Continuous flow synthesis of ZSM-5 zeolite on theorder of secondsZhendong 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 AdvancedIndustrial 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 reactorstakes a time so long (typically, on the order of days) that thecrystallization of zeolites has long been believed to be very slowin nature. We herein present a synthetic process for ZSM-5, anindustrially important zeolite, on the order of seconds in a contin-uous flow reactor using pressurized hot water as a heating medium.Direct mixing of a well-tuned precursor (90 °C) with the pressurizedwater preheated to extremely high temperature (370 °C) in the mil-limeter-sized continuous flow reactor resulted in immediate heatingto high temperatures (240–300 °C); consequently, the crystallizationof ZSM-5 in a seed-free system proceeded to completion within tensof or even several seconds. These results indicate that the crystalli-zation of zeolites can complete in a period on the order of seconds.The subtle design combining a continuous flow reactor with pres-surized hot water can greatly facilitate the mass production of zeo-lites in the future.

crystal growth | continuous flow synthesis | zeolites | ultrafast synthesis |ZSM-5

Zeolites have typically been synthesized via hydrothermaltreatment, a process designed to artificially mimic the geo-logical formation conditions of natural zeolites (1, 2). Dependingon the targeted framework and composition, several hours to days(or even several weeks) are required to complete the synthesis of acrystalline product (3). As a great improvement over the hundredsor thousands of years required for the formation of natural zeolites,artificial synthesis on the order of days allows us more opportu-nities to obtain zeolites with tunable properties (4, 5). Nevertheless,the hydrothermal synthesis of zeolites has remained a time-con-suming process. Long periods of hydrothermal treatment, for onething, cause a burden on both energy efficiency and operationalcosts. Furthermore, pressure-tight vessels used for the hydrother-mal treatment as well as harsh conditions therein make mechanisticstudies using in situ techniques difficult to perform (6, 7). Thecomplexity of crystallization, in turn, hinders the attempts at short-ening the synthesis period. This dilemma explains to some extent thereason behind the long-held belief that crystallization of zeolites isextremely slow in nature.The other major hurdle for the widespread application of zeolites

is the difficulty in mass production. Batchwise synthesis using au-toclaves is convenient for basic laboratory investigations but posesseveral challenges––in terms of economic efficiency, quality control,and operation flexibility––when large-scale manufacturing is con-sidered (8). In contrast, continuous flow process has proved to bebeneficial in the production of various chemicals (9, 10). Due to thetime-consuming hydrothermal treatment used for the synthesis ofzeolites, however, making a direct switch from batchwise synthesisto continuous flow manufacturing is not easy. To realize continuousflow production, shortening the synthesis period is therefore anissue of the highest priority. Recently, we have developed anultrafast route to synthesize industrially important zeolitic materialson the order of minutes (e.g., 10 min for aluminosilicate SSZ-13),based on which the continuous flow synthesis was successfully

established (11–13). The ultrafast route on the order of minutesnot only enabled the continuous flow synthesis but also demon-strated that the crystallization of zeolites can proceed much fasterthan generally observed.Owing to specific physical and chemical properties, supercritical

water or pressurized water with extremely high temperatures(usually near or above 300 °C) offers several potential advantagesfor hydrothermal processing (14). Continuous flow hydrothermalsynthesis of functional nanostructured materials in those mediumshas been extensively exploited in recent years, and the associatedmerits, such as rapid formation of crystalline materials and faciletuning of the properties, have been demonstrated (15, 16). Thehydrothermal synthesis of zeolites using supercritical water orpressurized water with extremely high temperatures, however,has rarely been reported. The challenge arises from the intrinsiccomplexity of zeolite formation. Zeolites are formed as a metastablephase compared with their dense-phase counterparts, which impliesthat overly high temperatures may only lead to the formation ofnonporous materials (17, 18). Meanwhile, the complicated assemblyof inorganic–organic species leading to the formation of zeolitesmay not be favored under the less hydrophilic conditions of waterwith too high temperatures (19). Finally, the organic structure-directing agent (OSDA) tends to decompose at high temperatures,which imposes further restrictions (20). To realize a synthesis insuch environments, it is of paramount importance that the crystal-lization of the target phase should be completed before the for-mation of a dense phase as well as the complete decompositionof OSDA.

