Near 100% ethene selectivity achieved by tailoring dual active sites to isolate dehydrogenation and oxidation

Material Engineering Solutions toward Selective Redox Catalysts for Chemical-Looping-Based Olefin Production Schemes: A Review

Chemical looping (CL) has emerged as a promising approach in the oxidative dehydrogenation (ODH) of light alkanes, offering an opportunity for significant reductions in emissions and energy consumption in the ethylene and propylene production industry. While high olefin yields are achievable via CL, the material requirements (e.g., electronic and geometric structures) that prevent the total conversion of alkanes to CO x are not clearly understood. This review aims to give a concise understanding of the nucleophilic oxygen species involved in the selective reaction pathways for olefin production as well as of the electrophilic oxygen species that promote an overoxidation to CO x products. It further introduces advanced characterization techniques such as X-ray photoelectron spectroscopy, Raman spectroscopy, electron paramagnetic resonance spectroscopy, and resonant inelastic X-ray scattering, which have been employed successfully in identifying such reactive oxygen species. To mitigate CO x formation and enhance olefin selectivity, material engineering solutions are discussed. Common techniques include doping of the bulk or surface and the deposition of functional coatings. In the context of energy consumption and CO2 intensity, techno-economic assessments of CL-ODH systems have predicted energy savings of up to 80% compared to established olefin production processes such as steam cracking or dehydrogenation. Finally, although their practical application has been limited to date, the potential advantages of the use of fluidized bed reactors in CL-ODH are presented.

Subsurface A-site vacancy activates lattice oxygen in perovskite ferrites for methane anaerobic oxidation to syngas

Tuning the oxygen activity in perovskite oxides (ABO3) is promising to surmount the trade-off between activity and selectivity in redox reactions. However, this remains challenging due to the limited understanding in its activation mechanism. Herein, we propose the discovery that generating subsurface A-site cation (Lasub.) vacancy beneath surface Fe-O layer greatly improved the oxygen activity in LaFeO3, rendering enhanced methane conversion that is 2.9-fold higher than stoichiometric LaFeO3 while maintaining high syngas selectivity of 98% in anaerobic oxidation. Experimental and theoretical studies reveal that absence of Lasub.-O interaction lowered the electron density over oxygen and improved the oxygen mobility, which reduced the barrier for C-H bond cleavage and promoted the oxidation of C-atom, substantially boosting methane-to-syngas conversion. This discovery highlights the importance of A-site cations in modulating electronic state of oxygen, which is fundamentally different from the traditional scheme that mainly credits the redox activity to B-site cations and can pave a new avenue for designing prospective redox catalysts.

Dual-Active-Sites Single-Atom Catalysts for Advanced Applications

Dual-Active-Sites Single-Atom catalysts (DASs SACs) are not only the improvement of SACs but also the expansion of dual-atom catalysts. The DASs SACs contains dual active sites, one of which is a single atomic active site, and the other active site can be a single atom or other type of active site, endowing DASs SACs with excellent catalytic performance and a wide range of applications. The DASs SACs are categorized into seven types, including the neighboring mono metallic DASs SACs, bonded DASs SACs, non-bonded DASs SACs, bridged DASs SACs, asymmetric DASs SACs, metal and nonmetal combined DASs SACs and space separated DASs SACs. Based on the above classification, the general methods for the preparation of DASs SACs are comprehensively described, especially their structural characteristics are discussed in detail. Meanwhile, the in-depth assessments of DASs SACs for variety applications including electrocatalysis, thermocatalysis and photocatalysis are provided, as well as their unique catalytic mechanism are addressed. Moreover, the prospects and challenges for DASs SACs and related applications are highlighted. The authors believe the great expectations for DASs SACs, and this review will provide novel conceptual and methodological perspectives and exciting opportunities for further development and application of DASs SACs.

Coupling acid catalysis and selective oxidation over MoO3-Fe2O3 for chemical looping oxidative dehydrogenation of propane

Redox catalysts play a vital role in chemical looping oxidative dehydrogenation processes, which have recently been considered to be a promising prospect for propylene production. This work describes the coupling of surface acid catalysis and selective oxidation from lattice oxygen over MoO₃-Fe₂O₃ redox catalysts for promoted propylene production. Atomically dispersed Mo species over γ-Fe₂O₃ introduce effective acid sites for the promotion of propane conversion. In addition, Mo could also regulate the lattice oxygen activity, which makes the oxygen species from the reduction of γ-Fe₂O₃ to Fe₃O₄ contribute to selectively oxidative dehydrogenation instead of over-oxidation in pristine γ-Fe₂O₃. The enhanced surface acidity, coupled with proper lattice oxygen activity, leads to a higher surface reaction rate and moderate oxygen diffusion rate. Consequently, this coupling strategy achieves a robust performance with 49% of propane conversion and 90% of propylene selectivity for at least 300 redox cycles and ultimately demonstrates a potential design strategy for more advanced redox catalysts.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

