Non-oxidative Dehydrogenation Research Articles

Non-Oxidative Propane Dehydrogenation by a Well-Defined Ga Catalyst Prepared by Surface Organometallic Chemistry

Non-Oxidative Propane Dehydrogenation by a Well-Defined Ga Catalyst Prepared by Surface Organometallic Chemistry

Oxidative dehydrogenation of ethane over boron-containing chabazite

We report that boron-containing zeolite chabazite (B-CHA) catalyzes the oxidative dehydrogenation of ethane (ODHE) with high selectivity (>70 %) and excellent stability in the temperature range of 500–600 °C. ODHE rates, in fact, increase over time on stream. Ethane consumption rate has an apparent activation energy of 126 kJ mol−1, with Langmuirian dependence on the oxygen partial pressure and first-order dependence on the ethane partial pressure. Investigation of the catalyst before and after reaction by one-dimensional 11B magic angle spinning (1D 11B MAS) nuclear magnetic resonance (NMR), two-dimensional 11B multiple quantum MAS (2D 11B MQMAS) NMR spectroscopy, and Fourier transform infrared (FTIR) spectroscopy identifies the B-OH group in defect trigonal boron (B(OSi)(OH)2) as the species initiating the ODHE reaction. This result could open a pathway to develop suitable catalysts for industrial ethylene production with lower greenhouse gas emissions than current non-oxidative dehydrogenation routes.

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.

Interpreting Nonoxidative Ethane Dehydrogenation in an Open Tube Reactor

Interpreting Nonoxidative Ethane Dehydrogenation in an Open Tube Reactor

Calcium additives enhance coke migration and catalyst stability for Pt-based catalysts in ethane dehydrogenation

Understanding and mitigating catalyst deactivation in high-temperature alkane dehydrogenation reactions is critical toward improving their energy and atom efficiency. In this work, we show that a partial overcoat of calcium oxide on a Pt-In alloy catalyst reduces the rate of catalyst deactivation by 10-fold during the non-oxidative dehydrogenation of ethane to ethylene. During 20 h of time-on-stream at 600 °C in 5 % C2H6, the Ca-doped PtIn2 catalyst retains >85 % of its initial activity while the undoped PtIn2 deactivates to <20 %. Temperature-programmed oxidation and spatially-resolved Raman spectroscopy reveal that the improved catalyst stability stems from enhanced migration of coke precursors away from the catalytically-relevant Pt sites to the silica support. Strikingly, the Ca overcoat does not alter the inherent rate and selectivity of coke formation but rather its crystallinity and localization, which nonetheless has a dramatic impact on the long-term performance of the catalyst.

The enhancing effect of Co2+ on propane non-oxidative dehydrogenation over supported Co/ZrO2 catalysts

Due to the increased production of shale gas containing propane, the share of non-oxidative propane dehydrogenation (PDH) in the large-scale production of propene is expected to continue to grow. There are, however, some ecofriendly and cost shortcomings associated with the currently applied Cr- or Pt-containing catalysts. Against this background, we present here Co/ZrO2 alternatives and reveal the fundamentals affecting their activity, which can be used for purposeful catalyst design. Co2+ species homogeneously distributed within the lattice of ZrO2 significantly increase the activity of coordinatively unsaturated Zr4+ for the PDH reaction. The increase is caused by accelerating both the cleavage of CH bonds in propane and the recombination of surface H species, with the latter reaction being the rate-limiting step. The best-performing catalyst outperformed an analogue of commercial K-CrOx/Al2O3 in terms of the rate of propene formation and demonstrated durable performance over a series of 10 PDH/regeneration cycles under industrially relevant conditions. It also outperformed most previously developed Co-containing catalysts in terms of propene productivity. The space–time yield of propene formation achieved at 57 % equilibrium propane conversion at 550 °C was 0.71 kg·h−1·kgcat-1.

Nonoxidative dehydrogenation of propane using boron-incorporated silica-supported Pt Sites synthesized by atomic layer deposition.

