Comprehensive Characterization of a Mesoporous Cerium Oxide Nanomaterial with High Surface Area and High Thermal Stability.

In the present study, the pore space of a mesoporous cerium oxide material is investigated, which forms by the self-assembly of primary particles into a spherical secondary structure possessing a disordered mesopore space. The material under study exhibits quite stable mesoporosity upon aging at high temperatures (800 °C) and is, thus, of potential interest in high-temperature catalysis. Here, different characterization techniques were applied to elucidate the structural evolution taking place between heat treatment at 400 °C and aging at 800 °C, i.e., in a water-containing atmosphere, which is usually detrimental to nanoscaled porosity. The changes in the mesoporosity were monitored by advanced physisorption experiments, including hysteresis scanning, and electron tomography analysis coupled with a 3D reconstruction of the mesopore space. These methods indicate that the 3D spatial arrangement of the primary particles during the synthesis under hydrothermal conditions via thermal hydrolysis is related to the thermal stability of the hierarchical mesopore structure. The assembly of the primary CeO2 particles (∼4 nm in size) results in an interparticulate space constituting an open 3D mesopore network, as revealed by skeleton analysis of tomography data, being in conformity with hysteresis scanning. At elevated temperatures (800 °C), sinter processes occur resulting in the growth of the primary particles, but the 3D mesopore network and the spherical secondary structure are preserved.

Amine-Functionalized Nanoporous Silica Monoliths for Heterogeneous Catalysis of the Knoevenagel Condensation in Flow.

Porous carrier materials functionalized with organocatalysts offer substantial advantages compared to homogeneous catalysts, e.g., easy separation of the catalyst, scalability, and an improved implementation in continuous operations. Here, we report the immobilization of (3-aminopropyl)trimethoxysilane (APTMS) onto self-prepared silica monoliths and its application as a heterogeneous catalyst in the Knoevenagel condensation between cyano ethylacetate and various aromatic aldehydes under continuous-flow conditions. The meso-macroporous silica monoliths (6-7 cm in length) were optimized to be used in flow taking advantage of their hierarchical meso- and macroporosity. The monoliths were cladded with a poly(ether ether ketone) (PEEK) tube by a refined procedure to guarantee tight connection between the carrier material and PEEK. Functionalization of the bare silica monoliths consisting of APTMS can be efficiently performed in flow in ethanol and toluene. While a large grafting gradient is obtained for toluene, the grafting in ethanol proceeds homogenously throughout the monolith, as evidenced by elemental analysis and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The silica monoliths exhibit high conversion up to 95% with concurrent low back pressures, which is of importance in flow catalysis. By connecting two monoliths, high conversions can be maintained for several flow rates. Two types of monoliths were synthesized, possessing different mesopore sizes. The monolith bearing the larger mesopore size showed an enhanced turnover frequency (TOF), while the monolith with the smaller mesopores allowed for larger quantities of the product to be synthesized, due to the higher surface area. A long-term stability test showed that the functionalized monoliths were still active after 66 h of continuous usage, while the overall yield decreased over time.

Pore Size Gradient Effect in Monolithic Silica Mesopore Networks Revealed by In-Situ SAXS Physisorption.

In disordered mesopore networks, the size distribution and connection between adjacent pores control desorption. How network characteristics can be extracted from corresponding physisorption isotherms is still a matter of research. To elucidate this, we study krypton physisorption (117.8 K) in the mesopore networks of "Nakanishi"-type monolithic silica. Combining physisorption in scanning acquisition mode with synchrotron-based in-situ SAXS provides complementary information on pore-filling states. These data reveal a mean pore size gradient in which pores grow smaller towards the material's network center. This structural motif cannot be derived through conventional isotherm analysis, but it is clearly exposed through scanning desorption curves which do not quite converge but merge individually with the main desorption isotherm before the lower hysteresis closing point. Hence, our findings provide the basis to build advanced models for analyzing scanning isotherms and extracting network characteristics through new descriptors, such as pore size and connectivity distributions as a function of the distance from the network center.

Tuning the Reactivity of Peroxo Anhydrides for Aromatic C-H Bond Oxidation.

Phenol moieties are key structural motifs in many areas of chemical research from polymers to pharmaceuticals. Herein, we report on the design and use of a structurally demanding cyclic peroxide (spiro[bicyclo[2.2.1]heptane-2,4'-[1,2]dioxolane]-3',5'-dione, P4) for the direct hydroxylation of aromatic substrates. The new peroxide benefits from high thermal stability and can be synthesized from readily available starting materials. The aromatic C-H oxidation using P4 exhibits generally good yields (up to 96%) and appreciable regioselectivities.