Selective Hydrogen Combustion Research Articles

Selective Catalytic Combustion of Hydrogen under Aerobic Conditions on Na2WO4/SiO2

AbstractNa2WO4/SiO2, a material known to catalyze alkane selective oxidation including the oxidative coupling of methane (OCM), is demonstrated to catalyze selective hydrogen combustion (SHC) with >97 % selectivity in mixtures with several hydrocarbons (CH4, C2H6, C2H4, C3H6, C6H6) in the presence of gas‐phase dioxygen at 883–983 K. Hydrogen combustion rates exhibit a near‐first‐order dependence on H2 partial pressure and are zero‐order in H2O and O2 partial pressures. Mechanistic studies at 923 K using isotopically‐labeled reagents demonstrate the kinetic relevance of H−H dissociation and absence of O‐atom recombination. In situ X‐ray diffraction (XRD) and W LIII‐edge X‐ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na2WO4 crystallinity and lack of second‐shell coordination with respect to W6+ cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase at temperatures below the bulk melting point of Na2WO4 (971 K) during SHC catalysis. The Na2WO4/SiO2 SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox‐active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes.

Selective Catalytic Combustion of Hydrogen under Aerobic Conditions on Na2WO4/SiO2.

Na2WO4/SiO2, a material known to catalyze oxidative coupling of methane, is demonstrated to catalyze selective hydrogen combustion (SHC) with >97% selectivity in mixtures with several hydrocarbons (CH4, C2H-6, C2H4, C3H6, C6H6) in the presence of gas-phase dioxygen at 883-983 K. Hydrogen combustion rates exhibit a near-first-order dependence on H2 partial pressure and are zero-order in H2O and O2 partial pressures. Mechanistic studiesusing isotopically-labeled reagents demonstrate the kinetic relevance of H-H dissociation and absence of O-atom recombination. In situ X-ray diffraction and W LIII-edge X-ray absorption spectroscopystudies demonstrate, respectively, a loss of Na2WO4 crystallinity and lack of second-shell coordination with respect to W6+ cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phaseduring SHC catalysis. The Na2WO4/SiO2 SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox-active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes.

Catalytic conversion of methane into benzene over ceria–zirconia‐promoted molybdenum‐supported zeolite for methane dehydroaromatization

The promotional effect of mixed ceria–zirconia oxides (CZO) over the Mo/HZSM‐5 catalyst for methane dehydroaromatization (MDA) reaction was studied. The surface and structural properties of the synthesized catalyst were thoroughly exemplified using various microscopic and spectroscopic tools, and the correlation between catalytic properties and its performance for MDA reaction is discussed. The impregnation of CZO solid solution on Mo/HZSM‐5 was monitored to give an excellent catalytic performance and improved benzene production rate (4.5 μmol/gcat.s) related to the conventionally prepared Mo/HZSM‐5 (3.1 μmol/gcat.s) catalyst. In addition, a significant reduction in coke formation was examined in the CZO‐modified Mo/HZSM‐5 catalyst. The prevailing comprehension for higher catalytic activity could be because of the redox properties of CZO‐deposited Mo/HZSM‐5, which acts as a selective oxygen supplier and performs hydrogen combustion during the reaction, which is indirectly probed by residual gas analysis‐mass spectroscopy, oxygen‐temperature programmed desorption, and hydrogen‐temperature programmed reduction analysis. The selective hydrogen combustion prevents the over‐oxidation of aromatic species formed during the reaction, whereas the generated steam helps in reducing the coke content deposition in the MDA reaction. Thus, the advantage of CZO‐incorporated Mo/HZSM‐5 is manifested as it promotes the reaction equilibrium to shift towards the formation of benzene which is favourable for MDA reaction.