Significance

Zeolites have greatly contributed to modern industries. Con-sumption of zeolites is expected to increase with the emergenceof newly commercialized applications. Typical synthesis of zeo-lites relies on batchwise hydrothermal synthesis, which usuallytakes tens of hours or even several days to complete. People havethus long believed that the crystallization of zeolites is very slowin nature. We herein demonstrate the continuous flow synthesisof ZSM-5, an industrially important zeolite, on the order of sec-onds. Crystallization from amorphous state to full crystallinitycould be completed in tens of or even several seconds. The syn-thesis on the order of seconds provides a great potential to fa-cilitate the mass production as well as to deepen the fundamentalunderstanding of zeolite crystallization.

Author contributions: T.W. designed research; Z.L., K. Okabe, C.A., Y. Yonezawa, and J.Z.performed research; A.E., Y. Yanaba, T.Y., and K. Ohara contributed new reagents/analytictools; Z.L., H.Y., T.O., and T.W. analyzed data; and Z.L., T.O., and T.W. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Freely available online through the PNAS open access option.1To whom correspondence should be addressed. Email: wakihara@chemsys.t.u-tokyo.ac.jp.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1615872113/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1615872113 PNAS | December 13, 2016 | vol. 113 | no. 50 | 14267–14271

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We present herein a continuous flow method for the synthesisof ZSM-5 using pressurized hot water with extremely high tem-perature (370 °C) as the heating medium. ZSM-5 is an alumi-nosilicate zeolite with intensive industrial applications (21, 22),yet typically its synthesis consumes tens of hours. In this work, wedemonstrate that the direct mixing of synthesis precursor and thepressurized hot water in a millimeter-sized continuous flow reactorcould result in immediate heating up to high temperatures (Fig. 1;for details of the continuous flow reactor see Fig. S1); conse-quently, the crystallization of ZSM-5, from amorphous state to fullcrystallinity, proceeded to completion at a remarkably fast rate in asystem without the addition of any seed. The well-tuned synthesisprecursor, obtained by aging the initial aluminosilicate gel at 90 °Cfor a certain period, triggered the nucleation and ensured theformation of ZSM-5 without any byproduct at extremely hightemperatures. Consequently, the crystallization rate surpassed thedecomposition rate of OSDA because of the creation of a favor-able environment for ultrafast crystallization. This design enabledthe synthesis of ZSM-5 within tens of or even several seconds,depending on the aging conditions. Due to the complex natureof zeolite crystallization, it is not possible to realize a synthesison the order of seconds using conventional methods carried outin conventional autoclaves. The continuous flow reactor to-gether with pressurized hot water as a heating medium com-prises a feasible technique to explore the time limit of zeolitecrystallization. Further novel findings, such as the discovery ofnew structures and in situ tuning of the properties of zeolites,will be expected using this synthetic platform. Moreover, with aremarkably short synthesis period, the continuous flow syn-thesis would be particularly advantageous for the mass pro-duction of industrially important zeolites.The initial synthesis mixture had a composition of 50 NaOH:

Al2O3:300 SiO2:20 TPAOH:2,300 H2O (where TPAOH is tetra-propylammonium hydroxide). Using this composition, conven-tional synthesis of ZSM-5 was carried out in a Teflon-linedautoclave at 170 °C for 24 h. The synthesized product was con-firmed to be completely crystalline ZSM-5 and was chosen as areference sample hereafter. Fig. 2A shows the X-ray diffraction(XRD) patterns of the products obtained from the continuous flowreactor after different residence times at 260 °C. Before the high-temperature synthesis, the synthesis mixture was aged at 90 °C for16 h. After aging, the collected solid phase was still amorphous.When this aged gel was subjected to high-temperature synthesis,apparent peaks due to the MFI (a three letter structure codeassigned by the International Zeolite Association Structure Com-mission) structure could be observed after a synthesis time of only1 s, demonstrating that considerable crystal growth had occurred.After a synthesis time of 6 s, fully crystalline ZSM-5 was obtained.Fig. 2B shows the SEM image of the ZSM-5 synthesized after 6 s,which had a smaller particle size compared with the referenceZSM-5 (Fig. 2C). The atomic-level local structure of the ZSM-5products was analyzed using high-energy X-ray total scattering(HEXTS) and 29Si solid-state magic-angle spinning NMR (29SiMAS NMR), as shown in Fig. 3A and Fig. S2, respectively (for

