Unusual selectivity of oxygenate synthesis: Formation of acetic acid from syngas over unpromoted Rh in NaY zeolite

Syngas conversion over Rh/zeolite-NaY catalysts at high-pressure lead to high yields of acetic acid. This unusual selectivity toward one oxygenate in the absence of any catalyst promoter is most pronounced at lower temperature; the apparent activation energy for overall CO-hydrogenation is 23.7 kcal/mol, but for the formation of acetic acid it is 11.9 kcal/mol. The selectivity is little affected by the protons formed during the reduction of Rh. In stiu FT-IR measurement reveals that changes in activity and selectivity during the start-up period are caused by thorough catalyst reconstruction, converting the original Rh⁰ clusters to multinuclear Rh₆(CO)₁₆ and CH₃Rh_y(CO)_x and/or mononuclear CH₃Rh(CO)_x carbonyl complexes, and smaller Rh⁰ clusters. Stable acetate groups, but not the surface bound acetyls, are formed and detected by FT-IR. Most of the cooperating Rh species survive when the pressure is lowered from 1.0 to 0.1 MPa, maintaining a high acetic acid selectivity that is vastly superior to that of the fresh catalyst.     

Selective synthesis of mixed alcohols from syngas over catalyst Fe2O3/Al2O3 in slurry reactor

The Fe₂O₃/Al₂O₃ catalyst was studied to selectively synthesize mixed alcohols from syngas in a continuously stirred slurry reactor with the oxygenated solvent Polyethylene Glycol-400 (PEG-400). The selectivity of mixed alcohols in the products reached as high as 95 wt.% and the C₂₊ alcohols (mainly ethanol) was more than 40 wt.% in the total alcohol products at the reaction conditions of 250 °C, 3.0 MPa, H₂/CO = 2 and space velocity = 360 ml/g_cat h. The hydrogen temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) measurements of the catalyst confirmed that the FeO phase was responsible for the high selectivity to mixed alcohols in the process. And the oxygenated solvent PEG-400 was also necessary for the selective synthesis of mixed alcohols in the reaction system.     

Syngas methanation with municipal solid waste char supported Ni catalyst: Choice of key parameters and catalyst's response

The low quality municipal solid wastes (MSW) derived char has a potential to be used as a methanation catalyst to achieve low-cost methanation of the syngas derived from MSW, and its performance with varying reaction parameters should be explored for better application. In this study, a MSW char supported Ni-based catalyst (Ni-MSWC) was prepared by impregnation; the influences of CO₂ and CH₄ impurities in the raw syngas on methanation and the feasibility of conducting methanation in a low-pressure condition were investigated, then the catalyst's response to the changing parameters was identified. The results showed that the presence of low concentration CO₂ in the H₂-rich syngas is favorable to the gasification of the char carrier and activates Ni-MSWC catalyst. However, it also promotes F–T synthesis reaction and leads to a decrease in the methane yield (Y_CH4). The decrease in reaction pressure causes a decrease in Y_CH4 and results in coke formation; but inhibits F–T synthesis reaction and increases methane selectivity (S_CH4). A higher reaction pressure increases system complexity and accelerates char carrier consumption. Moreover, presence of methane by 2.8% (vol.) promotes the methanation of CO₂ through the methane dry reforming reaction, but it inevitably causes coke formation and affects the catalyst's stability. Based on Y_CH4 and Ni-MSWC's responses, CO:CO₂ ≥ 3:2 and reaction pressure of 1 MPa (gauge pressure) are recommended, which help to inhibit the side reactions and maintain a high Y_CH4 (>95%).     

Gasification performance of olive pomace in updraft and downdraft fixed bed reactors

This study aims to investigate the gasification potential of olive pomace with using different fixed-bed gasifier systems. Olive pomace as a dried form was supplied from a chemical industry plant working on olive oil soap, located in Izmir, Türkiye. After a complete characterization of olive pomace, gasification experiments by using fixed bed reactor systems were done at three different gasifier temperatures as 700, 800 and 900 °C. As a gasification agent, dry air was used with four different flowrates (0.4, 0.2, 0.1, 0.05 L/min) while pure oxygen experiments were carried out with a flow rate of 0.01 L/min. Syngas with H₂ content of 48% and 45% (volumetric) were obtained in updraft and downdraft gasifiers, respectively, by using dried air as a gasifying agent. Heating value of syngas was around 12.4 MJ/Nm³. In the pure oxygen atmosphere, H₂ contents of the syngas were measured as 53% and 39%vol. In the updraft and downdraft gasifiers. This paper presents the research results on the olive pomace gasification study as a part of a large-scale research project and discuss them in the context of hydrogen production from the fixed bed reactors.     

Influence of a reaction mixture streamline on partial oxidation of methane in an asymmetric microchannel reactor

Partial oxidation of methane (POM) has been tested in an asymmetric microchannel reactor with different inlet configurations. One inlet of the reactor provided successive splitting of an inlet flow into parallel channels, whereas the opposite inlet allowed the inlet flow to enter the parallel channels simultaneously. It was found that concentrations of carbon monoxide and carbon dioxide changed by 20–30% and the conversion of methane changed by 5–20%, depending on the rate and direction of the inlet flow. The hydrogen production rate practically did not depend on the inlet configuration and equaled 15 l/h at the inlet flow rates from 600 to 1400 cm³/min and at the methane conversion of 80%. The data obtained demonstrated that the use of different operating modes of the asymmetric microreactor allows changing the composition of produced syngas.     

