•A new, fast screening method for surface polarity estimation of porous materials is proposed.•The method relies on the competitive liquid-phase adsorption of 1,4-dioxane from solvents of different polarity.•Structural modifications of MOFs effecting the surface polarity are sensitively reflected by the proposed polarity index.A new methodology for surface polarity screening of MOFs is proposed based on quantitative 1H NMR spectroscopic measurements of liquid-phase adsorption. The influence of the surface polarity on the adsorption process was studied on several materials, e.g. activated carbons, PAF-1, MIL-101(Cr), HKUST-1, and a UiO-67-series. The surface polarity was characterized through the difference in the 1,4-dioxane adsorption uptake from two solvents of opposite polarity, namely n-heptane and N,N-dimethylformamide. An NMR-derived polarity index was defined where the polarity of the MOF corresponds to its affinity to polar substances. It is demonstrated that the structural modifications of MOF materials, which should affect the polarity of these MOFs, are indeed reflected by the polarity index.Download high-res image (310KB)Download full-size image

Iron incorporation into diatom biosilica was investigated for the species Stephanopyxis turris. It is known that several “foreign” elements (e.g., germanium, titanium, aluminum, zinc, iron) can be incorporated into the siliceous cell walls of diatoms in addition to silicon dioxide (SiO2). In order to examine the amount and form of iron incorporation, the iron content in the growth medium was varied during cultivation. Fe:Si ratios of isolated cell walls were measured by ICP-OES. SEM studies were performed to examine of a possible influence of excess iron during diatom growth upon cell wall formation. The chemical state of biosilica-attached iron was characterized by a combination of infrared, 29Si MAS NMR, and EPR spectroscopy. For comparison, synthetic silicagels of variable iron content were studied. Our investigations show that iron incorporation in biosilica is limited. More than 95% of biosilica-attached iron is found in the form of iron clusters/nanoparticles. In contrast, iron is preferentially dispersedly incorporated within the silica framework in synthetic silicagels leading to Si–O–Fe bond formation.

Highlights•Synthesis of 13C,29Si-labeled tetraphenoxysilane.•H1–C13–Si29 double CP with labeled tetraphenoxysilane.•Double CP and 13C{29Si} REDOR experiments on diatom biosilica cell walls.•Most likely polyamines are the interface species in diatom biosilica.Triple resonance solid-state NMR experiments using the spin combination 1H–13C–29Si are still rarely found in the literature. This is due to the low natural abundance of the two heteronuclei. Such experiments are, however, increasingly important to study hybrid materials such as biosilica and others. A suitable model substance, ideally labeled with both 13C and 29Si, is thus very useful to optimize the experiments before applying them to studies of more complex samples such as biosilica. Tetraphenoxysilane could be synthesized in an easy, two-step synthesis including double isotope labelling. Using tetraphenoxysilane, we established a 1H–13C–29Si double CP-based HETCOR experiment and applied it to diatom biosilica from the diatom species Thalassiosira pseudonana. Furthermore, we carried out 1H–13C{29Si} CP-REDOR experiments in order to estimate the distance between the organic matrix and the biosilica. Our experiments on diatom biosilica strongly indicate a close contact between polyamine-containing parts of the organic matrix and the silica. This corroborates the assumption that the organic matrix is essential for the control of the cell wall formation.Graphical abstract

•Porous carbons were prepared by chlorination of silicon oxycarbides.•Electronic and pore structure of carbon was controlled via synthesis temperature.•EDLC electrodes were prepared with aqueous slurry and doctor blade technique.•Resulting electrodes show high power and energy density.In this study, we report on the preparation of new silicon oxycarbide-derived carbons (SiOCDC) obtained by pyrolysis and chlorination of a polyphenylsilsesquioxane pre-ceramic precursor. Wet-chemical conversion of phenyltrimethoxysilane (PhTMS) to the organosilica material was conducted using a two-step acid/base sol–gel process in aqueous medium. The resulting material was subsequently pyrolysed at 700, 1000 and 1300 °C to obtain a non-porous silicon oxycarbide ceramic. Chlorination at 700 and 1000 °C led to carbons having large surface areas exceeding 2000 m2 g−1 as well as large micro-/mesopore volumes up to 1.4 cm3 g−1. The temperature of the thermal treatment significantly influences the carbon and final pore structure. Pyrolysis at 700 °C and subsequent chlorination at 700 °C led to a mainly microporous material, whereas pyrolysis at 1300 °C and subsequent chlorination at 1000 °C generated a hierarchically porous SiOCDC with micro- and mesopores, respectively. All SiOCDC materials were prepared as supercapacitor electrodes using an aqueous slurry containing polytetrafluoroethylene (PTFE) and sodium carboxymethyl cellulose (CMC) as binder. With an organic electrolyte (1 M TEABF4 in acetonitrile) capacitances of up to 110 F g−1 and good long term stabilities could be observed.

