Microporous Organic Polyimides for CO₂ and H₂O Capture and Separation from CH₄ and N₂ Mixtures: Interplay between Porosity and Chemical Function

Porous polyimides have been considered to be a promising material class for gas capture and sequestration, leading to the synthesis of a substantial number of individual networks with noteworthy sorption properties. In spite of these efforts, the vision of a chemical control of adsorption and desorption of small molecules, in particular, for the competing uptake of technical relevant gas mixtures, is still hardly investigated. Here, we present a systematic study of five new polyimide networks based on a set of linkers with chemical functionalities covering the full range from hydrophobic to hydrophilic interactions. The corresponding microporous organic polyimides (MOPI-I to -V) were synthesized successfully based on a condensation reaction between amino and anhydride linker molecules in <i>m</i>-cresol at high temperatures, resulting in cross-linking degrees beyond 95% in all cases. Argon and carbon dioxide isotherms reveal surface areas up to 940 m²/g with ultramicroporosity, about 50% microporosity and high thermal stabilities under air with decomposition temperatures up to 480 °C. Sorption screening for variable temperatures revealed remarkable uptakes for carbon dioxide up to 3.8 mmol/g and water vapor up to 19.5 mmol/g combined with a smooth gate opening around 0.25 <i>p</i>/<i>p</i>₀ for MOPI-IV. In contrast, for MOPI-V the water vapor uptake decreases down to 7 mmol/g. Interestingly, the trend of the selectivities calculated by IAST and Henry does not correlate with the uptake behavior. For instance, MOPI-I and MOPI-III exhibit with 78 and 13 the highest CO₂ over N₂ and CH₄ Henry selectivities, although their CO₂ uptake is around 3.0 mmol/g. In total, we attribute the sorption properties for this class of materials mainly to the void size and shape within the ultramicroporous region. The chemical environment of the surfaces seems to have little influence on the uptake and a stronger effect on the separation behavior

Identifying Selective Host–Guest Interactions Based on Hydrogen Bond Donor–Acceptor Pattern in Functionalized Al-MIL-53 Metal–Organic Frameworks

We present a study analyzing the selectivity of host–guest interactions in a series of functionalized Al-MIL-53-X metal–organic frameworks with X = H, NH₂, and NHCHO using acetone, ethanol, and water as probe molecules. While the amino group introduces additional hydrogen bond donor centers the NHCHO anchors function as donor and acceptor. The guests were chosen due to their ability to act solely as an acceptor in the case of acetone, whereas ethanol and water provide acceptor and donor qualities with a gradual decrease of the acceptor strength toward ethanol. The characterization of the host–guest interactions includes a comprehensive solid-state NMR spectroscopic study based on a full assignment of ¹H and ¹³C high-resolution spectra using CRAMPS decoupling schemes to enhance ¹H resolution combined with advanced 2D HETCOR (¹H–¹³C, ¹H–²⁷Al, and ¹H–¹⁴N) spectra at high magnetic fields. In spite of a pronounced dynamical disorder of the guests, we could identify a preferred binding of the acetone via a NH···OC hydrogen bond for the NH₂ and the NHCHO anchor groups by analyzing trends in the ¹³C isotropic chemical shifts. At the same time ¹H–¹H through-space connectivities reveal a close vicinity of the acetone methyl groups to the benzene rings of the linkers. In contrast, for ethanol and water, the interaction with the anchor groups is too weak to compete with the thermal disorder at room temperature

Highly Efficient Supramolecular Nucleating Agents for Poly(3-hexylthiophene)

Controlling the solid-state morphology of semiconducting polymers is crucial for the function and performance of optoelectronic and photonic devices. Nucleation is a commonly used and straightforward approach to tailor the solid-state morphology of semi-crystalline polymers. However, efficient nucleating agents for semiconducting polymers are still rare. Here, we present a conceptual approach to tailor supramolecular nucleating agents for the semiconducting polymer, poly(3-hexylthiophene) (P3HT). Using this approach, we developed a class of supramolecular nucleating agents, which can achieve outstanding nucleation efficiencies of more than 95% at concentrations as low as 0.1 wt %. Such efficiencies can be achieved by combining an exceptionally high epitaxial match with highly regularly arranged donor-acceptor interactions between the nucleating agent and the polymer. Notably, the supramolecular agents do not induce trap states in thin films of P3HT and are beneficial for the film stability by controlling the solid-state morphology. We anticipate that this approach can be transferred to other semi-crystalline conjugated polymers, resulting in defined solid-state morphologies

Hydroxyl Defects and Oxide Vacancies within Ringwoodite-toward Understanding the Defect Chemistry of Spinel-Type Oxides

Contains fulltext : 221475.pdf (publisher's version ) (Closed access

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO4 and SiO4 structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and non-framework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al2O3-SiO2 (CMAS) glass composition was modified by substituting MgO for CaO and SiO(2 )for Al2O3 to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass- ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), Si-29 and Al-27 magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO, and AlO4 tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of Si-29 MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and A1-0-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO, and AlO4 tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al-4] O-Al-4] bonds; this region coexists with a predominantly SiO4 -containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 degrees C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO4 and AlO4 structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO4 -rich region at higher temperatures, leading to the formation of additional crystalline phases

