Aerobic syngas conversion: opportunities, challenges, and solutions.

Integration of carbon capture with utilization technologies can lead the way to a net-zero carbon economy. Nevertheless, direct chemical conversion of chemically captured CO2 remains challenging due to its thermodynamic stability. Here, we demonstrate CO2 capture from flue gas/air and its direct conversion into syngas under solar irradiation without any externally applied voltage. The system captures CO2 with an amine/hydroxide solution and photoelectrochemically converts it into syngas (CO:H2 1:2 (concentrated CO2), 1:4 (simulated flue gas), and 1:30 (air)) using a perovskite-based photocathode with an immobilized molecular Co-phthalocyanine catalyst. At the anode, plastic-derived ethylene glycol is oxidized into glycolic acid over a Cu26Pd74 alloy catalyst. The overall process uses flue gas/air as carbon source and discarded plastic waste as electron donor, opening avenues for integrated carbon-neutral/negative solar fuel and waste upcycling technologies.

Gasification is a process to transform solids, such as agricultural and municipal waste, into gaseous feedstock for making transportation fuels. The so‐called coarse solid residue (CSR) that remains after this conversion process is currently discarded as a process solid residue. In the context of transitioning from a linear to a circular society, the feasibility of using the solid process residue from waste gasification as a solid catalyst for light olefin production from CO, CO2, and H2 mixtures was investigated. This CSR‐derived catalyst converted biomass‐derived syngas, a H2‐poor mixture of CO, CO2, H2, and N2, into methane (57 %) and C2–C4 olefins (43 %) at 450 °C and 20 bar. The main active ingredient of CSR was Fe, and it was discovered with operando X‐ray diffraction that metallic Fe, present after pre‐reduction in H2, transformed into an Fe carbide phase under reaction conditions. The increased formation of Fe carbides correlated with an increase in CO conversion and olefin selectivity. The presence of alkali elements, such as Na and K, in CSR‐derived catalyst increased olefin production as well.

Syngas, a gaseous mixture of CO, H2 and CO2, can be produced by gasification of carbon-containing materials, including organic waste materials or lignocellulosic biomass. The conversion of bio-based syngas to chemicals is foreseen as an important process in circular bioeconomy. Carbon monoxide is also produced as a waste gas in many industrial sectors (e.g., chemical, energy, steel). Often, the purity level of bio-based syngas and waste gases is low and/or the ratios of syngas components are not adequate for chemical conversion (e.g., by Fischer-Tropsch). Microbes are robust catalysts to transform impure syngas into a broad spectrum of products. Fermentation of CO-rich waste gases to ethanol has reached commercial scale (by axenic cultures of Clostridium species), but production of other chemical building blocks is underexplored. Currently, genetic engineering of carboxydotrophic acetogens is applied to increase the portfolio of products from syngas/CO, but the limited energy metabolism of these microbes limits product yields and applications (for example, only products requiring low levels of ATP for synthesis can be produced). An alternative approach is to explore microbial consortia, including open mixed cultures and synthetic co-cultures, to create a metabolic network based on CO conversion that can yield products such as medium-chain carboxylic acids, higher alcohols and other added-value chemicals.

2LiH + MgB2 composite doped with TiO2 (Li-RHC-Ti) is employed with a two-fold purpose: hydrogen purification under a H2–CO (0.1 mol%) mixture and CO methanation. Upon dynamic cycling under CO–H2 mixture, hydrogen release curves display a quite stable amount of pure hydrogen of about 10 wt%, short release times of around 60 min, and minor degradation. Gas analysis by Fourier transform infrared spectroscopy (FTIR) after a thermal dehydrogenation process of MgH2 and LiBH4 under CO evidence the conversion of CO to CH4. Li-RHC-Ti dehydrogenated under CO shows the simultaneous formation of CH4, CH3OH, and B(CH3)3 in the gas phase. X-ray powder diffraction (XRPD) and FTIR characterizations of the solid phases of Li-RHC-Ti after both H2–CO mixture and CO interactions demonstrate the formation of MgO, LiBO2, and HCOO− species. Li-RHC-Ti acts as a hydrogen source and promoter for the CO conversion. Reaction pathways are proposed based on experimental results and equilibrium composition calculations.

