Ultrafast Continuous-Flow Synthesis of Crystalline Microporous

Article pubs.acs.org/cm

Ultrafast Continuous-Flow Synthesis of Crystalline Microporous Aluminophosphate AlPO4‑5 Zhendong Liu,† Toru Wakihara,† Daisuke Nishioka,‡ Kazunori Oshima,‡ Takahiko Takewaki,‡ and Tatsuya Okubo*,† †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Mitsubishi Chemical Group, Science and Technology Research Center, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan

‡

S Supporting Information *

ABSTRACT: Crystalline microporous materials have been typically synthesized by long-time hydrothermal treatment in a batch reactor, which suffers from drawbacks like frequent start-up and shut-down operations and low energy efficiency. The development of a continuous flow process for the synthesis of crystalline microporous materials is extremely challenging due to the slow crystallization of the microporous materials. In this work, we demonstrate the continuous flow synthesis of an important crystalline microporous aluminophosphate material, AlPO4-5. The continuous synthesis of AlPO4-5 was achieved by combining the seed-assisted method with a continuous flow reactor that could provide a much higher heating rate. The results showed that single phase AlPO4-5 was obtained after one-minute synthesis in the continuous flow reactor. A stable continuous process was maintained, because any hydrodynamic failure from the precipitation of the solid product could be minimized thanks to the ultrafast synthesis and the small particle size of the AlPO4-5 product. In addition, the reuse of the product from the continuous flow synthesis as a seed is demonstrated. This easily designed, efficient route can result in the significant cost and energy savings and thus has huge potential for the industrial-level production of AlPO4-5 crystals in the future.

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INTRODUCTION Crystalline microporous materials such as zeolite and its aluminum-phosphate counterparts have greatly contributed to the petrochemical and fine chemical processes as catalysts, adsorbents, and ion exchangers.1−3 The roles of crystalline microporous materials have recently been extended to new application fields such as heat pump process,4,5 emission control,6 and so on. Existing and potential applications make the more economic production of crystalline microporous materials an important issue. Many works have been undertaken to develop a low-cost process for the synthesis of these microporous crystals. For example, remarkable progress has been made toward the successful synthesis of important zeolites such as ZSM-5, beta, and RUB-13 without using the organic structure directing agent (OSDA).7−11 This novel route avoids the use of high cost OSDA as well as a calcination procedure to remove it from the as-synthesized products. Improving the “atom efficiency” is another important issue since a higher yield of the reaction will prevent wasting expensive resources and will also lead to a simpler purification process followed by the reaction.12 Despite the recent progress on the economic production of crystalline microporous materials, problems still remain. One of them is that these materials are typically synthesized by the long-time hydrothermal treatment in a batch manner owing to the slow crystallization, regardless of the presence or absence of OSDA. The batch process essentially suffers from drawbacks like frequent start-up and shut-down © 2014 American Chemical Society

operations and low energy efficiency. To address this issue, the development of a continuous process is highly demanded for the efficient synthesis of crystalline microporous materials. Although controversy about the evolution from a batch process to a continuous flow process still remains, many successful shifts have already been achieved in a wide range of fields, for example, organic synthesis, polymerization, and the preparation of nanoparticles − just to name a few.13−16 Remarkable improvements in continuous flow processes over batch processes have been demonstrated in emerging publications. The advantages of a continuous flow process are as follows: improved yield, high energy efficiency, low consumption of expensive chemicals, precise temperature management, moderate operating conditions, and ease of scale-up.17−21 Research into continuous chemical processes has already contributed to and will further promote the development of more efficient, environmentally friendly processes, making it an important topic in materials science.22−25 However, whether a chemical reaction can benefit from a continuous flow process depends on the characteristics of the reaction in question. More importantly, a successful design requires elaborate considerations from both perspectives on chemistry and engineering. Received: January 24, 2014 Revised: March 7, 2014 Published: March 9, 2014 2327

