A viologen-functionalized metal-organic framework for efficient CO2 photoreduction reaction

Multivariate Tuning of Photosensitization in Mixed-Linker Metal-Organic Frameworks for Efficient CO2 Reduction

Photosensitization is a powerful approach for enhancing the photocatalyst performance by improving light absorption, energy transfer, and charge separation. However, achieving high efficiency requires precise control over photosensitizers, catalytic centers, and their interactions, which remain challenging in heterogeneous systems. Herein, we develop multivariate zirconium metal-organic frameworks (MOFs) with mixing linkers and tunable defects that enable unprecedented control over photosensitizers, catalytic centers, and their ratios, creating an efficient platform for CO2 reduction. These MOFs integrate triphenylamine, phenoxazine, or phenothiazine-based linkers as photosensitizers and metal porphyrin linkers (metal = Fe, Co, Ni, and Zn) as CO2 reduction catalytic centers. Furthermore, the defect tolerance of robust Zr6 nodes allows for a systematic variation in linker ratios by introducing missing linker defects. By fine-tuning the photosensitizers, catalytic metal centers, and their ratios, we achieved an optimized photocatalyst with CO2-to-CO reduction rates of 247.8 μmol gcat.-1 h-1, representing a 17-fold enhancement over homogeneous analogues. Transient spectra and density functional theory calculations reveal the critical role of the framework structure in promoting efficient intrareticular energy transfer and charge separation. This study highlights the unique advantage of MOF platforms in the multivariate tuning of photocatalysts, paving the way for advanced artificial photosynthetic systems.

Beyond Tradition: A MOF-On-MOF Cascade Z-Scheme Heterostructure for Augmented CO2 Photoreduction

Metal-organic frameworks (MOFs) are rigorously investigated as promising candidates for CO2 capture and conversion. MOF-on-MOF heterostructures integrate bolstered charger carrier separation with the intrinsic advantages of MOF components, exhibiting immense potential to substantially escalate the efficiency of photocatalytic CO2 reduction (CO2RR). However, the structural and compositional complexity poses significant challenges to the controllable development of these heterostructures. Herein, a conventional MOF-on-MOF nanocomposite is readily optimized from a type II heterojunction to a state-of-the-art cascade Z-scheme configuration via the encapsulation of Pt nanoparticles (Pt NPs), establishing synergistic MOF-MOF and metal-MOF heterojunctions with reinforced built-in electric field. A cascade electron flow is thus propelled, vigorously separating the photogenerated charge carriers and profoundly extending their lifetimes. Collectively, the photocatalytic activity of the cascade Z-scheme is drastically promoted, exhibiting a nearly quintuple enhancement in the CO production rate over the original type II heterostructure. Moreover, the anti-sintering capacity of the developed nanocomposite is unveiled, elucidating its simultaneously improved activity and stability. These findings present unprecedented regulation over the configuration of a MOF-on-MOF heterojunction, substantially enriching the fundamental understanding and rational design strategies of composite materials.

Ba/Ti MOF: A Versatile Heterogeneous Photoredox Catalyst for Visible-Light Metallaphotocatalysis

The field of sustainable heterogeneous catalysis is evolving rapidly, with a strong emphasis on developing catalysts that enhance efficiency. Among various heterogeneous photocatalysts, metal-organic frameworks (MOFs) have gained significant attention for their exceptional performance in photocatalytic reactions. In this context, contrary to the conventional homogeneous iridium or ruthenium-based photocatalysts, which face significant challenges in terms of availability, cost, scalability, and recyclability, a new Ba/Ti MOF (ACM-4) is developed as a heterogeneous catalyst that can mimic/outperform the conventional photocatalysts, offering a more sustainable solution for efficient chemical processes. Its redox potential and triplet energy are comparable to or higher than the conventional catalysts, organic dyes, and metal semiconductors, enabling its use in both electron transfer and energy transfer applications. It facilitates a broad range of coupling reactions involving pharmaceuticals, agrochemicals, and natural products, and is compatible with various transition metals such as nickel, copper, cobalt, and palladium as co-catalysts. The effectiveness of the ACM-4 as a photocatalyst is supported by comprehensive material studies, photophysical, and recycling experiments. These significant findings underscore the potential of ACM-4 as a highly versatile and cost-effective photoredox catalyst, providing a sustainable, one-material solution for efficient chemical processes.

