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

Tracking Active Phase Behavior on Boron Nitride during the Oxidative Dehydrogenation of Propane Using Operando X-ray Raman Spectroscopy

Hexagonal boron nitride (hBN) is a highly selective catalyst for the oxidative dehydrogenation of propane (ODHP) to propylene. Using a variety of ex situ characterization techniques, the activity of the catalyst has been attributed to the formation of an amorphous boron oxyhydroxide surface layer. The ODHP reaction mechanism proceeds via a combination of surface mediated and gas phase propagated radical reactions with the relative importance of both depending on the surface-to-void-volume ratio. Here we demonstrate the unique capability of operando X-ray Raman spectroscopy (XRS) to investigate the oxyfunctionalization of the catalyst under reaction conditions (1 mm outer diameter reactor, 500 to 550 °C, P = 30 kPa C3H8, 15 kPa O2, 56 kPa He). We probe the effect of a water cofeed on the surface of the activated catalyst and find that water removes boron oxyhydroxide from the surface, resulting in a lower reaction rate when the surface reaction dominates and an enhanced reaction rate when the gas phase contribution dominates. Computational description of the surface transformations at an atomic-level combined with high precision XRS spectra simulations with the OCEAN code rationalize the experimental observations. This work establishes XRS as a powerful technique for the investigation of light element-containing catalysts under working conditions.

Self-evolved BOx anchored on Mg2B2O5 crystallites for high-performance oxidative dehydrogenation of propane

Oxidative dehydrogenation of propane (ODHP) is a promising process for producing propene. Recently, some boron-based catalysts have exhibited excellent olefin selectivity in ODHP. However, their complex synthetic routes and poor stability under high-temperature reaction conditions have hindered their practical application. Herein, we report a self-evolution method rather than conventional assembly approaches to acquire structures with excellent stability under a high propane conversion, from a single precursor-MgB2. The catalyst feasibly prepared and optimized exhibited a striking performance: 60% propane conversion with a 43.2% olefin yield at 535°C. The BOx corona pinned by the strong interaction with the borate enabled zero loss of the high conversion (around 40%) and olefins selectivity (above 80%) for over 100 h at 520°C. This all-in-one strategy of deriving all the necessary components from just one raw chemical provides a new way to synthesize effective and economic catalysts for potential industrial implementation.

Rationalizing kinetic behaviors of isolated boron sites catalyzed oxidative dehydrogenation of propane

Boron-based catalysts exhibit high alkene selectivity in oxidative dehydrogenation of propane (ODHP) but the mechanistic understanding remains incomplete. In this work, we show that the hydroxylation of framework boron species via steaming not only enhances the ODHP rate on both B-MFI and B-BEA, but also impacts the kinetics of the reaction. The altered activity, propane reaction order and the activation energy could be attributed to the hydrolysis of framework [B(OSi≡)3] unit to [B(OSi≡)3-x(OH···O(H)Si≡)x] (x = 1, 2, "···" represents hydrogen bonding). DFT calculations confirm that hydroxylated framework boron sites could stabilize radical species, e.g., hydroperoxyl radical, further facilitating the gas-phase radical mechanism. Variations in the contributions from gas-phase radical mechanisms in ODHP lead to the linear correlation between activation enthalpy and entropy on borosilicate zeolites. Insights gained in this work offer a general mechanistic framework to rationalize the kinetic behavior of the ODHP on boron-based catalysts.

Photocatalytic Oxidative Dehydrogenation of Propane for Selective Propene Production with TiO2

Oxidative dehydrogenation of propane (ODHP) as an exothermic process is a promising method to produce propene (C₃H₆) with lower energy consumption in chemical industry. However, the selectivity of the C₃H₆ product is always poor because of overoxidation. Herein, the ODHP reaction into C₃H₆ on a model rutile(R)-TiO₂(110) surface at low temperature via photocatalysis has been realized successfully. The results illustrate that photocatalytic oxidative dehydrogenation of propane (C₃H₈) into C₃H₆ can occur efficiently on R-TiO₂(110) at 90 K via a stepwise manner, in which the initial C-H cleavage occurs via the hole coupled C-H bond cleavage pathway followed by a radical mediated C-H cleavage to the C₃H₆ product. An exceptional selectivity of ∼90% for C₃H₆ production is achieved at about 13% propane conversion. The mechanistic model constructed in this study not only advances our understanding of C-H bond activation but also provides a new pathway for highly selective ODHP into C₃H₆ under mild conditions.

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.

