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"Thomas, Arne"
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Much ado about nothing – a decade of porous materials research
Research on porous materials has produced intriguing novel materials in terms of composition, porosity and structures recently. This perspective aims to provide a short overview on some of the highlights reported within the last decade in this field.
Journal Article
Ultralight covalent organic framework/graphene aerogels with hierarchical porosity
The fabrication of macroscopic objects from covalent organic frameworks (COFs) is challenging but of great significance to fully exploit their chemical functionality and porosity. Herein, COF/reduced graphene oxide (rGO) aerogels synthesized by a hydrothermal approach are presented. The COFs grow in situ along the surface of the 2D graphene sheets, which are stacked in a 3D fashion, forming an ultralight aerogel with a hierarchical porous structure after freeze-drying, which can be compressed and expanded several times without breaking. The COF/rGO aerogels show excellent absorption capacity (uptake of >200 g organic solvent/g aerogel), which can be used for removal of various organic liquids from water. Moreover, as active material of supercapacitor devices, the aerogel delivers a high capacitance of 269 F g
−1
at 0.5 A g
−1
and cycling stability over 5000 cycles.
Macroscopic architectures of covalent organic frameworks (COF) allow to fully exploit their chemical functionality and porosity but achieving three-dimensional hierarchical porous COF architectures remains challenging. Here, the authors present a COF/reduced graphene oxide aerogel which is synthesized by growing COF during a hydrothermal process along the surface of graphene sheets.
Journal Article
Constitutional isomerism of the linkages in donor–acceptor covalent organic frameworks and its impact on photocatalysis
When new covalent organic frameworks (COFs) are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage (imine, boroxine, vinyl, etc.) between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation,
i.e
., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine-linked donor-acceptor (D-A) COFs are synthesized, which are different in the orientation of the imine bonds (D-C=N-A (DCNA) and D-N=C-A (DNCA)). The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H
2
evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COFs.
Systematic investigation of isomerism in covalent organic frameworks (COFs) can provide key insights into their properties. Here, the authors reveal that the constitutional isomerism of the linkage i.e., linkage orientations distinctly impact COFs’ structural and photophysical properties.
Journal Article
Metal–Organic Framework and Covalent–Organic Framework‐Based Aerogels: Synthesis, Functionality, and Applications
Metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs)‐based aerogels are garnering significant attention owing to their unique chemical and structural properties. These materials harmoniously combine the advantages of MOFs and COFs—such as high surface area, customizable porosity, and varied chemical functionality—with the lightweight and structured porosity characteristic of aerogels. This combination opens up new avenues for advanced applications in fields where material efficiency and enhanced functionality are critical. This review provides a comparative overview of the synthetic strategies utilized to produce pristine MOF/COF aerogels as well as MOF/COF‐based hybrid aerogels, which are functionalized with molecular precursors and nanoscale materials. The versatility of these aerogels positions them as promising candidates for addressing complex challenges in environmental remediation, energy storage and conversion, sustainable water‐energy technologies, and chemical separations. Furthermore, this study discusses the current challenges and future prospects related to the synthesis techniques and applications of MOF/COF aerogels. This review paper focuses on metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs)‐based aerogels, which provide an extensive description of the synthetic strategies and structural features of MOF/COF‐based aerogels, along with their cutting‐edge applications in the realm of environmental remediation, gas storage and separation, sustainable energy‐water technology, energy storage and conversion.
Journal Article
Quantifying the density and utilization of active sites in non-precious metal oxygen electroreduction catalysts
Carbon materials doped with transition metal and nitrogen are highly active, non-precious metal catalysts for the electrochemical conversion of molecular oxygen in fuel cells, metal air batteries, and electrolytic processes. However, accurate measurement of their intrinsic turn-over frequency and active-site density based on metal centres in bulk and surface has remained difficult to date, which has hampered a more rational catalyst design. Here we report a successful quantification of bulk and surface-based active-site density and associated turn-over frequency values of mono- and bimetallic Fe/N-doped carbons using a combination of chemisorption, desorption and
57
Fe Mössbauer spectroscopy techniques. Our general approach yields an experimental descriptor for the intrinsic activity and the active-site utilization, aiding in the catalyst development process and enabling a previously unachieved level of understanding of reactivity trends owing to a deconvolution of site density and intrinsic activity.
Iron and nitrogen doped carbon materials are widely studied electrocatalysts, however measurement of features such as intrinsic turn-over frequency and active site utilization has proved difficult. Here, the authors use a combination of chemisorption and spectroscopy techniques to determine these properties.
