Effect of Al content on number and location of hydroxyl acid species in zeolites: a DRIFTS quantitative protocol without the need for molar extinction coefficients

Abstract

In this work, we have systematically investigated the effect of the Al content on the number and location of bridging hydroxyls (Brønsted acid sites) and terminal hydroxyls (terminal silanol sites) in ZSM-5 zeolites with varying silica-alumina ratio (SAR ¼ SiO 2 /Al 2 O 3 ) using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) without molar extinction coefficients. Two base probe molecules with different kinetic diameters, pyridine (5.7Å) and collidine (7.4Å), were used. The total content of Brønsted acid sites is obtained by elemental analysis of the proton form and sodium exchanged form of ZSM-5. To quantify the number of internal and external Brønsted acid sites the change in area of the Brønsted acid site peak in the O-H stretching region is then used; thus, the need for molar extinction coefficients to quantify Brønsted acid sites is thereby eliminated. The results reported here show that for ZSM-5 the number of external Brønsted sites, in absolute terms, increases with increasing total Al content, as evidenced by collidine adsorption. The number of terminal silanol acid sites was found to be proportional to the external surface area probed by argon sorption experiments. In summary, this work shows that the use of DRIFTS in conjunction with elemental analysis is a valid alternative to qualitatively and quantitatively probe the number and location of hydroxyl acid sites without the use of molar extinction coefficients and gives a comprehensive picture of the effect of Al content on number and location of hydroxyl acid species in ZSM-5 zeolites with varying SAR. The work contributes to advancing the range of characterisation protocols for solid acid catalysts.

Mesoporous H‐ZSM‐5 for the Conversion of Dimethyl Ether to Hydrocarbons

“…The ν (O – H) absorption bands involving more the aluminum sites can be found, for both measured zeolites, at ca. 3590 cm −1 for Brønsted acid sites Si‐O(H)‐Al and at 3663 cm −1 for so‐called extra‐framework aluminum species (EFAL) that behave more like Lewis acids in agreement with already documented studies . Interestingly these absorption bands are broader in the spectra originally recorded for the unloaded zeolites Z16D and A, suggesting the presence of water traces in the samples able to interact via H‐bonds with Lewis and Brønsted sites .…”

Lean and rich aging of a Cu/SSZ-13 catalyst for combined lean NOx trap (LNT) and selective catalytic reduction (SCR) concept

“…The degreened sample displayed negative bands at 3584 and 3608 cm −1 , which could be assigned to the Brønsted acid sites Al-OH-Si. [34][35][36][37] The bands at 3740 cm −1 corresponded to the terminal silanols (Si-OH) vibration. 34,35,37 The band at 3652 cm −1 was less clearly assigned.…”

“…[34][35][36][37] The bands at 3740 cm −1 corresponded to the terminal silanols (Si-OH) vibration. 34,35,37 The band at 3652 cm −1 was less clearly assigned. Aufdembrink et al reported the appearance of a peak at 3650 cm −1 after the activation at 550°C of an NH 4 + -exchanged chabazite zeolite.…”

Defining aluminum-zoning during synthesis of ZSM-5 zeolites

“…14,15 Recently, Bräuer et al observed that with a decrease of silicon-to-aluminum ratios in ZSM-5 crystals, the fraction of the external Brønsted sites decreases, which indicates that the distribution of aluminum over a crystal varies with the framework aluminum content. 16,17 By adopting different combinations of inorganic and organic structure-directing agents in ZSM-5 crystallization, Chawla et al found different (homogenous and heterogeneous) aluminum distributions over crystals. 18 These results suggest that the distribution of aluminum over a crystal depends on multiple factors.…”