Proton sites in Keggin heteropoly acids from17O NMR

Kozhevnikov, I. V.; Sinnema, A.; Bekkum, H. van

doi:10.1007/bf00808336

Cited by 64 publications

(51 citation statements)

References 17 publications

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“…In contrast, the intensity at 3,605 cm -1 changed a lot. According to the literature [21,22], the peak that appeared at 3,245 cm -1 is assigned to the terminal (W = O-H) stretching, and those at 3,605 cm -1 to the edge-bridging (W-O-H) stretching. We think that the peak at 3,245 cm -1 assigned to constitution water of PW 12 and the protons are the predominant activated species.…”

Section: Effect Of the Calcination Temperaturementioning

confidence: 99%

Keggin heteropolyacids as catalyst for the polymerization of β-pinene

Zhu

Liu

Xin-nan

et al. 2010

Reac Kinet Mech Cat

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A series of Keggin-type heteropolyacids (HPAs) have been explored for cationic polymerization of b-pinene in an attempt to substitute traditional polymerization catalyst Lewis acids. The catalysts were characterized by XRD, FT-IR, TGA, acid strengths of the catalysts were measured with Hammett indicators. Among them, 12-phosphotungstic acid (PW 12 ) exhibits much higher catalytic activity. The effects of solvent, calcination temperature and reaction temperature on the catalytic performance were investigated.

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Section: Effect Of the Calcination Temperaturementioning

confidence: 99%

Keggin heteropolyacids as catalyst for the polymerization of β-pinene

Zhu

Liu

Xin-nan

et al. 2010

Reac Kinet Mech Cat

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“…Much debate has arisen from inconclusive Brønsted‐acid‐proton locations at the different oxygen sites, such as terminal,24–26 bridging,18, 27, 28 or both,29, 30 based on various experimental and theoretical studies. On the other hand, intensive studies have also been undertaken to explore the acidic properties of HPW by using a variety of different techniques, such as Hammett indicators,31–33 thermal desorption,16, 33, 34 microcalorimetry,15–18, 35–38 hydrogen/deuterium exchange,39 FTIR19, 28, 40–43 and NMR spectroscopy,13–15, 20, 25, 26, 30, 38, 44–46 and theoretical density functional theory (DFT) calculations 47. Nevertheless, inconsistent conclusions can be found frequently in the literature, particularly regarding the strength and heterogeneity of acid sites in HPW.…”

Section: Introductionmentioning

confidence: 99%

New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from ³¹P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

Huang

Yang

Zheng

et al. 2010

Chemistry An Asian Journal

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The acid and transport properties of the anhydrous Keggin-type 12-tungstophosphoric acid (H(3)PW(12)O(40); HPW) have been studied by solid-state (31)P magic-angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, (31)P NMR resonances arising from various protonated complexes, such as TMPOH(+) and (TMPO)(2)H(+) adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)(2)H(+) adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to (31)P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH(+) complexes are associated with protons located at three different terminal oxygen (O(d)) sites of the PW(12)O(40)(3-) polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.

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“…From this spectrum, four characteristic IR absorption peaks of the Keggin units are observed in the range from 700 to 1100 cm -1 [17]. According to the literature [18,19], the peak that appears at 1079 cm -1 is assigned to the stretching vibrations of Si-O, those at 981 cm -1 to the W-Ot (Ot, terminal oxygen atoms) bonds stretching, the peaks at 894 and 807 cm -1 correspond to the W-Oe (Oe, edge-bridging oxygen atoms) and W-Oc (Oc, cornerbridging oxygen atoms) bonds stretching respectively. We think that the peak at The results were obtained after 3 h polymerization at 50°C, using 5 mL a-pinene in 5 mL solvents, 7 wt% SiW 12 (pretreated at 200°C for 4 h)…”

Section: Ir Characterization Of Catalysts and Polymersmentioning

confidence: 95%

Cationic polymerization of α-pinene using Keggin silicotungstic acid as a homogeneous catalyst

Liu

Zhang

Zeng

et al. 2011

Reac Kinet Mech Cat

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Cationic polymerization of a-pinene with silicotungstic acid (SiW 12 ) as a catalyst was investigated. The structure of polymers and the catalysts were characterized by FT-IR, 1 H-NMR and GPC. The experiments show that SiW 12 is more active than both phosphotungstic acid and phosphomolybdic acid. The protons dissociating from the catalysts are the reactive species. SiW 12 is found to be both the polymerization initiator and the counter-anion of the growing cationic center. The polymerization conditions are optimized by the single factor method as follows: monomer concentration is 50% (v/v), the dosage of catalysts is 7 wt% referred to a-pinene, reaction temperature is 50°C, reaction time is 1 h. Under these conditions, the overall conversion of a-pinene is up to 90.87%, the polymer yield is 62.46% and its average number molecular weight is about 600. GC-MS analysis shows that there are six isomers of a-pinene after the reaction, but their content is very low. Most of a-pinene are changed to poly(a-pinene).

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Proton sites in Keggin heteropoly acids from17O NMR

Cited by 64 publications

References 17 publications

Keggin heteropolyacids as catalyst for the polymerization of β-pinene

Keggin heteropolyacids as catalyst for the polymerization of β-pinene

New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from ³¹P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

Cationic polymerization of α-pinene using Keggin silicotungstic acid as a homogeneous catalyst

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Proton sites in Keggin heteropoly acids from17O NMR

Cited by 64 publications

References 17 publications

Keggin heteropolyacids as catalyst for the polymerization of β-pinene

Keggin heteropolyacids as catalyst for the polymerization of β-pinene

New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

Cationic polymerization of α-pinene using Keggin silicotungstic acid as a homogeneous catalyst

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New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from ³¹P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations