Structured reactors based on 3D Fe/SiC Catalysts: understanding the effects of mixing

The application of structured reactors provides a number of advantages in chemical processes. In this paper, two different three-dimensional (3D) Fe/SiC catalysts with a square cell geometry have been manufactured by Robocasting: monoliths (D = 14 and H = 15 mm) and meshes (D = 24 and H = 2 mm) and studied in the catalytic phenol oxidation by hydrogen peroxide (H2O2) for the sustainable production of dihydroxybenzenes (DHBZ). The fluid dynamics, catalytic performance, reaction rates, external mass transport limitation, and catalyst stability have been compared in three different reactors, monolithic fixed-bed reactor, multimesh fixed-bed reactor, and monolithic stirrer reactor, at selected operating conditions. The results show that the mechanical stirring of the 3D Fe/SiC monoliths avoids the external mass transfer limitation caused by the presence of oxygen bubbles in the channels (produced from the HOx· species in autoscavenging radical reactions). In addition, the backmixing has a positive effect on the efficient consumption of H2O2 but an adverse effect on the phenol selectivity to DHBZ since they are overoxidized to tar products at longer contact times. On the other hand, the wall porosity, and not the backmixing, affects the susceptibility of the 3D Fe/SiC catalyst to the Fe leaching, as occurs in the mesh structures. In conclusion, the monoliths operating under plug-flow and external mass transfer limitation in the monolithic fixed-bed reactor (MFB) provide an outstanding phenol selectivity to DHBZ and catalyst stabilityThis work is supported by the following agencies and grants: the Spanish Government under projects RTI2018-095052-BI00 (MICINN/AEI/FEDER, UE) and EIN2020-112153 (MCINN/AEI/10.13039/501100011033), the latter was also supported by the European Union through “NextGenerationEU/PRTR”, Community of Madrid under project S2018/ EMT-4341, and CSIC project I-COOP+ 2019 (ref COOPB20405). P.L. acknowledges the Community of Madrid and the European Social Fund for the financial support received through the contract PEJ-2019-AI/IND-14385. The authors thank Juliana Mejía for her technical assistanc

Treatment of real winery wastewater by wet oxidation at mild temperature

This study explores the treatment of high-strength real winery wastewater (COD0 ≈ 35 g/L, TOC0 ≈ 11 g/L) by wet oxidation processes. Wet air oxidation (WAO), catalytic wet air oxidation (CWAO), H 2O2-promoted CWAO, wet peroxide oxidation (WPO) and catalytic wet peroxide oxidation (CWPO) were the options tested using different carbon-based catalysts, viz. activated carbon, carbon black and graphite. Their suitability was analyzed in terms of polyphenol, chemical oxygen demand (COD) and total organic carbon (TOC) abatement upon 4 h reaction time. The results showed that hydrogen peroxide was the unique oxidant capable of achieving an effective reduction of the organic load. The graphite tested was the most active catalyst, most probably due in great part to its Fe content (0.4 wt.%), resistant to leaching. CWPO with that graphite was tested at different conditions following the evolution of COD, TOC and ecotoxicity. The best results were obtained by using graphite at 5 g/L, the original pH of the wastewater (3.8), 125 °C and the stoichiometric amount of hydrogen peroxide distributed in stepwise additions. Under those conditions, 80% COD and TOC removals with 85% of hydrogen peroxide efficiency were achieved after 4 h reaction time, giving rise to colorless effluents of very low Microtox ecotoxicityThe authors wish to thank the Spanish MICINN for the financial support through the projects CTQ2008-03988/PPQ and CTQ2010-14807. The Comunidad Autónoma de Madrid is also gratefully acknowledged for the financial support through the project S2009/AMB-158

The use of cyclic voltammetry to assess the activity of carbon materials for hydrogen peroxide decomposition

