Copper‐exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time‐resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono‐μ‐oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono‐μ‐oxo species, while the activation energy is two times lower.

Direct methane conversion to methanol via chemical looping using copper-exchanged zeolites has attracted considerable attention during the last decades and is one of the most-actively studied processes. Despite the significant progress that has been made in the design of active systems and the elucidation of active sites, the effects of zeolite topology and the structure of copper species on the nature of the reaction products are yet unclear. Herein, we show that oxygen-activated copper-exchanged zeolites of different framework types, namely, MOR, MFI, BEA, and FAU, yield different products, as detected by in situ Fourier-transform infrared and nuclear magnetic resonance spectroscopy. Molecular methanol, methoxy species, and dimethyl ether prevail at lower reaction temperatures (<473–523 K), and CuI carbonyls and gaseous carbon oxides were detected above 573 K. Methane coupling to C2 and C3 hydrocarbons was shown for the first time over CuMOR, CuBEA, and CuFAU. The nature and relative fraction of formed products strongly depend on the structure of the copper active sites, which is governed by the topology of the zeolite host. Several pathways of methane transformation over copper-exchanged zeolites are identified, opening opportunities for tuning the properties of the materials to achieve the best performance.

The one-step valorization of natural gas remains a challenge. Methane conversion to methanol via chemical looping over copper-containing zeolites is a promising route, and CuMFI is among the earliest successfully applied. However, the structure of the active sites in CuMFI, as well as the effect of copper loading and Si/Al ratio on the copper speciation, are yet to be understood. We found that for CuMFI, the Cu/Al ratio determines the selectivity of methane conversion by governing the structure of the active dicopper sites. At a Cu/Al ratio below 0.3, copper-containing MFI materials host dimeric centers with a Cu–Cu separation of 2.9 Å and a UV/vis absorption band at 27 200 cm–1 capable of selective oxidation of methane to methanol in a wide temperature range (450–550 K). A higher Cu/Al ratio leads to the formation of mono-μ-oxo dicopper sites with Cu–Cu = 3.2 Å, which exhibit a characteristic band at 21 900 cm–1 and react with methane at lower temperatures (<450 K), yielding overoxidation products. Identifying distinctions in the structure of selective and nonselective copper sites will aid in the design of better-performing materials.

Oxygen isotope exchange for sodium- and copper-containing mordenite was studied in the isothermal regime using a plug-flow reactor. No prominent exchange was observed over sodium mordenite at 873 K. In contrast, the presence of copper significantly increases the lability of oxygen atoms resulting in a substantial rate of isotope exchange in the temperature range between 793 and 873 K. A novel numerical model accounting for two kinetically distinct sources of exchangeable oxygen atoms was proposed and shown to provide high quality of fitting of experimental data. Two types of exchangeable atoms correspond to about 9 and 25% of the total amount of oxygen atoms, respectively, and these values do not change with the exchange temperature. The exchange mechanism involving two oxygen atoms from the surface dominates in the case of the first source of exchangeable atoms, while the second source undergoes exchange mainly via a single-atom exchange mechanism.

Gas-phase continuous synthesis of butadiene from propene and formaldehyde has been studied over heteropolyacids (HPA) supported on silica. Silicotungstic acid (SiW) is found to be the most active and selective among other HPA. At low SiW loadings, catalysts have low selectivity due to the formation of a butanal by-product, while at high loadings (≥40 wt %), selective butadiene formation and stable performance are achieved. Strong Brønsted acid sites associated with intrinsic SiW protons are shown to be more active in butadiene synthesis as compared with Si–OH2 + sites formed by the interaction of HPA protons with a silica support. The kinetic study and product cofeeding experiments suggest that propene activation or C–C bond formation is the rate-determining step and that both butadiene and butanal are formed over strong acid sites starting from a common intermediate. Over weaker acid sites, condensation of butanal with formaldehyde leads to 2-ethylacrolein. Besides that, the reaction is complicated by butadiene interaction with formaldehyde leading to 2,3-dihydropyran, propene oligomerization and cracking, formaldehyde decomposition toward CO and H2, and extensive coke formation leading to catalyst deactivation. However, the optimization of reaction temperature, SiW loadings, and the content of water in reaction media allows us to achieve a butadiene selectivity up to 64% and stable catalyst performance with time on stream.

Methane conversion over copper-containing zeolites can lead to the formation of C–C bonds, yielding hydrocarbons. By employing in situ MAS NMR spectroscopy, we elucidated the pathways of the transformation of methane and its partial oxidation and coupling products over copper-containing mordenite. Below 773 K, the direct coupling of methane is not possible, while the transformation of methanol, methoxy species, and dimethyl ether takes place via the methanol-to-hydrocarbons (MTH) process. In the presence of carbon monoxide, surface acetate species are formed from methanol via the Koch carbonylation reaction. The nature of the hydrocarbon pool species and concomitantly the formed hydrocarbons are strongly affected by the reactants: conversion of pure methanol leads to methyl-substituted cyclopentenyl cations, and the presence of carbon monoxide results in methyl-substituted benzenes. The study clarifies the mechanism of C–C bond formation during the conversion of methane and methanol over copper-containing mordenite and provides insights into the mechanism of the MTH process.

Copper‐exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time‐resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono‐μ‐oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono‐μ‐oxo species, while the activation energy is two times lower.

The structure of copper sites formed under an oxidative environment and their evolution in the course of the reaction with methane at elevated temperature was investigated by means of Cu K-edge X-ray absorption spectroscopy for a series of copper-containing MFI, MOR, and FAU zeolites. The pretreatment in oxygen at 723 K leads to the formation of copper(II)-oxo sites, whose nature depends on the framework type. Dimeric species are formed in CuMFI material, dimeric and monomeric sites coexist in CuMOR, and agglomerated copper-oxo nanoclusters are found in large-pore copper-containing faujasite (CuFAU). For all studied materials, the reaction with methane resulted in the exclusive formation of copper(I) species; no formation of metallic copper was detected even at 748 K. The nature of formed copper(I) species is governed by the structure of corresponding copper(II) centers. In particular, monomeric and dimeric copper(II)-oxo sites hosted in CuMOR and CuMFI are transformed into isolated copper(I) cations coordinated to ion-exchange positions of the zeolite. Contrarily, copper(II)-oxo clusters present in CuFAU undergo restructuring with only a partial loss of extra-framework oxygen and form aggregated species with a structure similar to that of bulk copper(I) oxide.