Influence of the nitric acid treatment on Al removal, framework composition and acidity of BEA zeolite investigated by XRD, FTIR and NMR

摘要

Two different procedures were used to prepare dealuminated BEA zeolite. The first procedure realized in two steps consisted of first calcination of original tetraethylammonium (TEA) BEA(17) zeolite in air at 823 K for 15 h to remove organic template and then treatment of as resulted organic-free HA1BEA(17) zeolite with concentrated nitric acid solution for different times to remove part or all Al atoms. Two periods was distinguished in the removal of Al atoms from HA1BEA(17) framework during the first procedure. During initial period (up to 0.08 h), the dealumination was very fast and 94% of Al was removed. This period corresponded to the easy removal of extra framework octahedral and framework tetrahedral aluminum from T3-T9 sites. In the second period (from 0.08 to 4 h), the dealumination was much slower. It corresponded to a much more difficult removal of framework tetrahedral aluminum from Tl and T2 sites. The second procedure realized in one step consisted of direct treatment of original TEABEA zeolite with concentrated nitric acid solution to remove simultaneously organic template and Al atoms. To obtain partly or totally dealuminated BEA zeolite the treatment time was 0.08 and 4 h respectively. The influence of the nitric acid treatment on Al removal, framework composition and acidity of BEA zeolite were investigated by XRD, FTIR and NMR. 2D ~(27)Al 3Q MAS NMR allowed distinguishing two kinds of framework tetrahedral Al atoms (Al_(Td)(2a) and Al_(Td)(2b)) in BEA zeolite which relative amounts depended on the Si/Al ratio. The Bronsted and Lewis acidic centers were evidenced in dealuminated BEA zeolites by FTIR spectra of adsorbed pyridine and CO use as probe molecules. Their relative amounts depended on the degree of dealumination and procedure used for removal Al atoms.