Catalysts For Dehydrogenation Of Alkanes Research Articles

Breaking the stability limit of zeolite-based alkane dehydrogenation catalysts

Metal catalysts confined inside zeolite micropores or crystals are widely applied in the chemical industry because of their distinctive catalytic functions, especially with regard to the conversion of small and stable alkane molecules [1,2]. Silicalite-1 (S-1), a pure silica zeolite with an MFI topology, has a robust framework capable of withstanding harsh thermal and chemical conditions. Recently, S-1-based catalysts have demonstrated significant advantages in the construction and stabilization of various active metal centers and have attracted considerable attention for their potential in alkane dehydrogenation reactions. The absence of acidic sites prevents various acid-catalyzed side reactions, rendering S-1 a promising candidate for high-temperature reactions such as non-oxidative propane dehydrogenation (PDH). Sun et al. revealed that encapsulating subnanometer bimetallic PtZn clusters in S-1 resulted in a high propylene selectivity of 99%, with no catalyst deactivation even after 200h in the PDH reaction [3]. However, sustaining high stability at high conversion rates is still challenging for PDH reactions, and commercial processes are still based on Pt- and Cr-based catalysts, which need frequent regeneration.

Unraveling the function of monolayer shell structure over Pt-based alloy catalysts in tuning ethane dehydrogenation reactivity and coking resistance

Ethane direct dehydrogenation (EDH) operated at high temperature easily leads to coke formation. The new core–shell catalysts with monolayer shell exhibited better catalytic performance, and the function of monolayer shell structure is of great importance. This work fully investigated catalytic performance of EDH over six types of core–shell Pt3M@Pt and Pt@Pt3M (M = Fe, Co, and Ni) catalysts with Pt3M and Pt monolayer shell under the experimental conditions. Here, density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations were employed. The results show that the metal M over Pt3M@Pt surface decreases coordination number of Pt-Pt and restricts deep dehydrogenation reactions. The Pt3M@Pt catalysts with d-band center close to Fermi level exhibit higher C2H4(g) formation activity than Pt@Pt3M catalysts at 873.15 K. H2(g) co-feeding significantly decreases the coverage of coking C*+CC* species. The screened Pt3Ni@Pt catalyst presents the highest C2H4(g) formation activity and 100 % selectivity at the H2(g) partial pressure of 0.1 bar and reaction temperature 873.15 K, which is superior to previously reported Pt, Pt3Sn, 1Pt3Sn@4Pt, Pd1-NDG, Pt2/Al2O3 and Rh/ZrO2 catalysts. H* adsorption energy is proposed as a descriptor to quickly evaluate C2H4(g) formation activity. This work provides a useful strategy for designing alkane dehydrogenation catalysts by tuning monolayer shell structure and coordination environment of active site.

Stable anchoring of single rhodium atoms by indium in zeolite alkane dehydrogenation catalysts.

Maintaining the stability of single-atom catalysts in high-temperature reactions remains extremely challenging because of the migration of metal atoms under these conditions. We present a strategy for designing stable single-atom catalysts by harnessing a second metal to anchor the noble metal atom inside zeolite channels. A single-atom rhodium-indium cluster catalyst is formed inside zeolite silicalite-1 through in situ migration of indium during alkane dehydrogenation. This catalyst demonstrates exceptional stability against coke formation for 5500 hours in continuous pure propane dehydrogenation with 99% propylene selectivity and propane conversions close to the thermodynamic equilibrium value at 550°C. Our catalyst also operated stably at 600°C, offering propane conversions of >60% and propylene selectivity of >95%.

