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

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.