TY - JOUR A1 - Hemmann, F. A1 - Scholz, G. A1 - Scheurell, K. A1 - Kemnitz, E. A1 - Jäger, Christian T1 - Time-optimized 15N qNMR determination of Lewis and Brønsted site concentrations and chemical bonding of pyridine at Brønsted sites in amorphous aluminium hydroxide fluoride JF - The journal of physical chemistry / C N2 - 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. KW - Catalysis KW - Lewis/Brønsted acid sites KW - Acid site concentration KW - Pyridine KW - Quantitative NMR KW - 15N KW - REDOR PY - 2012 DO - https://doi.org/10.1021/jp212045w SN - 1932-7447 SN - 1089-5639 VL - 116 IS - 19 SP - 10580 EP - 10585 PB - Soc. CY - Washington, DC AN - OPUS4-27724 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Hemmann, Felix A1 - Agirrezabal-Telleria, Iker A1 - Kemnitz, E. A1 - Jäger, Christian T1 - Probing slow chemical exchange of pyridine molecules at acid magnesium hydroxide fluoride surfaces by 15N NMR JF - The journal of physical chemistry / C N2 - 15N NMR of pyridine has been used to study Lewis and Brønsted sites at the acid surface of magnesium hydroxide fluoride at two different pyridine loadings with ratios of excess pyridine compared to pyridine molecules bound to Lewis and Brønsted sites of about 1:1 (highly loaded) and 0.15:1 (low loaded), respectively. The Lewis and Brønsted sites can be distinguished by the 15N chemical shift of pyridine at about –100 ppm (LPy) and –175 ppm (BPy). In both samples a very broad 15N resonance is found around –88 ppm assigned to weakly adsorbed (excess) pyridine molecules via hydrogen bridges (HPy). Additionally, another signal at about –49 ppm is observed in the highly loaded sample only representing physisorbed pyridine (PPy). A slow chemical exchange process of strongly bound LPy and BPy molecules with excess HPy and PPy molecules can be monitored using 15N exchange NMR. It takes place on a time scale of about 50–100 ms at room temperature. All pyridine molecules have the same 15N T1 and its value increases from 800 ms (highly loaded sample) to 5.3 s (low loading) hinting on changed pyridine mobility depending on the loading level. KW - Catalysis KW - Acid surfaces KW - 15N NMR KW - Chemical exchange KW - Pyridine PY - 2014 DO - https://doi.org/10.1021/jp405213x SN - 1932-7447 SN - 1089-5639 VL - 117 IS - 28 SP - 14710 EP - 14716 PB - Soc. CY - Washington, DC AN - OPUS4-30439 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -