Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- 2017 (21) (entfernen)
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- Mechanochemistry (13)
- In situ (8)
- XRD (4)
- Crystallization (2)
- Metal phosphonate (2)
- Polymorphism (2)
- Raman spectroscopy (2)
- 4'-hydroxyacetophenone (1)
- Acoustic levitator (1)
- Admixtures (1)
Eingeladener Vortrag
- nein (4)
We present a first direct measurement of the temperature during milling combined with in situ Raman spectroscopy monitoring. The data reveal a low temperature increase due to the mechanical impact and clear temperature increases as a consequence of the reaction heat. Based on the data, temperature rises as postulated in the magma plasma and hot spot theory can be excluded for soft matter milling syntheses.
Two strontium-based dicarboxylate systems [Sr(oBDC-F4)(H2O)2] (1) and [{Sr(oBDC)(H2O)2)·H2O] (2) were synthesized mechanochemically via milling of Sr(OH)2·8H2O with tetrafluorophthalic acid (H2oBDC-F4) or phthalic acid (H2oBDC), respectively. The new structures were determined ab initio from the powder X-ray diffraction (PXRD) data. Both compounds 1 and 2 crystallize in the monoclinic space group P21 /c as two-dimensional coordination polymers (2D-CPs). The determined structures were validated by extended X-ray absorption (EXAFS) data. Compounds 1 and 2 show different thermal stabilities. The fluorinated CP 1 is decomposed at 300 °C while the nonfluorinated CP 2 transforms into a new phase after thermal treatment at 400 °C. The two hydrated CPs exhibit small surface areas which increase after the thermal posttreatment for 1 but remains unchanged for the dehydrated sample of 2. Dynamic vapor sorption (DVS) experiments indicate that both the dehydrated and hydrated samples of 2 depict no significant differences in their adsorption isotherms. The DVS of water indicates that the phase transition after thermal posttreatment of 2 is irreversible.
Recent advances in our understanding of CaCO, nucleation from solution have provoked new and challenging questions. We have studied CaC03 formation using precipitation by carbonate ester hydrolysis which ensures precipitation from a strictly homogeneous solution state and allows “titrating” carbonate to a solution with a given Ca2+ concentration on a timescale suited for kinetic studies. Nucleation and crystallization were traced by combining dynamic light Scattering (DLS) and transmission electron microscopy (TEM). DLS served as in situ technique to identify the nucleation time, to monitor particle size evolution, to discriminate different precipitation mechanisms and to validate reproducibility. TEM snapshots taken during different stages of the precipitation process identified different phases and morphologies. At a high level of supersaturation homogeneous nucleation in solution led to the formation of amorphous CaC03 particles (diameter=30 nm), which transformed via vaterite to calcite. Nucleation occurred uniformly in solution which appears to be unique for the CaC03 System. In the presence of Na-polymethacrylate (Na-PMA), heterogeneous nucleation was suppressed and Ca-polymer aggregates were formed in the prenucleation stage. Beyond a critical threshold supersaturation CaC03 particles formed in solution outside of these aggregates. The nucleation process resembled that without additive, indicating that Na-PMA exerts only a minor effect on the CaC03 nucleation. In the postnucleation stage, the polymer led to the formation of extended liquid-like networks, which served as a precursor phase for solid ACC particles that formed alongside the network.
Particularly relevant in the context of polymorphism is understanding how structural, thermodynamic, and kinetic factors dictate the stability domains of polymorphs, their tendency to interconvert through phase transitions, or their possibility to exist in metastable states. These three aspects were investigated here for two 4′-hydroxyacetophenone (HAP) polymorphs, differing in crystal system, space group, and number and conformation of molecules in the asymmetric unit. The results led to a ΔfGm°-T phase diagram highlighting the enantiotropic nature of the system and the fact that the Z′=1 polymorph is not necessarily more stable than its Z′=2 counterpart. It was also shown that the form II → form I transition is entropy driven and is likely to occur through a nucleation and growth mechanism, which does not involve intermediate phases, and is characterized by a high activation energy. Finally, although it has been noted that conflicts between hydrogen bond formation and close packing are usually behind exceptions from the hypothesis of Z′=1 forms being more stable than their higher Z′ analogues, in this case, the HAP polymorph with stronger hydrogen bonds (Z′=2) is also the one with higher density.
The process of silicification in plants and the biochemical
effects of silica in plant tissues are largely unknown. To
study the molecular changes occurring in growing cells
that are exposed to higher than normal concentration of
silicic acid, Raman spectra of germinating pollen grains of
three species (Pinus nigra, Picea omorika, and Camellia
japonica) were analyzed in a multivariate classification
approach that takes into account the variation of biochemical
composition due to species, plant tissue structure,
and germination condition. The results of principal
component analyses of the Raman spectra indicate differences
in the utilization of stored lipids, a changed mobilization
of storage carbohydrates in the pollen grain bodies,
and altered composition and/or structure of cellulose of
the developing pollen tube cell walls. These biochemical
changes vary in the different species.
