Randall Q. Snurr

Ultrahigh Porosity in Metal-Organic Frameworks

Network Approaches to Highly Porous Materials Metal-organic frameworks (MOFs), in which inorganic centers are bridged by organic linkers, can achieve very high porosity for gas absorption. However, as the materials develop larger void spaces, there is also more room for growing interpenetrating networks—filling the open spaces not with gas molecules but with more MOFs. Furukawa et al. (p. 424, published online 1 July) describe the synthesis of a MOF in which zinc centers are bridged with long, highly conjugated organic linkers, but in which the overall symmetry of the networks created prevents formation of interpenetrating networks. Extremely high surface areas and storage capacities for hydrogen, carbon dioxide, and methane were observed. The large surface areas of these materials would correspond to that of dispersed nanocubes just 3 to 6 nanometers wide. Crystalline solids with extended non-interpenetrating three-dimensional crystal structures were synthesized that support well-defined pores with internal diameters of up to 48 angstroms. The Zn4O(CO2)6 unit was joined with either one or two kinds of organic link, 4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate (BTE), 4,4′,44″-[benzene-1,3,5-triyl-tris(benzene-4,1-diyl)]tribenzoate (BBC), 4,4′,44″-benzene-1,3,5-triyl-tribenzoate (BTB)/2,6-naphthalenedicarboxylate (NDC), and BTE/biphenyl-4,4′-dicarboxylate (BPDC), to give four metal-organic frameworks (MOFs), MOF-180, -200, -205, and -210, respectively. Members of this series of MOFs show exceptional porosities and gas (hydrogen, methane, and carbon dioxide) uptake capacities. For example, MOF-210 has Brunauer-Emmett-Teller and Langmuir surface areas of 6240 and 10,400 square meters per gram, respectively, and a total carbon dioxide storage capacity of 2870 milligrams per gram. The volume-specific internal surface area of MOF-210 (2060 square meters per cubic centimeter) is equivalent to the outer surface of nanoparticles (3-nanometer cubes) and near the ultimate adsorption limit for solid materials.

De novo synthesis of a metal–organic framework material featuring ultrahigh surface area and gas storage capacities

Metal-organic frameworks--a class of porous hybrid materials built from metal ions and organic bridges--have recently shown great promise for a wide variety of applications. The large choice of building blocks means that the structures and pore characteristics of the metal-organic frameworks can be tuned relatively easily. However, despite much research, it remains challenging to prepare frameworks specifically tailored for particular applications. Here, we have used computational modelling to design and predictively characterize a metal-organic framework (NU-100) with a particularly high surface area. Subsequent experimental synthesis yielded a material, matching the calculated structure, with a high BET surface area (6,143 m(2) g(-1)). Furthermore, sorption measurements revealed that the material had high storage capacities for hydrogen (164 mg g(-1)) and carbon dioxide (2,315 mg g(-1))--gases of high importance in the contexts of clean energy and climate alteration, respectively--in excellent agreement with predictions from modelling.

Development and Evaluation of Porous Materials for Carbon Dioxide Separation and Capture

The development of new microporous materials for adsorption separation processes is a rapidly growing field because of potential applications such as carbon capture and sequestration (CCS) and purification of clean-burning natural gas. In particular, new metal-organic frameworks (MOFs) and other porous coordination polymers are being generated at a rapid and growing pace. Herein, we address the question of how this large number of materials can be quickly evaluated for their practical application in carbon dioxide separation processes. Five adsorbent evaluation criteria from the chemical engineering literature are described and used to assess over 40 MOFs for their potential in CO(2) separation processes for natural gas purification, landfill gas separation, and capture of CO(2) from power-plant flue gas. Comparisons with other materials such as zeolites are made, and the relationships between MOF properties and CO(2) separation potential are investigated from the large data set. In addition, strategies for tailoring and designing MOFs to enhance CO(2) adsorption are briefly reviewed.

Screening of Metal−Organic Frameworks for Carbon Dioxide Capture from Flue Gas Using a Combined Experimental and Modeling Approach

A diverse collection of 14 metal-organic frameworks (MOFs) was screened for CO(2) capture from flue gas using a combined experimental and modeling approach. Adsorption measurements are reported for the screened MOFs at room temperature up to 1 bar. These data are used to validate a generalized strategy for molecular modeling of CO(2) and other small molecules in MOFs. MOFs possessing a high density of open metal sites are found to adsorb significant amounts of CO(2) even at low pressure. An excellent correlation is found between the heat of adsorption and the amount of CO(2) adsorbed below 1 bar. Molecular modeling can aid in selection of adsorbents for CO(2) capture from flue gas by screening a large number of MOFs.