Sub-Nanometer Mono-Layered Metal-Organic Frameworks Nanosheets for Simulated Flue Gas Photoreduction

The dilemma between the thickness and accessible active site triggers the design of porous crystalline materials with mono-layered structure for advanced photo-catalysis applications. Here, a kind of sub-nanometer mono-layered nanosheets (Co-MOF MNSs) through the exfoliation of specifically designed Co3 cluster-based metal-organic frameworks (MOFs) is reported. The sub-nanometer thickness and inherent light-sensitivity endow Co-MOF MNSs with fully exposed Janus Co3 sites that can selectively photo-reduce CO2 into formic acid under simulated flue gas. Notably, the production efficiency of formic acid by Co-MOF MNSs (0.85 mmol g-1 h-1) is ≈13 times higher than that of the bulk counterpart (0.065 mmol g-1 h-1) under a simulated flue gas atmosphere, which is the highest in reported works up to date. Theoretical calculations prove that the exposed Janus Co3 sites with simultaneously available sites possess higher activity when compared with single Co site, validating the importance of mono-layered nanosheet morphology. These results may facilitate the development of functional nanosheet materials for CO2 photo-reduction in potential flue gas treatment.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

MOF-Based Photocatalytic Membrane for Water Purification: A Review

Photocatalytic membranes can effectively integrate membrane separation and photocatalytic degradation processes to provide an eco-friendly solution for efficient water purification. It is of great significance to develop highly efficient photocatalytic membranes driven by visible light to ensure the long-term stability of membrane separation systems and the maximum utilization of solar energy. Metal-organic framework (MOF) is an emerging photocatalyst with a well-defined structure and tunable chemical properties, showing a broad application prospect in the construction of high-performance photocatalytic membranes. Herein, this work provides a comprehensive review of recent advancements in MOF-based photocatalytic membranes. Initially, this work outlines the main tailoring strategies that facilitate the enhancement of the photocatalytic activity of MOF-based photocatalysts. Next, this work introduces commonly used methods for fabricating MOF-based photocatalytic membranes. Subsequently, this work discusses the application and mechanisms of MOF-based photocatalytic membranes toward organic pollutant degradation, metal ion removal, and membrane fouling mitigation. Finally, challenges in developing MOF-based photocatalytic membranes and their practical applications are presented, while also pointing out future research directions toward overcoming these existing limitations.

Photochemical reduction of CO2 into CO coupling with triethanolamine decomposition

In this work, the impacts of triethanolamine (TEOA) on the performance of photochemical CO2 reduction were investigated in a simple homogeneous system. We demonstrates that CO2 can be converted into CO coupling with the decomposition of triethanolamine in TEOA aqueous solution without other additives under light irradiation. About 7.5 μmol CO product is achieved within 7 h with a maximum apparent quantum yield (AQY) of 0.086% at 254 nm. The isotope labelling experiment confirms that CO product originates from the reduction of CO2 rather than the decomposition of TEOA. In addition, the photochemical system exhibits excellent stability, no obvious inactivation is observed during long-term photochemical CO2 reduction reaction. This work provides a deep understanding of the effects of TEOA on the performance of photocatalytic CO2 reduction. Upon utilizing TEOA as a sacrificial electron donor in photocatalytic system, the contribution of TEOA must be considered once evaluating the catalytic activity of catalysts.

Viologen Guest-Mediated Luminescence Emission Tuning and Photochromic Behavior by a Series of Viologen@Zn-MOF Materials

The encapsulation of various guest molecules into the pores of metal-organic frameworks (MOFs) to form hybrid materials has attracted significant attention due to their unique spatial distribution and certain preferential geometry of the guests inside the MOFs. This arrangement often results in the guests exhibiting unique physical and chemical properties due to their intramolecular interactions with the host. In this article, five viologen derivatives were introduced as guests in a Zn-MOF with different benzene ring lengths, resulting in the formation of host-guest three-dimensional (3D) MOFs. The five compounds exhibited guest-dependent emission wavelength, color, and excellent photochromic behavior upon ultraviolet (UV) light radiation due to the distinct electronic transfer and π···π stacking interactions between the viologen guests and the host framework. This study provides a host-guest strategy for designing color-tunable luminescent and highly sensitive photochromic materials.

