Jonathan E. Bachman

Methane storage in flexible metal-organic frameworks with intrinsic thermal management.

As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures. Although activated carbons, zeolites, and metal–organic frameworks have been investigated extensively for CH4 storage, there are practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we use a reversible phase transition in a metal–organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp2− = 1,4-benzenedipyrazolate) are shown to undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp ‘step’. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.

Enhanced ethylene separation and plasticization resistance in polymer membranes incorporating metal–organic framework nanocrystals

The implementation of membrane-based separations in the petrochemical industry has the potential to reduce energy consumption significantly relative to conventional separation processes. Achieving this goal, however, requires the development of new membrane materials with greater selectivity, permeability and stability than available at present. Here, we report composite materials consisting of nanocrystals of metal-organic frameworks dispersed within a high-performance polyimide, which can exhibit enhanced selectivity for ethylene over ethane, greater ethylene permeability and improved membrane stability. Our results suggest that framework-polymer interactions reduce chain mobility of the polymer while simultaneously boosting membrane separation performance. The increased stability, or plasticization resistance, is expected to improve membrane utility under real process conditions for petrochemical separations and natural gas purification. Furthermore, this approach can be broadly applied to numerous polymers that encounter aggressive environments, potentially making gas separations possible that were previously inaccessible to membranes.

Plasticization-resistant Ni2(dobdc)/polyimide composite membranes for the removal of CO2 from natural gas

We demonstrate that the incorporation of Ni2(dobdc) metal–organic framework nanocrystals into various polyimides can improve the performance of membranes for separating CO2 from CH4 under mixed-gas conditions. Four upper-bound 6FDA-based polyimides, as well as the commercial polymer Matrimid®, show improved selectivity under mixed-gas feeds when loaded with 15–25 wt% Ni2(dobdc), while the neat polyimides show diminishing selectivity upon increasing feed pressure. This approach presents an alternative to chemical crosslinking for achieving plasticization resistance, with the added benefit of retaining or increasing permeability while simultaneously reducing chain mobility.

M2(m-dobdc) (M = Mn, Fe, Co, Ni) Metal–Organic Frameworks as Highly Selective, High-Capacity Adsorbents for Olefin/Paraffin Separations

The metal-organic frameworks M2(m-dobdc) (M = Mn, Fe, Co, Ni; m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate) were evaluated as adsorbents for separating olefins from paraffins. Using single-component and multicomponent equilibrium gas adsorption measurements, we show that the coordinatively unsaturated M2+ sites in these materials lead to superior performance for the physisorptive separation of ethylene from ethane and propylene from propane relative to any known adsorbent, including para-functionalized structural isomers of the type M2(p-dobdc) (p-dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate). Notably, the M2(m-dobdc) frameworks all exhibit an increased affinity for olefins over paraffins relative to their corresponding structural isomers, with the Fe, Co, and Ni variants showing more than double the selectivity. Among these frameworks, Fe2(m-dobdc) displays the highest ethylene/ethane (>25) and propylene/propane (>55) selectivity under relevant conditions, together with olefin capacities exceeding 7 mmol/g. Differential enthalpy calculations in conjunction with structural characterization of ethylene binding in Co2(m-dobdc) and Co2(p-dobdc) via in situ single-crystal X-ray diffraction reveal that the vast improvement in selectivity arises from enhanced metal-olefin interactions induced by increased charge density at the metal site. Moderate olefin binding enthalpies, below 55 and 70 kJ/mol for ethylene and propylene, respectively, indicate that these adsorbents maintain sufficient reversibility under mild regeneration conditions. Additionally, transient adsorption experiments show fast kinetics, with more than 90% of ethylene adsorption occurring within 30 s after dosing. Breakthrough measurements further indicate that Co2(m-dobdc) can produce high purity olefins without a temperature swing, an important test of process applicability. The excellent olefin/paraffin selectivity, high olefin capacity, rapid adsorption kinetics, and low raw materials cost make the M2(m-dobdc) frameworks the materials of choice for adsorptive olefin/paraffin separations.

Near-Perfect CO2/CH4 Selectivity Achieved through Reversible Guest Templating in the Flexible Metal–Organic Framework Co(bdp)

Metal-organic frameworks are among the most promising materials for industrial gas separations, including the removal of carbon dioxide from natural gas, although substantial improvements in adsorption selectivity are still sought. Herein, we use equilibrium adsorption experiments to demonstrate that the flexible metal-organic framework Co(bdp) (bdp2- = 1,4-benzenedipyrazolate) exhibits a large CO2 adsorption capacity and approaches complete exclusion of CH4 under 50:50 mixtures of the two gases, leading to outstanding CO2/CH4 selectivity under these conditions. In situ powder X-ray diffraction data indicate that this selectivity arises from reversible guest templating, in which the framework expands to form a CO2 clathrate and then collapses to the nontemplated phase upon desorption. Under an atmosphere dominated by CH4, Co(bdp) adsorbs minor amounts of CH4 along with CO2, highlighting the importance of studying all relevant pressure and composition ranges via multicomponent measurements when examining mixed-gas selectivity in structurally flexible materials. Altogether, these results show that Co(bdp) may be a promising CO2/CH4 separation material and provide insights for the further study of flexible adsorbents for gas separations.

Diamine-Appended Mg2(dobpdc) Nanorods as Phase-Change Fillers in Mixed-Matrix Membranes for Efficient CO2/N2 Separations

Despite the availability of chemistries to tailor the pore architectures of microporous polymer membranes for chemical separations, trade-offs in permeability and selectivity with functional group manipulations nevertheless persist, which ultimately places an upper bound on membrane performance. Here we introduce a new design strategy to uncouple these attributes of the membrane. Key to our success is the incorporation of phase-change metal-organic frameworks (MOFs) into the polymer matrix, which can be used to increase the solubility of a specific gas in the membrane, and thereby its permeability. We further show that it is necessary to scale the size of the phase-change MOF to nanoscopic dimensions, in order to take advantage of this effect in a gas separation. Our observation of an increase in solubility and permeability of only one of the gases during steady-state permeability measurements suggests fast exchange between free and chemisorbed gas molecules within the MOF pores. While the kinetics of this exchange in phase-change MOFs are not yet fully understood, their role in enhancing the efficacy and efficiency of the separation is clearly a compelling new direction for membrane technology.

Thermally Rearranged Polymer Membranes Containing Tröger's Base Units Have Exceptional Performance for Air Separations

The influence of segmental chain motion on the gas separation performance of thermally rearranged (TR) polymer membranes is established for TR polybenzoxazoles featuring Trögers base (TB) monomer subunits as exceptionally rigid sites of contortion along the polymer backbone. These polymers are accessed from solution-processable ortho-acetate functionalized polyimides, which are readily synthesized as high-molecular-weight polymers for membrane casting. We find that thermal rearrangement leads to a small increase in d-spacing between polymer chains and a dramatic pore-network reconfiguration that increases both membrane permeability and O2 /N2 selectivity, putting its performance above the 2015 upper bound.

Engineered Transport in Microporous Materials and Membranes for Clean Energy Technologies

Many forward-looking clean-energy technologies hinge on the development of scalable and efficient membrane-based separations. Ongoing investment in the basic research of microporous materials is beginning to pay dividends in membrane technology maturation. Specifically, improvements in membrane selectivity, permeability, and durability are being leveraged for more efficient carbon capture, desalination, and energy storage, and the market adoption of membranes in those areas appears to be on the horizon. Herein, an overview of the microporous materials chemistry driving advanced membrane development, the clean-energy separations employing them, and the theoretical underpinnings tying membrane performance to membrane structure across multiple length scales is provided. The interplay of pore architecture and chemistry for a given set of analytes emerges as a critical design consideration dictating mass transport outcomes. Opportunities and outstanding challenges in the field are also discussed, including high-flux 2D molecular-sieving membranes, phase-change adsorbents as performance-enhancing components in composite membranes, and the need for quantitative metrologies for understanding mass transport in heterophasic materials and in micropores with unusual chemical interactions with analytes of interest.