Time-Optimized ¹⁵N qNMR Determination of Lewis and Brønsted Site Concentrations and Chemical Bonding of Pyridine at Brønsted Sites in Amorphous Aluminum Hydroxide Fluoride

Abstract

Time-optimized 15N quantitative NMR (qNMR) is described for the precise determination of Lewis and Brønsted site concentrations per gram catalyst, enabling a measurement time reduction of more than a factor of 10. The proof-of-principle is demonstrated for amorphous aluminum hydroxide fluorides with their tunable Lewis to Brønsted functionalities and relevance to highly active and selective heterogeneous catalysis using pyridine as a widely accepted probe molecule for Lewis and Brønsted sites. Two different Lewis centers were found, and reliable site concentrations down to the micromole per gram region can be determined. Furthermore, 1H{15N} REDOR NMR for the acid proton at the Brønsted site yields a distance of the 15N atom of bonded pyridine to this acid proton of (0.112 ± 0.002) nm, such that fast motion/diffusion of the Brønsted site-bonded pyridine at the surface can be excluded.

Comparison of acidic site quantification methods for a series of nanoscopic aluminum hydroxide fluorides

“…The resonances at À120 and À180 ppm can be assigned to LS and BS, respectively. 24,27 The positions of the signals are the same for all samples in NMR and FTIR spectra suggesting that the acidic sites are almost of the same strength in all samples.…”

“…In order to avoid the temperature treatment during the quantication of acidic sites, other methods including titration methods as catalyst poisoning 13,14 and microcalorimetry, 15,16 Xray photoelectron, 17,18 Fourier Transformed Infrared (FTIR) [19][20][21][22][23] or Nuclear Magnetic Resonance (NMR) [24][25][26][27][28] spectroscopy have been used to determine numbers of Lewis and Brønsted acidic sites in solids using different probe molecules. 4,5 Especially solid state NMR is a unique technique for the characterization of solid acids and excellent reviews have been published from Brunner and Pfeifer, 24 Jiang et al 25 and Zheng et al 26 Pyridine is an oen used probe molecule in FTIR as well as in solid state NMR spectroscopy because it can be used qualitatively to distinguish Lewis and Brønsted sites and also quantitatively to determine concentrations of acidic sites.…”

“…37 Aluminum hydroxide uorides are mesoporous materials, which exhibit high surface areas and particle sizes in the nanoscale range. 11 Previous studies using 27 Al und 19 F NMR 11,12 had shown that samples synthesized with a stoichiometric amount of HF (F : Al ¼ 3) consist of aluminum atoms that are octahedrally coordinated by mostly uorine with a small amount of OH groups which are introduced due to the presence of different amount of water in the HF. Thus, by varying the water content of the HF employed in the synthesis the F to OH ratio can be tuned, and consequently also the ratio of Lewis to Brønsted sites changes.…”

Comparative study of the strongest solid Lewis acids known: ACF andHS-AlF3

“…In recent years, it has been demonstrated that solid-state 15 N NMR spectroscopy is a reliable method for the quantification of Lewis and Brønsted acid centers. 11,47 The adsorption of 15 Npyridine onto the matrix of a solid material such as, for example, ACF and HS-AlF 3 will form adducts that feature specific 15 N isotropic chemical shifts (δ iso ). The 15 N δ iso of pyridine and pyridinium adsorbed at aluminium chlorofluoride are −125 ppm and −178 ppm, respectively, typical chemical shift regions that can be used for the assessment of specific pyridine adducts that can be used for the characterization and quantification of Lewis versus Brønsted acid centers ( Fig.…”

“…This technique uses also probe molecules such as pyridine, phosphines or trialkylphosphine oxides, which interact with the Lewis and Brønsted sites of the solid material providing information which complements that obtained by vibrational spectroscopy. [11][12][13][14][15] ACF and HS-AlF 3 are very strong Lewis acids, equal or even stronger than SbF 5 in some cases. Well-known synthesis procedures make these materials available as heterogeneous catalysts for a large variety of reactions.…”