External Surface of Zeolite Imidazolate Frameworks Viewed Ab Initio

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External Surface of Zeolite Imidazolate Frameworks Viewed Ab Initio: Multifunctionality at the Organic-Inorganic Interface Celine Chizallet* and Nicolas Bats IFP, IFP-Lyon, Direction Catalyse et S eparation, BP3, 69360 Solaize, France

ABSTRACT The external surface sites of zinc based zeolite imidazolate frameworks (ZIFs) is investigated by cluster density functional theory calculations, which reveal a great variety of sites. Their stability as a function of temperature and pressure is resolved, thanks to a thermodynamic approach. Low-coordinated zinc cations ZnII and ZnIII, which can play the role of Lewis acid sites, are stabilized over a wide temperature range. Brønsted acid sites (NH groups), basic sites (N- extremities), as well as OH groups and hydrogenocarbonates (monodentate mainly, but also bidentate) are also stable. Their calculated vibrational feature is consistent with high-frequency vibrations observed in experimental spectra of ZIF-8 materials reported in the literature. This coexistence of groups of various expected reactivity opens perspectives to multifunctional catalysis at the external surface of ZIF-8. SECTION Surfaces, Interfaces, Catalysis

etal-organic framework (MOF) materials,1 and particularly zeolite imidazolate frameworks (ZIFs) have attracted considerable attention in recent years, because of their selective adsorption properties leading to potential applications in hydrogen and carbon dioxide storage.2-7 These features can usually be understood and predicted thanks to molecular modeling of the bulk material.8-11 However, in some other applications such as catalysis, the external surface of the material may play a determining role, as this is the place where the most defective and thus reactive sites may sit.12 In the case of zeolites, the external surface is indeed shown to play a very important role in some catalytic reactions.13 To the best of our knowledge, there are no study devoted to the investigation of the structure and reactivity of the external surface of MOF materials, probably because of the complexity of such systems. In the present work, we show how ab initio calculations enable us to elucidate the nature and stability of a great variety of sites likely to be present on the external surface of ZIFs. We focus on ZIF-8, the prototype of ZIF materials, constituted of zinc(II) metallic centers, each surrounded by four 2-methylimidazolate ligands, with a sodalite type structure.14,15 It can be expected that the behavior of the external surface of ZIF-8 is dominated by the variable coordination environment of zinc nucleus, which is affected upon crystal cleaving. Within the ZIF-8 periodic structure, the zinc ions coordinated to four imidazolate ligands represent the minimal building block of the solid, at the origin of its surface chemistry. Acluster approach was thus chosen in this preliminary work to model the local environment of reactive sites of ZIF-8, including partially decoordinated Zn sites (formally issued from crystal cleaving along Zn-N bonds) of the external surface (periodic

M

r 2009 American Chemical Society

tests were performed as reported in Supporting Information S1 so as to validate the present approach). The size of the clusters and the accuracy of the calculations (basis set) have been chosen to obtain optimal relevancy for a reasonable computing demand (see Supporting Information S1). Four main types of environments of Zn regarding the number of imidazolate ligands (C4H5N-, further denoted as Im-) can be invoked and have been modeled (Figure 1): {Zn-Imi}(2-i)þ are representative of the local environment of the Zn ions within the structure of the MOF (i=4) or at its external surface (i=1-3, depending on the number of links lost). For the sake of clarity, these clusters will be denoted {Zn-Imi} (i=1-4) in what follows, without any further mention of their charge. The geometry of {Zn-Im4} was first extracted from the structure of Huang et al.15 and then fully optimized. Then the Zn atom only, and the additional adsorbates, were allowed to relax in {Zn-Im1-3} to account for the rigidity of the real periodic ZIF8 structure. Lewis acid sites ZnIII (in {Zn-Im3}), ZnII (in {ZnIm2}) and ZnI (in {Zn-Im}) are thus revealed from ZnIV, as well as free N- extremities for some imidazolate ligands, likely to behave as bases. Water and CO2 of the ambient air can thus adsorb trough their oxygen atoms on the Zn ions. Water can also dissociate, giving rise to one OH group (hydroxylation of Zn) and one NH group (protonation of the N- extremities). Hydrogenocarbonates (monodentate or bidentate, called MH and BH in the following) can then be obtained from reaction of CO2 molecules with OH groups. Received Date: October 28, 2009 Accepted Date: December 1, 2009 Published on Web Date: December 07, 2009

