FUNCTIONALIZED ADSORBENT MATERIALS USING SUPERCRITICAL CO2

Abstract

Historically, porous silica (SiO2) is one of the most used adsorbents for a wide variety of processes in the industry. However, the fast grown on the demand of new nanotechnology based materials and sustainable green processes have made necessary the development of adsorbents with improved physico-chemical properties. One of the most applied options to modify porous silica is the incorporation on the surface of organic functional molecules, giving place to hybrid materials, in which the properties of both components are combined. In this doctoral thesis, supercritical carbon dioxide (scCO2) has been used as the solvent to carry out the functionalization processes. Carbon dioxide is a sustainable solvent and its use has been preferred in front of toxic organic liquid solvents, often applied in the traditional methods of synthesis. Amorphous silica matrices with structural ordered pores (MCM-41, 4 nm) and disordered pores (silica gel, 4-9 nm) were selected for the functionalization processes. Besides, the properties conferred by functionalization to microporous crystaline zeolites have been preliminary studied. The modifying agents applied in this thesis were either alkyl (octyltriethoxysialane) or amino (methylaminopropyltrimethoxisilane) silane and aziridine. The later compound is a monomer which polymerizes in presence of CO2, leading to hyperbranched polyethyleneimine (PEI) with multiple amino groups formed into the silica pores. This novel method only requires compressed CO2 as the reagent and the catalyst of the polymerization reaction of aziridine, which usually requires the use of organic solvents, a solid catalyst, high temperatures and long processing times. The functionalization of porous silica with aminosilane in scCO2 is more complex than the case of alkysilanes due to the high reactivity between amino groups and CO2 to form unsoluble carabamate species. However, in this study a protocol was designed to partially inhibit carabamate formation by controlling the pressure and temperature of the reaction media. The obtained materials were characterized using solid state characterization tools: low temperature N2 and CO2 adsorption, thermal analysis, infrared spectroscopy and X-Ray diffraction. Moreover, modeling and simulation methods were used as complementary tools that allowed the study of this complex systems with a high level of detail. The alkyl chain of the alkylsilane induced to the porous system a hydrophobic behavior, hence, obtaining materials candidates for oil adsorption. The functionalization with organic molecules containing the amino group allowed the preparation of materials for the adsorption and separation of CO2 from diluted gases (CO2 sequestration). The CO2 adsorption properties of the synthesized aminosilicas were evaluated combining experimental adsorption tools with molecular simulations. The characterization of these materials was based on the evaluation of the overall CO2 adsorption capacity and the influence of the temperature, the selectivity of the CO2 adsorption in gas mixtures, the stability in the cyclic adsorption/desorption process and the kinetics, which were determined by performing both microbalance and CO2 adsorption isotherms at different temperatures.

Date of Award	18 Nov 2014
Original language	English
Supervisor	Maria Concepción Domingo Pascual (Director) & María Lourdes Vega Fernández (Director)