Continuously Adjustable, Molecular-Sieving “Gate” on 5A Zeolite for Distinguishing Small Organic Molecules by Size

Abstract Zeolites/molecular sieves with uniform, molecular-sized pores are important for many adsorption-based separation processes. Pore size gaps, however, exist in the current zeolite family. This leads to a great challenge of separating molecules with size differences at ~0.01 nm level. Here, we report a novel concept, pore misalignment, to form a continuously adjustable, molecular-sieving “gate” at the 5A zeolite pore entrance without sacrificing the internal capacity. Misalignment of the micropores of the alumina coating with the 5A zeolite pores was related with and facilely adjusted by the coating thickness. For the first time, organic molecules with sub-0.01 nm size differences were effectively distinguished via appropriate misalignment. This novel concept may have great potential to fill the pore size gaps of the zeolite family and realize size-selective adsorption separation. Introduction Zeolites/molecular sieves are one of the most promising adsorbents that may help realize true molecular-sieving separation, because of their uniform, molecular-sized pores (0.3~1.3 nm) and high chemical, thermal, and mechanical stabilities1. Pore size gaps, however, exist in the current zeolite family, which leads to the difficulty in separating molecules with small size/shape differences, especially at the 0.01 nm level. Pore size of zeolites/molecular sieves can be adjusted by several techniques, including ion exchange2, framework control3,4,5,6,7, and zeolite external surface modification8,9,10. Ion exchanges have been used as an effective way of adjusting the pore sizes of LTA (Linde Type A) zeolites2. The framework of some zeolites, such as zeolite rho, may deform substantially upon adsorption of some molecules3. A molecular sieve, ETS-4, has been shown to contract gradually through dehydration at elevated temperatures so that its effective pore size can be adjusted at approximately 0.01 nm step4. Recently, a novel method, called ADOR (assembly-disassembly-organization-reassembly), was applied to chemically selectively remove germanium from germanosilicate zeolite UTL in a top-down strategy to prepare a series of IPC zeolites with continuously tuneable surface area and micropore volume5,6,7. Pore opening size of mordenite zeolite was reduced at 0.1 nm level by chemical vapor deposition (CVD) of silica coatings on the external surface of zeolites8. The CVD modified ZSM-5 zeolite showed increased shape selectivity of xylene isomers, and HZSM-5 zeolite showed enhanced para-selectivity in the methylation of toluene9,10. But, the pore opening reduction mechanism for CVD modified zeolite was not clear8,9,10. Despite a large selection pool of zeolites/molecular sieves and available techniques to adjust their pore sizes, not all desired pore sizes can be obtained for target separations. This is especially the case for separating molecules that are very close in size. In addition, pore modification and structure changes were always realized by sacrificing adsorption capacity or internal cavity4,11,12,13. Here, we report, for the first time, a bottom-up approach for precise pore mouth size adjustment for 5A zeolite from 0.5 to 0.46 nm without sacrificing internal cavity by pore misalignment; organic molecules with size differences as small as 0.01 nm were effectively distinguished by appropriate misalignment. We used molecular layer deposition (MLD) to form a conformal hybrid aluminum alkoxide (alucone) coating on the 5A zeolite surface (Supplementary Materials). The hybrid alucone coating was subsequently calcined in air to remove the organic compound to generate a porous alumina coating14. MLD provides exquisite control of the coating thickness at the sub-nanometer level and thus achieves conformal coating on substrates even with high-aspect-ratio features15,16,17,18,19. Figure 1a shows a transmission electron microscopy (TEM) image of 5A zeolite with 60 cycles of MLD; after calcination an approximately 20 nm thick coating was deposited on the 5A zeolite surface, corresponding to a nominal porous alumina deposition rate of 0.33 nm/cycle. The weight percentage of 60 cycles of MLD coating on 5A zeolite is estimated to be < 2% by applying the coating density19 and thickness, 5A zeolite solid density20, and external surface area of 5A zeolite crystals, estimated from the average particle size and shape (Figure S1 in Supplementary Materials). X-ray photoelectron (XP) spectra (Fig. 1b) shows after 120 cycles of MLD, silicon (2p binding energy at 102.3 eV) in 5A zeolite can hardly be seen due to the shorter excited electron mean free path than MLD coating thickness; the MLD coatings are composed of alumina (Table S1 and Figure S2 in Supplementary Materials). X-ray diffraction (XRD) confirmed LTA zeolite structure before and after MLD, and MLD coatings did not change zeolite structure (Figure S3 in Supplementary Materials). Brunauer–Emmett–Teller (BET) measurement and N2 sorption analysis show that 5A zeolites with and without MLD coatings had almost identical surface area (343.5 ± 8.3 m2/g) (Fig. 1c), and identical micropore volume (0.20 cm3/g) (Fig. 1c); argon sorption analysis further confirms there is no change in micropore volume after MLD coating deposition (Figure S4e). This suggests coatings were only on the external surface of 5A zeolite and the internal cavity of the zeolite was maintained. We also measured vapor adsorption isotherm of the MLD precursor, trimethyl aluminum (TMA), and found negligible adsorbed amounts (Figure S6 in Supplementary Materials). Therefore, MLD coatings are expected to be only on the external surface of 5A zeolite, instead of inside the zeolite pores. To further confirm the ultrathin MLD coating is on the 5A zeolite surface and has negligible effect on the internal cavity of 5A zeolite, we also measured CH4 adsorption isotherms on 5A zeolite and 5A zeolite with different cycles of MLD coatings (Fig. 1d); almost identical CH4 adsorbed amounts were found, indicating ultrathin MLD coating did not enter zeolite internal pores. These results demonstrate that ultrathin, porous MLD coatings were deposited only on the external surface of 5A zeolite. Figure 1: Characterization of 5A zeolite and 5A zeolite with molecular layer deposition (MLD) coatings. (a) Transmission electron microscopy (TEM) image of 5A-Zeolite-60. (b) X-ray photoelectron spectra (XPS) of Si 2P of 5A zeolite and 5A zeolite with different cycles of MLD coating on 5A zeolite. (c) BET surface area of 5A zeolite and 5A zeolite with different cycles of MLD coatings (•), and micropore volume of 5A zeolite and 5A zeolite with different cycles of MLD coatings (). Error bar is given automatically by Micromeritcs ASAP 2020 unit. (d) CH4 adsorption isotherms at 20 °C on 5A zeolite (), 5A-Zeolite-30 ( ○), and 5A-Zeolite-60 (∆). Solid black line is a fit of adsorption points of CH4 on 5A zeolite by the Langmuir model. All MLD coatings have been calcined in air following the procedure d (...truncated)