this end, synthetic metal-organic frameworks (MOFs) and covalent organic polymers (COPs) possess higher porosity and greater tunability leading to more substantial potential benefits for adsorption, compared to traditional sorbent materials. Similarly, hydrofluorocarbon refrigerants have more favorable applicability given the ease of operation above atmospheric pressures due to suitable saturated vapor pressures and boiling points. For these reasons, our work focuses on an ongoing strategy to promote sorption cooling via improvements in the sorbent/refrigerant pair. Specifically, we target the interaction of hydrofluorocarbon refrigerants with MOF/COP materials at a molecular level by interpreting the host-guest chemistry and the role of framework pore topology. These molecular-level differences translate to cooling performance, which is described herein. These strategies include engineering framework porosity (i.e., pore size, pore volume) by using elongated organic linkers and stereochemistry control during synthesis; manipulating the sorbate/sorbent interaction by introducing functional moieties or unsaturated metal centers to enhance working capacities in narrow pressure ranges; varying pore topology/morphology to impact adsorption isotherm behavior; and leveraging defective sites within the frameworks to further enhance adsorption capability. This atomic level understanding of sorbate-sorbent interactions is conducted using various in situ experimental techniques such as synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, in situ Fourier transform infrared spectroscopy, and direct sorption energies determinization with calorimetry. Moreover, the experimentally studied interactions and the corresponding adsorption mechanism are corroborated by computational studies using density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations. Using this approach, we have made strides toward engineering designed frameworks with precise molecular control to target refrigerant molecules and thereby enhance the performance of desired working pairs for sorption-based cooling.

Designing Porous Ion Emitters for Thermal Ionization Mass Spectrometry: Evaluating Metal-Organic Frameworks.

This work describes the first exploration of metal-organic frameworks (MOFs) as "next-generation" ion emitters for thermal ionization mass spectrometry (TIMS). MOFs were identified as promising candidates for this application given the synthetic control over their desired structural properties. This tunability results in well-ordered, high-surface-area, high-porosity frameworks with targeted sorption affinities. Here, we explored an aluminum-based, bipyridine-containing MOF (MOF-253) with and without incorporating a high work function metal, rhenium (Re). After analysis of an Nd-bearing MOF, we hypothesized that the well-dispersed, sponge-like interconnected network of the degraded structure would enhance Nd ionization more than traditional TIMS loading techniques (i.e., phosphoric acid). Compared to filaments loaded with phosphoric acid that require an additional benzene carburization step, the Nd ionization efficiencies (atoms detected relative to atoms loaded) for heated filaments loaded with MOF-253 were similar (∼1%). Electron microscopy after TIMS analysis demonstrated that the MOF was retained on the filament. While these results are preliminary, they demonstrate that MOFs have potential to enhance ionization and exceed the performance of traditional loading techniques by forming nanoporous ion emitters. Thus, further experimentation is likely to exceed this performance through more specific selection of the base MOF structure and modifications to porosity and composition. This work represents a novel application of MOFs and a next step in the pursuit of advanced thermal ionization with potential to expand across the periodic table.

The Surge of Metal-Organic-Framework (MOFs)-Based Electrodes as Key Elements in Electrochemically Driven Processes for the Environment.

Metal-organic-frameworks (MOFs) are emerging materials used in the environmental electrochemistry community for Faradaic and non-Faradaic water remediation technologies. It has been concluded that MOF-based materials show improvement in performance compared to traditional (non-)faradaic materials. In particular, this review outlines MOF synthesis and their application in the fields of electron- and photoelectron-Fenton degradation reactions, photoelectrocatalytic degradations, and capacitive deionization physical separations. This work overviews the main electrode materials used for the different environmental remediation processes, discusses the main performance enhancements achieved via the utilization of MOFs compared to traditional materials, and provides perspective and insights for the further development of the utilization of MOF-derived materials in electrified water treatment.

Porous Covalent Organic Polymers for Efficient Fluorocarbon-Based Adsorption Cooling.

Adsorption-based cooling is an energy-efficient renewable-energy technology that can be driven using low-grade industrial waste heat and/or solar heat. Here, we report the first exploration of fluorocarbon adsorption using porous covalent organic polymers (COPs) for this cooling application. High fluorocarbon R134a equilibrium capacities and unique overall linear-shaped isotherms are revealed for the materials, namely COP-2 and COP-3. The key role of mesoporous defects on this unusual adsorption behavior was demonstrated by molecular simulations based on atomistic defect-containing models built for both porous COPs. Analysis of simulated R134a adsorption isotherms for various defect-containing atomistic models of the COPs shows a direct correlation between higher fluorocarbon adsorption capacities and increasing pore volumes induced by defects. Combined with their high porosities, excellent reversibility, fast kinetics, and large operating window, these defect-containing porous COPs are promising for adsorption-based cooling applications.

