Sustainable Metallocavitand for Flue Gas-Selective Sorption: A

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C: Physical Processes in Nanomaterials and Nanostructures

Sustainable Metallocavitand for Flue Gas Selective Sorption: A Multiscale Study Biswajit Mohanty, and Natarajan Sathiyamoorthy Venkataramanan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11185 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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The Journal of Physical Chemistry

Sustainable Metallocavitand For Flue Gas Selective Sorption: A Multiscale Study Biswajit Mohanty and Natarajan Sathiyamoorthy Venkataramanan* School of Chemical and Biotechnology, SASTRA Deemed University, Thanjavur, Tamilnadu, India-613401 *To whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

We have carried out density functional theory (DFT); Grand canonical Monte Carlo (GCMC), and ideal adsorption solution theory (IAST) to understand the selective adsorption of flue within the cavity of porous metallocavitand pillarplex (PPX) molecule. Energies associated with the guest@PPX complex formation, depicts the effectiveness of encapsulation of guest within PPX. PPX is noted to have high selectivity towards the adsorption of Br2, HBr, CS2, H2S, and NO2 over their respective congeners. The strength of bonding and nature of the interaction is deciphered via quantum theory of atoms in molecule (QTAIM), noncovalent interaction (NCI), and energy decomposition analysis (EDA) scheme. The molecules containing acidic hydrogen viz. H2O, H2S, HF, HCl, and HBr makes hydrogen bonding with the ─N atom of the pyrazole ring and F2 and Cl2 make partially covalent bonding interaction with the ─Au atom of PPX. The interaction is mostly of van der Waals type, except in F2, Cl2, NO and HF, in which the cumulative contribution of orbital, electrostatic and dispersion terms are important. Furthermore, GCMC and IAST predict the quantitative capture of guest molecules and selectivity of the guest. Among the studied gases, Br2 is the potential candidate at ambient condition followed by CS2, Cl2, and H2S. At high pressure and temperature CS2 selectivity is more predominant.


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1.

Introduction

Emerging industrialization demands inherent energy resources, which stipulates the continuous supply of energy with zero carbon emission. Furthermore, exhaust gas, corrosive gas, and greenhouse gases in the atmosphere are a major concern on account of its antagonistic environmental effect. Unraveling these global assignments, immense interest has provoked on porous materials, sets a new paradigm in material science. Porous materials such as metal-organic frameworks (MOF), covalent organic framework (COF), and zeolites are well-studied systems for their guest uptake properties.1-13 MOF and COFs have a remarkable advantage over zeolites due to their pore size specificity, and tailor-made frameworks for target specific nature.14-18 Both MOF and COF have a plethora of application in energy storage, gas separation, catalysis, and electronics.19-30 In spite of the early success of these polymeric materials in the sequestration of toxic gases, their rigid nature in the presence of open metal sites limits them from robust use. To alleviate such problems, organic systems with discrete cavitand has been studied. They are flexible in nature, thereby facilitating the intrusion of the adsorbate deep inside the materials from the surface. The macrocyclic organic cavitands such as Pillar[n]arene (PA[n], n=5,6), Cucurbit[n]uril (CB[n], n=5,6,7,8) are the most versatile material in the supramolecular host-guest chemistry.31-36 Ganguly et al. studied CO2, CH4 and n-butane gases by PA[6] and found that maximum four CO2 molecule can reside within the cavity with moderate binding energy.37 Yang et al claim high CO2 selectivity over other flue gas, syngas, and natural gas in PA[5] and PA[6].38 Yamagishi et al. studied the PA[6] for both gas and vapor adsorption.39 Similarly, Nau et al., Isaacs et al, and Scherman et al. studied extensively on the cucurbit[n]urils, and their guest binding mechanism.40-48 Chattaraj et al investigated various common gas molecules and found that C2H2 and SO2 are the ideal candidates for CB[6] and CB[7] respectively.49,50 Masson et al. studied the