The performances and structure evolution of Pt-based catalysts for selective hydrogen combustion under propene-rich conditions

In the present study, the kinetic behaviour and active sites evolution processes of Pt-based catalysts were investigated. It was found that highly selective hydrogen combustion could be achieved over Sn modified Pt-based catalysts in presence of both propane and propene (over 98%). The stability tests, kinetic study and catalyst characterization revealed that the existence of oxygenated species is the reason for accelerated coking reactions. The formation of graphitized cokes serving as additional unselective active sites and the oxidation of tin in PtSn alloy phases are the primary reasons causing the catalytic selectivity loss during long-run tests under propene-rich condition.

The role of modified manganese perovskite oxide for selective oxidative dehydrogenation of ethane: Not only selective H2 combustion but also ethane activation

To break through the thermodynamic reaction limitation of ethane dehydrogenation to ethylene, the oxidative dehydrogenation in the presence of O2 and chemical looping oxidative dehydrogenation has been investigated with the oxygen storage oxide compound. LaMnO3 perovskite catalysts with large specific surface areas were prepared using citric acid-nitrate combustion. The oxygen vacancies and M4+ contents in the catalysts were improved by doping LaMnO3 as the active sites. The experimental results show that the catalyst not only promotes ethane dehydrogenation through selective hydrogen combustion but also promotes ethane conversion through activation of CH. The catalyst exhibits good stability in oxidative dehydrogenation reaction.

Redox oxide@molten salt as a generalized catalyst design strategy for oxidative dehydrogenation of ethane via selective hydrogen combustion

The current study demonstrates a redox oxide @ molten salt core-shell architecture as a generalized redox catalyst design strategy for chemical looping – oxidative dehydrogenation of ethane. 17 combinations of redox active oxides and molten salts were prepared, evaluated, and characterized. X-ray diffraction indicates that the redox oxides and molten salts are fully compatible, forming separate and stable phases. X-ray photoelectron spectroscopy demonstrates that the molten salts aggregate at the redox oxide surface, forming a core-shell structure to block the non-selective sites responsible for COx formation. Up to ∼74 % single-pass olefin yields were achieved using the proposed redox catalyst design strategy. Statistical analyses of the performance data indicate the potential to achieve up to 86.7 % single-pass yield by simply optimizing the operating conditions using the redox catalysts reported in this study. Meanwhile, the generalizability of the catalyst design strategy offers exciting opportunities to further optimize the composition and performance of the redox catalysts for ethane ODH under a chemical looping scheme with significantly reduced energy consumption and CO2 emissions.

Modeling of propane dehydrogenation combined with chemical looping combustion of hydrogen in a fixed bed reactor

A redox process combining propane dehydrogenation (PDH) with selective hydrogen combustion (SHC) is proposed, modeled, simulated, and optimized. In this process, PDH and SHC catalysts are physically mixed in a fixed-bed reactor, so that the two reactions proceed simultaneously. The redox process can be up to 177.0% higher in propylene yield than the conventional process where only PDH catalysts are packed in the reactor. The reason is twofold: firstly, SHC reaction consumes hydrogen and then shifts PDH reaction equilibrium towards propylene; secondly, SHC reaction provides much heat to drive the highly endothermic PDH reaction. Considering propylene yield, operating time, and other factors, the preferable operating conditions for the redox process are a feed temperature of 973 K, a feed pressure of 0.1 MPa, and a mole ratio of H2 to C3H8 of 0.15, and the optimal mass fraction of PDH catalyst is 0.5. This work should provide some useful guidance for the development of redox processes for propane dehydrogenation.

