Carbon- and Nitrogen-Based Organic Frameworks

ConspectusThis Account provides an overview of organic, covalent, porous frameworks and solid-state materials mainly composed of the elements carbon and nitrogen. The structures under consideration are rather diverse and cover a wide spectrum. This Account will summarize current works on the synthetic concepts leading toward those systems and cover the application side where emphasis is set on the exploration of those systems as candidates for unusual high-performance catalysis, electrocatalysis, electrochemical energy storage, and artificial photosynthesis.These issues are motivated by the new global energy cycles and the fact that sustainable technologies should not be based on rare and expensive resources. We therefore present the strategic design of functionality in cost-effective, affordable artificial materials starting from a spectrum of simple synthetic options to end up with carbon- and nitrogen-based porous frameworks. Following the synthetic strategies, we demonstrate how the electronic structure of polymeric frameworks can be tuned and how this can modify property profiles in a very unexpected fashion. Covalent triazine-based frameworks (CTFs), for instance, showed both enormously high energy and high power density in lithium and sodium battery systems. Other C,N-based organic frameworks, such as triazine-based graphitic carbon nitride, are suggested to show promising band gaps for many (photo)­electrochemical reactions. Nitrogen-rich carbonaceous frameworks, which are developed from C,N-based organic framework strategies, are highlighted in order to address their promising electrocatalytic properties, such as in the hydrogen evolution reaction, oxygen reduction reaction (ORR), and oxygen evolution reaction (OER). With careful design, those materials can be multifunctional catalysts, such as a bifunctional ORR/OER electrocatalyst.Although the majority of new C,N-based materials are still not competitive with the best (usually nonsustainable candidates) for each application, the framework/N approach as such is still in its infancy and has already moved organic materials to regions where otherwise only traditional noble metals or special inorganic semiconductors are found. As one potential way to enhance the properties of polymeric frameworks, the idea of catalysts having unique active surfaces based on Mott–Schottky heterojunctions and related concepts are addressed.In order to integrate all of the above versatile subjects from synthesis to applications on C,N-based organic frameworks, we begin the discussion with synthetic concepts and strategies for these frameworks to distinguish these systems from typical covalent organic frameworks based on boron oxide rings. Next we focus on the semiconducting properties of C,N-based organic frameworks in order to show a continuous transition between CTFs and other systems, such as graphitic carbon nitrides. At the end, applications of these materials are shown by highlighting their properties in electrochemical energy storage and photo- and electrocatalysis

Ultrafast Syntheses of Silver Foams from Ag₂NCN: Combustion Synthesis versus Chemical Reduction

Ultrafast Syntheses of Silver Foams from Ag₂NCN: Combustion Synthesis versus Chemical Reductio

Teaching New Tricks to an Old Indicator: pH-Switchable, Photoactive Microporous Polymer Networks from Phenolphthalein with Tunable CO₂ Adsorption Power

Switchable, organic microporous networks were synthesized by Sonogashira coupling of tetrabromophenolphthalein with 1,4-diethynylenebenzene using optimized reaction conditions. The resulting networks are microporous and have specific surface areas exceeding 800 m² g^–1. The microporosity and the pore polarity are sensitive to the pH value as evidenced by nitrogen and carbon dioxide physisorption experiments. The switching between the open and closed form of the lactone ring is reversible, but some porosity is lost throughout the process. The colored, alkaline salts of these networks are photochemically active, as shown by the effective heterogeneous photosensitization of the photopolymerization of methyl methacrylate with visible light

Thioimidazolium Ionic Liquids as Tunable Alkylating Agents

Alkylating ionic liquids based on the thioimidazolium structure combine the conventional properties of ionic liquids, including low melting point and nonvolatility, with the alkylating function. Alkyl transfer occurs exclusively from the <i>S</i>-alkyl position, thus allowing for easy derivatization of the structure without compromising specificity. We apply this feature to tune the electrophilicty of the cation to profoundly affect the reactivity of these alkylating ionic liquids, with a caffeine-derived compound possessing the highest reactivity. Anion choice was found to affect reaction rates, with iodide anions assisting in the alkylation reaction through a “shuttling” process. The ability to tune the properties of the alkylating agent using the toolbox of ionic liquid chemistry highlights the modular nature of these compounds as a platform for alkylating agent design and integration in to future systems

Self-Assembly of Metal Phenolic Mesocrystals and Morphosynthetic Transformation toward Hierarchically Porous Carbons

A facile and sustainable synthetic strategy based on the coordination of natural polyphenols with metal ions is developed for the textural engineering of mesocrystals and hierarchical carbon nanomaterials. The desired control of coordination between ellagic acid and zinc ions enables the macroscopic self-assembly behavior of crystalline nanoplatelets to be tailored into round and elongated “peanut”-like micron-sized mesostructured particles. Direct carbonization of these mesocrystals generates hierarchically porous carbon particles in good yields, possessing bimodal micro- and mesoporous architecture along with a well-preserved macroscopic structure. The pore system provides both small storage sites, demonstrated by high CO₂ uptake, and transport channels also accessible by larger molecules

