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Kinetics of H/D hydrogen exchange between deuterated isobutane-d 10 and Brønsted acid sites (BAS) of three zeolite samples (H-BEA, ZnO/H-BEA, Zn 2+ /H-BEA) were monitored with 1 H MAS NMR in situ at 343-468 K. The regioselective H/D exchange in the methyl groups detected on H-BEA can be rationalized in terms of the mechanism of indirect exchange, which involves protonation of the intermediate olefin and further hydride abstraction from the other alkane molecule by the formed carbenium ion. Loading of Zn species in the zeolite results in a decrease of the rate and an increase of the activation energy of the exchange. The loaded Zn species provide the tuning effect on the reaction occurrence, changing the mechanism from the indirect one to the mechanism of the direct exchange., (© 2022 Wiley-VCH GmbH.)

To clarify the effects of different Zn species, zeolite topology and acidity (quantity of Brønsted acid sites, BAS) on alkane aromatization, isobutane transformation on Zn 2+ /H-ZSM-5, Zn 2+ /H-BEA, and ZnO/H-BEA zeolites has been monitored with 13 C MAS NMR. The alkane transformation has been established to occur by aromatization and hydrogenolysis pathways. Zn 2+ species is more efficient for the aromatization reaction because aromatic products are formed at lower temperatures on Zn 2+ /H-BEA and Zn 2+ /H-ZSM-5 than on ZnO/H-BEA. The larger quantity of BAS in ZnO/H-BEA seems to provide a higher degree of the hydrogenolysis pathway on this catalyst. The mechanism of the alkane aromatization is similar for the zeolites of different topology and containing different Zn species, with the main reaction steps being the following: (i) isobutane dehydrogenation to isobutene via isobutylzinc; (ii) isobutene stabilization as a π-complex on Zn sites; (iii) isobutene oligomerization via the alkene insertion into Zn-C bond of methyl-σ-allylzinc formed from the π-complex; (iv) oligomer dehydrogenation with intermediate formation of polyene carbanionic structures; (v) aromatics formation via further polyene dehydrogenation, protonation, cyclization, deprotonation steps with BAS involvement., (© 2021 Wiley-VCH GmbH.)

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.

Kinetics of H/D hydrogen exchange between deuterated isobutane‐d10 and Brønsted acid sites (BAS) of three zeolite samples (H‐BEA, ZnO/H‐BEA, Zn2+/H‐BEA) were monitored with 1H MAS NMR in situ at 343–468 K. The regioselective H/D exchange in the methyl groups detected on H‐BEA can be rationalized in terms of the mechanism of indirect exchange, which involves protonation of the intermediate olefin and further hydride abstraction from the other alkane molecule by the formed carbenium ion. Loading of Zn species in the zeolite results in a decrease of the rate and an increase of the activation energy of the exchange. The loaded Zn species provide the tuning effect on the reaction occurrence, changing the mechanism from the indirect one to the mechanism of the direct exchange. [ABSTRACT FROM AUTHOR]

• The effect of Zn2+ and ZnO sites on n -butane conversion on Zn-BEA zeolites was established. • n -Butane conversion occurs via same reaction steps on both Zn2+ and ZnO containing zeolites. • Zn2+/H-BEA is more active showing higher rate and lower activation energies for main reaction steps. • Hydrogenolysis of n -butane occurs differently on Zn2+/H-BEA and ZnO/H-BEA. • Methane co-conversion with n -butane on Zn-BEA zeolite in non-oxidative conditions can be realized. Using solid-state 1H and 13C MAS NMR spectroscopy, the performance of H-BEA zeolite, modified with either Zn2+ cations (Zn2+/H-BEA) or ZnO clusters (ZnO/H-BEA), has been investigated with respect to n -butane transformation by aromatization and hydrogenolysis pathways. 13C-labeled n -butane has been used to follow the main stages of n -butane transformation on both Zn2+/H-BEA and ZnO/H-BEA with 13C MAS NMR at 298–623 K. Similar surface species, including n-butylzinc, n-butene, allyl-like oligomers, are formed as the intermediates on both zeolites. The kinetics of n -butane transformation has been monitored with 1H MAS NMR in situ at 543–573 K. Kinetics modeling reveals that Zn2+/H-BEA is more active for n -butane transformation than ZnO/H-BEA. A remarkable difference in the rates and the pathways of hydrogenolysis for Zn2+/H-BEA and ZnO/H-BEA has also been established. Propane and methane are hydrogenolysis products on ZnO/H-BEA whereas ethane is produced by the reaction on Zn2+/H-BEA. 13C NMR data and the kinetics analysis provide an insight on the occurrence of joint methane and n -butane conversion on Zn-modified zeolites under non-oxidative conditions. [ABSTRACT FROM AUTHOR]

