Emulsification/Internal Gelation Synthesis of Highly Porous Zeolite Granules for Improving Low-Frequency Acoustic Performance of Microloudspeakers

Acoustic porous medium is regarded as a promising filling material to enhance the acoustic volume and reproduce low frequency in modern closed-box microloudspeakers. Zeolites have attracted extensive attention because the sorption and diffusion of air molecules in their unique rigid-frame pores can lead to sound attenuation. However, these properties are hard to reserve after shaping micrometer-sized zeolite crystals into hundred-micrometer monolith particles. Herein, we report the synthesis of highly porous ZSM-5 zeolite spherical granules via a simple emulsification/internal gelation process. Aqueous alginate solution containing zeolites and CaCO3 was emulsified into paraffin oil, followed by the release of Ca2+ ions to cross-link with the polysaccharide residues and immobilize labile zeolites under weakly acidic conditions, forming granules with high sphericity. Highly porous zeolite granules with 301–427 μm in diameter were obtained by calcination, which preserved over 98% of the specific surface area and 93% of the porosity of original zeolites. In the acoustic test, 1.0 cm3 ZSM-5 granule filling demonstrated the largest resonance offset of 316 Hz. It was speculated that sound energy was converted into heat through the adsorption and diffusion of air molecules inside nanopores, as well as interscale diffusion, ultimately improving acoustic compliance. The present mild granulation method is expected to efficiently produce large-scale zeolite granules as promising filling materials for acoustic applications.