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CB[8] and their recognition properties towards various targets of biological interest.51 Stoddart and group have designed plenty of supramolecular cyclophanes with paraquat submit and demonstrated the potential host behavior towards targeting specific guests.52-55 However, a general drawback in the organic cavitand is the limited possibility for the post-synthetic modification.56 Therefore, the scientific community has shown interest in the design of new discrete metallocycles and metallocavitands for the host-guest interaction studies. Organometallic host-systems may overcome many of these above problems as they exhibit crucial advantages such as solubility, thermal stability, and flexibility.57,58 The major advantage of the organometallic host-guest system is the transformation of the single crystal of one morphology to another, which allows the exact monitoring of a crystal structure and orientations of the guest molecule upon their inclusion in the voids, resulting in the crystalline phase transformation process.59-61 Kang et al. found a cobalt cluster based supramolecular structure for guest adsorption and Janczak et al. claim a Zn(II) macrocyclic complex for gas adsorption properties.62-63 Coronado et al. synthesized a spin cross over (SCO) coordination polymer [Fe(btzx)3]2+ (btzx=1,4bis(tetrazol-1-ylmethyl)benzene) for CO2 adsorption and also observed the selectivity of CO2 over N2.64 Further Poater et al. carried out DFT calculations on the system and claim that the SCO polymer can accommodate only one CO2 molecule and for consistency they have also investigated H2, O2, H2O, CH4, C2H6, N2O, and NO molecules within the cavity.65 The stronger interaction found for CO2 than for N2 was in perfect agreement with the experimental of Cornado et al.64 Lin et al. observed that coinage metal complexes to retain its stability against air and moisture and the preparation of such complexes are straightforward with post-synthetic modifiability.66 Very recently Pöthig et al. synthesized a novel octanuclear N-heterocyclic carbene complexes of silver (I) and gold (I) center named pillarplexes (PPX) that has a tubular cavity which enables to host


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linear molecules.67 The structure has also resemblances with the organic cavitand pillar[n]arenes in the aspects of cavity size. However excellent adsorption of small guest molecules (gases) within the host cavity is observed. It is also essential to study the selectivity of the target specific molecules over the gaseous mixtures. Several reports claim that combined Grand canonical Monte Carlo (GCMC) and ideal adsorption solution theory (IAST) predicts gravimetric uptake of the gas molecules and their selective separation respectively, which are further validated by experimental data.68-70 To shed light on host capability of PPX, our group has already studied the electronic structure and the inclusion complex formation behavior of Ag, Au, and Cu metal centered PPX with the linear chain 1,8-diaminooctane and the guest molecule binds ideally within the cavity.71 Herein we have explored various gas molecules such as flue gas, biogas, syngas, corrosive gas, exhaust gas, greenhouse gas etc. within the Au-PPX cavity and their adsorption and separation behavior has been studied using multiscale modeling method. Density functional theory (DFT) has been used to determine the binding energy of the individual gas molecules.72 The natural bond orbital analysis (NBO) has been performed to study the charge transfer mechanism between the host-guest complexes.73 Interaction strength between the host and guest has been examined using wavefunction based QTAIM analysis.74 Noncovalent interaction (NCI) has been carried out to visualize the forces that exist between the host and guest in the complex system.75-77 Furthermore, the quantitative approach of the bond strength and the type of interaction is studied using energy decomposition analysis (EDA) scheme.78 The maximum gas uptake has been studied using Grand canonical Monte Carlo (GCMC) ensemble and the selectivity of gases over a mixture of gases has been performed using ideal adsorption solution theory (IAST) method.79,80


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Computational Details Geometry optimizations are carried out with the Grimme’s dispersion corrected pure functionals

B97-D2 in combination with the Los Almoes double-ξ effective core potential LanL2DZ using Gaussian09 program package.81-83 Gas phase optimization and the harmonic analysis has been performed to ensure that the obtained geometry is in the minima and not in the saddle point of the PES. The binding energy, zero-point energy for each complex are computed using the supermolecular approach and subsequently subjected to counterpoise correction proposed by Boys and Bernardi.84 The intermolecular charge transfer in the complex has been studied using NBO analysis and the calculations are performed by using hybrid functional B3LYP-D3 with Stuttgart/Dresden ECP designated as SDD basis set.73,85,86 Topological analysis based on the QTAIM has been performed with the open source Multiwfn programme with the wavefunctions obtained at the B3LYP-D3/SDD level of theory.74,87 The bond critical points (BCP) at the respective bond path is visualized using DAMQT code.88 The real space characteristics of interaction have been analyzed using NCI index.75-77 The nature of interaction in the complex system can be broadly understood by using two scalar quantities viz. electron density (ρ) and the reduced density gradient (RDG) (s) as given in the equation (1). s(r) =

1 2 1/3

2(3𝜋 )

|∇𝜌(𝑟)|

(1)

𝜌(𝑟)4/3

For weak interactions, the associated ρ(r) value is very small and approaches to zero (≈ 0). At this point, the noncovalent interactions can be isolated as regions with low density and low reduced gradient value. The density value at the low gradient indicates the interaction strength. However,


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both hydrogen bonding and steric crowding appear in the same region of reduced gradient space.89 The sign of Laplacian of electron density ∇2ρ(r) is used to distinguish between these two forces.90 There is an ambiguity upon the noncovalent interaction, the Laplacian in the interatomic region is dominated by the positive contribution from the nuclei, independently whether they are bonding or nonbonding interaction. To understand bonding in more details, the Laplacian is decomposed into eigenvalues (λi) of the electron density in the Hessian matrix and the second eigenvalue (λ2) is the key parameter for distinguishing bonded (λ2 0) interactions.90,91 ∇2 = λ1 + λ2 + λ3

(2)