It is known that carbon materials catalyze hydrogen peroxide decomposition in aqueous media. However, the catalytic activity of a particular carbon is dependent on various coupled structural, textural and chemical characteristics of the material, such that, formerly, the prediction of activity has not been possible. Here, the application of cyclic voltammetry (CV) is introduced as a rapid and conclusive technique in this respect. Three classes of carbon materials have been investigated: activated carbons, carbon blacks, and graphites, including some selected acid-washed samples which were used to examine the roles of mineral matter and surface oxygen. Characterization by electrochemical capacity measurements with CV, together with catalytic activity tests for hydrogen peroxide decomposition, reveal that the exchange current is directly proportional to the catalytic activity for hydrogen peroxide decomposition. That is, a linear dependence was found between this variable and the apparent first order catalytic decomposition rate constant. CV measurements with modified carbons also allow the elucidation of the effects of physicochemical characteristics of carbon materials on the rate of hydrogen peroxide decompositionThe authors wish to thank the Spanish MICINN for the financial support for the projects CTQ2008-03988/PPQ, CTQ2010-14807 and S2009/AMB-1588. Our gratitude to the anonymous reviewer for his/her helpful comments and recommendations which have significantly contributed to improve the quality of the pape

Highly efficient application of activated carbon as catalyst for wet peroxide oxidation

This paper addresses the improved performance of activated carbons in catalytic wet peroxide oxidation (CWPO) of phenol as target compound. Initial cyclic voltammetry experiments show that hydrogen peroxide and phenol compete for the same active sites on the carbon surface. Then, a significant coverage of the carbon surface by phenol molecules is the approach attempted to increase the efficiency of hydrogen peroxide and the performance of the oxidation process. In this work, two commercial activated carbons, with different physical and electrochemical properties have been tested. The results demonstrate that working at high phenol concentration (5. g/L) and phenol/carbon mass ratio (2), unprecedented hydrogen peroxide efficiencies of around 100% are achieved, allowing high oxidation and mineralization degrees, i.e. 97% phenol and 70% TOC conversions at 80. °C with the stoichiometric dose of hydrogen peroxide required for complete mineralization of phenol. The oxidation route of phenol in the presence of activated carbon is also studied and a reaction pathway proposed. Resorcinol was a new by-product detected whose formation occurs upon reaction on the carbon surface. Condensation by-products, typically formed in Fenton oxidation of phenol, were not found in the effluents but adsorbed on the carbon surface causing a progressive deactivation upon use. The activity can be easily recovered by oxidative thermal regeneration (350. °C, 24. h)The authors wish to thank the Spanish MICINN for the financial supportfor the projects CTQ2008-03988/PPQ, CTQ2010-14807 and S2009/AMB-158

Graphite and carbon black materials as catalysts for wet peroxide oxidation

This study explores the application of non-porous carbon materials, two graphites (G-F, G-S) and two carbon blacks (CB-V and CB-C) as catalysts for wet peroxide oxidation (CWPO). The activity, efficiency and stability of these carbon materials have been evaluated using phenol as target compound. The catalyst screening experiments were carried out batch-wise at CPhenol,0=1g/L, CH2O2,0=5g/L, Ccat=2.5g/L, T=80°C and pH0=3.5. The results allow concluding that CB-C was the most stable catalyst, although it showed a lower oxidation and mineralization activity than G-S and CB-V. Increasing the temperature up to 90°C allowed complete phenol conversion and around 70% TOC reduction with 100% efficiency of hydrogen peroxide consumption upon 20h reaction time at 5g/L CB-C load. As a consequence of the initial oxidation of the carbon surface, the electrochemical properties of CB-C were favorably changed upon CWPO and its catalytic performance was improved from the first to the second use and then maintained upon successive applications in a five-cycle testThe authors wish to thank the Spanish MICINN for the financial support through the projects CTQ2008-03988/PPQ and CTQ2010-14807. The Comunidad Autónoma de Madrid is also gratefully acknowledged for the financial support through the project S2009/AMB-158

Monolithic stirrer reactors for the sustainable production of dihydroxybenzenes over 3D printed Fe/γ-Al2O3 monoliths: kinetic modeling and CFD simulation

The aim of this work is to evaluate the performance of the stirring 3D Fe/Al2O3 monolithic reactor in batch operation applied to the liquid-phase hydroxylation of phenol by hydrogen peroxide (H2O2 ). An experimental and numerical investigation was carried out at the following operating conditions: CPHENOL,0 = 0.33 M, CH2O2,0 = 0.33 M, T = 75–95◦C, P = 1 atm, ω = 200–500 rpm and WCAT ~ 1.1 g. The kinetic model described the consumption of the H2O2 by a zero-order power-law equation, while the phenol hydroxylation and catechol and hydroquinone production by Eley–Rideal model; the rate determining step was the reaction between the adsorbed H2O2, phenol in solution with two active sites involved. The 3D CFD model, coupling the conservation of mass, momentum and species together with the reaction kinetic equations, was experimentally validated. It demonstrated a laminar flow characterized by the presence of an annular zone located inside and surrounding the monoliths (u = 40–80 mm s−1 ) and a central vortex with very low velocities (u = 3.5–8 mm s−1 ). The simulation study showed the increasing phenol selectivity to dihydroxybenzenes by the reaction temperature, while the initial H2O2 concentration mainly affects the phenol conversio

