•Hydrogels made from polysaccharide polymers are widely used.•Their functional properties depend on their molecular structure and mobility.•Dynamic and responsive hydrogels are challenging for ...molecular characterization.•Modern solid-state NMR spectroscopy reveals structure and dynamics of hydrogels.•Information is obtained on the polysaccharides and also the associated water.
Hydrogels find application in many areas of technology and research due to their ability to combine responsiveness and robustness. A detailed understanding of their molecular structure and dynamics (which ultimately underpin their functional properties) is needed for their design to be optimized and these hydrogels to be exploited effectively. In this review, we shed light on the unique capabilities of solid-state NMR spectroscopy to reveal this information in molecular detail. We review recent literature on the advancements in solid-state NMR techniques in resolving the structure, degree of grafting, molecular organization, water-biopolymer interactions and internal dynamical behavior of hydrogels. Among various solid-state NMR techniques, 13C cross polarization (CP) magic angle spinning (MAS) NMR is examined for its ability to probe the hydrogel and its trapped solvent. Although widely applicable to many types of polymeric and supramolecular hydrogels, the current review focuses on polysaccharide-based hydrogels.
Alginate hydrogels are versatile self-assembling biocompatible materials with diverse biomedical and food industrial applications, which includes uses in encapsulation, (drug) delivery and tissue ...engineering. Hydrogel formation requires cross-linking, which for alginates is often done with calcium ions that engage in specific interactions with the polysaccharide carboxylic acid groups. Water molecules also hydrate these alginate groups and fill macropores within the hydrogels, with implications for both mechanical properties and cargo encapsulation. Understanding these aspects of hydrogels requires the observation and characterization of the hydrogel waters, how they engage the alginate, and fill the macropores. Here we employed solid-state NMR (ssNMR) spectroscopy to detect and study water molecules in re-hydrated alginate hydrogels. 1H, 2H, and 13C magic angle spinning (MAS) NMR and relaxation measurements were combined to observe both water and alginate. Two different water phases were detected that vary upon gradual (re)hydration of the alginate hydrogels. These water pools differ in their chemical shifts and NMR relaxation properties, reflecting hydration waters directly associated with the carbohydrate polymers alongside dynamic waters in the macropores. Thus, the ssNMR detects the water-filled macropore water pools and how they vary upon calcium cross-linking. We also observe how calcium cross-linking selectively immobilizes the α-guluronate monosaccharides, but leaves the β-mannuronate units more flexible and prone to selective re-hydration. Thus, these ssNMR experiments can be used to probe cross-linking and hydration of alginate hydrogels, with implications for our understanding of design parameters that tune their performance in (drug) delivery and other food industrial applications.
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•Magic angle spinning NMR probed molecular aspects of alginate hydrogels.•Solid-state NMR used for detecting and quantifying water pools in hydrogels.•Solvent dynamics seen by NMR associate with distinct hydration water pools.•Impact of time-dependent cross-linking of alginate with calcium ions compared.•Molecular details of alginate dynamics, structure and re-hydration revealed.
Three zeolites (H‐Beta, H‐ZSM‐5 and H‐Y) were synthesized in the form of binder‐free macroscopic beads (d=215–840 μm) using a hydrothermal method employing anion‐exchange resin beads as hard ...template. The beads obtained after removal of the hard template by calcination consisted of crystalline zeolite domains connected with each other to form a hierarchical porous network in which the zeolitic micropores are accessible through meso‐ and macropores, as proven by characterization with XRD, N2 physisorption, SEM, and TEM. The composition, the nature and amount of acid sites and the degree of hydrophobicity of these beads were investigated by means of XRF, solid‐state NMR, pyridine‐FTIR and TGA. The zeolite beads were tested as heterogeneous catalysts in the Friedel‐Crafts acylation of anisole with acetic anhydride to produce para‐methoxyacetophenone. H‐Beta‐Beads displayed the best catalytic performance with 95 % conversion of acetic anhydride and 76 % yield of para‐methoxyacetophenone in a batch reactor test (90 °C, 6 h). Next, the catalytic performance of H‐Beta‐Beads was compared in both batch and continuous‐flow mode to extrudates prepared by mixing zeolite Beta powder with either kaolin or bentonite binders. H‐Beta‐Beads outperformed the extrudates in batch‐mode reactions and could be reused in multiple runs without discernible loss of activity. In the continuous‐flow test, H‐Beta‐Beads demonstrated higher average activity but deactivated more rapidly than the extrudates.
