Article pubs.acs.org/JPCC
Gradual Disordering of LTA Zeolite for Continuous Tuning of the Molecular Sieving Effect Hyeonbin Kim,† Hae Sung Cho,‡ Chaehoon Kim,† and Minkee Choi*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ Graduate School of Energy, Environment, Water and Sustainability, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: The “molecular sieving effect” of zeolites has enabled size-selective adsorption and catalysis. Although a large variety of zeolites have been developed thus far, it is still challenging to find zeolites that can separate molecules such as CO2, N2, CH4, and small organics that have kinetic diameters all closely located in the range of 0.3 to 0.4 nm. Here we demonstrate that controlled collapse or atomic disordering of NaA zeolite can systematically narrow the effective pore size below 0.4 nm and thus “tune” the molecular sieving effect. As the zeolite is gradually disordered, the adsorption amounts for all gas molecules decrease; however, larger molecules show a much faster decrease than that of the smaller ones. Consequently, the adsorption selectivities could be remarkably enhanced for various gas pairs.
1. INTRODUCTION Zeolites and related crystalline microporous oxides have played important roles in applications including ion-exchange, adsorption, and catalysis.1−3 The great industrial success of zeolites can be attributed to the various aspects of these materials such as a uniform microporous structure, high thermochemical stability, and ion-exchange capability as well as the economic feasibility of large-scale production. In particular, the “molecular sieving effect” of zeolites, which is the size-selective hosting of guest molecules with sizes smaller than the diameter of the micropore aperture, has enabled sizeselective adsorption and catalytic processes.4−9 Commercially important examples of size-selective molecular separation include the dehydration of gases and alcohols using KA (3A molecular sieve),6 separation of linear hydrocarbons from branched and cyclic hydrocarbons using CaA (5A molecular sieve),3 and xylene isomers separation using MFI zeolite.7,8 In principle, any mixtures of molecules can be separated by molecular sieving as long as the appropriate adsorbents with a micropore diameter between the sizes of the target molecules are available. Ordinary zeolites have micropore sizes ranging from 0.3 to 0.8 nm,10 which are generally determined by the number of T atoms (Si or Al) in the pore aperture. The micropore sizes of zeolites can be further tuned to some extent by the substitution of extra-framework cations.10−12 In recent years, assembly-disassembly-organization-reassembly (ADOR) has been proposed as a novel strategy to control the pore size of a zeolite.13,14 In ADOR process, a parent zeolite including a hydrolytically sensitive dopant element (e.g., Ge) is disassembled first; then, the resultant building blocks are reassembled to form new zeolite structures with different channel systems. The newly formed zeolites can have the pore © XXXX American Chemical Society
apertures having 8−12 T atoms, which means that the pore size is tailorable in the pore size range of > ∼0.4 nm. It is noteworthy, however, that industrially important gas separations generally involve molecules such as CO2, N2, CH4, and small organics that have kinetic diameters all closely located in the range of 0.3 to 0.4 nm. Although numerous zeolite structures have been discovered so far, finding zeolites with relevant pore diameters in that narrow size range is still extremely challenging. One notable achievement is the discovery of a titanosilicate, ETS-4, which is composed of a mixed octahedral/tetrahedral framework.9 The unique feature of ETS-4 is that the material framework can be systematically contracted through dehydration at elevated temperatures to tune the effective micropore size.3,9 Here we demonstrate another example of active control or tuning of the effective zeolite micropore size and thus the molecular sieving effect. The strategy is based on a very simple idea that the gradual framework collapse or atomic disordering of an NaA zeolite can systematically reduce the micropore aperture size below 0.4 nm, which is the original micropore aperture size of the zeolite (Figure 1). In zeolite chemistry, it has been known that full decationization (i.e., replacing extraframework Na+ cations with H+) of highly aluminous zeolites (Si/Al ≈ 1.0) results in a complete loss of zeolite crystallinity.10 For instance, zeolite A (LTA, Si/Al = 1.0)10,15,16 and zeolite X (FAU, Si/Al = 1.2)10,17 fully ion-exchanged with NH4+ start losing their crystallinity upon the decomposition of NH3 below 293 K due to extensive framework dealumination. Although Received: January 23, 2017 Revised: March 5, 2017 Published: March 13, 2017 A
DOI: 10.1021/acs.jpcc.7b00724 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
starting atomic coordinates for the NaA zeolite frameworks were adopted from the International Zeolite Association (IZA) database,21 and the starting positions for the Na atoms were obtained from a difference Fourier analysis.22 The Na atom positions were kept to follow the space group of the NaA to reduce the variables for the refinement. Because of the complexity of the structural model, constraints on the Si−O distances were applied. Their weight was totally removed in the final cycles. An isotropic atomic displacement parameter was imposed for individual atoms of Si, Al, O, and Na. The calculated atomic coordinates of all samples were visualized using a VESTA program.23 Elemental analyses were carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using an iCAP-6000 (Thermo Scientific). Scanning electron micrographs (SEM) were taken with a Magellan400 (FEI) at a low landing energy (2 kV) without a crushing or metal coating. TEM and energy-dispersive X-ray images were collected by JEM-2100F (JEOL) at a 200 kV acceleration voltage after mounting the samples on a copper grid (300 square mesh) using ethanol dispersion. Solid-state NMR spectra were recorded on a Bruker Avance 400 spectrometer equipped with a wide bore 9.4 T magnet, operating at a Larmor frequency, υ0, of 104.3 MHz for 27Al and 79.5 MHz for 29Si. 27 Al MAS NMR spectra were recorded with rf-field strengths of 50 kHz and a recycle delay of 1 s. 29Si MAS NMR spectra were recorded with high power 1H decoupling, rf-field strengths of 23.8 kHz, and a recycle delay of 25 s. Chemical shifts were reported in ppm relative to an external standard of 1 M aqueous Al(NO3)3 solution for 27Al and 4,4-dimethyl-4silapentane-1-sulfonic acid for 29Si. Gas Adsorption Experiment. Adsorption isotherms for various gas molecules were measured with an ASAP2050 (Micromeritics) volumetric adsorption analyzer. Before the adsorption measurements, ∼1 g of the zeolite samples was degassed at 673 K for 12 h. All adsorption experiments were carried out at 303 K up to 8 bar (criteria for equilibrium that the pressure change is
