Ever since Mobil researchers developed the self-assembled template method for synthesis of the M41S series mesoporous silica in 1992, this class of materials has been applied to numerous applications in catalysis, separation and purification, sensors, and biomaterials based upon its high porosity, ordered pore morphology, and tunable surface groups. Later, researchers developed the aerosol-assisted self-assembly method for the production of mesoporous silica in a process similar to spray drying. Until now, no high porosity mesoporous silica has been produced by flame aerosol technology because the high temperature typical of flame synthesis would destroy the organic template and destroy the pores. Moreover, flame synthesis of silica,
e.g., production of fumed silica, generally proceeds by a gas-to-particle conversion route that is inherently incompatible with templating. Here, we report the possibility of producing nanoparticles of mesoporous silica using a unique flame aerosol reactor configuration that separates the flame chemistry and particle formation process into different regions, which favors a much lower reaction temperature. In this process, a liquid solution of precursor and surfactant is injected into the throat of a converging-diverging nozzle, placed downstream of a hydrogen-oxygen flame. The nozzle accelerates the hot combustion gases, which atomize the precursor solution. Solvent evaporation, surfactant self-assembly and silica formation occur in each droplet, during the ~50 ms residence time in the aerosol reactor downstream of the nozzle. The droplets are much smaller and the residence time much shorter than in traditional aerosol-assisted self-assembly processes. With this approach, after calcination to remove the surfactant template, we produced mesoporous silica with a BET surface area of more than 1000 m
2/g, entirely in the form of submicron spheres, with a mean diameter near 100 nm. This approach is also generalizable to other materials, and in our ongoing research we are prepating other mesoporous metal oxides by the same process.
This aerosol synthesis route also allows single-step decoration of the mesoporous silica with another material, which is particularly advantageous for single-step preparation of supported catalysts. Here, as a first example of this, we add chromium nitrate into the silica precursor. As the mesoporous silica forms in droplets, CrOx nanoparticles are also generated. Upon calcination to remove the templating surfactant, this yields a supported CrOx catalyst structure. This catalyst shows very promising activity for CO2 oxidative dehydrogenation of propane, and the flame aerosol technology provides a scalable and continuous process for single-step production of such catalysts. CO2 oxidative dehydrogenation, in general, which reacts CO2 with natural gas liquids (ethane, propane, butane) to produce higher value products (ethylene, propylene, butene, and CO) can play an important role in CO2 utilization.
