Theoretical investigation for adsorption of CO2 and CO on MIL-101 compounds with unsaturated metal sites

Computational and Theoretical Chemistry 1055 (2015) 8–14

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Theoretical investigation for adsorption of CO2 and CO on MIL-101 compounds with unsaturated metal sites Xin-Juan Hou, Huiquan Li ⇑, Peng He Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, China

a r t i c l e

i n f o

Article history: Received 25 July 2014 Received in revised form 11 December 2014 Accepted 17 December 2014 Available online 26 December 2014 Keywords: MIL-101 Adsorption mechanism Preferential adsorption mode Non-covalent interaction analyses

a b s t r a c t In present study, the adsorption of CO2 and CO on the metal–organic framework chromium (III) terephthalate (MIL-101) were investigated using generalized gradient approximation with DFT-D correction. Non-covalent interaction analyses (NCI) were also performed to further investigate the interaction between MIL-101 and gas molecules. The theoretical results indicated that the preferential adsorption mode of CO2 on MIL-101 is different with that of CO. The preferential adsorption mode of CO2 on MIL101 involves one oxygen atom of CO2 (OCO2) coordinating with exposed Cr atoms through dominated Lewis acid–base interaction. The calculation results indicate that the transference from CCO to the exposed Cr atom causes the CO adsorption ability of MIL-101 to be much stronger than its corresponding CO2 adsorption ability. The theoretical investigation reveals the nature and strength of CO2 and CO adsorptions on MIL-101 and facilitates the development of new MOFs with high CO2 or CO adsorption capacity and selectivity. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The increasing atmospheric CO2 concentration is a worldwide concern because it greatly contributes to global climate change. CO and CO2 mixtures are produced from metallurgical plants, synthesis gas from steam reforming, CO2 conversion, partial oxidation of hydrocarbons, and so on [1]. The capture and separation of CO2 and CO are also important in their high-value utilization as raw material. Currently, porous materials that can reversibly capture and release CO2 by means of pressure or temperature swing are attracting great interest. Metal–organic frameworks (MOFs) belong to a family of hybrid porous materials that are formed by the coordination of metal ions with organic linkers. This new class of nanoporous materials exhibit excellent functionality. The wide choice of metal corners and organic linkers offer a theoretically infinite number of materials with a broad range of structural properties. [2–8] Thus, MOFs are applicable in gas storage and separation [9–20], catalysis [21–27], drug delivery [28–30], luminescence [31–33], and so on. Chromium (III) terephthalate (MIL-101), one of the most porous materials to date, is a prominent example of MOFs [34]. The major feature of MIL-101 is its stability at high temperatures, in the pres-

⇑ Corresponding author. Tel.: +86 10 82544835. E-mail address: [email protected] (H. Li). http://dx.doi.org/10.1016/j.comptc.2014.12.017 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.

ence of different organic solvents, or under ambient atmosphere. MIL-101 is an attractive candidate for adsorption studies of various pure and mixed gases [35–38]. Llewellyn et al. [39] indicated that the enthalpy of CO2 at zero coverage is about 44 kJ mol1, which is due to the coordination of CO2 molecules directly onto Lewis acid Cr sites of MIL-101. The pure gas adsorption properties of CO2, CH4, C3H8, SF6, and Ar were also measured by Chowdhury et al. [40] at different temperatures. Considering the differences of the experimental conditions, the heat of adsorption of CO2 from various experiments reveals a 20 kJ mol1 difference [39,40]. Both experimental results [39,40] indicated that the adsorbent is energetically heterogeneous in the adsorption of CO2 on MIL-101. Chen et al. [41] performed the grand canonical Monte Carlo (GCMC) calculation to study the adsorption of CO2 and CH4 on both dehydrated and hydrated MIL-101 at 303 K. Their results showed that adsorption first occurred in microporous supertetrahedra at low pressures and then in mesoscopic cages with increased pressure. Chowdhury et al. [42] studied the adsorption of CO2, CO, and CH4 on MIL-101, where the enthalpy of adsorption based on the DSL model were predicted to be 48.2 kJ mol1 for CO and 34.7 kJ mol1 for CO2. Munusamy et al. [38] also confirmed that the heat adsorption capability of CO on MIL-101 is higher than that of CO2. There are three types of functionalities that have been explored for enhancing CO2 capture performance, including amines, strongly polarizing organic functionalities and exposed metal cation sites.

