Washcoating of H-ZSM-5 zeolite onto steel microreactor plates – Filling the void space between zeolite crystallite agglomerates particles

Abstract

A method for washcoating zeolite catalysts onto the walls of stainless steel microreactor plate channels has been developed. The properties of the washcoating suspension have been optimised that were critical for the quality of the coating, such as good adherence and uniformity. The average size of the original zeolite crystallite agglomerate particles (milled zeolite cake from calcination) was further reduced by micronizer milling to below 10 μm. Ultra-fine γ-Al2O3 powder (<2.5 μm) was added to the zeolite washcoating suspension as a binder. These measures resulted in much improved coating morphology and adherence compared to a washcoat only made from the original zeolite agglomerates powder. The optimised zeolite washcoat (containing 75 wt% of particle size reduced zeolite crystallite agglomerates and 25 wt% of the ultra-fine γ-Al2O3 powder) was well adherent, suffering a loss of only 0.1 wt% upon adhesion testing by multiple drop tests, and also smooth and fairly uniform. The effect is seen as having filled the void spaces between the comparatively large zeolite crystallite agglomerate particles with the ultra-fine γ-Al2O3 particles, which measure increased washcoat compactness, interparticle and particle–wall contact, particle interlocking and, therefore, improved adhesion. No significant changes in zeolite crystallinity and catalytic performance were effected to the washcoat compared to the original crystallite agglomerates. In general, less than the optimum binder content (here of 25 wt%) would slightly improve catalytic activity per unit mass washcoat, but drastically impair on adherence, while more than the optimum binder content would only reduce catalytic activity with no benefit regarding adherence.