Proton Mobility, Intrinsic Acid Strength, and Acid Site Location in Zeolites Revealed by Varying Temperature Infrared Spectroscopy and Density Functional Theory Studies

Abstract

The intrinsic Brønsted acid strength in solid acids relates to the energy required to separate a proton from a conjugate base, for example a negatively charged zeolite framework. The reliable characterization of zeolites' intrinsic acidity is fundamental to the understanding of acid catalysis and setting in relation solid Brønsted acids with their activity and selectivity. Here, we report an infrared spectroscopic study with partial isotopic deuterium exchange of a series of 15 different acidic aluminosilicate materials, including ZSM-5 zeolites with very few defects. Varying Temperature Infrared spectroscopy (VTIR) permitted estimating activation energies for proton diffusion. Two different proton transfer mechanisms have been distinguished for two different temperature ranges. Si-rich zeolites appeared to be promising proton-transfer materials (E act. < 40 kJ mol −1 ) at temperatures above 150 °C (423 K). Further, a linear bathochromic shift of the Si−(OD)−Al stretching vibration as a function of temperature was observed. It can be assumed that this red-shift is related to the intrinsic O−(H/D) bond strength. This observation allowed the extrapolation and estimation of precise v(O−D)@0 K values, which could be attributed to distinct crystallographic locations through Density Functional Theory (DFT) calculations. The developed method was used to reliably determine the likelihood of the position of a proton in ZSM-5 zeolites under catalytically relevant conditions (T > 423 K), which has so far never been achieved by any other technique.

Understanding the Fundamentals of Microporosity Upgrading in Zeolites: Increasing Diffusion and Catalytic Performances

“…An atom‐pairwise D2 correction of Grimme [ 34 , 35 ] was used for this purpose, as applied previously to calculate a reasonable structure and energetics for zeolites. [ 36 , 37 , 38 ] The atomic positions were optimized until all forces were smaller than 0.005 eV Å −1 per atom. A primitive rhombohedral cell of FAU with two supercages containing total of 144 atoms was used for the calculations (Figure S8 , Supporting Information).…”

Studying Proton Mobility in Zeolites by Varying Temperature Infrared Spectroscopy

“…Our experimental setup, as explained in further detail in the experimental part, is based on a DRIFT cell. Experimental details concerning the used setup have also been reported in our previous work [24]. In short, all samples were activated in the cell under dry N 2 flow at 250 °C before the H 2 O:D 2 O (1:1) mixture (formal HDO) with a partial pressure of 3.16 kPa, namely, H 2 O vapor pressure at 25 °C in N 2 flow (~10 mL min −1 ) was fed into the cell.…”

“…Here, we present in detail our recently reported model relying on an isotopic H/D exchange infrared spectroscopic study with temperature variation (VTIR) [24]. The partial H/D exchange allowed for the use of an adapted VTIR model to estimate proton transfer activation energies in different temperature ranges (ambient −150 °C and 150–250 °C).…”

Theoretical Study of Zirconium Isomorphous Substitution into Zeolite Frameworks

“…Elements of different groups (III, IV, V, VI, and VII) have been introduced into zeolite frameworks. It has been shown that the local coordination environment around the positions where metal ions are incorporated have a great effect on the structure and acidity of the zeolites, and therefore on the catalytic reactivity [3][4][5][6][7][8][9]. Therefore, it is important to investigate the relationships among location, stability, and structural properties of different heteroatoms in frameworks [1].…”