Impact of Mineralizing Agents on Aluminum Distribution and Acidity of ZSM-5 Zeolites

Abstract

Intensive research on improving the catalytic properties of zeolites is focused on modulating their acidity and the distribution of associated Al sites. Herein, by studying a series of ZSM-5 zeolites over a broad range of Al content, we demonstrate how the nature of the mineralizing agent (F^- or OH^- ) used in hydrothermal syntheses directly impacts Al sites distribution. The proportions of Al sites, probed by ²⁷ Al NMR, depend on the Si/Al ratio for F^- , but remain identical for OH^- (from Si/Al=30 to 760). This leads to contrasting variations in weak and strong acidities. Such opposite effect of mineralizers is explained by the spatial location of negative charges and the resulting balance between short- and long-range electrostatic interactions. This understanding paves the way for additional and simple opportunities to control zeolites' acidity.

Keywords: Acidity; Electrostatic Interactions; NMR Spectroscopy; Temperature Programmed Desorption; Zeolites.

Figure 1

Variations in Al sites distributions probed by ²⁷Al NMR. (A–B) ²⁷Al spectra of TPA‐F‐[Al]‐ZSM‐5 (A) and TPA‐OH‐[Al]‐ZSM‐5 (B) zeolites obtained with F⁻ or OH⁻ anions as mineralizing agents, for three different Si/Al molar ratios (B ₀=14.1 T, ν _MAS=18 kHz). The onsets present overlapped comparisons. (C–D) Variations of Al sites percentages—A sites (○), B sites (Δ) and C sites (□)—as a function of Si/Al for TPA‐F/OH‐[Al]‐ZSM‐5 zeolites. Data is obtained by fitting ²⁷Al NMR spectra recorded at 14.1 T. (A) F⁻ route: symbols filled in grey correspond to Al(NO₃)₃⋅9 H₂O as Al precursor, others to Al(OH)₃. (B) OH⁻ route: symbols filled in grey correspond to Al(NO₃)₃⋅9 H₂O and tetraethoxysilane (TEOS) as Al and Si precursors, symbols filled in black to Al[OCH(CH₃)₂]₃ and fumed silica precursors, other symbols to Al[OCH(CH₃)₂]₃ and TEOS precursors. Dashed lines, obtained through a linear regression, are only guide for eyes.

Figure 2

NH₃‐TPD thermograms (rate=10 °C min⁻¹) of calcined H‐[Al]‐ZSM‐5 zeolites obtained with F⁻ and OH⁻ mineralizing agents (A and B respectively). Signal intensities are normalized over sample weight. The ratio w.a./s.a. refers to the ratio between weak and strong acidities expressed as NH₃ concentration. Colors are only used for visualization's purposes.

Figure 3

Al‐TPA spatial proximities. (A) ¹H{²⁷Al} D‐HMQC of a TPA‐F‐[Al]‐ZSM‐5 zeolite (Si/Al=30, T≈300 K, B ₀=20.0 T, ν _MAS=100 kHz). The spectrum is transposed for an easier comparison. (B) ²⁷Al{¹H} REDOR plots of TPA‐F‐[Al]‐ZSM‐5 zeolite with selective H_α pulses (Si/Al=30, T≈300 K, B ₀=20.0 T, ν _MAS=100 kHz). (C) ²⁷Al{³¹P} REDOR plots of an as‐synthesized zeolite obtained using TPP and a F⁻ route (Si/Al=27, T≈243 K, B ₀=14.1 T, ν _MAS=15 kHz). The dotted lines correspond to the REDOR curves for spin pairs with first and second order approximation that better model the initial build‐ups for A sites. See Supporting Information 7 for more details on these analyses.

Figure 4

Schematic visualization of the distributions of the positive charges coming from TPA (represented as point charges at N positions, blue circles), the negative charges brought by the mineralizing anions F⁻ and OH⁻, and the favored siting positions of Al in ZSM‐5 zeolites at high Si/Al. (A) F⁻ route: F atoms (green dots) are located in the [4¹5²6²] cages, favored Al atom is sitting at the T1 site (red dot), close to N and to the crystallographic position of F. (B) OH⁻ route: silanolates are distributed around TPA (SiO⁻, green spheres), Al atoms can be located in large areas (red sphere). The black lines represent the SiO bonds of the silica MFI framework projected onto the ac plane.