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

Advanced metal oxide catalysts for propane dehydrogenation: from design strategy to dehydrogenation performance

Propane dehydrogenation (PDH) technology has been considered an important breakthrough to cope with the ever-increasing demand for propylene. Developing high-performance non-noble metal catalysts has emerged as an effective approach for replacing the currently used commercial Pt- and Cr-based catalysts with high cost and toxicity. Metal oxides have attracted much attention as PDH catalysts due to their high C-H activity, abundant active sites, and desirable dehydrogenation pathways. Regulating the supports and active sites through the rational design of structure and composition provides a new promising platform to improve the dehydrogenation activity and stability of metal oxide catalysts. This review systematically summarizes the catalytic mechanism of PDH. The rational design of metal oxide catalysts with suitable supports and precisely modulated active sites is described with their catalytic performances. In addition, the important roles played by reaction conditions to promote PDH processes are discussed. Furthermore, combined with well-developed advanced characterization methods, the in-depth exploration of the metal oxide-based PDH catalysts is highlighted. Finally, some perspectives for metal oxide-based PDH catalysts are concisely proposed to achieve their future innovations and industrialization.

Facile and Environmentally Friendly Synthesis of Ga2O3/MFI Catalysts for Propane Dehydrogenation

A GaOx-based catalyst is recognized as a promising catalyst for dehydrogenation of light alkanes. Conventionally, impregnation and calcination are necessary processes for the loading of GaOx. However, the incomplete impregnation of gallium salt solution would generate wastewater, and the calcination for the dissociation of gallium salt would release harmful gases, such as SOx, NOx, and HCl. Meanwhile, the high temperature results in an excessive energy cost and undesired GaOx particle aggregation. Here, we report a facile and environmentally friendly method for the synthesis of GaOx-based catalysts. The gallium salt solution was replaced directly with liquid gallium (LG). Through a simple physical mixing method at room temperature, uniform GaOx nanoparticles with diameters of around 3.5 nm were loaded onto the surface of silicalite-1 (S-1). With the optimal GaOx/MFI catalyst, the propane conversion and propylene selectivity reached 22.9 and 90.1%, respectively, in the propane dehydrogenation reaction. The work offers a clean and economical strategy utilizing liquid metal (LM) as an impregnation solution for the preparation of GaOx-based catalysts.

Modifying the Charge-Density of Tetrahedral Cobalt(II) Centers through Carbon-Layer Modulation Promotes C-H Activation in the Propane Dehydrogenation Reaction (PDH)

The electronic structure of active metal centers plays an indispensable role in regulating catalytic reactivity in heterogeneous catalysis, developing other metals as promoters to decorate electronic state is a common strategy, while non-metal component of carbon as electronic additives to regulate d-band center has rarely been studied in thermal-catalysis field. Herein, we report electron-deficient tetrahedral Co(II) (Td-cobalt(II)) centers through carbon-layer modulation for propane dehydrogenation (PDH). It is indicated that bifunctional sites of both Td-cobalt(II) and metallic-cobalt are designed, and the in situ generated carbon through the disproportionation of CO on metallic-cobalt can cover the inactive metallic-cobalt and tailor d-band of active Td-cobalt(II) simultaneously. More importantly, the pre-deposited carbon-layer is proposed to decrease electron density of Td-cobalt(II) and make d-band center closer to Fermi level, consequently promotes C-H activation in PDH reaction. This study provides new perspective for the utilization of inactive carbon as electronic promoters and unlocks new opportunity to fabricate efficient PDH and other heterogeneous catalysts.

Well-Defined Supported ZnOx Species: Synthesis, Structure, and Catalytic Performance in Nonoxidative Dehydrogenation of C3-C4 Alkanes

