Cobalt-containing lignin-based fibers were synthesized in one step by electrospinning of Alcell lignin solutions as carbon precursor, a low-cost and renewable co-product of the paper making industry. The lignin fibers were thermostabilized in air to avoid the fusion during the carbonization process between 500 and 800 °C to obtain cobalt-containing porous carbon submicron fibers. These carbon fibers catalysts were studied for the Low-Temperature Fischer-Tropsch synthesis. The lignin-derived fibers containing Co catalyst located on the overall carbon fiber surface (internal and external) heat-treated at 500 °C (Co@CF-500) showed the best catalytic performance after 70 h on stream, with 75% and 60% selectivity to C5+ at 220 °C and H₂/CO ratios of 1 and 2, respectively, attributed to the high Co dispersion, optimal Co-particle size and better Co accessibility. Higher heat-treatment temperatures leaded to Co-containing carbon fibers with larger metallic cobalt nanoparticles encapsulated in graphitic-type carbon, which rendered them inaccessible for FTS.

@en

The Ti-containing metal organic framework (MOF) MIL-125 has been used as sacrificial precursor to obtain TiO₂ materials through the MOF-mediated synthesis route. In this study, Fe³⁺ was deposited on the surface of MIL-125 after its hydrothermal synthesis. Targeted Fe-doped titania photocatalysts were prepared through the direct calcination in air of Fe/MIL-125 crystals and/or by using a two-step method, including carbonization in inert atmosphere followed by calcination in air. The relationship between the synthesis conditions and the properties of the Fe-doped titania nanopowders, such as Fe content, porosity, phase composition and particle size was investigated. From elemental mapping, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, UV–Vis absorption spectroscopy and photoluminescence emission spectra, the presence of highly dispersed Fe³⁺ ions incorporated into the TiO₂ crystal lattice was confirmed, which led to a significant red shift of photoresponse towards visible light and reduced the recombination rate of electron-hole pairs at low iron content. By varying the pre-carbonization temperature, both crystal size and phase composition in the final materials were modulated. The performance of Fe-doped titania materials in photocatalytic water-splitting was tested for hydrogen evolution. Optimal photocatalytic performance was found at 0.15 and 0.5 wt% iron concentration and exceeded those of non-doped titania and commercial anatase both under visible and UV light irradiation, respectively, and among the highest reported in literature for these systems.

@en

A series of ruthenium-doped strontium titanate (SrTiO₃) perovskite catalysts were synthesized by conventional and microwave-assisted hydrothermal methods. The structure was analyzed by X-Ray diffraction (XRD) confirming the formation of the perovskite phase with some TiO₂ anatase phase in all the catalysts. Microwave irradiation decreases the temperature and time of synthesis from 220 °C for 24 h (conventional heating) to 180 °C for 1h, without affecting the formation of perovskite. A 7 wt. % ruthenium-doped SrTiO₃ catalyst showed the best dielectric properties, and thus its catalytic activity was evaluated for the methane dry reforming reaction under microwave heating in a custom fixed-bed quartz reactor. Microwave power, CH₄:CO₂ vol. % feed ratio and gas hourly space velocity (GHSV) were varied in order to determine the best conditions for performing dry reforming with high reactants conversions and H₂/CO ratio. Stable maximum CH₄ and CO₂ conversions of ∼99.5% and ∼94%, respectively, at H₂/CO ∼0.9 were possible to reach with the 7 wt. % ruthenium-doped SrTiO₃ catalyst exposed to maximum temperatures in the vicinity of 940 °C. A comparative theoretical scale-up study shows significant improvement in H₂ production capability in the case of the perovskite catalyst compared to carbon-based catalysts.

@en

A supported cobalt catalyst with atomically dispersed Co-N_x sites (3.5 wt% Co) in a mesoporous N-doped carbon matrix (named Co@mesoNC) is synthesized by hydrolysis of tetramethyl orthosilicate (TMOS) in a Zn/Co bimetallic zeolitic imidazolate framework (BIMZIF(Co,Zn)), followed by high-temperature pyrolysis and SiO₂ leaching. A combination of TEM, XRD XPS and X-ray absorption spectroscopy studies confirm the absence of cobalt nanoparticles and indicate that these highly dispersed cobalt species are present in the form of Co-N_x. The exclusive formation of Co-N_x sites in the carbon matrix is attributed to the presence of a large amount of Zn and N in the BIMZIF precursor together with the presence of SiO₂ in the pore space of this framework, extending the initial spatial distance between cobalt atoms and thereby impeding their agglomeration. The presence of SiO₂ during high-temperature pyrolysis is proven crucial to create mesoporosity and a high BET area and pore volume in the N-doped carbon support (1780 m² g⁻¹, 1.54 cm³ g⁻¹). This heterogeneous Co@mesoNC catalyst displays high activity and selectivity (>99%) for the selective hydrogenation of nitrobenzene to aniline at mild conditions (0.5–3 MPa, 343–383 K). When more challenging substrates (functionalized nitroarenes) are hydrogenated, the catalyst Co@mesoNC displays an excellent chemoselectivity to the corresponding substituted anilines. The presence of mesoporosity improves mass transport of reactants and/or products and the accessibility of the active Co-N_x sites, and greatly reduces deactivation due to fouling.

@en

High productivity towards C2–C4 olefins together with high catalyst stability are key for optimum operation in high temperature Fischer–Tropsch synthesis (HT-FTS). Here, we report the fabrication of Fe@C/Al composites that combine both the outstanding catalytic properties of the Fe–BTC MOF-derived Fe catalyst and the excellent mechanical resistance and textural properties provided by the inorganic AlOOH binder. The addition of AlOOH to Fe–BTC followed by pyrolysis in N2 atmosphere at 500 °C results in composites with a large mesoporosity, a high Fe/Fe3O4 ratio, 10–35 nm average Fe crystallite size and coordinatively unsaturated Al3+ sites. In catalytic terms, the addition of AlOOH binder gives rise to enhanced C2–C4 selectivity and catalyst mechanical stability in HT-FTS, but at high Al contents the activity decreases. Altogether, the productivity of these Fe@C/Al composites is well above most known Fe catalysts for this process.@en

Mesoporous H-ZSM-5-carbon composites, prepared via tetrapropylammonium hydroxide (TPAOH) post treatment of H-ZSM-5 followed by deposition of pyrolytic carbon, have been used as the support for the preparation of Co-based Fischer-Tropsch catalysts. The resulting catalysts display an improved performance during Fischer-Tropsch synthesis (FTS), with higher activity, higher selectivity towards C5-C9 (gasoline range) hydrocarbons and lower selectivity towards C1 (and C2) than Co/mesoH-ZSM5 (without pyrolytic carbon). This is due to the weaker metal-support interaction caused by the deposited carbon (as revealed by XPS) leading to a higher reducibility of the Co species. Further, the partial deactivation of the Brønsted acid sites by pyrolytic carbon deposition, as was observed by NH₃-TPD, allows the modification of the zeolite acidity. Both the olefin to paraffin (O/P) and the isoparaffin to normal paraffin (I/N) ratios decrease with the increase in the carbon content, opening the door to further tune the catalytic performance in multifunctional FTS operations.

@en