Zhirong Chen

Solvent-free aerobic oxidation of hydrocarbons and alcohols with Pd@N-doped carbon from glucose

The development of efficient systems for selective aerobic oxidation of hydrocarbons and alcohols to produce more functional compounds (aldehydes, ketones, acids or esters) with atmospheric air or molecular oxygen is a grand challenge for the chemical industry. Here we report the synthesis of palladium nanoparticles supported on novel nanoporous nitrogen-doped carbon, and their impressive performance in the controlled oxidation of hydrocarbons and alcohols with air. In terms of catalytic activity, these catalysts afford much higher turnover frequencies (up to 863 turnovers per hour for hydrocarbon oxidation and up to ~210,000 turnovers per hour for alcohol oxidation) than most reported palladium catalysts under the same reaction conditions. This work provides great potential for the application of ambient air and recyclable palladium catalysts in fine-chemical production with high activity. The development of efficient catalysts for the aerobic oxidation of hydrocarbons to high-value chemicals is industrially important. Here, the authors show that palladium nanoparticles loaded on porous nitrogen-doped carbon are highly active catalysts under laboratory and industrially relevant conditions.

Selective oxidation of benzene to phenol by FeCl3/mpg-C3N4 hybrids

Existing phenol production mostly by cumene processes is a three-step route with unwanted acetone as by-product. Here, we report the FeCl3 and mesoporous carbon nitride hybrid (FeCl3/mpg-C3N4) as an active and selective photocatalyst to activate H2O2 for the oxidation of benzene to phenol under visible-light illumination. By fine-optimizing FeCl3 loading amount in catalyst and reaction conditions, the one-step process achieved a 38% benzene conversion with 97% selectivity for phenol. The excellent catalytic performance of FeCl3/mpg-C3N4 should be attributed to the fast reduction of Fe3+ to Fe2+ by photo-irradiated electrons from mpg-C3N4. To further understand the reaction route, several electron spin resonance (ESR) tests were carried out, confirming mpg-C3N4 promoted redox cycle of Fe2+/Fe3+ in the FeCl3/mpg-C3N4 system. Based on these results, a catalytic mechanism for the oxidation of benzene by FeCl3/mpg-C3N4 hybrids was provided. This environmental-friendly and efficient method is expected to open up a new avenue for one-step phenol preparation.

Iron chloride supported on pyridine-modified mesoporous silica: an efficient and reusable catalyst for the allylic oxidation of olefins with molecular oxygen

A novel heterogeneous catalyst with iron chloride immobilized on pyridine-modified mesoporous silica has been developed. The supported Fe(III)/SiO2 catalyst displayed excellent catalytic properties for the allylic oxidation of 3,5,5-trimethylcyclohex-3-en-1-one (β-IP, 1) to 3,5,5-trimethlycyclohex-2-ene-1,4-dione (KIP, 2) with molecular oxygen as the oxidant under mild conditions. It can also catalyze the oxidation of other olefins, such as α-pinene and cyclohexene and its derivatives efficiently and selectively. In addition, the supported catalyst can be easily recycled without significant loss of activity and selectivity, which is a green alternative for practical applications.

A cobalt Schiff base with ionic substituents on the ligand as an efficient catalyst for the oxidation of 4-methyl guaiacol to vanillin

A cobalt Schiff base catalyst with ionic substituents on the ligand, N,N′-ethylenebis(acetylacetoniminato)-cobalt(II) hexafluorophosphoric pyridinium (Co-[Salen-Py][PF6]2), was synthesized. It displayed an excellent catalytic performance for the oxidation of 4-methyl guaiacol to vanillin (conversion = 100%, selectivity = 90%). Tentative reaction mechanism research indicated that the electron-withdrawing pyridinium substituent on the ligand of Co(acacen) is responsible for the high selectivity of vanillin. Meanwhile, utilizing ethylene glycol and water as solvent, vanillin can be isolated by simple crystallization in the form of a sodium salt, and the mother liquid of the crystallization, with a large amount of NaOH (the mole ratio of NaOH/4-methyl guaiacol = 2.38/1), can be successfully recycled at least three times, thereby decreasing the mole ratio of base/substrate from 3.3 : 1 to 1.05 : 1 when the mother liquid of crystallization was recycled. This strategy provides a potentially greener alternative for the synthesis of vanillin in industry.

Aerobic oxidation of β-isophorone catalyzed by N-hydroxyphthalimide: the key features and mechanism elucidated

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.

