Adsorption of Methanol on Brønsted Acid Sites in Zeolite H-ZSM-5 Investigated by Multinuclear Solid-State NMR Spectroscopy

Abstract

Adsorbate complexes formed by adsorption of methanol on zeolite H-ZSM-5 have been investigated by 1H, 2H, 13C, and 27Al NMR spectroscopy at temperatures between T = 425 K and T = 85 K. The data derived were explained by hydrogen-bonded neutral methanol molecules and partially protonated methanol clusters consisting of one and three methanol molecules per SiOHAl group, respectively. At T ≥ 295 K, the hydroxyl protons of adsorbate complexes formed by one methanol molecule per SiOHAl group exhibit a high mobility and undergo a rapid chemical exchange. The 1H MAS NMR shifts (δ1H = 4.1 and 14.2 ppm) of the OH groups after adsorption of CD3OH and the 2H quadrupole coupling constants (QCC = 230 and 140 kHz) of the OD groups after adsorption of CH3OD, obtained at T ≤ 153 K, correspond to those of rigid and hydrogen-bonded molecules. The 27Al quadrupole coupling constants of AlO4 tetrahedra involved in the local structure of bridging OH groups interacting with one and three methanol molecules (QCC = 8.0 and 2.5 MHz, respectively) depend on the number of adsorbate molecules and indicate a partial deprotonation of SiOHAl groups in the complexes formed by three methanol molecules per SiOHAl group.

Proton Transfer in Zeolites

“…The methoxonium protons undergo a huge downfield shift when interacting with the zeolite surface in the ion-pair complex. The calculated shift of 17.4 ppm is much larger than the observed value 9.4 ppm [25,27]. In the neutral complex the Brønsted site proton also undergoes a similarly large shift due to the strong hydrogen bond, while the methanol proton extends a weaker hydrogen bond to the zeolite framework and its NMR signal shifts less.…”

Motion of methanol adsorbed in porous coordination polymer with paramagnetic metal ions

“…Fig. 2(a) displays the observed 2 H NMR spectra for CD 3 OH adsorbed in CPL-1 from 173 to 293 K. The conditions of NMR measurement and the procedure for sample preparation are also described in Fig. 2.…”

In situ solid state NMR observation of the methanol-to-hydrocarbons (MTH) process over nanosized and microsized HZSM-5 zeolites

“…The number of adsorbed methanol molecules was obtained by comparison of the 1 H MAS NMR intensities of the unloaded and loaded samples. 16 All the NMR spectra were obtained at 9.4 T on a Bruker DRX-400 spectrometer using 4 mm ZrO 2 rotors at ambient temperature. The 1 H MAS NMR were measured at a resonance frequency of 400.1 MHz using single-pulse experiments with the p/10 pulse set at 1 ms and a 4 s recycle delay used.…”

“…It is well known that the chemical shift of the hydroxy resonance of methanol on HZSM-5 is strongly dependent upon loading. 16,[24][25][26][27] As the coverage increases, the resonance moves upfield to lower chemical shift values. 25 The relatively small value (6.8 ppm) of the methanol hydroxy resonance indicates that the coverage of methanol on nanosized HZSM-5 after 30 min adsorption is larger than that on microsized HZSM-5.…”

“…The variation of the 1 H MAS NMR shift of the hydroxy protons caused by an increase in the methanol coverage can be explained by a rapid chemical exchange between the hydroxy protons of bridging OH groups and those of methanol molecules in the physisorbed (hydrogen-bonded complexes) and/or chemisorbed state (methoxonium ions). 16,[24][25][26][27] Theoretical investigations 23,28 suggest that both hydrogen-bonded complexes and methoxonium ions will be formed upon methanol adsorption, and the former species have the most stable binding geometry. The internal H-H distance determined theoretically in the hydrogen-bonded complexes is about 192 pm.…”