Engineering O-O Species in Boron Nitrous Nanotubes Increases Olefins for Propane Oxidative Dehydrogenation

Boron nitride (BN) has been widely studied as an efficient catalyst for oxidative propane dehydrogenation (OPDH). Oxygen-containing boron species (e.g., BO·, B(OH)_xO_3-x) are generally considered as the active centers in BN for OPDH. Here, we show an effective progressive substitution strategy toward the development of boron-oxygen-nitrogen nanotubes (BONNTs) enriched with O-O species as a highly active, selective, and stable catalyst for OPDH. At 525 °C, an olefin yield of 48.6% is achieved over BONNTs with a propane conversion of 64.4%, 2.8 times that of boron nitrogen nanotubes (BNNTs). Even after reaction for 150 h (475 °C), BONNTs exhibit good olefin yield. Both the B(OH)_xO_3-x and O-O species that coexist in the BONNT catalyst are demonstrated as active centers, which differs from the B(OH)_xO_3-x one in BNNTs. Based on catalytic results, propane and oxygen alternate treatment experiments, and theoretical calculations, the O-O center is more favorable for producing both propylene (C₃₌) and ethylene (C₂₌), which experiences a dehydration pathway and two possible reaction paths with a lower energy barrier to yield olefins, while B(OH)_xO_3-x is mainly responsible for producing few C₃₌.

Water-Tolerant Boron-Substituted MCM-41 for Oxidative Dehydrogenation of Propane

Boron-based catalysts for oxidative dehydrogenation of propane (ODHP) have displayed excellent olefin selectivity. However, the drawback of deboronation leading to catalyst deactivation limited their scalable applications. Hereby, a series of mesoporous B-MCM-41 (BM-x, B/Si = 0.015-0.147) catalysts for ODHP were prepared by a simple hydrothermal synthesis method. It was found that propane conversion was increased and the initial reaction temperature was reduced with an increase of boron content, and the optimal values appeared on BM-2.0 (B/Si = 0.062), while olefins' (ethylene and propylene) selectivity was maintained at ca. 70-80%. Most importantly, BM-1.0 (B/Si = 0.048) exhibited favorable activity, stability, and water tolerance after washing treatment or long-time operation (e.g., propane conversion of ca. 15% and overall olefin selectivity of ca. 80% at 550 °C) because its high structural stability prevented boron leaches. These features were identified by X-ray diffraction (XRD), N₂ physisorption, inductively coupled plasma-mass spectrometry (ICP-MS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy studies. The tri-coordinated B-OH species incorporated into the mesoporous silica framework are considered to be the active sites for ODHP.

Efficient metal borate catalysts for oxidative dehydrogenation of propane

Boron-based catalysts have attracted attention due to their high selectivity to light olefins in the catalytic oxidative dehydrogenation of propane (ODHP) in the recent years, however, which still faced the...

Advances in Zeolite-Supported Metal Catalysts for Propane Dehydrogenation

Propylene is one of the building blocks of the modern industrial mansion, which is the feeding stock for polypropylene, acrylonitrile, and other important chemicals. Propane dehydrogenation (PDH) is one of...

Anchoring Boron Atom to the Specific Tetrahedral Sites of Borosilicate MFI by Imidazolium-based Molecules

Regulating heteroatom substituted tetrahedral sites (T sites) within a zeolite crystal significantly influences the zeolite properties. Herein, the as-synthesized MFI zeolite with tailored boron location at various T sites is...

Advances in Catalytic Applications of Zeolite-Supported Metal Catalysts

Zeolites possessing large specific surface areas, ordered micropores, and adjustable acidity/basicity have emerged as ideal supports to immobilize metal species with small sizes and high dispersities. In recent years, the zeolite-supported metal catalysts have been widely used in diverse catalytic processes, showing excellent activity, superior thermal/hydrothermal stability, and unique shape-selectivity. In this review, a comprehensive summary of the state-of-the-art achievements in catalytic applications of zeolite-supported metal catalysts are presented for important heterogeneous catalytic processes in the last five years, mainly including 1) the hydrogenation reactions (e.g., CO/CO₂ hydrogenation, hydrogenation of unsaturated compounds, and hydrogenation of nitrogenous compounds); 2) dehydrogenation reactions (e.g., alkane dehydrogenation and dehydrogenation of chemical hydrogen storage materials); 3) oxidation reactions (e.g., CO oxidation, methane oxidation, and alkene epoxidation); and 4) other reactions (e.g., hydroisomerization reaction and selective catalytic reduction of NO_x with ammonia reaction). Finally, some current limitations and future perspectives on the challenge and opportunity for this subject are pointed out. It is believed that this review will inspire more innovative research on the synthesis and catalysis of zeolite-supported metal catalysts and promote their future developments to meet the emerging demands for practical applications.

Understanding the Unique Antioxidation Property of Boron-Based Catalysts during Oxidative Dehydrogenation of Alkanes

Boron-based catalysts show excellent performance in oxidative dehydrogenation (ODH) of light alkanes to alkenes with high selectivity and extremely good antioxidation properties. However, the anti-deep-oxidation mechanism remains unclear. Herein, we chose h-BN and B₂O₃ as representative boron-based catalysts to investigate their reactions with two important intermediates in the light alkane ODH, Et· (evolving to ethene) and EtO· (evolving to ethene or CO_x), to elucidate the origin of the antioxidation of alkanes. The density functional theory calculations reveal that surface boron sites could eliminate alkoxy in their vicinity, resulting in exceptional inhibition of alkane deep-oxidation. The analysis of the electronic and geometric structures of key stationary points showed that the oxophilicity of B determined the low deep-oxidation of alkanes, and the homoleptic coordination of B with all three ligating atoms being O moderately enhanced its oxophilicity. This work represents a novel conceptual advance in the mechanistic understanding of alkane ODH.

Activation and conversion of alkanes in the confined space of zeolite-type materials

Microporous zeolite-type materials, with crystalline porous structures formed by well-defined channels and cages of molecular dimensions, have been widely employed as heterogeneous catalysts since the early 1960s, due to their wide variety of framework topologies, compositional flexibility and hydrothermal stability. The possible selection of the microporous structure and of the elements located in framework and extraframework positions enables the design of highly selective catalysts with well-defined active sites of acidic, basic or redox character, opening the path to their application in a wide range of catalytic processes. This versatility and high catalytic efficiency is the key factor enabling their use in the activation and conversion of different alkanes, ranging from methane to long chain n-paraffins. Alkanes are highly stable molecules, but their abundance and low cost have been two main driving forces for the development of processes directed to their upgrading over the last 50 years. However, the availability of advanced characterization tools combined with molecular modelling has enabled a more fundamental approach to the activation and conversion of alkanes, with most of the recent research being focused on the functionalization of methane and light alkanes, where their selective transformation at reasonable conversions remains, even nowadays, an important challenge. In this review, we will cover the use of microporous zeolite-type materials as components of mono- and bifunctional catalysts in the catalytic activation and conversion of C₁⁺ alkanes under non-oxidative or oxidative conditions. In each case, the alkane activation will be approached from a fundamental perspective, with the aim of understanding, at the molecular level, the role of the active sites involved in the activation and transformation of the different molecules and the contribution of shape-selective or confinement effects imposed by the microporous structure.

Recent Advances on Gallium-Modified ZSM-5 for Conversion of Light Hydrocarbons

Light olefins are key components of modern chemical industry and are feedstocks for the production of many commodity chemicals widely used in our daily life. It would be of great economic significance to convert light alkanes, produced during the refining of crude oil or extracted during the processing of natural gas selectively to value-added products, such as light alkenes, aromatic hydrocarbons, etc., through catalytic dehydrogenation. Among various catalysts developed, Ga-modified ZSM-5-based catalysts exhibit superior catalytic performance and stability in dehydrogenation of light alkanes. In this mini review, we summarize the progress on synthesis and application of Ga-modified ZSM-5 as catalysts in dehydrogenation of light alkanes to olefins, and the dehydroaromatization to aromatics in the past two decades, as well as the discussions on in-situ formation and evolution of reactive Ga species as catalytic centers and the reaction mechanisms.

Dehydrogenation of light alkanes to mono-olefins

In the past several decades, light alkane dehydrogenation to mono-olefins, especially propane dehydrogenation to propylene has gained widespread attention and much development in the field of research and commercial application. Under suitable conditions, the supported Pt-Sn and CrO_x catalysts widely used in industry exhibit satisfactory dehydrogenation activity and selectivity. However, the high cost of Pt and the potential environmental problems of CrO_x have driven researchers to improve the coking and sintering resistance of Pt catalysts, and to find new non-noble metal and environment-friendly catalysts. As for the development of the reactor, it should be noted that low operation pressure is beneficial for improving the single-pass conversion, decreasing the amount of unconverted alkane recycled back to the reactor, and reducing the energy consumption of the whole process. Therefore, the research direction of reactor improvement is towards reducing the pressure drop. This review is aimed at introducing the characteristics of the dehydrogenation reaction, the progress made in the development of catalysts and reactors, and a new understanding of reaction mechanism as well as its guiding role in the development of catalyst and reactor.

Isolated boron in zeolite for oxidative dehydrogenation of propane

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.

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

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.

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

Metal and metal oxide catalysts for non-oxidative ethane/propane dehydrogenation are outlined with respect to catalyst synthesis, structure–property relationship and catalytic mechanism.

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

Conversion of propane or isobutane from natural/shale gas into propene or isobutene, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes.