Steering the reaction pathway of syngas-to-light olefins with coordination unsaturated sites of ZnGaOx spinel

Ketene Conversion Chemistry within Mordenite Zeolite: Pore-Size-Dependent Reaction Mechanism, Product Selectivity, and Catalytic Activity

The oxide-zeolite bifunctional catalyst for ketene-bridged syngas conversion has gained great attention for addressing the selectivity challenge in light olefin production, where zeolite dominates the ketene conversion selectivity. However, the atomic-level mechanism underlying ketene-to-light olefin conversion within zeolite remains unclear. Herein, we focus on mordenite (MOR) zeolite and perform systematic first-principles calculations combined with microkinetic simulations to elucidate pore-type-dependent reaction networks for ketene-to-light olefin conversion. Our microkinetic results reveal that ketene conversion within MOR follows an autocatalytic process initiated by the Brønsted acid site, involving the generation and subsequent catalysis of reactive intermediates. Time-dependent dynamic evolution simulation shows that within the 12-membered-ring (12MR) pore, a thermodynamically stable five-membered-ring carbocation (FMR-CH3+) self-evolves and acts as the active center to convert CH2CO to multihydrocarbons. Instead, in the 8-membered-ring side pocket (8MR), direct CH3+ formation occurs via acetyl carbocation (CH3CO+) decarbonylation, inducing CH2CO conversion with exclusive ethylene selectivity. The distinct reaction mechanisms and product selectivities are attributed to the thermodynamic and kinetic constraints of cyclic/long-chain intermediate formation imposed by the smaller 8MR pore. Despite its higher free energy barrier, 8MR is identified as the key active site for light olefin formation due to its lower dependence on ketene pressure. We also highlight the critical factors influencing both the selectivity and activity of light olefin formation, offering valuable insights for the optimization of MOR catalysts. This study provides a quantitative mechanistic understanding of ketene conversion, emphasizing the role of pore structure in shaping catalytic activity and product selectivity, which may facilitate the design of efficient zeolite-based catalysts.

ZnOx overlayer confined on ZnCr2O4 spinel for direct syngas conversion to light olefins

ZnCrOx oxides coupled with zeolites (OXZEO) allow direct conversion of syngas into light olefins, while active sites in the composite oxides remain elusive. Herein, we find that ZnO particles physically mixed with ZnCr2O4 spinel particles can be well dispersed onto the spinel surfaces by treatment in syngas and through a reduction-evaporation-anchoring mechanism, forming monodispersed ZnOx species with uniform thickness or dimension on ZnCr2O4 up to a dispersion threshold ZnO loading of 16.0 wt% (ZnCr2O4@ZnOx). A linear correlation between CO conversion and surface ZnO loading clearly confirms that the ZnOx overlayer on ZnCr2O4 acts as the active structure for the syngas conversion, which can efficiently activate both H2 and CO. The obtained ZnCr2O4@ZnOx catalyst combined with SAPO-34 zeolite achieves excellent catalytic performance with 64% CO conversion and 75% light olefins selectivity among all hydrocarbons. Moreover, the ZnOx overlayer is effectively anchored on the ZnCr2O4 spinel, which inhibits Zn loss during the reaction and demonstrates high stability over 100 hours. Thus, a significant interface confinement effect is present between the spinel surface and the ZnOx overlayer, which helps to stabilize ZnOx active structure and enhance the catalytic performance.

