Disentangling Brønsted Acid Sites and Hydrogen-Bonded Silanol Groups in High-Silica Zeolite H-ZSM-5

Abstract

Hydrogen-bonded SiOH defects and Brønsted acidic bridging OH groups, defined as Brønsted acid sites (BAS), yield overlapping signals in 1H MAS NMR and FTIR spectroscopies. Here, a clear distinction in the distribution of SiOH defect sites between industrial ZSM-5 zeolites with Si/Al ratios of 14 and 20 as well as 60 is reported. At a high Si/Al = 60 ratio, hydrogen-bonded SiOH groups appear as signals in the 1H MAS NMR spectrum of H-ZSM-5 between 3 and 8 pm, thus severely overlapping with signals caused by unperturbed and hydrogen-bonded BAS. FTIR spectra are similarly affected. The hydrogen-bonded SiOH groups are paired, and they are clearly identified by 1H double-quantum single-quantum (DQ-SQ) MAS NMR correlation spectroscopy. A selective analysis was carried out by 1H­{27Al} rotational echo adiabatic pulse double resonance (REAPDOR) to yield H–Al distances for the BAS, whereas the silanols do not significantly contribute to this experiment due to short T2 relaxation times. Spectral overlap of BAS and defect sites is suggested to be of some relevance in particular for zeolites with high Si/Al ratios.

High Aluminum Ordering in SSZ-59: Residual 1H–27Al Dipolar Coupling Effects in 1H MAS NMR Spectra of Brønsted Acid Sites in Zeolites

“…The second coupling constant of 106 Hz is either representative of a second Al atom, which is 6.7 ± 0.3 Å apart from the BAS under investigation, or a combination of two additional Al atoms with longer distances, as will be further elaborated on below. We note that the goodness of the fit up to an evolution time of 4 ms is unprecedented in our previous library of such analyses for BAS. ,,,,,− We are also unaware of such a good fit using a 3-spin system at such long evolution times for distance analyses in zeolites that are available hitherto in the literature. This must be due to the high degree of Al ordering in this material, where the Al atoms are located at specific sites and adopt a fairly large separation from one another.…”

“…After drying at 323 K, the sample was placed inside a borosilicate glass tube and heated under vacuum (<10 27 All MAS NMR experiments were conducted with a MAS rate of 12.5 kHz and 4 mm Bruker HX double-resonance probes. Water contamination can be ruled out in the dehydrated sample (see ref 28 for further details). Experiments at 7.0 T were performed with a Bruker Avance III spectrometer, while for 9.4 T a Bruker Avance I was used.…”

The challenge of silanol species characterization in zeolites

“…1 H MAS NMR can therefore quantify and distinguish silanols based on their chemical shifts (non-H-bonded isolated silanols: ∼1.4 ppm, weakly H-bonded silanols: ∼2.2 ppm, Brønsted acid sites: ∼4.0 ppm and strongly H-bonded silanols and BAS above 4 ppm) although the chemical shift range is rather narrow and overlap occurs frequently due to dipolar couplings. [34][35][36] By comparing the relative quantities of different silanols in as synthesized zeolites, after template removal by calcination, and ion exchange procedures, the formation mechanism of internal silanols as a function of the template location can be elucidated. [37][38][39] A deuterium exchange of a ZSM-5 zeolite studied by both 1 H and 2 H NMR spectroscopy was used to reveal silanol species based on their geometry.…”

Crystal‐size–dependent external surface diffusion barriers in Pt/ZSM‐5 catalyzed n‐pentane isomerization

“…To give a direct proof of SiO 2 deposition on ZSM‐5 crystals, FTIR spectra are compared for parent and modified samples (Figure 6D). The absorbance peak at 3740 cm −1 can be attributed to SiOH on the external surface of Z5‐S and Z5‐S‐M, while the one at 3724 cm −1 corresponds to SiOH on the external surface of Z5‐L and Z5‐L‐M 53,54 . Z5‐S displays a higher SiOH peak intensity than Z5‐L.…”

“…The absorbance peak at 3740 cm À1 can be attributed to SiOH on the external surface of Z5-S and Z5-S-M, while the one at 3724 cm À1 corresponds to SiOH on the external surface of Z5-L and Z5-L-M. 53,54 Z5-S displays a higher SiOH peak intensity than Z5-L. This result is expected, since the external surface area of Z5-S is larger than that of Z5-L (Table 1) and thus Z5-S possesses more SiOH on the external surface.…”