Nonoxidative dehydrogenation of propane to propylene using Pt-based supported catalysts is an active research area in catalysis because catalyst attributes of Pt sites can be controlled by careful design of active sites. One way to achieve this is by the addition of a second metal that may impart a change in the electron density of active sites, which in turn affects catalytic performance. In this study, bimetallic Pt and B sites were deposited on powder SiO2 using atomic layer deposition (ALD). Boron was first deposited on SiO2 via half-cycle ALD using triisoproplyborate as the B source. Following calcination, Pt deposition was performed via half-cycle ALD using trimethyl(methylcyclopentadienyl)platinum(IV) as the Pt source. The synthesized catalysts were reduced under H2 at 550 °C and characterized using inductively coupled plasma optical emission spectroscopy for elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO to examine the properties of Pt, and time-resolved X-ray absorption near edge structure spectroscopy to examine the changes in the reducibility of Pt sites. The samples were then tested for nonoxidative dehydrogenation of propane at 550 °C using a fixed-bed plug-flow reactor to examine the role of B on the catalytic performance. Characterization results showed that the addition of B imparted an increase in electron density and affected the reducibility of Pt sites. In addition, incorporating B on SiO2 created anchoring sites for Pt ALD. The amount of Pt deposited on B/SiO2 was 2.2 times that on SiO2. Catalytic activity results revealed the addition of B did not change the initial activity of Pt sites significantly, but improved propylene selectivity from 80% to 87% and stability almost threefold. The enhanced selectivity and stability of PtB/SiO2 is most presumably due to favored desorption of propylene and mitigating coke formation under reaction conditions, respectively.

Support effects in vanadium incipient wetness impregnation for oxidative and non-oxidative propane dehydrogenation catalysis

Oxidative propane dehydrogenation with carbon dioxide (OPDH) is an attractive technology, co-producing carbon monoxide and propylene with a potentially negative greenhouse gas balance. Vanadium oxide containing materials are a promising class of catalysts for such process. In this study, we introduce vanadium into a range of micro- and mesoporous aluminosilicate materials by incipient wetness impregnation with NH4VO3 and the complexing agent oxalic acid, targeting a surface density of 0.6 V atoms per nm² of pristine support. Aspects of this impregnation procedure and its interaction with the support are discussed in the context of potential sources of variability in the dispersion of VOx species within the catalyst materials. The results indicate no formation of a monolayer of vanadium species, and differences in catalyst dispersion are discernible through comprehensive characterization techniques, including thermogravimetric analysis (TGA), X-ray diffraction, Raman spectroscopy, H2-TPR and UV-Vis spectroscopy. The materials are evaluated for the catalytic CO2-assisted propane dehydrogenation in two pressure regimes while being compared to classic non-oxidative propane dehydrogenation (PDH). Results are analyzed using a multivariable analysis of reactants and products, which provided valuable insights for catalyst design. The larger pore material with few acid sites (MCM-41 with Si/Al = 180) demonstrates the highest activity in terms of PDH and high pressure OPDH, i.e. high propane conversion and quasi-equivalent propylene production with similar CO2 co-conversion and CO co-production). Yet, in terms of responding to the introduction of CO2, the zeolites seem to respond better in terms of producing less H2 for only a small decrease in propylene. These microporous materials, except for the most acidic aluminous BEA, exhibit a significant increase in their production rates when increasing the total pressure in OPDH.

Novel Ge/SiO2 Catalysts for Nonoxidative Dehydrogenation of Propane

Novel Ge/SiO₂ Catalysts for Nonoxidative Dehydrogenation of Propane

Zeolite catalysts for non-oxidative ethane dehydrogenation to ethylene

The conversion of ethane to ethylene is crucial for deriving platform chemicals from non-petroleum feedstock.

Automatic mechanism generation involving kinetics of surface reactions with bidentate adsorbates

RMG was expanded with multidentate functionalities, which enables the automated discovery of mechanisms for the complex non-oxidative dehydrogenation of ethane.

