Hydrothermal synthesis of alkali-free chabazite zeolites
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Hydrothermal synthesis of alkali‑free chabazite zeolites Long V. Dang1 · Son T. Le1 · Raul F. Lobo2 · Trong D. Pham3
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Alkali-free siliceous CHA zeolites with a wide Si/Al ratio ranging from 11 to infinity have been synthesized in hydroxide and fluoride media using either N,N,N-Trimethyl-1-adamantammonium (TMAda), N,N,N-Dimethylethylcyclohexyl ammonium (DMECHA) cations, or a mixture thereof. TMAda, in combination with other low-cost organic templates, allowed the formation of pure silica CHA without any impurities in the synthesis gel with an H2O/SiO2 ratio of 3. Single-phase crystalline pure silica CHA was produced by DMECHA within a very narrow range of H2O/SiO2 ratio, and small deviations in the water content from this range led to the formation of a slightly less dense zeolite beta phase. Alkali-free siliceous CHA zeolites possessing a high crystallinity and a variety of particle sizes were successfully prepared for the first time using a low-cost DMECHA template, particularly by a direct interzeolite transformation of FAU zeolites having the same secondary building unit of double-6-rings (D6Rs). This makes DMECHA an attractive organic structure-directing agent (OSDA) for wide applications of CHA zeolites as catalysts and adsorbents. Keywords CHA · Zeolite · TMAda · DMECHA · Fluoride · Hydroxide
1 Introduction Eight-membered ring (8MR) zeolites, including silicoaluminophosphate and aluminosilicate forms such as AEI, AFX, CHA-type structures, have recently received considerable attention as catalysts for the methanol-to-olefins reaction (MTO) [1–5] and selective catalytic reduction (SCR) of NOx with NH3 [6–12]. Small pore zeolites have shown high SCR activity, improved hydrothermal stability, and better hydrocarbon tolerance compared to medium and large pore zeolite catalysts [13–15]. CHA-type zeolites have also reported to have high catalytic activities in the conversion of methane to methanol [16], in the production of methyl formate from methanol with H 2O2 as the oxidant [17], and be potential adsorbents in gas separations [18–22]. CHA-type * Trong D. Pham [email protected] 1
Faculty of Chemistry, Hanoi University of Science-Vietnam National University, Hanoi, Vietnam
2
Department of Chemical & Biomolecular Engineering, University of Delaware, Newark, Delaware, USA
3
Present Address: Corporate Strategic Research, ExxonMobil Research & Engineering Company Inc., 1545 Route 22 East, Annandale, NJ 08801, USA
zeolite belongs to the ABC-6 family, which is composed of 6MRs stacked in an AABBCC sequence (Fig. 1). Chabazite belongs to R3m (trigonal) space group [23], consists of three double 6-membered rings (D6Rs), three large ellipsoidal cavities (7 Å × 10 Å), and nine 8-membered ring windows (free aperture ∼ 3.8 Å) in one unit cell of 36 T-atoms. The synthesis of chabazites zeolites has been studied extensively due to their numerous industrial applications. CHA with a Si/Al ratio of less than three can be
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