Al-modified Silica Research Articles

Cesium Loaded on an Al-Modified Silica Support Catalyst for Methyl Acrylate Synthesis by Aldol Condensation of Methyl Acetate and Formaldehyde

An Al-modified silica-supported cesium catalyst was prepared by an incipient wetness impregnation method for the gas-phase aldol condensation reaction of methyl acetate and formaldehyde. The effect of the Al-modified silica support on the structure, specific surface area, and the acid property was studied using X-ray diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption, NH3-temperature programmed desorption, X-ray photoelectron spectroscopy, and Py-IR. Adding the cesium species, the Cs–Al/SiO2 catalyst performed well based on the evaluation results in a fixed-bed reactor. A combination of these characterization and testing results manifests the influence of acid–base properties on the catalytic activity. It was found that the selectivity was related to the amount of weak acid sites and medium-strong base sites. The highest selectivity of methyl acrylate (MA) was obtained at T = 355 °C, LHSV = 0.6 h–1, and Ma/FA/MeOH molar ratio = 1:1:2 over the 10% Cs–3% Al/SiO2 catalyst. Finally, the performance of the optimized catalyst did not exhibit an obvious decrease via three times in situ regeneration.

Waste glycerol conversion on pilot scale continuous process by Al-modified mesoporous SiO2 catalysts

This work describes the conversion of the waste glycerol (as generated in the biodiesel industry, supplied by Petrobras) to valuable chemicals (such as formic acid, diglycerol, and dioxanes). The reactions were evaluated in batch and continuous-flow fixed-bed reactors (PBR) on a pilot scale using high porosity mesoporous silica (≈ 1500 m2 g−1) and Al-modified silica catalysts (SiAl = 15/1 and 5/1). The Al addition increased the acidity of the catalysts and reduced their surface areas (698 m2 g−1 in SiAl = 5). High glycerol conversion (80%) was reached for 15 Si/Al and 5 Si/Al whereas SiO2 tends to deactivate in a continuous-flow fixed-bed reactor (PBR). Furthermore, the catalysts exhibited different selectivity: SiO2 produced mainly formic acid (≈ 65% average) in the liquid phase whereas the Al-modified catalysts generated a large amount of diglycerol, possibly due to the different acidity of the catalysts. This study indicates the potential industrial use of the catalysts and contributes to increasing the sustainability of the biodiesel industry.

Robust and conductive mesoporous reduced graphene oxide-silica hybrids achieved by printing and the sol gel route

Reduced graphene oxide (rGO) scaffoldings are used as templates to create lightweight 3D rGO/silica and rGO/silico-aluminate hybrids by a simple impregnation route and the sol-gel method. The printed rGO assemblies are infiltrated by the corresponding alkoxide precursor solution and gelled by exposure to ammonia vapours, producing an hybrid replica of the rGO structure. The hybrids show a significant prevalence of mesopores, with total porosity above 94 %, density of ∼ 0.1 g⋅cm−3 and high specific surface area (≥ 190 m2⋅g−1). As a result, the 3D composite materials show enhanced water adsorption capacity and hydrophilicity, display compressive strengths in the range 0.1 – 0.4 MPa, which scale with the proportion of silica (or Al-modified silica) on the hybrid scaffold, and electrical conductivities are above 60 S⋅m−1. These properties are very attractive for applications in the removal of pollutants, water filtering, catalysis, drug delivery, or energy production and storage.

Thermal stability of Al-modified silica aerogels through epoxide-assisted sol–gel route followed by ambient pressure drying

A novel method for preparing Al-modified silica aerogels with pretreated kaolin as the raw material is developed. Sols comprising silica and Al are prepared by an in situ reaction method from pretreated kaolin. The pretreatment step simplifies the process, saves preparation time, and reduces costs. A method for adjusting the Al content in the aerogel by changing the washing time of the sol during the ion impurity removal process is also presented. The volume stabilities and thermal properties of the Al-modified silica aerogels after high-temperature treatment are investigated. The results indicate that a small amount of modified Al improves the volumetric stability of an aerogel owing to viscous sintering of the Si–O–AlIV–(O–AlVI)y units, which increases the disorder of the SiO2 structure, building a barrier to viscous flow and hindering mass transfer.

Isolated Chromium(VI) Oxide Species Supported on Al-Modified Silica: A Molecular Description

Supported chromium oxide systems are widely used catalysts. Experimental data indicate that modification of the silica support with Al changes the properties of the surface Cr species. In this computational work, extensive density functional theory investigations of the monomeric Cr(VI) oxo species variously located on the Al-modified silica surface have been done using a large number of systematically generated periodic and cluster models. It is predicted that the surface modification can influence both the geometry and the relative stabilities of the Cr species. Monooxo Cr(VI) species located in the vicinity of two Al sites usually adopts a pseudotetrahedral geometry, in contrast to square pyramidal monooxo Cr(VI) species on silica. Under dehydrated conditions, the relative stabilities of the tetrahedral monooxo and dioxo Cr(VI) species are comparable, whereas the dioxo Cr(VI) species is thermodynamically preferred on silica. The obtained results explain the experimentally estimated higher ratio of the ...

Sodium silicate-based, alkali-activated slag mortars: Part I. Strength, hydration and microstructure

Alkali activation of ground granulated blast furnace slag (GGBFS) with sodium silicate gave clinker-free binders, with high strength and early strength development, although set times were short and somewhat variable. Isothermal calorimetry detected three heat evolution peaks (wetting, gelation of activator and bulk reaction of slag). X-ray diffraction (XRD) showed no crystalline products. Hydration was investigated by scanning electron microscopy (SEM; with quantitative image analysis) and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR). From early age, a uniform gel filled the initially water-filled space, and gradually densified as reaction proceeded. Microanalysis of outer product (OP) showed an Al-substituted C–S–H gel phase of widely variable (0.5–1.0) Ca/Si ratio. NMR showed long-chain substituted C–S–H with Al/Si ratio rising to 0.19 at 1 year, and also cross-linked material, consistent with a Ca- or Al-modified silica gel. Inner product (IP) regions around slag grains probably also contained hydrotalcite. Activation with KOH gave more rapid reaction of slag than for silicate activation, a less homogeneous microstructure, and lower strengths. The hydrates contained a substituted C–S–H gel of low Ca/Si ratio probably mixed with hydrotalcite, and occasional higher Al regions in the OP regions.