Hm. Kao et Gp. Grey, 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/, Journal of physical chemistry, 100(12), 1996, pp. 5105-5117
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 adsorp
tion of monomethylamine (MMA) on dehydrated zeolite HY. Monomethylammo
nium cations (MMAH(+)) are formed, which at temperatures below -40 deg
rees C are rigidly bound to the zeolite framework. At loading levels o
f MMA that exceed the number of Bronsted acid protons, H-1 resonances
intermediate in chemical shift between those of MMA and MMAH(+) are ob
served from species undergoing rapid proton transfer reactions between
MMA and MMAH(+). These exchange processes are frozen out at -140 degr
ees C, and MMAH(+) and physisorbed MMA are observed. The deprotonation
of the Bronsted acid SIO(H)Al site, and the consequent formation of M
MAH(+), results in a reduction of the Al-27 quadrupole coupling consta
nt (QCC) of the nearby aluminum atom by more than 12 MHz, and a QCC fo
r this site of 3.0 (+/-0.3) MHz is determined. Rehydration of HY and t
hen subsequent calcination at 400 degrees C results in the formation o
f 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 w
ith N-15 and H-1 MAS NMR. Significant mobility of the MMA species, and
proton transfer reactions between MMAH(+) and MMA coordinated to a Le
wis acid site, was detected in these samples at ambient temperatures;
these processes were frozen out at -150 degrees C, and discrete resona
nces from MMAH(+), a Lewis acid-MMA complex, and physisorbed MMA were
observed. N-15/Al-27 TRAPDOR NMR demonstrates that the MMA in the Lewi
s 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.