Unraveling the role of ultrasound in hydrothermal interzeolite conversion using a tubular ultrasound-integrated reactor

In the past years, ultrasound has been considered a sustainable process intensification technique for zeolite synthesis. However, understanding the link between ultrasound phenomena and their related effects has remained a challenge due to the limited availability of hydrothermal ultrasonic reactors and parameter standardization among the studies. In this work, a novel ultrasound-integrated tubular coiled reactor is presented, which enables fast and efficient ultrasonic hydrothermal zeolite synthesis. Specifically, the effect of ultrasound irradiation and its underlying mechanisms on high silica FAU-to-MFI interzeolite conversion is studied. Unseeded syntheses in the presence of an organic structure-directing agent (OSDA) are performed at 160 °C for residence times up to 2 h. The presence of hydroxyl radicals generated by ultrasound is assessed <i>via</i> terephthalic acid dosimetry at different temperatures and pressures as a measurement of the cavitation activity. The application of 20 W mL^-1 of suspension reveals an enhanced MFI growth rate and faster crystallization completion, resulting in an overall increase in the mean crystal size. Ultrasound is also successful in counteracting solid deposition on the walls of the coiled reactor. Applying hydrothermal conditions to this setup suppresses radical formation, indicating very weak transient cavitation activity. Therefore, these observations are attributed to the enhanced mass transfer <i>via</i> ultrasonic wave propagation, which renders the dissolved material more readily available for crystal growth.