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DOI: 10.1021/jz900192x |J. Phys. Chem. Lett. 2010, 1, 349–353

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energy (as provided by the DFT approach). ΔfU is analogous to adsorption (eventually dissociative) energy of water (n>0, m = 0) or CO2 (n = 0, m > 0). The temperature and pressure effects are included a posteriori, by calculating reaction Gibbs energy ΔrG according to eq 3.

Reaction energies evaluated by density functional theory (DFT)are computed at 0 K, and referred to isolated molecules in the gas phase. Formation energies ΔfU of species involving adsorbates (H2O, CO2) relate to the reaction depicted by eq 1, and is given in eq 2. fZn-Imi g þ nH2 OðgÞ þ mCO2ðgÞ ¼ fZn-Imi -nH2 O-mCO2 g

Δf GðfZn-Imi -nH2 O-mCO2 gÞ ¼ Δf U ðfZn-Imi -nH2 O-mCO2 gÞ - nμ°ðH2 OðgÞ ÞðT Þ ! P H2 O PCO2 þ m ln -mμ°ðCO2ðgÞ ÞðT Þ -RT n ln P° P°

ð1Þ

Δf U ¼ U ðfZn-Imi -nH2 O-mCO2 gÞ - U ðfZn-Imi gÞ - nUðH2 OðgÞ Þ - mUðCO2ðgÞ Þ

ð2Þ

ð3Þ

{Zn-Imi-nH2O-mCO2} is a surface species involving a number n of adsorbed water molecules (dissociated or not) and m adsorbed CO2 molecules (molecular or within hydrogenocarbonates). U is a generic terminology for internal

Calculated formation energies ΔfU(0K) of these various surface species are reported in Table 1. {Zn-Im3} is able to adsorb water, the most preferred adsorption mode being molecular adsorption (versus dissociation). The adsorption of CO2 on ZnIII in a linear mode is not as favorable as water adsorption. However, the reaction of CO2 with OH groups, giving rise to hydrogenocarbonates, is endothermic (-74 kJ.mol-1), the most stable species being MH (and not BH) on this site. The adsorption of water is much more endothermic on {Zn-Im2} than on {Zn-Im3}, and is moreover dissociative, which is preferred to molecular adsorbed water. As in the case of Zn-Im3, the linear adsorption of CO2 is not preferred, with hydrogenocarbonates (MH and BH) and associated NH being much more stable. As expected, {Zn-Im} behaves as the most reactive environment for Zn toward water and CO2. Formation energies reported in Table 1 correspond to the dissociation of at most one water molecule, since the modeled system contains only one ligand able to generate a NH group. The dissociation of one water molecule is in each case more favorable than its molecular adsorption and the adsorption of molecular CO2. Following the constraint of a maximal number of one OH group per complex, at most, one hydrogenocarbonate can be generated, the most stable one being MH. Complementary systems have been modeled, holding only dissociated OH groups (two or three) and up to

Figure 1. {Zn-Imi}(2-i)þ clusters used to model the ZIF-8 external surface: (a) i = 4, (b) i = 3, (c) i = 2, (d) i = 1.