3D-interconnected hierarchical porous N-doped carbon supported ruthenium nanoparticles as an efficient catalyst for toluene and quinoline hydrogenation

Ruthenium nanoparticles (2.6 nm) uniformly dispersed on a three-dimensional (3D) interconnected hierarchical porous N-doped carbon (Ru/NHPC) have been successfully developed, serving as a highly active and stable catalyst for the selective hydrogenation of aromatics under mild conditions. A novel “leavening” strategy, i.e. using biomass-derived α-cellulose as a carbon precursor and ammonium oxalate as both a nitrogen source and foaming agent, affords the NHPC material a large surface area (870 m2 g−1), an excellent hierarchical nanostructure which acts as a convenient mass transfer channel and a high ability in stabilizing and dispersing Ru nanoparticles. The Ru/NHPC catalyst exhibits a substantially enhanced activity for the hydrogenation of toluene (TOF up to 39 000 h−1) and quinoline (TOF up to 2858 h−1) in comparison with Ru/HPC (3D-hierarchical porous carbon without nitrogen doping) and Ru/AC (commercial activated carbon) under the same reaction conditions. Further investigations indicate that the 3D interconnected porous structure and N-doping contribute to the improved diffusion and mass transfer, homogeneous dispersion of ruthenium nanoparticles and high percentage of Ru0 (active sites), which results in considerable catalytic performance. This work offers great potential for the application of supported catalysts based on NHPC materials in fine chemical production with high activity.

Unexpected oxidation of β-isophorone with molecular oxygen promoted by TEMPO

A novel and efficient protocol for the oxidation of β-isophorone (β-IP) using molecular oxygen without any additives catalyzed by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been established. The generally accepted catalytic mechanism of alcohols by the oxoammonium cation (TEMPO+) derived from TEMPO indeed fails to explain our experimental observations, because a favorable radical-based process is confirmed by electron spin resonance measurements. Our results show that a plateau of the time-dependence curve is observed in the oxidation of β-IP with TEMPO at low temperature, which is quite different from that of N-hydroxyphthalimide (NHPI). The proposed mechanism of this catalytic process is also compared with that of NHPI. The theoretically characterized reaction pathways show that unlike the phthalimide N-oxyl radical, TEMPO promotes the oxidation via its interaction with the active intermediate hydroperoxide (ROOH) rather than its initial interaction with β-IP, and byproduct water also assists the α-H atom transfer from ROOH to TEMPO. In addition to the intensive oxidation of alcohols catalyzed by TEMPO, the present study widens its specific applications in the active C–H bonds of hydrocarbons, and also provides new insights into its promoted metal-free oxidation.

Reactivity and mechanism investigation of selective hydrogenation of 2,3,5-trimethylbenzoquinone on in situ generated metallic cobalt

Efficient and inexpensive catalysts are urgently desired for the hydrogenation of 2,3,5-trimethylbenzoquinone (TMBQ) to 2,3,5-trimethylhydroquinone (TMHQ), a key vitamin-E intermediate. In this study, a one-step method was developed to synthesize uniform cobalt-based NPs supported on porous nitrogen-doped carbon for the hydrogenation of TMBQ to TMHQ. The as-prepared catalyst shows a high yield (>90%) and selectivity (>99%) for TMBQ hydrogenation as well as α,β-unsaturated carbonyls. The satisfactory performance is attributed to the small particle size and homogeneous distribution. Meanwhile, metallic Co is proved to be responsible for the catalytic activity. Furthermore, density functional theory calculation discloses that the excellent chemoselectivity towards TMBQ is due to the preference for a desorption process over sequential hydrogenation of TMHQ. This novel material has great potential as a non-precious-metal catalyst for heterogeneous hydrogenation processes, due to its outstanding catalytic performance, simple preparation method and low production cost.

Mechanistic insights into the reaction of CF3CCl3 with SO3: Theory and experiment

Reaction mechanism of 1,1,1-trifluorotrichloroethane (CF3CCl3) and sulphur trioxide (SO3) in the presence of mercury salts (Hg2SO4 and HgSO4) was studied applying the density functional theory (DFT) at the UB3LYP/6-31+G(d,p) level. It was found that this reaction occurs in the free radical chain path as follows: mercury(I) sulphate free radical is generated by heat, causing CF3CCl3 to produce the CF3CCl2 free radical which reacts with SO3 leading to the formation of CF3CCl2OSO2 decomposing into CF3COCl and SO2Cl. The SO2Cl free radical triggers CF3CCl3 to regenerate CF3CCl2 which recycles the free radical growth reaction. This elementary reaction has the highest energy barrier and it is therefore the rate control step of the whole reaction path. Experiment data can confirm the existence of the mercury(I) salt free radical and the free radical initiation stage. So, mercury salts play the role of initiators not that of catalysts. The results agree well with our hypothesis.