pubs.acs.org/JPCL
Water Adsorption on Coordinatively Unsaturated Sites in CuBTC MOF Luk as Grajciar,† Ota Bludsk y,‡ 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, and ‡Institute of Organic Chemistry and Biochemistry, AS CR, v.v.i., Flemingovo nam. 2, 16610 Prague 6, Czech Republic
ABSTRACT We report a theoretical study of water adsorption on coordinatively unsaturated sites (cus's) in a metal-organic framework (MOF) compound CuBTC. The reliability of the density functional theory (DFT)-based methods and dispersion-corrected DFT-D schemes for the description of cus sites was investigated with respect to the accurate reference CCSD(T)/CBS data. The accuracy of both DFTand DFT-D methods was found to be insufficient. The proposed DFT/CC correction scheme gave the results in excellent agreement with the reference CCSD(T)/CBS data. DFT/CC calculations performed for the periodic CuBTC model gave RCu-OH2 = 2.19 Å and -ΔHads= 49 kJ mol-1, both in very good agreement with available experimental data. The interaction of the first water molecule with the paddlewheel unit is about 5 kJ mol-1 stronger than the interaction of the second water molecule with the same paddle-wheel unit. The DFT/CC scheme provides an accurate description of the extended MOF systems, and the results obtained with periodic DFT/CC model can be used for the testing and improvement of the force fields for classical simulations. SECTION Surfaces, Interfaces, Catalysis
KUST-1, Cu3(1,3,5-benzenetricarboxylate)2,1 often denoted as CuBTC, is one of the most studied MOFs for adsorption and separation of gases2-7 and even for catalytic8-10 applications. Adsorption isotherms for various gases in CuBTC were successfully modeled;11-15 nevertheless, the details of the adsorbate-adsorbent interactions at the low coverage limit and the role of coordinatively unsaturated sites (cus's) are not fully understood.4 Calculations on CuBTC were typically performed at the molecular mechanics level3,4,7 or at the density functional theory (DFT) level.15-17 However, the reliability of these calculations for characterization of adsorption complexes on cus's in CuBTC has not been established. For example, the interaction energy of CO2 with CuBTC (at the zero-coverage limit) depends on the level of theory used. The local density approximation (LDA) and a generalized gradient approximation (GGA) give -33 and -7 kJ/mol, respectively,17 while the NVT simulation employing a combination of DREIDING and TraPPE force fields gives -17 kJ/mol for molecules in a large pore.3 The reported experimental heats of adsorption for CO2 in CuBTC at low coverage are also somewhat ambiguous; the gravimetric and pulse response experiments in the temporal analysis of products (TAP) reactor gave -12.1 ( 0.3 and -14.6 ( 0.5 kJ mol-1, respectively,3 while the differential thermal analysis (DTA) and the sorption-isosteric method arrived at values of -30 and -35 kJ mol-1, respectively.18,19 It is therefore important to investigate the reliability of computational methods and
H
r 2010 American Chemical Society
models for the description of adsorbates' interaction with cus's in MOFs. An accurate theoretical description of adsorption properties of CuBTC is complicated by the large size of the unit cell (UC) and by the presence of pairs of coordinatively unsaturated CuII ions with open-shell electronic structure.20 The water adsorption on CuII cus's in CuBTC has been chosen as the case study for the following reasons: (i) Crystallographic data on H2O/CuBTC are available; (ii) water is usually present in the CuBTC samples, and it must be removed during the sample activation; (iii) the interaction of water with cus's in CuBTC is relatively strong; (iv) CuBTC is often used as a reference MOF, and a large amount of experimental and computational data are available in the literature; and (v) a pair of CuII cations in close proximity (about 2.5 Å) represents a particularly challenging problem due to the presence of two unpaired electrons, one on each Cu cation. The accuracy of methods for the description of water interaction with the CuII cus in CuBTC is first discussed based on the calculations performed for the copper formate (Cu2(HCOO)4 þ 2H2O) cluster model, and a selected method is then used in calculations employing a periodic model of CuBTC. The copper formate model has a paddle-wheel structure (inset Received Date: October 6, 2010 Accepted Date: November 9, 2010 Published on Web Date: November 12, 2010
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DOI: 10.1021/jz101378z |J. Phys. Chem. Lett. 2010, 1, 3354–3359
pubs.acs.org/JPCL
Table 1. Interaction Energies (in kJ mol-1) of Water with the Paddle-Wheel Cluster Model (inset of Figure 1) Evaluated with Various GGA, meta-GGA, and Hybrid DFT Functionalsa dispersion correction functional type
functional name
none
D2
D3
GGA
PBE RPBE
-32.4 -19.2
-39.5 -31.0
-39.7 -33.1
BLYP
-23.7
-35.0
-38.1
meta-GGA
TPSS
-30.2
-39.6
-40.2
B3LYP
-34.8
-44.7
BHLYP
-47.6
hybrid
-46.7 -57.4
a The corresponding value obtained at the reference level of theory, CCSD(T)/CBS, is -51.2 kJ mol-1.
It is well-known that GGA-type functionals do not account for dispersion interactions and that they need to be modified or augmented for the description of dispersion-stabilized complexes. Results obtained with the empirical dispersion correction schemes of Grimme (denoted as D223 and D324) are also shown in Figure 1. Clearly, the dispersion component accounts only for a fraction of the discrepancy between the PBE and CCSD(T) results. Besides the inability of DFT functionals to describe dispersion interactions, a problematic description of the Cu2þ cation with small ligands at the DFT level has been also reported.25-28 An unrealistically large spin and charge delocalization, which is often connected with the incomplete cancellation of the self-interaction included in the Coulomb energy by the exchange-correlation functional, was observed for GGA functionals in particular.26,28 This artificial delocalization decreases with the increasing amount of the exact exchange mixed in hybrid exchange-correlation functionals.26,28 Similar dependence of the spin and charge delocalization on the amount of the HF exchange is reported in Table S1 in the Supporting Information for Cu(HCOO)2 and H2O 3 3 3 Cu(HCOO)2 cluster models. This overestimated spin and charge delocalization leads to a too large charge density (and incorrect electronic structure description) on Cu2þ that is responsible for part of the DFT error in the description of the water adsorption on Cu2þ sites in CuBTC. Interaction energy, defined by eq 1, was evaluated for several commonly used GGA, meta-GGA, and hybrid functionals at the PBE/AVTZ equilibrium geometry (Table 1). Interaction energy is underestimated (compared to the reference level CCSD(T)/CBS value of -51.2 kJ mol-1) with all exchange-correlation functionals tested, except LDA, which gives a too large value of -57.5 kJ mol-1. The interaction energy increases with increasing amount of HF exchange in the functionals (-Eint(BLYP)
