Recent advances on simulation and theory of hydrogen storage in metal–organic frameworks and covalent organic frameworks

This critical review covers the application of computer simulations, including quantum calculations (ab initio and DFT), grand canonical Monte-Carlo simulations, and molecular dynamics simulations, to the burgeoning area of the hydrogen storage by metal–organic frameworks and covalent-organic frameworks. This review begins with an overview of the theoretical methods obtained from previous studies. Then strategies for the improvement of hydrogen storage in the porous materials are discussed in detail. The strategies include appropriate pore size, impregnation, catenation, open metal sites in metal oxide parts and within organic linker parts, doping of alkali elements onto organic linkers, substitution of metal oxide with lighter metals, functionalized organic linkers, and hydrogen spillover (186 references)

Electronic chemical potentials of porous metal-organic frameworks

The binding energy of an electron in a material is a fundamental characteristic, which determines a wealth of important chemical and physical properties. For metal-organic frameworks this quantity is hitherto unknown. We present a general approach for determining the vacuum level of porous metal-organic frameworks and apply it to obtain the first ionisation energy for six prototype materials including zeolitic, covalent and ionic frameworks. This approach for valence band alignment can explain observations relating to the electrochemical, optical and electrical properties of porous frameworks

Postsynthetic modification of zirconium metal-organic frameworks

Metal-organic frameworks (MOFs) have been in the spotlight for a number of years due to their chemical and topological versatility. As MOF research has progressed, highly functionalised materials have become desirable for specific applications, and in many cases the limitations of direct synthesis have been realised. This has resulted in the search for alternative synthetic routes, with postsynthetic modification (PSM), a term used to collectively describe the functionalisation of pre-synthesised MOFs whilst maintaining their desired characteristics, becoming a topic of interest. Advances in the scope of reactions performed are reported regularly; however reactions requiring harsh conditions can result in degradation of the framework. Zirconium-based MOFs present high chemical, thermal and mechanical stabilities, offering wider opportunities for the scope of reaction conditions that can be tolerated, which has seen a number of successful examples reported. This microreview discusses pertinent examples of PSM resulting in enhanced properties for specific applications, alongside fundamental transformations, which are categorised broadly into covalent modifications, surface transformations, metalations, linker and metal exchange, and cluster modifications

Solar Air Conditioning with Metal Organic Frameworks

Air conditioning is responsible for 5% of energy consumption in the United States as is increasingly in demand across the world as the global middle class continues to grow in size. During hotter months, electricity used to power cooling systems becomes taxing on electric grids, constituting approximately 40% of peak power demand. Traditional air conditioning (AC) systems are also associated with harmful environmental impacts. Both refrigerants used for cooling and fossil fuels used in power contribute to global warming by acting as green-house gases (GHG). Due to the negative effects associated with emissions, the ultimate goal of this research is to drastically reduce non-renewable energy consumption associated with AC units. Generations of technologies have been developed to address this ongoing issue. An emerging solution involves the integration of metal-organic frameworks (MOFs) sorbents into a solar air conditioning system. Because of MOF properties, this integration allows for a thermally driven cycle without requiring a non-renewable energy input. This thesis is comprised of six chapters geared towards assisting in the determination of the most efficient and effective means of incorporation of MOFs into AC systems. Primarily by conducting an extensive literature review, the third chapter discusses Metal Organic Frameworks in depth for determining the most suitable candidates for this research project. Specific needs for the system are examined with different MOFs that meet the criteria considered. In chapter four, feasibility of integrating MOFs into a membrane through sorption measurements is tested for candidate MOF CAU-10. Chapter five is centered around modeling a MOF-assisted indirect evaporative cooler using EES: Engineering Equation Solver. Modeling outputs give a preliminary understanding of the cooling process and its effect on temperature. Together, these chapters move toward showing the feasibility of operation and its applicability to the field of renewable AC. The study of MOF attributes in Chapter 3 focused on Relative Humidity (RH) at which the MOFs demonstrated a steep water uptake, water adsorption capacity, temperatures for MOF regeneration, long term stability, and cost to synthesize and fabricate. These investigations showed Co2Cl2(BTDD), MIL-101, MIL-100(M), MOF-841, and CAU-10 to be the most promising applicants. Through sorption measurements of MOF material CAU-10 its isotherm demonstrated a capacity at the adsorption step below 0.30 gH2O/gMOF but a maximum capacity over 0.5 gH2O/gMOF. The EES model results showed 80-90% of recycled air provides a supply temperature necessary for indoor cooling below 21 oC. Chapter six summarizes all results and gives recommendations focused on thermodynamic optimization.No embargoAcademic Major: Mechanical Engineerin

