Subscriber access provided by United Arab Emirates University | Libraries Deanship
Article
Adsorption and Diffusion Phenomena in Crystal Size Engineered ZIF-8 MOF Shunsuke Tanaka, Kosuke Fujita, Yoshikazu Miyake, Manabu Miyamoto , Yasuhisa Hasegawa, Takashi Makino, Stijin Van Der Perre, Julien Cousin Saint Remi, Tom R.C. Van Assche, Gino V. Baron, and Joeri F.M. Denayer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09520 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 4, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 39
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Adsorption and Diffusion Phenomena in Crystal Size Engineered ZIF-8 MOF Shunsuke Tanaka,*,†, ‡ Kosuke Fujita,† Yoshikazu Miyake,†, ‡ Manabu Miyamoto,§ Yasuhisa Hasegawa,‖ Takashi Makino,‖ Stijn van der Perre,⊥ Julien Cousin Saint Remi,⊥ Tom Van Assche,⊥ Gino V. Baron⊥ and Joeri F. M. Denayer*,⊥ †
Department of Chemical, Energy and Environmental Engineering, Kansai University, 3-3-35
Yamate-cho, Suita-shi, Osaka 564-8680 Japan. ‡
Organization for Research and Development of Innovative Science and Technology
(ORDIST), Kansai University. §
Department of Chemistry and Biomolecular Science, Gifu University, 1-1 Yanagido, Gifu
501-1193 Japan.
‖
Research Center for Compact Chemical System, National Institute of Advanced Industrial
Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551 Japan. ⊥
Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050
Brussel, Belgium.
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
KEYWORDS zeolitic imidazolate frameworks, structural transition, crystal size, surface barrier, intracrystalline diffusion
ABSTRACT
ZIF-8 is a flexible zeolitic imidazole-based metal-organic framework whose narrow pore apertures swing open by reorientation of imidazolate linkers and expand when probed with guest molecules. This work reports on the crystal size dependency of both structural transitions induced by N2 and Ar adsorption and dynamic adsorption behavior of n-butanol using well-engineered ZIF-8 crystals with identical surface area and micropore volume. It is found that the crystal downsizing of ZIF-8 regulates the structural flexibility in equilibrium adsorption and desorption of N2 and Ar. Adsorption kinetics of n-butanol in ZIF-8 are strongly affected by the crystal size, however, not according to a classical intracrystalline diffusion mechanism. Our results suggest that structural transitions and transport properties are dominated by crystal surface effects. Crystal downsizing increases the importance of such surface barriers.
2 ACS Paragon Plus Environment
Page 2 of 39
Page 3 of 39
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
INTRODUCTION Metal-organic frameworks (MOFs)1–5 offer many interesting opportunities in adsorption technology, with unprecedented capacities and chemical and structural tunability. Systematic variations in the metal and organic components allow the rational design of framework structures, resulting in the development of framework flexibility property. Among the many MOFs, zeolitic imidazolate framework-8 (ZIF-8) has attracted conspicuous attention for molecular sieving applications due to its facile synthesis coupled with its remarkable chemical and thermal stabilities as compared to other classes of MOFs.6–16 ZIF-8 crystallizes into the zeolite sodalite (SOD) topology, in which each bivalent Zn cation joins four 2-methylimidazole (mIm), generating a resistant structure with large cages (diameter of ~11.6 Å) interconnected via narrow 6-ring windows (~3.4 Å). Such framework structure gives ZIF-8 particularly interesting “gate-opening” functionality: it is interesting to note that ZIF-8 can adsorb molecules with kinetic diameter larger than its window size, like N2, CH4, C3H8, butanol and so on.17–21 This unexpected adsorption behavior has been speculated to be due to the flexible apertures that swing open by reorientation of imidazolate linkers enforced by guest adsorption.22–27 Another very interesting feature of the ZIF-8 MOF is that its crystal size can be varied over a wide range of values (typically from approximately 50 nm to 100 µm or more), something that is very hard to achieve with classical porous aluminosilicates.7,11–14 Such crystal size engineering might be exploited to tune the adsorptive and diffusion properties of the material for molecular separation processes. Although it is expected that smaller crystal sizes lead to faster uptakes, rendering them more efficient, only very few systematic studies explored the effect of crystal downsizing on adsorption equilibria and kinetics, and, moreover, on the adsorption-induced structural transition and framework flexibility of MOFs.27,28 Recently, Zhang et al. demonstrated a pronounced effect of crystal size in the low temperature 3 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
adsorption of N2 gas on ZIF-8 and proposed a core-shell model to explain the observations.27 The underlying mechanism remains, however, unclear. Therefore, in this work, we aimed at carefully preparing ZIF-8 samples of different crystal size with nearly identical structural and chemical properties, to allow an accurate study of the effect of crystal size on adsorption, desorption and diffusion of guest molecules into the ZIF-8 MOF. N2 and Ar isotherms and n-butanol uptake curves are measured on ZIF-8 crystals with sizes between 88 and 0.060 µm.
