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The impact of chain orientation on the melting of the alpha-form of isotactic polypropylene
No full textIsotactic polypropylene (iPP) exhibits polymorphism in the crystalline state that has long being leveraged to tune material properties. It has been established, that even the most prevailing α-iPP can exhibit significant variability with two distinct melting points T m often attributed to α1 and α2 forms. Several structural models with specific ordering in chain orientations and correlations along the crystals have been proposed. Precise structural investigation relies on inverse modeling of diffraction patterns but the impact of thermal history, a potential transition between the two forms and any existence of multiple solutions challenges characterization. Recent detailed atomistic simulations offered a direct route to link postulated structural arrangements to melting transitions with studies focusing to the α1 and β forms of iPP. In this study, we employ atomistic molecular dynamics to examine models of α1 and α2 with variable levels of ordering in terms of chain axis orientation. At rapid heating rates, we find high melting points that can differ up to 30 K depending on the fraction of the material that follows the P21/c structural arrangement. To contrast systems in the absence of superheating, we assembled configurations with free surfaces as well as models of semicrystalline iPP. We report melting kinetics above Tm for the P21/c and Cc crystals and provide Tm estimates in agreement to literature data. In addition, our study provides valuable microscopic information on the dynamics of melting of α-iPP along specific directions
Partial ordering of isovalent Cd and Mn atoms in the quater-nary phase Eu2Mn1–xCd1+xPt2 (x = 0.11) with LaNiAl type struc-ture
No full textAfter attempts to synthesize the imaginary ternary phase “EuMnPt” failed, the solid solutions EuMnxCd1–xPt were prepared following speculations that random mixing of the divalent Cd and Mn metals may be achieved in the previously reported analogue EuCdPt (TiNiSi type, Pnma–c3). However, an unexpected atomic ordering resulted in the quaternary phase Eu2Mn1–xCd1+xPt2 (x = 0.11) which adopts the Pd2(Mn,Pd)Ge2 (or LaNiAl, Pnma–c6) structure type instead, in which two mixed Mn/Cd sites in the structure show pronounced site preference, reminiscent to that of Mn/Pd sites in the prototype. Thus, the crystal structure of Eu2Cd1+xMn1–xPt2 (x = 0.11) represents a unique example of 4f-3d-4d-5d intermetallic system with nearly complete atomic ordering, and it may be best viewed as an intergrowth of hexagonal ZrNiAl type (previously reported for CeCdxMn1–xPt) and orthorhombic TiNiSi type (previously reported for YbCdxMn1–xPt). It is also the first Eu based compound to adopt the LaNiAl type structure, reported only with the earlier lanthanides (Ln = La, Ce, Pr, Nd), and all noble metals of the Pt group, but Os and Pt itself. The sublattice of paramagnetic Mn atoms consists of linear zigzag chains with Mn–Mn distance of 3.084(1) Å, while triangular magnetic geometrical frustration is realised in the Eu sublat-tice. Thus, the rather unexpected atomic segregation between two isovalent transition metals Mn (3d5) and Cd (4d10) may be ascribed, at first glance, to bonding preference for homonuclear Mn–Mn over heteronuclear Mn–Cd interactions, in agreement with the 18–n rule
Heterologous Biosynthesis of Cotylenol and Concise Synthesis of Fusicoccane Diterpenoids
No full textFusicoccane diterpenoids have attracted intensive attention due to their complex structures as well as diverse biological activities. Here we report a novel strategy for the synthesis of cotylenol and other fusicoccane diterpenoids. By harnessing the biosynthetic pathways of brassicicenes and fusicoccins, cotylenol was produced in an engineered Aspergillus oryzae strain. We further achieved the concise synthesis of three fusicoccane diterpenoids, including alterbrassicicene E and brassicicenes A and R in 4 or 5 chemical steps from brassicicene I. Our strategy may facilitate the preparation of fusicoccane diterpenoids and their synthetic analogues for biological studies
Phosphorylated Sporopollenin as a Sustainable Catalyst for Selective 5-Hydroxymethylfurfural Formation in Water: Insights into Phosphate Functionalization, Kinetics, and Mechanism
No full textMetal-free Brønsted acid catalysts in water are known to facilitate undesired side reactions such as polymers and formic acid synthesis, making it challenging to synthesize 5-hydroxymethylfurfural (5-HMF) from C6 sugars. Therefore, water-tolerant organic heterogeneous catalysts with high selectivity towards 5-HMF are of great interest. In this study, sporopollenin (exine), a natural biopolymer biomass, is employed as a heterogeneous support. We demonstrate for the first time that ortho-phosphoric acid-cleaning of the protoplasmic content on spores produces empty sporopollenin (ESP) functionalized with mono- and di-phosphoesters (41:59), (ESP-Phos) which can be used as a selective sustainable catalyst for the formation of 5-HMF from glucose. Activating ESP-Phos at 200 °C results in a