Balancing CO2 Adsorption and H2 Activation on Confined ZnOx Species for CO2 Hydrogenation

Many oxide catalysts exhibit high selectivity but low conversion in CO2 hydrogenation due to strong CO2 adsorption, which often impedes H2 dissociation and subsequent hydrogenation. Herein, we report that a ZnCr2O4@ZnOx catalyst featuring monodispersed ZnOx overlayer confined on ZnCr2O4 facilitates CO2 activation without compromising H2 activation. This catalyst demonstrates a dual-site mechanism in which ZnCr2O4 surface and/or ZnOx/ZnCr2O4 interface provide sites for CO2 activation and monodispersed ZnOx promote homolytic H2 dissociation and formation of stable metal─H species, enabling formate formation through hydrogen spillover to CO2 adsorption sites for hydrogenation at 303 K. In contrast, H2 activation on ZnO or ZnCr2O4 suffers from the poisoning effect of strong CO2 adsorption. Consequently, the ZnCr2O4@ZnOx catalyst shows 2-8 folds enhancement in CO2 hydrogenation between 623-773 K than ZnO and maintains 33% conversion and 100% CO selectivity at 723 K over 150 h. The established structure-performance relationship illustrates the critical role of dual-site catalysis and hydrogen spillover in hydrogenation reaction.

Accelerated Oxide-Zeolite Catalyst Design for Syngas Conversion by Reaction Phase Diagram Analysis and Machine Learning

Oxide-zeolite (OXZEO) catalyst design concept provides an alternative approach for the direct syngas-to-olefins (STO) with superior selectivity. Enhancing the activity of oxide components remains a critical and long-pursued target in this field. However, rational design strategies for optimizing oxides and improving the catalyst performance in such complex reaction networks are still lacking. We employed energetic descriptors such as the adsorption energies of CO* and O* (GadCO* and GadO*) through reaction phase diagram (RPD) analysis to predict the catalyst performance. The prediction was initially validated by the catalytic activity trends measured by experiments. Machine learning (ML) was further utilized to accelerate the screening of new catalysts. Ultimately, Bi-doped and Sb-doped ZnCrOx were theoretically predicted as optimized oxide candidates for the OXZEO reaction, which was experimentally verified to be more active than the currently best ZnCrOx counterpart. This work demonstrated enhanced OXZEO catalysts for STO as well as a research paradigm integrating theory and experiment to optimize bifunctional catalysts for complex reaction networks.

Synthesizing Liquid Fuels Over Carbon-Based Catalysts Via CO2 Conversion

The unique characteristics of carbon materials make them flexible for applications in heterogeneous catalysis. Their interest is expanding in the conscious efforts being made toward sustainable fuel production. A notable application is the heterogenous conversion of CO2 to liquid fuels, which exploits the characteristics of carbon materials, taking advantage of their electronic configurations, high surface area, pore properties, and synergistic role in catalysis. In this review, a critical overview of this rapidly developing field is presented. Various carbon allotropes and derivatives, as well as some strategies for fabricating carbon-based catalysts are keenly highlighted within thermal-, electro-, and photocatalytic CO2 conversion to liquid fuels. Distinct emphasis is placed on the role of different carbon materials by investigating the unique synergy attained at catalyst interfaces, the physicochemical properties attained, and their influence in enhancing the specific liquid fuels synthesis. Finally, the work is concluded, followed by an outlook detailing key challenges that need addressing.

Redefining the Symphony of Light Aromatic Synthesis Beyond Fossil Fuels: A Journey Navigating through a Fe-Based/HZSM-5 Tandem Route for Syngas Conversion

The escalating concerns about traditional reliance on fossil fuels and environmental issues associated with their exploitation have spurred efforts to explore eco-friendly alternative processes. Since then, in an era where the imperative for renewable practices is paramount, the aromatic synthesis industry has embarked on a journey to diversify its feedstock portfolio, offering a transformative pathway toward carbon neutrality stewardship. This Review delves into the dynamic landscape of aromatic synthesis, elucidating the pivotal role of renewable resources through syngas/CO2 utilization in reshaping the industry's net-zero carbon narrative. Through a meticulous examination of recent advancements, the current Review navigates the trajectory toward admissible aromatics production, highlighting the emergence of Fischer-Tropsch tandem catalysis as a game-changing approach. Scrutinizing the meliorated interplay of Fe-based catalysts and HZSM-5 molecular sieves would uncover the revolutionary potential of rationale design and optimization of integrated catalytic systems in driving the conversion of syngas/CO2 into aromatic hydrocarbons (especially BTX). In essence, the current Review would illuminate the path toward cutting-edge research through in-depth analysis of the transformative power of tandem catalysis and its capacity to propel carbon neutrality goals by unraveling the complexities of renewable aromatic synthesis and paving the way for a carbon-neutral and resilient tomorrow.

