Reactions, Volume 2, Issue 2 (June 2021) – 6 articles

Fischer–Tropsch synthesis (FTS) produces hundreds of hydrocarbons and oxygenates by simple reactants (CO + H₂) and the detailed chain propagation mechanism is still in dispute. An industrial iron-based catalyst was used to further clarify the mechanism by adding aldehyde, alcohol and alkene species into a fixed-bed tubular reactor. The added species were investigated in H₂ and syngas atmospheres, respectively. 1-alkene in the H₂ atmosphere presented an obvious hydrogenolysis, in which the produced C₁ species participated in C–C bond formation simultaneously. Co-feeding C_n alkene with syngas showed remarkable C_n+1 alcohol selectivity compared to the normal FTS reaction. In addition, the carbonyl group of aldehyde was extremely unstable over the iron-based catalyst and could easily be hydrogenated to an alcohol hydroxyl group, which could even undergo dehydration for hydrocarbon species formation. Experimental data confirmed that both heavier alkenes and alcohols added can be converted to chain growth intermediates and then undergo monomer insertion for chain propagation. These results provide strong evidence that the chain propagation in the FTS reaction is simultaneously controlled by the surface carbide mechanism and the CO insertion mechanism, with surface CH_x species and CO as monomers, respectively. The study is of guiding significance for FTS mechanism understanding and kinetic modeling. Full article

Different low-cost cobalt precursors (acetate, chloride) and thermal treatments (air calcination/H₂ reduction versus direct H₂-activation) were investigated to alter the interaction between cobalt and silica. H₂-activated catalysts prepared from cobalt chloride had large Co⁰ particles (XRD, chemisorption) formed by weak interactions between cobalt chloride and silica (temperature programmed reduction (TPR), TPR with mass spectrometry (TPR-MS), TPR with extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) techniques) and retained Cl-blocked active sites, resulting in poor activity. In contrast, unpromoted Co/SiO₂ catalysts derived from cobalt acetate had strong interactions between Co species and silica (TPR/TPR-MS, TPR-EXAFS/XANES); adding Pt increased the extent of the Co reduction. For these Pt-promoted catalysts, the reduction of uncalcined catalysts was faster, resulting in larger Co⁰ clusters (19.5 nm) in comparison with the air-calcined/H₂-activated catalyst (7.8 nm). Both catalysts had CO conversions 25% higher than that of the Pt-promoted catalyst prepared in the traditional manner (air calcination/H₂ reduction using cobalt nitrate) and three times higher than that of the traditional unpromoted Co/silica catalyst. The retention of residual cobalt carbide (observed in XANES) from cobalt acetate decomposition impacted performance, resulting in a higher C₁–C₄ selectivity (32.2% for air-calcined and 38.7% for uncalcined) than that of traditional catalysts (17.5–18.6%). The residual carbide also lowered the α-value and olefin/paraffin ratio. Future work will focus on improving selectivity through oxidation–reduction cycles. Full article

For a low-carbon society, it is necessary to extract hydrogen for fuel cells from biogas rather than from fossil fuels. However, impurities contained in the biogas affect the fuel cell; hence, there is a need for system and operation methods to remove these impurities. In this study, to develop a fuel cell system for the effective utilization of biogas-derived hydrogen, the compositional change and concentration of impurities in the hydrogen recirculation system under actual operation were evaluated using process simulation. Then, the mitigation operation for performance degradation using simple purification methods was evaluated on the proton exchange membrane fuel cells (PEMFC) stack. In the process simulation of the hydrogen recirculation system, including the PEMFC stack, the concentration of impurities remained at a level that did not pose a problem to the performance. In the constant voltage test for a simulated gas supply of biogas-derived hydrogen, the conditions for applying the methanation reforming and air bleeding methods were analyzed. As a result, methanation reforming is more suitable for supplying biogas-containing CO to the PEMFC stack for continuous operation. Full article

Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSⁿ) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal complexes [(phen)M(O₂CR)]⁺], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms involving group 10 metals differ from long-known charge-remote fragmentation reactions. Full article

The term LOHC stands for Liquid Organic Hydrogen Carriers. The term has been so well accepted by the scientific community that the studies published before the existence of this name are not very visible. In this mini-review, we have tried to rehabilitate various studies that deserve to be put back in the spotlight in the present context. Studies indeed began in the early 1980s and many publications have compared the use of various organic carriers, various catalysts and reactors. Recent reviews also include the economic aspects of this concept. Full article

A detailed study of the experimental issues involved in the design and operation of a methanol steam microreformer is presented in this paper. Micromachining technology was utilized to fabricate a metallic microchannel block coupling the exothermic and endothermic process. The microchannel block was coated with a Pd/ZnO catalyst in the reforming channels and with Pd/Al₂O₃ in the combustion channels by washcoating. An experimental system had been designed and fine-tuned allowing estimation of the heat losses of the system and to compensate for them by means of electric heating cartridges. In this way, the heat necessary for the reforming reaction is provided by methanol combustion, thanks to the temperature and flow cascade controller we developed. Thus, the coupling of both reactions in a block of microchannels without the interference caused by significant heat loss due to the small size of the laboratory microreactor could be studied. Runs of this microreformer device were carried out, varying the deposited catalyst amount, methanol steam reforming temperature and space velocity. When the reforming reaction was compensated by the combustion reaction and the heat losses by the electric heating, an almost isothermal behavior of the microchannel reactor was observed. In the less favorable case, with a 460 mg catalyst load, ΔT_MSR was about 8 K and ΔT_COMB was about 16 K. This confirmed good coupling of the methanol steam reforming and the methanol combustion. Full article