Probing thermal-induced structural evolution during the synthesis of layered Li-, Na-, or K-containing 3d transition-metal oxides

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.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis.

Capturing CO2 and converting it into fuels or fine chemicals is a promising way to deal with climate change issues and energy crises. However, the conversion of CO2 into hydrocarbons requires high activation energy owing to the stable CO bond. Therefore a catalyst with high performance is necessary to facilitate the chemical reactions. In recent years, it has been demonstrated that catalysts based on metal or metal oxides combined with zeolites have excellent performances in converting CO2 to various hydrocarbons and have the potential to be applied at an industrial scale. The present review article highlights the progress of zeolite-based catalysts in the CO2 conversion to hydrocarbon, i.e., gasoline, olefins, and aromatics products through modified Fischer-Tropcsh and methanol mediated pathways. The effect of zeolite properties, e.g., topology, acidity, morphology, crystallite size, extra framework cation and atom, and the pore structure, has been discussed. Also, several synthetic strategies for precisely adjusting the zeolite properties were demonstrated. Finally, the insight for future development was proposed. 

Zeolite-based catalyst for direct conversion of CO2 to C2+ hydrocarbon: A review

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis.

Direct catalytic hydrogenation of CO 2 is a highly promising route for olefin synthesis. However, achieving high olefin selectivity is predicated on the development of a selective catalyst with acceptable yield and stability. Herein, we report a Sr and Na co-decorated Fe catalyst that showed stable performance in the synthesis of high-valued olefins (ethene, propene and linear α-olefins) with space-time yield up to 290 mg g cat −1 h −1 without significant deactivation for over 500 h on-stream. The excellent catalytic performance is attributed to the cooperative effect of Sr and Na on the distinct sites (Fe 5 C 2 , Fe 3 C, Fe 3 O 4 ) in catalyst. In particular, SrCO 3 promoted the dispersion of binding Fe sites that facilitated the formation and stabilization of FeC x phase. Simultaneously, Sr contributed to the electron interaction between Na and Fe species. The dual effect of Sr as both structural and electronic promoters coordinatively improved C–O dissociative adsorption and subsequent C–C coupling, thus facilitated CO 2 hydrogenation. • Sr and Na co-decorated Fe catalyst showed excellent stability over500 h on-stream. • NaSrFe catalyst achieved high-valued olefin synthesis with yield up to 23.3 %. • SrCO 3 facilitated the formation, dispersion and stabilization of FeC x phase. • Sr species contributed to the electron interaction between Na and Fe species. 

Highly stable Sr and Na co-decorated Fe catalyst for high-valued olefin synthesis from CO2 hydrogenation

Cascade processes are gaining momentum in heterogeneous catalysis. The combination of several catalytic solids within one reactor has shown great promise for the one-step valorization of C1-feedstocks. The combination of metal-based catalysts and zeolites in the gas phase hydrogenation of CO2 leads to a large degree of product selectivity control, defined mainly by zeolites. However, a great deal of mechanistic understanding remains unclear: metal-based catalysts usually lead to complex product compositions that may result in unexpected zeolite reactivity. Here we present an in-depth multivariate analysis of the chemistry involved in eight different zeolite topologies when combined with a highly active Fe-based catalyst in the hydrogenation of CO2 to olefins, aromatics, and paraffins. Solid-state NMR spectroscopy and computational analysis demonstrate that the hybrid nature of the active zeolite catalyst and its preferred CO2-derived reaction intermediates (CO/ester/ketone/hydrocarbons, i.e., inorganic-organic supramolecular reactive centers), along with 10 MR-zeolite topology, act as descriptors governing the ultimate product selectivity. The reaction mechanism of carbon dioxide to high-value hydrocarbons over metal-zeolite bifunctional catalysts remains ambiguous. Here, the authors demonstrate that active zeolite catalysts’ topology and hybrid nature are descriptors; regulating the reaction mechanism and ultimate product selectivity.

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Stefan-Adrian F. Nastase

Papers

Selectivity descriptors for the direct hydrogenation of CO2 to hydrocarbons during zeolite-mediated bifunctional catalysis