Multivariate Flexible Framework with High Usable Hydrogen Capacity in a Reduced Pressure Swing Process

Abstract

Step-shaped adsorption–desorption of gaseous payloads by flexible metal–organic frameworks can facilitate the delivery of large usable capacities with significantly reduced energetic penalties. This is desirable for the storage, transport, and delivery of H2, as prototypical adsorbents require large swings in pressure and temperature to achieve usable capacities approaching their total capacities. However, the weak physisorption of H2 typically necessitates undesirably high pressures to induce the framework phase change. As de novo design of flexible frameworks is exceedingly challenging, the ability to intuitively adapt known frameworks is required. We demonstrate that the multivariate linker approach is a powerful tool for tuning the phase change behavior of flexible frameworks. In this work, 2-methyl-5,6-difluorobenzimidazolate was solvothermally incorporated into the known framework CdIF-13 (sod-Cd(benzimidazolate)2), resulting in the multivariate framework sod-Cd(benzimidazolate)1.87(2-methyl-5,6-difluorobenzimidazolate)0.13 (ratio = 14:1), which exhibited a considerably reduced stepped adsorption threshold pressure while maintaining the desirable adsorption–desorption profile and capacity of CdIF-13. At 77 K, the multivariate framework exhibits stepped H2 adsorption with saturation below 50 bar and minimal desorption hysteresis at 5 bar. At 87 K, saturation of step-shaped adsorption occurs by 90 bar, with hysteresis closing at 30 bar. These adsorption–desorption profiles enable usable capacities in a mild pressure swing process above 1 mass %, representing 85–92% of the total capacities. This work demonstrates that the desirable performance of flexible frameworks can be readily adapted through the multivariate approach to enable efficient storage and delivery of weakly physisorbing species.

Exploring the Influence of Linker Substitution and Ratios on Cooperative Framework Flexibility through the Mixed-Linker Approach

“…Depending on the kinetic stability of the adsorbate-saturated open phase, desorption upon lowering adsorbate pressure can occur in a Type I or V-like manner, typically with some hysteresis . If both adsorption and desorption occur in a step-shaped manner, low-energy storage, transport, and delivery of gaseous payloads are possible due to the reduced requisite swings in pressure and/or temperature relative to those necessitated by porous materials with Type I profiles. − …”

“…Characterization of the closed phase by single-crystal X-ray diffraction suggested that the latter two features result from closer interlinker C–H···π and π···π interactions (Figure ). However, for many relevant adsorbates, such as H 2 , the adsorption threshold pressure of CdIF-13 is prohibitively high, apparently due to these stronger interlinker interactions creating a larger energy barrier for adsorption-induced framework opening.…”

Adaptive Pore Opening to Form Tailored Adsorption Sites in a Cooperatively Flexible Framework Enables Record Inverse Propane/Propylene Separation

“…Previously, we reported a congener of ZIF-7, CdIF-13, ( sod– Cd­(benzimidazolate) 2 , (CdC 14 N 4 H 10 )), , which also exhibits cooperative flexibility and step-shaped adsorption, but with nearly 50% greater adsorption capacities, negligible prestep adsorption, and much higher adsorption threshold pressures. For example, whereas ZIF-7 adsorbs propane near 0.008 bar at 298 K, CdIF-13 does so at 0.2 bar, a much-preferred pressure range for ambient temperature adsorptive separation, , obviating the need for high temperatures. Moreover, CdIF-13 retains a high degree of crystallinity through the pressure-induced phase transition, which enabled single crystal X-ray diffraction measurements of the activated phase of CdIF-13 (Figure C), which in turn helped us answer the long-standing question of the nature of the dense phase of ZIF-7 .…”

Hydrogen Storage with Aluminum Formate, ALF: Experimental, Computational, and Technoeconomic Studies

“…Until recently, conventional MOFs for hydrogen storage required low temperatures to achieve high hydrogen uptake. , Most reported MOFs [e.g., Zn 4 O­(BDC) 3 (MOF-5, BDC 2– = 1,4-benzene­dicarboxylate), Cu 3 (BTC) 2 (HKUST-1, BTC 3– = 1,3,5-benzene­tricarboxylate), and CdIF-13 (sod-Cd­(benzimidazolate) 2 )] show maximum hydrogen uptake under cryogenic temperature and high-pressure conditions, whereas some emerging MOFs, such as Ni 2 ( m- dobdc) and V-btdd ( m -dobdc 4– = 4,6-dioxido-1,3-benzene­dicarboxylate; H 2 btdd = bis­(1 H -1,2,3-triazolo­[4,5- b ],[4′,5′- i ])­dibenzo­[1,4]­dioxin), exhibit record deliverable H 2 at near-ambient temperatures and high-pressure conditions (i.e., above 100 bar). , Initial findings suggested that Ni 2 ( m- dobdc) could achieve a lower levelized cost of storage ($/kWh) relative to cryogenic storage, but not compared to compressed gas storage at 350 bar without Ni 2 ( m- dobdc) being made cheaply and able to retain relatively high hydrogen uptake (i.e., below 10 $/kg and above 15 g/kg excess uptake) . It was also found that slower charging of the storage tank greatly reduced the cost of the associated compression and refrigeration units, further improving the promise of sorbent-based systems.…”