Unambiguously distinguishing Si[3Si,1Al] and Si[3Si,1OH] stuctural units in zeolite by 1H/29Si/27Al triple resonance solid state NMR spectroscopy

Luo, Qing; Yang, Jeong-Mo; Hu, Wei; Zhang, Mingjin; Yue, Yong; Ye, Chaohui; Deng, Feng

doi:10.1016/j.ssnmr.2005.02.004

Cited by 8 publications

(3 citation statements)

References 20 publications

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“…The bottom spectrum is acquired with the same spin–echo under the same MAS conditions, except for the application of 27 Al pulses during the echo evolution windows. This experimental approach, well-known in the literature as the spin–echo double resonance SEDOR or TRAPDOR experiment, has been used extensively to identify protons that are within a dipolar-coupling distance (2–6 Å) of an aluminum atom in inorganic oxides. − Of the four labeled peaks in the bottom spectrum of Figure , only the Si–OH peak intensity is the same in both experiments, which is consistent with the known fact that those hydroxyl groups are not close to Al atoms. The very small decrease in the SiOH intensity is due to the loss of the 2.8 ppm extra-framework AlOH peak with which it overlaps.…”

Section: Resultssupporting

confidence: 64%

Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5

et al. 2017

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Direct observation of multiple reactive sites in the zeolite HZSM-5, a member of the MFI family of zeolite structures, contradicts the traditional view of only one type of active protonic species in industrially important zeolites. In addition to the well-known Brönsted acid site proton, two other protonic species undergo room-temperature hydrogen-deuterium exchange with an alkane hydrocarbon reagent, including one zeolite moiety characterized by a broad H chemical shift at ca. 12-15 ppm that is reported here for the first time. Although the ca. 13 ppm chemical shift value is consistent with computational predictions from the literature for a surface-stabilized hydroxonium ion in a zeolite, data suggest that the signal does not arise from hydroxonium species but rather from hydroxyls on extra-lattice aluminol species proximate to Brönsted lattice sites, i.e., a small population of highly deshielded acid sites. Double-resonance experiments show that this species is proximate to Al atoms, similar to the Brönsted acid site proton. These sites can be removed by appropriate postsynthesis chemical treatment, yielding a catalyst with reduced activity for isotopic H/D exchange reactions. Additionally, other extra-lattice aluminum hydroxyl groups previously discussed in the literature but whose protons were considered unreactive are also shown for the first time to react with hydrocarbon probe molecules. Two-dimensional exchange NMR reveals direct proton exchange between the Brönsted site and these two types of extra-lattice Al-OH species, and it also reveals unexpected proton exchange between extra-lattice Al-OH species and an alkane reagent.

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Section: Resultssupporting

confidence: 64%

Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5

et al. 2017

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“…The top spectrum is acquired with a simple spin-echo pulse sequence and gives the same spectrum as those shown above that were acquired using a single pulse, while the bottom two spectra utilize the same spin-echo except for the application of 27 Al pulses during the echo evolution windows. This experimental approach, well-known in the literature as the spin-echo double resonance SEDOR or TRAPDOR experiment, has been used extensively to identify protons that are within a dipolar-coupling distance (2–6 Å) of an aluminum atom in inorganic oxides, as that coupling attenuates the refocused echo. − Signals that arise from protons coupled to Al atoms decrease or are eliminated, while those that arise from species like the 2.0 ppm terminal silanol peak are not affected. Clearly, Figure shows that the 5–7 ppm peak is significantly attenuated in the bottom two spectra.…”

Section: Resultsmentioning

confidence: 99%

Assessment, Control, and Impact of Brønsted Acid Site Heterogeneity in Zeolite HZSM-5

2018

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Acidic zeolites are solid aluminosilicate catalysts whose utility arises from Brønsted sites that predominately reside in their crystalline framework structure. Data described herein indicate that extra-framework aluminum (EFAl) moieties, often proposed as important species in overall catalyst activity via Brønsted−Lewis synergies, can themselves contribute protons that are also reactive Brønsted acid centers. While the MFI family of zeolites is a relatively simple channel-structure type, the quantitative spectroscopic detection of all protons shows that the distribution of reactive Brønsted acid site protons arising from framework and extraframework moieties can be complex. Experiments show that postsynthetic treatments can be used to modify this distribution, in theory enabling routes to HZSM-5 catalysts with only one type of reactive Brønsted site. Quantitative spin-counting NMR experiments combined with chemical washing using ammonium hexafluorosilicate (AHFS) show that the number of framework bridging acid sites (BAS) in typical commercial MFI catalysts (Si/Al equal 15 and 40) is between 50 and 60% of that expected based on the total Al content. Acidic protons from EFAl constitute the major fraction of remaining Brønsted sites. Probe-molecule reactions demonstrate that catalysts with only framework BAS are significantly less reactive than those with both extraframework and framework Brønsted acid sites. Various postsynthetic methods are compared to optimize the desired Brønsted acid site distribution in MFI catalysts, including both removal and re-introduction of acidic protons from EFAl sites.

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“…The latter is particularly important because the Si[3Si, 1Al] units are generally associated with the acidic SiOHAl groups, while the Si[3Si, 1OH] units correspond to nonacidic silanol groups in zeolite defects. Luo and co-workers 139 have shown how to apply the triple-resonance techniques based on the 1 H/ 27 Al/ 29 Si TRAPDOR-CP and 1 H/ 27 Al TRAPDOR pulse sequences. For example, the different peaks, observed in the 29 Si CP NMR spectra of materials MCM, can be distinguished and assigned by this triple-resonance NMR approach.…”

Section: Structural Features Of the Silica Lattice Bymentioning

confidence: 99%

Strategies for Solid-State NMR Studies of Materials: From Diamagnetic to Paramagnetic Porous Solids

Bakhmutov

2010

Chem. Rev.

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Unambiguously distinguishing Si[3Si,1Al] and Si[3Si,1OH] stuctural units in zeolite by 1H/29Si/27Al triple resonance solid state NMR spectroscopy

Cited by 8 publications

References 20 publications

Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5

Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5

Assessment, Control, and Impact of Brønsted Acid Site Heterogeneity in Zeolite HZSM-5

Strategies for Solid-State NMR Studies of Materials: From Diamagnetic to Paramagnetic Porous Solids

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