Enhanced catalytic performance with adjustable morphology and surface properties of alumina for propane dehydrogenation: Promote the catalytically active chromium sites, dispersion and anti-coking

To examine the effect of the morphology of alumina on the catalytic performance during non-oxidative dehydrogenation of propane, a series of Al2O3 supports with specific morphologies including sheet-like, rod, and nucleoshell were synthesized using a template-free hydrothermal route at different temperatures. The activity results show that catalysts with special morphology have better catalytic performance than the conventional alumina-based catalysts. Cr-AlNC catalyst with nucleoshell morphology cause a significant growth of Cr6+ species compared to other catalysts. The reduction of this species results in the creation of the most reactive Cr sites that are coordinatively unsaturated in the PDH reaction. This reduction also prevents the transformation of Cr species into α-Cr2O3. Cr-AlNC catalyst show anti-coking ability and great resistance against deactivation due to their unique morphology and more importantly, the strong metal-support interaction in addition to the low deactivation rate and coke deposition of 0.1 h−1 and 1.5 %, respectively. However, the corresponding values for the reference catalyst are 0.44 h−1 and 6.1 %, respectively.

Porous Single-crystalline Centimeter-sized α-Al2 O3 Monoliths for Selective and Durable Non-oxidative Dehydrogenation of Ethane.

Alpha alumina (α-Al2 O3 ) are inert materials with outstanding thermal, chemical and mechanical stability. Herein, we fabricate porous single-crystalline (PSC) α-Al2 O3 monoliths at centimeter scale to endow them with high catalytic activity while maintaining their stability. We reduce PSC α-Al2 O3 monoliths to create oxygen vacancies in lattice and stabilize them by the ordered lattice to construct unsaturated Al-O coordination structures for enhancing the catalytic activity. The generation of oxygen vacancy at 18e wyckoff position contributes to the unsaturated Al-O coordination. As a case study, we demonstrate the outstanding performance with conversion (≈34 %) and selectivity (≈95 %) toward non-oxidative dehydrogenation of ethane to ethylene at 700 °C. We achieve the outstanding performance without obvious degradation even after a continuous operation over 1000 hours at 700 °C.

Propane Dehydrogenation over Cobalt Aluminates: Evaluation of Potential Catalytic Active Sites

Non-oxidative propane dehydrogenation (PDH) is becoming an increasingly important approach to propylene production, while cobalt-containing catalysts have recently demonstrated great potential for use in this reaction, providing efficiencies comparable to those of industrially employed Pt- and Cr-based catalytic systems. It is therefore essential to clarify the nature of their active sites, especially since contradictory opinions on this issue are expressed in the literature. In this study, efforts were made to determine the state of Co in cobalt aluminates (CoAl2O4-Al2O3) responsible for PDH under typical operating conditions (600 °C, 1 atm). It is shown that the catalyst with a low cobalt content (Co/Al = 0.1) ensured the highest selectivity to propylene, ca. 95%, while maintaining significant propylene conversion. The structural motifs such as cobalt oxide and metallic cobalt nanoparticles, in addition to tetrahedral Co2+ species in the CoAl2O4 spinel system, were evaluated as potential active-site ensembles based on the obtained catalytic performance data in combination with the XRD, H2-TPR, TEM and XPS characteristics of as-synthesized, spent and spent–regenerated catalysts. It is revealed that the most likely catalytic sites linked to PDH are the Co-oxide forms tightly covering alumina or embedded in the spinel structure. However, additional in situ tuning is certainly needed, probably through the formation of surface oxygen vacancies rather than through a deeper reduction in Co0 as previously thought.