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Although the experimental and theoretical studies about the effect of functional groups connecting to organic linker are intensively studied [43–49], the related work about unsaturated sites of metal atom site is few. The microscopic adsorption mechanisms of CO2 and CO on MIL-101, including the difference of adsorption mode and adsorption ability remain unclear. In order to further illustrate the correlation between MOFs structures possessing exposed metal atoms and gas adsorption ability, a detailed theoretical investigation on the adsorption modes of CO2 and CO on MIL-101 is necessary. There are two crucial issues are addressed in this work. One issue is the interaction modes of CO2 and CO on MIL101, the other issue is the reason for the adsorption ability difference between CO2 and CO on MIL-101. 2. Calculation methods In present work, we investigated the adsorption of CO2 and CO on MIL-101 models. The model is a Cr3O trimer with a terminal functional group (MIL-101-R, R = –OH and –F). First, we constructed a Cr3O trimer of MIL-101 based on X-ray diffraction data [34]. In order to determine the adsorption site of CO2/CO on MIL101-R, a CO2/CO molecule was located at different positions around MIL-101-R models. For MIL-101-R, CO2/CO, and MIL-101R coordinated with CO2/CO (MIL-101-R  CO2/CO), full optimization and frequency analyses were firstly performed at the PBEPBE/6-31G(d,p) level by using Gaussian 03 program [50]. Yet standard DFT is not suitable for describing dispersion interactions such as Van der Waals and HB. Thus, to correct the dispersion in the DFT formalism, the dispersion correction scheme of Ortmann, Bechstedt, and Schmidt was adopted within DFT-D [51]. Based on the PBEPBE/6-31G(d,p) optimized structures, further full optimization were performed with the generalized gradient approximation (GGA) and Perdew and Wang functional 91 (PW91) [52] by using DMol3 code [53,54]. The calculations were performed using a double numerical plus polarization function (DNP) as basis set and

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DFT-D correction. The numerical basis sets implemented in DMol3 minimize or even eliminate basis set superposition error (BSSE) in contrast to Gaussian basis sets in which BSSE can be a serious problem [55]. It has been confirmed that the BSSE corrections in DNP are less than those in the Gaussian 6-311+G(3df,2pd) basis set [56]. The interaction energy (IE) of a gas molecule with an MIL-101 compound is calculated using the following formula:

IE ¼ EðMIL-101-R    CO2 =COÞ  EðMIL-101-RÞ  EðCO2 =COÞ where IE is the energy difference between the isolated MIL-101-R model with CO2/CO and MIL-101-R  CO2/CO. We also performed non-covalent interaction analysis (NCI) to further investigate the interaction between MIL-101-R and gas molecules by examining the reduced electron density gradient that comes from the electron density and its first derivative [57]. The reduced electron density gradient is defined (RDG) as:

RDGðrÞ ¼

jrqðrÞj

1 1=3

2ð3p2 Þ

ðqrÞ4=3

Weak interactions exist in the regions with low electron density and low RDG value. By multiplying the density by the sign of the second density Hessian eigenvalue (k2), different types of interactions (attraction and repulsion) can be distinguished. This noncovalent interaction analysis has been developed to visualize the non-covalent interaction by plotting the electron density versus the reduced density gradient. It is a powerful tool to study noncovalent attractive interaction. For NCI surfaces, red color is a sign of steric effect; blue color indicates strong attraction (between two bonded atoms, the blue region means the bonding); and green color means weak interaction such as van de Waals interaction. Yet, the analysis of sign k2 is necessary to help to discern between different types of non covalent interactions, whereas the density itself provides information about their strength. The related interaction analysis and plotting of figure are performed by Multiwfn program [58].

