The Nature of the Isolated Gallium Active Center for Propane Dehydrogenation on Ga/SiO2

Metastable gallium hydride mediates propane dehydrogenation on H2 co-feeding

In heterogeneous catalysis, the catalytic dehydrogenation reactions of hydrocarbons often exhibit a negative pressure dependence on hydrogen due to the competitive chemisorption of hydrocarbons and hydrogen. However, some catalysts show a positive pressure dependence for propane dehydrogenation, an important reaction for propylene production. Here we show that the positive activity dependence on H2 partial pressure of gallium oxide-based catalysts arises from metastable hydride mediation. Through in situ spectroscopic, kinetic and computational analyses, we demonstrate that under reaction conditions with H2 co-feeding, the dissociative adsorption of H2 on a partially reduced gallium oxide surface produces H atoms chemically bonded to coordinatively unsaturated Ga atoms. These metastable gallium hydride species promote C-H bond activation while inhibiting deep dehydrogenation. We found that the surface coverage of gallium hydride determines the catalytic performance. Accordingly, benefiting from proper H2 co-feeding, the alumina-supported, trace additive-modified gallium oxide catalyst GaOx-Ir-K/Al2O3 exhibited high activity and selectivity at high propane concentrations.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Supraparticles on beads for supported catalytically active liquid metal solutions - the SCALMS suprabead concept

A novel GaPt-based supported catalytically active liquid metal solution (SCALMS) material is developed by exploiting the suprabead concept: Supraparticles, i.e. micrometer-sized particles composed of nanoparticles assembled by spray-drying, are bonded to millimeter-sized beads. The suprabeads combine macroscale size with catalytic properties of nanoscale GaPt particles entrapped in their silica framework.

Recent Progress of Ga-Based Catalysts for Catalytic Conversion of Light Alkanes

The efficient and clean conversion of light alkanes is a research hotspot in the petrochemical industry, and the development of effective and eco-friendly non-noble metal-based catalysts is a key factor in this field. Among them, gallium is a metal component with good catalytic performance, which has been extensively used for light alkanes conversion. Herein, we critically summarize recent developments in the preparation of gallium-based catalysts and their applications in the catalytic conversion of light alkanes. First, we briefly describe the different routes of light alkane conversion. Following that, the remarkable preparation methods for gallium-based catalysts are discussed, with their state-of-the-art application in light alkane conversion. It should be noticed that the directional preparation of specific Ga species, strengthening metal-support interactions to anchor Ga species, and the application of new kinds of methods for Ga-based catalysts preparation are at the leading edge. Finally, the review provides some current limitations and future perspectives for the development of gallium-based catalysts. Recently, different kinds of Ga species were reported to be active in alkane conversion, and how to separate them with advanced in situ and ex situ characterizations is still a problem that needs to be solved. We believe that this review can provide base information for the preparation and application of Ga-based catalysts in the current stage. With these summarizations, this review can inspire new research directions of gallium-based catalysts in the catalysis conversion of light alkanes with ameliorated performances.

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.

Highly Active and Selective Sites for Propane Dehydrogenation in Zeolite Ga-BEA

A highly selective Ga-modified zeolite BEA for propane dehydrogenation has been synthesized by grafting Ga on Zn-BEA followed by removal of Zn in the presence of H₂. A propene selectivity of 82% at 19% propane conversion illustrates the high selectivity at 813 K. The kinetic model of the catalyzed dehydrogenation including the elementary steps of propane adsorption, first and second C-H bond cleavage, and propene and H₂ desorption demonstrates that the propane dehydrogenation rate is determined by the first C-H bond cleavage at low p_C3H8, while at high p_C3H8, the rate is limited by the desorption of H₂. The active sites have been identified as dehydrated and tetrahedrally coordinated Ga³⁺ in the *BEA lattice. The low selectivity toward aromatics is concluded to be associated with the high Lewis acid strength of lattice Ga³⁺ and the low Brønsted acid strength of the hydrated Ga sites.

