Purification of Waste-Generated Biogas Mixtures Using Covalent Organic Framework's High CO2 Selectivity.

Development of crystalline porous materials for selective CO2 adsorption and storage is in high demand to boost the carbon capture and storage (CCS) technology. In this regard, we have developed a ß-keto enamine-based covalent organic framework (VM-COF) via the Schiff base polycondensation technique. The as-synthesized VM-COF exhibited excellent thermal and chemical stability along with a very high surface area (1258 m2 g-1) and a high CO2 adsorption capacity (3.58 mmol g-1) at room temperature (298 K). The CO2/CH4 and CO2/H2 selectivities by the IAST method were calculated to be 10.9 and 881.7, respectively, which were further experimentally supported by breakthrough analysis. Moreover, theoretical investigations revealed that the carbonyl-rich sites in a polymeric backbone have higher CO2 binding affinity along with very high binding energy (-39.44 KJ mol-1) compared to other aromatic carbon-rich sites. Intrigued by the best CO2 adsorption capacity and high CO2 selectivity, we have utilized the VM-COF for biogas purification produced by the biofermentation of municipal waste. Compared with the commercially available activated carbon, VM-COF exhibited much better purification ability. This opens up a new opportunity for the creation of functionalized nanoporous materials for the large-scale purification of waste-generated biogases to address the challenges associated with energy and the environment.

Catching an Oxo Vanadate Porous Acetylacetonate Covalent Adaptive Catalytic Network that Renders Mustard-Gas Simulant Harmless.

In this work, we illustrated the design and development of a metal-coordinated porous organic polymer (POP) namely VO@TPA-POP via a post-synthetic metalation strategy to incorporate oxo-vanadium sites in a pristine polymer (TPA-POP) having acetylacetonate (acac) as anchoring moiety. The as-synthesized VO@TPA-POP exhibited highly robust and porous framework, which has been utilized for thioanisole (TA) oxidation to its corresponding sulfoxide. The catalyst demonstrated notable stability and recyclability by maintaining its catalytic activity over multiple reaction cycles without any significant loss in activity. The X-ray absorption spectroscopy (XAS) and density functional theory (DFT) analysis establish the existence of V(+4) oxidation state along with the VO(O)4 active sites into the porous network and the most energetically feasible mechanistic pathway involved in the TA oxidation, respectively, indicating the role of electron density associated with vanadium center during the catalytic transformation. Thus, this work aims at the demonstration of versatility and potential of VO@TPA-POP as a porous heterogeneous catalyst for the TA oxidation followed by decontamination of sulfur mustards (HD's) to their corresponding less toxic sulfoxides in a more efficient and greener way.

Pyrolysis Free Out-of-Plane Co-Single Atomic Sites in Porous Organic Photopolymer Stimulates Solar-Powered CO2 Fixation.

Herein, a facile strategy is illustrated to develop pyrolysis-free out-of-plane coordinated single atomic sites-based M-POP via a one-pot Friedel Craft acylation route followed by a post-synthetic metalation. The optimized geometry of the Co@BiPy-POP clearly reveals the presence of out-of-plane Co-single atomic sites in the porous backbone. This novel photopolymer Co@BiPy-POP shows extensive π-conjugations followed by impressive light harvesting ability and is utilized for photochemical CO2 fixation to value-added chemicals. A remarkable conversion of styrene epoxide (STE) to styrene carbonate (STC) (≈98%) is obtained under optimized photocatalytic conditions in the existence of promoter tert-butyl ammonium bromide (TBAB). Synchrotron-based X-ray adsorption spectroscopy (XAS) analysis reveals the single atom coordination sites along with the metal (Co) oxidation number of +2.16 in the porous network. Moreover, in situ diffuse reflectance spectroscopy (DRIFTS) and electron paramagnetic resonance (EPR) investigations provide valuable information on the evolution of key reaction intermediates. Comprehensivecomputational analysis also helps to understand the overall mechanistic pathway along with the interaction between the photocatalyst and reactants. Overall, this study presents a new concept of fabricating porous photopolymers based on a pyrolysis-free out-of-plane-coordination strategy and further explores the role of single atomic sites in carrying out feasible CO2 fixation reactions.

Tweaking Photo CO2 Reduction by Altering Lewis Acidic Sites in Metalated-Porous Organic Polymer for Adjustable H2 /CO Ratio in Syngas Production.

Herein, we have specifically designed two metalated porous organic polymers (Zn-POP and Co-POP) for syngas (CO+H2 ) production from gaseous CO2 . The variable H2 /CO ratio of syngas with the highest efficiency was produced in water medium (without an organic hole scavenger and photosensitizer) by utilizing the basic principle of Lewis acid/base chemistry. Also, we observed the formation of entirely different major products during photocatalytic CO2 reduction and water splitting with the help of the two catalysts, where CO (145.65 µmol g-1 h-1 ) and H2 (434.7 µmol g-1 h-1 ) production were preferentially obtained over Co-POP & Zn-POP, respectively. The higher electron density/better Lewis basic nature of Co-POP was investigated further using XPS, XANES, and NH3 -TPD studies, which considerably improve CO2 activation capacity. Moreover, the structure-activity relationship was confirmed via in situ DRIFTS and DFT studies, which demonstrated the formation of COOH* intermediate along with the thermodynamic feasibility of CO2 reduction over Co-POP while water splitting occurred preferentially over Zn-POP.

Photocatalytic H2O2 production from water and air using porous organic polymers.

Producing hydrogen peroxide (H2O2) from H2O and O2 under visible light irradiation is a promising solar-to-chemical energy conversion technology. Hydrogen peroxide has versatile applications as a green oxidant and liquid energy carrier but has been produced through energy-intensive and complex anthraquinone processes. Herein, we report the rational design of efficient and stable porous organic polymer (POP) containing redox centers, anthraquinone photocatalyst (ANQ-POP) for solar H2O2 production. ANQ-POP is readily synthesized with stable dioxin-linkages via efficient one-pot, transition-metal-free nucleophilic aromatic substitution reactions between 1,2,3,4,5,6,7,8-octafluoro-9,10-anthraquinone (OFANQ) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP). Exhibiting a fibrillar morphology, ANQ-POP boasts a high surface area of 380 m2∙g-1 and demonstrates thermal stability. With 10 % ethanol, ANQ-POP yields an H2O2 production rate of 320 µmol g-1 under visible light irradiation. Moreover, ANQ-POP alone can efficiently produce H2O2 without any photosensitizers and cocatalysts. Density functional theory calculations reveal that the quinone groups of the anthraquinone moieties can serve as redox centers for H2O2 production under light irradiation. Furthermore, unlike most conventional photocatalysts, it can produce H2O2 using only water and air by catalyzing both oxygen reduction and evolution reactions under light irradiation. Our findings provide an efficient, eco-friendly pathway for photocatalytic production of H2O2 under mild reaction conditions using a dioxin-derived POP-based photocatalyst.

Linker Independent Regioselective Protonation Triggered Detoxification of Sulfur Mustards with Smart Porous Organic Photopolymer.

The development of efficient metal-free photocatalysts for the generation of reactive oxygen species (ROS) for sulfur mustard (HD) decontamination can play a vital role against the stockpiling of chemical warfare agents (CWAs). Herein, one novel concept is conceived by smartly choosing a specific ionic monomer and a donor tritopic aldehyde, which can trigger linker-independent regioselective protonation/deprotonation in the polymeric backbone. In this context, the newly developed vinylene-linked ionic polymers (TPA/TPD-Ionic) are further explored for visible-light-assisted detoxification of HD simulants. Time-resolved-photoluminescence (TRPL) study reveals the protonation effect in the polymeric backbone by significantly enhancing the life span of photoexcited electrons. In terms of catalytic performance, TPA-Ionic outperformed TPD-Ionic because of its enhanced excitons formation and charge carrier abilities caused by the donor-acceptor (D-A) backbone and protonation effects. Moreover, the formation of singlet oxygen (1 O2 ) species is confirmed via in-situ Electron Spin Resonance (ESR) spectroscopy and density functional theory (DFT) analysis, which explained the crucial role of solvents in the reaction medium to regulate the (1 O2 ) formation. This study creates a new avenue for developing novel porous photocatalysts and highlights the crucial roles of sacrificial electron donors and solvents in the reaction medium to establish the structure-activity relationship.

Experimental Validation and Computational Predictions Join Forces to Map Catalytic C-H Activation in Ferrocene Metalated Porous Organic Polymers.

In recent times, a self-complementary balanced characteristic feature with the combination of both covalent bonds (structural stability) and open metal sites (single-site catalysis) introduced an advanced emerging functional nanoarchitecture termed metalated porous organic polymers (M-POPs). However, the development of M-POPs in view of the current interest in catalysis has been realized still in its infancy and remains a challenge for the years to come. In this work, we built benzothiazole-linked Fe-metalated porous organic polymer (Fc-Bz-POP) using ferrocene dicarboxaldehyde (FDC), 1,3,5-tris(4-aminophenyl) benzene (APB), and elemental sulfur (S8) via a template-free, multicomponent, cost-effective one-pot synthetic approach. This Fc-Bz-POP is endowed with unique features including an extended network unit, isolated active sites, and catalytic pocket with a possible local structure, in which convergent binding sites are positioned in such a way that substrate molecules can be held in close proximity. Prospective catalytic application of this Fc-Bz-POP has been explored in executing catalytic allylic "C-H" bond functionalization of cyclohexene (CHX) in water at room temperature. Catalytic screening results identified that a superior performance with a CHX conversion of 95% and a 2-cyclohexene-1-ol selectivity (COL) of 80.8% at 4 h and 25 °C temperature has been achieved over Fc-Bz-POP, thereby addressing previous shortcomings of the other conventional catalytic systems. Comprehensive characterization understanding with the aid of synchrotron-based extended X-ray absorption fine structure (EXAFS) analysis manifested that the Fe atom with an oxidation state of +2 in our Fc-Bz-POP catalytic system encompasses a sandwich structural environment with the two symmetrical eclipsed cyclopentadienyl (Cp) rings, featuring nearest-neighbor (NN) Fe-C (≈2.05 Å) intramolecular bonds, as validated by the Fe L3-edge EXAFS fitting result. Furthermore, in situ attenuated total reflection-infrared spectroscopy (ATR-IR) analysis data for liquid-phase oxidation of cyclohexene allow for the formulation of a molecular-level reaction mechanistic pathway with the involvement of specific reaction intermediates, which is initiated by the radical functionalization of the allyl hydrogen. A deep insight investigation from density functional theory (DFT) calculations unambiguously revealed that the dominant pathway from cyclohexene to 2-cyclohexene-1-ol is initiated by an allyl-H functionalization step accompanied by the formation of 2-cyclohexene-1-hydroperoxide species as the key reaction intermediate. Electronic properties obtained from DFT simulations via the charge density difference plot, Bader charge, and density of state (DOS) demonstrate the importance of the organic polymer frame structure in altering the electronic properties of the Fe site in Fc-Bz-POP, resulting in its high activity. Our contribution has great implications for the precise design of metalated porous organic polymer-based robust catalysts, which will open a new avenue to get a clear image of surface catalysis.

Putting forward a Ni-metallosalphen-based porous organic polymer for detoxification of sulfur mustard gas simulant.

We have introduced a Friedel-Crafts alkylation strategy of a Ni-salphen complex as derived from 2-hydroxy-5-methoxybenzaldehyde, an isomer of biomass derived vanillin, to construct a Ni-salphen based porous organic polymer (Ni@T-POP). The X-ray absorption spectroscopy (XAS) analysis revealed the existence of Ni-N2O2 core sites in the Ni@T-POP framework, which demonstrates unprecedented catalytic efficiency towards oxidative decontamination of sulfur mustards (HD's) compared to its complex precursor.

Engineering the Charge Density on an In2.77S4/Porous Organic Polymer Hybrid Photocatalyst for CO2-to-Ethylene Conversion Reaction.

The development of an efficient photocatalyst for C2 product formation from CO2 is of urgent importance toward the deployment of solar-fuel production. Here, we report a template-free, cost-effective synthetic strategy to develop a carbazole-derived porous organic polymer (POP)-based composite catalyst. The composite catalyst is comprised of In2.77S4 and porous organic polymer (POP) and is held together by induced-polarity-driven electrostatic interaction. Utilizing the synergy of the catalytically active In centers and light-harvesting POPs, the catalyst showed 98.9% selectivity toward the generation of C2H4, with a formation rate of 67.65 µmol g-1 h-1. Two different oxidation states of the In2.77S4 spinel were exploited for the C-C coupling process, and this was investigated by X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations. The role of POP was elucidated via several photophysical and photoelectrochemical studies. The electron transfer was mapped by several correlated approaches, which assisted in establishing the Z-scheme mechanism. Furthermore, the mechanism of C2H4 formation was extensively investigated using density functional theory (DFT) calculations from multiple possible pathways.

Tuning of Microenvironment in Covalent Organic Framework via Fluorination Strategy promotes Selective CO2 Capture.

Herein, we have designed and synthesized two heteroatom (N, O) rich covalent organic frameworks (COF), PD-COF and TF-COF, respectively, to demonstrate their relative effect on CO2 adsorption capacity and also CO2 /N2 selectivity. Compared to the non-fluorinated PD-COF (BET surface area 805 m2 g-1 , total pore volume 0.3647 ccg-1 ), a decrease in BET surface area and also pore volume have been observed for fluorinated TF-COF due to the incorporation of fluorine to the porous framework (BET surface area 451 m2 g-1 , total pore volume 0.2978 ccg-1 ). This fact leads to an enormous decrease in the CO2 adsorption capacity and CO2 /N2 selectivity of TF-COF, though it shows stronger affinity towards CO2 with a Qst of 37.76 KJ/mol. The more CO2 adsorption capacity by PD-COF can be attributed to the large specific surface area with considerable amount of micropore volume compared to the TF-COF. Further, PD-COF exhibited CO2 /N2 selectivity of 16.8, higher than that of TF-COF (CO2 /N2 selectivity 13.4).

Selective photocatalytic CO2 reduction to CH4 over metal-free porous polyimide in the solid-gas mode.

Using a catalyst-free one-pot polycondensation approach, a new donor-acceptor (D-A) based porous polyimide (PeTt-POP) photocatalyst was developed. PeTt-POP produced CH4 (125.63 ppm g-1 in 6 h) from CO2 under visible light irradiation in the gas-solid mode without the use of co-catalysts or sacrificial agents. The progress of the reaction and the corresponding intermediate species involved in the CO2 reduction were identified by operando DRIFTS experiments, from which a plausible reaction mechanism was proposed.

Correction: Metal-organic-framework derived Co-Pd bond is preferred over Fe-Pd for reductive upgrading of furfural to tetrahydrofurfuryl alcohol.

Correction for 'Metal-organic-framework derived Co-Pd bond is preferred over Fe-Pd for reductive upgrading of furfural to tetrahydrofurfuryl alcohol' by Saikiran Pendem et al., Dalton Trans., 2019, 48, 8791-8802, https://doi.org/10.1039/C9DT01190K.

Unlocking Molecular Secrets in a Monomer-Assembly-Promoted Zn-Metalated Catalytic Porous Organic Polymer for Light-Responsive CO2 Insertion.

