High Gravimetric and Volumetric Ammonia Capacities in Robust Metal-Organic Frameworks Prepared via Double Postsynthetic Modification.

Ammonia is a promising energy vector that can store the high energy density of hydrogen. For this reason, numerous adsorbents have been investigated as ammonia storage materials, but ammonia adsorbents with a high gravimetric/volumetric ammonia capacity that can be simultaneously regenerated in an energy-efficient manner remain underdeveloped, which hampers their practical implementation. Herein, we report Ni_acryl_TMA (TMA = thiomallic acid), an acidic group-functionalized metal-organic framework prepared via successive postsynthetic modifications of mesoporous Ni2Cl2BTDD (BTDD = bis(1H-1,2,3,-triazolo [4,5-b],-[4',5'-i]) dibenzo[1,4]dioxin). By virtue of the densely located acid groups, Ni_acryl_TMA exhibited a top-tier gravimetric ammonia capacity of 23.5 mmol g-1 and the highest ammonia storage of 0.39 g cm-3 at 1 bar and 298 K. The structural integrity and ammonia storage capacity of Ni_acryl_TMA were maintained after ammonia adsorption-desorption tests over five cycles. Temperature-programmed desorption analysis revealed that the moderate strength of the interaction between the functional groups and ammonia significantly reduced the desorption temperature compared to that of the pristine framework with open metal sites. The structures of the postsynthetic modified analogues were elucidated based on Pawley/Rietveld refinement of the synchrotron powder X-ray diffraction patterns and van der Waals (vdW)-corrected density functional theory (DFT) calculations. Furthermore, the ammonia adsorption mechanism was investigated via in situ infrared and vdW-corrected DFT calculations, revealing an atypical guest-induced binding mode transformation of the integrated carboxylate. Dynamic breakthrough tests showed that Ni_acryl_TMA can selectively capture traces of ammonia under both dry and wet conditions (80% relative humidity). These results demonstrate that Ni_acryl_TMA is a superior ammonia storage/capture material.

High Ammonia Uptake of a Metal-Organic Framework Adsorbent in a Wide Pressure Range.

Although numerous porous adsorbents have been investigated for NH3 capture applications, these materials often exhibit insufficient NH3 uptake, low NH3 affinity at the ppm level, and poor chemical stability against wet NH3 conditions. The NH3 capture properties of M2 (dobpdc) complexes (M=Mg2+ , Mn2+ , Co2+ , Ni2+ , and Zn2+ ; dobpdc4- =4,4-dioxidobiphenyl-3,3-dicarboxylate) that contain open metal sites is presented. The NH3 uptake of Mg2 (dobpdc) at 298 K was 23.9 mmol g-1 at 1 bar and 8.25 mmol g-1 at 570 ppm, which are record high capacities at both pressures among existing porous adsorbents. The structural stability of Mg2 (dobpdc) upon exposure to wet NH3 was superior to that of the other M2 (dobpdc) and the frameworks tested. Overall, these results demonstrate that Mg2 (dobpdc) is a recyclable compound that exhibits significant NH3 affinity and capacity, making it a promising candidate for real-world NH3 -capture applications.

Diamine Functionalization of a Metal-Organic Framework by Exploiting Solvent Polarity for Enhanced CO2 Adsorption.

Diamine-appended metal-organic frameworks (MOFs) exhibit exceptional CO2 adsorption capacities over a wide pressure range because of the strong interaction between basic amine groups and acidic CO2. Given that their high CO2 working capacity is governed by solvent used during amine functionalization, a systematic investigation on solvent effect is essential but not yet demonstrated. Herein, we report a facile one-step solvent exchange route for the diamine functionalization of MOFs with open metal sites, using an efficient method to maximize diamine loading. We employed an MOF, Mg2(dobpdc) (dobpdc4- = 4,4'-dioxido-3,3'-biphenyldicarboxylate), which contains high-density open metal sites. Indirect grafting with N-ethylethylenediamine (een) was performed with a minimal amount of methanol (MeOH) via multiple MeOH exchanges and diamine functionalization, resulting in a top-tier CO2 adsorption capacity of 16.5 wt %. We established the correlation between N,N-dimethylformamide (DMF) loading and infrared peaks, which provides a simple method for determining the amount of the remaining DMF in Mg2(dobpdc). All interactions among Mg, DMF, diamine, and solvent were analyzed by van der Waals (vdw)-corrected density functional theory (DFT) calculations to elucidate the effect of chemical potential on diamine grafting.