Mechanochemistry is increasingly used for synthesizing soft matter materials including metal organic compounds and cocrystals.1 The ever-increasing interest in this method is contrasted by a limited mechanistic understanding of the mechanochemical reactivity and selectivity. Time-resolved in situ investigations of milling reactions provide direct insights in the underlying mechanisms.2,3 We recently introduced a setup enabling in situ investigation of mechanochemical reactions using synchrotron XRD combined with Raman spectroscopy. The specific combination allows to study milling processes comprehensively on the level of the molecular and crystalline structure and thus obtaining reliable data for mechanistic studies. In this contribution, I will discuss our recent results investigating the formation of (polymorphic) cocrystals and metal phosphonates.4 First investigations of a mechanochemical synthesis under controlled temperature which allow determining the activation barrier are presented. Furthermore, in situ Raman spectroscopy coupled with thermography revealed a low temperature increase during milling reactions due to the mechanical impact and clear temperature increases as a result of the reaction heat.5 Our results indicate that in situ investigation of milling reactions offer a new approach to tune and optimize mechanochemically synthesized compounds.
The mechanochemical Knoevenagel condensation of malononitrile with p-nitrobenzaldehyde was studied in situ using a tandem approach. X-ray diffraction and Raman spectroscopy were combined to yield time-resolved information on the milling process. Under solvent-free conditions, the reaction leads to a quantitative conversion to p-nitrobenzylidenemalononitrile within 50 minutes. The in situ data indicate that the process is fast and proceeds under a direct conversion. After stopping the milling process, the reaction continues until complete conversion. The continuous and the stopped milling process both result in crystalline products suitable for single crystal X-ray diffraction.
Iron can form numerous oxides, hydroxides, and oxide−hydroxides. Despite their relevance, many of the transformation processes between these phases are still poorly understood. In particular the crystallization of quasi-amorphous hydroxides and oxide−hydroxides is difficult to assess, since typical diffraction and scattering methods provide only sampleaveraged information about the crystallized phases. We report a new approach for the investigation of the crystallization of oxide−hydroxides. The approach relies on model-type films that comprise a defined homogeneous nanostructure. The nanostructure allows quantitative linking of Information obtained by bulk-averaging diffraction techniques (XRD, SAXS) with locally resolved information, i.e., Domain sizes (SEM, TEM, LEEM) and phase composition (SAED). Using time-resolved imaging and diffraction we deduce mechanism and kinetics for the crystallization of ferrihydrite into hematite. Hematite forms via nucleation of hematite domains and subsequent Domain growth that terminates only upon complete transformation. A Johnson−Mehl−Avrami−Kolmogorov model describes the kinetics over a wide temperature range. The derived understanding enables the first synthesis of ferrihydrite films with ordered mesoporosity and quantitative control over the films’ hematite and ferrihydrite content.
Mechanochemistry is increasingly used as a ‘green alternative’ for synthesizing various materials including pharmaceutical cocrystals. Herein, we present the mechanochemical synthesis of three new cocrystals containing the API carbamazepine (cocrystals CBZ:Indometacin 1:1, CBZ:Benzamide 1:1, and CBZ:Nifedipine 1:1). The mechanochemical reaction was investigated in situ documenting a fast and complete reaction within one minute. Online NMR spectroscopy proved the direct influence of the dissolution behaviour of the coformers to the dissolution behaviour of the API carbamazepine. The dissolution behaviour of the organic cocrystals is compared to the behaviour of the pure drug indicating a general applicability of this approach for detailed cocrystal dissolution studies.
Mechanochemistry is increasingly used for synthesizing soft matter materials including metal organic compounds and cocrystals. The ever-increasing interest in this method is contrasted by a limited mechanistic understanding of the mechanochemical reactivity and selectivity. Time-resolved in situ investigations of milling reactions provide direct insights in the underlying mechanisms. We recently introduced a setup enabling in situ investigation of mechanochemical reactions using synchrotron XRD combined with Raman spectroscopy. The specific combination allows to study milling processes comprehensively on the level of the molecular and crystalline structure and thus obtaining reliable data for mechanistic studies. In this contribution, I will discuss our recent results investigating the formation of (polymorphic) cocrystals and metal phosphonates. First investigations of a mechanochemical synthesis under controlled temperature which allow determining the activation barrier are presented. Furthermore, in situ Raman spectroscopy coupled with thermography revealed a low temperature increase during milling reactions due to the mechanical impact and clear temperature increases as a result of the reaction heat.5 Our results indicate that in situ investigation of milling reactions offer a new approach to tune and optimize mechanochemically synthesized compounds.