Monolithic Zirconium-Based Metal-Organic Frameworks for Energy-Efficient Water Adsorption Applications.

Space cooling and heating, ventilation, and air conditioning (HVAC) accounts for roughly 10% of global electricity use and are responsible for ca. 1.13 gigatonnes of CO2 emissions annually. Adsorbent-based HVAC technologies have long been touted as an energy-efficient alternative to traditional refrigeration systems. However, thus far, no suitable adsorbents have been developed which overcome the drawbacks associated with traditional sorbent materials such as silica gels and zeolites. Metal-organic frameworks (MOFs) offer order-of-magnitude improvements in water adsorption and regeneration energy requirements. However, the deployment of MOFs in HVAC applications has been hampered by issues related to MOF powder processing. Herein, three high-density, shaped, monolithic MOFs (UiO-66, UiO-66-NH2 , and Zr-fumarate) with exceptional volumetric gas/vapor uptake are developed-solving previous issues in MOF-HVAC deployment. The monolithic structures across the mesoporous range are visualized using small-angle X-ray scattering and lattice-gas models, giving accurate predictions of adsorption characteristics of the monolithic materials. It is also demonstrated that a fragile MOF such as Zr-fumarate can be synthesized in monolithic form with a bulk density of 0.76 gcm-3 without losing any adsorption performance, having a coefficient of performance (COP) of 0.71 with a low regeneration temperature (≤ 100 °C).

Densified HKUST-1 Monoliths as a Route to High Volumetric and Gravimetric Hydrogen Storage Capacity.

We are currently witnessing the dawn of hydrogen (H2) economy, where H2 will soon become a primary fuel for heating, transportation, and long-distance and long-term energy storage. Among diverse possibilities, H2 can be stored as a pressurized gas, a cryogenic liquid, or a solid fuel via adsorption onto porous materials. Metal-organic frameworks (MOFs) have emerged as adsorbent materials with the highest theoretical H2 storage densities on both a volumetric and gravimetric basis. However, a critical bottleneck for the use of H2 as a transportation fuel has been the lack of densification methods capable of shaping MOFs into practical formulations while maintaining their adsorptive performance. Here, we report a high-throughput screening and deep analysis of a database of MOFs to find optimal materials, followed by the synthesis, characterization, and performance evaluation of an optimal monolithic MOF (monoMOF) for H2 storage. After densification, this monoMOF stores 46 g L-1 H2 at 50 bar and 77 K and delivers 41 and 42 g L-1 H2 at operating pressures of 25 and 50 bar, respectively, when deployed in a combined temperature-pressure (25-50 bar/77 K → 5 bar/160 K) swing gas delivery system. This performance represents up to an 80% reduction in the operating pressure requirements for delivering H2 gas when compared with benchmark materials and an 83% reduction compared to compressed H2 gas. Our findings represent a substantial step forward in the application of high-density materials for volumetric H2 storage applications.

Turning Molecular Springs into Nano-Shock Absorbers: The Effect of Macroscopic Morphology and Crystal Size on the Dynamic Hysteresis of Water Intrusion-Extrusion into-from Hydrophobic Nanopores.

Controlling the pressure at which liquids intrude (wet) and extrude (dry) a nanopore is of paramount importance for a broad range of applications, such as energy conversion, catalysis, chromatography, separation, ionic channels, and many more. To tune these characteristics, one typically acts on the chemical nature of the system or pore size. In this work, we propose an alternative route for controlling both intrusion and extrusion pressures via proper arrangement of the grains of the nanoporous material. To prove the concept, dynamic intrusion-extrusion cycles for powdered and monolithic ZIF-8 metal-organic framework were conducted by means of water porosimetry and in operando neutron scattering. We report a drastic increase in intrusion-extrusion dynamic hysteresis when going from a fine powder to a dense monolith configuration, transforming an intermediate performance of the ZIF-8 + water system (poor molecular spring) into a desirable shock-absorber with more than 1 order of magnitude enhancement of dissipated energy per cycle. The obtained results are supported by MD simulations and pave the way for an alternative methodology of tuning intrusion-extrusion pressure using a macroscopic arrangement of nanoporous material.

Monolithic metal-organic frameworks for carbon dioxide separation.