Numerical and experimental study on laminar burning velocity of syngas produced from biomass gasification in sub-atmospheric pressures

The laminar burning velocity of syngas mixtures has been studied by various researches. However, most of these studies have been conducted in atmospheric conditions at sea level. In the present study, the effect of sub atmospheric pressure was evaluated on the laminar burning velocity for a mixture of H₂, CO and N₂ (20:20:60 vol%) in real sub atmospheric condition. The measurements was conducted in an altitude of 2130 m.a.s.l (0.766 atm) and 21 m.a.s.l (0.994 atm) to evaluate the effect of pressure, the temperature and relative humidity were controlled using an air conditioning unit and was maintained in 295 ± 1 K and 62.6 ± 2.7% respectively. The Flames were generated using contoured slot-type nozzle burner, and an ICCD camera was used to capture chemiluminescence emitted by OH∗-CH∗ radicals present in the flame and thus obtain the flame front and determinate the laminar burning velocity using the angle method. The experimental results were compared with numerical calculations, conducted using the detailed mechanisms of Li et al. and the GRI-Mech 3.0. It was found that the laminar burning velocity increases at lower pressure, for an equivalence ratio of 1.1, the laminar burning velocity increases by almost 23% respect to the sea level conditions.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

Syngas production at intermediate temperature through H2O and CO2 electrolysis with a Cu-based solid oxide electrolyzer cell

Solid Oxide Electrolyzer Cells (SOECs) are promising energy devices for the production of syngas (H₂/CO) by H₂O and/or CO₂ electrolysis. Here we developed a Cu–Ce_0.9Gd_0.1O_2−δ/Ce_0.8Gd_0.2O_2−δ/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ-Ce_0.8Gd_0.2O_2−δ cell and performed H₂O and CO₂ electrolysis experiments in the intermediate temperature range (600°C–700 °C). As a baseline, the cell was first tested in fuel cell operation mode; the sample shows a maximum power density peak of 104 mW cm⁻² at 700 °C under pure hydrogen and air. H₂O electrolysis testing revealed a steady production of hydrogen with a Faraday's efficiency of 32% at 700 °C at an imposed current density of −78 mA cm⁻². CO production was observed during CO₂ electrolysis but higher cell voltages were required. A lower efficiency of about 4% was obtained at 700 °C at an imposed current density of −660 mA cm⁻². These results confirm that syngas production is feasible by water and carbon dioxide electrolysis but further improvements from both the manufacturing and the electrocatalytic aspects are needed to reach higher yields and efficiencies.     

Study of partial oxidation reforming of methane to syngas over self-sustained electrochemical promotion catalyst

A self-sustained electrochemical promotion (SSEP) catalyst is synthesized for partial oxidation reforming (POXR) of CH₄ to produce syngas (H₂ and CO) at a relatively low temperature ranging from 350 to 650 °C. The SSEP catalyst is comprised of 4 components: microscopic Ni/Cu/CeO₂ anode, La_0.9Sr_0.1MnO₃ cathode, copper as electron conductor, and yttria-stabilized-zirconia as oxygen ion conductor, which form microscopic electrochemical cells to enable the self-sustained electrochemical promotion for the POXR process. The SSEP catalyst exhibited much better catalytic performance in POXR of CH₄ than a Ni–Cu–CeO₂ catalyst and a commercial Pt–CeO₂ catalyst. The CH₄ conversion over the SSEP catalyst is 29.4% at 350 °C and reaches 100% at 550 °C and the maximum selectivity to H₂ is on the level of 90% at 450–650 °C under a GHSV of 42,000 h⁻¹. The mechanism of the SSEP is discussed.     

Dynamic and kinetic modeling of isotopic transient responses for CO insertion on Rh and Mn–Rh catalysts

Isotopic transient tracer techniques have been employed to study heterogeneous hydroformylation on Rh/SiO₂ and Mn–Rh/SiO₂. Pulse injection of D₂ and allowed tracing of the deuterium and CO incorporation pathway into the aldehyde product. The d₁- and d₂-propionaldehyde responses showed a double-peak, or two-hump, response to the D₂ pulse, while showed a single-hump response to the pulse. Analysis of the product responses to the D₂ pulse in CO/H₂/C₂H₄ and CO/H₂/C₂H₄/C₂H₅CHO suggests that the first hump of the d₁- and d₂-propionaldehyde responses was due to rapid H/D exchange with adsorbed propionaldehyde via enol intermediates. The decay of the second hump was due to reaction of adsorbed acyl with spillover hydrogen/deuterium. The response was due to CO insertion followed by acyl hydrogenation. Compartment modeling of the product responses from the and D₂ pulse inputs allowed determination of residence times of adsorbed intermediates, surface coverages of adsorbed intermediates, and the elementary rate constants for acyl hydrogenation and CO insertion. Elementary rate constants for acyl hydrogenation determined from this study were consistent with the value calculated by transition state theory (TST). The addition of Mn promoter to Rh/SiO₂ increased coverages of , , and and shifted the rate-limiting step for propionaldehyde formation. Acyl hydrogenation is the rate-limiting step on Rh/SiO₂ while CO insertion and acyl hydrogenation are both kinetically significant on Mn–Rh/SiO₂.