Atomic-scale changes of silica-supported catalysts with nanocrystalline or amorphous gallia phases: implications of hydrogen pretreatment on their selectivity for propane dehydrogenation

This work explores how H₂ pretreatment at 550 °C induces structural transformation of two gallia-based propane dehydrogenation (PDH) catalysts, viz. nanocrystalline γ/β-Ga₂O₃ and amorphous Ga₂O₃ (GaO_x) supported on silica (γ-Ga₂O₃/SiO₂ and Ga/SiO₂, respectively) and how it affects their activity, propene selectivity and stability with time on stream (TOS). Ga/SiO₂–H₂ shows poor activity and propene selectivity, no coking and no deactivation with TOS, similar to Ga/SiO₂. In contrast, the high initial activity and propene selectivity of γ-Ga₂O₃/SiO₂–H₂ decline with TOS but to a lesser extent than in calcined γ-Ga₂O₃/SiO₂. In addition, γ-Ga₂O₃/SiO₂–H₂ cokes less than γ-Ga₂O₃/SiO₂. Ga K-edge X-ray absorption spectroscopy suggests an increased disorder of the nanocrystalline γ/β-Ga₂O₃ phases in γ-Ga₂O₃/SiO₂–H₂ and the emergence of additional tetrahedral Ga sites (Ga_IV). Such Ga_IV sites are strong Lewis acid sites (LAS) according to studies using adsorbed pyridine and CO probe molecules, i.e., the abundance of strong LAS is higher in γ-Ga₂O₃/SiO₂–H₂ compared to γ-Ga₂O₃/SiO₂ but lower than in Ga/SiO₂ and Ga/SiO₂–H₂. Dissociation of H₂ on the Ga–O linkages in γ-Ga₂O₃/SiO₂–H₂ yields high-frequency Ga–H bands that are observed in Ga/SiO₂ and Ga/SiO₂–H₂ but not detected in γ-Ga₂O₃/SiO₂. We attribute the increased amount of Ga_IV sites in γ-Ga₂O₃/SiO₂–H₂ mostly to an increased disorder in γ/β-Ga₂O₃. X-ray photoelectron spectroscopy detects the formation of Ga⁺ and Ga⁰ species in both Ga/SiO₂–H₂ and γ-Ga₂O₃/SiO₂–H₂. Therefore, it is likely that a minor amount of Ga_IV sites also forms through the interaction of Ga⁺ (such as Ga₂O) and/or Ga⁰ with silanol groups of SiO₂.

We explore how H₂ pretreatment changes the structure of two gallia-based propane dehydrogenation catalysts, viz. crystalline γ/β-Ga₂O₃ and amorphous Ga₂O₃ supported on silica, and how it affects their activity, selectivity and stability on stream.

High-loading Ga-exchanged MFI zeolites as selective and coke-resistant catalysts for nonoxidative ethane dehydrogenation

A high-loading Ga-exchanged MFI zeolite was developed for efficient ethane dehydrogenation. Its high catalytic performance is ascribed to both the low amount of Brønsted acid sites and the major formation of [GaH2]+ ions among isolated Ga hydrides.

Activity Trends in the Propane Dehydrogenation Reaction Catalyzed by MIII Sites on an Amorphous SiO2 Model: A Theoretical Perspective

One class of particularly active catalysts for the Propane Dehydrogenation (PDH) reaction are well-defined M(III) sites on amorphous SiO2. In the present work, we focus on evaluating the catalytic trends of the PDH for four M(III) single-sites (Cr, Mo, Ga and In) on a realistic amorphous model of SiO2 using density functional theory-based calculations and the energetic span model. We considered a catalytic pathway spanned by three reaction steps taking place on selected MIII–O pair of the SiO2 model: σ-bond metathesis of propane on a MIII–O bond to form M-propyl and O–H group, a β-H transfer step forming M–H and propene, and the H–H coupling step producing H2 and regenerating the initial M–O bond. With the application of the energetic span model, we found that the calculated catalytic activity for Ga and Cr is comparable to the ones reported at the experimental level, enabling us to benchmark the model and the methodology used. Furthermore, results suggest that both In(III) and Mo(III) on SiO2 are potential active catalysts for PDH, provided they can be synthesized and are stable under PDH reaction conditions.

