Local Structure of Framework Aluminum in Zeolite H−ZSM-5 during Conversion of Methanol Investigated by In Situ NMR Spectroscopy

Abstract

In situ MAS NMR spectroscopy under continuous-flow conditions and spin−echo NMR experiments have been applied to investigate the quadrupolar interactions of framework aluminum atoms in zeolite H−ZSM-5 at elevated temperatures and during conversion of methanol. The strength of the 27Al quadrupolar interactions of framework aluminum atoms in zeolites depends on the electric field gradient caused by the charge distribution in the local structure of the aluminum sites. Adsorption of methanol on bridging OH groups at 295 K under flow conditions leads to weak 27Al quadrupolar interactions, corresponding to a quadrupole coupling constant of QCC = 4.4 MHz. After the temperature was raised to 573 K during methanol conversion, a strong decrease in the 27Al MAS NMR signal of framework aluminum at 54 ppm occurred due to a significant increase in the 27Al quadrupolar interactions. Comparison of 27Al echo NMR spectra of zeolite H−ZSM-5 loaded with methanol and dimethyl ether and data given in the literature showed that this increase in the 27Al quadrupolar interactions is caused by the formation of dimethyl ether (QCC = 11.2 MHz) and methoxy groups (QCC = 16.2 MHz) at bridging OH groups. Experiments performed during purging zeolite H−ZSM-5 with dry nitrogen at 573 K indicated a high hydroxyl proton mobility which has, however, no influence on the local structure of framework aluminum atoms.

Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity

“…Additional spectroscopic evidence in favor of side-chain methylation on H-ZSM-5 catalysts has been obtained by Hunger and co-workers, [89] who also have employed in situ NMR to examine several aspects of MTH chemistry. [90][91][92][93][94] It should also be noted that a lower homologue of the heptamethylbenzenium ion, the gemdimethyl isomer of the pentamethylbenzenium ion, has been observed in zeolite H-ZSM-5. [95]…”

Umwandlung von Methanol in Kohlenwasserstoffe: Wie Zeolith‐Hohlräume und Porengröße die Produktselektivität bestimmen

“…Dabei wurde die Reaktivität von verschiedenen Polymethylbenzolen und Butylbenzolisomeren untersucht. [90][91][92][93][94] Ferner sollte angeführt werden, dass ein hçheres Homolog des Heptamethylbenzeniumions, das gemi-nale Dimethylisomer des Pentamethylbenzeniumions, im H-ZSM-5-Zeolithen experimentell nachgewiesen wurde. [78][79][80][81] Bjørgen et al identifizierten das Heptamethylbenzenium-Kation als wichtiges Zwischenprodukt, indem sie die Reaktivität von Polymethylbenzolen über dem großporigen H-beta untersuchten.…”

“…[87,88] Die Alternative zum Abschälmodell ist das von Mole und Mitarbeitern [61,62] vorgeschlagene und später von Haw et al verfeinerte [76,77] [89] die sich ebenfalls der In-situ-NMR-Methode bedienten, um verschiedene Aspekte der MTH-Chemie zu untersuchen. [90][91][92][93][94] Ferner sollte angeführt werden, dass ein hçheres Homolog des Heptamethylbenzeniumions, das gemi-nale Dimethylisomer des Pentamethylbenzeniumions, im H-ZSM-5-Zeolithen experimentell nachgewiesen wurde. [95] 3.2.…”

Pervaporation dehydration of aqueous ethanol solution using H-ZSM-5 filled chitosan membranes

“…The most important acid-type center is the bridged framework formed by hydroxyl groups with Si-OH-Al configuration, which ensures the high acid strength of the H-ZSM-5 [14]. Due to the contribution of framework Al atoms to the hydroxyl groups of the Si-OH-Al configuration, the ratio of Si atoms to Al atoms will significantly influence the hydrophilicity of H-ZSM-5 [15]. In addition, H-ZSM-5 possesses the MFI-type structure with two kinds of channel structures.…”