Transient Kinetic Study of Ethane and Ethylene Aromatization over Zinc-Exchanged HZSM-5 Catalyst

Unraveling the Structure-Activity-Stability Relationship over Gallium-Promoted HZSM-5 Nanocrystalline Aggregates for Propane Aromatization

The aromatization of light alkane is an important process for increasing the aromatic production and utilization efficiency of light alkane resources simultaneously. Herein, Ga-modified HZSM-5 catalysts were prepared and investigated by a series of characterization techniques such as X-ray diffraction, nuclear magnetic resonance spectroscopy, transmission electron microscopy, N2 adsorption-desorption, and NH3 temperature-programmed desorption to study their physicochemical properties. The catalytic performance in propane aromatization was also tested. Importantly, the structure-activity relationship, reaction pathway, and coke formation mechanism in propane aromatization were systematically explored. It was found that different Ga introduction methods would affect the amounts of Brønsted and Lewis acid sites, and Ga-HZSM-5 prepared by the hydrothermal method exhibited higher amounts of Brønsted and Lewis acid sites but a lower B/L ratio. As a result, Ga-HZSM-5 showed higher propane conversion and benzene, toluene, and xylene yield compared with that of Ga2O3/HZSM-5. The propane aromatization reaction pathway indicated that propane dehydrogenation to propene was a crucial step for aromatic formation. The increase of the Lewis acid density in Ga-HZSM-5 can effectively improve the dehydrogenation rate and promote the aromatization reaction. Furthermore, the formation of coke species was studied by thermogravimetry-mass spectrometry and Raman approaches, the results of which indicated that the graphitization degree of coke formed over spent Ga-HZSM-5 is lower, resulting in enhanced anticoking stability.

Ultralow-Loading Pt/Zn Hybrid Cluster in Zeolite HZSM-5 for Efficient Dehydroaromatization

Minimizing Pt loading without sacrificing catalytic performance is critical, particularly for designing cost-efficient hydrocarbon transformation catalysts. Here, we show that ultralow-loading (0.001-0.05 wt %) Pt- and Zn-functionalized HZSM-5 catalysts, prepared through simple ion exchange and impregnation, are highly active and stable for light alkane dehydroaromatization (DHA). The specific activity of benzene, toluene, and xylene is up to 8.2 mol/g_Pt/min (or 1592 min^-1) over the 0.001 wt % Pt-Zn₂/HZSM-5 catalyst during ethane DHA at 550 °C under atmospheric pressure. Additionally, such bimetallic Pt_x-Zn_y/HZSM-5 catalysts are highly stable in contrast to the monometallic Pt/HZSM-5 catalysts. The rate constant of deactivation (k_deactiv), according to the first-order generalized power law equation model, for the bimetallic catalysts is up to 120 times lower than that of the monometallic counterparts, depending on the Pt loading. This breakthrough is achieved through the formation of the [Pt₁-Zn_n]^δ+ hybrid cluster, instead of Pt⁰ cluster-proton adducts, in the micropores of the ZSM-5 zeolite.

Catalytic Performances of Cu/MCM-22 Zeolites with Different Cu Loadings in NH3-SCR

The NH₃-SCR activities and hydrothermal stabilities of five xCu/MCM-22 zeolites with different Cu loadings (x = 2–10 wt%) prepared by incipient wetness impregnation method were systematically investigated. The physicochemical properties of xCu/MCM-22 zeolites were analyzed by XRD, nitrogen physisorption, ICP-AES, SEM, NH₃-TPD, UV-vis, H₂-TPR and XPS experiments. The Cu species existing in xCu/MCM-22 are mainly isolated Cu²⁺, CuO_x and unreducible copper species. The concentrations of both isolated Cu²⁺ and CuO_x species in xCu/MCM-22 increase with Cu contents, but the increment of CuO_x species is more distinct, especially in high Cu loadings (>4 wt%). NH₃-SCR experimental results demonstrated that the activity of xCu/MCM-22 is sensitive to Cu content at low Cu loadings (≤4 wt%). When the Cu loading exceeds 4 wt%, the NH₃-SCR activity of xCu/MCM-22 is irrelevant to Cu content due to the severe pore blockage effects caused by aggregated CuO_x species. Among the five xCu/MCM-22 zeolites, 4Cu/MCM-22 with moderate Cu content has the best NH₃-SCR performance, which displays higher than 80% NO_x conversions in a wide temperature window (160–430 °C). Furthermore, the hydrothermal aging experiments (xCu/MCM-22 was treated at 750 °C for 10 h under 10% water vapor atmosphere) illustrated that all the xCu/MCM-22 zeolites exhibit high hydrothermal stability in NH₃-SCR reactions.

Ammoxidation of Ethane to Acetonitrile and Ethylene: Reaction Transient Analysis for the Co/HZSM-5 Catalyst

Ethane ammoxidation to acetonitrile and ethylene over the Co/HZSM-5 catalysts was revisited based on both transient and steady-state performance evaluation to elucidate the structure/reactivity relationships. We suggested that the exchanged Co²⁺ cation encapsulated in the zeolite favors the formation of acetonitrile and ethylene, whereas nanosized cobalt oxide particles without close proximity with the HZSM-5 only favor CO₂ formation. Excess Brønsted acid sites of the zeolites may act as a reservoir for NH₃, which inhibits the CO₂ formation through the NH₃-mediated oxidative dehydrogenation mechanism. According to the transient kinetic analysis, the time constants τ from the back-transient decay for NH₃ and CO₂ are both 7.7 min, which decreased to 2.7 min for acetonitrile and further decreased to 3-4 s for ethane, ethylene, and O₂. Assuming first-order reaction kinetics, the rate constants for the formation of acetonitrile and CO₂ are 0.37 and 0.13 min^-1, respectively, from their corresponding reactive intermediates.