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Therefore, a question arises as to whether a continuous flow reactor can be applied to the synthesis of crystalline microporous materials. There is no definitive criterion to assess the applicability of the continuous flow reactor, and several screening methods have been proposed in previous studies.26−28 Generally, a continuous flow design may potentially benefit the reaction with high reaction rate, and a homogeneous reaction is more likely to realize a successful switch from a batch reactor to a continuous flow reactor than a heterogeneous reaction, in particular, reaction where solid reagents and/or products are involved.29−31 Thus, two main characteristics of the synthesis of crystalline microporous materials − the slow reaction rates and the formation of a solid product − make the continuous synthesis using a flow reactor extremely challenging. Additionally, the harsh operating conditions of hydrothermal synthesis under high temperature and high pressure cause further technical difficulties. Therefore, very limited successes have been achieved in the continuous hydrothermal synthesis of crystalline microporous materials. In a rare case, the continuous synthesis of zeolite A (LTA type structure) using a microreactor was successfully demonstrated, and this success was largely due to the fact that this material could be synthesized under moderate conditions (below 100 °C).32−35 Similarly, with the help of chemistry methods to manipulate the formation kinetics, an ordered mesoporous material, COK-12, could be synthesized within a short period of time under ambient temperature and pressure, and therefore a continuous synthesis was successfully demonstrated.36 The continuous synthesis of zeolites (ZSM-5, zeolite Y, and zeolite A) using microwave equipment was also reported.37,38 Although advantages in the use of microwave continuous synthesis were claimed, the drawbacks of microwave heating such as low controllability, limited penetration depth, and an unclear mechanism hinder the scale-up and wider application of this process.39,40 With this background, more efforts should be devoted to the development of a continuous process with ease of design and feasible scalability to realize the highly efficient synthesis of crystalline microporous materials. In this work, we demonstrate the continuous synthesis of an important crystalline microporous aluminophosphate material, AlPO4-5. Because of its characteristic one-dimensional, 12 membered-ring channel structure as well as its thermal stability, AlPO4-5 has been intensively studied.41,42 This material has already been commercialized as a vapor-adsorbent material used in adsorption chillers.4 Recently, we developed a rapid synthesis route for the preparation of AlPO4-5 by combining fast heating with a seed-assisted method.43 We found that the addition of seed is beneficial to skipping spontaneous nucleation and the fast heating induces much faster crystal growth. The combined effects lead to the ultrafast synthesis of AlPO4-5 in one minute. It is well-known that the continuous flow reactor is a useful system to achieve fast heating. Thus, the continuous process for the synthesis of AlPO4-5 was designed by combining the seed-assisted method and the continuous flow reactor that could provide a much faster heating rate. The fast crystallization as well as the small particle size of the synthesized AlPO4-5 crystals proved to be favorable for the hydrodynamic conditions of the flow reactor, which makes the flow process described in this work an easily designed continuous route and a feasible scale-up prospect for the synthesis of microporous crystals.

Article

EXPERIMENTAL SECTION

Preparation of Synthesis Gel. The following materials were used as provided: pseudoboehmite (Capatal C, Sasol), phosphoric acid (85 wt %, Wako), and tetrapropylammonium hydroxide (TPAOH, 40% aqueous solution, Merck). The gel was prepared as follows: pseudoboehmite was added to a phosphoric acid solution and stirred for 24 h upon which TPAOH was added and stirred for another 24 h to obtain the synthesis gel with a composition of 1 P2O5:1 Al2O3:1 TPAOH:50 H2O. In the seed-assisted synthesis, 10 wt % AlPO4-5 seed based on Al2O3 was added and stirred for 1.5 h before the addition of TPAOH. Synthesis of the AlPO4-5 Seed. Fifteen g of synthesis gel was added to a 23 mL Teflon-lined stainless steel autoclave (Parr, #4749) and heated at 190 °C in an oven under agitation at 20 rpm. After 24 h, the autoclave was cooled with cooling water. The product was then filtered, washed with water and dried at 80 °C overnight. The assynthesized AlPO4-5 was used as seed without any further treatment. Description of the Continuous Apparatus. A schematic layout of the continuous synthesis process is shown in Figure 1. A stainless

