Article pubs.acs.org/JPCC
Adsorption of Propane and Propylene on CuBTC Metal−Organic Framework: Combined Theoretical and Experimental Investigation Miroslav Rubeš,‡ Andrew D. Wiersum,§ Philip L. Llewellyn,§ Lukás ̌ Grajciar,‡ Ota Bludský,† and Petr Nachtigall*,‡ ‡
Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, CZ-128 40, Prague 2, Czech Republic § Laboratoire MADIREL (UMR 7246), Aix-Marseille Univ. & CNRS, Centre de St Jérôme, 13397 Marseille Cedex 20, France † Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, Prague 6, 162 10, Czech Republic S Supporting Information *
ABSTRACT: A combined experimental and theoretical investigation of propane and propylene adsorption in the metal− organic framework CuBTC is presented. The dependence of adsorption enthalpies on the adsorbed amount was determined by microcalorimetry up to 8 mmol g−1 coverage (roughly C3Hn/Cu2+ ratio of 1.5). Trends observed experimentally were interpreted on the basis of accurate calculations carried out at the hybrid DFT−ab initio level. Three types of adsorption sites were identified; however, qualitatively different results were obtained for propane and propylene. Propane preferentially adsorbs at the cage center sites (−ΔH° = 43 kJ mol−1), followed by adsorption at the cage window sites (31 kJ mol−1), while the interaction with the coordinatively unsaturated sites (CUS) is relatively weak (24 kJ mol−1). On the contrary, propylene preferentially interacts with the CUS (56 kJ mol−1), while the adsorption at the cage center and cage window sites was found to be only 45 and 34 kJ mol−1, respectively. Due to the topology of CuBTC, lateral interactions are significantly more important among the adsorbates located at the cage center and cage window sites (populated in the case of propane) than among adsorbates at the CUS and cage center sites (populated in the case of propylene). Therefore, adsorption energies obtained for coverages above 6 mmol g−1 of adsorbed amount were larger for propane than for propylene. Consequently, the presence of small cages makes the CuBTC MOF less suitable for propane/propylene separation than MOFs having the Cu2+ CUS but without small cages (e.g., CPO-27).
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INTRODUCTION Metal−organic frameworks (MOFs) are widely investigated due to their potential applications in various areas, including separation and purification, gas storage, catalysis, or drug delivery.1−4 There is a relatively large group of MOFs containing coordinatively unsaturated sites (CUS), e.g., MIL100, MIL-101, CPO-27, or HKUST-1, exhibiting unique properties in adsorption and even in catalysis.5−10 The CuBTC MOF (often denoted HKUST-1)11 is one of the earliest reported stable MOFs, and it has been often investigated experimentally and computationally. CuBTC is now considered as a reference MOF containing CUS. Adsorption of many gases in CuBTC, including small molecules such as H2, H2O, CO2, or CH4, has been investigated both experimentally and computationally, focusing on the adsorption on the CUS.12−15 It has been recognized that the agreement between experimental and computational results deteriorates with the increasing role of the CUS in adsorption. The CuBTC MOF has been also considered for propane/ propylene separation.1,16−20 The reported experimental isosteric heats for propane and propylene (mostly obtained by means of the simple Clausius−Clapeyron relation) show © XXXX American Chemical Society
significant differences; they are in the range of 27−38 and 33−51 kJ mol−1 for propane and propylene, respectively.17,18,20 The presence of the Cu2+ CUS makes the theoretical description of adsorption in CuBTC rather challenging. It has been shown that the accuracy of commonly employed force fields is not sufficient for methane adsorption in CuBTC,21 and the problem of force fields is mostly due to their inaccuracy in the description of the interaction with the CUS. In the case of propane/propylene adsorption in CuBTC, the situation is even more complicated due to the presence of the propylene double bond that can form a partial dative bond with the CUS. The preferential adsorption for propylene and isobutene has been explained by formation of partially dative bond between CUS and π-molecular orbitals.17,19,22 However, the accuracy of a force field (commonly Lennard-Jones) to describe correctly this partial dative bond is questionable even after the parameter readjustment. Received: February 14, 2013 Revised: April 29, 2013
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dx.doi.org/10.1021/jp401600v | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
The recently developed DFT/CC method23 has been successfully used to accurately describe adsorption in MOFs containing CUS; accurate adsorption enthalpies for H2O, CO, CO2, and CH4 in the CuBTC MOF were reported.15,21,24,25 It has been shown that the addition of a dispersion component missing in standard exchange-correlation functionals fixes only part of the problem in the density functional theory (DFT) description of the adsorbate−CUS interaction. It is thus the goal of the present study to describe the adsorption of propane and propylene in CuBTC MOF with high accuracy; the DFT/ CC correction scheme is adopted for that purpose. Adsorption enthalpies calculated for various adsorption sites and various adsorbate loadings are compared with newly acquired accurate experimental adsorption enthalpies obtained for a broad range of adsorbed amount using a Tian−Calvet-type microcalorimeter.
Figure 1. CuBTC adsorption sites: the 12 CUS sites in the unit cell (indicated by orange balls) are located just above the Cu2+ cations in the large (L3) cage; there are two small (S1) cages, each of them with one cage center site (green sphere) and four cage windows sites (blue spheres).
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EXPERIMENTAL AND THEORETICAL METHODS Sample Characterization. CuBTC MOF (HKUST-1) was provided by the Korean Research Institute for Chemical Technology and was prepared by microwave synthesis.26 The sample was activated by heating to 150 °C under secondary vacuum (
