Dehydrogenation of propane marches on

Propane dehydrogenation has emerged as a promising and booming method for direct propylene production. This preview highlights some recent important works that are moving forward the propane dehydrogenation process toward high performance with excellent propane conversion, propylene selectivity, and durability. Propane dehydrogenation has emerged as a promising and booming method for direct propylene production. This preview highlights some recent important works that are moving forward the propane dehydrogenation process toward high performance with excellent propane conversion, propylene selectivity, and durability. Propylene is the basic raw material for the production of polypropylene, propylene oxide, acrylonitrile, acrolein, acetone, and various high-value chemicals/intermediates, and it is mainly produced from petroleum-derived steam cracking, fluid catalytic cracking (FCC), and the methanol-to-propylene (MTP) process. In recent years, the success in the large-scale exploitation of natural gas and shale gas has motivated the development of industrial techniques toward the catalytically selective direct dehydrogenation of propane to obtain propylene. Currently, approximate 10% of the global propylene is from the direct dehydrogenation of propane technology. Compared with other propylene production methods, propane dehydrogenation possesses many outstanding advantages, such as superhigh propylene selectivity without hybrid products, low cost and plentiful source of feedstock, and abundant profits. Generally, there are mainly three pathways for performing propane dehydrogenation: (1) without oxidant (direct dehydrogenation, denoted as PDH), (2) with oxygen or air (oxidative dehydrogenation, denoted as ODH), and (3) using CO2 as a mild oxidant or for shifting the equilibrium in the pathway outlined in (1) via reaction with H2. To date, several commercial processes for PDH have been developed, including Oleflex, Catofin, FBD-4, PDH, STAR, ADHO, FCDh, and K-PROt. Among them, Oleflex and Catofin are the most commonly applied, wherein Pt-based and CrOx-based catalysts are involved, respectively.1Chen S. Chang X. Sun G. Zhang T. Xu Y. Wang Y. Pei C. Gong J. Propane dehydrogenation: catalyst development, new chemistry, and emerging technologies.Chem. Soc. Rev. 2021; 50: 3315-3354Crossref PubMed Google Scholar In the PDH reaction, it is well accepted that the activation of the first C–H bond of propane is the rate-determining step that determines the catalyst activity. In the past decades, a large number of catalysts including noble metal-based (Pt), non-noble metal oxide-based (CrOx, VOx, GaOx, ZrOx, and ZnOx), and non-oxide-based (carbides and nitrides) catalysts have been developed for PDH reaction because of their affinity for C–H bond of propane and low activity to C–C cleavage.1Chen S. Chang X. Sun G. Zhang T. Xu Y. Wang Y. Pei C. Gong J. Propane dehydrogenation: catalyst development, new chemistry, and emerging technologies.Chem. Soc. Rev. 2021; 50: 3315-3354Crossref PubMed Google Scholar In the commercialized process of dehydrogenation, deactivation of CrOx catalyst occurs due to coking in a short time and thus a frequent regeneration by coke-burning is required, while the deactivation of Pt-based catalyst is due to both coking and sintering, which needs to be regenerated by oxychlorination with air and chloride. In addition, the CrOx catalyst has a serious threat to health, while the Pt-based catalyst has the drawbacks of high cost and involving the ecologically harmful Cl2 or Cl-containing compounds for the re-dispersion of sintered Pt species in spent catalysts. Thus, developing new catalysts with improved coking and sintering resistance as well as eco-friendly and cost-efficient natures is highly desired. To date, many efforts have been devoted to developing new strategies for stabilizing the metal species against sintering and modulating the spatial and electronic properties of metal species against coking. These include the alloying strategy (i.e. introduction of the second metal), the encapsulation strategy, the strong metal-support interaction (SMSI) strategy, or a