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- (NH4)3InF6 (1)
- 115In and 19F solid state NMR (1)
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- 27Al NMR (1)
Organisationseinheit der BAM
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A series of crystalline aluminum hydroxy fluorides in cubic pyrochlore structure AlFx(OH)3-x·H2O with variable F-content x were investigated by solid-state NMR by applying different magnetic fields up to 21.1 T. Distinguishable octahedral species AlFx(OH)6-x (x = 1-6) were identified in the crystalline aluminum hydroxy fluorides. The subsequent analysis of the highfield 27Al MAS NMR data allows the derivation of the trend analysis graphs giving correlations between the 27Al chemical shifts and the quadrupolar frequencies and the F-content x in AlFx(OH)6-x. Clear trends were obtained for both, which are, along with the 19F MAS chemical-shift trend analysis presented earlier, valuable tools for the interpretation of MAS NMR spectra of amorphous AlFx(OX)3-x compounds (X = H, alkyl). Following the dehydration of the pyrochlores by solid-state NMR eventually reveals a remarkable influence of the incorporated solvent molecules (H2O) on the 19F chemical shift. On that basis, a new chemical-shift trend analysis for 19F chemical shifts in correlation with x in AlFx(OH)6-x units for proton-poor substances (in the Al, F, O, H system) was determined. By using this correlation, high-surface AlF3 has a mean bulk Al:F ratio similar to that found for ACF, namely, AlF2.8(O/OH)0.2.
This study reports three new crystalline aluminum isopropoxide oxide fluorides with molar ratios of Al:F equal to 1:1 and 1:1.25. These are the first three representatives isolated without the incorporation of external donor molecules. Compound 1 Al4F4(µ4-O)(µ-O'Pr)5[H(O'Pr)2] contains a tetranuclear unit consisting of two different five fold coordinated AlFO4-units, with F exclusively in the terminal position. Compound 2, Al4F4(µ4-O)(µ-O'Pr)5[H(O'Pr)2]·Al5F5(µ5-O)(µ-O'Pr)8, contains both a tetranuclear unit (as in 1) and a pentanuclear Al-unit. Al-atoms in the latter are five- and six fold coordinated. Compound 3, Al16F20(µ4-O)4(µ-O'Pr)20·2('PrOH), exhibits a slightly higher fluorination degree and contains an oligomeric chain of four F-linked tetranuclear Al-units. In addition to X-ray structure analysis, compound 1 was characterized by different solid state MAS NMR techniques, including 27Al triple quantum MAS NMR and 1H, 1H→13C CP, 19F and 27Al MAS NMR. On the basis of the collected data, a reliable decomposition of 27Al single pulse MAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO4-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol–gel mechanism previously discussed.
This study presents for the first time an NMR spectroscopic characterization of the room and high temperature phases of (NH4)3InF6 using 19F and 115In as probe nuclei. The reversible phase transition to the cubic phase at 353 K was followed by MAS NMR in situ. Static NMR experiments of the room temperature phase and MAS NMR experiments of the high temperature phase allowed the determination of the NMR parameters of both nuclei. Finally, the scalar In–F coupling, rarely observed in solid state NMR, is evidenced in both room and high temperature phases of (NH4)3InF6, and measured in the high temperature phase.
Time-optimized 15N quantitative NMR (qNMR) is described for the precise determination of Lewis and Brønsted site concentrations per gram catalyst, enabling a measurement time reduction of more than a factor of 10. The proof-of-principle is demonstrated for amorphous aluminum hydroxide fluorides with their tunable Lewis to Brønsted functionalities and relevance to highly active and selective heterogeneous catalysis using pyridine as a widely accepted probe molecule for Lewis and Brønsted sites. Two different Lewis centers were found, and reliable site concentrations down to the micromole per gram region can be determined. Furthermore, 1H{15N} REDOR NMR for the acid proton at the Brønsted site yields a distance of the 15N atom of bonded pyridine to this acid proton of (0.112 ± 0.002) nm, such that fast motion/diffusion of the Brønsted site-bonded pyridine at the surface can be excluded.
Functionalized partially hydroxylated MgF2 as catalysts for the dehydration of d-xylose to furfural
(2013)
Among the furan-based compounds, furfural (FUR) shows interesting properties as building-block or industrial solvent. It is produced from pentosan-rich biomass via xylose cyclodehydration. The current FUR production uses homogeneous catalysts and steam. According to recent studies, the reaction mechanism is different in the presence of Lewis (L) or Brønsted (B) sites. In this work, partially hydroxylated MgF2 catalysts, containing bifunctional Lewis and Brønsted properties, were further functionalized using different fluorosulfonic precursors. Its main objective was to test these solids as catalysts for xylose conversion to furfural. Extensive characterization techniques using TG–MS, 19F-MAS-NMR, or pyridine adsorption confirmed the substitution of surface OH groups by stronger Brønsted sites. The activity data showed a considerable change of the reaction kinetics and a final furfural selectivity of 90% at 160 °C in water/toluene for optimized L/B ratios by the one-step grafting technique. Moreover, the reaction analyses and the change of conversion pathways were studied.
Quantitative determination of acidic surface sites is highly important for the characterization of solid acids because the activity of a catalyst is often related to the concentration of these sites. A recently developed method using 15N Nuclear Magnetic Resonance spectroscopy (NMR) for the quantification of acidic Lewis and Brønsted sites has been tested for a series of nanoscopic aluminum hydroxide fluorides. Comparison with other methods for the quantitative determination of acidic sites shows that this 15N NMR quantification method is a promising technique for the comprehensive investigation of acidic sites. Three different acidic sites, one Brønsted and two Lewis sites, can be distinguished by their 15N chemical shifts of pyridine and simultaneously quantified under conditions corresponding to catalytic reaction conditions. Determination of the individual concentrations of acidic sites allows further insight into the catalytic process. It was found that the concentration of Brønsted sites correlates with catalyzed conversion of citronellal to isopulegol in the investigated series of catalysts. Additionally, investigations indicate that one of the Lewis sites become blocked during the reaction of citronellal.
15N NMR of pyridine has been used to study Lewis and Brønsted sites at the acid surface of magnesium hydroxide fluoride at two different pyridine loadings with ratios of excess pyridine compared to pyridine molecules bound to Lewis and Brønsted sites of about 1:1 (highly loaded) and 0.15:1 (low loaded), respectively. The Lewis and Brønsted sites can be distinguished by the 15N chemical shift of pyridine at about –100 ppm (LPy) and –175 ppm (BPy). In both samples a very broad 15N resonance is found around –88 ppm assigned to weakly adsorbed (excess) pyridine molecules via hydrogen bridges (HPy). Additionally, another signal at about –49 ppm is observed in the highly loaded sample only representing physisorbed pyridine (PPy). A slow chemical exchange process of strongly bound LPy and BPy molecules with excess HPy and PPy molecules can be monitored using 15N exchange NMR. It takes place on a time scale of about 50–100 ms at room temperature. All pyridine molecules have the same 15N T1 and its value increases from 800 ms (highly loaded sample) to 5.3 s (low loading) hinting on changed pyridine mobility depending on the loading level.
Lewis and Brønsted sites were quantified in a series of weak acidic hydroxylated magnesium fluorides by Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance spectroscopy (NMR) with pyridine as probe molecule. Molar extinction coefficients, which are necessary for quantitative FTIR measurements, were calculated by an easy approach. It utilizes the fact that both signals, used for the quantification by FTIR, are caused by the same deformation vibration mode of pyridine. Comparison of quantitative FTIR experiments and quantification by NMR shows that concentrations of acidic sites determined by FTIR spectroscopy have to be interpreted with caution. Furthermore, it is shown that the transfer of molar extinction coefficients from one catalyst to another may lead to wrong results. Molar extinction coefficients and concentrations of acidic sites determined by FTIR spectroscopy are affected by grinding and probably the particle size of the sample. High temperature during FTIR experiments has further impact on the quantification results.
Catalytic C–F bond activation reactions of mono- and polyfluoroalkanes at Lewis acidic amorphous aluminum chlorofluoride (ACF) are presented. The hydrogen sources Et3GeH or Et3SiH control the selectivity of the conversions. The immobilization of Et3GeH at ACF resulted in catalytic dehydrohalogenation reactions to yield olefins under very mild conditions. In contrast, if Et3SiH is immobilized at ACF, C–C coupling occured and the formation of Friedel–Crafts products was observed. MAS NMR spectroscopic studies revealed information about the surface binding of the substrates.
The very strong Lewis acid aluminium chlorofluo-ride (ACF) was loaded with anhydrous HF. The interactionbetween the surface of the catalyst and HF was investigatedusing a variety of characterization methods, which revealed he formation of polyfluorides. Moreover, the reactivity ofthe HF-loaded ACF towards the hydrofluorination of alkyneswas studied.