Timur Islamoglu

Postsynthetic Tuning of Metal–Organic Frameworks for Targeted Applications

Metal-organic frameworks (MOFs) are periodic, hybrid, atomically well-defined porous materials that typically form by self-assembly and consist of inorganic nodes (metal ions or clusters) and multitopic organic linkers. MOFs as a whole offer many intriguing properties, including ultrahigh porosity, tunable chemical functionality, and low density. These properties point to numerous potential applications, including gas storage, chemical separations, catalysis, light harvesting, and chemical sensing, to name a few. Reticular chemistry, or the linking of molecular building blocks into predetermined network structures, has been employed to synthesize thousands of MOFs. Given the vast library of candidate nodes and linkers, the number of potentially synthetically accessible MOFs is enormous. Nevertheless, a powerful complementary approach to obtain specific structures with desired chemical functionality is to modify known MOFs after synthesis. This approach is particularly useful when incorporation of particular chemical functionalities via direct synthesis is challenging or impossible. The challenges may stem from limited stability or solubility of precursors, unwanted secondary reactivity of precursors, or incompatibility of functional groups with the conditions needed for direct synthesis. MOFs can be postsynthetically modified by replacing the metal nodes and/or organic linkers or via functionalization of the metal nodes and/or organic linkers. Here we describe some of our efforts toward the development and application of postsynthetic strategies for imparting desired chemical functionalities in MOFs of known topology. The techniques include methods for functionalizing MOF nodes, i.e., solvent-assisted ligand incorporation (SALI) and atomic layer deposition in MOFs (AIM) as well as a method to replace structural linkers, termed solvent-assisted linker exchange (SALE), also known as postsynthethic exchange (PSE). For each functionalization strategy, we first describe its chemical basis along with the requirements for its successful implementation. We then present a small number of examples, with an emphasis on those that (a) convey the underlying concepts and/or (b) lead to functional structures (e.g., catalysts) that would be difficult or impossible to access via direct routes. The examples, however, are only illustrative, and a significant body of work exists from both our lab and others, especially for the SALE/PSE strategy. We refer readers to the papers cited and to the references therein. More exciting, in our view, will be new examples and new applications of the functionalization strategies-especially applications made possible by creatively combining the strategies. Underexplored (again, in our view) are implementations that impart electrical conductivity, enable increasingly selective chemical sensing, or facilitate cascade catalysis. It will be interesting to see where these strategies and others take this compelling field over the next few years.

Impact of post-synthesis modification of nanoporous organic frameworks on small gas uptake and selective CO2 capture

Porous organic polymers containing nitrogen-rich building units are among the most promising materials for selective CO2 capture and separation applications that impact the environment and the quality of methane and hydrogen fuels. In this study, we report on post-synthesis modification of nanoporous organic frameworks (NPOFs) and its impact on gas storage (H2, CH4, CO2) and selective CO2 binding over N2 and CH4 under ambient conditions. The synthesis of NPOF-4 was accomplished via acid catalyzed cyclotrimerization reaction of 1,3,5,7-tetrakis(4-acetylphenyl)adamantane in ethanol/xylenes. NPOF-4 is microporous and has high surface area (SABET = 1249 m2 g−1). Post-synthesis modification of NPOF-4 by nitration afforded NPOF-4-NO2 and its subsequent reduction resulted in an amine-functionalized framework NPOF-4-NH2 that exhibits improved gas storage capacities and very high CO2/N2 (139) and CO2/CH4 (15) selectivities compared to NPOF-4.

Targeted synthesis of a mesoporous triptycene-derived covalent organic framework

The synthesis and characterization of a highly porous triptycene-derived covalent organic framework (TDCOF-5) and its performance in small gas storage are reported. TDCOF-5 crystallizes into a 2D mesoporous network that contains accessible boron sites, exhibits high surface area (SALang = 3832 m2 g−1), and high gas uptake under low pressure settings.

