Synthesis of Nanocrystalline Mg-Al Hydrotalcites in the Presence of Starch-the Effect on Structure and Composition

Enhancement of Ni-NiO-CeO2 Interaction on Ni-CeO2/Al2O3-MgO Catalyst by Ammonia Vapor Diffusion Impregnation for CO2 Reforming of CH4

Ni-based catalysts have been widely used for the CO2 reforming of methane (CRM) process, but deactivation is their main problem. This study created an alternative electronic Ni-NiO-CeO2 interaction on the surface of 5 wt% Ni-5 wt% CeO2/Al2O3-MgO (5Ni5Ce(xh)/MA) catalysts to enhance catalytic potential simultaneously with coke resistance for the CRM process. The Ni-NiO-CeO2 network was developed on Al2O3-MgO through layered double hydroxide synthesis via our ammonia vapor diffusion impregnation method. The physical properties of the fresh catalysts were analyzed employing FESEM, N2 physisorption, and XRD. The chemical properties on the catalyst surface were analyzed employing H2-TPR, XPS, H2-TPD, CO2-TPD, and O2-TPD. The CRM performances of reduced catalysts were evaluated at 600 °C under ambient pressure. Carbon deposits on spent catalysts were determined quantitatively and qualitatively by TPO, FESEM, and XRD. Compared to 5 wt% Ni-5 wt% CeO2/Al2O3-MgO prepared by the traditional impregnation method, the electronic interaction of the Ni-NiO-CeO2 network with the Al2O3-MgO support was constructed along the time of ammonia diffusion treatment. The electronic interaction in the Ni-NiO-CeO2 nanostructure of the treated catalyst develops surface hydroxyl sites with an efficient pathway of OH* and O* transfer that improves catalytic activities and coke oxidation.

The Synthesis of Cu–Mn–Al Mixed-Oxide Combustion Catalysts by Co-Precipitation in the Presence of Starch: A Comparison of NaOH with Organic Precipitants

Cu–Mn mixed oxides are well known as active combustion catalysts. The common method for their synthesis is based on co-precipitation, with NaOH as a precipitant, and is burdened with the possibility of introducing undesired Na contamination. This work describes the use of two organic bases, tetrabutylammonium hydroxide and choline hydroxide, as precipitating agents in a novel alkali-free route for Cu–Mn–Al catalyst synthesis. To obtain fine crystalline precursors, which are considered advantageous for the preparation of active catalysts, co-precipitation was carried out in the presence of starch gel. Reference materials prepared with NaOH in the absence of starch were also obtained. Mixed oxides were produced by calcination at 450 °C. The precursors contained MnCO3 doped with Cu and Al, and an admixture of amorphous phases. Those prepared in the presence of starch were less crystalline and retained biopolymer residues. The combustion of these residues during calcination enhanced the formation of larger amounts of the Cu1.5Mn1.5O4 spinel phase, with better crystallinity in comparison to catalysts prepared from conventionally synthesized precursors. Tests of toluene combustion demonstrated that the catalysts prepared with starch performed better than those obtained in starch-free syntheses, and that the mixed oxides obtained by the alkali-free route were more active than catalysts prepared with NaOH. Catalytic data are discussed in terms of property–performance relationships.

Comparative Physicochemical and Catalytic Study of Nanocrystalline Mg-Al Hydrotalcites Precipitated with Inorganic and Organic Bases

Synthetic Mg-Al hydrotalcites (HT) are environmentally friendly solid bases frequently applied as catalysts in base catalyzed reactions. The most common synthesis method, using NaOH as precipitant, is problematized by the possibility of introducing undesired Na contamination. Alkali-free synthesis is usually performed with NH₃aq, a precipitant which is less efficient in incorporation of Mg into HT lattice. In the present work, organic bases, tetrabutylammonium hydroxide and choline hydroxide, were successfully employed as precipitating agents in a new alkali-free route of Mg-Al HT synthesis. HT solids were also obtained with inorganic bases, NH₃aq and NaOH. Characterization with X-ray diffraction, elemental analysis, scanning electron microscopy, Fourier-transform infrared spectroscopy and thermogravimetry/differential scanning calorimetry, confirmed the formation of nanocrystalline HT compounds with all employed bases. HT prepared with NH₃aq exhibited an Mg deficit, which was detrimental to the catalytic activity in base catalyzed reactions. The effect was attributed to the tendency of Mg²⁺ to form ammine complexes, a conclusion supported by quantum mechanical calculations. HT prepared with NaOH showed the highest crystallinity, which was unfavorable for catalytic application. The addition of starch to the synthesis medium provided a means by which to diminish the crystal size of all HT precipitates. Catalytic tests of the Baeyer-Villiger oxidation of cyclohexanone demonstrated that the highest yields of ε-caprolactone were obtained with fine-crystalline HT catalysts prepared with organic bases in the presence of a starch template.

Fine Crystalline Mg-Al Hydrotalcites as Catalysts for Baeyer-Villiger Oxidation of Cyclohexanone with H2O2

The catalytic activity of Mg-Al hydrotalcite (HT) materials in base-catalyzed reactions is known to be promoted by the low crystallinity of the HT solid. In the present work, two routes enabling the preparation of finely crystalline Mg-Al HT materials were explored: (1) the inverse microemulsion technique, and (2) co-precipitation in the presence of starch. Carbonate, chloride and bromide forms of HT were prepared, examined with X-ray diffraction, scanning electron microscopy/energy dispersive X-ray spectroscopy and infrared spectroscopy, and used as catalysts in the Baeyer–Villiger oxidation of cyclohexanone to ε-caprolactone with a H2O2/acetonitrile system. The bromide forms proved significantly less active than the chlorides and carbonates, as they promoted nonselective consumption of H2O2. The fine crystalline materials were more active than the more crystalline HT references obtained by conventional co-precipitation. Catalysts prepared by inverse microemulsion were less crystalline and more active than the starch-templated ones, but suffered stronger deactivation by the acidic reaction environment. Alkalization of the reaction medium with NaHCO3 stabilized the HT materials and increased the ε-caprolactone yield, which became comparable for both types of fine crystalline catalysts—thus pointing to the synthesis involving a simple and cheap starch templating approach as being a particularly attractive one.

Bio-DEE Synthesis and Dehydrogenation Coupling of Bio-Ethanol to Bio-Butanol over Multicomponent Mixed Metal Oxide Catalysts

Within the Waste2Fuel project, innovative, high-performance, and cost-effective fuel production methods from municipal solid wastes (MSWs) are sought for application as energy carriers or direct drop-in fuels/chemicals in the near-future low-carbon power generation systems and internal combustion engines. Among the studied energy vectors, C1-C2 alcohols and ethers are mainly addressed. This study presents a potential bio-derived ethanol oxidative coupling in the gas phase in multicomponent systems derived from hydrotalcite-containing precursors. The reaction of alcohol coupling to ethers has great importance due to their uses in different fields. The samples have been synthesized by the co-precipitation method via layered double hydroxide (LDH) material synthesis, with a controlled pH, where the M(II)/M(III) ≈ 0.35. The chemical composition and topology of the sample surface play essential roles in catalyst activity and product distribution. The multiple redox couples Ni2+/Ni3+, Cr2+/Cr3+, Mn2+/Mn3+, and the oxygen-vacant sites were considered as the main active sites. The introduction of Cr (Cr3+/Cr4+) and Mn (Mn3+/Mn4+) into the crystal lattice could enhance the number of oxygen vacancies and affect the acid/base properties of derived mixed oxides, which are considered as crucial parameters for process selectivity towards bio-DEE and bio-butanol, preventing long CH chain formation and coke deposition at the same time.

CO2 Hydrogenation to Methane over Ni-Catalysts: The Effect of Support and Vanadia Promoting

Within the Waste2Fuel project, innovative, high-performance, and cost-effective fuel production methods are developed to target the “closed carbon cycle”. The catalysts supported on different metal oxides were characterized by XRD, XPS, Raman, UV-Vis, temperature-programmed techniques; then, they were tested in CO2 hydrogenation at 1 bar. Moreover, the V2O5 promotion was studied for Ni/Al2O3 catalyst. The precisely designed hydrotalcite-derived catalyst and vanadia-promoted Ni-catalysts deliver exceptional conversions for the studied processes, presenting high durability and selectivity, outperforming the best-known catalysts. The equilibrium conversion was reached at temperatures around 623 K, with the primary product of reaction CH4 (>97% CH4 yield). Although the Ni loading in hydrotalcite-derived NiWP is lower by more than 40%, compared to reference NiR catalyst and available commercial samples, the activity increases for this sample, reaching almost equilibrium values (GHSV = 1.2 × 104 h–1, 1 atm, and 293 K).