Recent advances on the nanoporous catalysts for the generation of renewable fuels

Recent advances in nanoporous materials for renewable energy resources conversion into fuels

A consensus view in catalysis is that a higher density of catalytically active sites indicates a higher reaction rate. Using molecular dynamics simulations capable of mimicking the electrochemical formation of gas molecules, we herein demonstrate that this view is problematic for electrocatalytic gas production. Our simulation results show that a higher density of catalytic active sites does not necessarily indicate a higher reaction rate─a high density of active sites could lead to a reduction in the rate of reaction. Further analysis reveals that this abnormal phenomenon is ascribed to aggregation of the produced gas molecules near catalytic sites. This work challenges the consensus view and lays the groundwork for better developing gas-producing reaction electrocatalysts.

Enhancement of Catalytic Properties by Adjusting Molecular Diffusion in Nanoporous Catalysts

Advances in Emerging Crystalline Porous Materials.

ConspectusSolar-driven CO2 reduction into value-added chemicals, such as CO, HCOOH, CH4, and C2+ products, has been regarded as a potential way to alleviate environmental pollution and the energy crisis. In the past decades, numerous pioneered homogeneous catalytic systems composed of soluble photosensitizers (PSs) and catalytic active sites (CASs) have been explored for CO2 photoreduction. Nevertheless, inefficient electron migration based on random collision between CASs and PSs in homogeneous catalytic systems usually causes mediocre performance. Moreover, the relatively poor separation/recycling capability of the homogeneous systems has inevitably reduced their reusability and practicality. The rational combination of PSs and CASs have been proven to play critical roles in the development of highly efficient heterogeneous catalysts to improve their performance, such as anchoring them onto the solid matrixes or connecting them through bridging ligands. However, developing effective assembly strategies to achieve the ordered orientation and uniform heterogenization of PSs and CASs remains a great challenge, mainly due to the lack of crystallinity heterogeneous transformation and structural tailoring ability of traditional solid catalysts. Moreover, due to the lack of assembly and synthesis strategies, many efficient homogeneous photocatalytic systems are still unable to achieve high crystallinity heterogeneous transformation.Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) have recently attracted broad interest toward CO2 photocatalysis because of their diverse precursors, well-defined and tailorable structures, abundant exposed CASs and high surface areas, etc. Especially, the highly ordered orientation and uniform combination of PSs and CASs in MOFs and COFs are beneficial for improved light harvesting and charge separation, greatly helping to address the aforementioned challenges. Moreover, the well-defined crystalline structures of MOFs and COFs facilitate the establishment of the structure-activity relationship. Therefore, it is increasingly important to summarize the integration of PSs and catalysts to provide deep insight into MOF/COF-based photocatalysts.In this Account, we summarize the ordered integration of PSs and CASs in MOFs and COFs for CO2 photoconversion and describe the structure-activity relationships to guide the design of effective catalysts. Given the unique structural features of MOFs and COFs, we have emphasized the integration of PSs and CASs to optimize their photocatalytic performance, including the confinement of catalytic active nanoparticles (NPs) into photosensitizing frameworks, co-coordination of PSs and CASs, and ligand-to-metal charge-transfer and anchoring CASs on the secondary building units of the photosensitizing frameworks. The catalytic activity, selectivity, sacrificial agent, and stability of these systems were then discussed. More importantly, MOFs and COFs provide powerful platforms to understand the key steps for boosting CO2 photoreduction and exploring the catalytic mechanism, involving light harvesting, electron-hole separation/migration, and surface redox reactions. Finally, the perspective and challenge of CO2 photoreduction in MOF/COF platforms are further proposed and discussed. It is expected that this Account would provide deep insight into the integration of PSs and catalysts in COFs and MOFs with well-defined structures and afford significant inspiration toward enhanced performance in heterogeneous catalysis.

