Shengqian Ma

Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation

The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal–organic materials (MOMs). Here we report that a crystal engineering or reticular chemistry strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF62−) anions enables a ‘sweet spot’ of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area

Porous materials have been of intense scientific and technological interest because of their vital importance in many applications such as catalysis, gas separation, and gas storage. Great efforts in the past decade have led to the production of highly porous materials with large surface areas. In particular, the development of metal–organic frameworks (MOFs) has been especially rapid. Indeed, the highest surface area reported to date is claimed for a recently reported MOF material UMCM-2, which has a N2 uptake capacity of 1500 cm g at saturation, from which a Langmuir surface area of 6060 m g (Brunauer–Emmett–Teller (BET) surface area of 5200 m g) can be derived. Unfortunately, the high-surface-area porous MOFs usually suffer from low thermal and hydrothermal stabilities, which severely limit their applications, particularly in industry. These low stability issues could be resolved by replacing coordination bonds with stronger covalent bonds, as observed in covalent organic frameworks (COFs) or porous organic polymers. However, the COFs and porous organic polymers reported to date have lower surface areas compared to MOFs; the highest reported surface area for a COF is 4210 m g (BET) in COF103. Thus, further efforts are required to explore various strategies to achieve higher surface areas in COFs. Herein, we present a strategy that has enabled us to achieve, with the aid of computational design, a structure that possesses by far the highest surface area reported to date, as well as exceptional thermal and hydrothermal stabilities. We report the synthesis and properties of a porous aromatic framework PAF-1, which has a Langmuir surface area of 7100 m g. Besides its exceptional surface area, PAF-1 outperforms highly porous MOFs in thermal and hydrothermal stabilities, and demonstrates high uptake capacities for hydrogen (10.7 wt % at 77 K, 48 bar) and carbon dioxide (1300 mgg 1 at 298 K, 40 bar). Moreover, the super hydrophobicity and high surface area of PAF-1 result in unprecedented uptake capacities of benzene and toluene vapors at room temperature. It is well known that one of the most stable compounds in nature is diamond, in which each carbon atom is tetrahedrally connected to four neighboring atoms by covalent bonds (Figure 1a). Conceptually, replacement of the C C covalent bonds of diamond with rigid phenyl rings should not only retain a diamond-like structural stability but also allow sufficient exposure of the faces and edges of phenyl rings with the expectation of increasing the internal surface areas. By employing a multiscale theoretical method, which

Immobilization of MP-11 into a Mesoporous Metal–Organic Framework, MP-11@mesoMOF: A New Platform for Enzymatic Catalysis

Microperoxidase-11 has for the first time been successfully immobilized into a mesoporous metal-organic framework (MOF) consisting of nanoscopic cages and it demonstrates superior enzymatic catalysis performances compared to its mesoporous silica counterpart.

Enhancing H2 Uptake by “Close-Packing” Alignment of Open Copper Sites in Metal–Organic Frameworks†

Inspired by close-packing of spheres, to strengthen the framework-H{sub 2} interaction in MOFs (metal-organic frameworks), a strategy is devised to increase the number of nearest neighboring open metal sites ofe ach H{sub 2}-hosting cage, and to align the open metal sites toward the H{sub 2} molecules. Two MOF polymorphs were made, one exhibiting a record high hydrogen uptake of 3.0 wt% at 1 bar and 77 k.

Hydrogen Adsorption in a Highly Stable Porous Rare-Earth Metal-Organic Framework: Sorption Properties and Neutron Diffraction Studies

A highly stable porous lanthanide metal-organic framework, Y(BTC)(H2O).4.3H2O (BTC = 1,3,5-benzenetricarboxylate), with pore size of 5.8 A has been constructed and investigated for hydrogen storage. Gas sorption measurements show that this porous MOF exhibits highly selective sorption behaviors of hydrogen over nitrogen gas molecules and can take up hydrogen of about 2.1 wt % at 77 K and 10 bar. Difference Fourier analysis of neutron powder diffraction data revealed four distinct D2 sites that are progressively filled within the nanoporous framework. Interestingly, the strongest adsorption sites identified are associated with the aromatic organic linkers rather than the open metal sites, as occurred in previously reported MOFs. Our results provide for the first time direct structural evidence demonstrating that optimal pore size (around 6 A, twice the kinetic diameter of hydrogen) strengthens the interactions between H2 molecules and pore walls and increases the heat of adsorption, which thus allows for enhancing hydrogen adsorption from the interaction between hydrogen molecules with the pore walls rather than with the normally stronger adsorption sites (the open metal sites) within the framework. At high concentration H2 loadings (5.5 H2 molecules (3.7 wt %) per Y(BTC) formula), H2 molecules form highly symmetric novel nanoclusters with relatively short H2-H2 distances compared to solid H2. These observations are important and hold the key to optimizing this new class of rare metal-organic framework (RMOF) materials for practical hydrogen storage applications.

