Adam J. Matzger

A route to high surface area, porosity and inclusion of large molecules in crystals

One of the outstanding challenges in the field of porous materials is the design and synthesis of chemical structures with exceptionally high surface areas. Such materials are of critical importance to many applications involving catalysis, separation and gas storage. The claim for the highest surface area of a disordered structure is for carbon, at 2,030 m2 g-1 (ref. 2). Until recently, the largest surface area of an ordered structure was that of zeolite Y, recorded at 904 m2 g-1 (ref. 3). But with the introduction of metal-organic framework materials, this has been exceeded, with values up to 3,000 m2 g-1 (refs 4–7). Despite this, no method of determining the upper limit in surface area for a material has yet been found. Here we present a general strategy that has allowed us to realize a structure having by far the highest surface area reported to date. We report the design, synthesis and properties of crystalline Zn4O(1,3,5-benzenetribenzoate)2, a new metal-organic framework with a surface area estimated at 4,500 m2 g-1. This framework, which we name MOF-177, combines this exceptional level of surface area with an ordered structure that has extra-large pores capable of binding polycyclic organic guest molecules—attributes not previously combined in one material.

Porous, Crystalline, Covalent Organic Frameworks

Covalent organic frameworks (COFs) have been designed and successfully synthesized by condensation reactions of phenyl diboronic acid {C6H4[B(OH)2]2} and hexahydroxytriphenylene [C18H6(OH)6]. Powder x-ray diffraction studies of the highly crystalline products (C3H2BO)6·(C9H12)1 (COF-1) and C9H4BO2 (COF-5) revealed expanded porous graphitic layers that are either staggered (COF-1, P63/mmc) or eclipsed (COF-5, P6/mmm). Their crystal structures are entirely held by strong bonds between B, C, and O atoms to form rigid porous architectures with pore sizes ranging from 7 to 27 angstroms. COF-1 and COF-5 exhibit high thermal stability (to temperatures up to 500° to 600°C), permanent porosity, and high surface areas (711 and 1590 square meters per gram, respectively).

Dramatic Tuning of Carbon Dioxide Uptake via Metal Substitution in a Coordination Polymer with Cylindrical Pores

A series of four isostructural microporous coordination polymers (MCPs) differing in metal composition is demonstrated to exhibit exceptional uptake of CO2 at low pressures and ambient temperature. These conditions are particularly relevant for capture of flue gas from coal-fired power plants. A magnesium-based material is presented that is the highest surface area magnesium MCP yet reported and displays ultrahigh affinity based on heat of adsorption for CO2. This study demonstrates that physisorptive materials can achieve affinities and capacities competitive with amine sorbents while greatly reducing the energy cost associated with regeneration.

A Porous Coordination Copolymer with over 5000 m2/g BET Surface Area

New levels of surface area are achieved in a coordination polymer (UMCM-2, University of Michigan Crystalline Material) derived from zinc-mediated coordination copolymerization of a dicarboxylic and tricarboxylic acid. In addition to a large micropore contribution to the surface area, mesopores are also present. In contrast to the recently reported coordination copolymer UMCM-1, which has a mesoporous channel, UMCM-2 is built from three types of cages. In spite of exceptional porosity, both of these coordination polymers are thermally robust. Hydrogen uptake performance of UMCM-2 approaches 7 wt% at 77 K.

Effect of humidity on the performance of microporous coordination polymers as adsorbents for CO2 capture.

The CO(2)-capture performance of microporous coordination polymers of the M/DOBDC series (where M = Zn, Ni, Co, and Mg; DOBDC = 2,5-dioxidobenzene-1,4-dicarboxylate) was evaluated under flow-through conditions with dry surrogate flue gas (5/1 N(2)/CO(2)). The CO(2) capacities were found to track with static CO(2) sorption capacities at room temperature, with Mg/DOBDC demonstrating an exceptional capacity for CO(2) (23.6 wt %). The effect of humidity on the performance of Mg/DOBDC was investigated by collecting N(2)/CO(2)/H(2)O breakthrough curves at relative humidities (RHs) in the feed of 9, 36, and 70%. After exposure at 70% RH and subsequent thermal regeneration, only about 16% of the initial CO(2) capacity of Mg/DOBDC was recovered. However, in the case of Ni/DOBDC and Co/DOBDC, approximately 60 and 85%, respectively, of the initial capacities were recovered after the same treatment. These data indicate that although Mg/DOBDC has the highest capacity for CO(2), under the conditions used in this study, Co/DOBDC may be a more desirable material for deployment in CO(2) capture systems because of the added costs associated with flue gas dehumidification.

