A new unifying structural model of heteropolymolybdate salts: Microstructure and thermal stability of a series of molecular oxides

Investigation of the acrolein oxidation on heteropolyacid catalysts by transient response methods

Isotopic assessment by oxygen, water and acrolein labeling delivers more detailed mechanistic insights.

Observation of microporous cesium salts of 12-tungstosilicic acid using scanning transmission electron microscopy

Heteropolyanions and their arrays in microporous cesium salts of 12-tungstosilicic acid, Cs_2.5H_1.5[SiW₁₂O₄₀] and Cs_4.0[SiW₁₂O₄₀], were observed by aberration-corrected scanning transmission electron microscopy.

Thermal decomposition of lignocellulosic biomass in the presence of acid catalysts

Transformation of lignocellulosic biomass to biofuels involves multiple processes, in which thermal decomposition, hydrotreatment are the most central steps. Current work focuses on the impact of several solid acids and Keggin-type heteropolyacids on the decomposition temperature (Td) of pine wood and the characterization of the resulted products. It has been observed that a mechanical mixture of solid acids with pine wood has no influence on Td, while the use of heteropolyacids lower the Td by 100°C. Moreover, the treatment of biomass with a catalytic amount of H3PW12O40 leads to formation of three fractions: solid, liquid and gas, which have been investigated by elemental analysis, TGA, FTIR, GC-MS and NMR. The use of heteropolyacid leads, at 300°C, to a selective transformation of more than 50 wt.% of the holocellulose part of the lignocellulosic biomass. Moreover, 60 wt.% of the catalyst H3PW12O40 are recovered.

IR Spectroscopic Investigation of Heteropolymolybdate Catalysts: Acidic Properties and Reactivity towards Propene

Compounds of the general type CsxH3+y−xPVyMo12−yO40 (x = 0, 2–4 with y = 1, or x = 0, 2 with y = 0) were investigated by IR spectroscopy. After thermal treatment at 473–773 K in vacuum or N2, the probe molecules CO (at 77 K, in transmission) or CO2 (at 298 K, in diffuse reflectance) were adsorbed. On the surface of Cs2H2PVMo11O40 and Cs2HPMo12O40, Brønsted acid sites were indicated by a CO band at 2162 cm−1. All Cs containing compounds produced CO bands at 2152 cm−1 or CO2 bands at 2341 cm−1. Additional weak Lewis acid sites of Mo or V in at least two oxidation states were detected after treatment at 673 K. The fraction of the reduced species decreased in the order H4PVMo11O40 > Cs2H2PVMo11O40 > Cs3HPVMo11O40. In situ DRIFT spectra of these three compounds taken during interaction with propene at up to 673 K revealed that the pattern of IR bands typical of the Keggin anion was unaffected at x = 3, altered severely for the acid and to an intermediate extent for x = 2. The changes are consistent with a diminishing of the splitting of the P–O frequency. The Keggin structure represents the initial oxidized state of the heteropoly compound catalysts. Reduction affects but does not destroy the local structure as probed by IR spectroscopy. The more easily reducible H4PVMo11O40 is active for propene oxidation at 513 K, Cs2H2PVMo11O40 only at 617 K.

Characterization of nanostructured binary molybdenum oxide catalyst precursors for propene oxidation

High-resolution transmission electron microscopy characterization of molybdenum oxide catalyst precursors is carried out for a better understanding of the mechanisms leading to the active catalytic material. The precipitated material consists of both large-unit cell crystals and randomly orientated clusters on a 3-5 nm scale. The crystalline clusters are embedded in non-crystalline material, preventing the formation of large well-defined inactive orthorhombic MoO(3) crystals during activation of the catalyst material.

Gas-phase rate coefficients for the reactions of nitrate radicals with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene at room temperature

Rate coefficients for reactions of nitrate radicals (NO3) with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene were determined to be (6.55 +/- 0.78)x 10(-13) cm3 molecule(-1) s(-1), (3.78 +/- 0.45)x 10(-13) cm3 molecule(-1) s(-1), (5.30 +/- 0.73)x 10(-13) cm(3) molecule(-1) s(-1), (3.83 +/- 0.47)x 10(-13) cm(3) molecule(-1) s(-1), (4.37 +/- 0.49)x 10(-13) cm(3) molecule(-1) s(-1), (3.61 +/- 0.40)x 10(-13) cm3 molecule(-1) s(-1) and (8.9 +/- 1.5)x 10(-12) cm3 molecule(-1) s(-1), respectively. We performed kinetic experiments at room temperature and atmospheric pressure using a relative-rate technique with GC-FID analysis. The experimental results demonstrate a surprisingly large cis-trans(Z-E) effect, particularly in the case of the pent-2-enes, where the ratio of rate coefficients is ca. 1.7. Rate coefficients are discussed in terms of electronic and steric influences, and our results give some insight into the effects of chain length and position of the double bond on the reaction of NO3 with unsaturated hydrocarbons. Atmospheric lifetimes were calculated with respect to important oxidants in the troposphere for the alkenes studied, and NO3-initiated oxidation is found to be the dominant degradation route for (Z)-pent-2-ene, (Z)-hex-3-ene and (E)-3-methylpent-2-ene.

