Deanna M. D'Alessandro

Carbon Dioxide Capture: Prospects for New Materials

The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO(2) emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO(2) emissions to the atmosphere, namely postcombustion (predominantly CO(2)/N(2) separation), precombustion (CO(2)/H(2)) capture, and natural gas sweetening (CO(2)/CH(4)). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.

Enhanced carbon dioxide capture upon incorporation of N,N '-dimethylethylenediamine in the metal-organic framework CuBTTri

High capacity, high selectivity, and low-cost regeneration conditions are the most important criteria by which new adsorbents for post-combustion carbon dioxide capture will be judged. The incorporation of N,N′-dimethylethylenediamine (mmen) into H3[(Cu4Cl)3(BTTri)8 (CuBTTri; H3BTTri = 1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene), a water-stable, triazolate-bridged framework, is shown to drastically enhance CO2 adsorption, resulting in one of the best performing metal–organic frameworks for CO2 separation reported to date. High porosity was maintained despite stoichiometric attachment of mmen to the open metal sites of the framework, resulting in a BET surface area of 870 m2 g−1. At 25 °C under a 0.15 bar CO2/0.75 bar N2 mixture, mmen-CuBTTri adsorbs 2.38 mmol CO2 g−1 (9.5 wt%) with a selectivity of 327, as determined using Ideal Adsorbed Solution Theory (IAST). The high capacity and selectivity are consequences of the exceptionally large isosteric heat of CO2 adsorption, calculated to be −96 kJ mol−1 at zero coverage. Infrared spectra support chemisorption between amines and CO2 as one of the primary mechanisms of uptake. Despite the large initial heat of adsorption, the CO2 uptake was fully reversible and the framework could be easily regenerated at 60 °C, enabling a cycling time of just 27 min with no loss of capacity over the course of 72 adsorption/desorption cycles.

High-spin ground states via electron delocalization in mixed-valence imidazolate-bridged divanadium complexes

The field of molecular magnetism has grown tremendously since the discovery of single-molecule magnets, but it remains centred around the superexchange mechanism. The possibility of instead using a double-exchange mechanism (based on electron delocalization rather than Heisenberg exchange through a non-magnetic bridge) presents a tantalizing prospect for synthesizing molecules with high-spin ground states that are well isolated in energy. We now demonstrate that magnetic double exchange can be sustained by simple imidazolate bridging ligands, known to be well suited for the construction of coordination clusters and solids. A series of mixed-valence molecules of the type [(PY5Me(2))V(II)(micro-L(br)) V(III)(PY5Me(2))](4+) were synthesized and their electron delocalization probed through cyclic voltammetry and spectroelectrochemistry. Magnetic susceptibility data reveal a well-isolated S = 5/2 ground state arising from double exchange for [(PY5Me(2))(2)V(2)(micro-5,6-dimethylbenzimidazolate)](4+). Combined modelling of the magnetic data and spectral analysis leads to an estimate of the double-exchange parameter of B = 220 cm(-1) when vibronic coupling is taken into account.

Towards Conducting Metal-Organic Frameworks

The realization of metal-organic frameworks (MOFs) as electronic conductors is a highly sought after goal, which has the potential to revolutionize the areas of catalysis, solid-state sensors and solar energy conversion devices. To date, the design and synthesis of MOFs that exhibit through-framework conduction has been limited; however, significant interest is now emerging owing to the fascinating prospects for integrating multiple functions. This highlight article introduces the field of conducting nanoporous materials and discusses recent specific examples along with key design features that will underlie future developments in the area.

Controlling charge separation in a novel donor–acceptor metal–organic framework via redox modulation

Charge transfer metal–organic frameworks represent a versatile class of multifunctional materials that offer an unprecedented combination of physical properties. The framework [(Zn(DMF))2(TTFTC)(DPNI)] incorporating the donor and acceptor, tetrathiafulvalenetetracarboxylate (TTFTC) and N,N′-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (DPNI) respectively, exhibits charge transfer by virtue of donor–acceptor interactions within its crystalline structure. This through-space interaction is manifested by the formation of ligand-based radicals in the as-synthesised material and leads to a partial degree of charge separation. Five distinct electronic states of the framework can be accessed using solid state electrochemical and spectroelectrochemical techniques, including for the first time in application to metal–organic frameworks, EPR spectroelectrochemistry (SEC). The degree of charge transfer is controllable via redox modulation and has been quantified using complementary DFT modelling of the charge transfer states.

Microwave-assisted solvothermal synthesis of zirconium oxide based metal-organic frameworks.