Cobalt imidazolate framework as precursor for oxygen reduction reaction electrocatalysts.

We demonstrate a new approach of preparing a non-platinum group metal (PGM) electrocatalyst for oxygen reduction reaction through rational design by using cobalt imidazolate framework—a subclass of metal-organic framework (MOF) material—as the precursor with potential to produce uniformly distributed catalytic center and high active-site density. MOFs represent a new type of materials, and have recently been under broad exploration of various important applications due to their amenability to rational design for different functionalities at molecular level. In particular, their high surface areas, well-defined porous structures, and building block variety not only distinguish them from the conventional materials in gas adsorption and separation, but also offer new promises in catalysis application. However, the application of porous MOFs for electrocatalysis in fuel cell has yet to be exploited. The oxygen reduction reaction (ORR) at the cathode of a proton exchange membrane fuel cell (PEMFC) represents a very important electrocatalytic reaction. At present, the catalyst materials of choice are platinum group metals (PGMs). The high costs and limited reserves of PGMs, however, created a major barrier for large-scale commercialization of PEMFCs. Intensive efforts have been dedicated to the search of low-cost alternatives. The discovery of ORR activity on cobalt phthalocyanine stimulated extensive investigations of using Co–N4 or Fe–N4 macromolecules as precursors for preparation of transition metal (TM) based, non-PGM catalysts. The ORR activity over a cobalt–polypyrrole composite was observed, of which a Co ligated by pyrrolic nitrogens was proposed as the catalytic site. Activation in an inert atmosphere of the similar TM– polymer composite through pyrolysis further improved the catalytic activity. More recently, significant enhancement in ORR activity was demonstrated in a carbon-supported iron-based catalysts, and it was suggested that micropores (width <20 ) have critical influence on the formation of the active site with an ionic Fe coordinated by four pyridinic nitrogens after high-temperature treatment. The onset potential for an Fe-based catalyst is found to be 0.1 V higher than that of a Co-based system although the latter is more stable under PEMFC operating condition. These previous studies proposed the nitrogen-ligated TM entities either as the precursors or the active centers for the catalytic ORR process. Another challenge for non-PGM ORR catalysts is their relatively low turn-over-frequency in comparison with Pt. To compensate low activity without using excessive amount of catalyst, thus causing thick electrode layer and poor mass transport, it is desirable to produce the highest possible catalytic-site density, that are evenly distributed and accessible to gas diffusion through a porous framework. Herein we report the first experimental demonstration of porous MOF as a new class of precursor for preparing ORR catalysts. Different from previous approaches, MOFs have the following advantages when used to prepare non-PGM electrocatalysts: MOFs have clearly-defined three-dimensional structures. The initial entities such as TM–N4 can be grafted into MOFs with the highest possible volumetric density through regularly arranged cell structure. The MOF surface area and pore size are tunable by the length of the linker. The organic linkers would be converted to carbon during thermal activation while maintaining the porous framework, leading to catalysts with high surface area and uniformly distributed active sites without the need of a second carbon support or pore forming agent. Furthermore, the TM–ligand composition can be rationally designed with wide selection of metal–linker combinations for systematical investigation on the relationship between precursor structure and catalyst activity. Our studies demonstrate the initial step to achieve such advantages. [a] Dr. S. Ma, Dr. G. A. Goenaga, Dr. D.-J. Liu Chemical Sciences & Engineering Division Argonne National Laboratory, Argonne, IL 60439 (USA) Fax: (+1) 630-252-4176 E-mail : djliu@anl.gov [b] A. V. Call Department of Materials Science and Engineering Northwestern University, Evanston, IL 60208 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201003080.

A Coordinatively Linked Yb Metal–Organic Framework Demonstrates High Thermal Stability and Uncommon Gas‐Adsorption Selectivity

