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Cooperative Carbon Capture Capabilities in Multivariate MOFs Decorated with Amino Acid Side Chains: A Computational Study Michael L Drummond, Thomas Richard Cundari, and Angela K Wilson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4053538 • Publication Date (Web): 05 Jun 2013 Downloaded from http://pubs.acs.org on June 7, 2013
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Cooperative Carbon Capture Capabilities in Multivariate MOFs Decorated with Amino Acid Side Chains: A Computational Study Michael L. Drummond†, Thomas R. Cundari, and Angela K. Wilson* Center for Advanced Scientific Computing and Modeling (CASCaM), Department of Chemistry, University of North Texas, Denton, Texas 76203-5017 KEYWORDS Metal-organic frameworks (MOFs), carbon capture, bioinspired, Grand Canonical Monte Carlo (GCMC), mixed-component MOFs Supporting Information Placeholder ABSTRACT: Multivariate metal-organic frameworks (MTV-MOFs) allow for the incorporation of multiple functional groups appended to a uniform backbone, similar to the construction of proteins from an amino acid library. This analogy is explicitly developed through the simulation of CO2 adsorption isotherms of MOF-5 backbones decorated with amino acid side chains (AAMTV-MOFs). Differences in excess adsorption among these AA-MTV-MOFs can be explained by the varying functional groups at lower pressures and by structural properties of the framework at higher loadings. Synergistic carbon capture properties are observed in silico, with the degree of cooperativity governed by the strength of the CO2-AA interaction, the arrangement of the AA side chains, and the loading.
1. Introduction. Over the past decade, metalorganic frameworks (MOFs) have emerged as a subject of intense research, with potential applications in diverse areas such as energy gas storage,(1, 2) carbon capture,(3, 4) gas separation,(5, 6) catalysis,(7) chemical sensing,(8) and medicine(9). Scientific interest in MOFs can be attributed to two factors: their high porosity, with surface areas reaching over 7000 m2/g,(10) and the essentially limitless variety of possible MOF structures. The combination of metal-based clusters (often termed secondary building units, SBUs (11)) joined by a vast array of organic linkers leads to a combinatorial explosion in the number of resulting structures; postsynthetic modification(12) expands the possibilities even further by allowing directed in situ adjustments. Thus, the allure of MOFs is that among this practically infinite array of possible materials, a MOF with the structure and chemical functionality ideally suited for the task of interest can be identified and synthesized. A recent subset of MOFs will be particularly useful in the development of specifically tailored, rationally designed materials: mixed-component MOFs (MC-MOFs). As Burrows details,(13) in an MC-MOF, a mixture of different components (either linkers or metals) serves the same structural role throughout the MOF crystal. For
example, in one unit cell, Cd might be present, but Ni might take its place in the neighboring, otherwise identical unit cell (14). MC-MOFs can demonstrate many advantages over traditional pure component MOFs, such as affording the synthesis of otherwise inaccessible materials,(15) allowing for simultaneous separation and highcapacity gas storage,(16) and providing useful physical properties (e.g., hydrophobicity (17) or tunable photoluminescence (18)). Synergistic gas adsorption properties have also resulted from MC-MOFs, where mixing two linkers into a single MC-MOF generated improved storage capacity(19) or selectivity (20) compared to either pure component MOF. The most versatile MC-MOF system is the multivariate MOFs (MTV-MOFs) detailed by Yaghi and coworkers, who have shown that up to eight structurally identical but chemically distinct organic linkers can be incorporated into a single MOF-5-like structure, capable of yielding MTV-MOFs with gas adsorption properties superior to any analogous pure component MOF-5 (21). These MTV-MOFs were synthesized from mixtures of 1,4-benzenedicarboxylate (BDC)-based linkers bearing different substituent functionalities (e.g., –NH2, –Br, – NO2, –OCH2Ph, etc.). Thus, each MTV-MOF possesses
Figure 1. Sample AA-MTV-MOF supercell shown along the three crystallographic axes. Inner layers are labeled A-F (see below)
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Figure 2. Simulated excess adsorption isotherms calculated for multiple AA-MTV-MOFs. (Inset) A close-up view of the pressure region between 200 and 800 kPa.