Thus, analysis of the sign of λ2 will help to distinguish the different types of noncovalent interactions. The NCI analysis has been carried out using Multiwfn program with the previously obtained wavefunction. EDA analysis has been performed at the D3 dispersion corrected hybrid functional B3LYP-D3 in combination with TZ2P basis set using ADF (2017) program package.92-94 Zeroth order regular approximation (ZORA) with scalar relativistic effect has been invoked during the computation as Au exhibits a large relativistic effect.95 In EDA, the total binding energy is decomposed into attractive terms Electrostatic (Eelct), Dispersion (Edisp), Orbital (Eorb) and repulsive Pauli’s (Epau) component. The Eelct gives the electrostatic interaction between the two fragments (host and guest) which is calculated by electron density distribution in the complex system. Edisp is the dispersion interaction arises by the density fluctuation between the fragment units. Eorb is the orbital interaction occurs between the two interacting fragments when occupied orbital of one fragment interacts with the unoccupied orbital of other fragments. The Epau is the Paulis repulsion caused by the two electrons with the same spin cannot occupy the same region in space.96,97


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Grand canonical Monte Carlo simulations (GCMC) has been performed using a cell dimension of 29×23×28 Å to study the quantitative gas adsorption using sorption module of Material Studio. All GCMC calculations are computed at various temperatures such as 77, 273, 298, 340 K and a pressure range of 0.1 to 10 bar. Universal Force Field (UFF) parameters are used for Lennard-Jones (LJ) potential.98 The Ewald summation method with the cut-off distance of 12.5 Å is considered for the LJ interaction. The simulations include 1×105 cycle equilibration run followed by a 1×105 production run. Each step intends random translational, insertion/deletion moves with a probability of 0.20. Each cycle represents the number of the adsorbed molecule in the system, which fluctuates during the simulation and it changes with changing the physical variable of the system such as temperature and pressure. The selectivity of gas molecules has been studied using the Ideal adsorption solution theory (IAST) proposed by Myers and Prausnitz.80 The fitting approach has been assessed with various adsorption isotherm model and the dual site Langmuir model is found to be the best fit precisely with an R2 value of 0.99 and is given in equation 3. 𝑘1𝑝

𝑘2𝑝

Q = 𝑞11 + 𝑘1𝑝 + 𝑞21 + 𝑘2𝑝

(3)

Where p is the pressure of the bulk gas at equilibrium, Q is the gas uptake, q1 and q2 are the saturation capacities of sites 1 and 2. k1 and k2 are the affinity coefficients of the sites.

3.

Result and Discussions

3.1.

Structure and energetics

The initial geometry of pillarplex (PPX) adopted from the ref.65 has been optimized using B97D2/LanL2DZ level of theory. The height of the optimized PPX is 11.585 Å (expt. 11.700 Å)


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and the width of the PPX measured from face to face positioned Au···Au is 7.658 Å is shown in supporting information Figure S1. The interatomic distances between the Au-Au in the optimized structure is 3.190 Å (expt. 3.005 Å) is congruent with the experimental value. The cavity size of PPX is 4.300 Å is 0.400 Å lower and the height is 3.785 Å longer than analogous pillar[5]arene structure. A systematic investigation has been carried out by changing the orientation and bond length/angle of guest within the PPX cavity. We have considered fifteen common guest molecules such as H2, H2O, H2S, CO, CO2, CS2, N2, NO, NO2, F2, Cl2, Br2, HF, HCl and HBr and their corresponding optimized complex structures are shown in Figure 1. All the homonuclear diatomic molecules are oriented in a tilted manner from the horizontal axis. The maximum inclination is observed for Br2 with an angle of inclination 45.36° and a negligibly 3.52° inclination is observed for Cl2. Cl2 and Br2 are present at the center of PPX while the others are site-specific and orient partially at Au-Au site. F2 and Cl2 are present on the horizontal plane of two Au units, one end of F2 are covalently bonded with Au molecule while Cl2 molecule orients diagonally and covalently bonded with Au. Heteronuclear diatomic molecules such as CO, NO, HF, HCl, and HBr were also positioned in horizontal fashion, with the exception of CO, which is vertically oriented within the cavity. The angle of inclination is 24.44° found for NO while for other molecules the inclination is small. The hydrogen atom of halides is towards the pyrazole -N atom. Amid to others, CO2 and CS2 are vertically tilted with the degree of inclination less than 5.00°. H2O and H2S are orientated in such a manner that both the hydrogen atoms are at equidistance from the -N atom of the pyrazole ring to gain maximum H bonding interaction. The NO2 molecule is present at the center of the ring. The significant change of bond length is observed for di-halogens and the bond elongation of F2 is found to be maximum followed by Cl2 and Br2 upon complexation. The bond lengths of each guest