detailed explanation of the HEXTS results, see Fig. S3). The Si/Alratio 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). Themicropore volumes for both products were also comparable (0.128and 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 mainobstacles for the continuous flow synthesis involving solid reactantsand/or products. Continuous production having a high solid con-tent is susceptible to hydrodynamic failure caused by constriction,bridging, and random detachment of solid particles (23). In ourcase, the solid content in the synthesis mixture was significantlyhigh even in the presence of pressurized hot water. Despite a highsolid content, the compressed-air-driven vibrator, as sketched inFig. 1, proved to be a powerful technique, which can be easilyincorporated, to minimize precipitation or blockage problems.The ultrashort synthesis period required for complete crystalli-zation also helped prevent hydrodynamic failure. The yields forthe continuous flow synthesis are shown in Fig. S4, which dem-onstrates that high yields comparable to those in batch synthesescould be achieved.Instantaneous temperature increase generated by the direct

mixing of the synthesis precursor with the pressurized water atextremely high temperature is one of the key factors responsiblefor shortening the synthesis time within the order of seconds. Inour previous studies, we introduced a tubular reactor that helpsachieve fast heating in a preheated oil bath so as to minimize thethermal lag that usually exists in a conventional autoclave, andconsequently achieved complete synthesis on the order of minutes(11–13). Further shortening the synthesis period in the tubularreactor is hindered because it still takes almost 1 min for heattransfer from the external heating medium to the synthesis pre-cursor inside the tubular reactor. Direct mixing with the heatingmedium would be an ideal approach to prompt the heat transfer,and hence reduce the synthesis period further. In this study, directmixing of the synthesis precursor with water at extremely hightemperature (370 °C) remarkably facilitated a faster heating; as aresult, the crystallization of ZSM-5 could proceed immediatelyafter the synthesis precursor was fed. With this strategy, thethermal lag could almost be eliminated, which is very important torealize an ultrafast synthesis, because the crystallization can beinduced and completed before the complete decomposition ofOSDA under strongly basic, high-temperature conditions. Thesynthesis of ZSM-5 using the tubular reactor, heated in an

Fig. 1. Flowchart for the continuous flow synthesis of ZSM-5 on the orderof seconds.

Fig. 2. Continuous flow synthesis of ZSM-5 on the order of seconds. (A) XRDpatterns for the products synthesized over different periods at 260 °C (thereference ZSM-5 was synthesized in a conventional autoclave at 170 °C for24 h). (B and C) SEM images for the ZSM-5 product synthesized in 6 s at260 °C and the reference ZSM-5, respectively.

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aluminum block and operated in batch, was also attempted. Theresults show that it took around 5 min to obtain a fully crystallineZSM-5 product as opposed to 6 s in the continuous flow synthesis(Fig. S5). Measurement of the decomposition of TPAOH, theOSDA used in this study, was also carried out using the aluminumblock as a heating medium (Fig. S6). The result clearly shows thatconsiderable amount of oil phase (mainly comprising tripropyl-amine, one of the products of the TPAOH decomposition) wasgenerated after a heating period of 3 min at 260 °C, indicating thatapparent decomposition took place even within a short periodof heating.The duration of aging played an important role in determining

