Batch zeolite synthesis has seen major advances in the last 20 years thanks to the development of new synthesis methodologies, novel reactor designs, and machine learning tools [1,2]. One of the tools to optimize the synthesis process is the flow reactor. It is known to increase mass and heat transfer, especially in the case of zeolite synthesis, where transport is diffusion and conduction driven [3]. Moreover, in order to speed-up crystallization kinetics and avoid formation of viscous intermediates, interzeolite conversion (IZC) method is used, i.e., the use of crystalline materials as Si (and Al) source. However, IZC comes with the challenge of handling a solid suspension throughout the entire process, which can end up forming solid deposits until reactor clogging. Previous work in our group has shown that ultrasound is a useful tool not only to counteract this issue, but also to further enhance IZC crystallization kinetics, in particular the growth rate [4,5]. To overcome the challenging trade-off between a fast enough flow rate to prevent deposition with an adequate residence time to obtain a satisfactory production rate, a modular ultrasound-integrated reactor setup is designed to perform FAU-to-MFI IZC in flow.
Setup description
Two tubular coiled reactors made of PTFE tubing (ID=2 mm, OD=3 mm), of 12 mL each are connected in series, the first one (nucleation reactor) in horizontal position, and the second one (growth reactor) vertically placed, with the flow from top to bottom. Each reactor is surrounded by a sealed aluminium box where silicon oil recirculates internally for temperature control, and externally up to 6 Langevin-type transducers can be attached for ultrasound transmission. Pressure is controlled by a back pressure regulator placed after the second reactor. As previous work showed the stronger impact of ultrasound on the second part of the synthesis [4], ultrasonic transducers are attached to the growth reactor. Low frequency ultrasound is delivered at a resonance frequency of 44.8±0.2 kHz, and a total input power of 240 W.
Interzeolite conversion in flow
In the first stage, the batch, as well as the flow syntheses are performed in the growth ultrasound-integrated reactor as a stand-alone [4], at a temperature of 140 °C and a pressure of 4 bara. the molar ratio of the reagents used in all experiments is the following: 1 SiO2 : 385−1 AlO−2 : 0.35 OSDA+ : 0.08 Na+ : 0.43OH− : 20 H2O, with tetrapropylammonium hydroxide as Organic Structure-Directing Agent (OSDA). Batch syntheses show the strong presence of FAU after 30 min synthesis, which almost disappear when approaching 60 min residence time. On the other hand, thanks to the improved mass and heat transfer in flow, complete amorphization of the FAU occurs after 20 min, while MFI peaks are clearly visible after 30 min residence time, although with very low solid yield (0.8% and 1.3% for the silent and sonicated case, respectively). Therefore, with the addition of the second reactor, residence time in flow can be prolonged to 60 min, to reach an expected solid yield >50%, thanks to the growth rate enhancement. With such a high crystalline solids formation, the use of ultrasound in the second reactor is of paramount importance to avoid crystal deposition and reactor clogging.
Conclusions
A versatile ultrasound-integrated coiled tubular reactor setup is designed to perform interzeolite conversion of FAU to MFI in flow. The setup consists of two independent reactors of 12 mL that can be used as stand-alone, or in series enabling a residence time up to 60 min. Both reactors can individually transmit ultrasound at 240 W as external energy input. Allowing 60 min of residence time allows complete transformation to MFI, and the application of ultrasound in the second reactor increases the growth rate as well as counteracts solid deposition.
References
[1] A. Deneyer, Q. Ke, J. Devos and M. Dusselier, Chem. Mater., 2020, 32, 4884–4919.
[2] W. Chaikittisilp and T. Okubo, Science (80-. )., 2021, 374, 257–259.
[3] Z. Liu, J. Zhu, C. Peng, T. Wakihara and T. Okubo, React. Chem. Eng., 2019, 4, 1699–1720.
[4] E. Brozzi, M. Dusselier, and S. Kuhn, React. Chem. Eng., 2025, submitted.
[5] C. Devos, A. Bampouli, E. Brozzi, G. D. Stefanidis, M. Dusselier, T. Van Gerven and S. Kuhn, Chem. Soc. Rev., 2025, 54, 85–115.