Letters to the Editor

Extension of preparation methods employed with ceramic materials to carbon honeycomb monoliths

3D-printed high-surface carbon monoliths have been fabricated and tested as catalyst supports of CO₂ methanation active phases (NiO-CeO₂, 12 wt% Ni). The carbon carriers show a developed microporosity and good adherence to the catalytic phases of NiO-CeO₂, showing great stability and cyclability. Two monolith designs were used: a conventional parallel-channeled structure (honeycomb) and a complex 3D network of non-linear channels built upon interconnected circular sections (circles), where flow turbulences along the reactant gas path are spurred. The effect of the active phases particle size on the catalyst distribution and the overall performance has been assessed by comparing NiO-CeO₂ nanoparticles of 7 nm average (Np), with a reference counterpart of uncontrolled structure (Ref). The improved radial gases diffusion in the circles monolith design is confirmed, and nanoparticles show enhanced CO₂ methanation activity than the uncontrolled-size active phase at low temperatures (< 300 ºC). On the contrary, the Ref catalysts achieve higher CH₄ production at higher temperatures, where the reaction kinetics is controlled by mass transfer limitations (T > 300 ºC). SEM and Hg porosimetry evidence that nanoparticles are deposited at deeper penetration through the narrow micropores of the carbon matrix of the monolithic supports, which tend to accumulate on the channels surface remaining more accessible to the reactant molecules. Altogether, this study examines the impact of the channel tortuosity and the active phase sizing on the CO₂ methanation activity, serving as ground knowledge for the further rational and scalable fabrication of carbon monolith for catalytic applications.

The need to meet the new energy efficiency and productivity standards of several chemical manufacturing processes requires the redesign of reactor configurations. Particularly attractive is the integration of multistep processes, in which an effective process intensification can be achieved by integrating high-thermal conductive structured catalysts with highly efficient membrane separation methods. In this chapter, the main methods for preparing structured catalysts are briefly described, in terms of both the realization of the structure and the catalytic coatings. Moreover, the different types of membrane reactors and the main applications in processes based on both endothermic and exothermic reactions are illustrated. The structured catalysts allow to improve the processes, both in terms of conversion and selectivity to the desired products and of global process kinetics, which pass through an improved management of the reaction heat and the reduction of the process steps. The integration of the structured reactors with the membrane separation allows to further reduce the reactor devices, overcoming the thermodynamic limitations and further reducing the production times. However, further investigations are necessary to obtain high performance catalysts that integrate with the membranes, both in terms of operating temperatures and transport phenomena.

A natural Moroccan illite-smectite clay was investigated as adsorbent of nickel in water. Before its use, it was processed without additives as honeycomb monoliths, an advantageous design in many methods developed to control environmental pollution. The performance of the resulting monoliths was studied in dynamic conditions by flow recirculation through the honeycombs and using Atomic Absorption Spectroscopy for the determination of residual nickel concentration in the solutions. Design of experiments (DoE) was employed to evaluate the influence of three parameters and their interaction on the nickel adsorption capacity of the clay honeycombs: nickel concentration (10–206 ppm), salt concentration (4–36 g/L), and water flow (94–364 mL/min). Based on previous studies, recirculation time, pH and temperature were fixed at 1 h, 4.5 and room temperature, respectively. The maximum amount of nickel adsorbed (0.83 mg/g of clay) was obtained for a nickel concentration of 206 ppm, a 218 mL/min flow, and a salt concentration of 4 g/L. Experimental conditions for the highest repeatability were estimated, the respective values being 108.2 ppm, 94 mL/min and 20 g/L. Nickel retention by the clay was also confirmed after the adsorption tests by direct characterization of the used structured adsorbent through X-ray Fluorescence and SEM-EDS.