Figure 1. Schematic diagram of the continuous synthesis system. steel tube with inner/outer diameters of 2.18/3.18 mm was used as the continuous flow reactor. A high pressure syringe pump (Micro Feeder JP-H, Furue Science, Japan) was used to feed the synthesis gel. To increase the heating rate, a silicon oil bath was chosen as the heating medium. The length of the flow reactor immersed within the silicon oil bath was adjusted according to the required residence time. Pure water was supplied by a HPLC pump (MP311, Lab-Quatec, Japan) to dilute and cool the fluid flowing out of the oil bath. The system pressure was controlled by a back pressure regulator (TESCOM 26-1700, Emerson, US) installed before the outlet of the continuous flow reactor. The pressures at both ends of the flow reactor were measured using two pressure gauges, and the pressure was recorded by a homemade digital recorder. A safety valve and an alarm system were installed for safety considerations. Continuous Flow Synthesis. The tubular reactor was preheated to 190 °C using water as the circulating medium; accordingly, the system pressure was gradually adjusted to 1.4 MPa using a back pressure regulator. After the temperature and pressure conditions became steady, the synthesis gel was fed into the tubular reactor at a flow rate of 4 mL/min. At the outlet of the tubular reactor, cooling water was infused to dilute the synthesized product that flowed out. The flow rate of the cooling water was 10 mL/min. The mixed fluid was further quenched in an ultrasonic cleaner. The products during each theoretical residence time were collected, washed, centrifuged, and dried to get the final AlPO4-5 crystals. Characterization. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV diffractometer using CuKα radiation (λ = 0.15406 nm, 40 KV, 40 mA) at a scan rate of 4o/min between 5° and 35° (2θ). The crystallinity was calculated from the sum of all the peak intensities from 5° to 35° (2θ), and the AlPO4-5 seed was selected as the reference. The morphology of the products was observed by FE-SEM (Hitachi, S-900) with an accelerating voltage of 6 keV. Before the observations, the samples were sputter-coated with Pt. TEM observations were carried out using a JEM 2010 transmission electron microscope (JEOL, Japan) operated at 200 keV. Nitrogen adsorption−desorption measurements of the calcined samples were 2328

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performed on an Autosorb-1 instrument (Quantachrome Instruments) at 77 K. Before the measurements, the samples were pretreated at 400 °C for 6 h under vacuum.

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RESULTS AND DISCUSSIONS The AlPO4-5 crystals with AFI (the 3-letter structure type code assigned by the Structure Commission of the International Zeolite Association) structure was synthesized without any blockage problems because of the short synthesis time that was realized by the synergistic effect between the seed-assisted method and the fast heating provided by the continuous flow process. The XRD patterns for the seed and the product are shown in Figure 2, which demonstrates that the product of the

Figure 3. SEM and TEM images for the seed and the product synthesized using the continuous process. a, SEM image of the seed (inserted: low magnification micrograph to show the particle size of the seed). b, SEM image of the product (inserted: high magnification micrograph to show the ordered structure of the product). c, Low magnification TEM image and corresponding selected area electron diffraction (SAED) pattern of the product. d, High magnification TEM image of the product.

Figure 2. XRD patterns for the product and the seed. (Seed was synthesized in the autoclave for 24 h; the product was synthesized in a reactor with a length of 3.5 m, which corresponds to a residence time of 3.2 min.)

continuous flow reactor was single phase AlPO4-5. The N2 adsorption−desorption data (Table 1) shows that the BET surface area of this product was 416 m2/g, which is close to that of the seed (410 m2/g). However, the morphology of the product (Figure 3-b) was quite different from that of the seed (Figure 3-a). Compared with the seed, the size of the product synthesized in the continuous flow reactor (Figure 3-b) was smaller and more uniform. A low magnification TEM image and the corresponding selected area electron diffraction (SAED) demonstrate that the product had an ordered structure composed of parallel needle-like crystals with a crystalline lattice structure that was clearly observed by high magnification TEM (Figure 3-c and 3-d). The smaller particle size as well as the ordered structure of the product is considered due to the fast heating and the uniform crystallization environment, which was easily achieved in the continuous flow reactor. It is worth noting that the smaller particle size of the AlPO4-5 crystals was beneficial for the stability of the continuous synthesis process as any hydrodynamic failure from the precipitation of the solid product could be minimized.

The residence time plays an important role in the quality of the product in a continuous flow process. In this work, the crystallinity − residence time relationship was determined by changing the length of the flow reactor. As shown in Table 1, nearly full crystallinity was obtained when the residence time was one minute, which is consistent with the results from the batch reactor with tubular shape reported by us.43 This result was achieved thanks to the combined effects of the seedassisted method with the fast heating of the continuous flow reactor heated in an oil bath. A short synthesis time can result in large savings in energy and cost, because typically a much longer time is needed to obtain AlPO4-5 crystals in a conventional autoclave, even when the same amount of seed is used (see Figure S1). Additionally, the minute-level residence time can significantly benefit the hydrodynamic state within the flow reactor because a short residence time avoids the occurrence of precipitation-related problems. Figure 4-a shows the yield obtained during the continuous synthesis with a residence time of 3.2 min, and this had a stable fluctuation around the theoretical yield (100%, on the basis of

Table 1. Experimental Parameters for the Continuous Syntheses and the Properties of the Productsa no.

length of reactor (m)

residence timeb (min)

Crystallinity (%)XRDc

Crystallinity (%)micropored

BET surface area (m2/g)

