Continuous flow synthesis of ZSM-5 zeolite on the order of seconds Zhendong Liua, Kotatsu Okabea, Chokkalingam Ananda, Yasuo Yonezawaa, Jie Zhua, Hiroki Yamadaa, Akira Endob, Yutaka Yanabac, Takeshi Yoshikawac, Koji Oharad, Tatsuya Okuboa, and Toru Wakiharaa,1 aDepartment of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan; bNational Institute of Advanced Industrial Science and Technology, Ibaraki 305-8565, Japan; cInstitute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan; and dJapan Synchrotron Radiation Research Institute/SPring-8, Kouto 1-1-1, Sayo-gun, Hyogo 679-5198, Japan Edited by Mark E. Davis, California Institute of Technology, Pasadena, CA, and approved November 8, 2016 (received for review September 23, 2016) The hydrothermal synthesis of zeolites carried out in batch reactors established (11–13). The ultrafast route on the order of minutes takes a time so long (typically, on the order of days) that the not only enabled the continuous flow synthesis but also demon- crystallization of zeolites has long been believed to be very slow strated that the crystallization of zeolites can proceed much faster in nature. We herein present a synthetic process for ZSM-5, an than generally observed. industrially important zeolite, on the order of seconds in a contin- Owing to specific physical and chemical properties, supercritical uous flow reactor using pressurized hot water as a heating medium. water or pressurized water with extremely high temperatures Direct mixing of a well-tuned precursor (90 °C) with the pressurized (usually near or above 300 °C) offers several potential advantages water preheated to extremely high temperature (370 °C) in the mil- for hydrothermal processing (14). Continuous flow hydrothermal limeter-sized continuous flow reactor resulted in immediate heating synthesis of functional nanostructured materials in those mediums to high temperatures (240–300 °C); consequently, the crystallization has been extensively exploited in recent years, and the associated of ZSM-5 in a seed-free system proceeded to completion within tens merits, such as rapid formation of crystalline materials and facile of or even several seconds. These results indicate that the crystalli- tuning of the properties, have been demonstrated (15, 16). The zation of zeolites can complete in a period on the order of seconds. hydrothermal synthesis of zeolites using supercritical water or The subtle design combining a continuous flow reactor with pres- pressurized water with extremely high temperatures, however, surized hot water can greatly facilitate the mass production of zeo- has rarely been reported. The challenge arises from the intrinsic lites in the future. complexity of zeolite formation. Zeolites are formed as a metastable phase compared with their dense-phase counterparts, which implies crystal growth | continuous flow synthesis | zeolites | ultrafast synthesis | that overly high temperatures may only lead to the formation of ZSM-5 nonporous materials (17, 18). Meanwhile, the complicated assembly of inorganic–organic species leading to the formation of zeolites eolites have typically been synthesized via hydrothermal may not be favored under the less hydrophilic conditions of water Ztreatment, a process designed to artificially mimic the geo- with too high temperatures (19). Finally, the organic structure- logical formation conditions of natural zeolites (1, 2). Depending directing agent (OSDA) tends to decompose at high temperatures, on the targeted framework and composition, several hours to days which imposes further restrictions (20). To realize a synthesis in (or even several weeks) are required to complete the synthesis of a such environments, it is of paramount importance that the crystal- crystalline product (3). As a great improvement over the hundreds lization of the target phase should be completed before the for- or thousands of years required for the formation of natural zeolites, mation of a dense phase as well as the complete decomposition artificial synthesis on the order of days allows us more opportu- of OSDA. nities to obtain zeolites with tunable properties (4, 5). Nevertheless, the hydrothermal synthesis of zeolites has remained a time-con- Significance suming process. Long periods of hydrothermal treatment, for one thing, cause a burden on both energy efficiency and operational costs. Furthermore, pressure-tight vessels used for the hydrother- Zeolites have greatly contributedtomodernindustries.Con- mal treatment as well as harsh conditions therein make mechanistic sumption of zeolites is expected to increase with the emergence studies using in situ techniques difficult to perform (6, 7). The of newly commercialized