•Tailorable pore sizes of ordered mesoporous carbon•Pyrolysis of cobalt porphyrin supported on ordered mesoporous carbon.•Environmentally benign cobalt-based mesoporous carbon catalysts•Remarkable catalytic activity and reusability for arylalkanes oxidationOrdered mesoporous carbon supported Co-N-C catalysts (Co-N-C/OMC) were prepared through a combined method of impregnation and pyrolysis, in which the fabricated cobalt porphyrin and ordered mesoporous carbon (OMC) served as precursor and support, respectively. The catalysts displayed high activity toward the oxidation of arylalkanes. And the catalysts showed interconnected porous structure with tailorable pore sizes that could influence the catalytic performance. Furthermore, the catalysts showed an excellent catalytic activity and reusability, which was attributed to the synergistic effect between Co-N-C and OMC.Download high-res image (79KB)Download full-size image

The selective oxidation of hydrocarbons to the corresponding ketones with solvent-free and molecular oxygen as an oxidant is of great importance in academic and industrial fields in view of economy and environment. In this respect, we present the facile synthesis and characterization of excellent catalysts comprising cobalt nanoparticles encapsulated into graphitic nitrogen-doped carbon nanotubes (Co@GCNs) via one-pot pyrolysis of a chelate compound containing citric acid, melamine, and CoCl2·6H2O. The selective oxidation of ethylbenzene under molecular oxygen and solvent-free conditions is employed as a probe reaction to investigate the catalytic performance; the optimized catalyst shows the best conversion (68%) and selectivity for acetophenone (93%). Combination of the catalytic results of the control group and the different characterization methods demonstrates that high catalytic activity is due to the synergistic effect between metallic cobalt and nitrogen-doped carbon nanotubes. Moreover, the catalyst has high catalytic activity for the aerobic and solvent-free oxidation of other arylalkane substrates. The proposed mechanistic study illustrates that the reaction is a free radical reaction progressing through superoxide radical anions (˙O2−).

Mesoporous silica spheres with Mn–N–C materials integrated into the framework are synthesized via the surfactant (CTAB) template-assisted one-pot approach. A manganese porphyrin is used as the precursor of the Mn–N–C structure. The as-prepared catalyst exhibits remarkable activity and stability in heterogeneous catalytic systems for ethylbenzene oxidation.

Mesoporous hollow silica spheres have been drawing tremendous interest due to their special structure and properties and potential applications. Here we synthesized a nanoreactor via ship-in-bottle method, encapsulated with Mn–N-C by heating manganese porphyrin in nanocages of mesoporous hollow silica spheres. And manganese porphyrin is first encapsulated and confined in the hollow spheres. The nanoreactors are investigated through transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (HAADF-STEM) as well as nitrogen adsorption–desorption isotherms. The results demonstrate that the mesoporous hollow spheres with well-defined morphology hold large pore volumes (0.29–0.46 cm3 g–1), high specific surface areas (428–600 m2 g–1) and uniform pore sizes (4.0 nm). In addition, the ethylbenzene oxidation is conducted in order to explore the catalytic performance of the nanoreactors. And the nanoreactors are observed to possess remarkable catalytic activity and attractive stability for ethylbenzene oxidation, which should be ascribed to the special architectures and confined effect.Keywords: confined effect; encapsulation; ethylbenzene oxidation; mesoporous hollow silica spheres; Mn−N-C; nanoreactors; ship-in-bottle

In this paper, cobaltporphyrin is used as a precursor to synthesize carbon nitrides with metal active sites supported on silica spheres by heat treatment (i.e. M-N-C/SiO2). The catalytic performance of M-N-C/SiO2 for ethylbenzene oxidation has been investigated and techniques such as N2 adsorption/desorption isotherm, NH3-TPD, HRTEM, STEM mapping and X-ray photoelectron spectroscopy (XPS) are employed to explore the active sites for ethylbenzene oxidation. XPS results show that cobalt compounds, such as CoOx and metallic Co, as well as cobalt nitrides, such as Co-Nx, are formed after the pyrolysis of cobaltporphyrin. However, according to the NH3-TPD experiment, Co-Nx may be the primary active site. When Co-Nx is poisoned by KSCN, the significant loss of catalytic activity further proves and verifies that Co-Nx instead of CoOx is the primary active site of M-N-C/SiO2 for ethylbenzene oxidation.

