Investigation of the Strong Brønsted Acidity in a Novel SAPO-type Molecular Sieve, DNL-6

Abstract

By applying analysis of multiple solid-state MAS NMR spectra, the atomic coordination environment and acidity of a novel SAPO-type molecular sieve, DNL-6, are studied. 27Al MQ-MAS NMR and 31P27Al MQ-HETCOR NMR spectra reveal the existence of a SiAlSi region with Al(OSi)n (n = 14) species in the framework of DNL-6, explaining well the high concentration of single Si(OAl)4 species (3 mmol/g) accommodated in the sample. 13C MAS NMR of 2-13C-acetone adsorption indicates that there exist two kinds of strong Bronsted acid sites in DNL-6. One of them has similar strength as those in HZSM-5 and the other is even stronger, which is unusual in SAPO-type molecular sieves. Also, a discrepancy in Bronsted acid concentration between the theoretical and 1H NMR experimental results has been found, revealing the occurrence of a dehydoxylation process during the calcination. The origin of the extremely strong Bronsted acid sites in DNL-6 is investigated by density functional theory calculations, which suggest that the dehydroxylation process may lead to a local structure deformation and remarkably enhance the Bronsted acidity. More importantly, DNL-6 exhibits excellent catalytic activity in the synthesis of methylamines due to its stronger acidity.