Visualization of the Active Sites of Zinc-Chromium Oxides and the CO/H2 Activation Mechanism in Direct Syngas Conversion

Despite wide studies demonstrating the versatility of the metal oxide-zeolite (OXZEO) catalyst concept to tackle the selectivity challenge in syngas chemistry, the active sites of metal oxides and the mechanism of CO/H2 activation remain to be elucidated. Herein, we demonstrate experimentally the role of Cr in zinc-chromium oxides and unveil visually, for the first time, the active sites for CO activation employing scanning transmission electron microscopy-electron energy loss spectroscopy using the volumetric density of surface carbon species as a descriptor. The ZnCr2O4 spinel surface with atomic ZnOx overlayer is the most active site for C-O bond dissociation, particularly at the narrow ZnCr2O4(110) facets constrained between the (311) and (111) facets, followed by the Cr-doped wurtzite ZnO surface. In comparison, the surfaces of ZnCr2O4 with aggregated ZnOx overlayers, pure ZnO, and the stoichiometric ZnCr2O4 exhibit a significantly lower activity. In situ synchrotron-based vacuum ultraviolet photoionization mass spectrometric study on different temperature programmed surface reactions with isotopes of C18O, 13CO, and D2 validates direct CO dissociation over ZnCrn oxides in CO, forming CH2 and further to hydrocarbons if H2 is present and CH2CO intermediates in syngas. The activity of CO dissociation and hydrogenation over ZnCrn oxides correlates well with the syngas-to-light-olefins activity of ZnCrn-SAPO-18 composite catalysts as a function of the Cr/Zn ratio.

Homolytic H2 dissociation for enhanced hydrogenation catalysis on oxides

The limited surface coverage and activity of active hydrides on oxide surfaces pose challenges for efficient hydrogenation reactions. Herein, we quantitatively distinguish the long-puzzling homolytic dissociation of hydrogen from the heterolytic pathway on Ga2O3, that is useful for enhancing hydrogenation ability of oxides. By combining transient kinetic analysis with infrared and mass spectroscopies, we identify the catalytic role of coordinatively unsaturated Ga3+ in homolytic H2 dissociation, which is formed in-situ during the initial heterolytic dissociation. This site facilitates easy hydrogen dissociation at low temperatures, resulting in a high hydride coverage on Ga2O3 (H/surface Ga3+ ratio of 1.6 and H/OH ratio of 5.6). The effectiveness of homolytic dissociation is governed by the Ga-Ga distance, which is strongly influenced by the initial coordination of Ga3+. Consequently, by tuning the coordination of active Ga3+ species as well as the coverage and activity of hydrides, we achieve enhanced hydrogenation of CO2 to CO, methanol or light olefins by 4-6 times.

Promoting Syngas to Olefins with Isolated Internal Silanols-Enriched Al-IDM-1 Aluminosilicate Nanosheets

Selective conversion of syngas to value-added olefins has attracted considerable research interest. Regulating product distribution remains challenging, such as achieving higher olefin selectivity, propylene/ethylene (P/E) and olefin/paraffin (O/P) ratios. A new pentasil zeolite Al-IDM-1 with recently approved -ION structure, composed of 17-membered-ring (MR) extra-large lobed pores and intersected 10-MR medium pores, shows a C2-6 = selectivity up to 85 % and a high O/P value of 14 in the conversion of syngas when being combined with Zna Alb Ox oxide. Moreover, for the high-silica Al-IDM-1 with Si/Al ratio of 400, the selectivity of propylene and butene accounts for 88 % in C2-4 = , resulting in high P/E (>4) and butene/ethylene (B/E >3) ratios. The high C3-4 = selectivity is contributed by two main reasons, that is, the relatively weak acidity of Al-IDM-1 zeolite enhances the olefin-based cycle revealed by the probe reactions of methanol-to-propylene (MTP) and 1-hexene cracking, and the rich isolated internal SiOH groups in Al-IDM-1 promote the desorption of C3-4 = , once they are formed inside zeolite pores.