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after and before complexation is given in the supporting information Table S1. A additional bond length elongation of 0.256, 0.162, and 0.051 Å is observed for F2, Cl2, and Br2@PPX complex respectively. The higher bond elongation arises due to stronger affinity of F2 and Cl2 to form the covalent bonding interaction with the Au atom of the PPX. Like di-halogens, hydrogen halides also exhibit bond elongation upon complexation. HF and HCl molecules get elongated by 0.011 Å whereas HBr experience 0.004 Å elongation. A reduced bond length is observed for CO and its congeners, while no such change is observed in N2, NO, and NO2 molecules. The change of bond length except di-halogens and hydrogen halides are less than ± 0.010 Å. All the triatomic guest molecules encounter a minor change in bond angle after complexation and the extent of contraction is less than 1.00°. On contrary H2S shows 0.36° expansion of bond angle to attain the hydrogen bonding. It is noteworthy to investigate the Au-Au bond length in PPX where the average bond length decreased substantially afterward the complexation. On the contrary, an increase in Au-Au bond length (0.058 Å) is observed for Cl2@PPX complex and the increased bond distance is due to the sharing of electrons from Au→Cl2. The decrease in bond length is found higher for CS2@PPX (0.218 Å) and lower for Br2@PPX (0.004Å). In other complexes, the Au-Au bond contraction lies between 0.045 and 0.186 Å. The reduced bond length of Au-Au in guest@PPX other than X2@PPX (X=F, Cl, Br) is clearly understood by the effect of polarizability. The correlation of polarizability with the bond distances is shown in Figure 2a and the values are given in the supporting information Table S2. Furthermore, we have computed the deformation energy (DE), the energy difference between the isolated and the constrained species and is analyzed by single point energy calculation at B97-D2/LanL2DZ level of theory and is given in Table 1. The DE energy is higher for Cl2 (15.9 kcal/mol) and F2 (14.8 kcal/mol) associated complex. H2S (5.5 kcal/mol), CS2 (6.6 kcal/mol) and


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HCl (6.3 kcal/mol) complex also show appreciable deformation of the host structure while other complex hosts exhibit deformation of less than 2.0 kcal/mol. Similarly, the guest molecule upon complexation exhibits minor distortion and the DE is higher for F2 (2.3 kcal/mol) and Br2 (2.3 kcal/mol) while the remaining guest molecule experiences less than 1.0 kcal/mol. The higher DE of host and guest claims the stronger binding affinity of guest within the cavity. The binding energy (ΔE), basis set superposition corrected binding energy (BSSE) and zero-point (ZPE) corrected binding energy of each guest encapsulated PPX is calculated and the energy values are given in Table 1. The binding energy of each guest molecule provides information about the stability of the complex. Among the di-halogens, Br2 shows higher ΔE value of -30.7 kcal/mol followed by F2 (-22.5 kcal/mol) and Cl2 (-20.3 kcal/mol) and the same trend is also observed for their respective halides. The ΔE value of HBr (-21.7 kcal/mol) is 5.9 and 6.5 kcal/mol higher than HF and HCl system respectively. Among CS2, CO2, and CO, CS2 shows larger tendency to encapsulate within the cavity of PPX than CO2 and CO. The ΔE value of NO2@PPX is larger by 4.1 and -3.2 kcal/mol (-17.6 kcal/mol) than NO@PPX and N2@PPX respectively. On the other hand, H2O has a slightly higher ΔE value (by 0.2 kcal/mol) than H2S. The larger electronegative of O in H2O influences stronger H-bond in H2O@PPX than H2S@PPX making the former more stable. The H2 exhibits lower ΔE value of -4.7 kcal/mol, implying the weakest interaction compared to other guest molecules. Therefore, based on the ΔE values the possible arrangement of guest molecules within the cavity lies in the order of Br2> F2> CS2 > HBr > Cl2 > H2O> NO2> CO2> H2S> HF > HCl> N2 > NO > CO > H2. The ΔE of all the complexes has been studied at 298 K and the Gibbs free energy change for the complex formation (ΔG298) is computed. Subsequently, the entropy (ΔS298) and enthalpy (ΔH298) of complexation are also investigated and ΔS298 has minimal effect on the system, presumably due


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to the rigid nature of the PPX. For the reaction PPX + Guest → Guest@PPX, ΔG298 value reveals Br2 (-19.6 kcal/mol), F2 (-12.7 kcal/mol), HBr (-12.7 kcal/mol) and CS2 (-10.0 kcal/mol) encapsulated PPX are highly exergonic in nature. The other gases such as H2O, H2S, CO, CO2, N2, NO, NO2, Cl2, HF and HCl as guest within the cavity of PPX has ΔG298 values of -7.2, -8.9, -5.0, 9.0, -7.6, -5.4, -7.6, -9.1, -6.9, -6.8 kcal/mol respectively. The computed ΔG298 values correspond to H2 entrapped PPX is found to be endergonic in nature, implying the thermochemical instability at 298 K. The negative ΔS and ΔH suggesting the H2@PPX complex may attain stability at a lower temperature. Thus, in general, all the complexes are stabilized at 298 K and the enthalpy is the major driving force for complexation as shown in Figure 2b, while ΔS298 renders minimal effect on the associative complex formation. 3.2.

