Effect of the chromium precursor nature on the physicochemical and catalytic properties of Cr–ZSM-5 catalysts: Application to the ammoxidation of ethylene

Non-Noble FeCrOx Bimetallic Nanoparticles for Efficient NH3 Decomposition

Ammonia has the advantages of being easy to liquefy, easy to store, and having a high hydrogen content of 17.3 wt%, which can be produced without CO_x through an ammonia decomposition using an appropriate catalyst. In this paper, a series of FeCr bimetallic oxide nanocatalysts with a uniform morphology and regulated composition were synthesized by the urea two-step hydrolysis method, which exhibited the high-performance decomposition of ammonia. The effects of different FeCr metal ratios on the catalyst particle size, morphology, and crystal phase were investigated. The Fe_0.75Cr_0.25 sample exhibited the highest catalytic activity, with an ammonia conversion of nearly 100% at 650 °C. The dual metal catalysts clearly outperformed the single metal samples in terms of their catalytic performance. Besides XRD, XPS, and SEM being used as the means of the conventional characterization, the local structural changes of the FeCr metal oxide catalysts in the catalytic ammonia decomposition were investigated by XAFS. It was determined that the Fe metal and FeN_x of the bcc structure were the active species of the ammonia-decomposing catalyst. The addition of Cr successfully prevented the Fe from sintering at high temperatures, which is more favorable for the formation of stable metal nitrides, promoting the continuous decomposition of ammonia and improving the decomposition activity of the ammonia. This work reveals the internal relationship between the phase and structural changes and their catalytic activity, identifies the active catalytic phase, thus guiding the design and synthesis of catalysts for ammonia decomposition, and excavates the application value of transition-metal-based nanocomposites in industrial catalysis.

Light hydrocarbon conversion to acrylonitrile and acetonitrile - a review

Nitriles, particularly acrylonitrile and acetonitrile, are versatile chemicals that are used in various fields, such as polymer synthesis and pharmaceutical production. For a long time, acrylonitrile has been produced via propylene ammoxidation with acetonitrile as a byproduct. The depletion of crude reservoirs and the production of unconventional hydrocarbon resources (e.g., shale gas) renders light alkanes (including propane, ethane, and methane) to be potential feedstocks in the syntheses of acrylonitrile and acetonitrile. In this review, the processes of transforming light hydrocarbons to nitriles are surveyed, the developments in nitrile synthesis from alkanes are discussed, and the existing challenges and plausible solutions are addressed.

Screening of Adsorbent/Catalyst Composite Monoliths for Carbon Capture-Utilization and Ethylene Production

Combining CO₂ adsorption and utilization in oxidative dehydrogenation of ethane (ODHE) into a single bed is an exciting way of converting a harmful greenhouse gas into marketable commodity chemicals while reducing energy requirements from two-bed processes. However, novel materials should be developed for this purpose because most adsorbents are incapable of capturing CO₂ at the temperatures required for ODHE reactions. Some progress has been made in this area; however, previously reported dual-functional materials (DFMs) have always been powdered-state composites and no efforts have been made toward forming these materials into practical contactors. In this study, we report the first-generation of structured DFM adsorbent/catalyst monoliths for combined CO₂ capture and ODHE utilization. Specifically, we formulated M-CaO/ZSM-5 monoliths (M = In, Ce, Cr, or Mo oxides) by 3D-printing inks with CaCO₃ (CaO precursor), insoluble metal oxides, and ZSM-5. The physiochemical properties of the monoliths were vigorously characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N₂ physisorption, elemental mapping, pyridine Fourier transform infrared spectroscopy (Py-FTIR), H₂-temperature-programmed reduction (H₂-TPR), and NH₃-temperature-programmed desorption (NH₃-TPD). Their performances for combined CO₂ adsorption at 600 °C and ODHE reaction at 700 °C under 25 mL/min of 7% C₂H₆ were then investigated. The combined adsorption/catalysis experiments revealed the best performance in Cr-CaO/ZSM-5, which achieved 56% CO₂ conversion, 91.2% C₂H₄ selectivity, and 33.8% C₂H₄ yield. This exceptional performance, which was improved from powdered-state DFMs, was attributed to the high acidity and numerous oxidation states of the Cr₂O₃ dopant which were verified by NH₃-TPD and H₂-TPR. Overall, this study reports the first-ever proof-of-concept for 3D-printed DFM adsorbent/catalyst materials and furthers the area of CO₂ capture and ODHE utilization by providing a simple pathway to structure these composites.

