The selective oxo-functionalization of hydrocarbons under mild conditions with molecular oxygen as the terminal oxidant continues to be a hot topic in organic synthesis and industrial chemistry. Though many oxidation protocols in combination with transition metal salts, enzymes, organometallic catalysts, or organocatalysts have been summarized recently, a review that focuses solely on the metal-free allylic/ benzylic oxidation strategies with molecular oxygen is still unavailable. This critical review will summarize recent significant advances achieved in this important field under the scope of green chemistry, which covers the promising applications and brief mechanistic profiles involving three kinds of efficient catalysts, namely N-hydroxyimides, homogeneous/heterogeneous light-sensitive molecules, and heteroatomdoped carbon materials, and concerns the sustainability of these methods, as well as predicts the potential utilization of available but unreported analogous catalysts or catalytic systems in this field. Special emphasis will also be placed on the burgeoning metal-free strategies with visible light irradiation from the long-term greenness and sustainability of these oxidation processes due to their established appealing performances under ambient conditions.

Higher catalytic performances of N,N',N''-trihydroxyisocyanuric acid (THICA), N,N-dihydroxypyromellitimide (NDHPI), and N-hydroxynaphthalimide (NHNI) than that of N-hydroxyphthalimide (NHPI) have been demonstrated recently in aerobic oxidation. Herein, the rational design of reactive multi-nitroxyl organocatalysts has been addressed theoretically by using systematic analysis of some important properties and catalytic activities of yet-to-be-synthesized catalysts. Our results show that 1) NHNI and its analogue, similar to THICA, unlike NHPI and others, are unsuitable for solvent- or mediator-free catalysis due to their strong intramolecular hydrogen-bonding interactions; 2) increasing the reactive hydroxyimide groups on the same aromatic ring, or doped N atoms or ionic-pair groups onto the aromatic ring, can improve catalytic reactivity, whereas appropriate enlargement of conjugated aromatic systems results in unchanged activity; 3) the newly designed catalysts are more active than NHPI and NHNI and have catalytic activities comparable to NDHPI and THICA; 4) the ionic-pair supported case is suggested to be a very active catalyst, even towards inert propane, and can be used as a novel model catalyst for further improvements. The present work will be helpful in designing reactive hydroxyimide organocatalysts.

N,N-dihydroxypyromellitimide (NDHPI) and N,N',N''-trihydroxyisocyanuric acid (THICA) have been recently demonstrated to act as better carbon-radical-producing catalysts than the popular N-hydroxyphthalimide (NHPI). To gain a mature understanding of these particular catalysts, herein their geometrical, electronic, and thermochemical properties, as well as their catalytic activities, have been systemically investigated by a theoretical analysis. It appears that THICA, unlike NDHPI and NHPI, is unsuitable for solvent-free catalysis or catalysis in aprotic solvents due to its favorable coexistent planar conformer. Besides, the more remarkable catalytic efficiencies of NDHPI and THICA compared to NHPI can be ascribed to the lower barriers and the endothermicity in the H-abstraction processes by their radicals, especially by their multi-radicals which show stronger electron-withdrawing effects. Furthermore, the generation of THICA radicals would be much feasible at high temperature without co-catalysts. This study provides a new perspective towards the rational design of reactive hydroxyimide organocatalysts for industrial applications.

Quaternary ammonium-based polymeric ionic liquids (PILs) are novel CO2 sorbents as they have high capacity, high stability and high binding energy. Moreover, the binding energy of ionic pairs to CO2 is tunable by changing the hydration state so that the sorbent can be regenerated through humidity adjustment. In this study, theoretical calculations were conducted to reveal the mechanism of the humidity swing CO2 adsorption, based on model compounds of quaternary ammonium cation and carbonate anions. The electrostatic potential map demonstrates the anion, rather than the cation, is chemically preferential for CO2 adsorption. Further, the proton transfer process from water to carbonate at the sorbent interface is successfully depicted with an intermediate which has a higher energy state. By determining the CO2 adsorption energy and activation energy at different hydration states, it is discovered that water could promote CO2 adsorption by reducing the energy barrier of proton transfer. The adsorption/desorption equilibrium would shift to desorption by adding water, which constitutes the theoretical basis for humidity swing. By analyzing the hydrogen bonding and structure of the water molecules, it is interesting to find that the CO2 adsorption weakens the hydrophilicity of the sorbent and results in release of water. The requirement of latent heat for the phase change of water could significantly reduce the heat of adsorption. The special "self-cooling" effect during gas adsorption can lower the temperature of the sorbent and benefit the adsorption isotherms.

Due to the insufficient understanding of the selective oxidation mechanism of α/β-isophorones (α/β-IP) to ketoisophorone (KIP), the key features in the β-IP oxidation catalyzed by N-hydroxyphthalimide (NHPI) have been explored via theoretical calculations. β-IP is more favourable to being activated by phthalimide-N-oxyl radical (PINO˙) and peroxyl radical (ROO˙) than α-IP owing to the different C-H strengths at their reactive sites, thereby exhibiting selective product distributions. It was found that NHPI accelerates β-IP activation due to the higher reactivity of PINO˙ than ROO˙ and the equilibrium reaction between them, yielding considerable hydroperoxide (ROOH) and ROO˙. In addition, the ROOH decomposition is more favourable viaα-H abstraction by radicals than its self-dehydration and thermal dissociation. The strong exothermicity of this α-H abstraction, along with that from H-abstraction by co-yielded hot HO˙, is in favor of the straightforward formation of KIP, simultaneously leading to the isomerization of a few β-IP to α-IP and production of 4-hydroxyisophorone (HIP) and water. The proposed mechanisms, consistent with the experimental observations, allow for the deeper understanding and effective design of oxidation systems involving similar substrates or NHPI analogues that are of industrial importance.

Polymeric ionic liquids (PILs) are promising CO2 sorbents, as their behaviors are tunable by assembling ion pairs. This work aims to design CO2 sorbents with unique moisture-swing adsorption performance by assembling different anions on quaternary-ammonium-based PILs. Two aspects of the sorbent design were studied: the suitability of the CO2 affinity for different applications (e.g., direct air capture or flue gas capture) and capability for moisture-swing adsorption. Carbonate, fluoride, and acetate were chosen as counteranions, as they are representative anions with different basicity, valence, and water affinity. CO2 affinity was found to positively correlate with the pKa value of the counteranion, except for fluoride, which has an intrinsic character of attracting protons. The moisture swing capacity is determined by the difference between the hydration energies of the reactant and product after CO2 adsorption and followed the order carbonate > fluoride > acetate. Further investigations revealed that the repulsion between the two quaternary ammonium cations could promote the dissociation of hydrated water, which results in the lowest activation energy for CO2 adsorption for the PIL with carbonate. Therefore, the PIL with carbonate is potentially a desirable candidate for air capture and moisture-swing regeneration, while the PIL with acetate is suitable for CO2 capture under high partial pressure and regeneration through conventional approaches. This study provides a quantitative microscopic insight into the role of the anion in CO2 adsorption and paves the way toward the optimal PIL structure for CO2 capture under specific circumstances.

No abstract

No abstract