1 2 3 4 5

0.5 0.8 1.1 3.5 5.5

0.45 0.75 1.00 3.20 5.00

20 50 100 100 100

30 60 100 100 100

175 296 411 416 415

In all experiments the system pressure was 1.4 MPa, the temperature of the oil bath was 190 °C, and the flow rate was 4 mL/min. bTheoretical residence time. cCrystallinity calculated from the XRD results. dCrystallinity calculated from the micropore volume.

a

2329

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Figure 5. Reuse of the product as the seed. a, seed synthesized in a conventional autoclave; b, product synthesized in the continuous flow reactor using “a” as the seed; c, product synthesized in the continuous flow reactor using “b” as the seed. The XRD patterns and SEM micrographs correspond to each other as labeled. (For continuous syntheses, the experimental conditions were fixed as shown by entry no. 4 in Table 1.)

Figure 4. Yield (a) and pressure log (b) for the continuous synthesis. Both the yield and the system pressure fluctuated at the theoretical/ preset values (100% and 1.4 MPa for yield and pressure, respectively), indicating that the continuous synthesis was very stable. (The experimental parameters are shown by entry no. 4 in Table 1.)

reusability. The morphology of the AlPO4-5 product is determined by the heating environment and the presence or absence of the seed.43 When the product was used as a seed, it dissolved into the synthesis gel initially as did the normal seed synthesized using the autoclave. Consequently, AlPO4-5 product with uniform morphology and smaller particle size was obtained in the continuous flow reactor, since it can provide much higher heating rate. The reuse of the product can avoid the long-time requirement for the preparation of AlPO4-5 seed using a conventional autoclave. This will lay the foundation for the development of an integrated continuous synthesis of AlPO4-5 without additional seeding.

one theoretical residence time). If a blockage occurred, it would be detected by a pressure drop in the continuous flow reactor. A pressure log for the residence time of 3.2 min is shown in Figure 4-b, and no pressure difference was observed over the long running time. Both the yield of the solid material and the pressure log demonstrate that a stable continuous synthesis was maintained in this work. The combination of seed addition and fast heating was crucial and indispensable for the continuous synthesis. Without the presence of the seed, the crystallinity was around 20% when the residence time was 5 min, as indicated by the XRD patterns shown in Figure S2. Similarly, when only the seed was used with an oven instead of the oil bath, which means that the heating was much slower, nearly the same level of crystallinity (20%) was achieved for the residence time of 5 min. In addition, a blockage occurred in the continuous reactor when the residence time was further increased in both cases. In the oven-heated and seed-free syntheses, nearly no AFI structure was observed for the product synthesized over 5 min. These experimental results demonstrate that fast crystallization is important to achieve the continuous synthesis of AlPO4-5, and this requirement was easily achieved through a combination of seed addition and oil-bath heating in the continuous synthesis. The reusability of the product as a seed is another important concern for the seed-assisted synthesis. The reuse of the product synthesized in the continuous flow reactor as seed is shown in Figure 5. Using seed synthesized in a conventional autoclave (a in Figure 5), the AlPO4-5 product with different morphology (b in Figure 5) was synthesized in 3.2 min using the continuous flow reactor. Interestingly, when using that product (b in Figure 5) as seed, AlPO4-5 crystals with very similar morphology (c in Figure 5) were obtained after 3.2 min synthesis in the continuous flow reactor, indicating a good

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CONCLUSION In summary, we demonstrate a method to synthesize AlPO4-5 continuously using a flow reactor. The key to a successful continuous synthesis is the fast crystallization of AlPO4-5, which was realized by a synergy between the seed-assisted method and the fast heating provided by a flow reactor. This method enables the continuous synthesis of AlPO4-5 in one minute. Because of the fast heating and uniform environment for crystallization within the continuous flow reactor, the AlPO4-5 crystals synthesized using this continuous process showed well controlled morphology like a smaller particle size and an ordered structure. In addition, reuse of the product from the continuous synthesis as a seed is also demonstrated. When the product in the continuous flow reactor was used as a seed, the AlPO4-5 crystals with very similar morphology were obtained, indicating a good reusability. The reuse of the product in the continuous flow reactor can avoid the long-time reaction required for the preparation of seed using a conventional autoclave. This easily designed, efficient route can result in significant cost and energy savings, and it thus has huge potential for the industrial-level production of AlPO4-5 crystals. 2330

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Mitsubishi Chemical Corporation. The authors acknowledge Nano-Engineering Research Center at the University of Tokyo and the staffs there for their technical assistance with TEM observations. Z.L. is grateful to the Ministry of Education, Culture, Sports, Science and Technology, Japan, for a MonbuKagakusho Scholarship.

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