Elucidating the formation of Al-NBO bonds, Al-O-Al linkages and clusters in alkaline-earth aluminosilicate glasses based on molecular dynamics simulations

Exploring the reasons for the initiation of Al-O-Al bond formation in alkali-earth alumino silicate glasses is a key topic in the glass-science community. Evidence for the formation of Al-O-Al and Al-NBO bonds in the glass composition 38.7CaO-9.7MgO-12.9Al(2)O(3)-38.7SiO(2) (CMAS, mol%) has been provided based on Molecular Dynamics (MD) simulations. Analyses in the short-range order confirm that silicon and the majority of aluminium cations form regular tetrahedra. Well-separated homonuclear (Si-O-Si) and heteronuclear (Si-O-Al) cluster regions have been identified. In addition, a channel region (C-Region), separated from the network region, enriched with both NBO and non-framework modifier cations, has also been identified. These findings are in support of the previously proposed extended modified random network (EMRN) model for aluminosilicate glasses. A detailed analysis of the structural distributions revealed that a majority of Al, 51.6%, is found in Si-O-Al links. Although the formation of Al-O-Al and Al-NBO bonds is energetically less favourable, a significant amount of Al is found in Al-O-Al links (33.5%), violating Lowenstein's rule, and the remainder is bonded with non-bridging oxygen (NBO) in the form of Al-NBO (Al-O-(Ca, Mg)). The conditions necessary for the formation of less favourable bonds are attributed to the presence of a high amount of modifier cations in current CMAS glass and their preferable coordination

Probing Interactions of N‑Donor Molecules with Open Metal Sites within Paramagnetic Cr-MIL-101: A Solid-State NMR Spectroscopic and Density Functional Theory Study

Understanding host–guest interactions is one of the key requirements for adjusting properties in metal–organic frameworks (MOFs). In particular, systems with coordinatively unsaturated Lewis acidic metal sites feature highly selective adsorption processes. This is attributed to strong interactions with Lewis basic guest molecules. Here we show that a combination of ¹³C MAS NMR spectroscopy with state-of-the-art density functional theory (DFT) calculations allows one to unravel the interactions of water, 2-aminopyridine, 3-aminopyridine, and diethylamine with the open metal sites in Cr-MIL-101. The ¹³C MAS NMR spectra, obtained with ultrafast magic-angle spinning, are well resolved, with resonances distributed over 1000 ppm. They present a clear signature for each guest at the open metal sites. Based on competition experiments this leads to the following binding preference: water < diethylamine ≈ 2-aminopyridine < 3-aminopyridine. Assignments were done by exploiting distance sum relations derived from spin–lattice relaxation data and ¹³C­{¹H} REDOR spectral editing. The experimental data were used to validate NMR shifts computed for the Cr-MIL-101 derivatives, which contain Cr₃O clusters with magnetically coupled metal centers. While both approaches provide an unequivocal assignment and the arrangement of the guests at the open metal sites, the NMR data offer additional information about the guest and framework dynamics. We expect that our strategy has the potential for probing the binding situation of adsorbate mixtures at the open metal sites of MOFs in general and thus accesses the microscopic interaction mechanisms for this important material class, which is essential for deriving structure–property relationships

Identifying Selective Host-Guest Interactions Based on Hydrogen Bond Donor-Acceptor Pattern in Functionalized Al-MIL-53 Metal-Organic Frameworks

We present a study analyzing the selectivity of host guest interactions in a series of functionalized Al-MIL-53-X metal organic frameworks with X = H, NH2, and NHCHO using acetone; ethanol, and water as probe molecules. While the amino group introduces additional hydrogen bond donor centers the NHCHO anchors function as donor and acceptor. The guests were chosen due to their ability to act solely as an acceptor in the case of acetone, whereas ethanol and water provide acceptor and donor qualities with a gradual decrease of the acceptor strength toward ethanol. The characterization of the host guest interactions includes a comprehensive solid-state NMR spectroscopic study based on a full assignment of H-1 and C-13 high-resolution spectra using CRAMPS decoupling schemes to enhance H-1 resolution combined with advanced 2D HETCOR (H-1-C-13, H-1-Al-27, and H-1-N-14) spectra at high magnetic fields. In spite of a pronounced dynamical disorder of the guests, we could identify a preferred binding of the acetone via a NH center dot center dot center dot OC hydrogen bond for the NH2 and the NHCHO anchor groups by analyzing trends in the C-13 isotropic chemical shifts. At the same time H-1-H-1 through space connectivities reveal a close vicinity of the acetone methyl groups to the benzene rings of the linkers. In contrast, for ethanol and water, the interaction with the anchor groups is too weak to compete with the thermal disorder at room temperature