To clarify the issue on Brønsted acidity enhancement by the presence of zinc species in H‐ZSM‐5 zeolite (G.D. Qi, et al. Angew. Chem.‐Int. Edit. 2016, 55, 15826), the influence of Zn2+ cationic species, introduced into the zeolite, on the strength of Brønsted acid sites (BAS) has been analyzed by solid‐state NMR and FTIR techniques using trimethylphosphine (TMP) and CO as probe molecules. Comparative analysis of the acidity for H‐ZSM‐5 and Zn2+/H‐ZSM‐5 zeolites has been performed. TMP adsorbed on both zeolite samples exhibits same signals at 6.3 and −4.5 ppm in 1H and 31P MAS NMR spectra, respectively, assigned to protonated TMP. FTIR spectroscopy has shown that both zeolite samples, H‐ZSM‐5 and Zn2+/H‐ZSM‐5, display the same red frequency shift (ΔνOH⋅⋅⋅CO=−318 cm−1) of the O−H stretching band and the same blue frequency shift (ΔνCO=+32 cm−1) of the CO stretching band for CO adsorption complex with BAS. The results obtained do not provide any evidence for the existence of some specific BAS with enhanced acid strength for Zn2+‐exchanged zeolite compared to the strength inherent to pure acid‐form zeolite. It is thus inferred the strength of BAS in H‐ZSM‐5 zeolite is not affected by the modification of the zeolite with Zn2+ cationic species. [ABSTRACT FROM AUTHOR]

• The effect of Zn species on propane C H bonds activation on Zn-BEA zeolite was studied. • Both Zn2+ and ZnO species provide synergy with BAS for propane C H bond activation. • Zn2+ and ZnO species show dramatic acceleration of H/D exchange of propane with BAS. • H/D exchange, assisted by Zn2+ and ZnO, occurs regioselectively into CH 3 groups of propane. • The CH 2 group is involved in the exchange only on ZnO/H-BEA by intramolecular H-transfer. Zn-modified zeolites exhibit high activity for the aromatization of C 2+ alkanes. To understand the effect of Zn-species of different nature on propane molecules activation and transformation, the H/D hydrogen exchange of zeolite Brønsted acid sites (BAS) with deuterated propane- d 8 has been investigated. Two samples of the Zn-modified zeolite containing exclusively either isolated Zn2+ cations (Zn2+/H-BEA) or small (ZnO) n clusters (ZnO/H-BEA) have been studied. Zn-species of either type work jointly with BAS and may provide the synergy effect for propane C H bond activation resulting in dramatic acceleration of the H/D exchange between propane and BAS. The accelerating effect of isolated Zn2+ cations is more pronounced compared to zinc oxide clusters. Moreover, the H/D exchange occurs regioselectively into the methyl groups of propane on Zn2+/H-BEA zeolite. For ZnO/H-BEA, the methylene group becomes also involved in the exchange after some induction period. Various mechanisms providing the regioselective H/D exchange on Zn2+/H-BEA as well as the involvement of the methylene group after the induction period on ZnO/H-BEA are discussed. [ABSTRACT FROM AUTHOR]