Solvent-Free and Metal-Free Oxidation of Toluene Using O₂ and g‑C₃N₄ with Nanopores: Nanostructure Boosts the Catalytic Selectivity

Solvent-free oxidation of the primary C–H bonds in toluene to benzaldehyde has been achieved by using the metal-free catalyst g-C₃N₄ and O₂. It is the nanostructure of g-C₃N₄ that boosts the high selectivity by tuning the homogeneous oxidation to hetergeneous oxidation and capturing all free •O₂^– radicals to effectively suppress the overoxidation of aldehydes

Manipulation of Phase and Microstructure at Nanoscale for SiC in Molten Salt Synthesis

Silicon carbide (SiC) is a compound with strong covalent bonding, which gives its high mechanical strength and oxidation resistance, but also hinders its synthesis under moderate conditions. Herein, a facile route is presented for the synthesis of SiC nanomaterials from simple and abundant raw materials in an inorganic molten salt (MS). With this route, we are able to synthesis nanoscale 3C-SiC and 2H-SiC in a controlled manner, where the choice of cubic or hexagonal structure is coupled to nanocrystal size. By selection of the starting materials and tuning of the synthesis conditions, the MS-derived SiC can be isolated as nanoparticles (NPs), porous SiC/C composites with small primary crystals (2–4 nm), and as nanospheres. We also show that the SiC nanostructures are active for electrochemical hydrogen evolution reaction, and the activity can be remarkably improved by loading Pt (NPs) onto the structure

Template-Free One-Pot Synthesis of Porous Binary and Ternary Metal Nitride@N-Doped Carbon Composites from Ionic Liquids

Herein we present a straightforward synthesis approach toward composites of titanium, vanadium, and titanium–vanadium nitride nanoparticles embedded in nitrogen-doped carbon. These materials can be easily prepared via the heat treatment of mixtures of the corresponding metal precursors TiCl₄ or VOCl₃ dissolved in the ionic liquid 1-butyl-3-methyl-pyridinium dicyanamide (Bmp-dca) as the nitrogen/carbon source. WAXS diffractograms and TEM pictures of the resulting materials reveal the presence of highly crystalline metal nitride nanoparticles with an average diameter of 5 nm embedded in a graphitic carbon matrix. XPS measurements show that the carbon network is heavily doped with nitrogen; that is, it can be described as a nitrogen-doped carbon. Nitrogen sorption measurements show type I isotherms indicative of mainly microporous composites. The specific BET surface area increases with increasing amount of the metal precursor, and it is also dependent on the respective metal ion used. Under similar synthetic conditions, the specific surface areas for VN composites are higher than those of TiN materials, reaching values up to 550 m² g^–1 for VN. It is worth mentioning that these high surface areas can be reached without a template or etching; thus, it is an inherent structural feature of the composite. Furthermore, the use of ionic liquids as precursors offers the possibility for facile processing before material generation; that is, shaping, printing, or casting can be easily performed

Tuning the Pore Size in Gradient Poly(ionic liquid) Membranes by Small Organic Acids

Highly charged porous polymer membranes with adjustable pore size and gradient pore structure along the membrane cross-section were prepared by ammonia-triggered electrostatic complexation between an imidazolium-based cationic poly­(ionic liquid) (PIL) and multivalent benzoic acid derivatives. The PIL and the acid compound were first dissolved homogeneously in DMSO, cast into a thin film onto a glass plate, dried, and finally immersed into an aqueous ammonia solution. The diffusion of ammonia from the top to the bottom into the film neutralized the acid and introduced the gradient pore structure and in situ electrostatic cross-linking to fix the pores. The pore size and its distribution of the membranes were found controllable in terms of the multivalency of the acids, the imidazolium/carboxylate ratio, and the nature of the PIL counteranion

Uniform Graphitic Carbon Nitride Nanorod for Efficient Photocatalytic Hydrogen Evolution and Sustained Photoenzymatic Catalysis

Uniform graphitic carbon nitride nanorods (CNR) were facilely obtained by a morphology-preserving strategy by templating a chiral mesostructured silica nanorod. The hexagonal mesostructured pore structures of one-dimensional silica nanorods can provide nanoconfinement space for carbon nitride condensation to perfect layered structures. CNR demonstrated excellent photocatalytic capability in generating hydrogen from water even with a small specific surface area, compared with its mesoporous counterpart. For further application demonstration, the CNR was used for photocatalytic regeneration of NAD⁺ to NADH, the biological form of hydrogen. The in situ NADH regeneration system was further coupled with l-glutamate dehydrogenase for sustainable generation of l-glutamate from α-ketoglutarate. The high yield and high efficiency obtained here point a high-throughput and sustainable way for practical enzymatic applications