Structured catalysts – catalyst coatings on macroscopic supports – offer enhanced mass transfer, mechanical stability, and easy recyclability, making them attractive for applications such as catalytic distillation. However, conventional hydrothermal synthesis often suffers from uncontrolled bulk nucleation in the liquid phase, leading to low loading efficiency and poor adhesion. In a study published in ENGINEERING Chemical Engineering , researchers at Tianjin University and collaborators present a microwave assisted hydrothermal method that overcomes these limitations by exploiting the strong microwave absorption of silicon carbide (SiC) foam.
The team used NaA zeolite as the active phase and SiC foam as the support. Under microwave irradiation, the SiC support heated much faster (6.56 °C·min⁻¹) than the surrounding precursor solution (1.42 °C·min⁻¹), creating a persistent thermal gradient. This selective heating creates localized hot spots on the SiC surface, promoting directed nucleation and growth of zeolite crystals directly onto the support while suppressing homogeneous crystallization in the bulk solution. As a result, microwave synthesis achieved a comparable zeolite loading in only 15 minutes – a 93.75 % reduction in time compared to conventional hydrothermal synthesis (4 hours).
To address the issue of silicon dissolution from the SiC support – which can cause initial mass loss and incomplete coverage – three pretreatment methods were compared: calcination, slurry pretreatment, and silica sol pre coating. Silica sol pretreatment proved most effective. The silica sol layer transforms the inert SiC surface into an active interface rich in silanol groups, lowering the nucleation energy barrier and guiding preferential crystal growth. After five loading cycles, the mass variation (ratio of coated to bare support mass) reached 1.11, and the zeolite coating thickness was about 46 μm – far thicker than those obtained by calcination (∼5.8 μm) or slurry pretreatment (∼20.6 μm).
The adhesion and durability of the optimized NaA@SiC catalyst were rigorously tested. Ultrasonic treatment caused a mass loss of only about 0.36 %, and solvent flushing at 70 °C for four days resulted in a total mass loss of 0.62 %, demonstrating excellent robustness.
Catalytic performance was evaluated in the aldol condensation of dimedone and 3 bromobenzaldehyde. With a catalyst loading as low as 0.8 wt % active component (77 % lower than typical literature reports), the structured catalyst achieved a yield above 90 %. Over three consecutive cycles, the conversion decreased by only about 1 % and selectivity by about 0.9 %, while the cumulative catalyst mass loss was approximately 0.41 %, confirming excellent reusability.
This microwave assisted, silica sol pretreated method provides a rapid, energy efficient route to high performance structured zeolite catalysts on SiC supports, with strong potential for industrial applications such as catalytic distillation and continuous flow processes.
ENGINEERING Chemical Engineering
Experimental study
Not applicable
Directed growth of robust zeolite coatings on silicon carbide supports via microwave selective heating and silica sol pretreatment
15-Mar-2026