4 research outputs found
Structural Modifications of Residual Lignins from Sisal and Flax Pulps during Soda-AQ Pulping and TCF/ECF Bleaching
We
have studied the structural modifications of lignins from sisal and
flax during their soda-anthraquinone (AQ) pulping and subsequent totally
chlorine-free (TCF) and elementary chlorine-free (ECF) bleaching.
For this purpose, residual lignins were isolated from pulps, analyzed
by Py-GC/MS, 2D-NMR, and GPC, and their characteristics were compared
to the āmilled-woodā lignin of the raw materials. Soda-AQ
pulping caused a preferential removal of S-lignin and cleavage of
Ī²ā<i>O</i>ā4ā² linkages, but the
structure of the residual lignin remained relatively similar to native
lignin. TCF bleaching barely affected the lignin structure, and noticeable
amounts of Ī²ā<i>O</i>ā4ā² linkages
still occur in these pulps. In contrast, ECF bleaching caused strong
modifications in the lignin structure with the complete removal of
lignin markers in ECF-bleached sisal pulp. However, residual lignin
was still present in ECF-bleached flax pulp, with a predominance of
G- and H-lignin units and the presence of Ī²ā<i>O</i>ā4ā² linkages
Synthesis of 4H-SrMnO<sub>3.0</sub> Nanoparticles from a Molecular Precursor and Their Topotactic Reduction Pathway Identified at Atomic Scale
Stoichiometric
4H-SrMnO<sub>3.0</sub> nanoparticles have been synthesized
from thermal decomposition of a new molecular heterometallic precursor
[SrMnĀ(edta)Ā(H<sub>2</sub>O)<sub>5</sub>]Ā·<sup>3</sup>/<sub>2</sub>H<sub>2</sub>O whose crystal structure has been solved by single
crystal X-ray diffraction. From this precursor, highly homogeneous
4H-SrMnO<sub>3.0</sub> nanoparticles, with average particle size of
70 nm, are obtained. The agglomeration of these nanoparticles maintains
the sheet-assembling morphology of the metalāorganic compound.
Local structural information, provided by atomically resolved microscopy
techniques, shows that 4H-SrMnO<sub>3.0</sub> nanoparticles exhibit
the same general structural features as the bulk material, although
structural disorder, due to edge dislocations, is observed. The nanometric
particle size enables a topotactic reduction process at low temperature
stabilizing a metastable 4H-SrMnO<sub>2.82</sub> phase. The oxygen
deficiency is accommodated through extra cubic layers breaking the
...chch... 4H-sequence. These defect areas are Mn<sup>3+</sup> rich,
as evidenced by high energy resolution EELS data. Magnetic characterization
of nano-SrMnO<sub>3.0</sub> shows significant variations with respect
to the bulk material. Besides the dominant antiferromagnetic interactions,
a weak ferromagnetic contribution as well as exchange bias and a glassy-like
component are present. After the reduction process, the stabilization
of Mn<sup>3+</sup> in the 4H-structure gives rise to magnetic anomalies
in the 40ā60 K temperature range. The origin of such magnetic
features is discussed
Modification of the Lignin Structure during Alkaline Delignification of Eucalyptus Wood by Kraft, Soda-AQ, and SodaāO<sub>2</sub> Cooking
The modification of the lignin structure
of an eucalyptus feedstock
during alkaline delignification by kraft, soda-AQ, and soda-O<sub>2</sub> cooking processes has been investigated by different analytical
techniques (size exclusion chromatography (SEC), pyrolysis gas chromatographyāmass
spectroscopy (Py-GC/MS), <sup>1</sup>Hā<sup>13</sup>C two-dimensional
nuclear magnetic resonance (2D-NMR), and <sup>31</sup>P NMR). The
characteristics of the lignins were compared at different pulp kappa
levels, and with the native lignin isolated from the wood. The structural
differences between the kraft, soda-AQ, and soda-O<sub>2</sub> residual
lignins were more significant at earlier pulping stages. At the final
stages, all the lignin characteristics were similar, with the exception
of their phenolic content. Strong differences between lignins from
pulps and cooking liquors were observed, including enrichment in guaiacyl
units in pulp residual lignin and enrichment in syringyl units in
black liquor lignin. A comparison of the alkaline cookings indicate
that soda-O<sub>2</sub> process produced higher lignin degradation
and provided promising results as pretreatment for the deconstruction
of eucalyptus feedstocks for subsequent use in lignocellulose biorefineries