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1. Bio-MOF Construction

      We used our PRF funding to explore the use of simple biomolecules as linking molecules for constructing bio-MOFs.  We focused our attention on using adenine, a nucleobase, as a linking molecule.  Adenine has a high interaction potential with CO₂ by virtue of its abundance of Lewis-basic sites which can interact favorably with the Lewis-acidic carbon in CO₂.  Therefore, we reasoned that adenine-based bio-MOFs would have diverse structures capable of selectively adsorbing CO₂ and might potentially be useful for CO₂ capture applications. 

2. Microporous Solids

We isolated a new crystalline 3-D cobalt-adeninate MOF, Co₂(ad)₂(CO₂CH₃)₂•2DMF,0.5H₂O (bio-MOF-11), which exhibits this coordination mode.

 Bio-MOF-11 is permanently porous, with a BET surface area of 1000 m²/g calculated from N₂ adsorption studies. Its pores are densely lined with Lewis-basic amino and pyrimidine groups. This feature prompted us to examine its CO₂ adsorption properties. We first collected the CO₂ isotherm at 273 K. It is completely reversible, exhibits a steep rise at low pressures, and reaches a maximum of 6.0 mmol/g at 1 bar. Comparatively, the N₂ uptake at 273 K is only 0.43 mmol/g at 1 bar. At 298 K, the maximum CO₂ uptake is 4.1 mmol/g compared to only 0.13 mmol/g N₂. These CO₂ capacities are higher than some of the most promising MOF materials which have been studied for CO₂ storage and separation. The calculated selectivity for CO₂ vs. N₂ is 81:1 CO₂:N₂ at 273 K and 75:1 at 298 K. To our knowledge, these selectivity values are among the best reported to date for MOF materials.

3. Coordination Mode 3: metal-adeninate cage building blocks for constructing MOFs

We found that introducing biphenyldicarboxylic acid (H₂-BPDC) to reactions between adenine and zinc acetate in dimethylformamide (DMF) yielded a single crystalline material formulated as Zn₈(ad)₄(BPDC)₆O•2Me₂NH₂,8DMF,11H₂O, heretofore referred to as bio-MOF-1 (BPDC = biphenyldicarboxylate).

The framework structure is anionic, and dimethylammonium (DMA) cations reside in the channels. Bio-MOF-1 maintains its crystallinity after soaking for several weeks in various organic solvents and, importantly, water. Nitrogen adsorption studies yielded a type-I isotherm characteristic of a microporous material (BET SA = ~1700 m²/g).  

We recently showed that cation-exchange can be used to modulate the surface area and pore volume of bio-MOF-1 and its capacity for CO₂. Specifically, we replaced the DMA cations in the as-synthesized material (a) with either tetramethylammonium cations (TMA), tetraethylammonium cations (TEA), or tetrabutylammonium cations (TBA) to systematically reduce the pore volume and generate three new materials (b-d, respectively) (Table 1).

Table 1.

^a surface area, m²/g, ^b pore volume, cm³/g (cm³/mol); ^c mmol/g (molecules CO₂/formula unit); ^d isosteric heat of adsorption, kJ/mol at 1 bar.

We studied the CO₂ adsorption of a-d at 273 K (Table 1). Interestingly, the CO₂ capacity of a-d did not scale with pore volume and BET surface area. b adsorbed the largest amount of CO₂ (4.5 mmol/g at 1 bar), followed by c (4.2 mmol/g at 1 bar). The as-synthesized material, a, which has the largest BET surface area and pore volume, and d, which has the smallest BET surface area and pore volume adsorbed 3.41 mmol/g and 3.44 mmol/g CO₂, respectively, at 1 bar. From these data, we can conclude that pores with smaller volumes may be better suited for adsorbing CO₂, because b-d all adsorb more CO₂ per formula unit than a, even though they have smaller pore volumes.

      We have also derived conditions that yield discrete zinc-adeninate octahedral cages as building blocks (Figure 5). Three biphenyldicarboxylates coordinate to the Zn²⁺ tetrahedra on each open face of the cage and serve to connect the cages together into a diamond-like network, where each zinc-adeninate octahedral cage effectively serves as a tetrahedral four-connected building unit (Figure 5). By virtue of the length of the dicarboxylate linker and the size of the zinc-adeninate building unit, the resulting framework exhibits large mesoporous cavities.  This material has the highest pore volume of any known crystalline material.  We expect this strategy will yield more highly porous materials which can be exploited for various applications.

5. Summary

      The starter funds we received from the PRF allowed us to develop a new class of porous metal-adeninate materials.  These materials range from 0-D macrocycles, to 1-D polymers and 2-D sheets, to micro- and mesoporous 3-D solids.  We have shown that we can control the structure of these materials to control their CO₂ adsorption properties.  These results have been reported in four communications to the Journal of the American Chemical Society.