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Bio-MOF Construction

            For CO₂ capture, we have focused on using adenine as a linking biomolecule because it has an amino group and pyrimidine nitrogens.  We note that alkylamine-containing liquids are currently used as chemisorbents for CO₂ and therefore Lewis-basic amine sites within MOFs are expected to improve selectivity toward adsorption of CO₂.  In addition, adenine is rigid, which should facilitate formation of permanently porous frameworks, and it has multiple coordination modes.  We have begun exploring the coordination chemistry of adenine in the context of forming bio-MOFs and have successfully produced a number of new bio-MOF materials and other solid-state structures which are permanently porous and have interesting inclusion and adsorption properties.  These materials and their CO₂ adsorption properties are detailed below.

Zn₆(adeninate)₆(pyridine)₆(dimethylcarbamate)₆   

This material consists of macrocycles that self-assemble into a porous 3-D structure.  Single crystal X-ray diffraction data revealed that six Zn²⁺ occupy the vertices of the macrocycle and adeninates bridge the Zn²⁺ through their imidazolate nitrogens. Each Zn²⁺ binds in a tetrahedral fashion to two adeninates, one pyridine molecule, and one dimethylcarbamate anion (formed in situ). Since only the imidazolate nitrogens of the adeninate coordinate the Zn²⁺, N1 and the amino group remain available for hydrogen bonding.  The macrocycles self-assemble into an extended structure via cooperative adeninate-adeninate hydrogen bonding interactions. This stucture consists of alternating layers of macrocycles that stack in an a-b-c fashion. Each macrocycle forms a total of 12 hydrogen bonds (two per adeninate) with its six nearest-neighbor macrocycles within the structure. This packing motif results in the formation of cylindrical cavities (~ 5 × 20 Å) arranged peridically throughout the 3-D structure. The confines of each cavity are defined by one central macrocycle and fragments of the six nearest-neighbor macrocycles. Inspection of the structure reveals that three pyridine rings occlude the entrance to each cavity, resulting in an aperture measuring only ~1.2 Å. In order to access the cavities, we heated the material to remove a portion of the pyridine molecules to increase the pore aperture and allow diffusion of guest molecules within the structure. Remarkably, the powder X-ray diffraction pattern of the heated sample matches that of the as-synthesized material, indicating that the material maintains its structural integrity.  After removing the coordinated pyridine via heating, we found that the material can selectively adsorb CO₂ over N₂ and that we could tune the selectivity by carefully controlling the amount of pyridine stripped from the material.  More details can be found in a recent paper. ADDIN EN.CITE An20091211121117An, J.Fiorella, R.Geib, S. J.Rosi, N. L.Journal of the American Chemical SocietyJournal of the American Chemical SocietyJ. Am. Chem. Soc.J Am Chem Soc8401-84031312009¹ 

Bio-MOF-1

Unlike traditional MOFs, bio-MOF-1 consists of rigid metal-adeninate columnar building units which are connected together by biphenyldicarboxylate linking molecules to generate a 3-D MOF with 1-D pores.  Other structural details are included in a recent publication. ²  Bio-MOF-1 is anionic; therefore, small cations are distributed within the channels to balance the charge.  The cations are mobile and can be removed and replaced with a variety of other cationic molecules via simple cation exchange experiments.  This aspect enables facile tuning of the pore size, shape, and functionality.  Therefore, bio-MOF-1 can be used as a ‘universal scaffold' to produce a variety of functionally unique materials simply by changing the identity of the guest cation species (vide infra).   Powder diffraction experiments reveal that bio-MOF-1 is stable in a number of organic solvents, water, and biological buffers, indicating that it can be used in a variety of environments.  It is also permanently porous with a BET surface are of approximately 1700 m²/g, as evidenced by gas sorption measurements performed on the evacuated material.  These initial studies on bio-MOF-1 reveal several encouraging details regarding bio-MOFs:  1) they can exhibit high permanent porosity; 2) they can be exceptionally stable under a wide variety of conditions, including biological and environmental media; and 3) the functionality of their pores can be varied by performing facile cation-exchange experiments.

Anionic bio-MOF-1 and other anionic bio-MOFs present the opportunity to systematically study the influence of the counter-cation on the adsorption and recognition properties of the material.  Instead of preparing a new MOF for every target application, we can simply substitute new cations into the pores of bio-MOF-1 to endow the pores with the requisite dimensions and functionality needed for particular applications.  For example, we can introduce chiral cations to generate pores with chiral recognition capabilities, or we can introduce certain receptor molecules that endow the framework with sequestration capacity for specific target species.  All of the cation-exchanged varieties of bio-MOF-1 exhibit the structural rigidity and stability of the parent material.

We have found that the identity of the adsorbed cation can significantly impact CO₂ uptake capacity in a somewhat unusual fashion.  Specifically, we studied the CO₂ adsorption properties of bio-MOF-1 as a function of the size of the organic cation.  The organic cations we chose were dimethylammonium (H₂NMe₂), tetramethylammonium (NMe₄), and tetraethylammonium (NEt₄).  As the size of the organic cation increases, the BET surface area decreases, as expected.  However, the capacity for CO₂ uptake scales neither with the size of the organic cation nor the BET surface area.  In fact, we found that the NMe₄-exchanged material has the highest CO₂ capacity, followed by NEt₄.  The as-synthesized material, which has the smallest cation, adsorbs the least amount of CO₂.  This result suggests that there may be an optimal pore size for CO₂ adsorption.