Friedel Crafts: Zeolite beads as catalysts for anisole acylation.
Alginate hydrogels have garnered significant attention due to their promising applications in the food, biomedical, and pharmaceutical industries. The detection and quantification of distinct water ...phases within these hydrogels offer valuable insights into their dynamic, absorptive, and mechanical properties. Despite being comprised solely of 2 wt % polymeric materials, the alginate hydrogels exhibit a highly porous morphology, characterized by distinct water pools exhibiting varying mobility and dynamic behaviors. These phases can be delineated as largely free water phase with high mobility, which occupies the macropores, and bound water with restricted mobility, which interacts with the fibrous polymeric structure. Water pools interacting with their surrounding environments possess variable crystal structures on variable freezing points, this could be easily detected using X-ray scattering techniques. A comparative study was conducted based on the information derived from each technique, with differential scanning calorimetry (DSC) yielding quantitative information for the water phases in alginate hydrogels (i.e., 58 % free and 42 % bound water in 0.75 wt % - 6 h aging sample), whereas cryogenic scanning electron microscopy (Cryo-SEM), wide and small-angle X-ray scattering (WAXS and SAXS), Fourier Transform Infrared (FT-IR), and rheology provided valuable qualitative insights. In this study, deep insights into the molecular structure of alginates were obtained including the alteration in morphology and macropore distribution, increase in the wall thickness, density, and mechanical properties upon increasing the Ca2+ concentration and aging period.
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Spheres comprising 10 wt.% Mo2C/γ-Al2O3, synthesized through the sucrose route, exhibited unprecedented catalytic activity for olefin hydrogenation within an industrial naphtha feedstock that ...contained 23 wt.% olefins, as determined by supercritical fluid chromatography (SFC). The catalyst demonstrated resilience to sulfur, exhibiting no discernible deactivation signs over a tested 96 h operational period. The resultant hydrogenated naphtha from the catalytic process contained only 2.5 wt.% olefins when the reaction was conducted at 280 °C and 3.44 × 106 Pa H2, subsequently blended with Athabasca bitumen to meet pipeline specifications for oil transportation. Additionally, the carbide catalyst spheres effectively hydrogenated olefins under steam conditions without experiencing any notable hydrogenation in the aromatics. We propose the supported carbide catalyst as a viable alternative to noble metals, serving as a selective agent for olefin elimination from light petroleum distillates in the presence of steam and sulfur, mitigating the formation of gums and deposits during the transportation of diluted bitumen (dilbit) through pipelines.
Solid state nuclear magnetic resonance (ssNMR) is a powerful and attractive characterization method for obtaining insights into the chemical structure and dynamics of a wide range of materials. ...Current interest in cellulose-based materials, as sustainable and renewable natural polymer products, requires deep investigation and analysis of the chemical structure, molecular packing, end chain motion, functional modification, and solvent-matrix interactions, which strongly dictate the final product properties and tailor their end applications. In comparison to other spectroscopic techniques, on an atomic level, ssNMR is considered more advanced, especially in the structural analysis of cellulose-based materials; however, due to a dearth in the availability of a broad range of pulse sequences, and time consuming experiments, its capabilities are underestimated. This critical review article presents the comprehensive and up-to-date work done using ssNMR, including the most advanced NMR strategies used to overcome and resolve the structural difficulties present in different types of cellulose-based materials.