Carbon dioxide (CO2) is both a primary contributor to global warming and a major industrial impurity. Traditional approaches to carbon capture involve corrosive and energy-intensive processes such as liquid amine absorption. Although adsorptive separation has long been a promising alternative to traditional processes, up to this point there has been a lack of appropriate adsorbents capable of capturing CO2 whilst maintaining low regeneration energies. In the context of CO2 capture, metal-organic frameworks (MOFs) have gained much attention in the past two decades as potential materials. Their tuneable nature allows for precise control over the pore size and chemistry, which allows for the tailoring of their properties for the selective adsorption of CO2. While many candidate materials exist, the amount of research into material shaping for use in industrial processes has been limited. Traditional shaping strategies such as pelletisation involve the use of binders and/or mechanical processes, which can have a detrimental impact on the adsorption properties of the resulting materials or can result in low-density structures with low volumetric adsorption capacities. Herein, we demonstrate the use of a series of monolithic MOFs (monoUiO-66, monoUiO-66-NH2 & monoHKUST-1) for use in gas separation processes.

Bifunctional Pyridinium-Based Ionic-Liquid-Immobilized Diindium Tris(diphenic acid) Bis(1,10-phenanthroline) for CO2 Fixation.

A pyridinium-based ionic-liquid-decorated 1 D metal-organic framework (MOF; IL-[In2 (dpa)3 (1,10-phen)2 ]; IL=ionic liquid; dpa=diphenic acid; 1,10-phen=1,10-phenanthroline) was developed as a bifunctional heterogeneous catalyst system for CO2 -oxirane coupling reactions. An aqueous-microwave route was employed to perform the hydrothermal reaction for the synthesis of the [In2 (dpa)3 (1,10-phen)2 ] MOF, and the IL-[In2 (dpa)3 (1,10-phen)2 ] catalyst was synthesized by covalent postfunctionalization. As a result of the synergetic effect of the dual-functional sites, which include Lewis acid sites (coordinatively unsaturated In sites) and the I- ion in the IL functional sites, IL-[In2 (dpa)3 (1,10-phen)2 ] displayed a high catalytic activity for CO2 -epoxide cycloaddition reactions under mild and solvent-free conditions. Microwave pulses were employed for the first time in MOF-catalyzed CO2 -epoxide cycloaddition reactions to result in a high turnover frequency of 2000-3100 h-1 . The catalyst had an excellent reusability and maintained a continuous high selectivity. Furthermore, only a small amount of leaching was observed from the spent catalyst. A plausible reaction mechanism based on the synergistic effect of the dual-functional sites that catalyze the CO2 -epoxide cycloaddition reaction effectively is proposed.

Rapid, Microwave-Assisted Synthesis of Cubic, Three-Dimensional, Highly Porous MOF-205 for Room Temperature CO2 Fixation via Cyclic Carbonate Synthesis.

A dual-porous, three-dimensional, metal-organic framework [Zn4O(2,6-NDC)(BTB)4/3] (MOF-205, BET = 4200 m2/g) has been synthesized using microwave power as an alternative energy source for the first time, and its catalytic activity has been exploited for CO2-epoxide coupling reactions to produce five-membered cyclic carbonates under solvent-free conditions. Microwave synthesis was performed at different time intervals to reveal the formation of the crystals. Significant conversion of various epoxides was obtained at room temperature, with excellent selectivity toward the desired five-membered cyclic carbonates. The importance of the dual porosity and the synergistic effect of quaternary ammonium salts on efficiently catalyzed CO2 conversion were investigated using various experimental and physicochemical characterization techniques, and the results were compared with those of the solvothermally synthesized MOF-205 sample. On the basis of literature and experimental inferences, a rationalized mechanism mediated by the zinc center of MOF-205 for the CO2-epoxide cycloaddition reaction has been proposed.

A sustainable protocol for the facile synthesis of zinc-glutamate MOF: an efficient catalyst for room temperature CO2 fixation reactions under wet conditions.

A water stable zinc-MOF (ZnGlu) catalyst was facilely prepared from the proteinogenic amino acid, l-glutamic acid at room temperature in aqueous medium. CO2 fixations were promoted by the ZnGlu catalyst's inherently coordinated water and externally added water in yielding cyclic carbonate and cyclic urethane at room temperature. This eliminates the need for catalyst activation, making ZnGlu a ready-to-use catalyst. The enhanced CO2 cycloaddition with added water hints at the application of ZnGlu in wet flue gas conversions. This is the first reported attempt for the use of an MOF in the cycloaddition of aziridine and CO2.