combination of the above strategies. Plenty of studies have shown that the introduction of a second metal can effectively suppress coke formation and Pt sintering. The introduced metal species alloys with Pt around Pt nanoparticles, anchoring them on the surface of the support and hindering their easy migration. In addition, the second metal can also increase the electron density of the Pt sites in Pt alloys, which can weaken the Pt-propylene binding and therefore restrain the formation of coke precursors. For example, Gong and co-workers prepared a single-site [PtZn4] catalyst by assembling atomically ordered PtZn intermetallic alloys.2Chen S. Zhao Z.-J. Mu R. Chang X. Luo J. Purdy S.C. et al.Propane dehydrogenation on single-site [PtZn4] intermetallic catalysts.Chem. 2021; 7: 387-405Abstract Full Text Full Text PDF Scopus (34) Google Scholar The resultant catalyst enabled more than 95% selectivity of propylene over a temperature range of 520°C to 620°C and no obvious deactivation was observed within the 160-h industrial PDH test. The authors revealed that the geometrically isolated and electron-rich Pt in the [PtZn4] motif readily promoted the desorption of surface-bonded propylene and improved the stability by prohibiting coke side reactions. Additionally, the atomically ordered intermetallic alloy structure particularly assured superior anti-segregation and anti-sintering resistance. The encapsulation of ultrasmall metal species within nanoporous materials has been also regarded as an efficient strategy to improve the activity and stability of catalysts during PDH processes. For instance, Corma and co-workers prepared sub-nanometric Pt-Sn clusters (0.5–0.6 nm) encapsulated in the sinusoidal channels of pure silica MFI zeolite, and the resultant catalyst showed high stability and selectivity for propane dehydrogenation.3Liu L. Lopez-Haro M. Lopes C.W. Li C. Concepcion P. Simonelli L. Calvino J.J. Corma A. Regioselective generation and reactivity control of subnanometric platinum clusters in zeolites for high-temperature catalysis.Nat. Mater. 2019; 18: 866-873Crossref PubMed Scopus (171) Google Scholar,4Liu L. Lopez-Haro M. Lopes C.W. Rojas-Buzo S. Concepcion P. Manzorro R. Simonelli L. Sattler A. Serna P. Calvino J.J. Corma A. Structural modulation and direct measurement of subnanometric bimetallic PtSn clusters confined in zeolites.Nat. Catal. 2020; 3: 628-638Crossref Scopus (77) Google Scholar Yu and co-workers developed a ligand-protected direct hydrogen reduction method for encapsulating sub-nanometer bimetallic Pt-Zn clusters inside the pure silica MFI zeolite, which exhibited an extremely high propylene selectivity of 99.3% with a weight hourly space velocity (WHSV) of 3.6–54 h−1 at 550°C, and propane conversion of 40.4% even after 13,000 min on stream (WHSV = 3.6 h−1) without co-feeding H2.5Sun Q. Wang N. Fan Q. Zeng L. Mayoral A. Miao S. Yang R. Jiang Z. Zhou W. Zhang J. et al.Subnanometer bimetallic platinum–zinc clusters in zeolites for propane dehydrogenation.Angew. Chem. Int. Ed. Engl. 2020; 59: 19450-19459Crossref PubMed Scopus (99) Google Scholar Besides the alloying effect, a large amount of single-site Zn sites in catalysts could also play an important role in the improvement of the propylene selectivity and resistance to coke deposition and metal sintering during PDH reactions. Ryoo and co-workers prepared small Pt and rare-earth alloy particles (∼3 nm) in the pore walls of a mesoporous MFI zeolite with surface framework defects (called “silanol nests”).6Ryoo R. Kim J. Jo C. Han S.W. Kim J.-C. Park H. Han J. Shin H.S. Shin J.W. Rare-earth-platinum alloy nanoparticles in mesoporous zeolite for catalysis.Nature. 