(a1-1) IE= -42.0kJ/mol

(a1-2) IE= -31.2kJ/mol

(a1-3) IE= -35.4kJ/mol

(b1-1) IE= -39.7kJ/mol

(b1-2) IE= -28.5kJ/mol

(b1-3) IE= -33.8kJ/mol

Fig. 1. The optimized geometries of MIL-101-R  CO2 complexes using PW91/DZP method. The distances are in angstrom. (Cr, purple; O, red; C, grey; F, cyan; H, white). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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3. Results and discussion Various energy minima identified for MIL-101-R  CO2 and MIL101-R  CO (R = –OH and –F) at the PW91/DNP level are shown in Figs. 1 and 2, respectively. The gradient isosurfaces describing the non-covalent interaction of the MIL-101-F  CO2 and MIL-101F  CO complexes are shown in Fig. 3. Plots of the reduced density versus the electron density multiplied by the sign of the second

Hessian eigenvalue are shown in Fig. 1S of supporting information. As shown in Fig. 1S, one or more spikes in the low-density, lowgradient region, is a signature of non-covalent interactions. The density values of the low-gradient spikes also indicate the interaction strength. Low-density, low-gradient spike lying at negative values indicate stabilizing interaction; low-density, low-gradient spike near zero is an indication of weak attraction; low-density, low-gradient spike with positive values indicate the existence of

(a2-1) IE= -62.9kJ/mol

(a2-2) IE= -25.1kJ/mol

(a2-3a) IE= -22.2kJ/mol

(a2-3b) IE= -21.5kJ/mol

(a2-4) IE= -27.7kJ/mol

(a2-5) IE= -30.2kJ/mol

(b2-1) IE= -64.4kJ/mol

(b2-2) IE= -21.7kJ/mol

(b2-3) IE= -21.6kJ/mol

(d2-4) IE= -23.8kJ/mol

(d2-5) IE= -28.8kJ/mol

Fig. 2. The optimized geometries of MIL-101-R  CO complexes using PW91/DZP method. The distances are in angstrom.

X.-J. Hou et al. / Computational and Theoretical Chemistry 1055 (2015) 8–14

(d1-1)

(d1-2)

(d1-3)

(d2-1)

(d2-2)

(d2-3)

(d2-4)

(d2-5)

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Fig. 3. Gradient isosurfaces (s = 0.32au) for MIL-101-F  CO2 and MIL-101-F  CO complexes. (C and Cr, cyan; O, red; F, coral; H, grey). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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steric effect. In the following section, the low-density, low-gradient spike of MIL-101-F complex and the corresponding gradient isosurfaces will be analyzed in detail.

gradually gather near the functional groups or near the corners formed by two phenyl rings [as in mode (2) and (3)]. 3.2. CO adsorption on MIL-101 compounds