Na2WO4-tuned manganese ore as a high-effective redox catalyst for selective hydrogen combustion in the presence of methane and benzene

Selective hydrogen combustion in-situ can break the thermodynamic limit of methane dehydroaromatization, which enhances the yield of aromatic hydrocarbons at a given operating temperature. In this study, sodium tungstate (Na2WO4) was loaded on manganese ore by incipient wetness impregnation, which improved the selectivity of hydrogen combustion dramatically. The selectivity of hydrogen combustion (the percentage of lattice oxygen species involved in hydrogen combustion) is 86% and 53% for Na2WO4-tuned and pristine manganese ore at 750 °C, respectively. Introducing Na2WO4 causes a decrease in both the Mn(IV) fraction and oxygen vacancies in the surface layer as well as changes the crystal structure and the morphology of Mn ore. They are attributed to W ions substituting Mn ions, which makes benzene becomes more difficult to be activated compared with hydrogen at the metal oxide/Na2WO4 interface.

Selective hydrogen combustion over Rh-Sn/Al2O3 catalysts during propane dehydrogenation

Selective hydrogen combustion over Rh-Sn/Al2O3 catalysts during propane dehydrogenation

Selective hydrogen combustion as an effective approach for intensified chemical production via the chemical looping strategy

Demand continues to grow rapidly for commodity chemicals, such as light olefins, at a time when the chemicals sector must strive to reduce its energy consumption and greenhouse gas emissions. Process intensification provides a framework for producing the same chemical products with higher efficiency and lower emissions. In recent years, chemical looping has received increasing attention as a strategy for intensified chemical production. For example, the chemical looping oxidative dehydrogenation (CL-ODH) of ethane to ethylene offers the potential for near order-of-magnitude reductions in process energy usage and CO2 emissions in comparison to conventional ethane steam cracking, while chemical looping dehydroaromatization (CL-DHA) of methane offers a pathway for aromatics production at higher yields. In these examples, the CL processes rely on selective hydrogen combustion (SHC) to remove the co-produced H2 gas as water, providing several benefits, including yield increase, autothermal operation, and simplified downstream separation. As a yield-enhancing strategy, SHC is not new. However, the design of redox catalysts for SHC in a chemical looping context has only recently begun to be explored. In this perspective, we summarize previous research on SHC in chemical production schemes, and we attempt to outline priority areas of research in the years to come.

Tandem In2O3-Pt/Al2O3 catalyst for coupling of propane dehydrogenation to selective H2 combustion.

Tandem catalysis couples multiple reactions and promises to improve chemical processing, but precise spatiotemporal control over reactive intermediates remains elusive. We used atomic layer deposition to grow In2O3 over Pt/Al2O3, and this nanostructure kinetically couples the domains through surface hydrogen atom transfer, resulting in propane dehydrogenation (PDH) to propylene by platinum, then selective hydrogen combustion by In2O3, without excessive hydrocarbon combustion. Other nanostructures, including platinum on In2O3 or platinum mixed with In2O3, favor propane combustion because they cannot organize the reactions sequentially. The net effect is rapid and stable oxidative dehydrogenation of propane at high per-pass yields exceeding the PDH equilibrium. Tandem catalysis using this nanoscale overcoating geometry is validated as an opportunity for highly selective catalytic performance in a grand challenge reaction.

Overcoming propylene oxidation

Catalysis Catalysts for the oxidative dehydrogenation of propane to propylene and water become less selective with increasing conversion because propylene itself is more readily oxidized than propane. Yan et al. created nanoscale tandem catalysts by growing an ∼2-nanometer shell of indium oxide, a selective hydrogen combustion catalyst, over a propane dehydrogenation catalyst, platinum nanoparticles supported on alumina spheres (see the Perspective by Pei and Gong). This overcoat exposed the platinum nanoparticles for propane dehydrogenation. Surface hydrogen atoms were oxidized at the indium oxide–platinum interface. This approach boosted yields of propylene to up to 30%. Science , this issue p. [1257][1]; see also p. [1203][2] [1]: /lookup/doi/10.1126/science.abd4441 [2]: /lookup/doi/10.1126/science.abh0424

A tailored multi-functional catalyst for ultra-efficient styrene production under a cyclic redox scheme

