A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

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Abstract

Solid state NMR, calorimetry, and density functional theory (DFT) all provide a consistent interpretation of the acidity of the solid acid catalyst (SG)nAlCl2, which is prepared by reacting aluminum chloride with conditioned silica gel. These studies firmly establish that the acid sites are Brønsted in nature and that their strength is significantly greater than those in zeolites. Proton NMR results, including experiments exploiting 1H−27Al dipolar couplings, demonstrate that the Brønsted acid sites have an isotropic 1H chemical shift of 5.7 ppm and a concentration of 0.58 mmol/g. The strongest sites on this solid acid, present at 0.03 mmol/g, have −ΔHav values of 52 kcal/mol for reaction with pyridine. A value of 44 kcal/mol is maintained for incremental addition of pyridine up to 0.1 mmol/g. In comparison, −ΔHav for the strongest sites in zeolite HZSM-5 is only 42 kcal/mol. 15N magic angle spinning (MAS) NMR studies of adsorbed pyridine and 31P MAS NMR of trimethylphosphine confirm the Brønsted nature of these acid sites. The 13C isotropic chemical shift of acetone-2-13C on (SG)nAlCl2 (245 ppm) is identical to that measured in 100% H2SO4. 13C in situ NMR studies of ethylene and propene oligomerization show that the activity of (SG)nAlCl2 is far greater than that of zeolites. Cyclopentenyl carbenium ions are formed in significant yields in those reactions as well as during skeletal isomerization and cracking of cyclohexane at 433 K on (SG)nAlCl2. Local DFT calculations at the SVWN/DZVP2 level were used to provide predictions of the structure and energetics of the catalyst. The acidity (defined as the deprotonation energy corrected for zero-point and thermal contributions) obtained from these calculations ranges from 275.5 to 293.4 kcal/mol. Two of the three (SG)nAlCl2 models considered are more strongly acidic than a HZSM-5 cluster model treated at the same level of theory. The aggregate evidence from this study strongly supports classification of (SG)nAlCl2 as a catalyst with a Brønsted acid strength on the threshold of superacidity.

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