3.1. CO2 adsorption on MIL-101 compounds In general, the CO2 molecule is adsorbed on MIL-101-R via three modes. For adsorption mode (1), the IEs of MIL-101-R  CO2 complexes are about 42.0 kJ mol1 and 39.7 kJ mol1 for MIL-101F and MIL-101-OH, respectively. In mode (1), Lewis acid–base (LA–LB) interaction exists between one OCO2 and the exposed Cr atoms with a distance of about 2.4 Å, and weak interaction cooperatively exists between CO2 and the nearby phenyl rings. Combining Fig. 1 and Fig. 1S, for structure MIL-101-F  CO2 complex (a1-1), the spike near 0.03 is corresponding to the strong LA–LB interaction exists between one OCO2 and the exposed Cr atoms with a distance of 2.435 Å; the spikes near 0.007 and 0.005 indicate that van der Waals interactions also simultaneously exist between the other OCO2 and nearby phenyl rings; the spike near 0.017 indicates the steric effect existing between CO2 and MIL-101-F. For adsorption mode (2), CO2 is located near the functional groups in the presence of LA-LB interactions. In MIL-101-F  CO2 complex (a1-2), the spikes near 0.017 and 0.01 corresponds LA–LB interaction between the CCO2 and F atoms and weak HBs between two OCO2 atoms and hydrogen atoms of phenyl. In the adsorption mode (2) of all MIL-101-R  CO2 complexes, CO2 undergoes bending and deviates from its linear geometry. The O–C–O angles of CO2 in structure (a1-2) of MIL-101-OH  CO2 and (b1-2) of MIL-101-F  CO2 are 174.5° and 176.2°, respectively. The charges of Lewis base and the natural electron configuration of CCO2 in MIL-101-R  CO2 obtained from natural population analysis calculations are shown in Table 1. The bending of the O–C–O angle is caused by deviations from the sp-hybridization of the carbon atom and the presence of LA–LB interactions. For the MIL-101-R  CO2 complex with CO2 molecule adsorbed on MIL-101-R via mode (3), the LA–LB interactions exist between CCO2 and nearby oxygen atoms connected to the Cr atom with distances ranging from 2.8 Å to 3.4 Å and LA–LB interaction between OCO2 and C atoms of formates with distances about 3.4 Å. In structure (a1-3), the spikes near 0.01 and zero corresponding the weak LA–LB and van der Waals interactions between CCO2 and nearby O atoms and between OCO2 atom and the C atoms of formats. The spectra characterized the formation of CO2 coordinating with unsaturated Cr atoms [39]. Our theoretical study also confirmed that the preferential adsorption site is the unsaturated Cr atom. As the metal centers are occupied, CO2 molecules will gradually be located near the functional groups and on ‘‘top’’ of a plane formed by the three Cr atoms. In the initial stage, strong interaction occurs between the exposed Cr and the adsorbate molecule [as in mode (1)]. With the increase of pressure, the CO2 molecules

Table 1 The atomic partial charges (in e) of Lewis base and natural electron configuration of CCO2 in MIL-101-R  CO2 obtained from natural population analysis calculations by using the PW91/DNP method. Charges on lewis atoms of functional group

Natural electron configuration of CO2 in MIL-101-R  CO2

The angle of OCO in MIL-101-R  CO2

O(–OH) F(–F)

C:2s(0.940)2p(2.310) C:2s(0.934)2p(2.310)

174.5° 176.2°

Natural electron configuration of CO2 C:2s(0.935)2p(2.340)

