Synthesis of titanium oxide particles in supercritical CO2: Effect of operational variables in the characteristics of the final product

A new chemical precursor is proposed for the synthesis of TiO2 anatase nanoparticles in supercritical CO2: the organometallic diisopropoxititanium bis(acetylacetonate) (DIPBAT). DIPBAT thermohydrolysis in supercritical carbon dioxide (SC-CO2) has been studied in the range 10.0–20.0 MPa and 200–300 °C, and compared with that of titanium tetraisopropoxide (TTIP). The proposed reaction mechanism is a thermohydrolysis, where the hydrolysis of acetylacetonate groups is the limiting step of the reaction rate. The addition of water directly to the reaction favours the growth of formed particles, whereas ethanol offers better results as hydrolysing reactant, leading to smaller particles. Experiments have been performed first in a batch process and secondly in a semi-continuous one, varying the residence time in the reactor from 30 s to 2 min. The effect of operational variables in the final product and their influence in the different steps of the process have been studied. Results have shown that product crystallinity is related with temperature, and temperatures higher than 250 °C are necessary to obtain well-crystallized TiO2 anatase. In the same sense, area Brunauer–Emmett–Teller (BET) is connected with crystallinity, and amorphous product, Ti(OH)4, shows the highest surface area. Particle size and particle size distribution (PSD) are controlled by instantaneously supersaturation degree, and precursor concentration together with pressure are the main responsible of particle size control. Operational conditions influence solubility of species, mass transfer, chemical reaction and nucleation and particle growth and they mark the final characteristics of the product and its application. In such a way, good crystallized TiO2 anatase particles of about 200 nm in diameter have been obtained working at 300 °C, 20.0 MPa and residence time of 2 min, with a reaction medium composed by CO2/ethanol (80/20, v/v). Such particles present good optical properties and specific surface area BET of around 150 m2/g. At lower working temperatures the obtained particles present worse crystallinity; however, their specific surface area increases to 350 m2/g and they are suitable as support of metal clusters in heterogeneous catalysis.