Tunable CO2-to-syngas conversion via strong electronic coupling in S-scheme ZnGa2O4/g-C3N4 photocatalysts

The conversion of CO2 into syngas, a mixture of CO and H2, via photocatalytic reduction, is a promising approach towards achieving a sustainable carbon economy. However, the evolution of highly adjustable syngas, particularly without the use of sacrifice reagents or additional cocatalysts, remains a significant challenge. In this study, a step-scheme (S-scheme) 0D ZnGa2O4 nanodots (∼7 nm) rooted g-C3N4 nanosheets (denoted as ZnGa2O4/C3N4) heterojunction photocatalyst was synthesized vis a facial in-situ growth strategy for efficient CO2-to-syngas conversion. Both experimental and theoretical studies have demonstrated that the polymeric nature of g-C3N4 and highly distributed ZnGa2O4 nanodots synergistically contribute to a strong interaction between metal oxide and C3N4 support. Furthermore, the desirable S-scheme heterojunction in ZnGa2O4/C3N4 efficiently promotes charge separation, enabling strong photoredox ability. As a result, the S-scheme ZnGa2O4/C3N4 exhibited remarkable activity and selectivity in photochemical conversion of CO2 into syngas, with a syngas production rate of up to 103.3 μ mol g-1 h-1, even in the absence of sacrificial agents and cocatalyst. Impressively, the CO/H2 ratio of syngas can be tunable within a wide range from 1:4 to 2:1. This work exemplifies the effectiveness of a meticulously designed S-scheme heterojunction photocatalyst for CO2-to-syngas conversion with adjustable composition, thus paving the way for new possibilities in sustainable energy conversion and utilization.

Disentangling the activity-selectivity trade-off in catalytic conversion of syngas to light olefins

Breaking the trade-off between activity and selectivity has been a long-standing challenge in the field of catalysis. We demonstrate the importance of disentangling the target reaction from the secondary reactions for the case of direct syngas conversion to light olefins by incorporating germanium-substituted AlPO-18 within the framework of the metal oxide-zeolite (OXZEO) catalyst concept. The attenuated strength of the catalytically active Brønsted acid sites allows enhancing the targeted carbon-carbon coupling of ketene intermediates to form olefins by increasing the active site density while inhibiting secondary reactions that consume the olefins. Thus, a light-olefins selectivity of 83% among hydrocarbons and carbon monoxide conversion of 85% were obtained simultaneously, leading to an unprecedented light-olefins yield of 48% versus current reported light-olefins yields of ≤27%.

Combustion, Chemistry, and Carbon Neutrality

Combustion is a reactive oxidation process that releases energy bound in chemical compounds used as fuels─energy that is needed for power generation, transportation, heating, and industrial purposes. Because of greenhouse gas and local pollutant emissions associated with fossil fuels, combustion science and applications are challenged to abandon conventional pathways and to adapt toward the demand of future carbon neutrality. For the design of efficient, low-emission processes, understanding the details of the relevant chemical transformations is essential. Comprehensive knowledge gained from decades of fossil-fuel combustion research includes general principles for establishing and validating reaction mechanisms and process models, relying on both theory and experiments with a suite of analytic monitoring and sensing techniques. Such knowledge can be advantageously applied and extended to configure, analyze, and control new systems using different, nonfossil, potentially zero-carbon fuels. Understanding the impact of combustion and its links with chemistry needs some background. The introduction therefore combines information on exemplary cultural and technological achievements using combustion and on nature and effects of combustion emissions. Subsequently, the methodology of combustion chemistry research is described. A major part is devoted to fuels, followed by a discussion of selected combustion applications, illustrating the chemical information needed for the future.