ConspectusZinc oxide (ZnO) is a multipurpose material and finds its applications in various fields such as rubber manufacturing, medicine, food additives, electronics, etc. It has also been intensively studied in photocatalysis due to its wide band gap and environmental compatibility. Recently, heterogeneous catalysts with supported ZnOx species have attracted more and more attention for the dehydrogenation of propane (PDH) and isobutane (iBDH) present in shale/natural gas. The olefins formed in these reactions are key building blocks of the chemical industry. These reactions are also of academic importance for understanding the fundamentals of the selective activation of C-H bonds. Differently structured ZnOx species supported on zeolites, SiO2, and Al2O3 have been reported to be active for nonoxidative dehydrogenation reactions. However, the structure-activity-selectivity relationships for these catalysts remain elusive. The main difficulty stems from the preparation of catalysts containing only one kind of well-defined ZnOx species.In this Account, we describe the studies on PDH and iBDH over differently structured ZnOx species and highlight our approaches to develop catalysts with controllable ZnOx speciation relevant to their performance. Several methods, including (i) the in situ reaction of gas-phase metallic Zn atoms with OH groups on the surface of supports, (ii) one-pot hydrothermal synthesis, and (iii) impregnation/anchoring methods, have been developed/used for the tailored preparation of supported ZnOx species. The first method allows precise control of the molecular structure of ZnOx through the nature of the defective OH groups on the supports. Using this method, a series of ZnOx species ranging from isolated, binuclear to nanosized ZnOx have been successfully generated on different SiO2-based or ZrO2-based supports as demonstrated by complementary ex/in situ characterization techniques. Based on kinetic studies and detailed characterization results, the intrinsic activity (Zn-related turnover frequency) of ZnOx was found to depend on its speciation. It increases with an increasing number of Zn atoms in a ZnmOn cluster from 1 to a few atoms (less than 10) and then decreases strongly for ZnOx nanoparticles. The latter promote the formation of undesired C1-C2 hydrocarbons and coke, resulting in lower propene selectivity in comparison with the catalysts containing only ZnOx species ranging from isolated to subnanometer ZnmOn clusters. In addition, the strategy for improving the thermal stability of ZnOx species and the consequences of mass-transport limitations for DH reactions were also elucidated. The results obtained allowed us to establish the fundamentals for the targeted preparation of well-structured ZnOx species and the relationships between their structures and the DH performance. This knowledge may inspire further studies in the field of C-H bond activation and other reactions, in which ZnOx species act as catalytically active sites or promoters, such as the dehydroaromatization of light alkanes and the hydrogenation of CO2 to methanol.

Acidity of Isomorphic Substituted Zeolites with B, Al and Ga Revisited

Isomorphic substitution of zeolites with B, Al and Ga is a widely used approach in catalysis. The experimentally reported trend of their acidities decreases in the order: Al>Ga>B. However, a consistent explanation is still lacking in the literature. To bring more understanding of this trend, density functional theory computations were conducted on several model systems. First, the acidity of small clusters with two (2T) and five (5T) tetrahedral sites was analyzed. These systems were then projected onto three large void structures: H-[A]-BEA (52T), H-[A]-FAU (84T) and H-[A]-MOR (112T) with A=B, Al, Ga. Our electron density and Interacting Quantum Atom analyses show that the acidity of Al zeolites originates from the much stronger O-Al bond, which is dominated by the electrostatic attraction. The bridging hydroxyl therefore donates more charge density to the metal, the proton becomes more positive and consequently more acidic. Ga zeolites are more acidic than B zeolites due to the greater covalent nature on the O-Ga bond. The resulting acidity, as seen by ammonia, depends on both the acidic oxygen and the charge distribution of the surrounding oxygens exerted by the substituents.

Reduction of Cofed Carbon Dioxide Modifies the Local Coordination Environment of Zeolite-Supported, Atomically Dispersed Chromium to Promote Ethane Dehydrogenation

The reduction of CO2 is known to promote increased alkene yields from alkane dehydrogenations when the reactions are cocatalyzed. The mechanism of this promotion is not understood in the context of catalyst active-site environments because CO2 is amphoteric, and even general aspects of the chemistry, including the significance of competing side reactions, differ significantly across catalysts. Atomically dispersed chromium cations stabilized in highly siliceous MFI zeolite are shown here to enable the study of the role of parallel CO2 reduction during ethylene-selective ethane dehydrogenation. Based on infrared spectroscopy and X-ray absorption spectroscopy data interpreted through calculations using density functional theory (DFT), the synthesized catalyst contains atomically dispersed Cr cations stabilized by silanol nests in micropores. Reactor studies show that cofeeding CO2 increases stable ethylene-selective ethane dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra indicate that during reaction at 650 °C the Cr cations maintain a nominal 2+ charge and a total Cr-O coordination number of approximately 2. However, CO2 reduction induces a change, correlated with the CO2 partial pressure, in the population of two distinct Cr-O scattering paths. This indicates that the promotional effect of parallel CO2 reduction can be attributed to a subtle change in Cr-O bond lengths in the local coordination environment of the active site. These insights are made possible by simultaneously fitting multiple EXAFS spectra recorded in different reaction conditions; this novel procedure is expected to be generally applicable for interpreting operando catalysis EXAFS data.

Borosilicate-Based Framework: Synthesis, Single-Crystal Structure Study, and Physical Properties

Herein, we report the synthesis, crystal structure, and optical properties of a metal-free three-dimensional (3D) inorganic covalent framework ((H2en)[Si(B4O9)], named CityU-11, where H2en is the abbreviation for ethanediamine). With the assistance of a tiny amount of F- ions and the selection of SiO2 as Si sources, single crystals of CityU-11 can be successfully prepared under solvothermal conditions. The precise structure information on CityU-11 has been disclosed through both single-crystal X-ray diffraction (SCXRD) and low-dose high-resolution transmission electron microscopy (LD-HRTEM). The SCXRD results showed that CityU-11 crystallized in the noncentrosymmetric space group of Pnn2, while LD-HRTEM suggested that CityU-11 possessed almost the same interplanar distances of 0.6 nm for both (200) and (020) crystal planes, which finely matched with the double peaks of 2θ = 15° in the pattern of its powder X-ray diffraction (PXRD). CityU-11 also displayed an interesting optical property with a moderate birefringence of 0.0258@550 nm.

