Creating hierarchies promptly: Microwave-accelerated synthesis of ZSM-5 zeolites on macrocellular silicon carbide (SiC) foams

Hierarchical ZSM-5 coatings supported on macrocellular β-SiC foams were prepared by a microwave-accelerated secondary growth method. Under microwave irradiation, differential heating was enabled due to the high microwave-adsorbing ability of SiC leading to the fast assembly of ZSM-5 zeolites on the SiC foams. The effects of crystallisation temperature, time and agitation on the growth of ZSM-5 zeolites on SiC foams were also investigated under microwave heating. A specific surface area of ca. 52 m2 g−1 was obtained under the optimum condition of 150 °C and 4 h. Comparative studies of conventional hydrothermal and microwave-accelerated synthesis were conducted and the resulting materials were characterised extensively using XRD, SEM and N2 adsorption. The combination of microwave heating, high microwave absorption of SiC substrates and agitation allowed a much more focused heating across the surface of SiC foams than conventional heating, which suppressed the formation of ZSM-5 crystals in the liquid phase and the consequent sedimentation. Additionally, for the first time, mesoscopic features of ZSM-5 coatings on SiC foam composites (>10 μm) were studied in 3D by X-ray micro computed tomography (μ-CT), providing quantitative information on the quality of the ZSM-5 coating such as the global thickness and distribution of ZSM-5 across the SiC foam supports. It was concluded that microwave-accelerated secondary growth synthesis offered an efficient route to give high quality coatings on structured foams with the good crystal quality and uniform thickness as well as lower energy consumption, especially for the substrate with a high loss tangent.