Mobility of the Acidic Protons in H−ZSM-5 As Studied by Variable Temperature ¹H MAS NMR

Abstract

The dynamic nature of the protons in H−ZSM-5 was examined by the temperature dependence of 1H MAS NMR in the range 298−473 K. The line width of 1H MAS NMR of acidic protons increased, and through maximum it decreased. The intensity of the spinning sidebands monotonically decreased, and they almost disappeared upon raising the temperature. This temperature dependence of the spectrum was explained by the thermal motion of protons. The correlation times and the activation energies for proton mobility were estimated. The estimated values of the activation energy are 17−20 kJ mol-1.

DFT Cluster Modeling of Molecular and Dissociative Hydrogen Adsorption on Zn2+Ions with Distant Placing of Aluminum in the Framework of High-Silica Zeolites

“…These values are significant for the correct description of zeolites' acidity peculiarities and jump rates of proton translation motion, but have no effect (see figure 8) on our conclusion on the possibility of heterolytic dissociative adsorption of dihydrogen following the adsorption of dihydrogen by zinc ion. It is worth noting that for H-ZSM-5 zeolite, the experimental on-site proton jump barriers are in the range from 2.6 to 10.8 kcal/mol [27][28][29][30]. These values are at least two times smaller than those predicted from both cluster [22] and periodic [24] DFT calculations.…”

Density Functional Theory Study of Proton Mobility in Zeolites: Proton Migration and Hydrogen Exchange in ZSM-5

“…[29][30][31][32][33] Variable temperature 1 H magic angle spinning (MAS) NMR studies of dehydrated H-ZSM-5 zeolite provide evidence that acidic protons migrate between the four oxygen atoms surrounding the tetrahedral aluminum center in the following fashion reaction. This, in turn, makes us question whether the unconstrained geometry relaxation employed by Sauer et al 6,7 is appropriate in this case.…”

Adsorption of Hydrocarbons and Formation of Carbocations over Zeolites Studied by IR Spectroscopy

“…The dissociation equilibrium of protons on zeolites is balanced to the associated form (hydroxyl groups) in the low temperature range and moves to the dissociated form with increasing temperature. Experiments have confirmed the presence of mobile protons hopping across lattice oxygen anions in pores 3) . One specific characteristic of the solid acid catalytic properties of zeolites is shape selectivity, which is attributed to the ordered porous structures with pore sizes (0.4-0.8 nm) similar to molecular sizes.…”

“…Generally for H-ZSM-5 zeolites, the chemical shift of silanol groups occurs at 1.8-2.0 ppm and that of the acidic hydroxyl groups at 4.1-4.5 ppm, but these values are dependent on the type of zeolite and the Si/Al ratio. The activation energy for proton dissociation of the acidic hydroxyl groups can be estimated by the dependence of the peak width on temperature 3) .…”