Table 1. Calculated Formation Energies ΔfU (kJ 3 mol-1, eq 2) of Surface Sites on ZIF-8a H2O adsorption Cluster {Zn-Im3} {Zn-Im2}

{Zn-Im}

CO2 molecular adsorption

CO2 þ H2O adsorption

Δ fU

species

ΔfU

species

ΔfU

molecular H2O (1)

-31

molecular CO2 (1)

-3

MH (1) þ NH (1)

-74

OH (1) þ NH (1)

∼0 molecular CO2 (1)

-24

BH (1) þ NH (1)

-195

molecular CO2 (2)

-36

MH (1) þ OH (1) þ NH (2)

-233

molecular CO2 (1)

-90

MH (2) þ NH (2) BH (1) þ NH (1)

-261 -419

molecular CO2 (2)

-147

BH (1) þ molecular H2O (1) þ NH (1)

-512

molecular CO2 (3)

-197

MH (1) þ NH (1) þ molecular H2O (2)

-599

species

molecular H2O (1)

-85

OH (1) þ NH (1)

-159

molecular H2O (2)

-141

OH (2) þ NH (2) molecular H2O (1)

-184 -185

OH (1) þ NH (1)

-383

molecular H2O (2)

-314

molecular H2O (1) þ OH (1) þ NH (1)

-470

molecular H2O (3)

-421

molecular H2O (2) þ OH (1) þ NH (1)

-552

a The number of adsorbed molecules is given in brackets. OH and NH are obtained from dissociative adsorption of water. “MH” stands for monodentate hydrogenocarbonate, and “BH” is bidentate hydrogenocarbonate. For a given complex, the most stable species at 0 K is reported in bold.


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air. The reactivity of the {Zn-Imi} environments with respect to water and carbon dioxide logically increases in the order {Zn-Im1} > {Zn-Im2} > {Zn-Im3} as shown by the wider stability domains of the bare zinc species in the order ZnIII > ZnII . ZnI (this last species being absent from the stability diagram). The most stable adsorbates at the 0 K limit (MH groups associated with the same number of NH groups) are gradually eliminated upon thermal treatment, leading to OH groups in association with the same number of NH groups (CO2 desorption), and then to bare {Zn-Imi} complexes (H2O desorption, loss of Hþ on NH, and OH- on Zn for {ZnIm2} and {Zn-Im1}, or of molecular water on {Zn-Im3}). Figure 3 suggests very diverse surface sites and environments on the external surface of ZIF-8, depending on the operating conditions. At ambient temperature and pressure (mark A in Figure 3), ZnIII species are thus likely to be found on {Zn-Im3} and {Zn-Im2}, together with OH and NH groups on {Zn-Im2} and {Zn-Im}, and MH groups on {Zn-Im}. Under lower pressure (sample put into vacuum, mark B in Figure 3) at ∼298 K, the calculations predict the possible desorption of CO2 and H2O from {Zn-Im} leading to the formation of ZnIII and geminal OH groups. Upon further thermal treatment at ∼473 K (mark C in Figure 3), ZnII and isolated ZnII-OH groups should even be obtained on {Zn-Im}. Lowering the temperature at about 100 K (mark D in Figure 3) should produce MH groups on all kinds of {Zn-Imi} systems, converting all ZnIII into ZnIV. The calculated stability ranges at low temperature are however quite narrow, and this, together with the impact of the kinetics of readsorption of H2O and CO2, can lead to a great variety of plausible environments. The experimental infrared spectra of ZIFs are dominated by bands from the lattice of the bulk structure. However, when closely looking at the IR spectra of ZIF-8 published in the literature,7 several contributions can be seen in the 3500-3800 cm-1 frequency range, which can not be assigned to any structural vibration. Calculated anharmonic IR features (detailed in Supporting Information S3) show that

three MH groups. Results are reported in Supporting Information S2, and confirm that hydrogenocarbonates are the most stable species at 0 K, for this kind of environment as well. Taking as reference the energy of the systems without any adsorbate, the Gibbs energy of formation ΔfG of the other systems is reported as a function of the temperature for a given pressure, e.g., for given partial pressure of H2O and CO2, taking into account the composition of air (about 1% in H2O and 500 ppm CO2). Figure 2 depicts the example of the {Zn-Im2} based systems for a total pressure P = 1 bar. The minimum ΔfG curve provides the chemical nature of the most stable species for given T and P values. The same methodology has been applied to {Zn-Im3} and {Zn-Im}. Relevant structures are depicted in Figure 3 with data on their estimated thermal stability. P = 10-6 bar is typical of analytical conditions in vacuum, whereas P=1 bar represents ambient

Figure 2. Formation free energies ΔfG as a function of the temperature for a total pressure P = 1 bar, relative to the {ZnIm2}-type systems.