Highly Active Catalytic CO2 Hydrogenation to Lower Olefins via Spinel ZnGaOx Combined with SAPO-34

A key primary method for creating a carbon cycle and carbon neutrality is the catalytic hydrogenation of CO₂ into high value-added chemicals or fuels. In this work, ZnGaO_x oxides were prepared by parallel co-precipitation and physically mixed with SAPO-34 molecular sieves prepared by hydrothermal synthesis to produce ZnGaO_x /SAPO-34 bifunctional catalysts, which were evaluated for the catalytic synthesis of lower olefins (C₂ ⁼ -C₄ ⁼ ) from carbon dioxide hydrogenation. It was demonstrated that the reaction process requires oxygen defect activation, synergistic hydrogenation, and CO₂ alkaline adsorption of ZnGaO_x . The spinel structure of ZnGaO_x has more abundant oxygen defects and alkaline adsorption sites than the ZnGaO_x solid solution, which effectively enhances the catalytic performance. The CO₂ conversion was 28.52%, the selectivity of C₂ ⁼ -C₄ ⁼ in hydrocarbons reached 70.01%, and the single-pass yield of C₂ ⁼ -C₄ ⁼ was 10.95% at 370 °C, 3.0 MPa, and 4800 mL/g_cat /h.

High-Performance Photocatalytic Reduction of Nitrogen to Ammonia Driven by Oxygen Vacancy and Ferroelectric Polarization Field of SrBi4 Ti4 O15 Nanosheets

Photo-responsive semiconductors can facilitate nitrogen activation and ammonia production, but the high recombination rate of photogenerated carriers represents a significant barrier. Ferroelectric photocatalysts show great promise in overcoming this challenge. Herein, by adopting a low-temperature hydrothermal procedure with varying concentrations of glyoxal as the reducing agent, oxygen vacancies (Vo) are effectively produced on the surface of ferroelectric SrBi₄ Ti₄ O₁₅ (SBTO) nanosheets, which leads to a considerable increase in photocatalytic activity toward nitrogen fixation under simulated solar light with an ammonia production rate of 53.41 µmol g^-1 h^-1 , without the need of sacrificial agents or photosensitizers. This is ascribed to oxygen vacancies that markedly enhance the self-polarization and internal electric field of ferroelectric SBTO, and hence, facilitate the separation of photogenerated charge carriers and light trapping as well as N₂ adsorption and activation, as compared to pristine SBTO. Consistent results are obtained in theoretical studies. Results from this study highlight the significance of surface oxygen vacancies in enhancing the performance of photocatalytic nitrogen fixation by ferroelectric catalysts.

A Review on Green Hydrogen Valorization by Heterogeneous Catalytic Hydrogenation of Captured CO2 into Value-Added Products

The catalytic hydrogenation of captured CO2 by different industrial processes allows obtaining liquid biofuels and some chemical products that not only present the interest of being obtained from a very low-cost raw material (CO2) that indeed constitutes an environmental pollution problem but also constitute an energy vector, which can facilitate the storage and transport of very diverse renewable energies. Thus, the combined use of green H2 and captured CO2 to obtain chemical products and biofuels has become attractive for different processes such as power-to-liquids (P2L) and power-to-gas (P2G), which use any renewable power to convert carbon dioxide and water into value-added, synthetic renewable E-fuels and renewable platform molecules, also contributing in an important way to CO2 mitigation. In this regard, there has been an extraordinary increase in the study of supported metal catalysts capable of converting CO2 into synthetic natural gas, according to the Sabatier reaction, or in dimethyl ether, as in power-to-gas processes, as well as in liquid hydrocarbons by the Fischer-Tropsch process, and especially in producing methanol by P2L processes. As a result, the current review aims to provide an overall picture of the most recent research, focusing on the last five years, when research in this field has increased dramatically.