Specificity of the Thermal Stability and Reactivity of Two-Dimensional Layered Cu-Fe Sulfide-Mg-Based Hydroxide Compounds (Valleriites).

We recently synthesized prospective new materials composed of alternating quasi-atomic sheets of brucite-type hydroxide (Mg, Fe)(OH)2 and CuFe1-xS2 sulfide (valleriites). Herein, their thermal behavior important for many potential applications has been studied in inert (Ar) and oxidative (20% O2) atmospheres using thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses and characterization with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). In the Ar media, the processes are determined by the dehydroxylation of the hydroxide layers forming MgO, with the temperature of the major endothermic maximum of the mass loss at 413 °C. Sulfide sheets start to degrade below 500 °C and melt at nearly 800 °C, with bornite, chalcopyrite, and troilite specified as the final products. In the oxidative atmosphere, the exothermic reactions with the mass increase peaked at 345 and 495 °C, corresponding to the partial and major oxidations of Cu-Fe sulfide layers. Sulfur oxides captured in magnesium hydroxide layers to form MgSO4 compromised the layer integrity and promoted the oxidation of the sulfide entities. The final products also contained minor MgO, Cu2MgO3, Fe3O4, and MgFe2O4 phases. Samples doped with Al, which decreases the content of Fe in hydroxide layers, show notably impeded decay of valleriite in argon but facilitated the oxidation of Cu-Fe sulfides, while the impact of Li (it slightly increases the number of the Fe-OH sites) was less expressed. The mutual stabilization of the two-dimensional (2D) hydroxide and sulfide layers upon heating in an inert atmosphere but not in oxygen as compared with bulk brucite and chalcopyrite was suggested to explain high thermal resistance across the stacked incommensurate sheets, which slows down the endothermic reactions and accelerates the exothermic oxidation; the high number of Fe atoms in the hydroxide sheets are expected to promote the phonon exchange and heat transfer between the layers.

Fractionation of Aspen Wood to Produce Microcrystalline, Microfibrillated and Nanofibrillated Celluloses, Xylan and Ethanollignin.

A new method for extractive-catalytic fractionation of aspen wood to produce microcrystalline (MCC), microfibrillated (MFC), nanofibrilllated (NFC) celluloses, xylan, and ethanollignin is suggested in order to utilize all of the main components of wood biomass. Xylan is obtained with a yield of 10.2 wt.% via aqueous alkali extraction at room temperature. Ethanollignin was obtained with a yield of 11.2 wt.% via extraction with 60% ethanol from the xylan-free wood at 190 °C. The lignocellulose residue formed after the extraction of xylan and ethanollignin was subjected to catalytic peroxide delignification in the acetic acid-water medium at 100 °C in order to obtain microcrystalline cellulose. MCC is hydrolyzed with 56% sulfuric acid and treated with ultrasound to produce microfibrillated cellulose and nanofibrillated cellulose. The yields of MFC and NFC were 14.4 and 19.0 wt.%, respectively. The average hydrodynamic diameter of NFC particles was 36.6 nm, the crystallinity index was 0.86, and the average zeta-potential was 41.5 mV. The composition and structure of xylan, ethanollignin, cellulose product, MCC, MFC, and NFC obtained from aspen wood were characterized using elemental and chemical analysis, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analyses, Gas chromatography (GC), Gel permeation-chromatography (GPC), Scanning electron microscopy (SEM), Atomic force microscopy (AFM), Dynamic light scattering (DLS), Thermal gravimetric analysis (TGA).

Sulfation of Birch Wood Microcrystalline Cellulose with Sulfamic Acid Using Ion-Exchange Resins as Catalysts.

Cellulose sulfates are important biologically active substances with a wide range of useful properties. The development of new methods for the production of cellulose sulfates is an urgent task. In this work, we investigated ion-exchange resins as catalysts for the sulfation of cellulose with sulfamic acid. It has been shown that water-insoluble sulfated reaction products are formed in high yield in the presence of anion exchangers, while water-soluble products are formed in the presence of cation exchangers. The most effective catalyst is Amberlite IR 120. According to gel permeation chromatography, it was shown that the samples sulfated in the presence of the catalysts KU-2-8, Purolit s390 plus, and AN-31 SO42- underwent the greatest degradation. The molecular weight destribution profiles of these samples are noticeably shifted to the left towards low-molecular-weight compounds with an increase in fractions in the regions Mw ~2.100 g/mol and ~3.500 g/mol, indicating the growth of microcrystalline cellulose depolymerization products. The introduction of a sulfate group into the cellulose molecule is confirmed using FTIR spectroscopy by the appearance of absorption bands at 1245-1252 cm-1 and 800-809 cm-1, which correspond to the vibrations of the sulfate group. According to X-ray diffraction data, amorphization of the crystalline structure of cellulose is observed during sulfation. Thermal analysis has shown that with an increase in the content of sulfate groups in cellulose derivatives, thermal stability decreases.

Food Xanthan Polysaccharide Sulfation Process with Sulfamic Acid.

Xanthan is an important polysaccharide with many beneficial properties. Sulfated xanthan derivatives have anticoagulant and antithrombotic activity. This work proposes a new method for the synthesis of xanthan sulfates using sulfamic acid. Various N-substituted ureas have been investigated as process activators. It was found that urea has the greatest activating ability. BBD of xanthan sulfation process with sulfamic acid in 1,4-dioxane has been carried out. It was shown that the optimal conditions for the sulfation of xanthan (13.1 wt% sulfur content) are: the amount of sulfating complex per 1 g of xanthan is 3.5 mmol, temperature 90 °C, duration 2.3 h. Sulfated xanthan with the maximum sulfur content was analyzed by physicochemical methods. Thus, in the FTIR spectrum of xanthan sulfate, in comparison with the initial xanthanum, absorption bands appear at 1247 cm-1, which corresponds to the vibrations of the sulfate group. It was shown by GPC chromatography that the starting xanthan gum has a bimodal molecular weight distribution of particles, including a high molecular weight fraction with Mw > 1000 kDa and an LMW fraction with Mw < 600 kDa. It was found that the Mw of sulfated xanthan gum has a lower value (~612 kDa) in comparison with the original xanthan gum, and a narrower molecular weight distribution and is characterized by lower PD values. It was shown by thermal analysis that the main decomposition of xanthan sulfate, in contrast to the initial xanthan, occurs in two stages. The DTG curve has two pronounced peaks, with maxima at 226 and 286 °C.