Pt confined in Sn-ECNU-46 zeolite for efficient alkane dehydrogenation

Highly dispersed and stable Pt-based catalysts play a crucial role in constructing efficient catalytic systems for alkane dehydrogenation. In this study, a novel bimetallic Pt–Sn catalyst confined in extra-large-pore ECNU-46 zeolite (denoted as Pt/Sn-ECNU-46) is prepared by post-treatment. The open-site framework Sn species ((SiO)3Sn–OH) serve as anchors to interact with Pt species, favoring the high dispersion of Pt. On the other hand, the framework Sn species act as the second metal to regulate the geometrical and electronic environment of Pt species, thus suppressing their accumulation. Pt/Sn-ECNU-46 achieves a good performance in propane dehydrogenation reaction with high initial propane conversion (46%) and propylene selectivity (> 99%) as well as regeneration ability. In addition, Pt/Sn-ECNU-46 is also active in the dehydrogenation of n-hexane. This study explores the application of extra-large-pore zeolite as support in constructing metal-confined catalysts for alkane dehydrogenation.

High-Density Coordinatively Unsaturated Zn Catalyst for Efficient Alkane Dehydrogenation.

The exploration of non-noble metal catalysts for alkane dehydrogenation and their catalytic mechanisms is the priority in catalysis research. Here, we report a high-density coordinatively unsaturated Zn cation (Zncus) catalyst for the direct dehydrogenation (DDH) of ethylbenzene (EB) to styrene (ST). The catalyst demonstrated good catalytic performance (∼40% initial EB conversion rate and >98% ST selectivity) and excellent regeneration ability in the reaction, which is attributed to the high-density (HD) distribution and high-stability structure of Zncus active sites on the surface of zinc silicate (HD-Zncus@ZS). Density functional theory (DFT) calculations further illustrated the reaction pathway and intermediates, supporting that the Zncus sites can efficiently activate the C-H bond of ethyl on ethylbenzene. Developing the high-density Zncus catalyst and exploring the catalytic mechanism laid a good foundation for designing practical non-noble metal catalysts.

Catalytic membrane reactors for alkane dehydrogenation applications: an integration of catalysis and separation process

Abstract Catalytic dehydrogenation of saturated hydrocarbons to corresponding alkenes by the release of the stoichiometric amount of hydrogen is the paramount solution for safe storage of hydrogen. The utilization of a catalytic membrane reactor for this process enhances the reaction yield beyond thermodynamic equilibrium by selectively and simultaneously removing the produced H2 during the reaction. To this end, the present review is focused on the integration of H2 permeable membranes with the catalysts for dehydrogenation of lighter alkanes for coproduction of olefins and high-purity hydrogen in a single step. Besides, this review also covers dehydrogenation of liquid organic hydrogen carriers for safe storage of hydrogen. Herein, different types of H2 perm-selective membranes used for the dehydrogenation reaction are highlighted and the effect of hydrocarbon on H2 permeation through these membranes are discussed in detail. Furthermore, the simulation studies along with the experimental investigation performed on the membrane reactors for dehydrogenation of linear and cyclic alkanes are critically reviewed to find the coherence between simulation and experimental findings. Systematic discussion is done on the different types of alkane dehydrogenation reactions and the parameters affecting the reaction performance. Finally, directions are provided to prepare a cheaper and large industrial scale membrane reactor for dehydrogenation reaction. The concept of coupling an exothermic reaction with the endothermic dehydrogenation reaction is provided as a future direction study to enhance the overall yield and energy efficiency of the integrated membrane reactor.

IrGa Bimetallic Catalyst with Atomical Metal Dispersion for Propane Dehydrogenation with High Stability

AbstractIndustrial catalysts for alkane dehydrogenation to olefins suffer from fast deactivation due to the coking and sintering of active metal under harsh reaction conditions. Herein, iridium, a noble metal rarely applied in C−H activation, has been employed to construct an ultra‐stable IrGa/Al2O3 catalyst with atomical metal dispersion for non‐oxidative propane dehydrogenation (PDH). A synergetic effect between metallic Ir and GaOx is discovered which contributes to both high PDH activity and propylene selectivity. Notably, the Ir0.5Ga2/Al2O3 with 0.5 wt % of Ir and 2.0 wt % of Ga exhibited a propylene selectivity of 98 % at a 19 % propane conversion at 550 °C and 86 % at a 35 % propane conversion at 600 °C. A stable PDH performance of 100 h was observed after a deactivation process at the initial reaction stage. The catalytic performance of the IrGa/Al2O3 is comparable to that of Pt‐based catalysts, endowing the Ir‐based catalyst with great potential in dehydrogenation reactions.