Direct synthesis of dimethyl ether as a green fuel from syngas over nanostructured CuO–ZnO–Al2O3/HZSM-5 catalyst: Influence of irradiation time on nanocatalyst properties and catalytic performance

The activity and selectivity in the catalytic reduction of NO by a mixture of CO and H2 of three PdO-MoO3/γ-Al2O3 catalysts are compared in the presence of varying amounts of oxygen at reaction temperatures from 100 to 550°C. The catalysts were prepared by different methods and contain about 2% Mo and 2% Pd. Results are compared with those for PdO/γ-Al2O3, PdO-MoO3/γ-Al2O3 containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. The PdO-MoO3/γ-Al2O3 catalysts are more active for the selective reduction of NO to N2 and N2O than PdO/γ-Al2O3 under slightly oxidizing conditions at temperatures from 300 to 550°C. At these reaction conditions, the fresh PdO-MoO3/γ-Al2O3 catalysts are comparable with a commercial Pt-Rh catalyst. The improved activity of PdO-MoO3/γ-Al2O3 relative to PdO/γ-Al2O3 is believed to be due to the interaction between Pd and Mo. The effect of O2 on the activity and selectivity of these catalysts is different in the reduction of NO by H2, by CO, and by a mixture of H2 and CO. The results using the mixture of reductants cannot be inferred from the results with the single reductants.

Chapter 4 - Syngas conversion to biofuels: Recent progress

ConspectusThe hydrogenative conversion of both CO and CO2 into high-value multicarbon (C2+) compounds, such as olefins, aromatic hydrocarbons, ethanol, and liquid fuels, has attracted much recent attention. The hydrogenation of CO is related to the chemical utilization of various carbon resources including shale gas, biomass, coal, and carbon-containing wastes via syngas (a mixture of H2 and CO), while the hydrogenation of CO2 by green H2 to chemicals and liquid fuels would contribute to recycling CO2 for carbon neutrality. The state-of-the-art technologies for the hydrogenation of CO/CO2 to C2+ compounds primarily rely on a direct route via Fischer-Tropsch (FT) synthesis and an indirect route via two methanol-mediated processes, i.e., methanol synthesis from CO/CO2 and methanol to C2+ compounds. The direct route would be more energy- and cost-efficient owing to the reduced operation units, but the product selectivity of the direct route via FT synthesis is limited by the Anderson-Schulz-Flory (ASF) distribution. Selectivity control for the direct hydrogenation of CO/CO2 to a high-value C2+ compound is one of the most challenging goals in the field of C1 chemistry, i.e., chemistry for the transformation of one-carbon (C1) molecules.We have developed a relay-catalysis strategy to solve the selectivity challenge arising from the complicated reaction network in the hydrogenation of CO/CO2 to C2+ compounds involving multiple intermediates and reaction channels, which inevitably lead to side reactions and byproducts over a conventional heterogeneous catalyst. The core of relay catalysis is to design a single tandem-reaction channel, which can direct the reaction to the target product controllably, by choosing appropriate intermediates (or intermediate products) and reaction steps connecting these intermediates, and arranging optimized yet matched catalysts to implement these steps like a relay. This Account showcases representative relay-catalysis systems developed by our group in the past decade for the synthesis of liquid fuels, lower (C2-C4) olefins, aromatics, and C2+ oxygenates from CO/CO2 with selectivity breaking the limitation of conventional catalysts. These relay systems are typically composed of a metal or metal oxide for CO/CO2/H2 activation and a zeolite for C-C coupling or reconstruction, as well as a third or even a fourth catalyst component with other functions if necessary. The mechanisms for the activation of H2 and CO/CO2 on metal oxides, which are distinct from that on the conventional transition or noble metal surfaces, are discussed with emphasis on the role of oxygen vacancies. Zeolites catalyze the conversion of intermediates (including hydrocracking/isomerization of heavier hydrocarbons, methanol-to-hydrocarbon reactions, and carbonylation of methanol/dimethyl ether) in the relay system, and the selectivity is mainly controlled by the Brønsted acidity and the shape-selectivity or the confinement effect of zeolites. We demonstrate that the thermodynamic/kinetic matching of the relay steps, the proximity and spatial arrangement of the catalyst components, and the transportation of intermediates/products in sequence are the key issues guiding the selection of each catalyst component and the construction of an efficient relay-catalysis system. Our methodology would also be useful for the transformation of other C1 molecules via controlled C-C coupling, inspiring more efforts toward precision catalysis.