Selectivity in encapsulation in the congeners

From the above results, it obvious that PPX can act as a perfect host which can accommodate the guest molecule and all the complex formation is feasible at 298 K temperature, other than H2@PPX. Now one may ask, what is the selectivity of a guest over the mixtures? For example, if the intent is to separate H2S gas selectively among others by employing PPX as trapping agent, Br2, HBr, F2, CS2, and CO2 must be absent from the mixture, otherwise the substitution reaction will take place exergonically with the ΔG298 values of -10.8 (Br2), -3.8 (HBr), -3.8 (F2), -1.1 (CS2) and -0.1 (CO2) kcal/mol. H2S + PPX → H2S@PPX

ΔG298 = -8.9 kcal/mol

H2S@PPX + M → M@PPX +H2S

Exergonic reaction

H2S@PPX + N → H2S@PPX + N

Endergonic reaction


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where M= Br2, HBr, F2, CS2, and CO2 and N= H2, H2O, CO, N2, NO, NO2, F2, Cl2, HF, and HCl. Thus, H2S can be separated out selectively in the presence of N species. A graphical representation in which H2S, CO2, NO2, CS2, and Cl2, gas separation has been shown in Figure 3. Similarly, in the case of CO2 separation Br2, HBr, F2, CS2, and Cl2 should be removed from the gas mixture at 298 K. The more negative ΔG298 values for Br2 and HBr, implies that, in the presence of Br2 and HBr the other gas molecules are unlikely to be encapsulated within the cavity of PPX. In cases where the difference between the adsorption free energy of two or more gas molecules are negligibly small, then the possibility of equal proportions of the guest to be adsorbed by the PPX is likely.

3.3.

Nature of intramolecular interaction in Guest@PPX

NBO analysis reveals, the guest@PPX complex experiences electronic charge transfer and is significant from the guest to PPX. The observed NBO charge transfer for all the complex is given in the supporting information Table S1. This can be understood by using Pearson hard soft theory, which claims Au in the PPX is behaving like a soft acid. The lower charge density on the Au tends to accept the π electrons from the surrounding. On the contrary, in the F2 and Cl2@PPX, the halogen molecules accept electrons from the Au of PPX by shared paired of interaction. F2@PPX and Cl2@PPX gain an additional charge of 0.999 and 0.391 e- respectively which reflects from their increase in bond length. In H2 (0.042), H2O (0.047), H2S (0.100) and HF (0.019), HCl (0.090), HBr (0.095) the charge transfer increases with increasing the softness of the central atom. Presence of higher δ- charge on O atom of CO2 (0.064) renders lowering the charge transfer contribution whereas CS2 (0.186) and CO (0.125) shows appreciable charge transfer. In N2, NO, and NO2, -N atom behaves as an electron donor. A δ+ charge is developed on N while the O has a δ- charge. Therefore, the charge transfer is significant in N2 (0.122) followed by NO (0.103) and NO2 (0.092).


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3.3.1.

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Quantum Theory of Atoms in Molecule Analysis (QTAIM)

The quantitative strength of intermolecular interactions has been analyzed by virtue of electron density and energy terms obtained from their respective BCP is shown in the supporting information Figure S2. QTAIM unveils electron density ρ(r), Laplacian of electron density ∇2ρ(r), kinetic G(r), and potential V(r) energy density terms at the corresponding BCP. A positive ∇2ρ(r) value suggests the kinetic energy density dominates and the ρ(r) is locally depleted in these regions. Thus, we have considered kinetic, potential energy density to distinguish between the bonding and nonbonding interactions. The value of │V(r)/G(r)│ < 1 illustrates the pure closed-shell type electrostatic (dominant kinetic energy) and │V(r)/G(r)│ > 1 represents regular closed-shell intermediate between electrostatic and covalent bond (dominant potential energy) and │V(r)/G(r)│>2 depicts shared shell type of covalent in nature. The regular closed shell type interaction observed in CS2@PPX (S127···Au4), F2@PPX (F125···Au8), HF@PPX (H126···N25) complexes and the other interactions are pure closed-shell in nature. The analyzed ρ(r), ∇2ρ(r), G(r) and V(r) values are provided in the supporting information Table S3. The regularly closed shell type interactions considered as partial-covalent in nature. 3.3.2. Non-covalent interaction (NCI) The real space visualization of nature of the interaction obtained from NCI index is shown in Figure 4. The analysis of NCI index portrays a representation of van der Waals interactions, hydrogen bonding and steric clashes in the complex system. In PPX the steric strains are visualized in the pyrazole and imidazole ring center and van der Waals surface is observed between two methylene units, faced opposite to each other. All the guest molecules within the PPX are surrounded by fully/partially green isosurfaces implies the van der Waals contact therein and it increases with the