Supported boron-based catalysts for oxidative dehydrogenation of light alkanes to olefins

Boron-based catalysts have shown great fundamental and practical value in oxidative dehydrogenation (ODH) of light alkanes to olefins.

A New Paradigm in Pincer Iridium Chemistry: PCN Complexes for (De)Hydrogenation Catalysis and Beyond.

The discovery and development of organometallic catalysts is of paramount importance in modern organic synthesis, among which the ligand scaffolds play a crucial role in controlling the activity and selectivity. Over the past several decades, d8 transition-metal complexes of pincer ligands have been developed extensively thanks to their easy structural modification, versatile reactivities, and high stability. One paradigm is the bis(phosphine)-based pincer iridium complexes PCP-Ir, which are highly active for alkane dehydrogenation, partly due to their high thermostability. However, except for alkane dehydrogenation and related transformations, few applications of pincer iridium catalysis have been seen in organic synthesis. This mainly arises from the low functional-group compatibility and poor substrate scope and the limited catalytic chemistry that invariably involves Ir(I/III) redox processes initiated by oxidative addition of substrates to 14-electron (PCP)Ir fragments (the proposed catalytically active intermediates). In this Account, we describe our endeavor on the development of a new family of PCN-Ir complexes with initial intention on creating more efficient alkane dehydrogenation catalysts. The replacement of a soft, σ-donor phosphine arm in the PCP ligands by a harder, π-acceptor N-heteroarene (pyridine or oxazoline) not only provides an additional platform to modify the structural properties but also offers new modes of bond activation and novel reactivities and catalysis. One uniqueness of the PCN-Ir system lies in the formation, via ortho-C(sp2)-H cyclometalation of the pyridine unit in the PCNPy ligand, of the neutral monohydride (PCC)IrIIIHL (L = neutral ligand), which catalyzes positional and stereoselective 1-alkene-to-(E)-2-alkene isomerization. Moreover, the PCN-Ir catalysts effect ethanol dehydrogenation without decarbonylation, allowing for transfer hydrogenation of unactivated alkenes and trans-selective semihydrogenation of internal alkynes with user-friendly ethanol as the H-donor. Another feature originates from the ability of the pentacoordinate hydrido chloride complex (PCN)IrIIIHCl to undergo reversible solvent-coordination-induced-ionization (SCII), furnishing a cationic monohydride [(PCN)IrIIIH(Sol)]+Cl- bearing an uncoordinated Cl anion that effects selective hydrometalation of internal alkynes over the corresponding (Z)-alkenes; the resulting (PCN)IrIII(vinyl)Cl complex undergoes amine-assisted formal alcoholysis involving the protonation of the Cl anion by the activated IrIII-bound EtOH, again via the SCII pathway. Together these elementary reactions lay the foundation for cis-selective semihydrogenation of alkynes with EtOH. Further, the design of the oxazoline-containing chiral complexes (PCNOxa)IrIIIHCl enables asymmetric transfer hydrogenation of alkenes/ketones with ethanol. The efficient catalytic α-alkylation of unactivated esters/amides with alcohols is another case showing the benefit that the PCN-Ir catalyst can offer. These examples illustrate the profound impact of the pincer ligands on the reactivities and catalysis. We hope this Account will provide an in-depth view into the fundamentals of pincer iridium chemistry and ultimately broaden its applications in organic synthesis.

Electronic interaction between Cr3+ ions in chromia-alumina catalysts for light alkane dehydrogenation

In this work chromia-alumina catalysts have been studied by the electron paramagnetic resonance (EPR) spectroscopy in the temperature range -120–200 °C. It was observed, that the activity of the catalysts in the dehydrogenation of n-butane and i-butane correlates with the amount of strongly interacting Cr3+ ions of the amorphous chromia phase. A disproportionate increase in the antiferromagnetic coupling energy ΔE between Cr3+ ions was shown in the concentration range 4.7–9.8%wt.Cr (from 210 to 420 cm−1) and 9.8–13.3%wt.Cr (from 420 to 450 cm−1). It was established, that an increase in the exchange interaction between Cr3+ ions is more favourable for the dehydrogenation of i-butane as compared to n-butane.

