Catalytic nanosponges of acidic aluminosilicates for plastic degradation and CO2 to fuel conversion

The synthesis of solid acids with strong zeolite-like acidity and textural properties like amorphous aluminosilicates (ASAs) is still a challenge. In this work, we report the synthesis of amorphous “acidic aluminosilicates (AAS)”, which possesses Brønsted acidic sites like in zeolites and textural properties like ASAs. AAS catalyzes different reactions (styrene oxide ring-opening, vesidryl synthesis, Friedel−Crafts alkylation, jasminaldehyde synthesis, m-xylene isomerization, and cumene cracking) with better performance than state-of-the-art zeolites and amorphous aluminosilicates. Notably, AAS efficiently converts a range of waste plastics to hydrocarbons at significantly lower temperatures. A Cu-Zn-Al/AAS hybrid shows excellent performance for CO2 to fuel conversion with 79% selectivity for dimethyl ether. Conventional and DNP-enhanced solid-state NMR provides a molecular-level understanding of the distinctive Brønsted acidic sites of these materials. Due to their unique combination of strong acidity and accessibility, AAS will be a potential alternative to zeolites

1H CSA parameters by ultrafast MAS NMR: measurement and applications to structure refinement

A 1H anisotropic-isotropic chemical shift correlation experiment which employs symmetry-based recoupling sequences to reintroduce the chemical shift anisotropy in ν1 and ultrafast MAS to resolve 1H sites in ν2 is described. This experiment is used to measure 1H shift parameters for L-ascorbic acid, a compound with a relatively complex hydrogen-bonding network in the solid. The 1H CSAs of hydrogen-bonded sites with resolved isotropic shifts can be extracted directly from the recoupled lineshapes. In combination with DFT calculations, hydrogen positions in crystal structures obtained from X-ray and neutron diffraction are refined by comparison with simulations of the full two-dimensional NMR spectrum. The improved resolution afforded by the second dimension allows even unresolved hydrogen-bonded sites 1H to be assigned and their shift parameters to be obtained

Dynamic nuclear polarization enhanced solid-state NMR studies of surface modification of gamma-alumina

Dynamic nuclear polarization (DNP) gives large (>100-fold) signal enhancements in solid-state NMR spectra via the transfer of spin polarization from unpaired electrons from radicals implanted in the sample. This means that the detailed information about local molecular environment available for bulk samples from solid-state NMR spectroscopy can now be obtained for dilute species, such as sites on the surfaces of catalysts and catalyst supports. In this paper we describe a DNP-enhanced solid-state NMR study of the widely used catalyst gamma-alumina which is often modified at the surface by the incorporation of alkaline earth oxides in order to control the availability of catalytically active penta-coordinate surface Al sites. DNP-enhanced 27Al solid-state NMR allows surface sites in gamma-alumina to be observed and their 27Al NMR parameters measured. In addition changes in the availability of different surface sites can be detected after incorporation of BaO

Ion exchange and binding in selenium remediation materials using DNP-enhanced solid-state NMR spectroscopy

Selenate-loaded selenium water remediation materials based on polymer fibres have been investigated by dynamic nuclear polarization (DNP) enhanced solid-state NMR. For carbon-13 a significant reduction in experiment time is obtained with DNP even when compared with conventional carbon-13 NMR spectra recorded using larger samples. For the selenium remediation materials studied here this reduction allows efficient acquisition of {1H}-77Se heteronuclear correlation spectra which give information about the nature of the binding of the remediated selenate ions with the grafted side chains which provide the required ion exchange functionality

How does dense phase CO2 influence the phase behaviour of block copolymers synthesised by dispersion polymerisation?

Block copolymers synthesised in supercritical CO2 dispersion undergo in situ self-assembly which can result in a range of nanostructured microparticles. However, our previous study revealed that copolymers with different block combinations possessed different microphase separated morphologies at identical block volume fractions. In this paper, we follow up those initial observations. By examining the phase behaviour of a selection of structurally diverse block copolymers, we explore the structural factors which influence the conflicting self-assembly behaviours. The composition dependence of the morphology is found to be strongly related to the CO2-philicity of the second block relative to poly(methyl methacrylate) (PMMA). Whilst PMMA-b-poly(benzyl methacrylate) (PBzMA) and PMMA-b-poly(N,N-dimethylaminoethylmethacrylate) (PDMAEMA) phase behaviour follows traditional diblock copolymer phase diagrams, PMMA-b-poly(styrene) (PS) and PMMA-b-poly(4-vinyl pyridine) (P4VP), which comprise blocks with the greatest contrast in CO2-philicity, self-assemble into unexpected morphologies at several different block volume fractions. The morphology of these copolymers in the microparticulate form was found to revert to the predicted equilibrium morphology when the microparticles were re-cast as films and thermally annealed. These findings provide strong evidence that CO2 acts as a block-selective solvent during synthesis. The CO2-selectivity was exploited to fabricate various kinetically trapped non-lamellar morphologies in symmetrical PMMA-b-PS copolymers by tuning the ratio of polymer:CO2. Our data demonstrate that CO2 can be exploited as a facile process modification to control the self-assembly of block copolymers within particles