Application of pyrene-derived benzimidazole-linked polymers to CO2 separation under pressure and vacuum swing adsorption settings

Pyrene-derived benzimidazole-linked polymers (BILPs) have been prepared and evaluated for selective CO2 uptake and separation under pressure and vacuum swing conditions. Condensation of 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) with 2,3,6,7,10,11-hexaaminotriphenylene, 2,3,6,7,14,15 hexaaminotriptycene, and 3,3′-diaminobenzidine afforded BILP-11, BILP-12 and BILP-13, respectively, in good yields. BILP-12 exhibits the highest specific surface area (SABET = 1497 m2 g−1) among all known BILPs and it also has very high CO2 uptake 5.06 mmol g−1 at 273 K and 1.0 bar. Initial slope selectivity calculations indicate that BILP-11 has high selectivity for CO2/N2 (103) and CO2/CH4 (11) at 273 K. IAST selectivity calculations of BILPs at 298 K also showed high CO2/N2 (31–56) and CO2/CH4 (6.6–7.6) selectivity levels. The isosteric heats of adsorption for CO2 fall in the range of 32 to 36 kJ mol−1 and were considerably higher than those of CH4 (16.1–21.7 kJ mol−1). More importantly, the performance of pyrene-based BILPs in CO2 removal from flue gas and methane-rich gases (natural gas and landfill gas) under different industrial conditions was investigated according to evaluation criteria suggested recently by Bae and Snurr. The outcome of this study revealed that BILPs are among the best known porous materials in the field; they exhibit high working capacity, regenerability, and sorbent selection parameters. Collectively, these properties coupled with the remarkable physicochemical stability of BILPs make this class of polymers very promising for CO2 separation applications.

Synthesis and evaluation of porous azo-linked polymers for carbon dioxide capture and separation

A series of new azo-linked polymers (ALPs) was synthesized via copper(I)-catalyzed oxidative homocoupling of 2D and 3D aniline-like monomers. ALPs have moderate surface areas (SABET = 412–801 m2 g−1), narrow pore sizes (<1 nm), and high physiochemical stability. The potential applications of ALPs for selective CO2 capture from flue gas and landfill gas at ambient temperature were studied. ALPs exhibit high isosteric heats of adsorption for CO2 (28.6–32.5 kJ mol−1) and high CO2 uptake capacities of up to 2.94 mmol g−1 at 298 K and 1 bar. Ideal adsorbed solution theory (IAST) selectivity studies revealed that ALPs have good CO2/N2 (56) and CO2/CH4 (8) selectivities at 298 K. The correlation between the performance of ALPs in selective CO2 capture and their properties such as surface area, pore size, and heat of adsorption was investigated. Moreover, the CO2 separation ability of ALPs from flue gas and landfill gas under pressure-swing adsorption (PSA) and vacuum-swing adsorption (VSA) processes was evaluated. The results show that ALPs have promising working capacity, regenerability, and sorbent selection parameter values for CO2 separation by VSA and PSA processes.

Enhanced Carbon Dioxide Capture from Landfill Gas Using Bifunctionalized Benzimidazole-Linked Polymers

Tuning the binding affinity of small gases and their selective uptake by porous adsorbents are vital for effective CO2 removal from gas mixtures for environmental protection and fuel upgrading. In this study, an amine-functionalized benzimidazole-linked polymer (BILP-6-NH2) was synthesized by a combination of pre- and postsynthetic modification techniques in two steps. Presynthetic incorporation of nitro groups resulted in stoichiometric functionalization (1 nitro/phenyl) in addition to noninvasive functionalization, where more than 80% of the surface area maintained compared to BILP-6. Experimental studies presented enhanced CO2 uptake and CO2/CH4 selectivity in BILP-6-NH2 compared to BILP-6, which are governed by the synergetic effect of benzimidazole and amine moieties. DFT calculations were used to understand the interaction modes of CO2 with BILP-6-NH2 and confirmed the efficacy of amine groups. Encouraged by the enhanced uptake and selectivity in BILP-6-NH2, we have evaluated its performance in landfill gas separation under vacuum swing adsorption (VSA) settings, which resulted in very promising working capacity and sorbent selection parameters outperforming most of the best solid adsorbent in the literature.