Light olefins are important feedstocks and platform molecules for the chemical industry. Their synthesis has been a research priority in both academia and industry. There are many different approaches to the synthesis of these compounds, which differ by the choice of raw materials, catalysts and reaction conditions. The goals of this review are to highlight the most recent trends in light olefin synthesis and to perform a comparative analysis of different synthetic routes using several quantitative characteristics: selectivity, productivity, severity of operating conditions, stability, technological maturity and sustainability. Traditionally, on an industrial scale, the cracking of oil fractions has been used to produce light olefins. Methanol-to-olefins, alkane direct or oxidative dehydrogenation technologies have great potential in the short term and have already reached scientific and technological maturities. Major progress should be made in the field of methanol-mediated CO and CO₂ direct hydrogenation to light olefins. The electrocatalytic reduction of CO₂ to light olefins is a very attractive process in the long run due to the low reaction temperature and possible use of sustainable electricity. The application of modern concepts such as electricity-driven process intensification, looping, CO₂ management and nanoscale catalyst design should lead in the near future to more environmentally friendly, energy efficient and selective large-scale technologies for light olefin synthesis.

Main-Group Catalysts with Atomically Dispersed In Sites for Highly Efficient Oxidative Dehydrogenation

Transition metal oxides are well-known catalysts for oxidative dehydrogenation thanks to their excellent ability to activate alkanes. However, they suffer from an inferior alkene yield due to the trade-off between the conversion and selectivity induced by more reactive alkenes than alkanes, which obscures the optimization of catalysts. Herein, we attempt to overcome this challenge by activating a selective main-group indium oxide considered to be inactive for oxidative dehydrogenation in conventional wisdom. Atomically dispersed In sites with the local structure of [InOH]²⁺ anchored by substituting the protons of supercages in HY are enclosed to be active centers that enable the activation of ethane with a metal-normalized turnover number of almost one magnitude higher than those of their supported In₂O₃ counterparts. Furthermore, the structure of isolated [InOH]²⁺ sites could be stabilized by in situ formed H₂O from the selective oxidation of hydrogen by In₂O₃ nanoparticles. As a result, the as-designed main-group In catalysts exhibit 80% ethene selectivity at 80% ethane conversion, thus achieving 60% ethene yield due to active isolated [InOH]²⁺ sites and selective In₂O₃ nanoparticles, outperforming state-of-the-art transition metal oxide catalysts. This study unlocks new opportunities for the utilization of main-group elements and could pave the way toward a more rational design of catalysts for highly efficient selective oxidation catalysis.

Mixed Metal Oxides of M1 MoVNbTeOx and TiO2 as Composite Catalyst for Oxidative Dehydrogenation of Ethane

Composite catalysts of mixed metal oxides were prepared by mixing a phase-pure M1 MoVNbTeOx with anatase-phase TiO2. Two methods were used to prepare the composite catalysts (the simple physically mixed or sol-gel method) for the improvement of the catalytic performance in the oxidative dehydrogenation of ethane (ODHE) process. The results showed that TiO2 particles with a smaller particle size were well dispersed on the M1 surface for the sol-gel method, which presented an excellent activity for ODHE. At the same operating condition (i.e., the contact time of 7.55 gcat·h/molC2H6 and the reaction temperature of 400 °C), the M1-TiO2-SM and M1-TiO2-PM achieved the space time yields of 0.67 and 0.52 kgC2H4/kgcat/h, respectively, which were about ~76% and ~35% more than that of M1 catalyst (0.38 kgC2H4/kgcat/h), respectively. The BET, ICP, XRD, TEM, SEM, H2-TPR, C2H6-TPSR, and XPS techniques were applied to characterize the catalysts. It was noted that the introduction of TiO2 raised the V5+ abundance on the catalyst surface as well as the reactivity of active oxygen species, which made contribution to the promotion of the catalytic performance. The surface morphology and crystal structure of used catalysts of either M1-TiO2-SM or M1-TiO2-PM remained stable as each fresh catalyst after 24 h time-on-stream tests.