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

Abstract

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.

Silica-supported Phosphorus Chloride: An Efficient and Recyclable Catalyst for Beckmann Rearrangement of Ketoximes and Dehydration of Aldoximes Under Microwave Irradiation

“…Silica gel was first dried at 100°C in vacuum to remove excess water without dehydrating the silanol groups [24]. Then silica gel reacted with excess phosphorus trichloride at room temperature to give silica-supported phosphorus chloride as the product in high yield after the release of hydrogen chloride gas.…”

Isomerization of cyclohexane and hexane over silica- embedded triflate derivative catalysts

“…Summarizing, the immobilization of triflate derivative catalysts on silica led to systems able to catalyze the skeletal isomerization of cyclohexane at temperatures much lower than reported for zeolites. 27 The system is comparable with silica-supported aluminum chloride 24 but it is environmentally more friendly. The difference of the catalytic reactivity between cyclohexane and n-hexane is not a thermodynamical one because the free Gibbs energies of the reactions are similar, about -1 kcal/mol in both cases.…”

Styrene and styrene dimer derivatives, cyclohexene, tert-butylbenzene and cumene as test reactions for acid strength measurements of crystalline and amorphous silica-aluminas, sulfated oxides and Amberlyst

“…The basis of these measurements is the correlation of d 13 C chemical shifts with calculated deprotonation energy of acid sites [32]. Some results are presented below [20,32,33] [9] reported, in the case of acetone applied as probe molecule, that the acid strength of HZSM-5 was in the proximity of that of 70 % H 2 SO 4 (H O = -5.8) and higher than the values for HY and HX zeolites as well as SZ. Moreover, the observed 13 C chemical shift value for adsorbed acetone is a mean of the values of adsorption on several acid sites of varying acid strengths.…”