Alkyne Hydrogenation Catalysis across a Family of Ga/In Layered Zintl Phases

Transition-metal-free Zintl-Klemm phases have received little attention as heterogeneous catalysis. Here, we show that a large family of structurally and electronically similar layered Zintl-Klemm phases built from honeycomb layers of group 13 triel (Tr) or group 14 tetrel (Tt) networks separated by electropositive cations (A) and having a stoichiometry of ATr2 or ATrTt (A = Ca, Ba, Y, La, Eu; Tr = Ga, In; Tt = Si, Ge) exhibit varying degrees of activity for the hydrogenation of phenylacetylene to styrene and ethylbenzene at 51 bar H2 and 40-100 °C across a variety of solvents. The most active catalysts contain Ga with, formally, a half-filled pz orbital, and minimal bonding between neighboring Tr2 or TrTt layers. A 13-layer trigonal polytype of CaGaGe (13T-CaGaGe) was the most active, cyclable, and robust catalyst and under modest conditions (1 atm H2, 40 °C) had a surface specific activity (590 h-1) comparable to a commercial Lindlar's catalyst. Additionally, 13T-CaGaGe maintained 100% conversion of phenylacetylene to styrene at 51 bar H2, even after 5 months of air exposure. This work reveals the structural design elements that lead to particularly high catalytic activity in Zintl-Klemm phases, further establishing them as a promising materials platform for hydrogen-based heterogeneous catalysis.

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Well-defined Ga(III) sites on SiO₂ are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO₂ by means of first-principles calculations using realistic amorphous periodic SiO₂ models. We evaluated the three reaction steps in PDH, namely, the C-H activation of propane to form propyl, the β-hydride (β-H) transfer to form propene and a gallium hydride, and the H-H coupling to release H₂, regenerating the initial Ga-O bond and closing the catalytic cycle. Our work shows how Brønsted-Evans-Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO₂ and highlights the role of the strain of the reactive Ga-O pairs on such sites of realistic amorphous SiO₂ models. It also shows how transition-state scaling holds very well for the β-H transfer step. While highly strained sites are very reactive sites for the initial C-H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO₂ for the PDH reaction.

Olefin oligomerization by main group Ga3+ and Zn2+ single site catalysts on SiO2

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni²⁺ and Cr³⁺. Here we report that silica-supported, single site catalysts containing immobilized, main group Zn²⁺ and Ga³⁺ ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni²⁺. The molecular weight distribution of products formed on Zn²⁺ is similar to Ni²⁺, while Ga³⁺ forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.

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.

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.

Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl3

In this study, the mechanism and kinetics of C₃H₈ dehydrogenation and cracking are examined over Ga/H-MFI catalysts prepared via vapor-phase exchange of H-MFI with GaCl₃. The present study demonstrates that [GaH]²⁺ cations are the active centers for C₃H₈ dehydrogenation and cracking, independent of the Ga/Al ratio. For identical reaction conditions, [GaH]²⁺ cations in Ga/H-MFI exhibit a turnover frequency for C₃H₈ dehydrogenation that is 2 orders of magnitude higher and for C₃H₈ cracking, that is 1 order of magnitude higher than the corresponding turnover frequencies over H-MFI. C₃H₈ dehydrogenation and cracking exhibit first-order kinetics with respect to C₃H₈ over H-MFI, but both reactions exhibit first-order kinetics over Ga/H-MFI only at very low C₃H₈ partial pressures and zero-order kinetics at higher C₃H₈ partial pressures. H₂ inhibits both reactions over Ga/H-MFI. It is also found that the ratio of the rate of dehydrogenation to the rate of cracking over Ga/H-MFI is independent of C₃H₈ and H₂ partial pressures but weakly dependent on temperature. Measured activation enthalpies together with theoretical analysis are consistent with a mechanism in which both the dehydrogenation and cracking of C₃H₈ proceed over Ga/H-MFI via reversible, heterolytic dissociation of C₃H₈ at [GaH]²⁺ sites to form [C₃H₇-GaH]⁺-H⁺ cation pairs. The rate-determining step for dehydrogenation is the β-hydride elimination of C₃H₆ and H₂ from the C₃H₇ fragment. The rate-determining step for cracking is C-C bond attack of the same propyl fragment by the proximal Brønsted acid O-H group. H₂ inhibits both dehydrogenation and cracking over Ga/H-MFI via reaction with [GaH]²⁺ cations to form [GaH₂]⁺-H⁺ cation pairs.

Identification of the structure of the Bi promoted Pt non-oxidative coupling of methane catalyst: a nanoscale Pt3Bi intermetallic alloy

Identification of a Pt₃Bi nanoscale, surface intermetallic alloy catalyst for non-oxidative coupling of methane (NOCM).

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.