Doping transition metals to modulate the chirality and photocatalytic activity of rare earth clusters

In this paper, we report the successful assembly of achiral {Ln6M} ([Ln6M(μ3-OH)8(acac)12(CH3O)x(CH3OH)y], Ln = La, M = Mn, Co, Fe) and chiral {Nd9Fe2} ([Nd9Fe2(μ4-O)(μ3-OH)14(acac)16(NO3)(CH3OH)2(H2O)3]) rare earth clusters using achiral rigid ligands and a transition metal doping strategy. {Ln6M} can be viewed as the fusion of two {Ln3M} tetrahedrons by sharing vertices. {Nd9Fe2} results from the fusion of four {Ln3M} tetrahedrons by vertice and edge sharing. The substitution of Ln with transition metal leads to changes in the coordination pattern around neighboring Ln, which triggers the switch of metal center chirality. This study demonstrates the potentiality of utilizing transition metal doping and rigid ligand to control the chirality of rare earth clusters. In addition, the photocatalytic CO2 activity of these transition metal-doped rare earth clusters has been studied.

Tailoring the Nanoporosity and Photoactivity of Metal-Organic Frameworks With Rigid Dye Modulators for Toluene Purification

Facile synthesis of hierarchically porous metal-organic frameworks (MOFs) with adjustable porosity and high crystallinity attracts great attention yet remains challenging. Herein, a micromolar amount of dye-based modulator (Rhodamine B (RhB)) is employed to easily and controllably tailor the pore size of a Ti-based metal-organic framework (MIL-125-NH2 ). The RhB used in this method is easily removed by washing or photodegradation, avoiding secondary posttreatment. It is demonstrated that the carboxyl functional group and the steric effects of RhB are indispensable for enlarging the pore size of the MIL-125-NH2 . The resulting hierarchically porous MIL-125-NH2 (RH-MIL-125-NH2 ) exhibits optimized adsorption and photocatalytic activity because the newly formed mesopore with defects concurrently facilitates mass transport of guest molecules (toluene) and photogenerated charge separation. This work offers a meaningful basis for the construction of hierarchically porous MOFs and demonstrates the superiority of the hierarchical pore structure for adsorption and heterogeneous catalysis.

A viologen-based Cd(II) coordination polymer as a multifunctional platform for photochromism, chemochromism and a broad range of fluorescence pH sensing

A multi-responsive Cd(II) coordination polymer (1) has been constructed by introducing a viologen derivative as both the framework backbone and ligand side pendant. Notably, compound 1 exhibits intriguing properties, including photochromism, methanol-assisted photochromism and chemochromism to ammonia. Furthermore, compound 1 also displays fluorescence pH sensing ability in a wide pH range.

Oxidation of N-Alkyl(iso)quinolinium Salts Over TEMPO@Metal-Organic Framework Heterogeneous Photocatalyst†

Quinolones and isoquinolones are of particular importance to pharmaceutical industry due to their diverse biological activities. However, their synthetic protocols were limited by high toxicity, high energy consumption, poor functional group tolerance and noble metal catalyst. This study concerns the development of a series of TEMPO@PCN-222 (TEMPO: 2,2,6,6-tetramethylpiperidinyl-1-oxy; PCN: porous coordination network) composite photocatalysts by coordinating different amount of 4-carboxy-TEMPO with the secondary building units of PCN-222. Upon visible-light irradiation, photogenerated holes in the highest occupied molecular orbital of PCN-222 can smoothly transfer to TEMPO, which can significantly boost the photosynthesis of bioactive (iso)quinolones from readily available N-alkyl(iso)quinolinium salts. TEMPO@PCN-222 exhibits an outstanding catalytic stability and substrate tolerance with a 1-methyl-2-quinolinone yield of 86.7 %, over four times that with PCN-222 (21.4 %). This work provides a new route to construct composite photocatalysts from abundant starting materials for efficient photosynthesis of high value-added chemicals.