substantial increase in the di-phosphoester ratio of ESP-Phos200 (29:71), as shown by 31P NMR. This results in improved activity (92% yield) and selectivity (96%) for the synthesis of 5-HMF. The DFT calculations further suggest that glucose to di-phosphoester interactions are stronger (–86.1 kJ mol–1) than mono-phosphoester interactions (–72.5 kJ mol–1), explaining the potential enhancement in glucose to 5-HMF catalysis. Kinetics analysis indicated that the catalytic system follows a pseudo-first-order reaction. ESP-Phos200 increased the glucose to HMF formation rate 23-fold (rate constant k = 0.0046 min–1) higher under optimized conditions (180 oC, 12 h) as compared to the humin formation rate. A detailed mechanistic study involving isotopic labelling, 13C NMR, and DFT calculations suggested that ESP-Phos200 followed a direct glucose dehydration mechanism rather than glucose isomerization to fructose and its subsequent dehydration. Although humin polymer deposition was observed after each run (up to 4 cycles), causing deactivation of active sites, subsequent calcining of ESP-Phos200 at 200°C restored the activity. The catalyst and reaction method used here for the synthesis of 5-HMF are environmentally friendly, sustainable, and promising for large-scale production of 5-HMF
Microfluidic platform for screening the activity of immobilized photocatalysts for degradation of organic water pollutants
No full textPhotochemistry screening platforms have the potential to accelerate the discovery and development of new photocatalysts. This study presents the design and characterization of a novel activity screening platform of immobilized photocatalytic films for degrading water pollutants. The compact testing system is engineered with four 3D-printed microreactors and a rotative multi-wavelength LED light source, which is capable of emitting at the 395, 409, 413, and 443 nm. Despite the different LEDs being placed in a compact space, 95% of the light that reaches the photocatalytic films is emitted by the LEDs directly opposite them. Therefore, the design allows for a minimum of 16 distinct testing conditions by simply rotating the light source. The performance of the microfluidic platform was characterized using the photocatalytic degradation of a pesticide, imidacloprid, in the presence of P25 TiO2, immobilized as thin film on glass plates. The results demonstrated a consistent degradation efficiency of around 35 % at 395 nm, with negligible variation across the four microreactors and no influence of the testing order at 395, 409, 413 and 443 nm. Notably, the photocatalytic film activity did not decrease after 6 hours of operation and under five successive illumination conditions. The screening conditions were optimized using the dynamic water infusion which increased the degradation efficiency of the imidacloprid to 71 %. In addition, the dynamic illumination allowed the sequential operation of the 4 types of LEDs, and led to a halved degradation efficiency despite the LEDs were lighted up for only a quarter of the time. This microfluidic platform diminishes the manual labor and the quantities of photocatalyst and polluted water required per test compared to the batch screening, consequently, it emerging as an efficient and sustainable tool that is suitable for the automated screening of immobilized photocatalysts
A BN-Benzvalene
No full textThe synthesis and crystallographic characterization of BN-benzvalene, the first light-element hetero-benzvalene, is described. BN-benzvalenes are produced via photoexcitation of C5-aryl-substituted 1,2-azaborines under flow conditions. Mechanistic studies support a boron-specific, two-step photoisomerization pathway involving a BN-Dewar benzene intermediate, which is distinct from the photoisomerization pathway proposed in benzene and phospha- and sila-benzenes for the formation of their respective benzvalene analogues
On the Interplay Between Force, Temperature, and Electric Fields in the Rupture Process of Mechanophores
No full textThe use of oriented external electric fields (OEEFs) shows promise as an alternative method for catalyzing chemical reactions. The ability to target a specific bond by aligning it with a bond-weakening electric field may be beneficial in mechanochemical reactions, which use mechanical force to selectively rupture specific bonds. Previous computational studies have focused primarily on a static description of molecules in OEEFs, while the crucial influence of thermal oscillations on the stability of the molecules has been neglected. Here, we performed ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT) to investigate the behaviour of a model mechanophore under the simultaneous influence of thermal and electric field effects. We examine and compare the changes to the bond and its thermal oscillations in strong OEEFs at various temperatures, without and with mechanical stretching forces applied to the molecule. We show that the change in bond length caused by an electric field is largely independent of the temperature, both without and with applied mechanical force. The amplitude of the thermal oscillation increases with increasing field strength and with increasing temperature, but at low temperatures, the application of mechanical force leads to an additional increase in amplitude. Our research shows that methods for applying mechanical force and OEEFs can be safely combined and included in an AIMD simulation at both low and high temperatures, allowing researchers to computationally investigate mechanochemical reactions in realistic application scenarios