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size of guest molecules. A blue sphere is observed in F2@PPX and Cl2@PPX which delineate the strong attractive forces exhibits between Au···F and Au···Cl and the interaction strength is intermediate between electrostatic and covalent bond. A blue sphere also perceived in HF@PPX, indicates a hydrogen bonding interaction between the N atom of PPX and the H atom of HF (PPXN···H-F) molecule. A low intense red circle is noticed in the bond center of Cl2 and Br2 in Cl2@PPX and Br2@PPX respectively are due to the electronic overlap in the molecule. The plot between the RDG (s) versus sign(λ2)ρ of guest@PPX (shown by blue color) reveals attractive (-ve region), steric (+ve region) and van der Waals interaction (≈0 regions) as shown in the supporting information Figure S3 and is merged with the PPX isosurface (shown by red color). In RDG plot, regions marked with a green rectangular box with the sign(λ2)ρ value range of 0 to -0.01 corresponds electrostatic (high intensity, -0.005 to -0.01 a.u.) and dispersion (low intensity, -0.005 a.u.) interactions. Hydrogen bonding is observed in H2O, H2S, HF, HCl and HBr complex and the strength is in the order of HF > HCl > HBr > H2O > H2S encapsulated complex. A yellow square box in the Br2@PPX and Cl2@PPX represents the strain produced by electronic overlap. This implies that the gas molecules bound with PPX with van der Waals type of interaction. 3.3.3. Energy Decomposition analysis (EDA) EDA has been performed to understand the relative contribution of energy terms such as Eelct, Edisp, Eorb, and Epau to the total binding energy by considering the guest as one fragment and PPX as another. The results of EDA are shown in Figure 5 along with the percentage of contribution and the values of each term are given in the supporting information Table S4. H2O (47.9%) exhibits lesser contribution of Edisp compared to H2 (57.3%) and H2S (59.8%) while the former has maximum Eelct (28.5%) and Eorb (23.6%) contribution than the latter. This claims the interaction of acidic hydrogen of H2O with the N portal of PPX leads a strong hydrogen bonding (N···H = 2.542


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Å) which also responsible for the tilted parallel orientation of H2O molecule to attain maximum Hbonding strength. An ion-quadrupole interaction in H2 molecule results in slightly higher (around 2%) Eelct and Eorb contribution compared to H2S. The adsorption energy of CO, CO2, and CS2 are engendered by Edisp (ca. 64.0―74.7%). Increasing the size of the guest (CO2, CS2), resulting in induced-dipole to access maximum contact with the walls of the host, hence guests approach vertically within the PPX cavity. The CO molecule adsorbed by dipole-dipole interaction which reflects from Eelct (22.4%) and Eorb (13.6%) value compared to CO2 and CS2. The Edisp contribution is strong enough to stabilize the N2@PPX (62.7%) complex, whereas, cumulative Edisp and Eorb interactions of NO and NO2 contributes to the total binding energy. The Eorb of NO is 46.7% is 20.5% higher than NO2 and the counter effect is observed for Edisp term. The -N atom of N2 and NO are positioned end-on with the Au atom at 3.562 and 3.409 Å respectively allowing π-back bonding which is stronger in the latter case. An effective π-back bonding prompts reasonably higher Eorb in NO and NO2 compared N2. Moving from F2 to Br2, the Eorb reduces from 64.7─18.8%, contrary Edisp increases within the range of 8.8─53.1%. The formation of partial-covalent bond in F2@PPX controls the total adsorption strength and in di-halogens, the strength is reduced with decreasing the electronegativity. HCl and HBr@PPX are stabilized by Edisp contribution of 59.3 and 63.8% respectively and Eorb, Eelct terms have minimal effect. The equivalent contribution of Eelct (32.7%), Edisp (31.0%) and Eorb (36.2%) terms are responsible to stabilize HF@PPX complex, this can be noticed from the hydrogen bonding interaction between PPX―N···H―F with an effective length of 2.026 Å. The hydrogen bonding strength decreases gradually in HCl (2.451Å) and HBr (2.722 Å) which, is accountable to understand the HX orientation. In nearly all the cases, except NO, F2, Cl2, and HF, Edisp is the largest contribution to the total binding energy term ranging from 8.8% to 74.7% whereas Eelct contribution is 15.1% to 32.7% for the same system. Eorb contribution in HF,


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Cl2, NO, and Cl2 is higher among all other guest molecules which is 36.2% to 64.7% and for remaining complex the contribution is lesser than 26.2%. Thus, all the guests within the PPX exhibit mostly van der Waals type interaction, except F2 and Cl2, in which the interaction is balanced between electrostatic and covalent nature. 3.4.