Propene transformation on copper-modified ZSM-5 zeolite has been studied by 13C MAS NMR and FTIR spectroscopy. Two major reaction pathways are identified, aromatization to the simple aromatic hydrocarbons (benzene, toluene) and oxidation to acrolein. Copper-allyl species is inferred to be the key intermediate in both processes. π-Complex of propene with copper sites is the primary intermediate, preceding the formation of copper-allyl species. The role of different active sites is established. Cu2+–O2– Lewis acid-base pairs are responsible for the allylic species formation, whereas [Cu 3 (μ-O) 3 ]2+ oxo-clusters provide the oxidation of copper-allyls. The results obtained are of importance for developing zeolite-based catalysts for propene aromatization to the simple aromatic compounds and partial oxidation to acrolein. [Display omitted] • Propene is transformed to simple aromatics and acrolein on Cu/ZSM-5. • Copper sites provide strong stabilization of propene in the form of π- complexes. • Copper-allyl species represents the key intermediate in propene aromatization and oxidation. • [Cu 3 (μ-O) 3 ]2+ oxo-clusters are responsible for oxidizing allylic species. [ABSTRACT FROM AUTHOR]

[Display omitted] • Methane reactions with benzene and CO on Cu-ZSM-5 has been studied. • The nature of Cu sites, cations or oxo-clusters, affects the reaction performance. • [Cu 3 (µ-O) 3 ]2+ oxo-clusters facilitate benzene methylation with methane. • Cu2+ cations are more effective for methane carbonylation with CO. Three types of methoxy-like intermediates were generated at methane interaction with Cu-loaded ZSM-5 zeolite, containing copper in the form of either Cu2+ cations (Cu2+/H-ZSM-5) or [Cu 3 (µ-O) 3 ]2+ oxo-clusters (CuO/H-ZSM-5). The methoxy species were tested for the reaction with benzene and carbon monoxide. 13C MAS NMR spectroscopy reveals that the methoxy species formed on CuO/H-ZSM-5 easily interact with benzene yielding toluene, whereas the methoxy species on Cu2+/H-ZSM-5 are less effective for benzene methylation. At the same time, the methoxy species on Cu2+/H-ZSM-5 can interact with CO forming surface acetate species, while strong adsorption of CO by [Cu 3 (µ-O) 3 ]2+ oxo-clusters of CuO/H-ZSM-5 inhibits the reaction. [ABSTRACT FROM AUTHOR]

Abstract: 13C CP MAS NMR analysis of the products of the interaction of methane, ethane and propane with α-Ga2O3 or Ga-modified zeolite BEA at 523–623K shows that dissociative adsorption of C1–C3 alkanes on the surface of gallium oxide or Ga-modified zeolite BEA results to the formation of Ga-methyl, Ga-ethyl and Ga-propyl species. This observation allows one to conclude that Ga-alkyls, rather than earlier suggested alkoxy species, could be the intermediates in small alkane dehydrogenation and aromatization on these catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Acidic OH groups of acid-form and Zn and Ga-modified zeolite beta (BEA) have been characterized with 1H MAS NMR and by IR spectroscopy of adsorbed carbon monoxide. It is demonstrated that OH groups, which exhibit a vibration band at 3740cm−1, reveal acidity, which is similar to that of the OH groups with the band at 3610cm−1 according to the value of the low frequency shift of OH vibrations with adsorbed CO (Δν OH/CO =300cm−1). The IR band 3740cm−1 corresponds to the signal at ca. 2.1ppm in 1H MAS NMR spectrum. The OH groups with signal 2.1ppm are involved in H/D exchange with methane-d 4 similar to acidic OH groups with the signals 4.0–5.1ppm. The signals at 2.1ppm in 1H MAS NMR and at 3740cm−1 in IR are attributed to the strongly acidic silanol groups of the faulted structure of the zeolite. The silanols with the signals at 1.8ppm and 3745cm−1 are weakly acidic (ΔνOH/CO =85cm−1) and are not involved in the H/D exchange. Loading of the zeolite with Zn affords a notable decrease of the concentration of strongly acidic SiOHAl groups, whereas the quantity of these groups does not decrease upon loading the zeolite with Ga. [Copyright &y& Elsevier]