Styrene is an important commodity chemical that is highly energy and CO2 intensive to produce. We report a redox oxidative dehydrogenation (redox-ODH) strategy to efficiently produce styrene. Facilitated by a multifunctional (Ca/Mn)1−xO@KFeO2 core-shell redox catalyst which acts as (i) a heterogeneous catalyst, (ii) an oxygen separation agent, and (iii) a selective hydrogen combustion material, redox-ODH auto-thermally converts ethylbenzene to styrene with up to 97% single-pass conversion and >94% selectivity. This represents a 72% yield increase compared to commercial dehydrogenation on a relative basis, leading to 82% energy savings and 79% CO2 emission reduction. The redox catalyst is composed of a catalytically active KFeO2 shell and a (Ca/Mn)1−xO core for reversible lattice oxygen storage and donation. The lattice oxygen donation from (Ca/Mn)1−xO sacrificially stabilizes Fe3+ in the shell to maintain high catalytic activity and coke resistance. From a practical standpoint, the redox catalyst exhibits excellent long-term performance under industrially compatible conditions.

Effect of Sodium Tungstate Promoter on the Reduction Kinetics of CaMn0.9Fe0.1O3 for Chemical Looping – Oxidative Dehydrogenation of Ethane

Reduction kinetics of unpromoted and Na2WO4-promoted CaMn0.9Fe0.1O3 redox catalysts are measured in the context of chemical looping-oxidative dehydrogenation (CL-ODH). CL-ODH is a promising alternative for ethylene production compared to steam cracking, as it reduces the energy demand and increases the single-pass ethane conversion. The ability of a redox catalyst for selective hydrogen combustion (SHC), i.e. selectively oxidizing hydrogen co-product from ethane dehydrogenation, represents an effective strategy for CL-ODH because it can shift the reaction equilibrium and facilitate exothermic overall reaction. In this work, kinetic models and parameters of unpromoted and Na2WO4-promoted, Fe-doped CaMnO3 (CaMn0.9Fe0.1O3) under H2 and C2H4 were investigated. The reduction of unpromoted CaMn0.9Fe0.1O3 follows reaction-order models. C2H4 reduction has a higher energy barrier and a greater dependence on active lattice oxygen concentration, resulting in an order-of-magnitude decrease in the reduction rate. In comparison, the reduction of Na2WO4-promoted CaMn0.9Fe0.1O3 follows an Avrami-Erofe’ev nucleation and nuclei growth model. The addition of Na2WO4 more significantly suppressed C2H4 combustion relative to H2 oxidation. As a result, the reduction rate of Na2WO4-promoted CaMn0.9Fe0.1O3 under H2 was three orders of magnitude greater than that under C2H4, demonstrating its excellent SHC properties. The resulting redox catalyst was shown to be effective for ethane CL-ODH with measured 90.5% ethylene selectivity and 41.6% ethylene yield at 750 °C. The kinetics models and parameters provide useful information for CL-ODH reactor design and further development of the redox catalyst.

Selective hydrogen combustion in the presence of propylene and propane over Pt/A-zeolite catalysts