3D printing of cubic zirconia lattice supports for hydrogen production

The demand for hydrogen has extraordinarily grown during the last years, being one of the most attractive forms of fuels to produce green energy. Cubic zirconia ceramics are considered promising catalytic supports, and the additive manufacturing of porous 3D structures based on these ceramics could enhance their catalytic performance. Herein, lightweight highly porous (up to 88%) 3D patterned 8 mol% yttria-stabilized cubic zirconia (8YSZ) scaffolds are manufactured by robocasting from pseudoplastic aqueous-based inks to produce catalytic supports for the hydrogen (H2) production. These scaffolds are thermally treated at temperatures ranging between 1000 and 1400 ◦C and, hence, mechanically and electrically characterized. 3D 8YSZ structures sintered at 1200 ◦C, with an appropriate balance between high porosity (86%) and compressive strength (3.7 MPa), are impregnated with palladium (Pd) catalytic nanoparticles and employed in the catalytic dehydrogenation of renewable formic acid (FA) using a fixed-bed reactor. 3D Pd/8YSZ catalyst leads to the continuous production of CO-free H2 with a FA conversion of 32% at T =55 ◦CThis work was supported by the Spanish Government through RTI2018-095052-B-I00, PID2019-105079RB-I00 (MICINN/AEI/FEDER, UE), PID2021-125427OB-I00 (MICINN/AEI/FEDER, UE) and EIN2020- 112153 (MCINN/AEI/10.13039/501100011033) projects, the latter also supported by the European Union through “NextGenerationEU/ PRTR”. M. Koller gratefully acknowledges funding within “Support for International Mobility of Researchers of the Institute of Thermomechanics, Czech Academy of Sciences, part II”, no. CZ.02.2.69/0.0/ 0.0/18_053/0017555 of the Ministry of Education, Youth and Sports of the Czech Republic funded from the European Structure and Investment Funds (ESIF). G. Vega acknowledges the Universidad Aut´onoma de Madrid for the Predoctoral contract. The authors thank J. Mejía for her permanent technical assistance in the catalytic experiment

Formic acid-to-hydrogen on Pd/AC catalysts: Kinetic study with catalytic deactivation

A kinetic model for formic acid (FA) decomposition over a commercial 10 wt% Pd/AC catalyst has been developed to describe the hydrogen production and to understand the deactivation mechanism. The kinetic data were obtained in a batch slurry reactor in absence of mass transfer limitation at: CFA,0 = 0.25–2 M, CCAT = 1 g L−1, T = 25–85 ºC and P = 1 atm. The catalyst stability was studied in successive cycles at different temperatures. Fresh, used and regenerated Pd/AC catalysts were deeply characterized to gain insight into the activity, selectivity and stability. H2 and CO2 were the only reaction products detected. The reaction follows a first order kinetic for FA while the activity shows exponential decay with the initial FA concentration and reaction temperature. This paper represents a step forward in the on-site hydrogen production technology by using FA as liquid organic hydrogen carrierThe authors thank the financial support by the Community of Madrid through the project S2018/EMT-4341 and the Government of Spain through the project PID2019-105079RB-I00 (MCIU/AEI/FEDER, UE). Also, C. Martin acknowledges the Community of Madrid and the European Social Fund for the financing received through the contract PEJ2020-AI/AMB18976. G. Vega acknowledges the Universidad Autonoma ´ de Madrid for the Predoctoral contract. The authors thank Juliana Mejía for her permanent technical assistance, the "Servicio Interdepartamental de Investigacion" ´ (Sidi) of the Universidad Autonoma de Madrid (UAM), and in particular Luis Larumbe from FTIR lab and Josu´e Friedrich from TXRF lab, the “Centro Nacional de Microscopía Electronica ´ ” (ICTSCNME) of the Universidad Complutense de Madrid (UCM), in particular to Esteban Urones and the “Servicios Centrales de Apoyo a la Investigacion´ ” (SCAI), in particular to Maria del Valle Martínez de Yuso and María Dolores Marqu´es from Solidos Porosos La