Biomass-derived cobalt-coordinated N-doped carbon (CoNC) for C–H bond oxidation is synthesized by a facile procedure based on pyrolysis of cobaltporphyrin with natural, amino acid-rich biomass casein as a supplementary nitrogen source. The catalysts are characterized by techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The obtained CoNC catalyst has a high metal content and dispersion and shows superior catalytic performance in C–H bond oxidation with molecular oxygen as oxidant under solvent-free conditions. This is attributed to the promotion of the Co–Nx moiety originating from cobaltporphyrin. Moreover, the catalyst shows remarkable stability and can be recycled several times without losing its activity.

•Noble-metal-free Co-N-C immobilized carbon nanotubes.•The catalysts synthesized via heating nitrogen-rich cobalt tetraphenyl porphyrin supported on CNTs in N2 atmosphere.•The remarkable catalytic activity of Co-N-C/CNTs for ethylbenzene oxidation.•The well-dispersed Co-N-C species and the enhanced interactions between substrate and active sites with the introduction of CNTs.The catalysts, namely noble-metal-free Co-N-C immobilized carbon nanotubes (CNTs), are synthesized via heating nitrogen-rich cobalt tetraphenyl porphyrin (CoTPP) supported on CNTs in N2 atmosphere. The obtained catalysts have been characterized by BET, Raman, XRD, TEM, HRTEM and XPS. It is found that the synergistic effect between carrier and active sites plays an important role in the catalytic performance of Co-N-C/CNTs for ethylbenzene oxidation. When the mass ratio of CoTPP to CNTs is 0.15, the catalyst exhibits the highest catalytic performance for ethylbenzene oxidation (i.e. 19.9% for ethylbenzene conversion, 72.9% selectivity of acetophenone). It can be attributed to the well-dispersed Co-N-C species and the enhanced interaction chances between substrate and active sites with the introduction of CNTs.

Cobalt-coordinated N-doped carbon (CoNC) was fabricated by heating cobalt porphyrins and ionic liquids. The prepared catalysts displayed superior catalytic capacity for the direct oxidation of ethylbenzene, and unprecedented stability was obtained even after six runs with a similar catalytic performance.

In this study, metalloporphyrin has been immobilized on a core–shell structured SiO2@CeO2 doped with transition metals such as Fe, Cu, Co and Mn. The as-prepared catalysts have been characterized via N2 adsorption–desorption, XRD, TEM, FT-IR spectroscopy, and UV-vis spectroscopy. It is found that metalloporphyrin is anchored onto a SiO2 shell with a thickness of about 20 nm and on a MOx/CeO2 core (M = Fe, Cu, Co and Mn) with a diameter about 120 nm, which may benefit the diffusion of substrates through the pores in the thin shell into the metal oxide cores and the formation of a synergistic effect between metalloporphyrin and metal oxides. Moreover, CoTPP-(MOx/CeO2)@SiO2 catalysts (M = Fe, Cu, Co and Mn) exhibit different physical and chemical properties, such as surface areas, particle sizes and catalytic performances owing to the addition of transition metals into CeO2. Moreover, the catalyst doped with Co exhibits a higher catalytic performance than the other catalysts for ethylbenzene oxidation. Thus, the addition of transition metals, such as Co, Cu, Fe and Mn, plays an important role in the catalytic performance of the catalysts for ethylbenzene oxidation via adjusting the physical and chemical properties of the core–shell catalysts.

In this study, Co–N–C supported on silica spheres is prepared through heat-treatment of supported metalloporphyrin in an N2 atmosphere. The catalytic performance of the catalysts for ethylbenzene oxidation is investigated and these catalysts are characterized by techniques such as BET, FT-IR, UV-vis, TEM and XPS. In comparison with other catalysts such as supported cobalt porphyrin and unsupported cobalt porphyrin, Co–N–C supported on silica spheres shows much higher catalytic activity for ethylbenzene oxidation (15.7%) and selectivity to acetophenone (76.5%). In addition, the catalyst can retain its high catalytic activity after several reuses. The results show that the high catalytic performance of the catalyst may be attributed to the formation of Co–N4–C sites during the heat-treatment and supported Co–N–C catalysts may be beneficial to yield more Co–N4–C sites.

CeO2@SiO2 core-shell nanoparticles were prepared by microemulsion method, and metalloporphyrins were immobilized on the CeO2@SiO2 core-shell nanoparticles surface via amide bond. The supported metalloporphyrin catalysts were characterized by N2 adsorption-desorption isotherm (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), ultraviolet and visible spectroscopy (UV-Vis), and Fourier transform infrared spectroscopy (FT-IR). The results show that the morphology of CeO2@SiO2 nanoparticles is core-shell microspheres with about 30 nm in diameter, and metalloporphyrins are immobilized on the CeO2@SiO2 core-shell nanoparticles via amide bond. Especially, the core-shell structure contains multi CeO2 core and thin SiO2 shell, which may benefit the synergistic effect between the CeO2 core and the porphyrin anchored on the very thin SiO2 shell. As a result, this supported metalloporphyrin catalysts present comparably high catalytic activity and stability for oxidation of ethylbenzene with molecular oxygen, namely, ethylbenzene conversion remains around 12% with identical selectivity of about 80% for acetophenone even after six-times reuse of the catalyst.

•Co–N–C/SiO2 catalysts prepared through supported metalloporphyrin.•Formation of Co–N–C on the surface of SiO2 via heat treatment.•High activity for ethylbenzene oxidation achieving over Co–N–C/SiO2 catalyst heated at 500 °C.•Well-dispersed Co–N–C acting as active centers for the activation of C–H bonds.•Co–N4–C sites responsible for the high catalytic activity.The effect of heat treatment on the Co–N–C/SiO2 catalysts prepared through supported metalloporphyrin has been investigated for selective oxidation of ethylbenzene. And techniques such as BET, XRD, FT-IR, UV–vis, TEM, TG–DTA and XPS are used to explore the relationship between heat temperature and the catalytic performance of the catalysts. The results show that Co–N–C/SiO2 catalyst heated at 500 °C exhibits much higher activity for ethylbenzene oxidation compared with its counterparts such as Co–N–C/SiO2 catalysts heated at 300 °C, 400 °C, 600 °C and 700 °C, respectively. This may be attributed to the formation of much more Co–N4–C active sites on the surface of Co–N–C/SiO2 catalyst heated at 500 °C.

We have studied CO oxidation over a ternary CuOx/Co3O4–CeO2 catalyst and employed the techniques of N2 adsorption/desporption, XRD, TPR, TEM, in situ DRIFTS, and QMS (quadrupole mass spectrometry) to explore the origin of active oxygen. DRIFTS-QMS results with labeled 18O2 indicate that the origin of active oxygens in CuOx/Co3O4–CeO2 obeys a model, called a queue mechanism. Namely gas-phase molecular oxygens are dissociated to atomic oxygens and then incorporated in oxygen vacancies located at the interface of Co3O4–CeO2 to form active crystalline oxygens, and these active oxygens diffuse to the CO–Cu+ sites thanks to the oxygen vacancy concentration magnitude and react with the activated CO to form CO2. This process, obeying a queue rule, provides active oxygens to form CO2 from gas-phase O2 via oxygen vacancies and crystalline oxygen at the interface of Co3O4–CeO2.

In this study, CeO2@SiO2 core–shell nanoparticles have been synthesized and used as supports to graft cobalt porphyrin via an amide bond. The catalyst was characterized using techniques such as FT-IR, UV–vis, SEM, TEM and BET. The results show that the catalyst was composed of regular nanoparticles (around 50 nm) with a core–shell structure. In addition, the catalyst exhibits an excellent activity, selectivity and stability for solvent-free selective oxidation of diphenylmethane with atmospheric pressure of oxygen. The conversion of diphenylmethane was as high as 41.6% with selectivity to diphenyl ketone of 96.3%. Even after reused up to 6 times, the catalyst maintained stable working ability.Graphical abstractCobalt tetraphenylporphyrin was immobilized onto CeO2@SiO2 nanoparticles via an amide bond. The catalyst exhibits an excellent catalytic activity, selectivity and stability for solvent-free highly selective oxidation of diphenylmethane by oxygen at ambient pressure.Highlights► CeO2 was modified by citrate sodium. ► CeO2@SiO2 core–shell nanoparticles have been synthesized. ► Metalloporphyrin was immobilized on CeO2@SiO2. ► The catalyst exhibits excellent catalytic performance for solvent-free oxidation of diphenylmethane to diphenyl ketone. ► The catalysts possess remarkable reusability.