Solid-state NMR has proven to be a versatile technique for studying the chemical structure, 3D structure and dynamics of all sorts of chemical compounds. In nanotechnology and particularly in thin ...films, the study of chemical modification, molecular packing, end chain motion, distance determination and solvent-matrix interactions is essential for controlling the final product properties and applications. Despite its atomic-level research capabilities and recent technical advancements, solid-state NMR is still lacking behind other spectroscopic techniques in the field of thin films due to the underestimation of NMR capabilities, availability, great variety of nuclei and pulse sequences, lack of sensitivity for quadrupole nuclei and time-consuming experiments. This article will comprehensively and critically review the work done by solid-state NMR on different types of thin films and the most advanced NMR strategies, which are beyond conventional, and the hardware design used to overcome the technical issues in thin-film research.
This study focuses on detailed characterizations of asphaltene fractions extracted from the Algerian Hassi-Messaoud oil field. It was found that the extracted asphaltenes are not completely soluble ...in toluene, instead two fractions of asphaltenes were obtained upon solubilizing the heptane-precipitated neat asphaltenes in toluene. Extensive characterizations of the toluene-soluble and insoluble fractions were carried out using elemental analysis, Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (ssNMR). It was suggested that the high oxygen content and uneven compositional structures are the main contributors to asphaltene instability. The toluene-insoluble fractions were found to have higher polarity and aromaticity as well as more oxygen content than the neat asphaltenes and toluene-soluble fractions.
Two multifunctional catalytic systems comprising Sn-based/doped crystalline zeolite Beta were synthesized, and they were employed as heterogeneous catalysts in the selective conversion of glycerol to ...methyl lactate. The first catalytic system, named Au-Pd-Sn-deAl-7.2-Beta-DP, was created through the post-synthesis dealumination of the parent zeolite Beta (Si/Al = 10) using 7.2 M HNO3. Subsequently, it was grafted with 27 mmol of SnCl4, resulting in Sn-deAl-7.2-Beta. Following this, Au and Pd nanoparticles were supported on this catalyst using the deposition–precipitation (DP) method. The second catalytic system was a physical mixture of Au and Pd nanoparticles supported on functionalized carbon nanotubes (Au-Pd-F-CNTs) and Sn-containing zeolite Beta (Sn-deAl-7.2-Beta). Both catalytic systems were employed in glycerol partial oxidation to methyl lactate under the following conditions: 140 °C for 4.5 h under an air pressure of 30 bar. The Au-Pd-Sn-deAl-7.2-Beta-DP catalytic system demonstrated 34% conversion of glycerol with a 76% selectivity for methyl lactate. In contrast, the physical mixture of Au-Pd-F-CNTs and Sn-deAl-7.2-Beta exhibited higher activity, achieving 58% glycerol conversion and a nearly identical selectivity for methyl lactate (77%). The catalytic results and catalyst structure were further analyzed using various characterization techniques, such as X-ray diffraction (XRD), N2 physisorption, scanning electron microscopy (SEM), X-ray fluorescence (XRF), transmission electron microscopy (TEM), UV-vis spectroscopy, and pyridine Fourier transform infrared (FTIR). These analyses emphasized the significance of adjusting the quantity of active sites, particle size, and active sites proximity under the chosen reaction conditions.
The self-assembly of cetyltrimethylammonium bromide, sodium dodecylsulfate, Triton X-100, and sulfobetaine surfactants in aqueous solutions was examined by dynamic light scattering, both in the ...presence and absence of 0.1 M NaCl salt, across various temperatures. For each surfactant, critical parameters, such as concentration and phase transition temperatures, of micelles were determined by monitoring changes in the hydrodynamic diameter with concentration and temperature. Additionally, we explored the self-assembly behavior of these surfactants when they are introduced alongside polystyrene nanoparticles. Our findings enabled the elucidation of surfactant molecule adsorption mechanisms onto polystyrene nanoparticle surfaces. Furthermore, by analyzing variations in the z-average diameter and zeta potential, we were able to establish the Krafft point, a parameter that remains imperceptible when polystyrene nanoparticles are absent from the solution.
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