Recent advances in preparations and applications of carbon aerogels: A review

Catalytic non-oxidative direct dehydrogenation of light alkanes serves as an effective reinforcement to selectively produce the corresponding olefins, and the heterogeneous metals and metal oxides, not limited to the commercially used Pt- and Cr-based catalysts, are widely investigated to enhance the efficiency. In this review, we outline the progress of these dehydrogenation catalysts that have been mainly developed in the past five years. For different classes of the most-promising catalysts in the selective dehydrogenation of ethane-to-ethylene and propane-to-propylene, their syntheses, structural information, catalytic properties and mechanisms are comparatively summarized.

Recent progress in heterogeneous metal and metal oxide catalysts for direct dehydrogenation of ethane and propane

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.

Conversion of propane or butanes from natural/shale gas into propene or butenes, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes. Herein, we critically analyse recent developments in the non-oxidative, oxidative, and CO2-mediated dehydrogenation of propane and isobutane to the corresponding olefins over metal oxide catalysts. Particular attention is paid to (i) comparing the developed catalysts in terms of their application potential, (ii) structure-activity-selectivity relationships for tailored catalyst design, and (iii) reaction-engineering aspects for improving product selectivity and overall process efficiency. On this basis, possible directions for further research aimed at the development of inexpensive and environmentally friendly catalysts with industrially relevant performance were identified. In addition, we provide general information regarding catalyst preparation and characterization as well as some recommendations for carrying out non-oxidative and CO2-mediated dehydrogenation reactions to ensure unambiguous comparison of catalysts developed in different studies.

Current status and perspectives in oxidative, non-oxidative and CO2-mediated dehydrogenation of propane and isobutane over metal oxide catalysts.

Oxidative dehydrogenation of propane (ODHP) is a key technology for producing propene from shale gas, but conventional metal oxide catalysts are prone to overoxidation to form valueless CO x Boron-based catalysts were recently found to be selective for this reaction, and B-O-B oligomers are generally regarded as active centers. We show here that the isolated boron in a zeolite framework without such oligomers exhibits high activity and selectivity for ODHP, which also hinders full hydrolysis for boron leaching in a humid atmosphere because of the B-O-SiO x linkage, achieving superior durability in a long-period test. Furthermore, we demonstrate an isolated boron with a -B[OH…O(H)-Si]2 structure in borosilicate zeolite as the active center, which enables the activation of oxygen and a carbon-hydrogen bond to catalyze the ODHP.

Isolated boron in zeolite for oxidative dehydrogenation of propane.

Metal-free boron- and carbon-based catalysts have shown both great fundamental and practical value in oxidative dehydrogenation (ODH) of light alkanes. In particular, boron-based catalysts show a superior selectivity toward olefins, excellent stability and atom-economy to valuable carbon-based products by minimizing CO2 emission, which are highly promising in future industrialization. The carbonaceous catalysts also exhibited impressive behavior in the ODH of light alkanes helped along by surface oxygen-containing functional groups. This review surveyed and compared the preparation methods of the boron- and carbon-based catalysts and their characterization, their performance in the ODH of light alkanes, and the mechanistic issues of the ODH including the identification of the possible active sites and the exploration of the underlying mechanisms. We discussed different boron-based materials and established versatile methodologies for the investigation of active sites and reaction mechanisms. We also elaborated on the similarities and differences in catalytic and kinetic behaviors, and reaction mechanisms between boron- and carbon-based metal-free materials. A perspective of the potential issues of metal-free ODH catalytic systems in terms of their rational design and their synergy with reactor engineering was sketched.

Oxidative dehydrogenation of light alkanes to olefins on metal-free catalysts

Oxidative dehydrogenation of propane using layered borosilicate zeolite as the active and selective catalyst

Development of porous sorbents that can selectively capture carbon dioxide (CO2) through adsorption technology from the flue gas is essential to reduce emission of CO2 to the atmosphere. However, under typical flue gas conditions (∼0.15 bar of CO2 at 40–70 °C), the adsorption capacity and selectivity for CO2 over those for nitrogen (N2) remain poor on traditional porous adsorbents due to weak adsorbent–adsorbate interactions. Here, we report the synthesis of hierarchically structured porous carbon composites via an interfacial assembling strategy using nanoclay LAPONITE®, resorcinol and formaldehyde as the precursors. This fabrication strategy allows fine tuning of the surface chemistry and pore network of porous carbons with the aim of enhancing the CO2 capture capacity at elevated temperatures (e.g., 50–150 °C). The obtained carbon composites reach a record-high CO2/N2 selectivity (114.3) at 70 °C, according to equilibrium gas adsorption analysis and dynamic breakthrough measurement associated with a high adsorption capacity of 1.7 mmol g−1 at 1.1 bar and 70 °C, and 0.5 mmol g−1 at 0.17 bar and 70 °C. The inorganic substance integrated 3D carbon framework is responsible for such superior CO2 capture at 70 °C by enhancing adsorbent–adsorbate interactions.

Interfacial assembled preparation of porous carbon composites for selective CO2 capture at elevated temperatures

Coproduction of styrene and benzaldehyde over a boron nitride-supported monomeric MoOx catalyst

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Oxidative dehydrogenation of light alkanes to olefins on metal-free catalysts

Oxidative dehydrogenation of propane using layered borosilicate zeolite as the active and selective catalyst

Interfacial assembled preparation of porous carbon composites for selective CO2 capture at elevated temperatures

Coproduction of styrene and benzaldehyde over a boron nitride-supported monomeric MoOx catalyst