Probing the Brønsted and Lewis acidity of zeolite HY: A 1H/27Al and 15N/27Al TRAPDOR NMR study of monomethylamine adsorbed on HY

Hsien Ming Kao, Clare P. Grey

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Abstract

A combination of variable temperature 1H, 15N, and 27Al MAS NMR and 1H/27Al and 15N/27Al TRAPDOR NMR has been used to study the adsorption of monomethylamine (MMA) on dehydrated zeolite HY. Monomethylammonium cations (MMAH+) are formed, which at temperatures below -40°C are rigidly bound to the zeolite framework. At loading levels of MMA that exceed the number of Brønsted acid protons, 1H resonances intermediate in chemical shift between those of MMA and MMAH+ are observed from species undergoing rapid proton transfer reactions between MMA and MMAH+. These exchange processes are frozen out at -140°C, and MMAH+ and physisorbed MMA are observed. The deprotonation of the Brønsted acid SiO(H)Al site, and the consequent formation of MMAH+, results in a reduction of the 27Al quadrupole coupling constant (QCC) of the nearby aluminum atom by more than 12 MHz, and a QCC for this site of 3.0 (±0.3) MHz is determined. Rehydration of HY and then subsequent calcination at 400°C results in the formation of silanols, two aluminum hydroxide species, and Lewis acid sites. The aluminum hydroxide species with associated 1H resonances at 2.8 and 0.9 ppm have 27Al QCCs of ≥ 11.7 and 10.0 (±0.8) MHz, respectively. MMA bound to the Lewis acid site was observed with 15N and 1H MAS NMR. Significant mobility of the MMA species, and proton transfer reactions between MMAH+ and MMA coordinated to a Lewis acid site, was detected in these samples at ambient temperatures; these processes were frozen out at -150°C, and discrete resonances from MMAH+, a Lewis acid-MMA complex, and physisorbed MMA were observed. 15N/27Al TRAPDOR NMR demonstrates that the MMA in the Lewis acid-MMA complex is directly bound to an aluminum atom at the Lewis acid site, with a 27Al QCC of 8.3 (±0.3) MHz. The Al-N internuclear distance was estimated to be shorter than 2.7 Å.

Original languageEnglish
Pages (from-to)5105-5117
Number of pages13
JournalJournal of Physical Chemistry
Volume100
Issue number12
DOIs
StatePublished - 21 Mar 1996

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