Methane activation by ZSM-5-supported transition metal centers

This review focuses on recent fundamental insights about methane dehydroaromatization (MDA) to benzene over ZSM-5-supported transition metal oxide-based catalysts (MO_x/ZSM-5, where M = V, Cr, Mo, W, Re, Fe). Benzene is an important organic intermediate, used for the synthesis of chemicals like ethylbenzene, cumene, cyclohexane, nitrobenzene and alkylbenzene. Current production of benzene is primarily from crude oil processing, but due to the abundant availability of natural gas, there is much recent interest in developing direct processes to convert CH₄ to liquid chemicals. Among the various gas-to-liquid methods, the thermodynamically-limited Methane DehydroAromatization (MDA) to benzene under non-oxidative conditions appears very promising as it circumvents deep oxidation of CH₄ to CO₂ and does not require the use of a co-reactant. The findings from the MDA catalysis literature is critically analyzed with emphasis on in situ and operando spectroscopic characterization to understand the molecular level details regarding the catalytic sites before and during the MDA reaction. Specifically, this review discusses the anchoring sites of the supported MO_x species on the ZSM-5 support, molecular structures of the initial dispersed surface MO_x sites, nature of the active sites during MDA, reaction mechanisms, rate-determining step, kinetics and catalyst activity of the MDA reaction. Finally, suggestions are given regarding future experimental investigations to fill the information gaps currently found in the literature.

Lean and rich aging of a Cu/SSZ-13 catalyst for combined lean NOx trap (LNT) and selective catalytic reduction (SCR) concept

In the combined lean NO_x trap (LNT) and selective catalytic reduction (SCR) concept, the SCR catalyst can be exposed to rich conditions during deSO_x of the LNT.

Probing the nature of active chromium species and promotional effects of potassium in Cr/MCM-41 catalysts for methyl mercaptan abatement

The introduction of K enables a large number of CrO₄²⁻ active species to be anchored and dispersed on the surface of Cr-based catalysts.

Synthesis of CrOx/C catalysts for low temperature NH3-SCR with enhanced regeneration ability in the presence of SO2

Chromium oxide nano-particles with an average diameter of 3 nm covered by amorphous carbon (CrO_x/C) were successfully synthesized. The synthesized CrO_x/C materials were used for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR), which shows superb NH₃-SCR activity and in particular, satisfactory regeneration ability in the presence of SO₂ compared with Mn-based catalysts. The as-prepared catalysts were characterized by XRD, HRTEM, Raman, FTIR, BET, TPD, TPR, XPS and in situ FTIR techniques. The results indicated presence of certain amounts of unstable lattice oxygen exposed on the surface of CrO_x nano-particles with an average size of 3 nm in the CrO_x/C samples, which led to NO being conveniently oxidized to NO₂. The formed NO₂ participated in NH₃-SCR activity, reacting with catalysts via a “fast NH₃-SCR” pathway, which enhanced th NH₃-SCR performance of the CrO_x/C catalysts. Furthermore, the stable lattice of the CrO_x species made the catalyst immune to the sulfation process, which was inferred to be the cause of its superior regeneration ability in the presence of SO₂. This study provides a simple way to synthesize stable CrO_x nano-particles with active oxygen, and sheds light on designing NH₃-SCR catalysts with highly efficient low temperature activity, SO₂ tolerance, and regeneration ability.

Novel CrO_x@C catalyst with both remarkable NH₃-SCR activity and satisfactory regeneration ability in the presence of SO₂.

Ethanol gas-phase ammoxidation to acetonitrile: the reactivity of supported vanadium oxide catalysts

The gas-phase ammoxidation of ethanol, a bio-based platform molecule, has been investigated as a possible more sustainable route for the production of acetonitrile, using supported vanadium oxide catalysts. The nature of the interaction between the support and the active species greatly affected the catalytic performance.

Efficient conversion of cotton stalks over a Fe modified HZSM-5 catalyst under microwave irradiation

Doped amounts of Fe species to HZSM-5 distinctly improved the catalytic performance of the catalyst for the liquefaction of cotton stalk to bio-oil due to the enhanced total and weak acid sites.