Effect of supports on the kind of in-situ formed ZnOx species and its consequence for non-oxidative propane dehydrogenation

Non-oxidative propane dehydrogenation (PDH) to propene is the basis of various large-scale processes suffering however from high costs or environmental incompatibility of currently applied Pt- or Cr-containing catalysts. Herein, we demonstrate that active and selective catalysts can be obtained from cheap and commercially available Zr- or Ti-based supports and ZnO without producing any liquid or solid waste. Catalytically active species formed in situ under PDH conditions are composed of isolated ZnOx as concluded from X-ray absorption spectroscopic analysis. The kind of support affects the geometry of such species that is probably decisive for catalyst activity. ZnOx on the surface of LaZrOx revealed the highest Zn-related TOF of propene formation. However, the following activity order in terms of space time yield of propene formation (STYC3H6) at 550 °C and about 50% equilibrium propane conversion using a feed with 40 vol% propane was obtained: ZnO//TiZrOx > ZnO//SiZrOx > ZnO//LaZrOx > ZnO//TiO2. The best-performing catalyst showed STYC3H6 of 2 kg kgcat−1 h−1 and was durable in 8 PDH/regeneration cycles. Temporal analysis of products with submillisecond resolution suggests that H2 formation should be the rate-determining step in the course of the PDH reaction.

Controlling Metal-Oxide Reducibility for Efficient C-H Bond Activation in Hydrocarbons.

Knowing the structure of catalytically active species/phases and providing methods for their purposeful generation are two prerequisites for the design of catalysts with desired performance. Herein, we introduce a simple method for precise preparation of supported/bulk catalysts. It utilizes the ability of metal oxides to dissolve and to simultaneously precipitate during their treatment in an aqueous ammonia solution. Applying this method for a conventional VOx -Al2 O3 catalyst, the concentration of coordinatively unsaturated Al sites was tuned simply by changing the pH value of the solution. These sites affect the strength of V-O-Al bonds of isolated VOx species and thus the reducibility of the latter. This method is also applicable for controlling the reducibility of bulk catalysts as demonstrated for a CeO2 -ZrO2 -Al2 O3 system. The application potential of the developed catalysts was confirmed in the oxidative dehydrogenation of ethylbenzene to styrene with CO2 and in the non-oxidative propane dehydrogenation to propene. Our approach is extendable to the preparation of any metal oxide catalysts dissolvable in an ammonia solution.

Highly stable and selective Pt/TS-1 catalysts for the efficient nonoxidative dehydrogenation of propane

Highly stable and selective Pt/TS-1 catalysts for the efficient nonoxidative dehydrogenation of propane

Microwave-assisted, performance-advantaged electrification of propane dehydrogenation.

Nonoxidative propane dehydrogenation (PDH) produces on-site propylene for value-added chemicals. While commercial, its modest selectivity and catalyst deactivation hamper the process efficiency and limit operation to lower temperatures. We demonstrate PDH in a microwave (MW)-heated reactor over PtSn/SiO2 catalyst pellets loaded in a SiC monolith acting as MW susceptor and a heat distributor while ensuring comparable conditions with conventional reactors. Time-on-stream experiments show active and stable operation at 500°C without hydrogen addition. Upon increasing temperature or feed partial pressure at high space velocity, catalysts under MWs show resistance in coking and sintering, high activity, and selectivity, starkly contrasting conventional reactors whose catalyst undergoes deactivation. Mechanistic differences in coke formation are exposed. Gas-solid temperature gradients are computationally investigated, and nanoscale temperature inhomogeneities are proposed to rationalize the different performances of the heating modes. The approach highlights the great potential of electrification of endothermic catalytic reactions.

Effect of Deposition Sequence on Catalytic Activity of CrOx–ZrO2–SiO2 in Nonoxidative Propane Dehydrogenation

Effect of Deposition Sequence on Catalytic Activity of CrOx–ZrO2–SiO2 in Nonoxidative Propane Dehydrogenation

Effect of Deposition Sequence on Catalytic Activity of CrOx–ZrO2–SiO2 in Nonoxidative Propane Dehydrogenation

Effect of Deposition Sequence on Catalytic Activity of CrOx–ZrO2–SiO2 in Nonoxidative Propane Dehydrogenation