Anthropogenic carbon dioxide (CO2) emission is soaring day by day due to fossil fuel combustion to fulfill the daily energy requirements of our society. The CO2 concentration should be stabilized to evade the deadly consequences of it, as climate change is one of the major consequences of greenhouse gas emission. Chemical fixation of CO2 to other value-added chemicals requires high energy due to its stability at the highest oxidation state, creating a tremendous challenge to the scientific community to fix CO2 and prevent global warming caused by it. In this work, we have introduced a novel monomer-assembly-directed strategy to design va isible-light-responsive conjugated Zn-metalated porous organic polymer (Zn@MA-POP) with a dynamic covalent acyl hydrazone linkage, via a one-pot condensation between the self-assembled monomer 1,3,5-benzenetricarbohydrazide (TPH) and a Zn complex (Zn@COM). We have successfully explored as-synthesized Zn@MA-POP as a potential photocatalyst in visible-light-driven CO2 photofixation with styrene epoxide (SE) to styrene carbonate (SC). Nearly 90% desired product (SC) selectivity has been achieved with our Zn@MA-POP, which is significantly better than that for the conventional Zn@TiO2 (∼29%) and Zn@gC3N4 (∼26%) photocatalytic systems. The excellent light-harvesting nature with longer lifetime minimizes the radiative recombination rate of photoexcited electrons as a result of extended π-conjugation in Zn@MA-POP and increased CO2 uptake, eventually boosting the photocatalytic activity. Local structural results from a first-shell EXAFS analysis reveals the existence of a Zn(N2O4) core structure in Zn@MA-POP, which plays a pivotal role in activating the epoxide ring as well as capturing the CO2 molecules. An in-depth study of the POP-CO2 interaction via a density functional theory (DFT) analysis reveals two feasible interactions, Zn@MA-POP-CO2-A and Zn@MA-POP-CO2-B, of which the latter has a lower relative energy of 0.90 kcal/mol in comparison to the former. A density of states (DOS) calculation demonstrates the lowering of the LUMO energy (EL) of Zn@MA-POP by 0.35 and 0.42 eV, respectively, for the two feasible interactions, in comparison to Zn@COM. Moreover, the potential energy profile also unveils the spontaneous and exergonic photoconversion pathways for the SE to SC conversion. Our contribution is expected to spur further interest in the precise design of visible-light-active conjugated porous organic polymers for CO2 photofixation to value-added chemicals.

Ferrocene-derived Fe-metalated porous organic polymer for the core planarity-triggered detoxification of chemical warfare agents.

Herein, we demonstrate the successful construction of two Fe-metalated porous organic polymers having planar (Fe-Tt-POP) and non-planar (Fe-Rb-POP) geometry via the ternary copolymerization strategy for the catalytic oxidative decontamination of different sulfur-based mustard gas simulants (HD). Fe-Tt-POP exhibits superior catalytic performance for the oxidation of thioanisole (TA) in comparison with Fe-Rb-POP. Interestingly, this activity difference can be further explored by in situ operando DRIFTS and DFT computational studies.

Electrocatalytic water oxidation performance in an extended porous organic framework with a covalent alliance of distinct Ru sites.

The rational synthesis of durable, earth-abundant efficient electrocatalysts for the oxygen evolution reaction (OER) from water is one of the most important routes for storing renewable energy and minimizing fossil fuel combustion. The prime hurdles for effectively utilizing commercial RuO2 as (OER) electrocatalysts are its very low stability, catalyst deactivation, and high cost. In this work, we explored a Ru-integrated porous organic polymer (Ru@Bpy-POP) by a facile one-pot Friedel-Crafts alkylation strategy between redox-active (Ru(demob)3Cl2) and a carbazole unit, which is composed of unique features including an extended framework unit, isolated active sites, and tunable electrode kinetics. Ru@Bpy-POP can serve as a bridge between a Metal-Organic Framework (MOF) and POP-based catalytic systems with a balanced combination of covalent bonds (structural stability) and open metal sites (single site catalysis). Ru@Bpy-POP, deposited on a three-dimensional nickel foam electrode support, exhibits a promising electrocatalytic OER activity with an ultra-low ruthenium loading compared to a benchmark RuO2 catalyst, providing an overpotential of about 270 mV to reach 10 mA cm-2 in an alkaline medium. Moreover, a high current density of 248 mA cm-2 was achieved for the Ru@Bpy-POP catalyst at only 1.6 V (vs. RHE), which is much higher than 91 mA cm-2 for commercial RuO2. The robust, albeit highly conjugated, POP framework not only triggered facile electro-kinetics but also suppressed aggregation and metallic corrosion during electrolysis. In particular, the benefits of covalent integration of distinct Ru sites into the framework can modulate intermediate adsorption and charge density, which contributes to its exceptional OER activity. All of the critical steps involved in OER are complemented by Density Functional Theory (DFT) calculations, which suggest that electrocatalytic water oxidation proceeds from a closed-shell configuration to open-shell electronic configurations with high-spin states. These open-shell configurations are more stable than their closed-shell counterparts by 1 eV, improving the overall catalytic activity.

Organogel-assisted porous organic polymer embedding Cu NPs for selectivity control in the semi hydrogenation of alkynes.

