Magnetic Aerogels for Room-Temperature Catalytic Production of Bis(indolyl)methane Derivatives.

The potential of aerogels as catalysts for the synthesis of a relevant class of bis-heterocyclic compounds such as bis(indolyl)methanes was investigated. In particular, the studied catalyst was a nanocomposite aerogel based on nanocrystalline nickel ferrite (NiFe2O4) dispersed on amorphous porous silica aerogel obtained by two-step sol-gel synthesis followed by gel drying under supercritical conditions and calcination treatments. It was found that the NiFe2O4/SiO2 aerogel is an active catalyst for the selected reaction, enabling high conversions at room temperature, and it proved to be active for three repeated runs. The catalytic activity can be ascribed to both the textural and acidic features of the silica matrix and of the nanocrystalline ferrite. In addition, ferrite nanocrystals provide functionality for magnetic recovery of the catalyst from the crude mixture, enabling time-effective separation from the reaction environment. Evidence of the retention of species involved in the reaction into the catalyst is also pointed out, likely due to the porosity of the aerogel together with the affinity of some species towards the silica matrix. Our work contributes to the study of aerogels as catalysts for organic reactions by demonstrating their potential as well as limitations for the room-temperature synthesis of bis(indolyl)methanes.

Nanoporous Au Behavior in Methyl Orange Solutions.

Nanoporous (NP) gold, the most extensively studied and efficient NP metal, possesses exceptional properties that make it highly attractive for advanced technological applications. Notably, its remarkable catalytic properties in various significant reactions hold enormous potential. However, the exploration of its catalytic activity in the degradation of water pollutants remains limited. Nevertheless, previous research has reported the catalytic activity of NP Au in the degradation of methyl orange (MO), a toxic azo dye commonly found in water. This study aims to investigate the behavior of nanoporous gold in MO solutions using UV-Vis absorption spectroscopy and high-performance liquid chromatography. The NP Au was prepared by chemical removal of silver atoms of an AuAg precursor alloy prepared by ball milling. Immersion tests were conducted on both pellets and powders of NP Au, followed by examination of the residual solutions. Additionally, X-ray photoelectron spectroscopy and electrochemical impedance measurements were employed to analyze NP Au after the tests. The findings reveal that the predominant and faster process involves the partially reversible adsorption of MO onto NP Au, while the catalytic degradation of the dye plays a secondary and slower role in this system.

Aluminosilicate-Supported Catalysts for the Synthesis of Cyclic Carbonates by Reaction of CO2 with the Corresponding Epoxides.

Functionalized aluminosilicate materials were studied as catalysts for the conversion of different cyclic carbonates to the corresponding epoxides by the addition of CO2. Aluminum was incorporated in the mesostructured SBA-15 silica network. Thereafter, functionalization with imidazolium chloride or magnesium oxide was performed on the Al_SBA-15 supports. The isomorphic substitution of Si with Al and the resulting acidity of the supports were investigated via 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and NH3 adsorption microcalorimetry. The Al content and the amount of MgO were quantified via inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The anchoring of the imidazolium salt was assessed by 29Si and 13C MAS NMR spectroscopy and quantified by combustion chemical analysis. Textural and structural properties of supports and catalysts were studied by N2 physisorption and X-ray diffraction (XRD). The functionalized systems were then tested as catalysts for the conversion of CO2 and epoxides to cyclic carbonates in a batch reactor at 100 or 125 °C, with an initial CO2 pressure (at room temperature) of 25 bar. Whereas the activity of the MgO/xAl_SBA-15 systems was moderate for the conversion of glycidol to the corresponding cyclic carbonate, the Al_SBA-15-supported imidazolium chloride catalysts gave excellent results over different epoxides (conversion of glycidol, epichlorohydrin, and styrene oxide up to 89%, 78%, and 18%, respectively). Reusability tests were also performed. Even when some deactivation from one run to the other was observed, a comparison with the literature showed the Al-containing imidazolium systems to be promising catalysts. The fully heterogeneous nature of the present catalysts, where the inorganic support on which the imidazolium species are immobilized also contains the Lewis acid sites, gives them a further advantage with respect to most of the catalytic systems reported in the literature so far.

Synthesis of Dimethyl Carbonate by Transesterification of Propylene Carbonate with Methanol on CeO2-La2O3 Oxides Prepared by the Soft Template Method.

