Hierarchization of USY Zeolite by NH₄OH. A Postsynthetic Process Investigated by NMR and XRD

Abstract

Ammonia treatment of USY zeolite has led to a new hierarchical material. The local and global structural changes during the transformation have been monitored by XRD and by 1 H, 29 Si, and 27 Al solid-state NMR. A wealth of 1D and 2D NMR protocols were applied, including 1 H DQ-SQ, 27 Al MQMAS, 29 Si MAS and CPMAS as well as 1 H-29 Si HETCOR. The effects of aqueous ammonia treatment, different thermal post-treatments and rehydration were studied. An increasing loss of crystallinity was observed upon increasing duration of ammonia treatment. Under the experimental conditions a few percent of silica was lost into solution and no loss of aluminum was observed. But, increasing numbers of silanol groups were detected. The progressive transformation induces formation of mesopores, reduction of the fraction of the sample exhibiting Bragg crystallinity and apparition of a dense, amorphous aluminosilicate phase. The latter contains ammonium ions and strongly bound water which both are resistant to thermal decomposition up to 350 °C. After about 24 hours of treatment the zeolite fraction has been completely transformed into the amorphous phase. At intermediate stages a complexhierarchical material is obtained with mesopores and zeolitic micropores next to a dense amorphous aluminosilicate, containing ammonium ions, highly structured water and silanols nests.

Catalyst Design by NH4OH Treatment of USY Zeolite

“…[50][51][52] Multiple-quantum MAS NMR experiments confi rmed its tetrahedral coordination. [ 30 ] The occurrence of distorted tetrahedral aluminum as Lewis acid sites has been proposed earlier in a publication on hydrothermally dealuminated Y zeolites. [ 52 ] Also partial reintegration of leached Al into the zeolitic framework, leading to a broadening of the framework Al MAS NMR signal, was reported to create Lewis acid sites.…”

“…These results confi rm that the NH 4 OH treatment transforms the original USY material into a less ordered structure with a high condensation state (Q 4 ), containing silanol groups. [ 30 ] These silanol groups were also characterized by FTIR spectroscopy of the OH region ( Figure S5, Supporting Information). As H-USY(40) is obtained via severe dealumination, a signifi cant amount of lattice terminating silanol groups is present at 3745 cm −1 .…”

“…[ 29 ] An X-ray diffraction (XRD) and magicangle spinning nuclear magnetic resonance (MAS NMR) study has been performed on these materials, revealing the partial and gradual formation of amorphous phases containing silanol nests, strongly bound ammonium and highly structured water molecules in close interaction with each other. [ 30 ] However, the functionality of the resulting materials was never assessed.…”

Preparation and characterization of hierarchical USY by post-treatment

“…The resonance peak at 60 ppm is assigned to framework tetra-coordinated Al (Al(IV) a ). The tail resonance peak at *50 ppm is marked as Al(IV) b , but its assignment is unclear [16,[25][26][27][28]: (1) distorted tetra-coordinated framework Al atoms, (2) penta-coordinated aluminum species, and (3) non-framework four-coordinated Al species. The resonance peak at *35 ppm is attributed to nonframework penta-coordinated Al species (Al(V)).…”

“…Nanosized zeolite and hierarchical (i.e., micro-, mesoporous) materials can substantially increase the external surface area and shorten the diffusion path length in micropores [7,8]. Therefore, various strategies have been developed to obtain extra-large-pore zeolites [9,10], nanosized zeolites [11][12][13] and hierarchical zeolites [14][15][16]. For Y zeolite, two approaches have been developed to obtain hierarchical structures, involving direct synthesis using dual templates [17] and post-treatment of Y zeolite by desilication or dealumination using acid or alkali.…”