•Metalloporphyrins were encapsulated in hollow silica microspheres.•Hollow silica microspheres were prepared using Fe3O4 as hard templates.•MnCP@SiO2 exhibits high activity for oxidation of ethylbenzene.•This catalyst possesses attractive stability than their homogeneous counterpart.In this work, manganese porphyrins were immobilized on hybrid nanocomposite microspheres through amide bond using Fe3O4 as hard templates. The catalyst was characterized by N2 adsorption–desorption isotherm (BET specific surface area), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and ultraviolet and visible spectroscopy (UV–vis) analysis. The results showed the average size of the hybrid nanocomposite microspheres is 450 nm in diameter and the supported metalloporphyrin catalysts remained reactive after removing Fe3O4 leaving the silica support with micropores. The catalytic performance of the as-prepared catalysts without Fe3O4 core instead of with the core was significantly enhanced as a result of the homogeneous catalysis in a heterogeneous catalyst, and the conversion of ethylbenzene was up to 22.9%. The catalyst also possessed high stability, and could be reused 6 times without remarkable loss of the catalytic activity, compared to the unsupported counterpart.

AbstractIn the present study, we have investigated the reducibility of CuO species on CuO-CeO2 catalysts and the influence of CuO species on the catalytic performance for CO preferential oxidation (CO PROX) in excess hydrogen. It is revealed that the smaller the difference of reduction temperature (denoted as T) for two adjacent CuO species is, the higher the catalytic activity of CuO-CeO2 for the PROX in excess hydrogen may be obtained. It means that if the reduction energy of Cu0-Cu2+ pairs matched better, the reduction-oxidation recycle of Cu0-Cu2+ pairs would go on more easily, then the transferring energy of Cu0-Cu2+ pairs would be lesser. Therefore, the CuO-CeO2 catalysts will be largely improved in their catalytic performance if the different CuO species on the catalysts have matched the reduction energy, which would allows them to cooperate effectively.

Mesoporous silica spheres with Mn–N–C materials integrated into the framework are synthesized via the surfactant (CTAB) template-assisted one-pot approach. A manganese porphyrin is used as the precursor of the Mn–N–C structure. The as-prepared catalyst exhibits remarkable activity and stability in heterogeneous catalytic systems for ethylbenzene oxidation.

Cobalt-coordinated N-doped carbon (CoNC) was fabricated by heating cobalt porphyrins and ionic liquids. The prepared catalysts displayed superior catalytic capacity for the direct oxidation of ethylbenzene, and unprecedented stability was obtained even after six runs with a similar catalytic performance.

In this paper, cobaltporphyrin is used as a precursor to synthesize carbon nitrides with metal active sites supported on silica spheres by heat treatment (i.e. M-N-C/SiO2). The catalytic performance of M-N-C/SiO2 for ethylbenzene oxidation has been investigated and techniques such as N2 adsorption/desorption isotherm, NH3-TPD, HRTEM, STEM mapping and X-ray photoelectron spectroscopy (XPS) are employed to explore the active sites for ethylbenzene oxidation. XPS results show that cobalt compounds, such as CoOx and metallic Co, as well as cobalt nitrides, such as Co-Nx, are formed after the pyrolysis of cobaltporphyrin. However, according to the NH3-TPD experiment, Co-Nx may be the primary active site. When Co-Nx is poisoned by KSCN, the significant loss of catalytic activity further proves and verifies that Co-Nx instead of CoOx is the primary active site of M-N-C/SiO2 for ethylbenzene oxidation.