Pristine and Aged Microplastics Can Nucleate Ice Through Immersion Freezing
No full textMicroplastics (MP) are ubiquitous in the environment; their atmospheric relevance is increasingly recognized. Because of their atmospheric concentrations, a question exists as to whether MP can act as ice nucleating particles in the atmosphere. This study investigates the immersion freezing activity of lab-prepared MP of four different compositions—low density polyethylene (LDPE), polypropylene (PP), poly(vinyl chloride) (PVC), and polyethylene terephthalate (PET)—using droplet freezing assays. The MP are also exposed to ultraviolet light, ozone, sulfuric acid, and ammonium sulfate to mimic environmental aging of the plastics to elucidate the role that these processes play in the ice nucleating activity of MP. Results show that all studied MP act as immersion nuclei and aging processes can modify this ice nucleating activity, leading, primarily, to decreases in ice nucleating activity for LDPE, PP, and PET. The ice nucleating activity of PVC generally increased following aging which we attribute to a cleaning of chemical defects present on the surface of the stock material. Chemical changes were monitored with infrared spectroscopy (ATR-FTIR) and the growth of a peak at 1650-1800 cm-1 was associated with a decrease in ice nucleating activity while loss of an existing peak in that region was associated with an increase in ice nucleating activity. The studied MP have ice nucleating activities sufficient to be a non-negligible source of ice nucleating particles in the atmosphere if present in sufficiently high concentrations
Mechanochemical ATRP: an asset for the bulk copolymerization of solid and liquid monomers
No full textAtom Transfer Radical Polymerization (ATRP) is one breakthrough technique to obtain well-defined and controlled polymers or copolymers. However, this technique has been mainly considered in solution and alternatives such as solvent-free mechanochemistry in a ball-mill, an emerging approach for a greener and safer chemistry, is left aside as a tool for polymer synthesis. Nevertheless, mechanochemistry in a ball-mill offers many advantages and permits to avoid the use of solvent, with shorter reaction times and simplified treatment steps making it easier to recover the product of interest. Herein, interest was put onto the development of mechanochemical ARGET-ATRP conditions for styrene polymerization, one solid styrenic derivative sodium styrene sulfonate, one liquid styrenic derivative 4-vinylbenzyl chloride and a mixture of both monomers. A fine tuning of the catalyst quantity and easily removable ascorbic acid/Na2CO3 reducing agent, never describe in the ball-mill previously and more practical than other reducing agent, gave polymers and copolymers in good to excellent yield (up to >99 %), with a control over dispersity (typically Đ < 1.5), as analyzed by 1H NMR and size exclusion chromatography, respectively. As a further matter, these unprecedented conditions allow to render null and void all considerations concerning the physical state, properties (hydrophilic, hydrophobic) and miscibility/solubility of monomers which are key parameters in solution based ATRP for the synthesis of polymer and copolymer
Unhindered Biplane-Shaped N-Heterocyclic Carbenes as Powerful Ligands in Challenging Ru-Catalysed Olefin Metathesis of Sterically-Crowded Substrates
No full textDespite notable progress, olefin metathesis methods for preparing sterically crowded C-C double bonds remain scarce. They are commonly based on specialised ruthenium catalysts with sterically reduced N-heterocyclic carbene (NHC) ligands, able to accommodate more crowded olefinic substrates during the catalytic steps. Yet, although being highly active, these complexes are rather unstable. In the case of Ru complexes containing NHC ligands with N-aryl groups, the catalyst deactivation is mainly due to intramolecular C-H activation at the ortho position of the N-aryl group of the NHC ligand. Considering that the deleterious C-H activation process requires the rotation of the N-aryl arm of the NHC ligand, we introduced a second decker of aromatic groups in benzimidazolylidene-based N-phenyl NHC ligands, which led to robust and highly efficient ruthenium metathesis catalysts in challenging metathesis reactions of tri-and tetra-substituted olefins. The beneficial effect of these upper aromatic “wings” on the stability and activity of the Ru-complexes is rationalised through the experimental determination of the stereoelectronic properties of the NHC ligands, complemented by DFT calculations on the nature of the through-space interactions between the aromatics and on the decomposition pathway of these second-generation Hoveyda precursors