Yttrium doped phosphate-based glasses: structural and degradation analyses

This study investigates the role of yttrium in phosphate-based glasses in the system 45(P 2 O 5)-25(CaO)-(30-x)(Na 2 O)-x(Y 2 O 3) (0≤ x≤ 5) prepared via melt quenching and focuses on their structural characterisation and degradation properties. The structural analyses were performed using a combination of solid-state nuclear magnetic resonance (NMR), Fourier transform infrared spec-troscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). 31 P NMR analysis showed that depolymerisation of the phosphate network occurred which increased with Y 2 O 3 content as metaphosphate units (Q 2) decreased with subsequent increase in pyrophosphate species (Q 1). The NMR results correlated well with structural changes observed via FTIR and XPS analyses. XRD analysis of crys-tallised glass samples revealed the presence of calcium pyrophosphate (Ca 2 P 2 O 7) and sodium metaphosphate (NaPO 3) phases for all the glass formulations explored. Yttrium-containing phases were found for the formulations containing 3 and 5 mol% Y 2 O 3. Degradation analyses performed in Phosphate buffer saline (PBS) and Milli-Q water revealed significantly reduced rates with addition of Y 2 O 3 content. This decrease was attributed to the formation of Y-O-P bonds where the octahedral structure of yt-trium (YO 6) cross-linked phosphate chains, subsequently leading to an increase in chemical durability of the glasses. The ion release studies also showed good correlation with the degradation profiles

Halide Mixing and Phase Segregation in Cs2AgBiX6 (X=Cl, Br, I) Double Perovskites from Cesium-133 Solid-State NMR and Optical Spectroscopy

All-inorganic double perovskites (elpasolites) are a promising potential alternative to lead halide perovskite in optoelectronic applications. While halide mixing is a well-established strategy for band gap tuning, little is known about halide mixing and phase segregation phenomena in double perovskites. Here, we synthesize a wide range of single− and mixed−halide Cs2AgBiX6 (X=Cl, Br, I) double perovskites using mechanosynthesis and probe their atomic-level microstructure using 133Cs solid-state MAS NMR. We show that mixed Cl/Br materials form pure phases for any Cl/Br ratio while Cl/I and Br/I mixing is only possible with-in a narrow range of halide ratios (<3 mol% I) and leads to a complex mixture of products for higher ratios. We characterize the optical properties of the resulting materials and show that halide mixing does not lead to an appreciable tunability of the PL emission. We find that iodide incorporation is particularly pernicious in that it quenches the PL emission intensity and radiative charge carrier lifetimes for iodide ratios as low as 0.3 mol%. Our study shows that sol-id-state NMR, in conjunction with optical spectroscopies, provides a comprehensive understanding of the structure-activity relationships, halide mixing and phase segregation phenomena in Cs2AgBiX6 (X=Cl, Br, I) double perovskites

Measuring 19F shift anisotropies and 1H–19F dipolar interactions with ultrafast MAS NMR

A new 19F anisotropic–isotropic shift correlation experiment is described that operates with ultrafast MAS, resulting in good resolution of isotropic 19F shifts in the detection dimension. The new experiment makes use of a recoupling sequence designed using symmetry principles that reintroduces the 19F chemical shift anisotropy in the indirect dimension. The situations in which the new experiment is appropriate are discussed, and the 19F shift anisotropy parameters in poly(difluoroethylene) (PVDF) are measured. In addition, similar recoupling sequences are shown to be effective for measuring 1H–19F distances via the heteronuclear dipolar interaction. This is demonstrated by application to a recently synthesized zirconium phosphonate material that contains one-dimensional chains linked by H–F hydrogen bonds

Porous macromolecular dihydropyridyl frameworks exhibiting catalytic and halochromic activity

New porous macromolecular frameworks (PMFs) have been designed and prepared by the condensation of dialdehydes with aminoacrylonitriles. Two porous materials were prepared by reacting 3,3′-benzene-1,4-diylbis(3-aminoprop-2-enenitrile) with benzene-1,4-dicarbaldehyde and biphenyl-4,4′-dicarbaldehyde to give PMF-NOTT-1 and PMF-NOTT-2, respectively. Adsorption and desorption studies of N2 (77 K) and CO2 (273–303 K and 20 bar) were used to characterize the porosity of these materials. CO2 adsorption measurements indicate that these PMFs have similar porosity with Dubinin–Radushkevich micropore volumes of 0.142 and 0.144 cm3 g−1 and CO2 surface excess uptakes of 28.4 and 28.9 wt% at 20 bar, 273 K for PMF-NOTT-1 and PMF-NOTT-2, respectively. The isosteric heats of adsorption for CO2 at zero surface coverage were 31.9 kJ mol−1 (for PMF-NOTT-1) and 33.1 kJ mol−1 (for PMF-NOTT-2). However, N2 adsorption studies at 77 K indicated that PMF-NOTT-2 shows activated diffusion effects due to the presence of some narrow ultramicroporosity. The conjugated systems of these frameworks can be reversibly switched by varying proton concentration in solution and these materials thus demonstrate halochromic properties. PMF-NOTT-1, constructed from shorter building blocks than PMF-NOTT-2, exhibits higher catalytic activity and selectivity in Knoevenagel condensation reactions of malononitrile with benzaldehydes. The advantages of using PMFs as catalysts or adsorbents are their excellent thermal and chemical stabilities and they can be recovered and regenerated for re-use