Diatoms—unicellular algae with silicified cell walls—have become model organisms for investigations of biomineralization processes. Numerous studies suggest the importance of biosilica-associated or even embedded biomolecules for the biosilica formation. Such molecules are peptides, polyamines, and even saccharides. However, the role of the latter class of biomolecules is only poorly understood yet. Therefore, we investigated the saccharide composition of the biosilica-associated organic material of the diatom Stephanopyxis turris. This species exhibits a considerably high saccharide content in its siliceous cell walls. Gas chromatography-mass spectrometry analysis revealed that mannose-6-phosphate is strongly associated to the cell walls. This phosphorylated saccharide has not yet been found in diatom biosilica. In vitro studies on the polyallylamine-induced silica precipitation were carried out in the presence of mannose-6-phosphate. Compared to inorganic phosphate, mannose-6-phosphate significantly influenced the precipitation behavior of this model system suggesting a possible contribution of mannose-6-phosphate to the biomineralization process of Stephanopyxis turris.Graphical abstractHighlights► Investigation of the saccharide fraction of the biosilica from Stephanopyxis turris. ► We report for the first time a biosilica-associated phosphorylated saccharide. ► Our studies suggest a contribution to the biomineralization of this diatom.

The synthesis and structural flexibility of the metal–organic frameworks M2(2,6-ndc)2(dabco) (DUT-8(M), M = Ni, Co, Cu, Zn; 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) as well as their characterization by gas adsorption, 129Xe NMR and 13C MAS NMR spectroscopy are described. Depending on the integrated metal atom the compounds show reversible (DUT-8(Ni), DUT-8(Co)), non-reversible (DUT-8(Zn)) or no (DUT-8(Cu)) structural transformation upon solvent removal and/or physisorption of several gases. DUT-8(Co) exhibits a similar structural transformation by solvent removal and adsorption behavior as observed for DUT-8(Ni). DUT-8(Zn) undergoes an irreversible structural change caused by solvent removal. The non-flexible copper containing MOF reveals the best performance concerning porosity and gas storage capacities within the DUT-8 series. Xenon adsorption studies combined with 129Xe NMR spectroscopy are used to study the flexibility of the DUT-8 compounds. 129Xe chemical shift and line width strongly depend on the metal atom. Solid-state 13C NMR spectroscopy has been applied in order to further characterize the organic parts of the DUT-8 frameworks. While DUT-8(Ni) exhibits narrow, well-resolved lines in its “as made” state, the signals of DUT-8(Co) are broadened and shifted over an unusually wide chemical shift range (−72 to 717 ppm). No detectable signals are found in DUT-8(Cu) indicating significantly changed internal dynamics compared to DUT-8(Ni) and DUT-8(Co).

The synthesis and structure of a new flexible metal–organic framework Ni2(2,6-ndc)2(dabco) (DUT-8(Ni), DUT = Dresden University of Technology, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) as well as its characterization by gas adsorption and 129Xe NMR spectroscopy is described. The compound shows reversible structural transformation without loss of crystallinity upon solvent removal and physisorption of several gases. Xenon adsorption studies combined with 129Xe NMR spectroscopy turn out to be favorable methods for the detection and characterization of the so-called “gate-pressure” effect in this novel MOF material. The linewidth and chemical shift of the 129Xe NMR signal are shown to be very sensitive parameters for the detection of this structural transition from a narrow pore system with low porosity to a wide pore state. The transition and threshold temperature is clearly detected.

The synthesis and structure of a new flexible metal–organic framework Ni2(2,6-ndc)2(dabco) (DUT-8(Ni), DUT = Dresden University of Technology, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) as well as its characterization by gas adsorption and 129Xe NMR spectroscopy is described. The compound shows reversible structural transformation without loss of crystallinity upon solvent removal and physisorption of several gases. Xenon adsorption studies combined with 129Xe NMR spectroscopy turn out to be favorable methods for the detection and characterization of the so-called “gate-pressure” effect in this novel MOF material. The linewidth and chemical shift of the 129Xe NMR signal are shown to be very sensitive parameters for the detection of this structural transition from a narrow pore system with low porosity to a wide pore state. The transition and threshold temperature is clearly detected.