Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200400655

Controlled microwave heating in modern organic synthesis.

Graphitic carbon nitride, g-C3N4, can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C3N4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.

/pdf/graphitic-carbon-nitride-materials-variation-of-structure-2o5pjcr9g6.pdf

Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.

Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry

Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

https://link.springer.com/content/pdf/10.1007%2Fs10853-010-5113-0.pdf

Review of the anatase to rutile phase transformation

A review on g-C3N4-based photocatalysts

Nitrogen-rich carbon nitrides are produced as amorphous, bulk solids from the slow thermal decomposition of 2,4,6-triazido-1,3,5-triazine [(C3N3)(N3)3]. This energetic molecular azide is thermally unstable and readily decomposes at 185 °C in a high-pressure reactor to produce carbon nitride materials, e.g., C3N4. Under applied nitrogen gas pressure, (C3N3)(N3)3 decomposes to yield a solid with one of the highest reported nitrogen-to-carbon ratios corresponding to C3N5. This azide precursor also decomposes upon rapid heating to 200 °C to form graphite nanoparticles without any retained nitrogen. Spectroscopic evidence (infrared, nuclear magnetic resonance, and ultraviolet−visible) demonstrates that the carbon−nitrogen solids have significant sp2 carbon bonding in a conjugated doubly bonded network. Electron microscopy reveals that these powders have a glassy microstructure with large irregular pores and voids. C3N4 and C3N5 are thermally stable up to 600 °C and sublime to produce carbon nitride thin films ...

Synthesis of Nitrogen-Rich Carbon Nitride Networks from an Energetic Molecular Azide Precursor

Nitrogen-rich carbon nitride (CNx, x ≥ 1) network materials have been produced as disordered structures by a variety of precursor-based methods, many that involve solid-state thermolysis at or above 500 °C. One popular precursor building block is the triazine unit (C3N3), and most postulated amorphous CNx network structures are based on cross-linked triazine units. Since hydrogen is most often observed in the product, these materials are usually more appropriately described as CNxHy materials. Results from recent carbon nitride studies using larger conjugated heptazine (C6N7) precursors and from rigorous structural investigations of triazine to heptazine thermal conversion processes have prompted a reexamination of likely local structures present in amorphous carbon nitride networks formed by triazine thermolysis reactions. In the present study, the formation and local structure of a CNxHy material formed via the rapid and exothermic decomposition of a reactive triazine precursor, C3N3(NHCl)3, was examine...

From triazines to heptazines: deciphering the local structure of amorphous nitrogen-rich carbon nitride materials.

Chemical exchange (metathesis) reactions are used in many syntheses of important solids. While metathesis reactions in the liquid and gas phase are well-known, metathesis reactions from solid-state precursors have received much less attention. This review details work on the synthesis of refractory ceramics via rapid metathesis reactions between solid metal halides and alkali (or alkaline earth) metal main group compounds (e.g., Li3N or MgB2). The discussion includes thermodynamic considerations in choosing appropriate precursor couples. Through a careful choice of precursors, rapid, highly exothermic reactions can reach high temperatures (>1000 °C) on very short time scales (<1 s). The products are often crystalline and single phase with crystallite sizes varying from tens of angstroms to a few microns, depending on the refractory nature of the material and the reaction conditions (i.e., scale and the use of inert additives). The utility of this metathesis route is demonstrated by metal nitride, boron ni...

Synthesis of Refractory Ceramics via Rapid Metathesis Reactions between Solid-State Precursors

Lanthanum carbide (La2C80): a soluble dimetallofullerene

Derivatized s-triazine (C3N3) precursors have seen significant recent use in the production of carbon nitride materials. Larger polycyclic molecular precursors, such as those containing an s-heptazine core (C6N7 or tri-s-triazine), may improve stability and order in carbon nitride products. In this Communication, we describe the synthesis and crystal structure of 2,5,8-triazido-s-heptazine (2). Synthesis of 2 was achieved from melon, an oligomeric s-heptazine synthesized by the pyrolysis of NH4SCN. Melon was converted to molecular 2,5,8-trichloro-s-heptazine, which was then transformed to the triazide upon reaction with (CH3)3SiN3. The crystal structure of 2 verifies that the s-heptazine is planar and the azides adopt a pinwheel-like C3h arrangement around the periphery. The s-heptazine core shows pi delocalization in the C-N bonds around the periphery (av. 1.33 A), while the internal planar C-N bonds are longer (1.40 A). The heptazine units pack into parallel, but offset, layered sheets in the crystal. The triazide 2 exhibits photoluminescence at 430 nm and rapidly and exothermically decomposes upon heating at 185 degrees C to produce a tan thermally stable carbon nitride powder with a formula near C3N4.

Edward G. Gillan

Papers

Synthesis of Nitrogen-Rich Carbon Nitride Networks from an Energetic Molecular Azide Precursor

From triazines to heptazines: deciphering the local structure of amorphous nitrogen-rich carbon nitride materials.

Synthesis of Refractory Ceramics via Rapid Metathesis Reactions between Solid-State Precursors

Lanthanum carbide (La2C80): a soluble dimetallofullerene

Synthesis and Structure of 2,5,8-Triazido-s-Heptazine: An Energetic and Luminescent Precursor to Nitrogen-Rich Carbon Nitrides