Molecular Insight into Fluorocarbon Adsorption in Pore Expanded Metal-Organic Framework Analogs.

The rapid growth in the global energy demand for space cooling requires the development of more efficient environmental chillers for which adsorption-based cooling systems can be utilized. Here, in this contribution, we explore sorbents for chiller use via a pore-engineering concept to construct analogs of the 1-dimensional pore metal-organic framework MOF-74 by using elongated organic linkers and stereochemistry control. The prepared pore-engineered MOFs show remarkable equilibrium adsorption of the selected fluorocarbon refrigerant that is translated to a modeled adsorption-based refrigeration cycle. To probe molecular level interactions at the origin of these unique adsorption properties for this series of Ni-MOFs, we combined in situ synchrotron X-ray powder diffraction, neutron powder diffraction, X-ray absorption spectroscopy, calorimetry, Fourier transform infrared techniques, and molecular simulations. Our results reveal the coordination of fluorine (of CH2F in R134a) to the nickel(II) open metal centers at low pressures for each Ni-MOF analog and provide insight into the pore filling mechanism for the full range of the adsorption isotherms. The newly designed Ni-TPM demonstrates exceptional R134a adsorption uptake compared to its parent microporous Ni-MOF-74 due to larger engineered pore size/volume. The application of this adsorption performance toward established chiller conditions yields a working capacity increase for Ni-TPM of about 400% from that of Ni-MOF-74, which combined with kinetics directly correlates to both a higher coefficient of performance and a higher average cooling capacity generated in a modeled chiller.

Molecular Intermediate in the Directed Formation of a Zeolitic Metal-Organic Framework.

Directed synthesis promises control over architecture and function of framework materials. In practice, however, designing such syntheses requires a detailed understanding of the multistep pathways of framework formations, which remain elusive. By identifying intermediate coordination complexes, this study provides insights into the complex role of a structure-directing agent (SDA) in the synthetic realization of a promising material. Specifically, a novel molecular intermediate was observed in the formation of an indium zeolitic metal-organic framework (ZMOF) with a sodalite topology. The role of the imidazole SDA was revealed by time-resolved in situ powder X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS).

Transition-Metal Nitroprussides Examined for Water Harvesting and Sorption Cooling.

Transition-metal pentacyanonitrosylferrates, commonly known as nitroprussides, have a long and documented history. Here, we synthesize cobalt and nickel nitroprussides (NPs) in order to probe their use as sorbents for water and fluorocarbon uptake for potential water harvesting and cooling applications. These NPs show stable and reversible equilibrium sorption isotherms at room temperature with peak uptake values of ∼40 wt % for H2O and ∼30 wt % for fluorocarbon R134a. At water harvesting conditions, working capacities of ∼19 wt % were obtained for NPs. At sorption cooling conditions, the working capacities favored nickel NP. Given the advantages of an easy, inexpensive, and scalable synthesis, this study demonstrates the potential for using nitroprussides for future water harvesting and adsorption cooling systems.

Probing the Sorption of Perfluorooctanesulfonate Using Mesoporous Metal-Organic Frameworks from Aqueous Solutions.

One approach to reduce increasing concentrations of toxic per- and polyfluoroalkyl substances (PFAS) involves the capture of PFAS from aqueous media using porous materials. The use of highly porous, tunable metal organic framework (MOF) materials is appealing for targeted liquid phase sorption. In this work, we demonstrate the excellent capture of perfluorooctanesulfonate (PFOS) using both the chromium and iron analogs of the MIL-101 framework. Experimental characterization of PFOS uptake reveals unique differences in sorption properties between these two analogs, providing key implications for future PFOS sorbent design. Specifically, STEM-EDS and IR spectroscopy show definitive proof of sorption. Furthermore, XPS analysis shows evidence of a strong interaction between sulfur atoms of the polar headgroup of PFOS and the metal center of the framework in addition to the fluorinated nonpolar tail. Additionally, in situ 19F NMR reveals higher PFOS affinity for Cr-MIL-101 versus Fe-MIL-101 based on sorption kinetics. Surprisingly, at these relatively high PFOS concentrations, activated acetylene black carbon is severely outperformed by both MOFs.

Role of Transition Metals in Metal-Organic Frameworks as Nanoporous Ion Emitters for Thermal Ionization Mass Spectrometry.

