Ultrasonically Activated Liquid Metal Catalysts in Water for Enhanced Hydrogenation Efficiency

Hydride (H-) species on oxides have been extensively studied over the past few decades because of their critical role in various catalytic processes. Their syntheses require high temperatures and the presence of hydrogen, which involves complex equipment, high energy costs, and strict safety protocols. Hydride species tend to decompose in the presence of atmospheric oxygen and water, which reduces their catalytic activities. These challenges highlight the need for further research to improve the stability and efficiency of catalytic processes and develop safer and cost-effective synthesis methods. This paper introduces an ultrasonic fabrication method for gallium hydride species on liquid metal (LM) nanoparticles (Ga-H@LM NPs) in water and describes the evaluation of their catalytic properties. The Ga-H@LM NPs were synthesized by dispersing liquid metals of eutectic gallium-indium in water using a two-step ultrasonication process in an ice bath. The presence of Ga-H species was confirmed by Fourier-transform infrared spectroscopy. The Ga-H@LM NPs demonstrated the rapid catalytic hydrogenation of 4-nitrophenol and reductive degradation of azo dyes within minutes without the need for external reducing agents like NaBH4. The proposed mechanism involves high-energy ultrasonic cavitation at the interface between LM NPs and water, which promotes the formation of H2 from water and its activation to form Ga-H on particles surface during ultrasonication. This study has significant implications for advancing the field of catalysis because it provides a novel and efficient catalytic method for the synthesis of stable hydride species on gallium oxides.

Isolated Pt Atoms Stabilized by Ga2O3 Clusters Confined in ZSM-5 for Nonoxidative Activation of Ethane

Selective activation of light alkanes is an essential reaction in the petrochemical industry for producing commodity chemicals, such as light olefins and aromatics. Because of the much higher intrinsic activities of noble metals in comparison to non-noble metals, it is desirable to employ solid catalysts with low noble metal loadings to reduce the cost of catalysts. Herein, we report the introduction of a tiny amount of Pt (at levels of hundreds of ppm) as a promoter of the Ga2O3 clusters encapsulated in ZSM-5 zeolite, which leads to ∼20-fold improvement in the activity for ethane dehydrogenation reaction. A combination of experimental and theoretical studies shows that the isolated Pt atoms stabilized by small Ga2O3 clusters are the active sites for activating the inert C-H bonds in ethane. The synergy of atomically dispersed Pt and Ga2O3 clusters confined in the 10MR channels of ZSM-5 can serve as a bifunctional catalyst for the direct ethane-benzene coupling reaction for the production of ethylbenzene, surpassing the performances of the counterpart catalysts made with PtGa nanoclusters and nanoparticles.

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.

Ga+-Chabazite Zeolite: A Highly Selective Catalyst for Nonoxidative Propane Dehydrogenation

Ga-chabazite zeolites (Ga-CHA) have been found to efficiently catalyze propane dehydrogenation with high propylene selectivity (96%). In situ Fourier transform infrared spectroscopy and pulse titrations are employed to determine that upon reduction, surface Ga₂O₃ is reduced and diffuses into the zeolite pores, displacing the Brønsted acid sites and forming extra-framework Ga⁺ sites. This isolated Ga⁺ site reacts reversibly with H₂ to form GaH_x (2034 cm^-1) with an enthalpy of formation of ∼-51.2 kJ·mol^-1, a result supported by density functional theory calculations. The initial C₃H₈ dehydrogenation rates decrease rapidly (40%) during the first 100 min and then decline slowly afterward, while the C₃H₆ selectivity is stable at ∼96%. The reduction in the reaction rate is correlated with the formation of polycyclic aromatics inside the zeolite (using UV-vis spectroscopy) indicating that the accumulation of polycyclic aromatics is the main cause of the deactivation. The carbon species formed can be easily oxidized at 600 °C with complete recovery of the PDH catalytic properties. The correlations between GaH_x vs Ga/Al ratio and PDH rates vs Ga/Al ratio show that extra-framework Ga⁺ is the active center catalyzing propane dehydrogenation. The higher reaction rate on Ga⁺ than In⁺ in CHA zeolites, by a factor of 43, is the result of differences in the stabilization of the transition state due to the higher stability of Ga³⁺ vs In³⁺. The uniformity of the Ga⁺ sites in this material makes it an excellent model for the molecular understanding of metal cation-exchanged hydrocarbon interactions in zeolites.