Design of highly active Ni catalysts supported on carbon nanofibers for the hydrolytic hydrogenation of cellobiose.

The direct transformation of cellulose into sugar alcohols (one-pot conversion) over supported nickel catalysts represents an attractive chemical route for biomass valorization, allowing the use of subcritical water in the hydrolysis step. The effectiveness of this process is substantially conditioned by the hydrogenation ability of the catalyst, determined by design parameters such as the active phase loading and particle size. Herein, mechanistic insights into catalyst design to produce superior activity were outlined using the hydrolytic hydrogenation of cellobiose as a model reaction. Variations in the impregnation technique (precipitation in basic media, incipient wetness impregnation, and the use of colloidal-deposition approaches) endowed carbon-nanofiber-supported catalysts within a wide range of Ni crystal sizes (5.8-20.4 nm) and loadings (5-14 wt%). The link between the properties of these catalysts and their reactivity has been established using characterization techniques such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectroscopy (ICP-OES). A fair compromise was found between the Ni surface area (3.89 m2/g) and its resistance against oxidation for intermediate crystallite sizes (∼11.3 nm) loaded at 10.7 wt%, affording the hydrogenation of 81.2% cellobiose to sorbitol after 3 h reaction at 190°C and 4.0 MPa H2 (measured at room temperature). The facile oxidation of smaller Ni particle sizes impeded the use of highly dispersed catalysts to reduce the metal content requirements.

Direct conversion of almond waste into value-added liquids using carbon-neutral catalysts: Hydrothermal hydrogenation of almond hulls over a Ru/CNF catalyst.

The almond industry leaves behind substantial amounts of by-products, with almond hulls being the primary residue generated. Given that one way to improve food security is by decreasing waste to reduce environmental impacts, developing sustainable processes to manage this by-product is necessary. Herein, we report on the hydrothermal hydrogenation of almond hulls over a carbon-neutral Ru supported on carbon nanofibres (Ru/CNF) catalyst, addressing the temperature, H2 pressure, time and catalyst loading. These variables controlled the distribution of the reaction products: gas (0-5%), liquid (49-82%) and solid (13-51%), and ruled the composition of the liquid effluent. This aqueous fraction comprised oligomers (46-81 wt%), saccharides (2-7 wt%), sugar alcohols (2-15 wt%), polyhydric alcohols (1-8 wt%) and carboxylic acids (7-31 wt%). The temperature and reaction time influenced the extension of hydrolysis, depolymerisation, deamination, hydrolysis, hydrogenation and dehydration reactions. Additionally, the initial H2 pressure and catalyst loading kinetically promoted these transformations, whose extensions were ruled by the amount of H2 effectively dissolved in the reaction medium and the prevalence of hydrogenations over dehydration/decarboxylation reactions or vice versa depending on the catalyst loading. Process optimisation revealed that it is feasible to convert up to 67% of almond hulls into merchantable oligomers at 230 °C, 35 bar initial H2, using 1 g catalyst/g biomass (0.4 g Ru/g biomass) for 360 min. Additionally, decreasing the temperature to 187 °C without modifying the other parameters could convert this material into oligomers (31 wt%) and small oxygenates (17 wt% carboxylic acids, 11 wt% sugar alcohols and 6 wt% polyhydric alcohols) concurrently. The theoretical energy assessment revealed that the total and partial combustion of the spent solid material could provide the required energy for the process and allow catalyst recovery and reutilisation. This environmental friendliness and holistic features exemplify a landmark step-change to valorising unavoidable food waste.

Analysis and optimisation of a novel 'almond-refinery' concept: Simultaneous production of biofuels and value-added chemicals by hydrothermal treatment of almond hulls.

For the first time, this work investigates the achievability of developing a biorefinery concept around almond hulls by hydrothermal treatment (HTT), thoroughly scrutinising the influence of the temperature (200-300 °C), pressure (100-180 bar), time (20-180 min) and solid loading (5-25 wt%). This process allowed the conversion of almond hulls into four main products: gas (2-13%), bio-oil (2-12%), aqueous (4-69%) and hydro-char (17-89%). The gas consisted of a mix of H2, CO2, CO and CH4 with a LHV fluctuating from 1 to 13 MJ/m3 STP. The bio-oil comprised a mixture of alkanes, aldehydes, ketones, phenols, furans, benzenes and nitrogen compounds with a HHV between 21 and 31 MJ/kg. The solid product resembled an energetic hydro-char material (HHV 21-31 MJ/kg), while the aqueous effluent comprised a mixture of value-added chemicals, including saccharides and small oxygenated compounds. The production of biofuels can be maximised at 256 °C and 100 bar, using a 5 wt% solid loading for 157 min, conditions at which 43% of the original feedstock can be converted into an elevated energy-filled bio-oil (11% yield, 30 MJ/kg), along with a high energetic hydro-char (32% yield, 29 MJ/kg). Regarding value-added chemicals, up to 10% of the almond hulls can be converted into a bio-oil with a high proportion (45%) of phenolic species at 250 °C and 144 bar with a solid loading of 5 wt% for 167 min. In comparison, a sugar-rich (81 C-wt%) solution can be produced in high yield (54%), by treating a 24 wt% suspension at 252 °C and 180 bar for 153 min. Therefore, the versatility, novelty and intrinsic green and holistic nature of this 'almond-refinery' concept exemplify a landmark achievement in future energy and chemicals production from biomass, which might help render the complete bio-refinery for almond hulls more cost-effectively and ecologically feasible.

