Fast synthesis of submicron all-silica CHA zeolite particles using a seeding method

Abstract

Ultra-fast synthesis of all-silica CHA zeolite was achieved using a seeding method. The CHA particles size was reduced from 10 μm to 500 nm.

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Zeolite membranes, which are thermally and chemically stable, have great potential to separate gas or liquid mixtures through the difference in adsorption and diffusion.^1–5 Among various types of zeolite membranes, small pore zeolite membranes (zeolite with 8-MR) are especially interesting, because their pore sizes are perfect for many important industrial separation processes. For example, the pore aperture of zeolite SAPO-34 (CHA structure with 8-MR pores) is ∼0.38 nm, which allows the selective diffusion of CO₂ (kinetic diameter 0.33 nm) to the inner pore structure, while excluding the larger CH₄ (0.38 nm) molecule.⁶ Among 8-MR zeolites, SAPO-34 (CHA type),^6–14 CHA^15–17 and DDR membranes^18–22 have been extensively studied for CO₂ separation. Great CO₂–CH₄ separation performance was realized on SAPO-34 zeolite membrane; however, the hydrophilic nature of SAPO-34 zeolite makes it difficult for the application of SAPO-34 membrane for natural gas separation. The adsorption of water in the hydrophilic SAPO-34 zeolite is much stronger than CO₂, which will block the pores of SAPO-34 crystals, and lead to flux reduction and decrease of selectivity. For real industrial application, the membrane will require frequent regeneration (dehydration), which increases system cost and maintenance. Thus, it is desirable to have a hydrophobic zeolite membrane, which may minimize the unpleasant adsorption of water. The hydrophobicity of zeolite is related to its Si/Al ratio and all-silica zeolite is the most hydrophobic. All-silica zeolite also has better thermal and chemical stability compared to its aluminumsilicate count part, which can be used in harsh environment.

All-silica CHA zeolite membrane has high potential for CO₂–CH₄ separation, since it shares the same structure as SAPO-34. However, the synthesis of all-silica membrane is very challenging. Zeolite membrane usually is prepared by secondary growth method, which requires the deposition of a uniform thin seed layer on the porous support. The seed crystals should have narrow size distribution in the submicron range. Despite many reports on the synthesis of low-silica CHA zeolite,²³ the synthesis of sub-micron all-silica CHA remains unsolved. Traditionally, all-silica CHA zeolite particles prepared in fluoride media with low H₂O/SiO₂ ratio are 5–10 μm in diameter.²⁴ Eilertsen et al. studied the synthesis of high silica CHA zeolite in fluoride media and the particle size was 3–4.5 μm for all-silica CHA.²⁵ Bohström et al. prepared sub-micron high-silica CHA (SSZ-13) in non-fluoride media with high H₂O/SiO₂ ratio. The particle size can be controlled from 190 nm to several microns, however, the Si/Al ratio is only 50.²⁶ Kim et al. systematically studied the synthesis of all-silica CHA in fluoride media. They found that sub-micron plate-like Si-CHA particles were obtained along with micron size cubic CHA particles and the low H₂O/SiO₂ ratio was the key to the co-synthesis of plate-like CHA particles.²⁷ Miyamoto found the H₂O/SiO₂ ratio significantly influenced the specific surface and pore volume of CHA, and they cannot make submicron all-silica CHA crystals.²⁸

Seeding is a common method for shortening synthesis time and reducing crystal size.¹ Surprisingly, no one ever tried seeding method for all-silica CHA synthesis, maybe the solid-like precursor make seeding difficult. However, Xiao et al.'s pioneering work about solvent-free and template-free synthesis of ZSM-5 and beta zeolites involved the seeding in solid-like precursor.²⁹

In this communication, we successfully prepared sub-micron all-silica CHA zeolite particles by a simple seeding method. The addition of all-silica CHA seeds in the mother gel not only shortened the synthesis time, but also reduced the particle size significantly.

