Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

Abstract

We observed surprising difference in the FTIR (Fourier transform infrared) hydroxyl spectra of the structurally isomorphous, proton exchanged H-CHA and H-SAPO-34 molecular sieves when measured by transmission (TR) or diffuse reflectance (DRIFT) techniques. Experimental and density functional theory (DFT) based model evidence is presented in this paper to prove that the essential reason for this spectral difference is that DRIFT emphasizes the vibrations of surface hydroxyl sites. Vibrations of the bulk Brønsted acidic hydroxyls shift to higher frequencies when they become surface species, the IR beam is reflected from approximately the top ~15 to 20 Å thick layer of the particles, hence the proportion of surface related IR bands becomes significant compared to the bulk related ones in the DRIFT spectra while the opposite is valid for the TR spectra. We demonstrate that the surface hydroxyls are Brønsted acidic both on the H-CHA and the H-SAPO-34 particles and the upshifted vibrations noticed primarily in the DRIFT spectra are Al-OH vibrations on the surface even of H-SAPO-34, not P-OH groups as most researchers believe. We also show that the bulk Brønsted sites might involve HO1, HO2 and HO4 type hydroxyls associated with the known geometrically different oxygen positions on both molecular sieves, but only HO1 surface hydroxyls are associated with the red-shifted vibration intensified in the DRIFT spectra. Moreover, a single surface model cannot account for every vibration observed in DRIFT spectra. From the combination of IR vibrations of three adequate surface models one can as properly match the experimental DRIFT spectra as the TR spectra from the combination of the calculated bulk HO1…HO4 vibrations of these molsieve crystals.

Origin of Water-Induced Brønsted Acid Sites in Sn-BEA Zeolites

“…In further experiments we followed the transformation of water upon adsorption over Sn-BEA using infrared spectroscopy. In the present study, we used DRIFT spectroscopy because of its high sensitivity toward OH groups in the case of large zeolite particles, where conventional transmission FTIR suffers from a low signal-to-noise ratio …”

“…In the present study, we used DRIFT spectroscopy because of its high sensitivity toward OH groups in the case of large zeolite particles, where conventional transmission FTIR suffers from a low signal-to-noise ratio. 54 First, the surface site evolution through water adsorption on Sn-BEA and SnO 2 /Si-BEA samples at different temperatures was studied (Figure 5). The adsorption of small doses of water over Sn-BEA leads to immediate changes in the spectra within the range of OH vibrations (Figure 5a).…”

Water‐Induced Structural Dynamic Process in Molecular Sieves under Mild Hydrothermal Conditions: Ship‐in‐a‐Bottle Strategy for Acidity Identification and Catalyst Modification

“…3600 and 3627 cm À1 . [53] Herein, with the encapsulation of TMP to achieve sensitive probing, the slight differences of bridge hydroxy groups are revealed and fine identification is realized experimentally.…”

Understanding SO2 Poisoning over Different Copper Species of Cu-SAPO-34 Catalyst: A Periodic DFT Study

“…All of the supercell DFT calculations were carried out using CASTEP code in Materials Studio. , The PBE form for generalized gradient approximation (GGA) was used to optimize the periodic structure and to describe electron exchange and correlation. − We used ultrasoft pseudopotentials and a plane wave cutoff energy of 400 eV for core and valence states, respectively. The spin polarized and van der Waals (VDW) forces were considered in all calculations.…”