Thermal ionization mass spectrometry is a powerful analytical technique that allows for precise determination of isotopic ratios. Analysis of low abundance samples, however, can be limited by the ionization efficiency. Following an investigation into a new type of metal-organic hybrid material, nanoporous ion emitters (nano-PIEs), devised to promote the emission of analyte ions and reduce traditional sample loading challenges, this work evaluates the impact that changing the metal in the material has on the ionization of uranium (U). Being derived from metal-organic frameworks (MOFs), nano-PIEs inherit the tunability of their parent MOFs. The MOF-74 series has been well studied for probing the impact various framework metals (i.e., Mg, Mn, Co, Ni, Cu, Zn, and Cd) have on material properties, and thus, a series of nano-PIEs with different metals were derived from this isoreticular MOF series. Trends in ionization efficiency were studied as a function of ionization potential, volatility, and work function of the framework metals to gain a better understanding of the mechanism of analyte ionization. This study finds a correlation between the analyte ionization efficiency and nano-PIE framework metal volatility that is attributed to its tunable thermal stability and degradation behavior.

Organoalkoxysilane-grafted silica composites for acidic and basic gas adsorption.

With the prevalence of air quality issues in our society, the ability to remove toxic gases from air is a necessity. This work addresses the development of biphasic, nanostructured, organoalkoxysilane-grafted, siliceous materials for use in single pass filters of various types for the removal of acidic and basic gases from humid air. Materials exhibit high single pass capacities for sulfur dioxide, a representative acid-forming gas, or ammonia, a representative basic gas. The nanostructured siliceous support provides initial ammonia capacity, and grafted amine and carbonyl groups provide desired functional chemistries for sulfur dioxide and enhanced ammonia capacities. Methacryloxypropyltrimethoxysilane (MAPS)-MCM-41 has the highest ammonia capacity at about 7 mol/kg at 1500 ppmv and 23 °C, and 3-aminopropyltriethoxysilane (APTES)-MCM-41 has the highest sulfur dioxide capacity at 0.85 mol/kg at 500 ppmv and 23 °C. These biphasic materials exhibit high adsorption capacity for two distinct gases and are promising candidates as adsorbents for protection from toxic industrial gases.

Structure-Property Correlation of Hierarchically Porous Carbons for Fluorocarbon Adsorption.

Although traditional commercially available porous carbon-fluorocarbon working pairs have shown promising applicability for adsorption cooling, advancements in engineered carbons may further improve the performance. Moreover, insights into structure-property relationships that target higher sorption capacities within these synthesized carbons may guide such materials' future design. We utilized hierarchically porous carbons (HPCs), synthesized with colossal microporous and mesoporous content characterized by high surface areas (up to 2689 m2/g) and pore volume values (up to 10.31 cm3/g) toward fluorocarbon R134a adsorption. This unique pore topology leads to exceptional R134a uptake, ∼250 wt %, outperforming the highest uptake carbon material to date, Maxsorb III (∼220 wt %). Material characterizations reveal that the outstanding R134a capacity may be attributed to textural properties and oxygen-terminated functional groups more than graphitization of the material. Most importantly, HPCs are efficiently utilized in a two-bed model chiller device, where the performance shows excellent working capacity (105 wt %, ∼2 times the value of reported carbon materials/R134a). Fluorocarbon adsorption on HPCs also displays fast kinetics (equilibrium time: ∼2 min) mainly driven by physical adsorption (Qst: ∼27 kJ/mol), characteristic of swiftly reversible behavior adsorption-desorption behaviors. This work provides a fundamental understanding of the applicability of HPCs/R134a working pair for adsorption cooling.

Understanding Time Dependence on Zinc Metal-Organic Framework Growth Using in Situ Liquid Secondary Ion Mass Spectrometry.

The abundance of novel metal-organic framework (MOF) materials continues to increase as more applications are discovered for these highly porous, well-ordered crystalline structures. The simplicity of constituents allows for the design of new MOFs with virtue of functionality and pore topology toward target adsorbates. However, the fundamental understanding of how these frameworks evolve during nucleation and growth is mostly limited to speculation from simulation studies. In this effort, we utilize a unique vacuum compatible system for analysis at the liquid vacuum interface (SALVI) microfluidic interface to analyze the formation and evolution of the benchmark MOF-74 framework using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Principal component analysis of the SIMS mass spectra, together with ex situ electron microscopy, powder X-ray diffractometry, and porosimetry, provides new insights into the structural growth, metal-oxide cluster formation, and aging process of Zn-MOF-74. Samples collected over a range of synthesis times and analyzed closely with in situ ToF-SIMS, transmission electron microscopy, and gas adsorption studies verify the developing pore structure during the aging process.

Kinetics and Mechanisms of ZnO to ZIF-8 Transformations in Supercritical CO2 Revealed by In†Situ X-ray Diffraction.