In this study, CeO2, La2O3, and CeO2-La2O3 mixed oxide catalysts with different Ce/La molar ratios were prepared by the soft template method and characterized by different techniques, including inductively coupled plasma atomic emission spectrometry, X-ray diffraction, N2 physisorption, thermogravimetric analysis, and Raman and Fourier transform infrared spectroscopies. NH3 and CO2 adsorption microcalorimetry was also used for assessing the acid and base surface properties, respectively. The behavior of the oxides as catalysts for the dimethyl carbonate synthesis by the transesterification of propylene carbonate with methanol, at 160 °C under autogenic pressure, was studied in a stainless-steel batch reactor. The activity of the catalysts was found to increase with an increase in the basic sites density. The formation of dimethyl carbonate was favored on medium-strength and weak basic sites, while it underwent decomposition on the strong ones. Several parasitic reactions occurred during the transformation of propylene carbonate, depending on the basic and acidic features of the catalysts. A reaction pathway has been proposed on the basis of the components identified in the reaction mixture.

Nanostructured Ni/CeO2-ZrO2 Catalysts for CO2 Conversion into Synthetic Natural Gas.

NiO-CeO2-ZrO2 mixed oxides, with Ni/(Ce + Zr) = 1 mol/mol and different Ce/Zr molar ratios, were prepared by the soft-template method. The chemical composition, texture, structure, and redox features of the synthesized systems were investigated by different techniques. All samples were nanocrystalline (NiO nanocrystal average size 4 nm) and had high surface area and quite an ordered mesoporous system. The catalytic performances in the CO2 conversion into methane were studied at atmospheric pressure, 300 °C, and stoichiometric H2/CO2 molar ratio. Prior to reaction the catalysts were submitted to a mild reduction pretreatment (H2 at 400 °C for 1 h). XRD analysis of the samples after pretreatment showed the presence of small Ni crystals (4-7 nm) on all the samples as well as of some unreduced NiO nanocrystals on the systems with high Zr content, in accordance with H2-TPR experiments, which indicated that NiO reduction is promoted by CeO2 but hindered by ZrO2. The catalytic tests were performed at two different space velocities (72000 and 900000 cm³ h-1 g-1cat) on a series of Ni-based catalysts supported on CeO2-ZrO2 systems with different Ce/Zr ratios, including the two pure oxides. CO2 conversion and selectivity to CH4 (which was always close to 100 mol%) were constant throughout the 6-hour runs. CO2 conversion resulted to increase with CeO2 content in the catalyst, thus indicating the role of the CeO2 component of the support in activating CO2, whereas H2 is activated on the Ni nanoparticles.

Copper-Based Catalysts Supported on Highly Porous Silica for the Water Gas Shift Reaction.

Copper-based nanoparticles, supported on either a silica aerogel or cubic mesostructured silicas obtained by using two different synthetic protocols, were used as catalysts for the water gas shift reaction. The obtained nanocomposites were thoroughly characterised before and after catalysis through nitrogen adsorption-desorption measurements at -196 °C, TEM, and wide- and low-angle XRD. The samples before catalysis contained nanoparticles of copper oxides (either CuO or Cu2 O), whereas the formation of metallic copper nanoparticles, constituting the active catalytic phase, was observed either by using pre-treatment in a reducing atmosphere or directly during the catalytic reaction owing to the presence of carbon monoxide. A key role in determining the catalytic performances of the samples is played by the ability of different matrices to promote a high dispersion of copper metal nanoparticles. The best catalytic performances are obtained with the aerogel sample, which also exhibits constant carbon monoxide conversion values at constant temperature and reproducible behaviour after subsequent catalytic runs. On the other hand, in the catalysts based on cubic mesostructured silica, the detrimental effects related to sintering of copper nanoparticles are avoided only on the silica support, which is able to produce a reasonable dispersion of the copper nanophase.

Modifications induced by pretreatments on Au/SBA-15 and their influence on the catalytic activity for low temperature CO oxidation.

SBA-15 functionalization with mercaptopropyltrimethoxysilane has been used to prepare supported gold catalysts for the low temperature CO oxidation reaction. Supports and catalysts have been characterized by chemical analysis, CHS analysis, XRD, TGA, nitrogen adsorption-desorption at 77 K, TEM, CPMAS NMR, XPS and EPR. Catalytic runs have been carried out at atmospheric pressure and 313-623 K and the influence of diverse thermal treatments of the samples prior to reaction has been investigated. The presence of organic residues and the size of the gold nanoparticles strongly affect catalytic activity. Only high-temperature calcination in air followed by treatment under H2 atmosphere leads to active catalysts. After complete elimination of the functionalizing agent, caused by the calcination step, a gold-mediated "activation" process of the silica support takes place during the hydrogen treatment. As a consequence, active catalysts for the low temperature CO oxidation are obtained, even though the size of the Au particles is too large for establishing direct Au-oxygen interactions, usually assumed to be essential for the reaction over silica-supported gold catalysts.