the crystallization rate. Fig. 4A shows the crystallinity curves (on thebasis of XRD peaks, shown in Fig. S7) for the syntheses using thealuminosilicate gels with different aging periods at 90 °C. Clearly,extremely fast synthesis within 16 s could also be achieved evenwhen gels with an aging period as short as 2 h were used. Syn-thesis of zeolites starts from the mixing of raw materials, which isusually followed by an aging period, performed either at roomtemperature or at an elevated temperature. During the agingprocess, not only physical mixing occurs but also several impor-tant chemical reactions toward the formation of precursors arealso triggered, leading to the formation of species that can ini-tiate the nucleation and significantly influence the crystallizationrate as well as the properties of the resultant crystals (24). TheXRD patterns in Fig. 4B show that peaks due to aluminumsource [gibbsite Al(OH)3] rather than MFI structure are ob-served, indicating that samples collected after aging were largelyamorphous. The 29Si MAS NMR spectra in Fig. 4C demonstratethe formation of aluminosilicates and silicates [Q4(2Al) and Q3 spe-cies, as represented by the broad peak centered around −101 ppm].No long-range order could be detected, which is further con-firmed by the results obtained from HEXTS (shown in Fig. S8).The high-resolution transmission electron microscopy (HRTEM)images shown in Fig. 4 D–F demonstrate that long-range orderhas not yet formed, as no lattice can be found. However, thermalgravimetric analysis (TGA) results show that the weight loss forthe samples collected after certain aging periods became highercompared with the one separated from the initial gel, implying astronger interaction between inorganic and organic species alongwith the aging period (Fig. S9). Because the gibbsite Al(OH)3remained partially undissolved (Fig. 4B and Fig. S9) after aging,the inorganic–organic species may mainly come from the

tetrapropylammonium-containing silicates (21). Meanwhile, it isworth noting that the particle size of the solid intermediates be-came larger with the aging period, which is consistent with pre-vious reports (25, 26). These results indicate that the short-rangeorder might have started to form during the aging stage, whichcreates an environment favorable for the immediate crystal growthunder high temperatures, resulting in the synthesis on the orderof seconds.We further studied the effect of the synthesis temperature on

the crystallization rate of ZSM-5. Fig. 5 A–D shows the 27Al MASNMR spectra of the products synthesized at 240, 260, 280, and300 °C, respectively. The 27Al MAS NMR spectrum can serve asan accurate indicator of crystallinity because aluminum hydroxidewith a gibbsite structure, whose peak appears at around 0 ppm,totally dissolves and is consumed only on complete crystallization(14). Clearly, the fastest crystallization rate was observed at asynthesis temperature of 260 °C. The crystallization rate becameslower when the synthesis was performed at 240 °C. Interestingly,synthesis temperatures higher than 260 °C did not further promptthe crystallization, and on the contrary, slower crystallization rateswere observed. A higher temperature indeed prompted the rate ofcrystal growth during the initial stage, according to the NMRspectra for the samples with a short synthesis period (2 s, for ex-ample). In contrast, the enhancement of crystallization rate withtemperature was not sustained, as seen from the NMR spectra forthe longer synthesis periods. One possible reason for this obser-vation is the low stability of OSDA at high temperatures, thatis, higher temperatures cause faster decomposition of OSDA,resulting in a slower crystal growth rate during the later stages ofcrystallization. Overly high temperatures are not good for zeolitecrystallization as a dense phase might be formed due to the de-composition of OSDA. Although the formation of a dense phasewas not observed in this study, the results obtained from the batchexperiments clearly prove that adverse effects, including slowercrystallization rates, were observed at 300 °C (Fig. S5) primarilybecause of the enhanced decomposition of OSDA.Although it is considered that zeolites are susceptible to hot water

owing to the accelerated hydrolysis of silanol groups at high tem-peratures (27, 28), they can withstand the hot water environment

Fig. 4. Effect of aging. (A) Crystallinity curves for the syntheses using thesynthesis mixture aged for different periods at 90 °C (synthesis temperature,260 °C). (B) XRD patterns of the solids collected after different aging periods[the circles denote the peaks due to gibbsite Al(OH)3]. (C) The

29Si MAS NMRspectra of the solids collected after different aging periods. (D–F) HRTEMimages for the samples aged for 2, 6, and 16 h, respectively.

Fig. 3. Confirmation of the properties of the ZSM-5 product synthesized in6 s. (A) Comparison of pair distribution function, G(r), of the product syn-thesized in 6 s and the reference ZSM-5 (for detailed explanation, see Fig.S3). (B), Comparison of N2 adsorption–desorption isotherms and microporevolumes of the product synthesized in 6 s and the reference ZSM-5.