Benzothiazole- and benzoxazole-linked porous polymers for carbon dioxide storage and separation

Incorporation of CO2-philic heteroatoms (i.e. N, S, and O) into porous organic polymers has been instrumental in achieving selective CO2 capture. Here, we report the synthesis of porous benzothiazole and benzoxazole linked polymers which have sulfur and oxygen atoms, respectively, in addition to the nitrogen functionality. Their structural properties have been analyzed and compared to their analogous benzimidazole linked polymers which have only nitrogen heteroatoms. The polymers exhibit high surface areas (SABET = 698–1011 m2 g−1), high physicochemical stability, and considerable CO2 storage capacity. Low pressure gas uptake experiments were used to calculate the binding affinity of small gas molecules and revealed that the polymers have high heats of adsorption (Qst) for CO2 (28.7–33.6 kJ mol−1). Comparison of CO2 uptakes and Qst values of benzothiazole-, benzoxazole- and benzimidazole-linked polymers demonstrated that smaller pores facilitate CO2 adsorption with higher Qst values and the total CO2 uptake capacity mainly depends on the surface areas provided that the pore sizes are significantly small in lower micropore regions. The reported polymers also show moderate to high adsorption selectivity for CO2/N2 (40–78) and CO2/CH4 (5.7–7.8) as determined from the Ideal Adsorbed Solution Theory (IAST) calculation using pure gas isotherms at 298 K.

A cost-effective synthesis of heteroatom-doped porous carbons as efficient CO2 sorbents

Carbon dioxide release into the atmosphere from fossil fuel burning has been identified as the major reason behind global warming. This has triggered immense interest in developing cost-efficient methods and materials for CO2 capture in recent years. In this study, benzimidazole was used as an inexpensive and commercially available single source precursor for carbon and nitrogen to successfully produce heteroatom-doped porous carbons (BIDCs) with remarkable CO2 capture properties. Our synthetic protocol involves solid-state addition of potassium hydroxide to the benzimidazole precursor to prevent its evaporation and subsequent carbonization–activation of the nonvolatile powdery mixture to ensure simultaneous porosity formation and heteroatom doping. The porous structure and heteroatom doping level can be controlled by tuning the KOH/benzimidazole ratio and carbonization temperature. The highest CO2 uptake values at 0.1 bar (1.60 mmol g−1), 1 bar (5.46 mmol g−1), and 30 bar (28.10 mmol g−1) were realized for three different carbons at 298 K. These diverse CO2 capture performances originate from a unique combination of surface chemistry and porous architecture for each BIDC. The carbon materials are regenerable and highly promising for CO2 separation or storage.

Presence versus Proximity: The Role of Pendant Amines in the Catalytic Hydrolysis of a Nerve Agent Simulant

Amino-functionalized zirconium-based metal-organic frameworks (MOFs) have shown unprecedented catalytic activity compared to non-functionalized analogues for hydrolysis of organophosphonate-based toxic chemicals. Importantly, the effect of the amino group on the catalytic activity is significantly higher in the case of UiO-66-NH2 , where the amino groups reside near the node, compared to UiO-67-m-NH2 , where they are directed away from the node. Herein, we show that the proximity of the amino group is crucial for fast catalytic activity towards hydrolysis of organophosphonate-based nerve agents. The generality of the observed amine-proximity-dictated catalytic activity has been tested on two different MOF systems which have different topology. DFT calculations reveal that amino groups on all the MOFs studied are not acting as Brønsted bases; instead they control the microsolvation environment at the Zr6 -node active site and therefore increase the overall catalytic rates.

Improvement of Methane–Framework Interaction by Controlling Pore Size and Functionality of Pillared MOFs

The rational design of functionalized porous metal-organic frameworks (MOFs) for gas adsorption applications has been applied using three spacer ligands H2DPT (3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine), DPT (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine), and BPDH (2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) to synthesize TMU-34, [Zn(OBA)(H2DPT)0.5]n·DMF, TMU-34(-2H), [Zn(OBA)(DPT)0.5]n·DMF, and TMU-5, [Zn(OBA)(BPDH)0.5]n·1.5DMF, respectively. By controlling the pore size and chemical functionality of these three MOFs, we can improve the interactions between CO2 and especially CH4 with the frameworks. Calculated Qst(CH4) for TMU-5, TMU-34, and TMU-34(-2H) are 27, 23, and 22 kJ mol-1, respectively. These Qst values are among the highest for CH4-framework interactions. For systematic comparison, two reported frameworks, TMU-4 and TMU-5, have been compared with TMU-34 and TMU-34(-2H) in CO2 adsorption.