Solid state NMR, calorimetry, and density functional theory (DFT) 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 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 −ΔH 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, −ΔH av 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-2-13 C on (SG) n AlCl2 (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) 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 Brønsted acid strength on the threshold of superacidity.
Since its discovery, sulfated zirconia and metal-doped sulfated zirconia have been the subject of numerous reports on their catalytic activity. Their ability to perform low-temperature hydrocarbon isomerizations has led to claims that sulfated zirconia and metal-doped sulfated zirconia are superacids or at least very strong acids. This has led to many investigations on the acid strength of sulfated zirconia and metal-doped sulfated zirconia producing varying results. We report the use of cal-ad (which uses combined information from calorimetry and adsorption of pyridine onto the solid acid) to determine the acidity of sulfated zirconia and metal-doped sulfated zirconia. We find that sulfated zirconia has two types of acid sites: 24 µmol of a strong site, which has a strength of 31.2 kcal/mol, and 52 µmol of a weaker site of 25.8 kcal/mol, which places the acidity of sulfated zirconia lower than that of HZSM-5 (41 kcal/mol) and about the same as that of HY (34 kcal/mol). Doping sulfated zirconia with 0.2 wt % Pt does not change the acidity, but doping with iron and manganese (which increases the catalytic activity) results in a lower measured acidity, indicating that iron and manganese have occupied the strongest acid sites.
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