Compared to other porous materials, task-specific site-engineered ionic covalent organic polymers (iCOPs) represent promising scaffolds for sustainable catalysis and environmental remediation. In particular, high-temperature selective scavenging and co-catalyst-free benign fixation of carbon dioxide (CO2) are critical to carbon-footprint reduction. On the other hand, multiphasic iodine sequestration is important as it deciphers real-time nuclear hazard mitigation. Herein, we report devising of heteroatom, π-conjugation, and pyridinum ion enriched novel iCOP that displays reasonable CO2 adsorption at diverse temperatures with highly recurrent multicyclic operation. Remarkably, rise in sorption temperature to 313 K drastically improve the CO2/N2 selectivity by 233%, signifying the materials’ great potential in point-source carbon capture. This cationic organic polymer acts as solvent, co-catalyst and metal-free CO2 cycloaddition catalyst that works effectively under atmospheric pressure with appreciable reusability. Unprecedentedly, the non-redox CO2 fixation is exclusive towards heteroatom-containing epoxides. The crucial role of pore-affixed task-specific sites in the activation of ether-based substrates is rationalized from reaction profile energy diagram using density functional theory (DFT) study, diverse control experiments and performance comparison of an isoskeletal iCOP that precludes essential functionalities. The iCOP further demonstrates rapid incarceration of iodine with ultrahigh uptake in vapor (wt%) and solution (1117 mg·g−1) phase, which surpasses several contemporary iodine adsorbents. A battery of experimental validations together with DFT-derived electrostatic potential mapping elaborate the synergistic role of multiple polar sites in the iCOP for bi-phasic iodine adsorption via charge transfer-induced generation of polyiodide species. Notably, the iodine-loaded material can be quickly recycled, promoting an environment-friendly route to high-performance practical sequestration of nuclear waste.
Unprecedented atmospheric emission of carbon dioxide (CO2) by anthropogenic activities is one of the main reprobates for major environmental problems like global warming, climate change, and ocean acidification. In carbon capture and utilization (CCU) technology, solid porous adsorbents with low heat capacities are more promising. Conventional amine-based scrubbing suffers from its corrosive nature, chemisorption and high energy penalty in regeneration. Importantly, high-temperature selective CO2 capture is demanding in view of practical flue gas separation. However, this feat is tricky as uptake capacity and selectivity are enormously reduced with rising temperature, which demands tailor made adsorbents with molecule-specific interaction. As an integral part of adsorption, utilization of inexpensive, and nontoxic CO2 as a C1 feedstock for producing chemicals is of particular importance as it offers the economical, sustainable, and renewable synthesis of value-added products. In particular, atom-efficient conversion of CO2 to cyclic carbonate represents a promising fixation route. The product is widely used in fine chemicals for industrial purposes like electrolytes for lithium-ion cells, precursor of polycarbonate monomer, and utilization as polar aprotic solvents. However, the lower reactivity of thermodynamically stable CO2 requires the backing of a suitable catalyst. Homogeneous catalysts, including ionic liquids (ILs) and their derivatives, have shown promise in CO2 conversion. But, their non-recyclability is a great obstacle to practical applicability. Even in a heterogeneous system, metal-free catalysts are more desirable as they devoid leaching of toxic metals during repeated usages. Over the last few decades, researchers have emphasized on developing task-specific functional site-grafted porous organic systems like covalent organic frameworks (COFs) and porous organic polymers (POPs) for CO2 capture and fixation. However, catalytic performances of majority of these materials are limited by backing of an external co-catalyst and/ or cumbersome multi-step post-functionalization. Then again, many of such organic polymers include assistance of toxic solvents and/ or higher CO2 pressure during the progress of reaction. On the other hand, achieving high selectivity for CO2 over other gases under practically relevant condition is a significant challenge. In this scenario, synthesis of purely organic polymers with in-situ fabricated antagonistic functionality can surpass the disputes for organocatalysts and represent as innovative solution towards carbon-footprint reduction. Based on the above notions, coupled with the fact that efficient CO2 fixation requires both acidic and basic sites, fabrication of bi-modal organo-catalysts is of utmost importance. However, such antagonistic sites often neutralize each other during self-assembly, which is a major challenge in accelerating CO2 fixation. Therefore, the construction of metal-free systems with multiple polar functional sites for atmospheric pressure CO2 conversion in the absence of solvent and co-catalyst is considered as most important global agenda.
On a different note, the nuclear power is the paradigm route to solve the energy demand of a rapidly growing population in a carbon-free manner. However, the continuous banishing of nuclear energy wastes into the atmosphere severely affects normal life and human health. Specifically, molecular iodine is one of the volatile nuclear pollutants that emerges during the processing of nuclear fuel rods. While non-radioactive isotopes (125 I) are harmless, the release of long-lived (half-life∼15.7×10 6 years) and highly volatile 129 I poses deadly risks, as seen from incidents like Fukushima in 2011 over land and ocean of the Pacific region in Europe. Unlike complex chemical precipitation method, physical adsorption is the most promising and widely acceptable conduit for encapsulating radioiodine to prevent future hazardous effects from industrial nuclear waste. Hence, diverse adsorbents like Ag-loaded zeolites, porous carbon, metal–organic frameworks (MOFs), and COFs are employed to scavenge iodine. However, high cost of silver-based materials, and poor tailorability of carbon do not allow their usage over the real-time platform. Though MOFs and COFs have shown high iodine uptake; their studies are often restricted to single phase (vapor or solution). Further, they usually lack systematic investigations of capture mechanism at molecular level to upgrade performance characteristics. In this context, covalent organic polymers (COPs) gained massive attention owing to their synthetic efficacy and simplicity in the introduction of required functionality without tedious post-synthetic modification. These features make COPs more appropriate for a wide range of applications, and bestow them as excellent candidates to tackle difficulties in the energy, and environmental sectors. Ionic covalent organic polymer (iCOP) is a subset of charged organic polymer that attracted significant attention for numerous applications because of their tunable structural parameters as per the chosen application. Importantly, charged backbone of iCOPs with free counter balancing ions confer notable ion exchange performance. Specifically, the ionic moieties in halogen-based iCOPs can exert strong dipole-quadrupole interaction with CO2 to improve capture performance of this major greenhouse gas. In addition, these anions of iCOPs can act as active nucleophilic reagents to facilitate the epoxide-CO2 cycloaddition, thereby excluding the role of external co-catalyst. On the other hand, combined presences of Lewis basic nitrogen sites, polarizable pore surface (conjugated aromatic rings), and an ionic backbone (existence of exchangeable anions) synergistically serve as an interactive host matrix for iodine/ polyiodide in both solid and solution phase.
Based on these notions, we fabricated an iCOP via incisive amalgamation of three different functionalities including abundant π-electrons, heteroatoms, and pyridinum ion. Such fascinating attributes benefit the material exhibiting temperature variant CO2 adsorption with a notable CO2/N2 selectivity enhancement upon rise in temperature. Further, the iCOP acts as a recyclable organo-catalyst in solvent and co-catalyst free cycloaddition of epoxides at atmospheric CO2 pressure under less harsh condition. Selective cycloaddition of heteroatom containing epoxides is one of the unique features of the catalyst that is explicitly validated from a battery of control experiments, and theoretical studies. Assimilation of ample polar-site allows the polymeric structure displaying substantial iodine uptake in vapor and organic phase, which surpasses many porous polymers. High iodine incarceration by this iCOP was mainly attributed to multilayer adsorption via polyiodide formation, as supported from experimental and theoretical analyses. Overall, the work contributes to the forefront of sustainable catalysis and environmental remediation through purposeful engineering of multimodal task-specific
