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In this paper we describe a new approach for the formation of nanoporous particles in which a thermally stable, water-soluble, and therefore easily leached second component is employed during spray pyrolysis to produce a composite particle. We demonstrate the new synthetic approach for synthesizing nanoporous aluminum oxide (Al2O3) by spray pyrolysis of aluminum nitrate (Al(NO3)3·9H2O) and sodium chloride (NaCl) solutions. The characteristics of the nanoporous material such as surface area, pore volume, and particle morphology are investigated as a function of relative humidity, temperature, and salt fractions. Specific surface area of the product particles increased significantly from 10 m2/g for the produced composite particles to over 370 m2/g for the remaining Al2O3 particles following aqueous leaching to remove the NaCl filler. Nanoporous particles could be produced at relatively low temperature (<550 °C) and high relative humidity (or low evaporation rate of aerosolized droplets) when followed by the aqueous leaching process of final product powder. However, we observed only solid particles at reactor temperatures above 700 °C and residence times of about 1 s. We attribute this behavior to evaporation of salt from the composite particles as evidenced through particles size distribution measurements, which showed the formation of a nuclei mode at these temperatures. At these higher temperatures we suspect a collapse of the porous network in the absence of the supporting salt network and results in particles with low surface area. Intermediate levels of salt fractions result in the formation of nanoporous particles, while high salt fractions result in a loosely connected network and the formation of individual nanoparticles in the salt matrix.

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We describe a new salt-assisted aero-sol-gel approach to produce spherical nanosized mesoporous silica particles. As an alternative to expensive templating mediums in prior works, salt (NaCl) was employed as a templating medium because it is thermally stable, recyclable, and easily leached. Furthermore, we demonstrate the ability to carry out traditional sol-gel chemistry within an aerosol droplet. The role of salt in sol-gel chemistry and aerosol processing was investigated as a function of hydrolysis time. It was verified that salt accelerates the kinetics of silica gelation, and simultaneously becomes an excellent templating medium to support nano-sized pores inside silica structures in the aerosol processing route. The presence of salt results in a roughly ten-fold increasing in the pore specific surface area and pore volume, subsequent to leaching of the salt matrix. The surface area and pore volume of the as-produced nanoporous silica particles was found to increase with increasing sol-gel hydrolysis time.

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