Julio Fraile

Preparation of silane-coated TiO2 nanoparticles in supercritical CO2

Nanometric inorganic pigments are widely used as fillers for hybrid composite materials. However, these nanometric powders are hydrophilic in nature and their surface must be functionalized before use. In this work, titanium dioxide (TiO2) nanoparticles were coated using silane coupling agents with alkyl functionality. A supercritical carbon dioxide (scCO2) method was used for surface silanization. Five alkylalkoxysilanes with different alkyl chain length and structure were studied: methyltrimethoxy, isobutyltriethoxy, octyltriethoxy, octyldimethylmethoxy and octadecyltrimethoxysilane. The microstructure and thermal stability of deposited monolayers were characterized using thermogravimetric analysis, ATR-IR spectroscopy, transmission electron microscopy, wettability characterization and low-temperature N2 adsorption/desorption analysis. The use of scCO2 as a solvent provided an effective approach to functionalize individual inorganic nanoparticles due to the enhanced diffusivity of the solution molecules in the aggregates interparticle voids. The trifunctional silanes employed here yielded surfaces with better thermal stabilities and greater hydrophobicities than the used monofunctional silane.

Hybrid aminopolymer–silica materials for efficient CO2 adsorption

The present work focuses on the development of a new eco-efficient chemical method for the polymerization of aziridine to hyperbranched polyethyleneimine (PEI) into mesoporous silica by using compressed CO2 as a solvent, reaction medium and catalyst. PEI was in situ grafted into MCM-41 and silica gel substrates, with pore diameters of 3.8 and 9.0 nm, respectively. The optimal polymerization conditions were found by varying the reaction pressure (1.0–10 MPa), temperature (25–45 °C) and time (20–400 min). The thermal stability analysis indicated that aminopolymer chains were covalently attached on the amorphous silica surface. The described compressed CO2 route for the synthesis of high amine content hybrid products (6–8 mmolN g−1) is a very fast method, with processing times in the order of few minutes even at very low working pressures (1.0 MPa), being a step forward in the design of efficient hybrid aminopolymer nanocomposites for CO2 capture. The adsorptive behavior of the prepared hybrid materials was experimentally established by recording the N2 (−196 °C) and CO2 (25, 50 and 75 °C) adsorption isotherms. Results were compared to molecular simulation studies performed using Grand Canonical Monte Carlo for either N2 or CO2 adsorbed on amino modified MCM-41, thus helping to elucidate the predominant PEI configuration present in the functionalized materials.

Fractal porosity in metals synthesized by a simple combustion reaction

A simple modification of a combustion method has been used for the production of ultraporous metals in air. Nitrates of different metallic elements were reacted with glycine as a reducing fuel. The glycine to nitrate ratio can be simply used to control the formation of oxides or, in the case of fuel-rich mixtures, the formation of metals such as Ni, Co, Cu or Ag. Furthermore, the metallic monoliths obtained present a remarkable porosity of fractal nature (from macro to nano scales) with pores ranging from many microns down to at least 5 nm. This exceedingly simple approach shows the way for the design and synthesis of complex porous microstructures of metals for the wide variety of applications where interface optimization is crucial.

Preparation and Characterization of Graphene Oxide Aerogels: Exploring the Limits of Supercritical CO2 Fabrication Methods

The supercritical carbon dioxide (scCO2 ) synthesis of non-reduced graphene oxide (GO) aerogels from dispersions of GO in ethanol is here reported as a low-cost, efficient, and environmentally friendly process. The preparation is carried out under the mild conditions of 333 K and 20 MPa. The high aspect ratio of the used GO sheets (ca. 30 μm lateral dimensions) allowed the preparation of aerogel monoliths by simultaneous scCO2 gelation and drying. Solid-state characterization results indicate that a thermally-stable mesoporous non-reduced GO aerogel was obtained by using the supercritical procedure, keeping most of the surface oxygenated groups on the GO sheets, thus, facilitating further functionalization. Moreover, the monoliths have a very low density, high specific surface area, and excellent mechanical integrity; characteristics which rival those of most light-weight reduced graphene aerogels reported in the literature.