2020; 585: 221-224Crossref PubMed Scopus (84) Google Scholar The authors revealed that the silanol nests enable the rare-earth elements to exist as single atomic species with a substantially higher chemical potential compared with that of the bulk oxide, making it possible for them to diffuse onto Pt. Thanks to the alloying effect and SMSI effect of different metal components as well as the confinement effect of zeolites, the obtained catalyst was stable, highly active, and selective for the propane dehydrogenation reaction. Strikingly, unlike the direct PDH process in which the propane conversion is limited by the thermodynamic, the ODH process wherein the H2 produced is oxidized to H2O during dehydrogenation not only transforms the endothermic process into an exothermic process, but also breaks the equilibrium limitation of the reaction and avoids the thermal cracking. As such, the ODH process has attracted more and more attention. However, the ODH process is not commercialized yet because the selectivity to propylene is far lower than that of the PDH process and there exist rich side reactions and serious deep oxidation of propane to COx. Recently, Notestein and co-workers reported the preparation of a single nanoscale tandem catalyst for the ODH process.7Yan H. He K. Samek I.A. Jing D. Nanda M.G. Stair P.C. Notestein J.M. Tandem In2O3-Pt/Al2O3 catalyst for coupling of propane dehydrogenation to selective H2 combustion.Science. 2021; 371: 1257-1260Crossref PubMed Scopus (47) Google Scholar The In2O3 over Pt/Al2O3 catalysts were prepared with an atomic layer deposition technique and the resultant nanostructures kinetically coupled with the domains through surface hydrogen atom transfer, leading to enhanced propane dehydrogenation to propylene over Pt, then selective hydrogen combustion over In2O3, avoiding excessive hydrocarbon combustion. The composite catalyzed the rapid and stable oxidative dehydrogenation of propane at high single-pass conversion exceeding the PDH equilibrium. Later, Xiao and co-workers discovered that the isolated boron in the zeolite framework exhibited high activity and selectivity and superior durability for ODH.8Zhou H. Yi X. Hui Y. Wang L. Chen W. Qin Y. Wang M. Ma J. Chu X. Wang Y. et al.Isolated boron in zeolite for oxidative dehydrogenation of propane.Science. 2021; 372: 76-80Crossref PubMed Scopus (42) Google Scholar It was found that the isolated boron with a –B[OH…O(H)–Si]2 structure in borosilicate zeolite is the active center. These discoveries move us a big step forward toward the potential industrial application of the ODH process. However, a trade-off needs to be made between the PDH and ODH processes because both the product of propylene (C3H6) and hydrogen (H2) in the PDH process are high value-added chemicals, while the hydrogen (H2) is oxidized to cheap water (H2O) in the ODH process, assuming the over-oxidization of hydrocarbon is completely prohibited. In addition to the development of new catalysts, the separation issue in the PDH reaction is also needed to be taken into account in future research. As is well acknowledged, the PDH reaction is highly endothermic and the conversion for a single pass is limited by the thermodynamic equilibrium. As a result, the process consumes a high amount of energy and a large amount of unconverted propane must be recycled back to the reactor after separation, leading to large equipment investment and high energy consumption. In fact, the separation section in a PDH process is even more energy-intensive than the dehydrogenation reaction. Therefore, a highly efficient separation process for propylene and propane is also extremely desired for the PDH process. In addition, there exists a trade-off between the conversion and the selectivity of the PDH process. Appropriate reaction conditions are important not only for the reactor design, but also for the exploration of new catalysts, i.e., the reaction conditions must be considered when conducting a deep investigation on a catalyst possessing good performance. In summary, we consider that the challenges in future propane dehydrogenation application involve the further development of new catalysts with excellent conversion, selectivity, stability, and durability; understanding the reaction mechanism with the assistance of in situ characterizations and theoretical calculations; the rational design of reactors under optimal conditions; and the highly efficient separation of propylene and propane (Figure 1). We acknowledge financial support from the National Natural Science Foundation of China ( 21920102005 , 21835002 , U1967215 , and 21621001 ) and the 111 Project of China ( B17020 ).