Porous metal–organic frameworks (MOFs), which have emerged as new zeolite analogues, have attracted considerable research interest in the past decade as, compared to traditional zeolites, they possess a high surface area, modifiable surface, and tunable pore size. These characteristics have led to an enormous application potential for MOFs in catalysis, gas storage, and adsorptive separation. One of the main concerns regarding porous MOFs is their limited thermal stability, which prevents them from competing with inorganic zeolites in practical applications. Most porous MOFs can only be heated up to 150–350 8C without losing their framework integrity. Interpenetration, which often arises from weak interactions, has been widely used to improve the thermal stability of porous MOFs, and interpenetrated porous MOFs that are stable up to 400 8C have been reported. Interpenetration increases the wall thickness and reduces the pore size of an MOF, both of which lead to enhanced thermal stability. If two interpenetrated frameworks can be linked through coordinative bonds, the thermal stability should be boosted still further (Scheme 1). Herein we report such a coordinatively linked, doubly interpenetrated Yb MOF with improved thermal stability (up to 500 8C) and uncommon gas-adsorption selectivity. We have previously reported a cobalt-based porous MOF with doubly interpenetrated, (8,3)-connected nets (PCN-9; PCN: porous coordination network). PCN-9 contains a square-planar Co4(m4-O) secondary building unit (SBU) where each Co center is five-coordinate with a coordination site open toward the channel. As a consequence of this interpenetration, PCN-9 is thermally stable up to 400 8C (by thermogravimetric analysis (TGA)). If the interpenetrated, (8,3)-connected nets can be linked at the open metal sites by a bridging ligand, the thermal stability of the resulting MOF should be still higher. A short bridge is the best candidate due to the proximity of the two nets, and we chose SO4 2 as the bridging ligand because it can chelate the two metal centers and stabilize the MOF further. In addition, it can be generated slowly under solvothermal conditions through decomposition of DMSO (dimethyl sulfoxide), thereby facilitating the formation of the coordinatively linked interpenetrated MOF. Initial attempts to use sulfates to bridge the doubly interpenetrated, (8,3)-connected nets in PCN-9 failed. There are two possible reasons for this failure: the limited coordination number (maximum of six) of the cobalt center and the need for additional counterions to balance the overall charge. The coordination number of the metal center can be increased by using Ln cations instead of Co and no additional counterions will be needed in this case to balance the overall charge. With these considerations in mind, a ytterbium MOF with coordinatively linked, doubly interpenetrated, (8,3)-connected nets (PCN-17) was synthesized. Studies of similar MOFs containing other lanthanides are currently underway and will be reported in due course elsewhere. PCN-17 is stable up to 480 8C and exhibits selective adsorption of H2 and O2 over N2 and CO. Crystals of PCN-17 were obtained upon heating a mixture of H3TATB (TATB= 4,4’,4’’-S-triazine-2,4,6-triyl tribenzoate) and ytterbium nitrate in DMSO at 145 8C for 72 hours. The formula of PCN-17 (Yb4(m4-H2O)(C24H12N3O6)8/3(SO4)2·3H2O·10DMSO) was determined by X-ray crystallography, elemental analysis, and thermogravimetric analysis (TGA). X-ray structural analysis revealed that PCN-17 crystallizes in the space group Im3m. As expected, it adopts a square-planar Yb4(m4H2O) SBU, with the m4-H2O molecule, which is probably disordered over two or more orientations (see below), residing at the center of a square of four Yb atoms (Figure 1a). The four Yb atoms in the SBU lie in the same plane and each coordinates to seven O atoms (four from four carboxylate groups of four different TATBs, two from the bridging sulfate generated in situ, and one from the m4H2O). The Yb···m4-H2O distance (2.70 D) indicates very weak Scheme 1. a) A single net. b) Two doubly interpenetrated nets. c) Interpenetrated nets linked by a coordinative bond. The vertical gold dotted line represents a p–p interaction; the blue solid line represents coordinative bonding.

How can proteins enter the interior of a MOF? Investigation of cytochrome c translocation into a MOF consisting of mesoporous cages with microporous windows.

It has been demonstrated for the first time that the heme protein cytochrome c (Cyt c) can enter the interior of a MOF despite the larger molecular dimension of the protein relative to the access pore sizes. Mechanistic studies suggest that the Cyt c molecules must undergo a significant conformational change during translocation into the MOF interior through the relatively small nanopores.

Introduction of π-Complexation into Porous Aromatic Framework for Highly Selective Adsorption of Ethylene over Ethane

In this work, we demonstrate for the first time the introduction of π-complexation into a porous aromatic framework (PAF), affording significant increase in ethylene uptake capacity, as illustrated in the context of Ag(I) ion functionalized PAF-1, PAF-1-SO3Ag. IAST calculations using single-component-isotherm data and an equimolar ethylene/ethane ratio at 296 K reveal that PAF-1-SO3Ag shows exceptionally high ethylene/ethane adsorption selectivity (Sads: 27 to 125), far surpassing benchmark zeolite and any other MOF reported in literature. The formation of π-complexation between ethylene molecules and Ag(I) ions in PAF-1-SO3Ag has been evidenced by the high isosteric heats of adsorption of C2H4 and also proved by in situ IR spectroscopy studies. Transient breakthrough experiments, supported by simulations, indicate the feasibility of PAF-1-SO3Ag for producing 99.95%+ pure C2H4 in a Pressure Swing Adsorption operation. Our work herein thus suggests a new perspective to functionalizing PAFs and other types of advanced porous materials for highly selective adsorption of ethylene over ethane.

Biomimetic Catalysis of a Porous Iron-Based Metal− Metalloporphyrin Framework

A porous metal-metalloporphyrin framework, MMPF-6, based upon an iron(III)-metalated porphyrin ligand and a secondary binding unit of a zirconium oxide cluster was constructed; MMPF-6 demonstrated interesting peroxidase activity comparable to that of the heme protein myoglobin as well as exhibited solvent adaptability of retaining the peroxidase activity in an organic solvent.