Acidity and Local Confinement Effect in Mordenite Probed by Solid-State NMR Spectroscopy

Gong, Kecheng; Liu, Zhengmao; Liu, Liang; Zhao, Zhenchao; Guo, Meiling; Liu, Xuebin; Han, Xiuwen; Bao, Xinhe; Hou, Guangjin

doi:10.1021/acs.jpclett.0c03610

Cited by 18 publications

(12 citation statements)

References 50 publications

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“…Even higher relative concentrations of acid sites in the 12-ring are obtained from dosing CD 3 CN and probing with 1 H MAS NMR. Qualitatively, the spectra are comparable to those reported by Zheng et al, 49,50 yet typical acid site distributions (12-ring vs 8-ring) in MOR are usually reported as 2:1 to 1:1. 44 This discrepancy could be associated with unknown molar extinction coefficients in IR spectra or a degree of sites in the 8-ring that cannot be titrated.…”

Section: Resultssupporting

confidence: 85%

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

et al. 2022

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In the pursuit of controlling the propensity of Cumordenite (MOR) for the selective oxidation of CH 4 , we take a closer look at intrinsic zeolite parameters. Via synthesis design, we vary the relative proportion of Al situated near the 8-rings and 12rings of MOR zeolite. This is accomplished using different Al sources impacting the local degree of silica dissolution and zeolite formation as evidenced by crystallization times and morphological differences. Interrogating the crystalline system with steric probe molecules in conjunction with spectroscopic techniques such as 1 H magic angle spinning (MAS) NMR, infrared spectroscopy, as well as temperature-programmed desorption confirms discrete changes of the Al within the unit cell. The subsequent copper exchange allows for the generation of Cu-MOR materials of different inclinations for the activation of methane in the stepwise formation of MeOH. Here, an increasing degree of acid sites in more easily accessible locations (e.g., 12-ring) correlates with increasing maximum productivity toward MeOH at moderate exchange degrees. X-ray absorption spectroscopy supports this notion, finding a higher concentration of self-reduction-resistant framework-associated Cu 2+ species, previously established as the active sites in the selective oxidation of CH 4 .

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Section: Resultssupporting

confidence: 85%

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

et al. 2022

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“…Mordenite (MOR) zeolite consists of both parallel 12-membered-ring (12-MR) (7.0 × 6.7 Å) and 8-MR (5.7 × 2.6 Å) channels, connected by 8-MR openings (4.8 × 3.4 Å) known as side pockets. , The BASs at different positions show distinct confinement effects for reactant molecules, especially reflected in a MOR zeolite catalyzing a dimethyl ether (DME) carbonylation reaction, which is a key step in converting syngas to ethanol. − It has been demonstrated that the 8-MR channels are preferred for the selective carbonylation of DME, where the local environment of BASs could stabilize the transition state involved in CO inserting into the bonded methyl group. − Corma et al proposed from theoretical studies that the T3-O33 site in the 8-MR channel is the only position selective for carbonylation . In contrast, as the larger 12-MR channels could accommodate more reaction routes, such as DME to hydrocarbons via a hydrocarbon pool (HCP) mechanism, a further HCP transformation to a coke reaction, etc., and the 12-MR channels provide transport channels for the reactions; the BASs in the 12-MR channel will cause rapid deactivation of the MOR zeolite. − Therefore, it is necessary to selectively remove the BASs in 12-MR channels and to control the BASs preferentially located in 8-MR channels of MOR to improve its catalytic performance in the DME carbonylation reaction.…”

Section: Introductionsupporting

confidence: 92%

Selective Removal of Acid Sites in Mordenite Zeolite by Trimethylchlorosilane Silylation to Improve Dimethyl Ether Carbonylation Stability

et al. 2022

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Catalysis research always pursues more efficient catalysts and realizes selectivity-controlled conversion. The local environment of acid sites in a zeolite is regarded as the vital reason for its catalytic selectivity in many chemical reactions. Herein, we have demonstrated that the acid sites in the 12-membered-ring (12-MR) channels of mordenite zeolite could be selectively covered by a trimethylchlorosilane (TMCS) silylation treatment, which could significantly improve the dimethyl ether (DME) carbonylation performance. Detailed mechanism studies by in situ DRIFT, 1H MAS NMR, and FTIR spectra analyses indicate that the TMCS species replace the Brønsted H+ in the bridging hydroxyl groups when chloro groups are rapidly hydrolyzed by the protons, accompanied by the formation of siloxane bonds. Due to the space limitation, the silylation reaction mainly occurred in the 12-MR channels and created most of the remaining acid sites (80%) in the 8-MR channels of the MOR zeolite, which gave better selectivity and a much longer lifetime in the DME carbonylation reaction. This work realizes a conceptual pathway to selectively control the distribution of acid sites within different confinement positions of zeolites to improve the catalytic selectivity of catalysts.

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“…In addition to the different confinement effects of MOR-12MR and MOR-8MR to ketene, the stronger acidic strength of BAS in MOR-8MR than that in MOR-12MR may be the other reason for the different protonation states of ketene. 43,44 The diverse behaviors of ketene and acylium ion in MOR-12MR and MOR-8MR differentiates the activity of the various channels toward MA and AcOH formation. The long life of ketene and its high mobility in MOR-12MR would facilitate the migration of ketene from the 12MR channel to the 8MR channel via the 10MR window of the side pocket and then protonation to the acylium ion, which is located at the side pocket with low mobility as illustrated in Figure S3.…”

Section: Introductionmentioning

confidence: 99%

Molecular Understanding of the Catalytic Consequence of Ketene Intermediates under Confinement

Chen

et al. 2021

J. Am. Chem. Soc.

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Neutral ketene is a crucial intermediate during zeolite carbonylation reactions. In this work, the roles of ketene and its derivates (viz., acylium ion and surface acetyl) associated with direct C−C bond coupling during the carbonylation reaction have been theoretically investigated under realistic reaction conditions and further validated by synchrotron radiation X-ray diffraction (SR-XRD) and Fourier transformed infrared (FT-IR) studies. It has been demonstrated that the zeolite confinement effect has significant influence on the formation, stability, and further transformation of ketene. Thus, the evolution and the role of reactive and inhibitive intermediates depend strongly on the framework structure and pore architecture of the zeolite catalysts. Inside side pockets of mordenite (MOR), rapid protonation of ketene occurs to form a metastable acylium ion exclusively, which is favorable toward methyl acetate (MA) and acetic acid (AcOH) formation. By contrast, in 12MR channels of MOR, a relatively longer lifetime was observed for ketene, which tends to accelerate deactivation of zeolite due to coke formation by the dimerization of ketene and further dissociation to diene and alkyne. Thus, we resolve, for the first time, a long-standing debate regarding the genuine role of ketene in zeolite catalysis. It is a paradigm to demonstrate the confinement effect on the formation, fate, and catalytic consequence of the active intermediates in zeolite catalysis.

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Acidity and Local Confinement Effect in Mordenite Probed by Solid-State NMR Spectroscopy

Cited by 18 publications

References 50 publications

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

Selective Removal of Acid Sites in Mordenite Zeolite by Trimethylchlorosilane Silylation to Improve Dimethyl Ether Carbonylation Stability

Molecular Understanding of the Catalytic Consequence of Ketene Intermediates under Confinement

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