2,6-Di-tert-butylpyridine Sorption Approach to Quantify the External Acidity in Hierarchical Zeolites

Abstract

This work was aimed to evidence that substituted pyridine, 2,6-ditert-butylpyridine, is a suitable probe for the quantitative investigation of the external acidity in hierarchically structured zeolites. The 2,6-di-tert-butylpyridine was too large to enter the micropores, even in wide pore zeolites, and nearly no sites in nonmesoporous zeolites were available. Accessibility studies of acid sites in zeolites TNU-9 and BEA involving quantitative IR measurements with hindered 2,6-di-tertbutylpyridine as a probe were performed. The extinction coefficients of the 1615 cm −1 diagnostic bands of 2,6-di-tert-butylpyridine interacting with Brønsted acid sites were determined. Lewis acid sites were not detected with the probe. The accessibility factor (AF) for the 2,6-di-tert-butylpyridine probe molecule was defined as the number of sites detected by adsorption of the dTBPy (external sites) divided by the total amount of acid sites in the studied zeolites as quantified by pyridine sorption. Upon desilication resulting in the fabrication of the secondary mesopores, the enhanced accessibility of the protonic sites was observed. In comparison to the mesoporous zeolites with the secondary system of mesopores generated by alkaline leaching, considerably higher accessibility of protonic sites was evidenced in both ultrathin ZSM-5 and delaminated ITQ-2 zeolite.

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

“…FTIR spectroscopy of adsorbed probe molecules is a powerful tool for systematic studies of acidity of hierarchical zeolites providing information on the influence of desilication conditions on location, content, acid strength and accessibility of both Brønsted and Lewis sites. [21][22][23] In this study, CO was used to assess the acidic properties of the hierarchical zeolites and a larger basic molecule such as 2,4,6-trimethylpyridine (collidine), which cannot assess the micropores, was used to get information on the enhanced accessibility to the active sites. The hierarchical H-ZSM-5 zeolites were evaluated in the industrially relevant acid-catalyzed Beckmann rearrangement of cyclohexanone oxime to -caprolactam in the liquid-phase and at low temperatures, specifically to avoid the formation of large molecular weight condensation products and other by-products that are generated in the conventional gas-phase process.…”

Mesoporous Silica Scaffolds as Precursor to Drive the Formation of Hierarchical SAPO‐34 with Tunable Acid Properties

“…In order to discriminate the Brønsted acid sites located inside the micropores from those present on the mesopore surface, probe molecules of significantly larger kinetic diameter were employed. Substituted pyridines are appropriate probe molecules as their inherent basicity and steric bulk (which restricts diffusion through small pore apertures) can be exploited in qualitative and quantitative analysis of acid site accessibility ,,…”

“…To gain further insight into the fraction of accessible Brønsted sites in HierSAPO‐34, other strongly‐basic probe molecules, of different kinetic diameter were deployed. In particular, NH 3 was used to monitor the total Brønsted acid sites, as it readily enters both the micropores and mesopores of the HierSAPO‐34 framework,, whereas pyridine (Py), with a kinetic diameter of 0.54 nm, and 2,6‐di‐ tert ‐butylpyridine (2,6‐dTBP), with a kinetic diameter of 1.05 nm, cannot enter the micropores and can therefore probe only the more accessible acid sites . The FTIR difference spectra of NH 3 (Figure A), pyridine (Figure B) and 2,6‐dTBP (Figure C) adsorbed on HierSAPO‐34 at room temperature are reported in the low frequency region where the signals of protonated species are observed.…”

Effect of Structural, Size, and Acid Characteristics of Hierarchical BEA and MOR Zeolites on Their Activity in the Catalytic Reduction of N2O and no by Propylene

“…The adsorption of pyridine and 2,6-di-tert-butylpyridine was carried out for 15 min at 150°C, while the desorption was carried out until reaching less than 1 Pa residual equilibrium pressure while stepwise increasing the evacuation temperature. The IR spectra were recorded at room temperature and the concentration of acid sites was calculated according to the procedure of Gora-Marek et al [19].…”

“…We should thus note that the Lewis acid site concentration determined relative to the adsorption of 2,6-di-tert-butylpyridine is highly underestimated and cannot reflect the actual concentration of these sites. This finding is attributed to inaccessibility of a portion of the Lewis acid sites located in the segments with high surface curvature due to shielding of the unshared nitrogen electron pair by the bulky di-tert-butyl substituents [19].…”