Fischer Tropsch Synthesis (FTS) is an exothermic chemical reaction in which synthesis gas (or ‘syngas’- a mixture of H2 and CO) is converted into hydrocarbons or value-added chemicals. In this process, a catalyst (typically cobalt based or Iron based) is used in a Fixed Bed (FB) or Slurry Bed (SB) reactor for the conversion process. Qatar hosts both the technologies in its world's largest Gas to Liquid (GTL) facilities (Shell Pearl GTL and Sasol Oryx GTL). Although both the technologies have been commercially implemented in a large scale, further process intensification by radial scale-up has been a challenging task due to certain process limitation associated with transport characteristics of both the beds. In particular, the FB technology has issues related to hotspot formation owing to exothermicity of the FTS process which is significantly better in its SB counterpart. Our efforts in the current study are invested to understand the FB performance when it is radially scaled-up to a higher reactor geometry, and to possibly mitigate the effect of hotspot formation. In particular, the objective of this work is to utilize the merits of nonconventional Supercritical Fluids FTS (SCF-FTS) to consolidate the benefits of both the beds (FB and SB) to address the challenges related to hotspot formation. For this, we have developed a multi-dimensional computational fluid dynamics (CFD) model in COMSOL® to facilitate a high-resolution understanding of both the SCF-FTS and conventional Gas Phase (GP)-FTS from the perspective of bed thermal management. As an extension to our previous modeling efforts in development of 1-D and 2-D FB-FTS model [1-2], we are currently involved in development of a multiscale 2D model to investigate the pore characteristics using both the modes of operation. Comprehensive experimental investigations were carried out at different operating conditions to support the modeling efforts. A conventional cobalt catalyst with inferior thermal conductivity was investigated in both GP-FTS and SCF-FTS. Later, a novel Micro-fibrous Entrapped Cobalt Catalyst (MFECC) with superior thermal conductivity was investigated in both GP-FTS and SCF-FTS. Conventional catalyst operated SCF-FTS conditions gave a very high value (0.90) than its GP-FTS. The MFECC catalytic bed on the other hand when operated in SCF-FTS conditions gave a slightly lower value (0.86), but six-fold % CO conversion than in GP-FTS. MFECC catalytic bed also exhibited higher C5+ selectivity & higher catalyst activity in SCF-FTS. In order to closely understand the intricate difference in thermal performance shown by the MFECC bed compared to conventional FB, we have performed a detailed CFD calculation. Results of the MFECC bed have shown to provide orders of magnitude improvement in bed thermal conductivity and proved its capability to control hotspot formation. In particular, the results of conventional FB at 20 bar and at a gas hourly space velocity of 5000 1/h in a reactor tube of 0.59 inch ID shows hotspot formation of about at the centerline. On the other hand, the temperature rise in MFECC bed for same operating condition was only. Further, a very recent outcome of this work enabled us to investigate the potential of scaling-up the radial geometry of the MFECC reactor to a 4” ID reactor to improve its throughput while maintaining temperature homogeneity in the reactor bed [2]. The proposed study is a part of a broader project involving both experimental and modeling studies, and is performed at multiple stages to enable mitigation of challenges related to reactor scale up, and runaway hotspot formation in a fixed bed FT reaction.

