Kinetic and spectroscopic study of 1O2 generation from H2O2 catalyzed by LDH-MoO4(2-) (LDH=layered double hydroxide).

Layered double hydroxides (LDHs), exchanged with molybdate, decompose H2O2 to form one molecule of singlet-state dioxygen (1O2) from two molecules of H2O2. The dependence of the kinetics of H2O2 decomposition on Mo and H2O2 concentrations and on temperature has been related to structural characteristics of the material (X-ray diffraction (XRD), scanning electron microscopy (SEM), IR spectroscopy, N2 adsorption, thermogravimetry) and to molybdate speciation as revealed by in-situ studies in the presence of peroxide (FT Raman, diffuse reflectance UV/visible spectroscopy). The H2O2 decomposition rate is linearly correlated with the amount of LDH-exchanged molybdate, except when a considerable fraction of the molybdate occupies less accessible interlayer positions. A maximum in the H2O2 decomposition rate as the H2O2 concentration is increased is due to the successive formation of mono-, di-, tri-, and tetraperoxomolybdates. This behavior was modeled successfully by using the equilibrium constants for formation of the Mo-peroxo complexes, and the rate constants for decay of the peroxomolybdates with 1O2 liberation. Time-resolved diffuse reflectance and Raman observations of the various MoO4(2-)-peroxide adducts are in line with the proposed kinetic scheme. Of all the Mo-peroxo species on the LDH, the triperoxomolybdate has the highest rate for decay to 1O2. Comparison with the kinetics of dissolved molybdate shows that the monomolecular decay of all peroxomolybdate species proceeds much faster at the LDH surface than in solution. Consequently, maximal rates per Mo atom are at least twice as high for the heterogeneous LDH catalyst as for the homogeneous systems.

Behavior of Triticum durum Desf. arabinoxylans and arabinogalactan peptides during industrial pasta processing.

Three industrial pasta processing lines for different products (macaroni, capellini and instant noodles) were sampled at three subsequent stages (semolina, extruded, and dried end products) in the process. Arabinoxylans (AX) and arabinogalactan peptides (AGP) were analyzed. Although very low endoxylanase activities were measured, the level of water-extractable AX (WE-AX) increased, probably because of mechanical forces. No change was observed in the level and structural characteristics of AGP. The WE-AX molecular weight (MW) profiles showed a very small shift toward lower MW profiles; those of AGP revealed no changes as a result of the production process. After separation of WE-AX and AGP, (1)H NMR analysis and gas chromatography of the alditol acetates obtained following hydrolysis, reduction, and acetylation revealed no changes in the arabinose substitution profile of the WE-AX samples during pasta processing. At optimal cooking times, WE-AX losses in the cooking water are small (maximally 5.9%). However, the loss of AGP is more pronounced (maximally 25.0%). Overcooking led to more losses of both components.

Variation in the degree of D-xylose substitution in water-extractable European durum wheat (Triticum durum desf.) semolina arabinoxylans.

Durum wheat (Triticum durum Desf.) semolina water-extractable arabinoxylan (TWEAX) (yield 0.28%, arabinose-to-xylose ratio (A/X) 0. 62) was fractionated by a stepwise increase in ethanol concentration (up to 65%). The A/X ratios of the resulting fractions varied between 0.42 and 0.80. With increasing ethanol concentrations, increasing A/X ratios went hand in hand with a relative increase of low molecular weight compounds, indicating that high molecular weight compounds with a low A/X ratio are preferentially precipitated from alcohol/water mixtures. (1)H NMR showed that, whereas in TWEAX the levels of unsubstituted xyloses (X(0)), monosubstituted (X(1)), and disubstituted (X(2)) xyloses were 63.1%, 11.8%, and 25.1%, respectively, fractions that precipitated with increasing ethanol concentrations had decreasing levels of X(0). Simultaneously, the level of X(1) decreased equally until it leveled of at ca. 10%. Concomitantly, the level of X(2) increased. The levels of X(0), X(1), and X(2) varied between 69.7% and 53.4%, 18.2% and 10.7%, and 12.2% and 35.9%, respectively.

Distribution and structural variation of arabinoxylans in common wheat mill streams.

In 19 wheat-milling fractions total pentosan content, calculated as 0.88 x (% L-arabinose + % D-xylose), varied between 1.44 and 30.66% on dry matter (dm). It increased with ash content once the latter exceeded 0.6% (dm basis). Water-extractable arabinoxylans were recovered by saturating water extracts to 65% ethanol. Their contents in the milling fractions varied between 0.35 and 1.38%, and above 0.6% ash content also increased with this parameter. Their L-arabinose-to-D-xylose ratios ranged between 0.65 and 0.39, with the lowest values found for the fractions with highest ash content, indicating that the ash-rich tissues contain more arabinoxylans that are less branched. (1)H NMR spectroscopy revealed that the decrease in L-arabinose-to-D-xylose ratio was accompanied by an increase in unsubstituted xylose residues and a decrease in disubstituted xylose residues, while the contents of monosubstituted xyloses were virtually constant.

The 27Al DOR NMR characterization of the molecular sieve AlPO4-8.

27Al double rotation (DOR) NMR spectroscopy has been applied to investigate the framework ordering in the aluminophosphate molecular sieve AlPO4-8 during the hydration process. Relative well-resolved peaks in the DOR spectra of both dehydrated and successively rehydrated AlPO4-8 allow the isotropic shifts and the quadrupolar shifts to be correlated with the local framework structure. The rather complex interaction of water with AlPO4-8 occurs in a specific way, not randomly. For dehydrated AlPO4-8 efforts are shown to correlate respectively the isotropic shifts to the mean Al-O-P angles and the quadrupolar coupling constants to the shear strains of the different aluminium sites.