ZIF-8 was synthesized in supercritical carbon dioxide (scCO2 ). In situ powder X-ray diffraction, ex situ microscopy, and simulations provide an encompassing view of the formation of ZIF-8 and intermediary ZnO@ZIF-8 composites in this nontraditional solvent. Time-resolved imaging exposed divergent physicochemical reaction pathways from previous studies of the growth of anisotropic ZIF-8 core@shell structures in traditional solvents. Synthetically relevant physiochemical properties of scCO2 were integrated into classical nucleation theory, relating interfacial forces, calculated through DFTB+ based molecular dynamics (MD), with 3D nucleation outcomes. The kinetics of crystallization were examined and displayed a characteristic signature of time- and temperature-dependent mechanisms over the extent of the reaction. Lastly, it is shown that subtle factors, such as the extent of reaction and the size/shape of sacrificial templates can tailor ZIF-8 composition and size, eliciting control over hierarchical porosity in a nonconventional green solvent.

Metal-Organic Framework-Based Microfluidic Impedance Sensor Platform for Ultrasensitive Detection of Perfluorooctanesulfonate.

The growing global concerns to public health from human exposure to perfluorooctanesulfonate (PFOS) require rapid, sensitive, in situ detection where current, state-of-the-art techniques are yet to adequately meet sensitivity standards of the real world. This work presents, for the first time, a synergistic approach for the targeted affinity-based capture of PFOS using a porous sorbent probe that enhances detection sensitivity by embedding it on a microfluidic platform. This novel sorbent-containing platform functions as an electrochemical sensor to directly measure PFOS concentration through a proportional change in electrical current (increase in impedance). The extremely high surface area and pore volume of mesoporous metal-organic framework (MOF) Cr-MIL-101 is used as the probe for targeted PFOS capture based on the affinity of the chromium center toward both the fluorine tail groups as well as the sulfonate functionalities as demonstrated by spectroscopic (NMR and XPS) and microscopic (TEM) studies. Answering the need for an ultrasensitive PFOS detection technique, we are embedding the MOF capture probes inside a microfluidic channel, sandwiched between interdigitated microelectrodes (IDµE). The nanoporous geometry, along with interdigitated microelectrodes, increases the signal-to-noise ratio tremendously. Further, the ability of the capture probes to interact with the PFOS at the molecular level and effectively transduce that response electrochemically has allowed us achieve a significant increase in sensitivity. The PFOS detection limit of 0.5 ng/L is unprecedented for in situ analytical PFOS sensors and comparable to quantification limits achieved using state-of-the-art ex situ techniques.

Insight into Fluorocarbon Adsorption in Metal-Organic Frameworks via Experiments and Molecular Simulations.

The improvement in adsorption/desorption of hydrofluorocarbons has implications for many heat transformation applications such as cooling, refrigeration, heat pumps, power generation, etc. The lack of chlorine in hydrofluorocarbons minimizes the lasting environmental damage to the ozone, with R134a (1,1,1,2-tetrafluoroethane) being used as the primary industrial alternative to commonly used Freon-12. The efficacy of novel adsorbents used in conjunction with R134a requires a deeper understanding of the host-guest chemical interaction. Metal-organic frameworks (MOFs) represent a newer class of adsorbent materials with significant industrial potential given their high surface area, porosity, stability, and tunability. In this work, we studied two benchmark MOFs, a microporous Ni-MOF-74 and mesoporous Cr-MIL-101. We employed a combined experimental and simulation approach to study the adsorption of R134a to better understand host-guest interactions using equilibrium isotherms, enthalpy of adsorption, Henry's coefficients, and radial distribution functions. The overall uptake was shown to be exceptionally high for Cr-MIL-101, >140 wt% near saturation while >50 wt% at very low partial pressures. For both MOFs, simulation data suggest that metal sites provide preferable adsorption sites for fluorocarbon based on favorable C-F ··· M+ interactions between negatively charged fluorine atoms of R134a and positively charged metal atoms of the MOF framework.

Dynamic Adsorption of CO2/N2 on Cation-Exchanged Chabazite SSZ-13: A Breakthrough Analysis.

Alkali-exchanged SSZ-13 adsorbents were investigated for their applicability in separating N2 from CO2 in flue gas streams using a dynamic breakthrough method. In contrast to IAST calculations based on equilibrium isotherms, K+ exchanged SSZ-13 was found to yield the best N2 productivity, comparable to Ni-MOF-74, under dynamic conditions where diffusion properties play a significant role. This was attributed to the selective, partial blockage of access to the chabazite cavities, enhancing the separation potential in a 15/85 CO2/N2 binary gas mixture.

The effect of pyridine modification of Ni-DOBDC on CO2 capture under humid conditions.

The metal-organic framework Ni-DOBDC was modified with pyridine molecules to make the normally hydrophilic internal surface more hydrophobic. Experiments and molecular simulations show that the pyridine modification reduces H2O adsorption while retaining substantial CO2 capacity under the conditions of interest for carbon capture from flue gas.