Design of bifunctional 1D nanostructures for the catalytic conversion of carbon dioxide into cyclic carbonates

One dimensional silica-based nanotubes represent an innovative and promising morphology in the context of heterogeneous catalysis. Here these nanostructures were prepared for the first time as bifunctional materials, with hafnium or tin atoms inserted as single sites in the silica structure and imidazolium moieties anchored at the surface. The low dimensional solids thus present both acid sites owing to the presence of metal cations in tetrahedral coordination (co-catalyst) and nucleophilic species coming from the counterion of the imidazolium moieties (catalyst). The design of the catalysts consisted of two main steps. The Hf- or Sn-doped silica solids were initially prepared using a straightforward sol-gel method including a pH adjustment step allowing a quantitative insertion of the metal cations in the silica framework. These materials were post-functionalized with imidazolium moieties. The solids were extensively characterized thus confirming the presence of well-defined and open tubular structure, high specific surface area, successful insertion of Hf and Sn in the silica framework, and a correct functionalization with imidazolium salts. The different catalysts were tested in the valorization of CO2 with styrene oxide to give the corresponding cyclic carbonate. The bifunctional solids were stable and recyclable. The versatility of the best catalyst, represented by the Hf-based material, was confirmed using different epoxides. Finally, by tuning the reaction conditions or changing the imidazolium salt, a further boost of the catalytic performances was achieved.</p

Low-Dimensional Hollow Nanostructures:From Morphology Control to the Release of an Active Pharmaceutical Ingredient

The mechanism guiding the growth of hollow silica nanotubes or nanospheres was unveiled for the first time through in situ liquid-phase transmission electron microscopy, allowing visualization of the morphology of the hydrated species involved in the early stage of material formation. The combined action of a surfactant (F127) and a swelling agent (toluene) was essential for the development of targeted nanostructures, owing to the formation of surfactant-stabilized toluene droplets in the aqueous phase. The quantity of surfactant-stabilized toluene droplets was unambiguously pointed as the key parameter guiding the formation of either tubular or spherical nanostructures. Leveraging on the fundamental understanding of the parameters guiding the formation of hollow silica nanostructures, tubular and spherical carriers were prepared and exploited for the release of an active pharmaceutical ingredient (API). The impregnation of nanotubes and nanospheres with a poorly water-soluble API (curcumin) was investigated, leading to an optimal loading of 20 wt %. The accessibility of nanotubes and nanospheres showed to be highly beneficial to increase the release kinetics of the targeted API in simulated intestinal fluid, opening promising perspectives in the field. Release was more efficient than with other conventional mesoporous silica-based carriers

Hafnium-doped silica nanotubes for the upgrading of glycerol into solketal: Enhanced performances and in-depth structure-activity correlation

An unprecedented type of Hf-doped silica nanotubes was synthesized using a straightforward one-pot sol–gel procedure. The well-defined nanotubes with a diameter of 14–20 nm exhibited high specific surface area and a widely open texture. The method – involving a key pH adjustment step – allowed a quantitative insertion of hafnium in the materials (Si/Hf = 74) and favored the insertion of Hf as dispersed species. Depending on the synthesis parameters, the chemical environment around Hf was modified, as evidenced by XPS, NH3-TPD and NH3-IR. Hf-doped silica nanotubes showed excellent activity in the conversion of glycerol to solketal, a reaction of high relevance in the context of biorefineries. Importantly, the turnover frequency and the acidity were unambiguously correlated with the insertion of Hf in the silica matrix. The best catalyst was proven to be stable and recyclable, and this sustainable reaction was also amenable to further catalytic enhancement upon optimized reaction conditions

Surface-functionalized mesoporous gallosilicate catalysts for the efficient and sustainable upgrading of glycerol to solketal

Two series of functionalized mesoporous Ga silicates were prepared in a straightforward and sustainable one-pot procedure using different alkyl silanes. The efficacy of the adopted co-synthetic approach based on aerosol processing has been proved by 29Si solid-state NMR experiments revealing a degree of functionalization close to the theoretical value. The successful incorporation of gallium as single sites within the silica framework was confirmed via71Ga solid-state magic-angle-spinning NMR measurements. These materials were tested as catalysts for the synthesis of solketal from glycerol at low temperature and under solventless conditions. A systematic study evidenced the importance of a careful tuning of surface polarity, achievable with surface functionalization as well as with different thermal treatments. The solids functionalized with a low degree of methyl groups (5%) displayed enhanced performances compared to the non-functionalized analogues, highlighting the highly beneficial role of surface hydrophobicity as well as the importance of the careful tuning of the hydrophilic/hydrophobic balance. The best functionalized catalysts proved to be easily reusable for multiple catalytic runs. With such a high-performance catalyst in hand, we propose a process which shows a favorable E-factor, indicating that the production of solketal can be envisaged in a sustainable way

Catalytic conversion of glycerol into solketal with Ga-silicate nanoparticles prepared by aerosol process

Here, the aerosol assisted sol-gel process was used as a powerful tool to synthesize silica-based solids with Ga inserted as single-site in the structure. The influence of different parameters on both morphological properties and catalytic activity were studied. Three different materials bearing a Si/Ga ratio of 34, 74 and 148 were synthesized and extensively characterized via N2 physisorption, XRD, TEM, ICP-OES spectroscopy, XPS and 29Si solid state NMR. All materials displayed promising features for catalytic applications such as high surface area (~ 400 m2/g), controlled mesoporosity and narrow particles size distribution. To investigate the coordination number/geometry of the metal center inserted as single site within the silica matrix, a deep structural investigation of the materials was performed via solid state NMR of 71Ga under magic angle spinning (MAS) or static conditions. The understanding and the quantification of the different species is of fundamental importance since it allows to strongly correlate the final properties of the solids with the metal coordination. The challenging study of quadrupolar 71Ga allowed to observe a signal with a maximum at around 140 ppm that can be assigned to a predominant contribution of tetrahedral gallium present as single site in the silica framework

Synthesis and In-Depth Characterization of Ga-Based Structured Catalysts: Enhancing Glycerol Conversion

Mesoporous Ga-silicates were efficiently prepared using a sustainable and continuous synthesis procedure. Three different Si/Ga ratios of 34, 74 and 148. The insertion of Ga predominantly as single site in tetrahedral coordination in the silica framework was elucidated by XPS, using the Auger parameter of Ga in a Wagner plot representation. The speciation of Ga was further clarified using solid state 71Ga NMR spectroscopy, confirming the formation of mainly isolated Ga species. Consistently, the aerosol-made Ga-silicates displayed outstanding turnover frequencies (up to 677 h-1) and selectivity, markedly outcompeting other reference metallosilicate catalysts reported in literature. Moreover, the most active catalyst was successfully reused in multiple catalytic cycles thus proving its stability under the selected reaction conditions.<br /