Acidic Properties of H−β Zeolite As Probed by Bases with Proton Affinity in the 118−204 kcal mol^-1 Range:  A FTIR Investigation

Abstract

The interaction of Brønsted sites of H-β (AH) with bases B with proton affinity (PA) ranging in the 118 (N 2 ) to 204 (NH 3 ) kcal mol -1 interval leads to the formation of 1:1 adducts. The vibrational manifestations of these adducts change considerably as a function of the proton potential. In 1:1 neutral adducts characterized by weak-medium hydrogen bonds, the shift of the ν(AH) mode (∆ν j) is proportional to the shift caused by the same bases on the Si-OH (weak) Brønsted acid used as a standard. As far as the 1:1 adducts formed with ethers and tetrahydrofuran (THF; PA ) 196 kcal mol -1 ), the situation is more complex owing to high ionicity acquired by the hydrogen bond. When stronger bases are used (pyridine, Py), the true proton transfer is observed. The ν(AH‚‚‚B) modes of the medium-strong 1:1 adducts are modulated by Fermi resonances with 2δ and 2γ overtones and with internal modes of A and B moieties. A general review of the phenomenon is given. Also the spectra of the ionic 1:1 adducts formed with Py and NH 3 are modulated by complex Fermi resonance effects. The situation occurring at higher dosages of B is also examined: it is concluded that in the presence of an excess of H 2 O (and CH 3 OH) the originally formed neutral 1:1 adducts are transformed into protonated clusters.

Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

“…The magnitudes of the shifts in the ν(OH) vibrational frequencies of the Brønsted sites upon perturbation by adsorbed CO are listed in Table 1. These values constitute a convenient measure of the acid strength of the catalysts and are very similar to previously reported values for H-ZSM-5 and H-beta [48,49], thus confirming the presence of strong Brønsted acidity and that the catalysts employed here are representative for the respective topologies. Elemental analysis by ICP-MS indicates very similar Si/Al ratios, and this is confirmed by NH 3 -TPD.…”

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

“…3300 cm −1 due to the OH···CO H-bonded adducts. The frequency shift (ΔνOH) was about 315 cm -1 , typical of microporous HZSM-5 zeolite, [42,43] hence indicating that the acid strength of the Brønsted sites is not affected by the desilication treatment. Increasing the CO coverage, a broad signal centred at ca.…”

“…At higher CO coverage, 80P shows two negative bands at 3745 cm -1 (SiOHext) and 3710 cm -1 (SiOHint) and in parallel, two broad signals grow at 3650 cm -1 , due to CO adsorbed on Si-OHext (ΔνOH of 95 cm -1 ) and 3590 cm -1 , due to CO adsorbed on SiOHint (ΔνOH of 120 cm -1 ). [43] The values of the shift induced by CO on the O-H stretching mode are reported in Table S1 in the SI. In the hierarchical catalysts (80H and 30H), the band at 3590 cm -1 is less evident due to the higher contribution of external Si-OH produced by desilication.…”

Quantum chemical calculation of infrared spectra of acidic groups in chabazite in the presence of water

“…Such an approach is not adequate to describe the idea of Fermi resonance, which is between the fundamental of the OH stretch and the two overtones of the OH bendings. 27 This requires at least a cubic coupling term. Moreover, the large shift of the OH stretch itself is also caused in large part by anharmonicities in the potential energy surface.…”