Detection of Brønsted acid sites in zeolite HY with high-field 17O-MAS-NMR techniques

Abstract

The acidity and unique porous structures of zeolites play an important role in controlling the activity and selectivity of many zeolite-based catalysts. Although (27)Al, (29)Si and (1)H NMR spectroscopy represent standard analytical tools with which to study these materials, (17)O-NMR investigations are much less routine, owing to the very low natural abundance of (17)O (0.037%), its relatively low resonant frequency and its large quadrupole moment. (17)O-NMR resonances from framework oxygen sites in a variety of zeolites have been detected, but the (17)O-NMR resonance from oxygen directly bound to the Brønsted acid site (Si-O(H)-Al) has remained elusive. Here we report the direct observation of this resonance in dehydrated zeolite HY, by using high magnetic-field strengths. (17)O-(1)H double-resonance NMR experiments are used to prove unambiguously that the (17)O signal arises from O nearby H atoms. A large quadrupolar coupling constant, the measure of the local distortion of this site, of 6.6 MHz is determined, which is similar to that obtained in ab initio calculations of zeolite HY-like clusters; this value drops to 5 MHz on acetone binding. The results presented in this paper open up methods for characterizing zeolite acidity and investigating H(+)-sorbent interactions.

Investigating Local Structure in Layered Double Hydroxides with 17O NMR Spectroscopy

“…[ 26 ] Consistent with this assertion, the 17 O single pulse NMR data can also be well fi t with these Mg 3 OH/Mg 2 AlOH ratios and the NMR parameters in Table 1 ( Figure S10, Supporting Information). The mole percentages of Mg 3 OH and Mg 2 AlOH sites in the hydroxide layers of OH − anion-containing LDHs extracted from the 17 O NMR data are in good agreement with the predictions made assuming cation ordering with Al-O-Al linkage avoidance and thus the results from 25 Mg NMR and 1 H NMR [ 14,15 ] investigations of NO 3 − -containing LDHs (Figure 5 b). No MgAl 2 OH environment was observed and a different trend in the percentage of Mg 2 AlOH was found in our study compared to the results from Cadars and co-workers [ 13 ] at high Al mole percentages (>30%), again we believe their observation of the Al-O-Al defects originates from the different preparation methods.…”

“…Both Mg 3 OH and Mg 2 AlOH environments could be observed with parameters consistent with ab initio calculations. The intensities of these two sites in LDHs with hydroxide ions as the interlayer species can be quantitatively determined and the results agree well with the Al-O-Al avoidance model demonstrated previously by 1 H and 25 Mg NMR for NO 3 − -containing LDHs. The resolution of the signals of the different M 3 17 OH local environments is better than the 1 H signals of the same species obtained by ultrafast MAS NMR spectroscopy.…”

“…Ab initio calculations have been performed for hydroxidelike clusters Mg 12 (OH) 25 − and Mg 11 Al(OH) 25 , corresponding to Mg 3 OH and Mg 2 AlOH environments, respectively, to support the spectral assignment (Table S2 and (Table 1 and Table S2, Supporting Information). The calculated values are all slightly larger than the values measured at room temperature, in agreement with previous 17 O NMR calculations of zeolitic clusters.…”

“…The calculated values are all slightly larger than the values measured at room temperature, in agreement with previous 17 O NMR calculations of zeolitic clusters. [ 20,24,25 ] The small differences between the experimental results and calculations may also come from the fact that the interlayer anions and water molecules are not included in these calculations.…”

“…Cadars et al then argued based on their 1 H double quantum (DQ) MAS NMR spectroscopy results that even within a largely ordered structure, a small concentration of defects, for example, MgAl 2 OH, could exist in Al rich LDHs. [ 13 ] Cation ordering was further supported by the 25 Mg NMR data obtained by Sideris et al at an ultrahigh magnetic fi eld, where it was argued that the presence of defects in some samples was likely a consequence of the synthesis method. [ 14,15 ] Large 1 H-1 H homonuclear dipolar couplings are present in these systems, [ 17 ] and ultrafast spinning rates (i.e., >40 kHz) [ 18 ] were required for the NO 3 − -containing LDHs to allow resolution of the major resonances.…”