the same common backbone, BDC. MTV-MOFs are therefore constructured in a manner conceptually similar to how proteins are assembled from an alphabet of amino acids, each of which contains the same common backbone (H2N-CH-COOH) but bearing different side chain moieties. Combining a common amino acid backbone with variable side chains confers two simultaneous advantages to proteins: the variability allows construction of tailored protein frameworks that can perform functions as diverse as catalysis, signaling, motion, etc., but due to the common peptide backbone, only a single uniform process (viz. translation) is needed to construct any protein, regardless of its ultimate task (22). An artificial MOF system following this bifunctional principle would enjoy these same advantages: the ability to perform disparate tasks would be imparted by a localized variable element (e.g., varying BDC substituents), but a common backbone would allow for one synthetic protocol or infrastructure to construct multiple varied structures. In this Communication, computational simulations are applied to explore the analogy between proteins and MOFs by considering one task – CO2 adsorption (3, 4) – along with one common backbone – the previously utilized BDC of the MTV-MOFs (21) – combined with a subset of Nature’s amino acid side chain alphabet to impart variability. 2. Computational Details. The method applied to model the adsorption of CO2 by these amino acid-
MTV-MOFs (AA-MTV-MOFs) is Grand Canonical Monte Carlo (GCMC) theory, a technique routinely applied (23) to predict the adsorption of gas molecules by porous sorbents, as described in greater detail elsewhere (24). All structures were built using MOE2010 (25), starting with the experimental coordinates for MOF-5 packaged with the Music code (26). Amino acid side chain models were attached at the 2 and 5 positions of all internal BDC linkers, as shown in the sample AA-MTV-MOF supercell in Figure 1. These supercells contain amino acid substituents in all interior unit cells, with one layer of MOF-5 on all sides. The amino acid side chain models studied were Ala, Cys, Asp, Ser, and Thr, as well as His, Phe, and Tyr without their Cβ methylene groups and Arg without the propyl chain (i.e., only guanidinium), due to steric constraints. Arg and His were chosen due to previously demonstrated abilities (27, 28) to adsorb CO2 well, and the others were selected to facilitate interesting comparisons (vide infra). To maintain overall charge neutrality, the positive Arg moiety was always paired with a negative Asp moiety. Only physical adsorption was considered (i.e., chemisorption of CO2 by amine via carbamate formation (29) was not investigated), as is common for computational studies of carbon capture with MOFs (3032). The CO2 adsorbate was modeled using the parameters and atomic charges of the TraPPE force field (33). For the MOF-5 backbone atoms (i.e., the BDC linker and the Zn4O SBU), partial charges were assigned as the DDEC(c2) charges of Manz and Sholl (34), with minor
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adjustments to maintain overall charge neutrality (see SI the for final values); Lennard-Jones parameters were taken from the Universal Force Field (35). For the amino acid side chains, Lennard-Jones parameters and partial charges were taken from the AMBER99 force field (36), as implemented in MOE2010 (25), again with minor adjustments to atomic changes to maintain neutrality in the final supercell. Cross-interaction parameters were determined with the Lorentz-Berthelot mixing rules, i.e., the arithmetic mean for σ and the geometric mean for ε. Combining parameters from different force fields in this fashion may affect the absolute quantitative accuracy, but as the overall goal of this work is to make comparisons in the CO2 adsorption of different AA-MTV-MOFs, it is essential that the side chains – the only variable portion – be modeled accurately, and thus a force field suitable for amino acids (e.g., Amber) must be used. Unfortunately, there are no MOF parameters in Amber, and so a general force field like UFF must be applied; the fact that each AA-MTV-MOF contains identical cores treated with UFF should thereby minimize the relative error from this mixing of force fields. To generate the initial AA-MTV-MOF structures, the coordinates of the MOF-5 backbone were fixed, side chains were added as described above, AMBER99 parameters were assigned by MOE2010, and side chain geometries were minimized to a root-mean-square gradient of
