Flame assisted synthesis of catalytic ceramic membranes

Membranes consisting of one or more metal oxides can be synthesized by flame pyrolysis. The general principle behind flame pyrolysis is the decomposition and oxidation of evaporated organo-metallic precursors in a flame, thereby forming metal oxide monomers. Because of the extreme supersaturation created in the flame, the monomers will nucleate homogeneously and agglomerate to form aggregates of large ensembles of monomers.

The aggregates will then sinter together to form single particles. If the flame temperature and the residence time are sufficiently high, the formed oxide particles will be spherical due to the fast coalescence at the high temperatures in the flame. The primary product from the flame pyrolysis is an aerosol of metal oxide nanoparticles.

The aerosol gas from the flame can be utilized for several different purposes, depending on the precursors fed to the flame. With the present technology it is possible to make supported catalysts, composite metal oxides, catalytically active surfaces, and porous ceramic membranes. Membrane layers can be formed by using a porous substrate tube (or surface) as a nano-particle filter.

The aerosol gas from the flame is led through a porous substrate tube, where a part of the gas is sucked through the wall of the substrate, thereby creating a thin filter cake on the inner surface of the substrate tube. The top-layer can be deposited directly on a coarse pore structure. Since the Brownian motion of the aerosol particles is fast compared to the fluid velocity through the substrate, the particles will not penetrate very deep into the substrate.

The pore diameter of the deposited top-layer depends on the size of the particles, which can be controlled by changing the fed rate of precursor to the flame. Using a macro-porous alpha-alumina substrate, membranes with pore sizes below 5 nm have been produced by this continuous filtration of nano-particles.

In this way, top-layers with Knudsen separation have been achieved by a reduction of the pore size of three orders of magnitude within an hour. It has previously been shown that it also is possible to produce both composite metal oxides and supported catalysts by flame pyrolysis, simply by feeding suited precursors to the flame.

Together with the membrane deposition technique, this leads the way for flame assisted synthesis of catalytic membranes.