Mechanism research reveals the role of Fen (n = 2-5) supported C2N as single-cluster catalysts (SCCs) for the non-oxidative propane dehydrogenation in the optimization of catalytic performance

Single cluster catalysts show excellent potential for propane dehydrogenation, compensating for the limited catalytic performance of single-atom catalysts in reactions involving multiple reaction steps and intermediates. Herein, density functional theory is used to investigate the catalytic activity and mechanism for non-oxidized propane dehydrogenation on Fen-C2N (n = 2-5). Firstly, the stability of Fen-C2N (n = 2-5) is evaluated by comparing the mean values of binding energy and cohesive energy. The results show that Fen-C2N (n = 2-4) can exist stably, which is also verified by the molecular dynamics calculation at 873 K. Band structure analysis shows that the screened catalysts have metal properties, which are conducive to charge transfer. Fukui function analysis is used to predict the optimal adsorption site. The electronic properties of propane and propylene adsorbed on catalysts are further studied by the partial density of states and deformation charge density. The activation barrier (Ea) and reaction energy (ΔE) of the main reaction steps are evaluated. The results show that Fe2-C2N (Ea = 0.97 eV, ΔE= 0.22 eV) has the best catalytic activity. The Ea for further propylene dehydrogenation is also used to evaluate the yield of propylene. Compared with Fe-C2N, Fe2-C2N can regulate the adsorption strength of propane and propylene, showing better catalytic ability and higher selectivity for propylene. The above research provides ideas for the design of new catalysts with high selectivity and activity for non-oxidative propane dehydrogenation.

Site Diversity and Mechanism of Metal-Exchanged Zeolite Catalyzed Non-Oxidative Propane Dehydrogenation

Metal-exchanged zeolites are well-known propane dehydrogenation (PDH) catalysts; however, the structure of the active species remains unresolved. In this review, existing PDH catalysts are first surveyed, and then the current understanding of metal-exchanged zeolite catalysts is described in detail. The case of Ga/H-ZSM-5 is employed to showcase that advances in the understanding of structure-activity relations are often accompanied by technological or conceptional breakthroughs. The understanding of Ga speciation at PDH conditions has evolved owing to the advent of in situ/operando characterizations and to the realization that the local coordination environment of Ga species afforded by the zeolite support has a decisive impact on the active site structure. In situ/operando quantitative characterization of catalysts, rigorous determination of intrinsic reaction rates, and predictive computational modeling are all significant in identifying the most active structure in these complex systems. The reaction mechanism could be both intricately related to and nearly independent of the details of the assumed active structure, as in the two main proposed PDH mechanisms on Ga/H-ZSM-5, that is, the carbenium mechanism and the alkyl mechanism. Perspectives on potential approaches to further elucidate the active structure of metal-exchanged zeolite catalysts and reaction mechanisms are discussed in the final section.

CO2-Assisted Dehydrogenation of Propane to Propene over Zn-BEA Zeolites: Impact of Acid–Base Characteristics on Catalytic Performance

Research results about the influence of BEA zeolite preliminary dealumination on the acid–base characteristics and catalytic performance of 1% Zn-BEA compositions in propane dehydrogenation with CO2 are presented. The catalyst samples, prepared through a two-step post-synthesis procedure involving partial or complete dealumination of the BEA specimen followed by the introduction of Zn2+ cations into the T-positions of the zeolite framework, were characterized using XRD, XPS, MAS NMR, SEM/EDS, low-temperature N2 ad/desorption, C3H8/C3H6 (CO2, NH3)-TPD, TPO-O2, and FTIR-Py techniques. Full dealumination resulted in the development of a mesoporous structure and specific surface area (BET) with a twofold decrease in the total acidity and basicity of Zn-BEA, and the formation of Lewis acid sites and basic sites of predominantly medium strength, as well as the removal of Brønsted acid sites from the surface. In the presence of the ZnSiBEA catalyst, which had the lowest total acidity and basicity, the obtained selectivity of 86–94% and yield of 30–33% for propene (at 923 K) exceeded the values for ZnAlSiBEA and ZnAlBEA. The results of propane dehydrogenation with/without carbon dioxide showed the advantages of producing the target olefin in the presence of CO2 using Zn-BEA catalysts.