Capacitance Enhancement of Hydrothermally Reduced Graphene Oxide Nanofibers.

Nanocarbon materials present sp2-carbon domains skilled for electrochemical energy conversion or storage applications. In this work, we investigate graphene oxide nanofibers (GONFs) as a recent interesting carbon material class. This material combines the filamentous morphology of the starting carbon nanofibers (CNFs) and the interlayer spacing of graphene oxide, and exhibits a domain arrangement accessible for fast transport of electrons and ions. Reduced GONFs (RGONFs) present the partial removal of basal functional groups, resulting in higher mesoporosity, turbostratic stacking, and surface chemistry less restrictive for transport phenomena. Besides, the filament morphology minimizes the severe layer restacking shown in the reduction of conventional graphene oxide sheets. The influence of the reduction temperature (140-220 °C) on the electrochemical behaviour in aqueous 0.5 M H2SO4 of RGONFs is reported. RGONFs present an improved capacitance up to 16 times higher than GONFs, ascribed to the unique structure of RGONFs containing accessible turbostratic domains and restored electronic conductivity. Hydrothermal reduction at 140 °C results in the highest capacitance as evidenced by cyclic voltammetry and electrochemical impedance spectroscopy measurements (up to 137 F·g-1). Higher temperatures lead to the removal of sulphur groups and slightly thicker graphite domains, and consequently a decrease of the capacitance.

Enhanced Reduction of Few-Layer Graphene Oxide via Supercritical Water Gasification of Glycerol.

A sustainable and effective method for de-oxygenation of few-layer graphene oxide (FLGO) by glycerol gasification in supercritical water (SCW) is described. In this manner, reduction of FLGO and valorization of glycerol, in turn catalyzed by FLGO, are achieved simultaneously. The addition of glycerol enhanced FLGO oxygen removal by up to 59% due to the in situ hydrogen generation as compared to the use of SCW only. Physicochemical characterization of the reduced FLGO (rFLGO) showed a high restoration of the sp²-conjugated carbon network. FLGO sheets with a starting C/O ratio of 2.5 are reduced by SCW gasification of glycerol to rFLGO with a C/O ratio of 28.2, above those reported for hydrazine-based methods. Additionally, simultaneous glycerol gasification resulted in the concurrent production of H2, CO, CH4 and valuable hydrocarbons such as alkylated and non-alkylated long chain hydrocarbon (C12-C31), polycyclic aromatic hydrocarbons (PAH), and phthalate, phenol, cresol and furan based compounds.

On-site production of hydrogen from mineral waste oils by thermocatalytic decomposition: an Aragon case study.

On-site production of hydrogen and carbon nanofibers by thermocatalytic decomposition (TCD) of mineral oil waste (MWO) is analyzed. An experimental study carried out at lab scale to estimate the yields that can be expected from TCD of the MWO collected in the Aragon area is presented. Based on these results, mass and energy balance have been carried out to have a preliminary estimation on the products that could be obtained by processing the 10 000 tonnes/year of MWO that can be collected in the Aragon region. The process would consist of four steps: (1) drying, (2) vaporization, (3) primary decomposition, and (4) catalytic decomposition. After drying and vaporization, MWO is converted in step 3 into fuel grade carbon and a gas mixture that mainly contains hydrogen and methane. Methane is partially converted in step 4 into hydrogen and a carbon material that contains carbon nanofibers which could be used to manufacture utilities with high added value. The 10 000 tonnes/year of MWO would yield 705 t/y of H2, 4962 t/y of fuel grade carbon, and 1016 t/y of pure carbon. The mixture obtained (71% H2: 23% CH4) could be used as a hydrogen source to obtain pure hydrogen or hydrogen-natural gas mixtures to fuel a captive fleet of public urban vehicles powered by fuel cells or dedicated ICE, respectively.