Mobility of the Acidic Proton in Broensted Sites of H-Y, H-Mordenite, and H-ZSM-5 Zeolites, Studied by High-Temperature 1H MAS NMR

Abstract

The mobility of the acidic proton of the Bransted site in three zeolites has been investigated by means of high temperature (up to 660 K) MAS Nh4R. At high temperatures, spinning sidebands (SSB), caused by the dipolar interaction between the acidic proton and the neighboring aluminum nucleus, gradually disappear. This means that the acidic protons become mobile. At the same time protons from terminal SiOH groups remain stationary over the time scale of the spinning speed. Fitting of the temperature dependence of the second moment of the SSB pattern with known models enables the estimates for the activation energy of proton mobility to be given. These were found to be 45, 54, and 61 kJ/mol for HZSM-5, H-mordenite, and H-Y, respectively. In our interpretation the activation energy corresponds to the average proton affinity difference of oxygen atoms in the first coordination sphere of the aluminum atom at the Bransted center.

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.…”

Formation of the V Center on H-Type Zeolites with Thermal Treatment

“…27 Al MAS NMR Measurement. 27 Al MAS NMR spectra were measured at room temperature on a Bruker DRX-400 spectrometer with a BBO (broadband observe) MAS probe operating at 104.3 MHz. Typically, 400 scans were accumulated using a 0.75 µs π/12 pulse with a 3 s recycle delay.…”

“…Typically, 400 scans were accumulated using a 0.75 µs π/12 pulse with a 3 s recycle delay. 27 Al MAS NMR spectra were recorded with samples spun at 10 kHz, and the chemical shift was referenced to 1% aqueous Al (H2O)6 3+ . Figure 1 shows the ESR spectra of the H-MCM-22 sample after helium purging at different temperatures.…”

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.…”