A solid acid catalyst at the threshold of superacid strength: NMR, calorimetry, and density functional theory studies of silica-supported aluminum chloride

Solid state NMR, calorimetry, and density functional theory (DPT) all provide a consistent interpretation of the acidity of the solid acid catalyst (SG)(n)AlCl2 which is prepared by reacting aluminum chloride with conditioned silica gel. These studies firmly establish that the acid sites are Bronsted in nature and that their strength is significantly greater than those in zeolites. Proton NMR results, including experiments exploiting H-1-Al-27 dipolar couplings, demonstrate that the Bronsted acid sites have an isotropic H-1 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 -Delta H-av, 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, -Delta H-av for the strongest sites in zeolite HZSM-5 is only 42 kcal/mol. N-15 magic angle spinning (MAS) NMR studies of adsorbed pyridine and P-31 MAS NMR of trimethylphosphine confirm the Bronsted nature of these acid sites. The C-13 isotropic chemical shift of acetone-2-C-13 on (SG)(n)AlCl2 (245 ppm) is identical to that measured in 100% H2SO4. C-13 in situ NMR studies of ethylene and propene oligomerization show that the activity of (SG)(n)AlCl2 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)(n)AlCl2. 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)(n)AlCl2 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)(n)AlCl2 as a catalyst with a Bronsted acid strength on the threshold of superacidity.