A Q- and X-band pulsed electron nuclear double resonance study of the structure and location of the vanadyl ions in the Cs salt of heteropolyacid PVMo11O40

The location and coordination geometry of vanadium(IV) ions in the cesium salt of molybdovanadophosphoric heteropolyacid Cs(4)PVMo(11)O(40) were studied using orientation-selective pulsed ENDOR (electron nuclear double resonance) experiments. To enhance the orientation selectivity for the paramagnetic vanadyl species, these investigations were done at Q-band frequencies. It was possible to resolve interactions of the paramagnetic vanadyl ions (VO(2+)) with all relevant nuclei, (1)H, (31)P, (51)V, and (133)Cs. The location of the vanadyl species was studied by determination of the complete (31)P hyperfine tensor. This approach was done for both the fresh and the calcined Cs(4)PVMo(11)O(40) materials, and no differences in the structures of the VO(2+) complexes were found. The ENDOR results give experimental evidence for the location of the V(IV) ions. For both samples, it was possible to exclude the incorporation of V(IV) at the Mo sites. The VO(2+) species are directly attached to the outer surface of the heteropolyanion and coordinated to four of the outer oxygen atoms with a V-P distance of 3.96 A.

Influence of dehydration effects on the optical spectra of H4PVMo11O40 in the visible and near infrared range: intra- and intercenter optical transitions in the V-Mo cluster

UV-Vis-NIR diffuse reflectance spectra of heteropoly acids of the type of H4PVMo11O40.xH2O were recorded with improved spectroscopic equipment for different times on helium stream at room temperature. A broad asymmetric absorption band with a maximum at 740-780 nm in the visible range and a shoulder at 900-1000 nm in the near infrared range is identified. With increasing time on stream the intensity of this band grows appreciably: simultaneously, the water peaks at 1430 and 1925 nm decrease. Focusing on a single mixed-valence V4+-Mo6+-cluster, a model is suggested for the description of the experimental results. The model takes into account electron transfer, low symmetry crystal fields and vibronic interaction. The absorption band shape calculation is performed within the semiclassical limits. The band shape is shown to be determined both by the optical d-d transitions in V4+ and Mo5- and the intervalence V4+-Mo6+ optical transition. Appreciable changes in the band intensity are attributed to the increase in the electron transfer parameter due to water loss. Quite good agreement with the experiment was found for a reasonable set of transfer and vibronic parameters.

Elucidation of structure and location of V(IV) ions in heteropolyacid catalysts H4PVMo11O40 as studied by hyperfine sublevel correlation spectroscopy and pulsed electron nuclear double resonance at W- and X-band frequencies

Electron spin resonance, pulsed electron nuclear double resonance (ENDOR) spectroscopy at W- and X-band frequencies, and hyperfine sublevel correlation (HYSCORE) spectroscopy have been employed to determine the location of the V(IV) ions in H4PVMo11O40 heteropolyacid catalysts. In these materials the heteropolyanions have the well-known structure of the Keggin molecule. Interactions of the unpaired electrons of the paramagnetic vanadyl ions (VO(2+)) with all relevant nuclei 1H, 31P, and 51V) could be resolved. The complete analysis of the hyperfine coupling tensor for the phosphorus nucleus in the fourth coordination sphere of the V(IV) ion allowed for the first time a detailed structural analysis of the paramagnetic ions in heteropolyacids in hydrated and dehydrated catalysts. The 31P and 1H ENDOR results show that V(IV) ions are incorporated as vanadyl pentaaqua complexes [VO(H2O)5](2+) in the void space between the heteropolyanions in the hydrated heteropolyacid. For the dehydrated H4PVMo11O40 materials the distance between the V(IV) ion and the central phosphorus atom of the Keggin molecule could be determined with high accuracy on the basis of orientation-selective 31P ENDOR experiments and HYSCORE spectroscopy. The results give a first direct experimental evidence that the paramagnetic vanadium species are not incorporated at molybdenum sites into the Keggin structure of H4PVMo11O40 and also do not act as bridges between two Keggin units after calcination of the catalyst. The vanadyl species are found to be directly attached to the Keggin molecules. The VO(2+) ions are coordinated to four or three outer oxygen atoms from one PVMo11 heteropolyanion in a trigonal-pyramidal or slightly distorted square-pyramidal coordination geometry, respectively.