The behavior of selective hydrogen combustion (SHC) in the presence of propylene and propane changing with reaction temperature in a range of 100–600 °C has been investigated over the Pt catalysts supported on A-zeolites. The effect of Pt loading varying from 0.01 to 2 wt% on the catalytic SHC performance has been studied in the conditions with a feed gas molar composition of C3H8/C3H6/H2/O2 = 4/4/4/2 balanced with N2 and gas hourly space velocity of 15,000 h−1. The results show that for each Pt/3A catalyst having a different Pt loading there is a maximum of H2 conversion by combustion appearing between 300 and 400 °C, while the selectivity to comprehensive H2 conversion can maintain 100% when the temperature lower than 300 °C. Moreover, the Pt/3A catalyst with a Pt loading of 0.5 wt % performs better than the others at the temperatures higher than 300 °C. The maximal H2 combustion achieved over the 0.5 wt% Pt/3A catalyst is as high as 96.6% along with a selectivity of 100% at 300 °C, and a 92.4% H2 combustion with 98.5% selectivity can be obtained even if at 500 °C. The characterization of the catalysts reveals that the distribution of Pt atoms and the number of atoms in Pt clusters may be the key factors for giving rise to the good SHC performance. The influence of three types of A-zeolite supports on the Pt catalyzed SHC process has also been investigated. 3A zeolite is superior to 4A and 5A for supporting 0.5 wt% Pt catalyst in terms of both activity and selectivity. The lower C3H6 conversion on the 0.5 wt% Pt/3A catalyst compared to the 0.5 wt% Pt/5A may be ascribed to the insufficient sites for the C3H6 activation on the surface of Pt/3A due to the limitation of 3A channels inaccessible to C3H6. This contrarily brings about the better SHC performance on the 0.5 wt% Pt/3A catalyst.

Progress in chemical looping-based transformations of small molecules

Chemical looping is a process in which one reaction is split into two or more separate reactions that occur sequentially in one reactor or simultaneously in different reactors. The transport of matter and energy between reactants is achieved via a medium, and the valence of one element of the medium swings between two values. Compared to co-feed mode in conventional reaction technologies, alternative or separate feeding has the advantages of high exergy efficiency, excellent process safety, and flexible operation. It was ever commercialized to produce oxygen, hydrogen, and carbon dioxide before viable processes were developed. Since the 21st century, the application of chemical looping technology in transformations of small molecules like light hydrocarbons and nitrogen has attracted extensive attention all over the world. In this paper, the recent progress in the field is reviewed. The paper mainly focuses on solar CO2 and H2O splitting, methane reforming, selective hydrogen combustion coupled with dehydrogenation, and ammonia synthesis. The main challenges of the chemical looping-based transformations of small molecules are also briefly discussed.

Reduction Kinetics of Perovskite Oxides for Selective Hydrogen Combustion in the Context of Olefin Production

Chemical looping represents a novel approach for generating light olefins in which thermal cracking or catalytic dehydrogenation is coupled with selective hydrogen combustion (SHC) by a metal oxide redox catalyst, which enables autothermal operation, increased per‐pass conversion, and greater‐than‐equilibrium yields. Recent studies indicate that Na2WO4‐promoted perovskite oxides are effective redox catalysts with high olefin selectivity. Herein, kinetic parameters, rates, and reaction models for the reduction of unpromoted and Na2WO4‐promoted CaMnO3 redox catalysts by H2, C2H4, and C2H6, is reported. Reduction rates of CaMnO3 under ethylene and ethane are significantly lower than under H2. Model fitting of reduction kinetics show good agreement with reaction order–controlled models for CaMnO3 reduction and predict greater oxygen site dependence and higher activation energy for CaMnO3 reduction by C2H4 as compared with H2. Avrami–Erofe'ev nucleation and growth models provide the best fit to the reduction of Na2WO4/CaMnO3 in H2 and in C2H4. After Na2WO4 promotion, the reduction rate of CaMnO3 is three orders of magnitude lower in ethylene in comparison to hydrogen, consistent with its superior selectivity to hydrogen combustion. The models developed can be applied toward reactor design and optimization in the context of enhanced olefin production via SHC under a cyclic redox scheme.

Clariant launches CATOFIN 311, a propane dehydrogenation catalyst

A co-fed process combining propane dehydrogenation (PDH) with selective hydrogen combustion (SHC) is proposed, simulated, and optimized. The co-fed process uses adiabatic moving bed radial flow reactors as PDH reactors and adiabatic fixed bed reactors as SHC reactors. This co-fed process is proven to be very advantageous over the Oleflex process in some aspects, showing a propylene yield 6.0-46.1% higher and saving 2.86-7.24 × 106 kJ per ton of propylene under different operating conditions. These advantages are attributed to the utilization of SHC reactors: they consume some hydrogen in the process, which shifts the reaction equilibrium towards propylene; they also provide much heat to drive the highly endothermic PDH reaction. For the co-fed process, a feed temperature of 873 K, a total pressure of 1 bar, and using four PDH reactors are preferable; the optimal oxygen input is significantly affected by feed temperature and number of PDH reactors. Besides, under these preferable conditions, a SHC catalyst should have a minimum hydrogen combustion selectivity of 60.4% to reproduce and exceed the Oleflex efficiency. This work should provide useful guidelines for the development of co-fed processes for propane dehydrogenation.

Process simulation and optimization of propane dehydrogenation combined with selective hydrogen combustion

A co-fed process combining propane dehydrogenation (PDH) with selective hydrogen combustion (SHC) is proposed, simulated, and optimized. The co-fed process uses adiabatic moving bed radial flow reactors as PDH reactors and adiabatic fixed bed reactors as SHC reactors. This co-fed process is proven to be very advantageous over the Oleflex process in some aspects, showing a propylene yield 6.0-46.1% higher and saving 2.86-7.24 × 106 kJ per ton of propylene under different operating conditions. These advantages are attributed to the utilization of SHC reactors: they consume some hydrogen in the process, which shifts the reaction equilibrium towards propylene; they also provide much heat to drive the highly endothermic PDH reaction. For the co-fed process, a feed temperature of 873 K, a total pressure of 1 bar, and using four PDH reactors are preferable; the optimal oxygen input is significantly affected by feed temperature and number of PDH reactors. Besides, under these preferable conditions, a SHC catalyst should have a minimum hydrogen combustion selectivity of 60.4% to reproduce and exceed the Oleflex efficiency. This work should provide useful guidelines for the development of co-fed processes for propane dehydrogenation.

Perovskite oxides for redox oxidative cracking of n-hexane under a cyclic redox scheme

Steam cracking of naphtha is a commercially proven technology for light olefin production and the primary source of ethylene in the Europe and Asia-Pacific markets. However, its significant energy consumption and high CO2 intensity (up to 2 tons CO2/ton C2H4), stemming from endothermic cracking reactions and complex product separations, make this state-of-the-art process increasingly undesirable from an environmental standpoint. We propose a redox oxidative cracking (ROC) approach as an alternative pathway for naphtha conversion. Enabled by perovskite oxide-based redox catalysts, the ROC process converts naphtha (represented by n-hexane) in an auto-thermal, cyclic redox mode. The addition of 20 wt.% Na2WO4 to SrMnO3 and CaMnO3 created highly selective redox catalysts capable of achieving enhanced olefin yields from n-hexane oxy-cracking. This was largely attributed to the redox catalysts’ high activity, selectivity, and stability towards selective hydrogen combustion (SHC) under a redox mode. Na2WO4/CaMnO3 demonstrated significantly higher olefin yield (55–58%) when compared to that from thermal cracking (34%) at 725 °C and 4500 h−1. COx yield as low as 1.7% was achieved along with complete combustion of H2 over 25 cycles. Similarly, Na2WO4/SrMnO3 achieved 41% olefin yield, 0.4% COx yield, and 73% H2 combustion at this condition. Oxygen-temperature-programmed desorption (O2-TPD) indicated that Na2WO4 hindered gaseous oxygen release from CaMnO3. Low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) revealed an outermost perovskite surface layer covered by Na2WO4, which suppressed near-surface Mn and alkaline earth metal cations. The formation of non-selective surface oxygen species was also inhibited. XPS analysis further confirmed that promotion of SrMnO3 with Na2WO4 suppressed surface Sr species by 90%, with a similar effect also observed on CaMnO3. These findings point to the promoting effect of Na2WO4 and the potential of promoted SrMnO3 and CaMnO3 as selective redox catalysts for efficient production of light olefins from naphtha via the ROC process.