applications. Typical synthesis of zeo- complexity of crystallization, in turn, hinders the attempts at short- lites relies on batchwise hydrothermal synthesis, which usually takes tens of hours or even several days to complete. People have ening the synthesis period. This dilemma explains to some extent the thus long believed that the crystallization of zeolites is very slow reason behind the long-held belief that crystallization of zeolites is in nature. We herein demonstrate the continuous flow synthesis extremely slow in nature. of ZSM-5, an industrially important zeolite, on the order of sec- The other major hurdle for the widespread application of zeolites onds. Crystallization from amorphous state to full crystallinity is the difficulty in mass production. Batchwise synthesis using au- could be completed in tens of or even several seconds. The syn- toclaves is convenient for basic laboratory investigations but poses thesis on the order of seconds provides a great potential to fa- several challenges––in terms of economic efficiency, quality control, –– cilitate the mass production as well as to deepen the fundamental and operation flexibility when large-scale manufacturing is con- understanding of zeolite crystallization. sidered (8). In contrast, continuous flow process has proved to be ENGINEERING beneficial in the production of various chemicals (9, 10). Due to the Author contributions: T.W. designed research; Z.L., K. Okabe, C.A., Y. Yonezawa, and J.Z. time-consuming hydrothermal treatment used for the synthesis of performed research; A.E., Y. Yanaba, T.Y., and K. Ohara contributed new reagents/analytic zeolites, however, making a direct switch from batchwise synthesis tools; Z.L., H.Y., T.O., and T.W. analyzed data; and Z.L., T.O., and T.W. wrote the paper. to continuous flow manufacturing is not easy. To realize continuous The authors declare no conflict of interest. flow production, shortening the synthesis period is therefore an This article is a PNAS Direct Submission. issue of the highest priority. Recently, we have developed an Freely available online through the PNAS open access option. ultrafast route to synthesize industrially important zeolitic materials 1To whom correspondence should be addressed. Email: [email protected]. on the order of minutes (e.g., 10 min for aluminosilicate SSZ-13), This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. based on which the continuous flow synthesis was successfully 1073/pnas.1615872113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1615872113 PNAS | December 13, 2016 | vol. 113 | no. 50 | 14267–14271 Downloaded by guest on September 29, 2021 detailed explanation of the HEXTS results, see Fig. S3). The Si/Al ratio of the product, analyzed by inductively coupled plasma– atomic emission spectrometer (ICP-AES), was 105, which is sim- ilar to that of the reference ZSM-5 (with a Si/Al ratio of 108). The micropore volumes for both products were also comparable (0.128 and 0.124 cm3/g for the product synthesized in 6 s and the refer- ence ZSM-5, respectively; Fig. 3B). A stable hydrodynamic condition was maintained in the con- tinuous flow synthesis, as indicated by the temperature and pres- sure profiles (Fig. S1). Hydrodynamic failure is one of the main Fig. 1. Flowchart for the continuous flow synthesis of ZSM-5 on the order obstacles for the continuous flow synthesis involving solid reactants of seconds. and/or products. Continuous production having a high solid con- tent is susceptible to hydrodynamic failure caused by constriction, We present herein a continuous flow method for the synthesis bridging, and random detachment of solid particles (23). In our of ZSM-5 using pressurized hot water with extremely high tem- case, the solid content in the synthesis mixture was significantly perature (370 °C) as the heating medium. ZSM-5 is an alumi- high even in the presence of pressurized hot water. Despite a high nosilicate zeolite with intensive industrial applications (21, 22), solid content, the compressed-air-driven vibrator, as sketched in yet typically its synthesis consumes tens of hours. In this work, we Fig. 1, proved to be a powerful technique, which can be easily demonstrate that the direct mixing of synthesis precursor and the incorporated, to minimize precipitation or blockage problems. pressurized hot water in a millimeter-sized continuous flow reactor The ultrashort synthesis period required for complete crystalli- could result in immediate heating up to high temperatures (Fig. 1; zation also helped prevent hydrodynamic failure. The yields for for details of the continuous flow reactor see Fig. S1); conse- the continuous flow synthesis are shown in Fig. S4, which dem- quently,