ConspectusChemical reactions can be promoted at lower temperatures and pressures, thereby reducing the energy input, by introducing suitable catalysts. Despite its significance, the quest for efficient and stable catalysts remains a significant challenge. In this context, addressing the efficiency of catalysts stands out as a paramount concern. However, the challenges posed by the vague structure and limited tailorability of traditional catalysts would make it highly desirable to fabricate optimized catalysts based on the understanding of structure-activity relationships. Covalent organic frameworks (COFs), a subclass of fully designed crystalline materials formed by the polymerization of organic building blocks through covalent bonds have garnered widespread attention in catalysis. The precise and customizable structures of COFs, coupled with attributes such as high surface area and facile functional modification, make COFs attractive molecular platforms for catalytic applications. These inherent advantages position COFs as ideal catalysts, facilitating the elucidation of structure-performance relationships and thereby further improving the catalysis. Nevertheless, there is a lack of systematic emphasis on and summary of structural regulation at the atomic/molecular level for COF catalysis. Consequently, there is a growing need to summarize this research field and provide deep insights into COF-based catalysis to promote its further development.In this Account, we will summarize recent advances in structural regulation achieved in COF-based catalysts, placing an emphasis on the molecular design of the structures for enhanced catalysis. Considering the unique components and structural advantages of COFs, we present the fundamental principles for the rational design of structural regulation in COF-based catalysis. This Account starts by presenting an overview of catalysis and explaining why COFs are promising catalysts. Then, we introduce the molecular design principle for COF catalysis. Next, we present the following three aspects of the specific strategies for structural regulation of COF-based catalysts: (1) By designing different functional groups and integrating metal species into the organic unit, the activity and/or selectivity can be finely modulated. (2) Regulating the linkage facilitates charge transfer and/or modulates the electronic structure of catalytic metal sites, and accordingly, the intrinsic activity/selectivity can be further improved. (3) By means of pore wall/space engineering, the microenvironment surrounding catalytic metal sites can be modulated to optimize performance. Finally, the current challenges and future developments in the structural regulation of COF-based catalysts are discussed in detail. This Account provides insight into the structural regulation of COF-based catalysts at the atomic/molecular level toward improving their performance, which would provide significant inspiration for the design and structural regulation of other heterogeneous catalysts.

Integrating Superwettability within Covalent Organic Frameworks for Functional Coating

Covalent organic frameworks (COFs) as crystalline polymers possess ordered nanochannels. When their channels are adorned with catalytically active functional groups, their highly insoluble and fluffy powder texture makes them apt heterogeneous catalysts that can be dispersed in a range of solvents and heated to high temperatures (80–180 °C). This would mean very high catalyst density, facile active-site access, and easy separation leading to high isolated yields. Different approaches have been devised to anchor or disperse the catalytic sites into the nanospaces offered by the COF pores. Such engineered COFs have been investigated as catalysts for many organic transformation reactions. These range from Suzuki–Miyaura coupling, Heck coupling, Knoevenagel condensation, Michael addition, alkene epoxidation, CO2 utilization, and more complex biomimetic catalysis. Such catalysts employ COF as a “passive” support that merely docks catalytically active inorganic clusters, or in other cases, the COF itself participates as an “active” support by altering the electronics of the inorganic catalytic sites through the redox activity of its framework. Even more, catalytic organic pockets or metal complexes have been directly tethered to COF walls to make them behave like single-site organocatalysts. Here, we have listed most COF-based organic transformations by categorizing them as metal-free non-noble-metal@COF and noble-metal@COF. The initial part of this review highlights the advantages of COFs as a component of a heterogeneous catalyst, while the latter part discusses all of the current literature on this topic.

Despite their potential for biodiesel synthesis, directly calcined minerals rich in magnesium/calcium carbonates are used at the expense of large amounts of the catalyst, high alcohol loading, long processing times, and severe conditions. We report a facile and low-cost route for preparing calcined porous calcite (CPC) and dolomite (CPD) via simple thermal decomposition of the reaction mixtures of natural calcite and dolomite with stearic acid. CPC and CPD, both of which have porous structures, were compared with previously reported catalysts and examined as promising heterogeneous base catalysts under mild conditions. A 40% increase in conversion through CPC and CDC was achieved compared with that achieved through directly calcined minerals. A systematic understanding of the enhancement and deactivation relationships of the products was established by catalyst characterization, including XRD, BET, CO2-TPD, SEM, TEM, and XPS. The obtained CPC and CPD showed large pore sizes and high active site densities and provided an advantageous environment for transesterification of triglyceride with methanol. CPC and CPD are re-usable and show no loss of activity after regeneration.

Transition metal dichalcogenides (TMDs) have been intensely investigated as attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER). However, it still remains a great challenge to enhance electrical conductivity, maintain high intrinsic activity and increase active site density of TMD-based electrocatalysts in practice. Herein, aided by first-principles calculations, a novel composite electrocatalyst in which the TMD clusters are immobilized on the surface of defective black phosphorus (BP) has been designed, with the promise to combine excellent intrinsic activity, high active site density, and high conductivity. The strong X–P (X = S, Se, or Te) covalent bonds between TMD clusters and defects (single and double vacancies) on the BP surface stabilize the composite structure and avoid the long-standing deactivation problem caused by cluster desorption. TMD clusters possess high active sites density, while BP acts as conducting layer to efficiently transport electrons from the electrode to active sites to ensures high conductivity. In addition, the ab-initio molecular dynamics simulations have been performed on cluster diffusion on the defective BP, illustrating the possibility of synthesizing such composite electrocatalyst by a simple drop-casting procedure at finite temperatures.