The angle of OCO in CO2 180.0°

0.76 0.50

CO adsorption on MIL-101-R compounds are classified into five modes: (1) CCO coordinates with exposed Cr atoms with charge transfer from CO to the exposed Cr atom; (2) OCO coordinates with exposed Cr atoms through LA–LB interaction; (3) CO locates near the functional groups through LA–LB interaction; (4) CO positions on top of a plane formed by three Cr atoms with LA–LB interaction between CCO and nearby oxygen atoms connected with the Cr atom; and (5) CO locates on top of a plane formed ‘‘parallel’’ with Cr–O bond through LA–LB interactions between CO with nearby formates. The IEs of mode (1) of MIL-101-R  CO complexes are much higher than the corresponding values of the other four adsorption modes. For mode (2), the LA–LB interaction between the exposed Cr atom and OCO is the only stabilizing interaction. For mode (3), the complex stabilization arises from the LA–LB interaction between the functional groups and the CCO atom and the HB between the OCO and the Haromatic atoms. In adsorption mode (4) of MIL-101-R  CO, the two stabilizing interaction are the LA–LB interactions between CCO and oxygen atoms of formate and the HB interaction formed between OCO and Haromatic. In adsorption mode (5), only the LA–LB interactions between CO and formates exist. Using the adsorption of CO on MIL-101-F as example, we analyzed the different adsorption modes of CO. The charge analysis indicated the charges on the unsaturated Cr atom of MIL-101-F as well as CCO and OCO of the CO molecule are +1.074, +0.105 and 0.105, respectively. In adsorption mode (1), the atomic charges of the exposed Cr atom in MIL-101-F  CO is +0.848, whereas the corresponding value in MIL-101-F is +1.074, and the atomic charges of CCO in MIL-101-F  CO complex and isolated CO molecule are +0.318 and +0.105, respectively. The obvious differences in the atomic charges in MIL-101-F  CO, MIL-101-F, and CO indicated that charge transfer from the Cr atom to CCO exists when CO is adsorbed on MIL-101-F as in mode (1), causing the strong IE of structure (b2-1). In structure (b2-2), the atomic charges in unsaturated Cr and OCO are 1.058 and 0.083, respectively, which indicates the absence of charge transfer from the Cr atom to CO. The CO adsorption in structure (b2-2) is mainly contributed by the LA–LB interaction with a Cr  OCO distance of 2.559 Å. The spike near zero also indicates the existence of van der Waal interaction in structure (b2-1) and (b2-2). The configuration of (b2-3) is stabilized by weak LA–LB interactions between F and CCO atoms with a distance of 2.941 Å and weak vdW interaction between OCO and phenyl rings. For structure (b2-4), weak LA–LB interaction exists between the CCO and the nearby oxygen atoms, and weak vdW interaction exists between OCO and nearby phenyl rings. In structure (b2-5) the atomic charges in C atoms of three formates are +0.617, +0.623, and +0.613, whereas the distances between the C atom of the formates and OCO are 3.853, 3.820, and 3.784 Å. The atomic charges of the oxygen atom near CCO are 0.521 and 0.546, whereas the distances between CCO and the two nearby oxygen atoms are 4.012 and 3.881 Å. Obviously, in structure (b25), weak LA-LB interaction and vdW interaction between CO and MIL-101-F stabilized this configuration, which causes the CO adsorption and the positioning of the CO molecule on ‘‘top’’ of the Cr–O bond. The highest IE of MIL-101-F  CO is obviously larger than that of MIL-101-F  CO2. This phenomenon is verified by the differences in the enthalpy of adsorption at zero loading of CO and CO2 on MIL-101 [42]. Our calculation results indicate that charge transfer between CO and MIL-101-F results in stronger IE of MIL-101-F  CO2 and the interaction between CO and unsaturated Cr is much stronger than that of CO2.

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4. Conclusions In order to study the adsorption mechanism of CO2 and CO on MOF-101, theoretical calculations are performed. The main conclusions can be summarized as following: (1) The heterogeneous interaction process between CO2 and MIL-101 at the microscopic level is confirmed. There are mainly three kinds of CO2 adsorption modes in MIL-101. The LA-LB interactions between CO2 and MIL-101 play the key role in the adsorption process. (2) The heterogeneous interaction processes between CO and MIL-101 were investigated. There are mainly five kinds of adsorption modes for CO on MIL-101. The strongest IE of CO with MIL-101 is caused by the charge transfer from Cr atom to CCO in the MIL-101  CO complex. In the other four adsorption modes, LA–LB interaction between CO and MIL101 dominate CO adsorption on MIL-101. (3) Although both CO2 and CO molecules are preferentially adsorbed near the exposed Cr atom, their adsorption mechanisms are different. For the CO2 preferential adsorption mode, the interaction between CO2 and MIL-101 are mainly contributed by the strong LA–LB interaction between OCO2 and the exposed Cr atom. In CO preferential adsorption mode, the charge transfer from CO to the exposed Cr atom causes the highest interaction energy. The theoretical investigation reveals the adsorption mechanism of CO2 and CO on MIL-101, namely, unsaturated Cr atom and the functional group connecting with Cr atom or organic linker play the key role in determining the adsorption ability and selectivity of MIL-101. In order to design some MIL-101 derivatives which possess special adsorption ability, the experiment work should firstly try to change the status of Cr atom and the functional group.

Acknowledgements We acknowledge the National Natural Science Foundation of China (Grant No. 20903099) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry for their financial support. The results described in this paper are obtained on the Deepcomp7000 of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2014. 12.017.

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