Cerium oxide was synthesized by pyrolysis of the salt Ce(NO) · 6HO (P), co-combustion of Ce(NO) · 6HO with urea (M), sol-gel method (Z-G) and decomposition of the hydroxide precipitated by mixing aqueous solutions of Ce(NO) · 6HO and ammonia (G). Samples with manganese oxide MnO/CeO and MnCeO were obtained by impregnation and using urea. The effect of specific surface area on the activity of CuO/CeO CuO/MnO/CeO CuO/MnCeO samples in the reaction of CO oxidation in a mixture of CO + O + H in the range of 30 – 400°C was studied. It was found that the supported catalysts nCuO/nMnO/CeO(P) (n = 5,7.5,10 wt%) provide the highest conversion of CO to CO compared to that in the presence of the systems (5 – 10)% MnO/CeO(P) MnCeO(M) (x = 0.1 – 0.5) 7.5% CuO/MnCeO. On nCuO/nMnO/CeO(P) samples, 100% conversion of CO to CO is achieved at 120°C, the temperature window ΔT, in which this value remains unchanged, is 40°C, which is worse than the values for (7.5 – 10)% CuO/CeO(P) catalysts, for which 100% conversion of CO to CO is recorded at 100°C and is maintained up to 160°C in the temperature window ΔT = 60°C. Based on X-ray diffraction (XRD) and temperature-programmed reduction with hydrogen (TPR H) data, it is concluded that oxygen from the interacting oxides MnO/MnO CeO in binary systems participates in CO oxidation. The introduction of copper oxide into them increases oxygen activity as a result of the formation of copper-manganese oxide structures in ternary systems.

An experimental study on the fuel conversion efficiency and NOx emissions of a spark-ignition gas engine for power generation by fuel mixture of methane and model syngas (H2/CO)

Given the continuous and excessive CO2 emission into the atmosphere from anthropomorphic activities, there is now a growing demand for negative carbon emission technologies, which requires efficient capture and conversion of CO2 to value-added chemicals. This review highlights recent advances in CO2 capture and conversion chemistry and processes. It first summarizes various adsorbent materials that have been developed for CO2 capture, including hydroxide-, amine-, and metal organic framework-based adsorbents. It then reviews recent efforts devoted to two types of CO2 conversion reaction: thermochemical CO2 hydrogenation and electrochemical CO2 reduction. While thermal hydrogenation reactions are often accomplished in the presence of H2, electrochemical reactions are realized by direct use of electricity that can be renewably generated from solar and wind power. The key to the success of these reactions is to develop efficient catalysts and to rationally engineer the catalyst-electrolyte interfaces. The review further covers recent studies in integrating CO2 capture and conversion processes so that energy efficiency for the overall CO2 capture and conversion can be optimized. Lastly, the review briefs some new approaches and future directions of coupling direct air capture and CO2 conversion technologies as solutions to negative carbon emission and energy sustainability.

Conversion of CO2 by reverse water gas shift (RWGS) reaction using a hydrogen oxyflame

Real-time spectroscopic ellipsometry (RTSE) has been applied to study the growth of nanocrystalline diamond thin films by microwave plasma-enhanced chemical vapor deposition on silicon substrates. The goal of this research is to characterize the diamond film growth process as a function of the gas source and substrate temperature, comparing the results obtained using various mixtures of CO and H2 with those obtained from a standard mixture of CH4 highly diluted in H2. The capabilities of RTSE have been exploited to establish the true near-surface substrate temperature under the specific diamond film growth conditions, as well as the deposition rates for a succession of films prepared on the same substrate under different conditions. The latter capability allows large regions of parameter space to be scanned expeditiously. As a result of this study, a low-temperature growth process has been identified that yields high deposition rates (up to 2.5 μm/h) at relatively low microwave plasma powers (0.5 kW). In contrast to the commonly-used H2-rich mixtures of CH4 or CO and H2 that exhibit monotonic reductions in the growth rate with decreasing substrate temperature from 800 to 400 °C, CO-rich mixtures of CO and H2 exhibit an increase and a well-defined maximum as the temperature is reduced over this range. At a CO/H2 gas flow ratio of 18, for example, the growth rate peaks near 450 °C and is a factor of ∼20 higher than that obtained with the standard H2-rich mixtures of CH4/H2 and CO/H2. These observations suggest a different diamond growth mechanism from the CO-rich mixtures of CO/H2 with potentially important applications for low-temperature substrate materials.

Tailor-Made Zeolitic Water Nanochannels for Liquid Fuel Production