Covalent organic frameworks (COFs) with high porosity have garnered considerable interest for various applications owing to their robust and customizable structure. However, conventional COFs are hindered by their narrow pore size, which poses limitations for applications such as heterogeneous catalysis and guest delivery that typically involve large molecules. The development of hierarchically porous COF (HP-COF), featuring a multi-scale aperture distribution, offers a promising solution by significantly enhancing the diffusion capacity and mass transfer for larger molecules. This review focuses on the recent advances in the synthesis strategies of HP-COF materials, including topological structure design, in-situ templating, monolithic COF synthesis, defect engineering, and crystalline self-transformation. The specific operational principles and affecting factors in the synthesis process are summarized and discussed, along with the applications of HP-COFs in heterogeneous catalysis, toxic component treatment, optoelectronics, and the biomedical field. Overall, this review builds a bridge to understand HP-COFs and provides guidance for further development of them on synthesis strategies and applications.

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well‐established as heterogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large‐pore COF as catalytic support in α,ω‐diene ring‐closing metathesis reactions, leading to increased macrocyclization selectivity. COF pore‐wall modification by immobilization of a Grubbs‐Hoveyda‐type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous olefin metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF‐catalyst (MMC:O=1.35) of up to 51 % compared to the homogeneous catalyst (MMC:O=0.90) was observed along with a substrate‐size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement.

Aqueous zinc‐iodine batteries (AZIBs) are gaining attention as next‐generation energy storage systems due to their high theoretical capacity, enhanced safety, and cost‐effectiveness. However, their practical application is hindered by challenges such as slow reaction kinetics and the persistent polyiodide shuttle effect. To address these limitations, we developed a novel class of covalent organic frameworks (COFs) featuring electron‐rich nitrogen sites with varied density and distribution (N1‐N4) along the pore walls. These nitrogen sites enhance iodine species confinement and mass transport. Our experimental and theoretical studies reveal that the continuous and optimized distribution of nitrogen sites within the COF structure significantly reduces internal resistance and boosts redox activity. Moreover, the N4‐COF demonstrates superior performance compared to other porous materials, due to its high density and strategic alignment of active sites. The I2@N4‐COF cathode achieves a remarkable specific capacity of 348 mAh g−1 at 1 C, almost 1.8 times greater than that of the I2@N1‐COF, while also maintaining excellent cycling stability. This integration of a porous framework with aligned nitrogen sites in the N4‐COF structure not only enhances iodine redox behavior but also offers a promising design strategy for developing high‐performance AZIB electrodes.

Sustainability in chemistry heavily relies on heterogeneous catalysis. Enzymes, the main catalyst for biochemical reactions in nature, are an elegant choice to catalyze reactions due to their high activity and selectivity, although they usually suffer from lack of robustness. To overcome this drawback, enzyme-decorated nanoporous heterogeneous catalysts were developed. Three different approaches for Candida antarctica lipase B (CAL-B) immobilization on a covalent organic framework (PPF-2) were employed: physical adsorption on the surface, covalent attachment of the enzyme in functional groups on the surface and covalent attachment into a linker added post-synthesis. The influence of the immobilization strategy on the enzyme uptake, specific activity, thermal stability, and the possibility of its use through multiple cycles was explored. High specific activities were observed for PPF-2-supported CAL-B in the esterification of oleic acid with ethanol, ranging from 58 to 283 U mg-1 , which was 2.6 to 12.7 times greater than the observed for the commercial Novozyme 435.

Covalent organic frameworks (COFs) are an emerging class of porous organic polymers that have been utilized as scaffolds for anchoring metal active species to act as heterogeneous catalysts. Though several examples of such COFs exist, a thorough experimental and computational analysis on such catalysts is limited. In this work, a series of two-dimensional (2D) imine COFs (TTA-DFB COF (N), TTA-TBD COF (N∧O), and TTA-DFP COF(N∧N)) were synthesized by using suitable building units to obtain three different coordination sites (N, N∧O, and N∧N). These were post-modified with Pd(II) to catalyze the Suzuki-Miyaura coupling reaction. Pd@TTA-DFB COF, where Pd(II) was coordinated to N sites, showed the fastest reactivity and lower stability. Pd@TTA-DFP COF showed highest stability but slowest reactivity. Pd@TTA-TBD COF was the best among the three with both high stability and fast reactivity. By combining both experimental and computational results, we conclude that the Pd(II) to Pd(0) reduction is a key step in the difference between the catalytic reactivities of the three COFs. This study demonstrates the importance of the building block approach to design COFs for efficient heterogeneous catalysis and to understand the fate of the reaction profile.