Liu et al. PNAS | December 13, 2016 | vol. 113 | no. 50 | 14269

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if the residence time is short. We demonstrate herein the con-tinuous flow synthesis of ZSM-5, where direct mixing of a well-tuned precursor with pressurized hot water at high temperaturewas used to quicken the crystallization rate, resulting in the syn-thesis on the order of seconds. These unexpectedly fast crystalli-zation rates in a continuous flow apparatus help us to approachthe limit of the zeolite crystallization and provide a unique op-portunity for carrying out mechanistic studies. Furthermore, the

technique applied here can greatly facilitate the mass productionand will be extended to the continuous flow synthesis of otherzeolites (and related materials) in the future. Compared with theextremely short hydrothermal synthesis time, the precursor tuningthrough aging takes time. We next aim to deeply understand thenature of aging by combining chemistry and engineering per-spectives and hope to further shorten the overall production timein the future.

Materials and MethodsMaterials. The following materials were used as provided: colloidal silica(LUDOX AS-40, 40 wt % suspension in H2O), TPAOH (40% aqueous solution,Merck), aluminum hydroxide (with gibbsite structure, 100%, Wako), andsodium hydroxide (particulate, Wako).

Preparation of Synthesis Mixture for ZSM-5. The synthesis mixture had acomposition of 50 NaOH:Al2O3:300 SiO2:20 TPAOH:2,300 H2O, which wasprepared as follows: TPAOH solution, NaOH aqueous solution (20 wt %), andaluminum hydroxide were mixed and stirred for 5 min, and thereafter col-loidal silica was added and stirred for an additional 30 min. Before high-temperature synthesis, this mixture was aged at 90 °C in a plastic bottle thatwas rotated in an oven with a speed of 20 rpm.

Detailed Description of the Continuous Flow Synthesis. The continuous flowapparatus contains the following key components: HPLC pumps, electronicheaters and temperature controller (MTCS, Misumi), pressure gauges (PGI-63B-MG10-LAQX, Swagelok), compressed-air-driven vibrator, back-pressure regu-lator (TESCOM 26–1700, Emerson, US) and an in-house-assembled computercontrol system. The continuous flow reactors and necessary connecting unitswere all made from stainless steel standard parts (purchased from Swagelok).A detailed illustration of the system can be found in Fig. S1. In a typical op-eration, the system was gradually preheated to target temperatures usingwater as a circulating medium, and accordingly, the system pressure was ad-justed to 23 MPa using the back-pressure regulator. After the temperatureand pressure conditions became steady, the synthesis gel was fed to replacethe circulating water and the continuous flow synthesis started. The flow ratesfor the synthesis gel and the hot water were maintained at 1 mL/min and1.6 mL/min, respectively, which resulted in a synthesis temperature of 260 °C.Considering the density of water at 260 °C (∼780 kg/m3), the actual velocity inthe continuous flow reactor (1/8-inch standard tube, with an inner diameter of2.18 mm) was ∼0.015 m/s, which means that the length of the continuous flowreactor required for a residence time of 6 s was ∼0.09 m. At the outlet of thecontinuous flow reactor, cooling water was fed to cool down the slurry at a flowrate of 10 mL/min. To minimize the precipitation of the solids, the compressed-air-driven vibrator kept shaking the continuous flow reactor at a frequency of∼100 Hz (under a pressure of 0.1 MPa). The slurry flowing out of the continuousflow reactor was centrifuged, washed, and dried to obtain the final ZSM-5crystals. Considering the gel flow rate (1 mL/min) used in the typical operationconditions, a production rate of 260 g ZSM-5 per day could be achieved. Becausethe actual reactor volumewas quite small (15.6 mL), the continuous flow processgenerated a very high space–time yield [∼7,000 kg/(m3h)].

ACKNOWLEDGMENTS. This work is partly supported by the New Energy andIndustrial Technology Development Organization. The high-energy X-raytotal scattering experiments at SPring-8 were approved by the Japan Syn-chrotron Radiation Research Institute under Proposals 2015B0115, 2016A0115,and 2016B0115.

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