Large-scale screening of hypothetical metal–organic frameworks

Metal-organic frameworks (MOFs) are porous materials constructed from modular molecular building blocks, typically metal clusters and organic linkers. These can, in principle, be assembled to form an almost unlimited number of MOFs, yet materials reported to date represent only a tiny fraction of the possible combinations. Here, we demonstrate a computational approach to generate all conceivable MOFs from a given chemical library of building blocks (based on the structures of known MOFs) and rapidly screen them to find the best candidates for a specific application. From a library of 102 building blocks we generated 137,953 hypothetical MOFs and for each one calculated the pore-size distribution, surface area and methane-storage capacity. We identified over 300 MOFs with a predicted methane-storage capacity better than that of any known material, and this approach also revealed structure-property relationships. Methyl-functionalized MOFs were frequently top performers, so we selected one such promising MOF and experimentally confirmed its predicted capacity.

Using molecular simulation to characterise metal–organic frameworks for adsorption applications

Molecular simulation is a powerful tool to predict adsorption and to gain insight into the corresponding molecular level phenomena. In this tutorial review, we provide an overview of how molecular simulation can be used to characterise metal-organic frameworks for adsorption applications. Particular attention is drawn to how these insights can be combined to develop design principles for specific applications.

A facile synthesis of UiO-66, UiO-67 and their derivatives

A scalable, reproducible method of synthesizing UiO-66- and UiO-67-type MOFs, entailing the addition of HCl to the reaction mixture, has been investigated. The new protocol requires a fraction of the time of previously reported procedures, yields exceptional porosities, and works with a range of linkers.

Vapor-phase metalation by atomic layer deposition in a metal-organic framework

Metal-organic frameworks (MOFs) have received attention for a myriad of potential applications including catalysis, gas storage, and gas separation. Coordinatively unsaturated metal ions often enable key functional behavior of these materials. Most commonly, MOFs have been metalated from the condensed phase (i.e., from solution). Here we introduce a new synthetic strategy capable of metallating MOFs from the gas phase: atomic layer deposition (ALD). Key to enabling metalation by ALD In MOFs (AIM) was the synthesis of NU-1000, a new, thermally stable, Zr-based MOF with spatially oriented -OH groups and large 1D mesopores and apertures.

Light-Harvesting and Ultrafast Energy Migration in Porphyrin-Based Metal–Organic Frameworks

Given that energy (exciton) migration in natural photosynthesis primarily occurs in highly ordered porphyrin-like pigments (chlorophylls), equally highly ordered porphyrin-based metal-organic frameworks (MOFs) might be expected to exhibit similar behavior, thereby facilitating antenna-like light-harvesting and positioning such materials for use in solar energy conversion schemes. Herein, we report the first example of directional, long-distance energy migration within a MOF. Two MOFs, namely F-MOF and DA-MOF that are composed of two Zn(II) porphyrin struts [5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphinato]zinc(II) and [5,15-bis[4-(pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II), respectively, were investigated. From fluorescence quenching experiments and theoretical calculations, we find that the photogenerated exciton migrates over a net distance of up to ~45 porphyrin struts within its lifetime in DA-MOF (but only ~3 in F-MOF), with a high anisotropy along a specific direction. The remarkably efficient exciton migration in DA-MOF is attributed to enhanced π-conjugation through the addition of two acetylene moieties in the porphyrin molecule, which leads to greater Q-band absorption intensity and much faster exciton-hopping (energy transfer between adjacent porphyrin struts). The long distance and directional energy migration in DA-MOF suggests promising applications of this compound or related compounds in solar energy conversion schemes as an efficient light-harvesting and energy-transport component.

Object-oriented Programming Paradigms for Molecular Modeling

This paper discusses the application of object-oriented programming (OOP) design concepts to the development of molecular simulation code. A number of new languages such as Fortran 90 (F90) have been developed over the last decade that support the OOP design philosophy. We briefly describe the salient features of F90 and some basic object-oriented design principles. As an illustration of the design concepts we implement a general interface in F90 for calculating pairwise interactions that can be extended easily to any number of different forcefield models. The ideas presented here are used in the development of a mu ltipurpose si mulation c ode, named Music. An example of the use of Music for grand canonical Monte Carlo (GCMC) simulations of flexible sorbate molecules in zeolites is given. The example illustrates how OOP allowed existing code for molecular dynamics and GCMC to be easily combined to perform hybrid GCMC simulations with minimal coding effort.