Heteroatom-rich porous-organic-polymers (POPs) comprising highly cross-linked robust skeletons with high physical and thermal stability, high surface area, and tunable pore size distribution have garnered significant research interest owing to their versatile functionalities in a wide range of applications. Here, we report a newly developed organogel-assisted porous-organic-polymer (POP) supported Cu catalyst (Cu@TpRb-POP). The organogel was synthesized via a temperature induced gelation strategy, employing Schiff-base coupling between 2,4,6-triformylphloroglucinol aldehyde (Tp) and pararosaniline base (Rb). The gel is subsequently transformed to hierarchical porous organic structures without the use of any additive, thereby offering advantageous features including extremely low density, high surface area, a highly cross-linked framework, and a heteroatom-enriched backbone of the polymer. During the semi-hydrogenation of terminal and internal alkynes, the Cu@TpRb-POP-B catalyst with Cu embedded in the TpRb-POP structure consistently demonstrated improved selectivity towards alkenes compared to Cu@TpRb-POP-A, which contains Cu NPs exposed at the exterior surfaces of the POP support. Additionally, Cu@TpRb-POP-B showed higher stability and reusability than Cu@TpRb-POP-A. The superior performance of the Cu@TpRb-POP-B catalyst is attributed to the steric hindrance effect, which controls the product selectivity, as well as the synergistic interaction between the heteroatom-rich POP framework and the embedded Cu NPs. Both the effects are corroborated by experimental characterization of the catalysts and density functional theory (DFT) calculations.

Nanoarchitectonics of Metal-Free Porous Polyketone as Photocatalytic Assemblies for Artificial Photosynthesis.

The main component of natural gas is methane, whose combustion contributes to global warming. As such, sustainable, energy-efficient, nonfossil-based methane production is needed to satisfy current energy demands and chemical feedstocks. In this article, we have constructed a metal-free porous polyketone (TPA-DPA PPK) with donor-acceptor (D-A) groups with an extensive π-conjugation by facile Friedel-Crafts acylation reaction between triphenylamine (TPA) and pyridine-2,6-dicarbonyl dichloride (DPA). TPA-DPA PPK is a metal-free catalyst for visible-light-driven CO2 photoreduction to CH4, which can be used as a solar fuel in the absence of any cocatalyst and sacrificial agent. CH4 production (152.65 ppm g-1) is ∼5 times greater than that of g-C3N4 under the same test conditions. Charge-density difference plots from excited-state time-dependent density functional theory (TD-DFT) calculations indicate a depletion and accumulation of charge density among the donor/acceptor functional groups upon photoexcitation. Most notably, binding energies from DFT demonstrate that H2O is more strongly bound with the pyridinic nitrogen group than CO2, which shed insight into mechanistic pathways for photocatalytic CO2 reduction.

Presenting porous-organic-polymers as next-generation invigorating materials for nanoreactors.

Porous organic polymers (POPs) represent an emerging class of porous organic materials which mainly comprise organic building blocks that are interconnected via strong covalent bonds, thereby offering highly cross-linked frameworks with rigid structures and specific void spaces for accommodating guest molecules. In the past few years, POPs have garnered colossal research interest as nanoreactors for heterogeneous catalysis (thermal, photochemical, electrochemical, etc.) because of their intriguing characteristic features, such as high thermal and chemical stabilities, adjustable chemical functionalities, large surface areas, and tunable pore size distributions. This feature article provides an overview of existing research relating to diverse POP synthetic approaches (COFs, CTFs, and some amorphous POPs), the possible modification of the functionality of POPs, and their exciting application as next-generation nanoreactors. These POPs are extremely interesting, as they offer the potential for either metal-free or metalated polymer catalysts allowing photocatalytic CO2 reduction to solar-fuel, biofuel upgrades, the conversion of waste cooking oil to bio-oil, and clean H2 production from water, addressing many scientific and technological challenges and providing new opportunities for various specific topics in catalysis. Finally, we emphasize that the integration of various synthetic approaches and the application of POPs as nanoreactors will provide opportunities in the near future for the precision synthesis of functional materials with significant impact in both basic and applied research areas.

Correction: Strongly coupled Mn3O4-porous organic polymer hybrid: a robust, durable and potential nanocatalyst for alcohol oxidation reactions.

[This corrects the article DOI: 10.1039/C6RA07200C.].