Probing the Acid Strength of Brønsted Acidic Zeolites with Acetonitrile:  Quantum Chemical Calculation of ¹H, ¹⁵N, and ¹³C NMR Shift Parameters

Abstract

Calculations have been carried out of 1 H, 15 N, and 13 C NMR chemical shift parameters, which are used to characterize the acid strength and accessibility of Brønsted acid sites in seven high-silica zeolites of structure types CHA, FAU, FER, MFI, MOR, MTW, and TON. The acid sites had previously been selected by a systematic minimization procedure, and an acetonitrile molecule inserted using a Monte Carlo method. The main interactions between acetonitrile and the zeolite are an N‚‚‚H-O type hydrogen bond to the Brønsted proton and van der Waals' interactions with the framework. Calculations using GIAO (with B3LYP/TZV) were performed on optimized clusters, which included the acid site and sufficient atoms to represent the surrounding pore topology. The isotropic shielding values were obtained, as well as the principal components of the shielding tensors and the chemical shift parameters. Both the isotropic shifts of the acid proton, δ iso ( 1 H), and δ iso ( 15 N) correlate well with the hydroxy bond length, r(OH), and hence to acid strength. In contrast, δ iso ( 13 C) is strongly dependent on the orientation of the acetonitrile molecule, as influenced by the local zeolite geometry. The principal tensor components, δ 11 , δ 22 , and δ 33 , exhibit recognizable trends only for the nitrogen atoms. To describe the powder pattern of the shift in a more illustrative way, we use the span, Ω, the skew, κ, the anisotropy, δ aniso , and the asymmetry, η. The symmetry of the powder pattern is related to the local symmetry of the nuclei but not at all to the Brønsted acidity.

Acid Properties of SAPO-34 Molecular Sieves with Different Si Contents Templated by Various Organic Structure-Directing Agents

“…When acetonitrile interacts with BA sites via hydrogen bonding (illustrated as an inset in Fig. 2), the 1 H resonance will be shifted to the lower field 26,27 . Therefore, this adsorbate-induced resonance shift ∆δH is utilized for probing the acid strength.…”

“…The solidstate NMR experiments were carried out on a Bruker Avance III 600 spectrometer equipped with a 14.1 T wide-bore magnet and a 4 mm WVT MAS probe. The resonance frequencies were selected at 600.13, 242.93, 156.4 and 119.2 MHz for 1 H, 31 P,27 Al, and 29 Si, respectively. 1 H MAS NMR spectra were recorded using a single pulse sequence with a π/4 pulse of 1.8 μs and a 10 s recycle delay at a spinning rate of 12 kHz, using adamantane (1.74 ppm) as the chemical shift reference.…”

AIM and NBO analyses on the interaction between SWCNT and cyclophosphamide as an anticancer drug: A density functional theory study

“…The magnetic¯eld arises from the circulation of electrons to surrounding, so the magnetic¯eld has been induced from the spin of electrons. [40][41][42][43][44][45][46][47] The magnetic¯eld induced by the B 0 external magnetic¯eld and i , so the e±cient¯eld B eff at the nucleus is followed by [46][47][48][49][50][51][52] :…”

Zeolites and Other Structurally Microporous Solids as Acid–Base Materials

“…Some authors suggested that a correlation exists between OH stretching frequency and the SieO(H)eAl bond angle. 70 As for 1 H MAS NMR peaks of protonic zeolites, the trend among different studies is for increased chemical shift corresponding to an increase in the intrinsic acid strength (proton affinity), 71 i.e. protons are more deshielded in zeolites perceived to be more acidic.…”