<scp>NMR</scp>
            Spectroscopy for the Characterization of Surface Acidity and Basicity

Hunger, Michael

doi:10.1002/9783527610044.hetcat0060

Cited by 13 publications

(4 citation statements)

References 108 publications

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“…In a recent paper on Pt/γ-Al 2 O 3 /zeolite composite catalysts, the effect of residual chlorine from the Pt precursor was studied in detail using (pyridine) FTIR and NH 3 -TPD, revealing that acidity from residual chlorine had no consequences for catalysis. 66 For ZSM-5, zeolite Beta, and zeolite Y, numerous acidity studies are available in the literature, using NMR 67 or FTIR with basic probe molecules (e.g., CO, pyridines, and acetonitrile) 68,69 sometimes in conjunction with catalytic tests, 70 to describe the various acidic sites of these zeolites. There is general agreement that the strong Brønsted acid sites, resulting from a bridging hydroxyl group of charge-balancing tetrahedrally coordinated aluminum atoms (Al 3+ ) with a silicon atom (Al−OH−Si), are the active sites for isomerization and cracking reactions of hydrocarbons.…”

Section: Molecular Modelingmentioning

confidence: 99%

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

et al. 2020

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In this study, Pt nanoparticles on zeolite/γ-Al 2 O 3 composites (50/50 wt) were located either in the zeolite or on the γ-Al 2 O 3 binder, hereby varying the average distance (intimacy) between zeolite acid sites and metal sites from “closest” to “nanoscale”. The catalytic performance of these catalysts was compared to physical mixtures of zeolite and Pt/γ-Al 2 O 3 powders, which provide a “microscale” distance between sites. Several beneficial effects on catalytic activity and selectivity for n -heptane hydroisomerization were observed when Pt nanoparticles are located on the γ-Al 2 O 3 binder in nanoscale proximity with zeolite acid sites, as opposed to Pt nanoparticles located inside zeolite crystals. On ZSM-5-based catalysts, mostly monobranched isomers were produced, and the isomer selectivity of these catalysts was almost unaffected with an intimacy ranging from closest to microscale, which can be attributed to the high diffusional barriers of branched isomers within ZSM-5 micropores. For composite catalysts based on large-pore zeolites (zeolite Beta and zeolite Y), the activity and selectivity benefitted from the nanoscale intimacy with Pt, compared to both the closest and microscale intimacies. Intracrystalline gradients of heptenes as reaction intermediates are likely contributors to differences in activity and selectivity. This paper aims to provide insights into the influence of the metal–acid intimacy in bifunctional catalysts based on zeolites with different framework topologies.

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Section: Molecular Modelingmentioning

confidence: 99%

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

et al. 2020

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“…corresponding to 3b) can be compared with ∆δ 1H values of 5.1, 6.2, and 7.9 ppm obtained after adsorption of acetonitrile on bridging SiOHAl groups in zeolites H-Y, H-MOR, and H-ZSM-5. 16,17 Considering the well-accepted rule that a large adsorbateinduced low-field shift, ∆δ 1H , of the 1 H MAS NMR signal of hydroxyl groups on solid materials indicates a large acid strength and vice versa, 18,19 the bridging AlOH groups in MIL-53ht are characterized by a low acid strength. In a similar manner, also the pyridine-induced low-field shift of the AlOH signal of ∆δ 1H ) 2.3 ppm (Figure 3d) indicates a low acid strength of the above-mentioned hydroxyl protons.…”

Section: Characterization Of Mil-53lt and Of Mil-53htmentioning

confidence: 99%

Adsorbate Effect on AlO₄(OH)₂Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

et al. 2010

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1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, C Q, of the framework aluminum atoms in dehydrated MIL-53 from C Q = 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, C Q, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the C Q values (C Q = 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2 centers, the third change of the C Q value to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the C Q value of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility.

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“…An additional method for characterizing the strength of Brønsted acid sites, such as the acidic protons in dehydrated tungstophosphoric polyanions, is the adsorption of acetone interacting by hydrogen bonding with these surface sites. 25 The adsorbate-induced low-field shift ∆δ 13C of acetone-2- 13 C upon interaction with solid acids is a well-accepted scale of acid strength. 26 In Figures 8a,c, the 13 This assignment was obtained by comparing results of 13 C{ 31 P}REDOR NMR experiments and DFT calculations.…”

Section: Accessibility and Acid Strength Of Brønsted Acid Sites In Hmentioning

confidence: 99%

Structural Stability and Brønsted Acidity of Thermally Treated AlPW₁₂O₄₀in Comparison with H₃PW₁₂O₄₀

et al. 2008

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The thermal stability and hydroxyl coverage of aluminum salt of 12-tungstophosphoric acid, AlPW 12 O 40 , was studied by X-ray diffraction and solid-state NMR spectroscopy in comparison with the pure H 3 PW 12 O 40 heteropoly acid. The most important differences of these materials consist in the higher thermal stability of the local structure and the stronger temperature dependence of the concentration of acidic protons of AlPW 12 O 40 . H 3 PW 12 O 40 showed shrinkage of the unit cell in the X-ray patterns and changes of the local structure in the 31 P MAS NMR spectra already upon dehydration at 473 K, while no structural modifications were found for AlPW 12 O 40 up to 673 K. On AlPW 12 O 40 , acidic protons and AlOH groups are formed by dissociation of water molecules at the multivalent aluminum cations upon dehydration at temperatures of 373-423 K. After dehydration at temperatures higher than 423 K, the hydroxyl groups of AlPW 12 O 40 dehydroxylate and nonhydroxylated Al 3+ cations are formed. The accessibility of acidic protons as studied by adsorption of pyridine was found to be similar for dehydrated H 3 PW 12 O 40 and AlPW 12 O 40 , that is, a lower accessibility occurs with increasing dehydration temperature. The adsorption of probe molecules for studying the acid strength indicated that AlPW 12 O 40 dehydrated at 373-523 K is as superacidic as dehydrated H 3 PW 12 O 40 .

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NMR Spectroscopy for the Characterization of Surface Acidity and Basicity

Abstract: The sections in this article are Introduction Investigation of B rønsted Acidic Hydroxyl Groups by 1 H MAS NMR Spectroscopy Solid‐State NMR Characterization of the Accessibility and Strength of B rønsted Acid Sites by Probe Mol… Show more

Cited by 13 publications

References 108 publications

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

Adsorbate Effect on AlO₄(OH)₂Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Structural Stability and Brønsted Acidity of Thermally Treated AlPW₁₂O₄₀in Comparison with H₃PW₁₂O₄₀

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NMR Spectroscopy for the Characterization of Surface Acidity and Basicity

Abstract: The sections in this article are Introduction Investigation of B rønsted Acidic Hydroxyl Groups by 1 H MAS NMR Spectroscopy Solid‐State NMR Characterization of the Accessibility and Strength of B rønsted Acid Sites by Probe Mol… Show more

Cited by 13 publications

References 108 publications

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

Adsorbate Effect on AlO4(OH)2Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Structural Stability and Brønsted Acidity of Thermally Treated AlPW12O40in Comparison with H3PW12O40

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Adsorbate Effect on AlO₄(OH)₂Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Structural Stability and Brønsted Acidity of Thermally Treated AlPW₁₂O₄₀in Comparison with H₃PW₁₂O₄₀