EXPERIMENTAL SECTION Material synthesis and characterization. All ZIF-8 samples, apart from the sample with the largest crystal size, were synthesized in an aqueous system at room temperature.10–12 Detailed experimental procedures of this environmentally friendly synthesis process and crystal size estimation of all ZIF-8 samples are described in the Supporting Information. XPS spectra were recorded on a JPS-9000MX spectrometer (JEOL) using Mg Kα radiation (10 kV, 10 mA) as the energy source. The pressure in the instrumental chamber was less than 1 × 10–5 Pa. No radiation damage was observed during the data collection time. Binding energies were calibrated on Pt 5p3/2 (51.7 eV) and C1s (284.2 eV). Raman spectra were recorded on a NRS3100 spectrophotometer (JASCO) using a laser diode, of which the excitation wavelength was 531.96 nm, under atmospheric conditions. The samples were heated at 150 °C for about 6 h to remove any residual volatile compounds before the analyses. The laser was irradiated to the sample from the object lens, and the back scatter was detected with the same lens. The spectral resolution was about 1 cm–1 and Raman peaks were calibrated with the Ne emission lines in the air. N2 and Ar adsorption/desorption. Adsorption isotherms of N2 in ZIF-8 were measured at 77 K using a BELSORP-max (Bel Japan). The samples were degassed at 200 °C under 4 ACS Paragon Plus Environment
Page 4 of 39
Page 5 of 39
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
vacuum. Brunauer–Emmett–Teller (BET) model surface area, SBET, and Langmuir model surface area, SL, were calculated from the nitrogen adsorption branches. A part of the nitrogen adsorption isotherm in the P/P0 range 0.01–0.1 was fitted to the BET equation to estimate the SBET and the SL calculation was performed using all data points. The total pore volume Vtotal, was calculated from the amount of nitrogen adsorbed at P/P0 = 0.99. The micropore volume Vmicro, was calculated from the αs-plot method. The method was based on the comparison of the adsorption isotherm for samples with that of a standard nonporous carbon. Adsorption isotherms of Ar in ZIF-8 were measured at 79, 84, 87, and 90 K using an AUTOSORB-1 (Quantachrome) in cooperation with a cryostat (Oxford Instruments). Vapor phase adsorption (Equilibrium adsorption measurements). Adsorption isotherms of n-butanol in ZIF-8 were measured by the dynamic gravimetric method on an SGA-100H microbalance system (VTI Corporation, USA). A reservoir filled with n-butanol is temperature controlled through Peltier elements. N2 bubbling through the reservoir entrains the n-butanol vapor. This vapor stream continuously flows over the sample positioned in a sample holder connected to the microbalance. During the measurements, the total pressure is constant near atmospheric pressure and the adsorbate partial pressure is controlled by regulating the saturator temperature and/or the dilution rate. Low vapor pressures were obtained by diluting the saturator flow at the lowest saturator temperature with N2 flow. About 10 mg of the sample was placed in a stainless steel sample pan and positioned in the microbalance system. After activation by heating to 200 °C during 2h and at a heating rate of 2 °C/min under N2 flow, adsorption isotherms of n-butanol were determined at 323 K by weighing the adsorbate uptake at different partial pressures of the adsorbate. The equilibrium criterion was typically set at a mass change
