ARTICLE pubs.acs.org/JPCC
Understanding CO2 Adsorption in CuBTC MOF: Comparing Combined DFTab Initio Calculations with Microcalorimetry Experiments Lukas Grajciar,† Andrew D. Wiersum,‡ Philip L. Llewellyn,‡ Jong-San Chang,§ 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 Chimie Provence, Universite d’Aix-Marseille I, II et III - CNRS (UMR 6264), Centre de St Jer^ome, 13397 Marseille Cedex 20, France § Research Group for Nanocatalyst, Biorefinery Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Daejeon 305-600, Korea
bS Supporting Information ABSTRACT: A combined experimental and theoretical investigation of CO2 adsorption in the metalorganic framework CuBTC is presented. Adsorption enthalpies were measured as a function of coverage up to 13 mmol g1 adsorbed amount (corresponding roughly to CO2/Cu = 5:2) by a TianCalvet-type microcalorimeter. Experimetal data are interpreted based on accurate calculations employing a combined DFTab initio computational scheme. CO2 molecules adsorb preferentially on coordinatively unsaturated sites for coverages below CO2/Cu = 1:1; at higher coverages (up to CO2/Cu = 5:3), CO2 adsorbs in cage window sites; and at higher coverages, the sites in cage centers and in large cages start to be occupied. Experimental adsorption enthalpies are almost constant (29 kJ mol1) up to the CO2/Cu = 5:3 coverage, suggesting a homogeneity of adsorption sites. However, calculations clearly show that adsorption sites in CuBTC are rather heterogeneous. The experimentally observed independence of adsorption enthalpies with respect to coverage is due to the cancellation of two effects: the decrease in the adsorbateadsorbent interaction is compensated by an increase in the lateral interactions.
’ INTRODUCTION Metalorganic frameworks (MOFs) are a recent addition to the class of microporous and mesoporous materials. Made up of metal clusters connected by organic ligands, they form crystalline materials presenting surface areas and pore volumes that are able to compete with activated carbons while having a far more varied structure and adaptable surface chemistry even than zeolites.1 They are seen as potential replacements for activated carbons and zeolites in applications such as adsorption2 and catalysis.3,4 CuBTC, also known as HKUST-1, is one of the earliest reported stable MOFs with a very high surface area. Based on the well-known copper “paddlewheel”,5 it is formed of large interconnecting square channels surrounded by small tetrahedral side pockets (Figure 1) and presents a number of coordinatively unsaturated (cus) copper metal sites. This makes it particularly suited to the adsorption of gases, as the small cages and open metal sites provide strong adsorption sites at low pressure whereas the large channels allow high uptakes through pore filling at higher pressures.6,7 Because of its relatively straightforward synthesis and high adsorption capacity, CuBTC has been widely studied both experimentally and theoretically and can be considered a reference MOF. Early studies focused mainly on its potential for storing H2811 and CO2,12 whereas more recently, r 2011 American Chemical Society
CuBTC has been considered for gas separations, such as the removal of CO2 from carbon monoxide13 and methane,14 and even for catalysis.15 Several groups have also worked on different synthesis routes leading to higher surface areas,1618 resulting in the data published in the literature being highly dependent on the quality of the sample. Adsorption on CuBTC has been modeled for a number of gases using both grand-canonical Monte Carlo (GCMC)7,13,14,1922 and density functional theory (DFT)20,22 simulations, and the results have been used to investigate the adsorption mechanism and location of adsorbed species in CuBTC. However, both of these models are limited by the difficulties in accurately reproducing the interactions of adsorbed molecules with unsaturated metal sites at low coverages.7,20,23 In particular, large discrepancies have been observed in the calculated heats of adsorption. Recently, the DFT/coupled-cluster (CC) method,2426 a combined DFTab initio computational scheme, has been shown to successfully model the interactions of water with CuBTC23 and will be used here to investigate the adsorption of CO2 on CuBTC. Received: June 26, 2011 Revised: August 2, 2011 Published: August 02, 2011 17925
dx.doi.org/10.1021/jp206002d | J. Phys. Chem. C 2011, 115, 17925–17933
The Journal of Physical Chemistry C
Figure 1. View of the structure and connectivity of CuBTC containing three pores (cages): a small tetrahedral pore shown as an orange sphere and two large octahedral pores approximately depicted as red and yellow spheres. Cu, O, C, and H atoms are depicted in pink, red, gray, and white, respectively.
Microcalorimetry has been shown to be a powerful tool for studying adsorption phenomena in porous materials. It provides a direct measurement of the interaction energy between the adsorbent and the guest molecule and can be used to probe the chemical nature of the surface, highlight phase transitions and pore filling, and characterize specific adsorption sites.27 In particular, microcalorimetry has been used to illustrate the flexibility of the MIL53 series28 and the interaction of adsorbed species with open metal sites in the large-pore MIL100(Fe) system.29 In this work, we have used a combination of the ab initio calculations employing the recently developed DFT/CC method and microcalorimetry to study the adsorption mechanism of CO2 in CuBTC at various coverages, focusing especially on the different adsorption sites and the corresponding enthalpies.
’ MATERIALS AND METHODS Experimental Methods. Sample Characterization. CuBTC MOF (HKUST-1) was provided by the Korean Research Institute for Chemical Technology and was prepared by microwave synthesis.17 The sample was activated by heating to 150 C under secondary vacuum (