Liquid Phase Adsorption by Microporous Coordination Polymers: Removal of Organosulfur Compounds

The utility of microporous coordination polymers (MCPs) for the adsorption of large organosulfur compounds (benzothiophene, dibenzothiophene, 4,6-dimethyldibenzothiophene) found in fuels is demonstrated. Large capacities are obtained at both low and high sulfur concentrations. For 4,6-dimethyldibenzothiophene, the compound most difficult to remove using current industrial techniques, a capacity of 41 g S/kg MCP at 1500 ppmw S is achieved by UMCM-150. It was determined that the size/shape of the pores in the MCP, rather than the surface area or pore volume, is the most important factor controlling adsorption capacity.

Improved Stability and Smart-Material Functionality Realized in an Energetic Cocrystal†

Though energetic compounds, and explosives in particular, represent some of the most influential materials in human history, their modern evolution has been relatively slow. Aside from being hindered by the inherent dangers of the field, improving on the state-of-the-art is difficult as novel energetic materials (energetics) must achieve a challenging combination of properties including high explosive power, high stability, and low cost. Moreover, explosive power is highly dependent on solid-state density, and though chemical structures with high energies can be designed, engineering their crystal structures is not possible. In fact, crystal structure prediction, a problem closely related to crystal engineering, is fraught with challenges. This is especially so for compounds with nitro groups, a common moiety among energetic compounds. Though an admirable amount of work has gone into the discovery of novel compounds with high chemical potential energy, few of these materials have proven viable for explosives applications. Work to improve the material properties of existing energetics has focused primarily on exploring polymorphism in hopes to find more dense or less sensitive forms. This approach is attractive because it allows one to improve energetic compounds without the daunting task of implementing new, safe, and scalable chemistry. This advantage is present in another approach that is only starting to garner interest in solid-state energetics research: cocrystallization. Until recently, cocrystallization had been absent from the literature as a method for energetic solid form engineering despite its current success in engineering solid forms of pharmaceuticals. This imbalance is perhaps due to the chemical differences distinguishing energetics and pharmaceuticals. Most active pharmaceutical ingredients feature polar groups that are rich in predictable interactions conducive to cocrystal formation: primarily hydrogen bonding. Energetics, in contrast, are defined primarily by nitro groups, a solitary moiety that offers very few predictable interactions sufficiently strong or versatile to be useful in cocrystal design. Recently, we demonstrated that cocrystallization can be used to generate novel solid forms of energetic materials by presenting seventeen cocrystals containing 2,4,6-trinitrotoluene (TNT). Though the work proved the viability of cocrystallization to increase density and improve thermal stability in energetic materials, these improvements were achieved to the detriment of explosive power. When cocrystallized with a non-energetic compound, as was the case with these TNT cocrystals, the energetic component inevitably sees its explosive power diluted. Several measureable material properties are improved, but the resultant cocrystals are not viable explosives. Moreover, all of these cocrystals were formed by p–p stacking, a synthon available to the aromatic class of energetics, but not to the broader and more powerful non-aromatic class. Therefore, it remains unclear if energeticenergetic cocrystals can be realized based solely upon C H and nitro-group interactions. Presented here is an energetic–energetic cocrystal composed of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and TNT in a 1:1 molar ratio (1, Figure 1). CL-20 is a relatively new energetic compound

Enabling cleaner fuels: desulfurization by adsorption to microporous coordination polymers.

Microporous coordination polymers (MCPs) are demonstrated to be efficient adsorbents for the removal of the organosulfur compounds dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) from model diesel fuel and diesel fuel. For example, packed bed breakthrough experiments utilizing UMCM-150 find capacities of 25.1 g S/kg MCP for DBT and 24.3 g S/kg MCP for DMDBT from authentic diesel indicating that large amounts of fuel are desulfurized before the breakthrough point. Unlike activated carbons, where selectivity has been a problem, MCPs selectively adsorb the organosulfur compounds over other, similar components of diesel. Complete regeneration using toluene at modest temperatures is achieved. The attainment of high selectivities and capacities, particularly for the adsorption of the refractory compounds that are difficult to remove using current desulfurization techniques, in a reversible sorbent indicates that fuel desulfurization may be an important application for MCPs.

Heterogenization of Homogeneous Catalysts in Metal–Organic Frameworks via Cation Exchange

This paper describes the heterogenization of single-site transition-metal catalysts in metal-organic frameworks (MOFs) via cation exchange. A variety of cationic complexes of Pd, Fe, Ir, Rh, and Ru have been incorporated into ZJU-28, and the new materials have been characterized by optical microscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction. MOF-supported [Rh(dppe)(COD)]BF4 catalyzes the hydrogenation of 1-octene to n-octane. The activity of this supported catalyst compares favorably to its homogeneous counterpart, and it can be recycled at least four times. Overall, this work provides a new and general approach for supporting transition-metal catalysts in MOFs.

MOF@MOF: microporous core–shell architectures

Mixing two different linkers with the same topology has been applied to make metal-organic frameworks (MOFs) either in one batch or sequentially to generate coordination copolymers with either a randomly mixed or a core-shell composition of linkers.