Recent progress in Cr-based catalysts for oxidative dehydrogenation of light alkanes by employing CO2 as a soft oxidant

Abstract CO2 can be used as a soft oxidant for oxidative dehydrogenation of light alkanes (CO2-ODH), which is beneficial to realize the reuse of CO2 and meet the demand for olefins. The core of this reaction is the catalyst. Cr-based catalysts have attracted much attention for their excellent catalytic performance in CO2-ODH reactions due to their various oxidation states and local electronic structures. In this paper, the synthesis and modification methods of Cr-based catalysts for CO2-ODH are reviewed. The structure–activity relationship and reaction mechanism are also summarized. Moreover, the reasons for the deactivation of Cr-based catalysts are analysed and the main challenges faced by Cr-based catalysts in the CO2-ODH process, as well as the future development trend and prospect, are discussed.

Revealing the surface atomic arrangement of noble metal alkane dehydrogenation catalysts by a stepwise reduction-oxidation approach

Revealing the surface atomic arrangement of noble metal alkane dehydrogenation catalysts by a stepwise reduction-oxidation approach

Rhodium atoms for alkane dehydrogenation

Catalysis Nanoparticles of rhodium dispersed on metal oxides are generally poor catalysts for alkane dehydrogenation because the reactants bind too strongly to the metal. Hannagan et al. performed first-principle calculations indicating that single rhodium atoms in a copper surface should be stable and selective for conversion of propane to propene and hydrogen. Model studies of single rhodium atoms embedded in a copper (111) surface revealed a very high selectivity to propene and high resistance to the formation of surface carbon that would deactivate the catalyst. Science , abg8389, this issue p. [1444][1] [1]: /lookup/doi/10.1126/science.abg8389

First-principles design of a single-atom–alloy propane dehydrogenation catalyst

Rhodium atoms for alkane dehydrogenation Nanoparticles of rhodium dispersed on metal oxides are generally poor catalysts for alkane dehydrogenation because the reactants bind too strongly to the metal. Hannagan et al. performed first-principle calculations indicating that single rhodium atoms in a copper surface should be stable and selective for conversion of propane to propene and hydrogen. Model studies of single rhodium atoms embedded in a copper (111) surface revealed a very high selectivity to propene and high resistance to the formation of surface carbon that would deactivate the catalyst. Science , abg8389, this issue p. 1444

Stabilizing effect of α-Cr2O3 on highly active phases and catalytic performance of a chromium alumina catalyst in the process of isobutane dehydrogenation

• Stabilization of active phases of amorphous Cr 2 O 3 and polychromates by α-Cr 2 O 3 • Cr 2 O 3 -phases prevent agglomeration of active phase particles • Cr 2 O 3 -phases contribute of high catalytic performance The demand for C 3 -C 5 olefins is constantly growing, that is why it is important to improve the performance of catalysts for dehydrogenation of light alkanes. The study regards the influence of the supports porous system, the contribution of α-Cr 2 O 3 particles to the state of the active phase and the catalytic performance of the chromia-alumina catalysts in the isobutane dehydrogenation reaction. The catalysts were synthesized by impregnating the support with chromic acid. The supports and the catalysts were studied by the following techniques: low-temperature nitrogen adsorption, temperature-programmed desorption and reduction; UV-Vis- and Raman-spectroscopy, X-ray phase and X-ray fluorescence analyses. It was found that crystals of α-Cr 2 O 3 are formed on a support with 56 m 2 /g specific surface area and 7.1% (m/m) chromium content, that contributes to stabilization of the particles of highly active phases of amorphous Cr 2 O 3 and polychromates and catalytic performance. Meanwhile, no α-Cr 2 O 3 particles are formed on a support with a 103 m 2 /g specific surface area and 7.4% (m/m) chromium content. Therefore, in the course of 54 regeneration reactions cycles, the rates of isobutylene formation and isobutylene selectivity are significantly reduced due to agglomeration of amorphous Cr 2 O 3 particles, formation of di-, tri- chromates, and migration of part of chromium into the support structure.