The rampant global energy demand is predominantly met by fossil fuels, resulting in the reduction of their supply and an alarming increase in greenhouse gas (GHG) emissions. These two challenges have fascinated the scientific community in finding solutions to these problems. Hydrogen (H₂) as an energy carrier is considered as one of the solutions to address the challenges. Methane (CH₄) decomposition is considered a favourable technology for the production of H₂ and valuable by-products in the form of carbon nanomaterial. The H₂ could be directly used for various applications such as fuel cell technology, transportation fuel, and for the synthesis of chemicals such as ammonia (NH₃) and methanol (CH₃OH). In the current scenario, demand for CO_x-free H₂ has increased on an annual basis, either in terms of quantity or number of applications and the thermo-catalytic methane decomposition (CDM) process is gaining more attention to produce H₂. To improve the H₂ yield and quality of the structured carbon by-product, various approaches involving the catalyst and process parameters have been investigated. This review critically discusses various conventional and novel catalyst systems to identify the recent progress for CDM and the way forward. Furthermore, reactor system configurations powered by conventional and renewable energy sources for methane decomposition, as well as their working principles, technical advantages, and limitations are thoroughly discussed. Finally, the various potential applications of carbon nanomaterial are briefly devised.

Natural illite–smectite and stevensite Moroccan clays were used for the simultaneous removal of lead and cadmium from aqueous medium. The clays were employed in raw state and extruded as honeycomb monoliths form without any additives, which confirms the novelty of this approach in water treatment. The experiments were done in batch conditions with continuous stirring and using a recirculated flow, respectively. In addition to a characterization of the clays by XRF, XRD, TGA, laser granulometry, N₂ physisorption, FTIR spectroscopy, SEM-EDS and evaluation of the cation exchange capacity, special attention was paid to the influence on the co-adsorption of variables such as adsorbent dosage, contact time and initial concentration of Cd²⁺ and Pb${}^{2+}$. Pseudo-second order kinetics and good fitting to Redlich–Peterson model for both heavy metals were found. Our results also suggest that Pb${}^{2+}$ and Cd${}^{2+}$ uptake is controlled by chemisorption with predominance of Langmuir characteristics. No significant depletion of the metals retention attributable to competition was observed, particularly for the stevensite (maximum retention capacity of 1.2 mg Pb${}^{2+}$/g and 4.6 mg Cd${}^{2+}$/g) that showed higher specific surface area. For both clays, cadmium ions adsorption was relatively favoured in the bimetallic solution, and the honeycombs kept the powders performance. Honeycomb monoliths as a compact adsorbent offer a promising way of water treatment thanks to their stability and easy incorporation into dynamic processes avoiding the issues of pressure drop under wastewater circulation.

Whey, the main by-product of the dairy industry, is proposed as a sustainable binder for the production of extruded activated carbon. Coconut shell char was mixed with controlled quantities of either rehydrated whey powders or partially dehydrated liquid whey. After extrusion, extrudates were dried, and carbonised or activated under N₂ or CO₂ atmosphere, respectively. A comprehensive study of the effect of different parameters including binder proportion, coconut shell char particle size and carbonisation temperature was carried out. The composition of whey prompted Maillard reactions boosted with temperature that conferred a resin-like behaviour to the binder. Quality parameters of the resulting extrudates were their ball-mill hardness, real and bulk densities. All whey-bound pellets retained their original shape after the high temperature treatment. The hardness of those whey extrudates was particularly remarkable considering the biomass origin of this binder. The carbonisation temperature had little effect in the hardness or densities of the carbon pellets. The best pellet formulation comprised the use of coconut shell char powders with particle sizes < 212 µm, in mixtures having a 7/3 char/dry whey mass ratio. The properties of the pellets obtained after the activation of such pellets were comparable to those of commercial activated carbon extrudates. The use of partially dehydrated liquid whey rather than re-hydrated dry whey rendered very similar results. The alkalinity of the binder made the resulting activated carbons adequate for H₂S removal at room temperature. An economic estimation of the process placed whey in a practical position within the binder market.