Zirconium oxide based Metal-Organic Frameworks were synthesised using a rapid and efficient microwave-assisted solvothermal method that produced purer phases and higher quality crystalline products in significantly (>95%) less time than the conventional heating method. A new amino-functionalised analogue has been synthesised exclusively using this microwave-assisted methodology.

Toward carbon dioxide capture using nanoporous materials

The development of more efficient processes for CO2 capture from the flue streams of power plants is considered a key to the reduction of greenhouse gas emissions implicated in global warming. Indeed, several U.S. and international climate change initiatives have identified the urgent need for improved materials and methods for CO2 capture. Conventional CO2 capture processes employed in power plants world-wide are typically postcombustion “wet scrubbing” methods involving the absorption of CO2 by amine-containing solvents such as methanolamine (MEA). These present several disadvantages, including the considerable heat required in regeneration of the solvent and the necessary use of inhibitors for corrosion control, which lead to reduced efficiencies and increased costs for electricity production. This perspective article seeks to highlight the most recent advances in new materials for CO2 capture from power plant flue streams, with particular emphasis on the rapidly expanding field of metal–organic frameworks. Ultimately, the development of new classes of efficient, cost-effective, and industrially viable capture materials for application in carbon capture and storage (CCS) systems offers an immense opportunity to reduce atmospheric emissions of greenhouse gases on a national and international scale.

Tuning the functional sites in metal–organic frameworks to modulate CO2 heats of adsorption

Metal–organic frameworks (MOFs) have been targeted as solid state sorbents for postcombustion carbon dioxide capture due, in part, to the enormous tunability of their structures through the incorporation of different functional sites. The isosteric heat of adsorption (Qst) provides one measure of the interaction of a solid sorbent with guest molecules, and has a bearing on the low pressure (<1 bar) CO2 uptake, selectivity and regenerability of a material. It is a key factor in the design of adsorbents for gas separation; however, it is sometimes overlooked in the evaluation of MOFs for CO2 capture. This highlight article draws together the impact of various functional sites on the CO2 heat of adsorption, and examines the interplay between functional sites and other factors such as competing water adsorption that influence a materials suitability for CO2 capture from industrial streams.

A Mixed-Spin Molecular Square with a Hybrid [2×2]Grid/Metallocyclic Architecture†

The design and synthesis of molecular architectures displaying a range of tunable and potentially useful properties has been the subject of widespread attention over the past decade. Both discrete and polymeric metallosupramolecular assemblies have been shown to exhibit intriguing properties, with potential applications including gas separation and adsorption, catalysis, photoactivity, magnetism, electrochemistry, and host–guest behavior. Cyanidebridged metal–organic framework materials have attracted intense research interest for their magnetic properties; for example, Prussian Blue is known to be a molecule-based magnet with a Curie temperature (Tc) of 5.6 K, while some heterometallic analogues have been shown to function as high Tc magnets with ordering temperatures up to 372 K. [7] Metallocycles and grids represent comparatively simple systems in which the tunability of the molecular structure and ensuing physico-chemical properties can be explored. Although a number of grids and metallocycles displaying interesting magnetic and spin-crossover properties have been developed, the design and successful construction of these systems, particularly those exhibiting multiple spins at room temperature, still represents a significant challenge. Herein we report the synthesis and characterization, and the magnetic and electronic properties, of a mixed-spin Fe4-molecular square with a hybrid grid/metallocyclic architecture. Based upon the decomposition of selenocyanate, we now report a rare example of a hybrid grid/metallocyclic architecture containing both L (2,3-bis[{3-(pyridin-2-yl)-1H-pyrazol-1-yl}methyl]quinoxaline) and cyanide-bridged Fe (highspin, HS)–Fe (low-spin, LS) molecular square at room temperature, which has been constructed through a metaldirected self-assembly strategy (Scheme 1).

Radicals in metal–organic frameworks

The burgeoning field of metal–organic frameworks (MOFs) has been marked by numerous key advances over the past two decades. An emerging theme is the incorporation of radical species which may be ligated as an integral structural component of, or simply appended to, the material, or else merely a guest within it. Radical incorporation has been shown to endow MOFs with a plethora of unique and fascinating magnetic, electronic and optical properties, paving the way towards their application as spin probes, and in magnetic/electronic devices, chemical sensing and molecular recognition. In view of the rapid growth of the literature in the area, this review highlights progress over the past three years (since 2011), and seeks to uncover promising ideas that will underscore future advancements at both the fundamental and applied levels.