(10d) Speeding up the Synthesis of Zeolites: From Several Days to Several Seconds

Porous crystalline materials, in particular zeolites, continue their momentum in finding new applications in fields of catalysis and separation.^[1] One example of the newly established uses of zeolites is ammonia selective catalytic reduction of nitrogen oxide (NO_x), which employs metal-ion-exchanged small pore zeolites as highly stable catalysts to selectively reduce NO_x to N₂ and H₂O.^[2] With the advent of the “shale gas revolution”,^[3] zeolites have also been extensively studied as potential catalysts in the direct conversion of methane.^[4] Such application prospects call for an efficient synthesis of high-quality zeolites. However, it usually takes several days or even several weeks for the conventional hydrothermal synthesis of zeolites, which is typically carried out in batch operation, to obtain a zeolite with desired structure and properties. Developing an ultrafast synthesis route is of high significance to boost the existing applications as well as to explore new benefits of zeolites in diverse fields. Recently, our group has established the ultrafast syntheses of a series of industrially important zeolites.^[5-8] The synthesis periods for those zeolites were shortened to tens of or several minutes, which are in stark contrast to several days that are required for the conventional approach to obtain the corresponding type of material. In this contribution, we present our recent progresses on the ultrafast synthesis of zeolite, with a focus on discussing how the remarkably enhanced crystallization rate could be achieved and its potential consequences in the mass production of zeolites.

In general, following essential prerequisites are needed to achieve an ultrafast synthesis of a zeolite: 1) a well pre-tuned synthesis mixture obtained through appropriate aging to promptly supply nutrient components for crystal growth, 2) the addition of seed crystals to skip nucleation, 3) fast heating to avoid negative “thermal lag” effects such as decomposition of organic structure-directing agent and formation of undesired, metastable phase, 4) a synthesis temperature as high as the system allows to fasten the crystal growth rate. These factors could generate a synergistic effect that led to the ultrafast synthesis on the order of minutes. The fast-synthesized zeolites have proven to possess comparable properties, in terms of crystallinity and microposity, and exhibit similar or ever better catalytic activities.^[5,8] Based on the remarkably shortened synthesis period, the continuous flow synthesis of zeolites has been established. This plug flow reactor (PFR) process holds a great potential to replace the currently dominating batch process for the mass production of zeolites. In addition, we recently designed a continuous flow reactor, which employed pressurized hot water as a heating medium in order to achieve fast heating within one second or less.^[9] Direct mixing of the pressurized water preheated to 370 °C with a properly aged synthesis mixture resulted in immediate heating to a high temperature (260 °C), and consequently, the crystallization of ZSM-5 with a seed-free system proceeded to completion in only 6 seconds. The synthesis on the order of seconds provides a great advantage to further facilitate the mass production of zeolites.

References:

[1] Davis, M. E. Ordered Porous Materials for Emerging Applications. Nature 2002, 417, 813−821.

[2] Fickel, D.; D’Addio, E.; Lauterbach, J.; Lobo, R. The Ammonia Selective Catalytic Reduction Activity of Copper-Exchanged SmallPore Zeolites. Appl. Catal., B 2011, 102, 441−448.

[3] Siirola, J. J. The Impact of Shale Gas in the Chemical Industry. AIChE J. 2014, 60 810 – 819.

[4] Narsimhan, K.; Iyoki, K.; Dinh, K.; Roman-Leshkov, Y. Catalytic Oxidation of Methane into Methanol over Copper-Exchanged Zeolites with Oxygen at Low Temperature. ACS Cent. Sci. 2016, 2, 424–429.

[5] Liu, Z.; Wakihara, T.; Oshima, K.; Nishioka, D.; Hotta, Y.; Elangovan, S. P.; Yanaba, Y.; Yoshikawa, T.; Chaikittisilp, W.; Matsuo, T.; Takewaki, T.; Okubo, T. Widening Synthesis Bottlenecks: Realization of Ultrafast and Continuous-Flow Synthesis of High-Silica Zeolite SSZ-13 for Nox Removal. Angew. Chem. 2015, 127, 5775−5779.

[6] Liu, Z.; Wakihara, T.; Nishioka, D.; Oshima, K.; Takewaki, T.; Okubo, T. One-Minute Synthesis of Crystalline Microporous Aluminophosphate (AlPO4-5) by Combining Fast Heating with A Seed-Assisted Method. Chem. Commun. 2014, 50, 2526−2528.

[7] Liu, Z.; Wakihara, T.; Nishioka, D.; Oshima, K.; Takewaki, T.; Okubo, T. Ultrafast Continuous-Flow Synthesis of Crystalline Microporous Aluminophosphate AlPO4-5. Chem. Mater. 2014, 26, 2327−2331.

[8] Zhu, J.; Liu, Z.; Iyoki, K.; Anand, C.; Yoshida, K.; Sasaki, Y.; Sukenaga, S.; Ando, M.; Shibata, H.; Okubo, T.; Wakihara, T. Ultrafast Synthesis of High-Silica Erionite Zeolites with Improved Hydrothermal Stability. Chem. Commun. 2017, 53, 6796–6799.

[9] Liu, Z.; Okabe, K.; Anand, C.; Yonezawa, Y.; Zhu, J.; Yamada, H.; Endo, A.; Yanaba, Y.; Yoshikawa, T.; Ohara, K.; Okubo, T; Wakihara, T. Continuous flow synthesis of ZSM-5 zeolite on the order of seconds. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 14267–14271.