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

A combination of variable temperature H-1,N-15, and Al-27 MAS NMR and H-1/Al-27 and N-15/Al-27 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 degrees C are rigidly bound to the zeolite framework. At loading levels of MMA that exceed the number of Bronsted acid protons, H-1 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 degrees C, and MMAH(+) and physisorbed MMA are observed. The deprotonation of the Bronsted acid SIO(H)Al site, and the consequent formation of MMAH(+), results in a reduction of the Al-27 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 degrees C results in the formation of silanols, two aluminum hydroxide species, and Lewis acid sites. The aluminum hydroxide species with associated H-1 resonances at 2.8 and 0.9 ppm have Al-27 QCCs of greater than or equal to 11.7 and 10.0 (+/-0.8) MHz, respectively. MMA bound to the Lewis acid site was observed with N-15 and H-1 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 degrees C, and discrete resonances from MMAH(+), a Lewis acid-MMA complex, and physisorbed MMA were observed. N-15/Al-27 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 Al-27 QCC of 8.3 (+/-0.3) MHz. The Al-N internuclear distance was estimated to be shorter than 2.7 Angstrom.