Journal Article
Synthesis of Functional Materials Using N‐heterocyclic Amines Beyond Melamine
Melamine, a nitrogen‐rich heterocycle with a 1,3,5‐triazine core and three exocyclic amines, has played a central role in materials chemistry, enabling the synthesis of supramolecular assemblies, covalent organic polymers, and carbon nitride semiconductors. However, structurally related N‐heterocyclic amines, differing in ring substitution or heterocycle, are far less applied in such fields despite their potential to overcome limitations associated with melamine‐based materials. In this review, recent advances in the synthesis of covalent and carbon‐based materials derived from melamine analogues, including 6‐Phenyl‐1,3,5‐triazine‐2,4‐diamine, 6‐Methyl‐1,3,5‐triazine‐2,4‐diamine and 2,4,6‐Triaminopyrimidine are highlighted. It is focused on how variations in heterocycle identity and functional group substitution influence monomer reactivity, condensation behavior, and the resulting material properties, such as chemical composition, dimensionality, porosity and morphology. Diverse applications of such materials are discussed, from fluorescent sensing platforms to CO2 capture and photo‐electrocatalysis, however, the primary emphasis lies on synthetic strategies and structure‐property relationships. It is concluded with a critical perspective on how these alternative building blocks, when leveraging sustainable synthesis methods coupled with emerging AI‐guided materials discovery, may enable the targeted design of next‐generation functional materials. Melamine has long enabled supramolecular assemblies, polymers, and carbon nitrides, yet related N‐heterocyclic amines remain underexplored. This review highlights advances with melamine analogues such as 6‐phenyl‐, 6‐methyl‐triazine diamines and 2,4,6‐triaminopyrimidine, focusing on how structural variation governs reactivity, condensation, and material properties, with implications for catalysis, sensing, and CO2 capture amongst others.
Journal Article
Unraveling surface sensitivity for generating metastable active sites in molybdenum-based catalysts for CO2 hydrogenation
The reverse water-gas shift (RWGS) reaction is crucial for sustainable CO
2
conversion, yet catalyst surface remodeling at high temperatures remains a complex and pivotal phenomenon. This study investigates the complex relationship between surface reconstruction and catalytic performance using a series of molybdenum-based catalysts, which can generate different catalytic MoO
3
surface layers under RWGS conditions. In-situ characterization techniques and theoretical analyses reveal that the MoO
3
layer on MoO
3
/MoO
2
-C and MoO
3
/Mo
2
N-C is in-situ reduced to MoO
2
and metastable MoO
x
(2
Journal Article
An anionic two dimensional covalent organic framework from tetratopic borate centres pillared by lithium ions
Non-covalent interactions play an important role for the framework formation of two-dimensional covalent organic frameworks. Until now, π–π interactions and hydrogen bonding are the main reported forces facilitating the stacking of framework layers. Here, we present a two-dimensional anionic covalent organic framework based on tetratopic borate linkages, where layers are connected by ionic interactions between the linkage site and counter cations. The crystalline covalent organic framework is accessed through the formation of an amorphous borate-based polymer and subsequent solvothermal treatment. The progress of crystallization is investigated, revealing the crystallite growth and morphological change from agglomerated dense particles to hollow crystallite spheres. Due to the pillared nature, the crystallites can be exfoliated into nanosheets by sonication of the material in the presence of methanol. The crystallization and ordered arrangement of the lithium ions in the interlayer space is shown to benefit the conductivity tenfold compared to the amorphous material.
Non-covalent interactions facilitate the stacking of framework layers in two dimensional covalent organic frameworks playing an important role for the framework formation. Here, the authors describe a covalent organic framework with tetratopic borate linkages whose counter cations promote the framework layer interaction.
Journal Article
Engineering Charge Heterogeneity in COF/Graphene Hydrogels for Salt‐Resistant Solar Evaporation
Solar‐driven interfacial evaporation technology offers a sustainable solution for water treatment, however, its efficiency in marine environments is often hindered by salt accumulation. Here, a series of covalent organic framework (COF)/graphene hydrogels with controllable charge distributions is reported, including homo‐ and hetero‐charged as well as zwitterionic surface functionalities. By adjusting the ratio of anionic and cationic building blocks during COF synthesis, the surface charge of the hydrogel evaporator is continuously modulated at the molecular level. The hetero‐charged and zwitterionic hydrogels enable faster water evaporation in seawater than in pure water by reducing the evaporation enthalpy. In particular, the hetero‐charged hydrogel achieves an evaporation rate of up to 3.23 kg m−2 h−1 in 3.5 wt.% saline water, 12% higher than that in pure water, and maintains strong salt resistance even in 20 wt.% brine. Beyond desalination, the hydrogel exhibits high potential for treating diverse wastewaters, providing a scalable platform for sustainable water treatment in complex environments. Hetero‐charged COF/graphene hydrogel (HCG‐1) with spatially separated cationic and anionic domains is engineered to achieve salt‐resistant solar evaporation. This unique charge architecture regulates Na⁺ and Cl− distribution, promotes intermediate‐water formation and effectively suppresses salt crystallization, enabling stable solar evaporation in saline water at rates exceeding those in pure water.
Journal Article