Grand canonical Monte Carlo Simulation and Ideal Adsorption Solution Theory for Selectivity of Gas Adsorption

To understands the maximum gas adsorption at various temperature and pressure, we have carried out Grand canonical Monte Carlo (GCMC) simulations at a temperature range of 77−340 K and up to 10 bar pressure. In Figure 6, we have shown the pore volume distribution and the density isosurfaces observed for the PPX system. Grey and blue colour code portray inter and intramolecular voids in the crystal structure. The density plot at 298 K unfolds the gas occupancies in the interior/exterior void of the PPX. Molecules viz. H2 (2.89Å), H2O (2.68 Å), N2 (3.64 Å), F2 (3.35 Å) and HF (3.14 Å) delineates higher density due to low kinetic diameter whereas CS2 (4.48 Å), NO2 (4.01 Å), Cl2 (4.2 Å) and Br2 (4.3 Å) are in the lower due to optimum kinetic diameter while the other molecules are in the intermediate state. High kinetic diameter gas molecules experience stronger intermolecular repulsion resulting in a low-density region. At ambient condition, the absorption maximum is observed for H2S, CS2, NO2, Br2, and HBr from their respective congeners and with increasing pressure the adsorption increases for all the guests. The adsorption isotherm plot of the individual guest is shown in Figure 7. An anomaly after 1 bar pressure claims a gradual increase of Cl2 adsorption over Br2 due to the chemisorption of Cl2 with Au atom. In general, at the ambient condition, the adsorption of CS2 is higher than other guests and after 2 bar pressure H2S, Cl2 increases. The adsorption at 77 K (Figure S4) is fully contrary to


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the 298 K (1 bar). Moving to 273 K (Figure S5), the adsorption of CS2 remains unchanged after 4bar and its congener’s increases upon increasing pressure. The co-adsorption of CO2 and CS2 is observed from 7 bar pressure. The adsorption of Br2 at 0.35 bar is found slightly higher than Cl2. At the higher temperature of 340 K (Figure S6), a strong discrepancy is observed between the Cl2 and Br2 adsorption. The adsorption of Br2 escalates up to 2.50 bar pressure and afterward, Cl2 sorption is dominant. GCMC simulation establishes the adsorption of Br2 in PPX is temperature dependent, increases with the rise of temperature. It is also found that an increase in temperature leads to a higher CS2 uptake. No such discrepancy is noticed at a different range of pressure (0.1─10 bar pressure and 77─340 K temperature) for the guests other than Cl2 and Br2. The results obtained at ambient condition corroborated with DFT study. This analysis imparts the separation of H2S, CS2, NO2, Br2, and HBr from their respective congeners at ambient condition. Further, we have calculated isosteric heats of adsorption (qst) which is shown in Figure 8. The qst helps to understand the strength of interaction between the guest molecules and the adsorbent surface. The qst under all pressures studied at 298 K temperature is in the range of 2─13 kcal/mol. The obtained results infer that H2S, CS2, NO2, Br2, and HBr molecules adhere strongly within the walls of PPX and have larger qst value than their corresponding congeners. In general, the qst is higher for CS2@PPX than others due to the strong attractive intermolecular interaction between the CS2 molecules while for remaining molecules the decrease of qst in the highlighted regime of 0―1 bar pressure depicts binding site heterogeneity. The selectivity of guest adsorption in the presence of guest mixtures can be calculated by using ideal adsorption solution theory (IAST) method.78 The IAST is computed with the previously obtained GCMC data by using a dual-site Langmuir isotherm model with the maximum fitting of R2 value greater than 99.0%. H2S, CS2, NO2, Br2 and HBr selectivity is prominent among their


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respective congeners at 298 K is shown in Figure 9, which also supports the binding energy value computed by using the DFT method. A comprehensive analysis of selectivity of all the guests within the PPX was studied at 1 bar pressure. The results indicate that the selectivity of Cl2 is dominated initially at a pressure of 0.30 bar while at higher pressure, CS2 shows remarkable selectivity. Successively at ambient condition the order of selectivity is CS2 > Cl2 > Br2 > H2S> HBr > CO2 > HCl > NO2 > CO > N2 > H2O > NO > F2 > H2 > HF. Thus CS2, Cl2, Br2, and H2S are the ideal candidate for adsorption and separation of from flue gas mixture.

3.5.