Figure 3. Structure and thermal stability of adsorbates on {Zn-Imi} environments, for a total pressure of 1 bar (lower axis) and 10-6 bar (upper axis) in air. (a) MH and NH on {Zn-Im3}, (b) molecular water on {Zn-Im3}, (c) {Zn-Im3} without any adsorbate, (d) two MH and two NH on {Zn-Im2}, (e) one MH, one OH, and two NH on {Zn-Im2}, (f) one BH and NH on {Zn-Im2}, (g) one OH and NH on {Zn-Im2}, (h) {Zn-Im2} without any adsorbate. (i) three MH on {Zn-Im}, (j) two MH and one OH on {Zn-Im}, (k) two OH on {Zn-Im}, (l) one OH and one NH on {Zn-Im}. The A to D marks refer to the experimental operating conditions discussed in the text.


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Zn-OH groups are expected in the ∼3720-3750 cm-1 range, whereas NH groups and hydrogenocarbonates give rise to ∼3530-3575 cm-1 and ∼3680 cm-1 stretching modes, respectively (in the absence of hydrogen bonding). This good agreement shows that the experimental bands can be explained by the species proposed in the present work. The present calculations thus provide a predictive tool to monitor the nature of surface sites on ZIF materials. ZnII and ZnIII are potentially Lewis acid sites, whereas N- extremities of imidazolate ligands are Lewis and Brønsted bases. NH groups can be considered as Brønsted acid sites. By analogy with the ZnO inorganic oxide, generally exhibiting basic sites, hydroxyl groups and presumably hydrogenocarbonates on a zinc nucleus might not be very acidic, and could even exhibit base properties. The conditions for multifunctional catalysis are thus gathered on ZIFs, provided that appropriate pretreatment conditions are chosen. Low coordinated zinc ions and Zn-OH groups could also be the place of nucleation of metal nanoparticles, particularly gold,16 providing extension of the multifunctional properties to a oxidation-reduction dimensionality. Some internal sites of ZIF-8 might also transitorily decoordinate from one or several imidazolate ligands, to allow the formation of species mentioned in Figure 3, which could also explain the stability of small gold particles within the pores.16 This aspect of ZIF chemistry has never been studied so far, as most molecular approaches focus on the bulk of the material itself. The sites revealed here may be of increasing relevance for small particle sizes or for metal-organic polyedra (MOPs) where the surface-to-volume ratio is maximal.17 This work thus opens new perspectives for the understanding of the behavior of these materials in operating conditions, and for their further applications with respect to their catalytic properties. Calculations have been performed thanks to the Turbomole18,19 code (B3LYP,20,21 def2-TZVPP basis set). ΔfG is calculated with the following approximations: (i) the variations of G for the condensed phase (ZIF-8) before and after reaction (with water for example) is simplified into the differences in internal energy U, so that the difference between ΔrU and ΔrG is reduced to the difference between G and U for the gas phase molecules involved in the adsorption; (ii) for the latter species, the ideal gas approximation is performed, and the vibrational, rotational, and translational partition functions are calculated to deduce the chemical potential μ°(T) of the given molecule as a function of temperature. This results in the expression of ΔfG given in eq 3.

ACKNOWLEDGMENT The authors thank Javier Perez Pellitero,

Vincent Lecocq, and Delphine Bazer-Bachi (Direction Catalyse et S eparation, IFP-Lyon) for fruitful discussions.

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SUPPORTING

INFORMATION AVAILABLE Additional details about the choice of the system type and size and of the basis set, ΔfU values, and vibrational frequencies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: celine. [email protected]. Fax: (þ33) 4 78 02 20 66.


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External Surface of Zeolite Imidazolate Frameworks Viewed Ab Initio

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