Increasing the Brønsted Acidity of Flame‐Derived Silica/Alumina up to Zeolitic Strength

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts

“…[99], indicate the presence of Al 2 O 3 phases in mesoporous aluminosilicates. However, AlOH groups of Al 2 O 3 phases cannot be responsible for a Brønsted acidity as recently shown by solid-state NMR studies of flame-derived silica-alumina and alumina [42].…”

“…Therefore, the 1 H MAS NMR signals of metal OH groups in small cages of zeolites or inside metal oxide clusters (e.g., CaOH, MgOH, AlOH, LaOH, MoOH, VOH, and TiOH) may occur at chemical shifts of up to d 1H ¼6.5 ppm. Examples are different structure types of Al 2 O 3 and amorphous aluminosilicates with internal AlOH groups occurring at d 1H E3 ppm [35,41,42]. In the 1 H MAS NMR spectra of dealuminated zeolites, signals of hydroxyl protons bound to extra-framework aluminum species cause signals at d 1H ¼2.4-3.6 ppm [38,[43][44][45].…”

“…Upon formation of ammonium ions ðNH 4 þ Þ by protonation of the ammonia molecules adsorbed at accessible and strongly acidic OH groups, a narrow 1 H MAS NMR signal occurs at d 1H ¼6.5-7.0 ppm [12,42,89,102,103]. Due to this resonance position, the ammonium signal does not overlap with most of the OH signals of solid catalysts.…”

“…In this case, the formation of Si(OH)Al and metal OH [105,108,127,128] a Rapid exchange of adsorbate molecules requires NMR measurements at low temperatures (Tr 120 K). [42], and on dealuminated zeolite deH,Na-Y/81.5 (framework n Si /n Al ¼ 6.0) prepared by steaming at 748 K with a water vapor pressure of 81.5 kPa and studied in the non-hydrated state [128]. Asterisks denote spinning side-bands.…”

“…Very recently, the Brønsted acidity of flame-derived silicaalumina was studied by solid-state NMR spectroscopy [42]. These materials were prepared by flame-spraying of solutions consisting of aluminum acetylacetonate and tetraethoxysilane in acetic acid and methanol [42].…”