Metal-based catalysts for the non-oxidative dehydrogenation of light alkanes to light olefins

This review provides an overview of metal-based catalysts, including Pt-, Pd-, Rh- and Ni-based bimetallic catalysts for non-oxidative dehydrogenation of light alkanes to olefins.

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.

Heterogeneous alkane dehydrogenation catalysts investigated via a surface organometallic chemistry approach.

The selective conversion of light alkanes (C2–C6 saturated hydrocarbons) to the corresponding alkene is an appealing strategy for the petrochemical industry in view of the availability of these feedstocks, in particular with the emergence of Shale gas. Here, we present a review of model dehydrogenation catalysts of light alkanes prepared via surface organometallic chemistry (SOMC). A specific focus of this review is the use of molecular strategies for the deconvolution of complex heterogeneous materials that are proficient in enabling dehydrogenation reactions. The challenges associated with the proposed reactions are highlighted, as well as overriding themes that can be ascertained from the systematic study of these challenging reactions using model SOMC catalysts.

Beyond the Reverse Horiuti–Polanyi Mechanism in Propane Dehydrogenation over Pt Catalysts

The catalytic dehydrogenation of light alkanes over Pt catalysts is generally accepted to follow a reverse Horiuti–Polanyi mechanism. Using a microkinetic analysis in combination with results from density functional theory calculations, we show that although propane dehydrogenation (PDH) occurs by two successive dehydrogenation steps on terraces, an unexpected non-reverse Horiuti–Polanyi mechanism accounts for more than half the propylene production at the under-coordinated active sites that dominate the kinetics of PDH. The main reaction is composed of three dehydrogenation steps that have two β-H atoms and one α-H atom removed from propane, followed by the hydrogenation of CH3CCH2; starting from this species, the formation of propylene and byproducts proceed by way of two parallel competing reactions. The proposed mechanism has been verified by exploring several key and general aspects of the kinetic behavior observed in the dehydrogenation of light alkanes, and it is found that only when adsorbate–adsorbate interactions are taken into consideration can the experimentally determined kinetics be properly reproduced. Increasing the H2 partial pressure from low values favors an increase in the coverage of free sites due to the gasification of adsorbed coke precursors, which in turn gives rise to lowered energy barriers for C–H bond breaking, thereby achieving an increased propane consumption rate. As the H2/C3H8 ratio increases, the rate of propylene production first goes up and then declines, and a maximum is observed at a H2/C3H8 ratio of 1.33, which occurs when the negative effect of the increased free 4-fold hollow sites that bring about deep dehydrogenation begins to dominate the positive effect of the increased free step sites that are responsible for activating propane. The mechanism formulated here proves to be valid even if the temperature, pressure, or the H2/C3H8 ratio is varied and hence provides a foundation for the rational design of metal and alloy catalysts for light alkane dehydrogenation.

Laser vaporized CrOx/Al2O3 nanopowders as a catalyst for isobutane dehydrogenation

Laser vaporized xCr/Al2O3 nanopowders with the particle size of ca. 15 nm were studied by XRF, EDX, HRTEM, XRD, DD, BET, TGA, UV–Vis DRS, Raman and PL spectroscopy methods as a catalytic system in dehydrogenation of isobutane. The nanopowders were found to contain not only the Cr6+ ions located on the surface of nanoparticles, but also two different types of Cr3+ sites as components of the γ-Al2O3 solid solution. One type consists of the bulk Cr3+b sites in a strong crystal field. Another type is represented by the Cr3+s sites residing near the surface of xCr/Al2O3 nanoparticles. Such sites are in a weak crystal field. It was shown that catalytic activity of the nanostructured xCr/Al2O3 system is provided by the high content of surface Cr3+s sites. The results obtained make it possible to propose the nanopowders obtained by laser synthesis as a promising catalytic system that can be used in the development of highly active catalysts for dehydrogenation of alkanes with the decreased chromium content.