An Ising Model for Metal-Organic Frameworks

We present a three-dimensional Ising model where lines of equal spins are frozen in such that they form an ordered framework structure. The frame spins impose an external field on the rest of the spins (active spins). We demonstrate that this "porous Ising model" can be seen as a minimal model for condensation transitions of gas molecules in metal-organic frameworks. Using Monte Carlo simulation techniques, we compare the phase behavior of a porous Ising model with that of a particle-based model for the condensation of methane (CH$_4$) in the isoreticular metal-organic framework IRMOF-16. For both models, we find a line of first-order phase transitions that end in a critical point. We show that the critical behavior in both cases belongs to the 3D Ising universality class, in contrast to other phase transitions in confinement such as capillary condensation.Comment: 11 pages, 9 figure

Selective gas capture via kinetic trapping

Conventional approaches to the capture of CO_2 by metal-organic frameworks focus on equilibrium conditions, and frameworks that contain little CO_2 in equilibrium are often rejected as carbon-capture materials. Here we use a statistical mechanical model, parameterized by quantum mechanical data, to suggest that metal-organic frameworks can be used to separate CO_2 from a typical flue gas mixture when used under {\em nonequilibrium} conditions. The origin of this selectivity is an emergent gas-separation mechanism that results from the acquisition by different gas types of different mobilities within a crowded framework. The resulting distribution of gas types within the framework is in general spatially and dynamically heterogeneous. Our results suggest that relaxing the requirement of equilibrium can substantially increase the parameter space of conditions and materials for which selective gas capture can be effected.Comment: 12 pages, 10 figure

Designing Kitaev spin liquids in metal-organic frameworks

Kitaev's honeycomb lattice spin model is a remarkable exactly solvable model, which has a particular type of spin liquid (Kitaev spin liquid) as the ground state. Although its possible realization in iridates and $\alpha$-RuCl$_3$ has been vigorously discussed recently, these materials have substantial non-Kitaev direct exchange interactions and do not have a spin liquid ground state. We propose metal-organic frameworks (MOFs) with Ru$^{3+}$ (or Os$^{3+}$) forming the honeycomb lattice as promising candidates for a more ideal realization of Kitaev-type spin models where the direct exchange interaction is strongly suppressed. The great flexibility of MOFs allows generalization to other three-dimensional lattices, for potential realization of a variety of spin liquids such as a Weyl spin liquid.Comment: 6+11 pages, 2+7 figures, 0+1 table; the final submitted version to appear in PR

Modeling Lewis catalyzed reactions in Metal Organic Frameworks

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Flexibility in metal–organic frameworks : a basic understanding

Much has been written about the fundamental aspects of the metal-organic frameworks (MOFs). Still, details concerning the MOFs with structural flexibility are not comprehensively understood. However, a dramatic increase in research activities concerning rigid MOFs over the years has brought deeper levels of understanding for their properties and applications. Nonetheless, robustness and flexibility of such smart frameworks are intriguing for different research areas such as catalysis, adsorption, etc. This manuscript overviews the different aspects of framework flexibility. The review has touched lightly on several ideas and proposals, which have been demonstrated within the selected examples to provide a logical basis to obtain a fundamental understanding of their synthesis and behavior to external stimuli

Two-dimensional band structure in honeycomb metal-organic frameworks

Metal-organic frameworks (MOFs) are an important class of materials that present intriguing opportunities in the fields of sensing, gas storage, catalysis, and optoelectronics. Very recently, two-dimensional (2D) MOFs have been proposed as a flexible material platform for realizing exotic quantum phases including topological and anomalous quantum Hall insulators. Experimentally, direct synthesis of 2D MOFs has been essentially confined to metal substrates, where the interaction with the substrate masks the intrinsic electronic properties of the MOF. Here, we demonstrate synthesis of 2D honeycomb metal-organic frameworks on a weakly interacting epitaxial graphene substrate. Using low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by density-functional theory (DFT) calculations, we show the formation of 2D band structure in the MOF decoupled from the substrate. These results open the experimental path towards MOF-based designer quantum materials with complex, engineered electronic structures