Surface Modification of Kraft Lignin by Mechanochemical Processing with Sodium Percarbonate.

In this article, we present a novel one-pot mechanochemical reaction for the surface activation of lignin. The process involves environmentally friendly oxidation with hydrogen peroxide, depolymerization of fractions with high molecular mass, and introduction of new carbonyl functions into the lignin backbone. Kraft lignin was ground with sodium percarbonate and sodium hydroxide in a ball mill at different time intervals. Analyses by infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), size exclusion chromatography (SEC), dynamic vapor sorption (DVS), and small-angle X-ray scattering (SAXS) showed significant improvements. After only 5 min of reaction, there was a 47% reduction in mass-average molecular weight and an increase in carboxyl functionalities. Chemical activation resulted in an approximately 2.8-fold increase in water adsorption. Principal component analysis (PCA) provided further insight into the correlations between IR spectra and SAXS parameters.

Thermally processed Ni-and Co-struvites as functional materials for proton conductivity.

Here, we describe how to synthesise proton-conductive transition metal phosphates (TMPs) by direct thermal processing of precursor M-struvites, NH4MPO4·6H2O, with M = Ni2+, Co2+. In the as-derived TMP phases their thermal history and bulk proton conductivity were linked with the structural information about the metal coordination, phosphate groups, and volatile compounds. These aspects were investigated with vibrational and synchrotron-based spectroscopic methods (FT-IR, FT-RS, XAS). We elucidated the structures of amorphous and crystalline Ni- and Co phosphate phases in association with different coordination changes and distortion degrees of the metal polyhedra as they developed upon heating. Ni-struvite transformed to a stable amorphous phase over a broad range of temperatures (90 °C < T < 600 °C), in which it remained in an octahedral coordination environment, but the degree of distortion changed with T. In contrast, heating of Co-struvite led to several successive crystalline phases with only unstable transitional and short-lived amorphous components. Among the as-occurring phases, a highly functional layered M-dittmarite NH4MPO4·H2O obtained at low temperatures (T < 200 °C) demonstrated high proton conductivity values of 4.2 × 10-5 S cm-1 for Ni-dittmarite and Co-dittmarite > 10-4 S cm-1 at room temperature. Even at low humidity, these values are comparable with those found for Nafion, MOFs, some perovskites or composite materials. Coprecipitation of phosphates and transition metal cations in the form of struvite is potentially a viable method to extract these elements from wastewater. Thus, we propose that recycled M-struvites could be potentially further directly upcycled into crystalline and amorphous TMPs useful for electrochemical applications.

Novel and easy access to highly luminescent Eu and Tb doped ultra-small CaF2, SrF2 and BaF2 nanoparticles - structure and luminescence.

A universal fast and easy access at room temperature to transparent sols of nanoscopic Eu3+ and Tb3+ doped CaF2, SrF2 and BaF2 particles via the fluorolytic sol-gel synthesis route is presented. Monodisperse quasi-spherical nanoparticles with sizes of 3-20 nm are obtained with up to 40% rare earth doping showing red or green luminescence. In the beginning luminescence quenching effects are only observed for the highest content, which demonstrates the unique and outstanding properties of these materials. From CaF2:Eu10 via SrF2:Eu10 to BaF2:Eu10 a steady increase of the luminescence intensity and lifetime occurs by a factor of ≈2; the photoluminescence quantum yield increases by 29 to 35% due to the lower phonon energy of the matrix. The fast formation process of the particles within fractions of seconds is clearly visualized by exploiting appropriate luminescence processes during the synthesis. Multiply doped particles are also available by this method. Fine tuning of the luminescence properties is achieved by variation of the Ca-to-Sr ratio. Co-doping with Ce3+ and Tb3+ results in a huge increase (>50 times) of the green luminescence intensity due to energy transfer Ce3+ → Tb3+. In this case, the luminescence intensity is higher for CaF2 than for SrF2, due to a lower spatial distance of the rare earth ions.