Two Kinds of Framework Al Sites Studied in BEA Zeolite by X-ray Diffraction, Fourier Transform Infrared Spectroscopy, NMR Techniques, and V Probe

Abstract

The dealumination of BEA zeolite by treatment with concentrated nitric acid is evidenced by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. Two-dimensional 27Al 3Q and 5Q magic-angle-spinning (MAS) NMR allow the detection of two kinds of tetrahedral Al atoms whose relative amounts depend on the Si/Al ratio and which correspond to two specific T-sites. 29Si MAS NMR and 1H MAS NMR measurements confirm these results. 29Si MAS NMR spectra evidence two resonances at around −114 and −111 ppm ascribed to Si sites of the Si(OSi)4 environment of two different crystallographic sites. Moreover, the presence of Si atoms associated with hydroxyl groups is confirmed by a resonance at −102 ppm when 1H−29Si CP is applied. The Brønsted and Lewis acidic sites in dealuminated BEA zeolites are evidenced by FTIR spectra of adsorbed pyridine which show two kinds of bridging hydroxyl groups (Si−O(H)−Al) of different acid strength. 51V MAS NMR confirms the incorporation of vanadium atoms into vacant T-atom sites of a fully dealuminated SiBEA zeolite leading to two kinds of tetrahedral V(V) sites (δiso = – 708 and – 766 ppm), with a V=O double bond and linked by V−OSi bonds to the framework. The two types of tetrahedral V(V) sites are in line with the two kinds of tetrahedral Al sites initially present in the zeolite. Moreover, the two bands at 3620 and 3645 cm−1 suggest that VSiBEA also contains V(V) sites with V(V)O−H groups, which exhibit Brønsted acidic character as shown by FTIR of adsorbed pyridine. Possible ways for the formation of tetrahedral V(V) in the BEA structure are proposed.

One-pot conversion of furfural to useful bio-products in the presence of a Sn,Al-containing zeolite beta catalyst prepared via post-synthesis routes

“…The crystalline structure was preserved during the acid treatments, which is in agreement with the literature for HNO 3 -treated beta materials [83,[85][86][87][88][89]. The SSIE process did not lead to measurable changes in the crystalline structure of the materials deAl-betan.…”

“…Powder X-ray diffraction (XRD) patterns of the dealuminated materials (deAl-betan) are similar to that of H-beta (Figure 1), displaying reflections characteristic of the crystalline structure with BEA topology [41,[82][83][84]. The crystalline structure was preserved during the acid treatments, which is in agreement with the literature for HNO 3 -treated beta materials [83,[85][86][87][88][89].…”

Performance of Cu-ZSM-5 in a Coupled Monolith NSR-SCR System for NOx Removal in Lean-Burn Engine Exhaust

“…Then, three different regions can be observed in the UV-VIS spectra for the three SCR catalysts. The presence of different exchanged Cu(II) cations is consequence of the existence of different kinds of framework sites in ZSM-5 zeolites [23,24] and according to our previous results, three different Cu 2? species were also detected by X-Ray photoelectronic spectroscopy [15].…”

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

“…Thus, the removal of framework Al atoms leads to change of acidity and catalytic activity of zeolites. So, the zeolite dealumination has therefore been a matter of a lot of work [13][14][15][16][17][18][19][20][21]. There are several factors that influence the zeolite dealumination process such as zeolite structure, synthesis conditions and dealumination method used.…”