All-silica CHA zeolite was prepared with N,N,N-trimethyl-1-adamantanammonium hydroxide as organic template in traditional fluoride media with low H₂O/SiO₂ ratio (1SiO₂ : 0.5TMAdaOH : 0.5HF : 3H₂O, at 433 K for 48 h). Fig. 1 and 2 shows the XRD patterns and SEM images of the obtained products. From the XRD patterns shown in Fig. 1, the gel was amorphous after 3 h, and CHA peaks began to appear after 6 h. After 12 h, well-developed CHA XRD pattern was obtained and no other phase was observed. The SEM images (Fig. 2) are quite consistent with the XRD patterns. Well-developed cubic crystals (∼8 μm) were obtained after 12 h. The obtained all-silica CHA particles were ball-milled into smaller particles. The XRD pattern in Fig. 3 exhibited major peaks of CHA zeolite with significantly weakened intensity, which indicated that the structure integrity of CHA zeolite was maintained. After ball-milling, the SEM images in Fig. 4 showed irregular shaped particles with diameter from 100 nm to microns.

Fig. 1 XRD patterns of all-silica CHA prepared without seeding.

Fig. 2 SEM images all-silica CHA samples prepared without seeding.

Fig. 3 XRD patterns of ball-milled all-silica CHA zeolite.

Fig. 4 SEM images of ball-milled all-silica CHA zeolite.

Then, the CHA seeds were mixed with the precursor gel to see the effect of seeding (20 mg seeds/1.5 g SiO₂). Usually, the addition of seeds leads to smaller crystals and short synthesis time because seeds provide more nuclei and shorten the induction time. Fig. 5 and 6 show the XRD patterns and SEM images of samples prepared with various synthesis times (with seeding). As shown in Fig. 5, the product was amorphous after 1.5 h and CHA peaks appeared after 2 h. After 3 h, the CHA peaks were well developed. This is consistent with the SEM images shown in Fig. 6. At 2 h, cubic CHA crystals were observed along with plenty amorphous material. Well-developed cubic CHA crystals were obtained after 3 h synthesis. Although not uniform, the crystals were generally in the sub-micron range. The non-uniformity of crystal size distribution might be the result of solid-like precursor, which makes seeds mixing difficult. Compared with CHA crystals prepared without seeding, the crystal size was significantly reduced from 8 μm to 500 nm and synthesis time was shortened from 12 h to 3 h, which indicated that seeding is an effective method in reducing crystal size and induction period.

Fig. 5 XRD patterns of all-silica CHA prepared by seeding method (effect of synthesis time).

Fig. 6 SEM images of all-silica CHA zeolites prepared with seeding (effect of synthesis time).

Fig. 7 and 8 show the effect of seed contents on the XRD patterns and crystal size. The seed concentration was increased in order to reduce the crystal size further, since more nuclei lead to smaller crystals. As shown in Fig. 7, pure CHA with good crystallinity was obtained for all cases. Based on Fig. 8, there is no clear connection between seed concentration and crystal size. The obtained crystals were in the submicron range, but more seeds didn't lead to smaller crystals. This might be the result of non-uniform distribution of seeds in the precursor gel. The solid-like precursor makes the mixing difficult and also the diffusion of secondary nuclei.

Fig. 7 XRD patterns of all-silica CHA by seeding method (effect of seed content).

Fig. 8 SEM of CHA by HT at 180 °C with different seed contents (20 mg, 40 mg, 80 mg and 160 mg).

Fig. S1† shows the nitrogen isotherm of obtained all-silica CHA samples prepared with and without seeding. Both sample show type I isotherm and have similar BET surface area (580 and 574 m² g⁻¹ for seeding). The elemental analysis (conducted on a Inductively Coupled Plasma Optical Emission Spectrometer) shows no presence of Al in both samples, which confirmed their all-silica nature. EDAX (energy dispersive X-ray analysis) analysis exhibited 0.54 wt% fluorine content of the CHA sample (20 mg seed). Previous publication indicated that the fluoride ions are incorporated into the small double six-ring cages in the CHA framework.³⁰

Conclusions

A simple seeding method was used for the synthesis of submicron all-silica CHA crystals. The addition of CHA seeds not only shortened the synthesis time, but also significantly reduced the crystal size from 8 μm to 500 nm.

Acknowledgements

The authors acknowledge financial support from National 863 Project (no. 2012AA050104), SARI (no. Y426473231), Advanced Coal Project (no. XDA07040400).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02561c

‡ Jianming Zhang and Xiangyan Liu contributed equally to this manuscript.

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