Comparison of present results with [Fe(btzx)3]2+ coordination polymer

A brief comparison of our result with the recently synthesized and theoretically studied spin cross over (SCO) coordination polymer (CP) [Fe(btzx)3]2+ has been carried out to understand the nature of interaction of gas molecules with the macrocycles.64 A profound computational analysis has been proposed by Poater et al. by considering various gas molecules such as H2, O2, N2, CO, CO2, H2O, CH4, C2H6, N2O and NO within the void of CP.65 Poater et al. also considered Ru and Os metal for the sorption studies and claim that change of the metal doesn’t have a determinant role. The cavities of the CP are 9.000 Å and can be able to accept only one molecule, whereas the inner diameter of PPX is 4.3Å and height is 11.7Å. Thus, PPX has high surface area for gas adsorption than CP. We have also compared the binding energy of guest@PPX with the guest@CP. The guests shows a higher affinity towards the PPX compared to CP. A comparison between the interaction terms obtained in the EDA results for the CO2 molecule, shows that for both [Fe(btzx)3]2+ coordination polymer and the PPX molecules, a high component of dispersion term was observed. The nature of interaction in guest@CP is compelled by the overlapping of π-orbitals of the gas molecule with the π-system of the aryl ring whereas N2 and O2 show attractive interaction with the metallic center and


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H2O molecule shows hydrogen bonding interaction. In guest@PPX, no such significant π-π interaction is observed rather a partial covalent bonding interaction is observed for F2 and Cl2. The oxygen portal (δ-) of CO, CO2, NO, and NO2 interacts with the pyrazole ring system via O(δ-)··· 𝜋𝑁 ― 𝑁 interactions and the N2 molecules shows an affinity towards the Au center. The acidic hydrogen containing guest molecules makes hydrogen bonding interaction with the nitrogen atom of PPX. Hence, the sorption of the gas molecule by PPX is more effective than the [Fe(btzx)3]2+ coordination polymer.

4.

Conclusions We have performed DFT calculations to examine the strength and binding nature of various guest

molecules within the cavity of Pillarplexe (PPX). CO, CO2, CS2 resides vertically with slight tilting position within the cavity of PPX, whereas H2, H2S, NO, F2, Cl2 and HX (X=F2, Cl2, Br2) molecules are preferred to orient almost in the horizontal fashion and the H-atom of HX and H2S directed towards the N-atom of pyrazole ring. H2O reside in the gap between two pyrazole unit to gain maximum H-bond strength. N2, NO2, and Br2 are in maximum tilted position near the center of PPX to utilize maximum van der Waals interaction. Binding energy (ΔE), ZPE and BSSE corrected binding energy, along with Gibbs free energy (ΔG298) change for the complex system, has been evaluated. Br2, Cl2, F2, CS2, and H2S are the most viable guest molecules encapsulated within the pores of the PPX system. Herein we have explained intricately with a single guest molecule within the PPX cavitand and the number will be vary depending upon the pore size and the intermolecular repulsion between the guests.


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QTAIM analysis depicts partial covalent bonding nature is observed in the F2, Cl2 and strong hydrogen bonding in HF molecule. The NCI analysis reveals the green isosurface, surrounded to guest molecule represents van der Waals type of interaction between them. Aside from NO, F2, Cl2 and HF system, in all other cases, the contribution from dispersion term obtained in the EDA analysis is found to be maximum, towards the strength of binding energy implying the van der Waals type interaction present in the host-guest system. The F2 and Cl2 molecule interacts with the Au atom center of PPX via partial covalent bonding interaction and consequently, the interaction is predominately supported by orbital and electrostatic interaction. Similarly, in NO molecule, the major electrostatic contribution arises by effective π back-bonding interaction while in HF, the presence of strong attractive hydrogen bonding interaction resulting equal contribution of electrostatic, dispersion and orbital contribution to the total binding energy. The GCMC simulations carried out at 273, 298 and 340 K portray the maximum gas adsorption is observed for H2S, CS2, NO2, and HBr molecules over 0.1 to 10 bar pressure from their respective congeners. A contrary observation for Cl2 and Br2 is perceived where dominant adsorption of Cl2 molecule is observed initially and the rise of temperature reinforces the adsorption of Br2 while F2 adsorption is lower among the di-halogens. The adsorption of CS2 increases with increasing temperature and pressure. The adsorption of gas molecules at 77 K is contrary to 298 K. A higher isosteric heat of adsorption (qst) value is observed for the guests like H2S, CS2, NO2, Br2, and HBr prevails the molecule strongly bound within the PPX cavity. The CS2 molecule qst is higher among all, due to the strong attractive interaction. Hence, it is ideal to assume CS2, Cl2, Br2, and H2S can be effectively adsorbed and selectively desorbed from the gaseous mixture.


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Supporting Information Includes physical parameter of pristine guest and encapsulated guest, thermodynamic properties, BCP vales, EDA analysis, NCI-RDG plot, GCMC adsorption isotherm plot etc.

Acknowledgments The author thanks the CCMS, Institute for Materials Research, Tohoku University, Japan and CDAC Pune, India for providing supercomputing facility. We acknowledge the department of science and technology (DST), India for a research grant, project No. SB/S1/PC-047/2013. References 1. Kirchon, A.; Dey, G. S.; Fang, Y.; Banerjee, S.; Ozdemir, O. K.; Zhou, H. C. Suspension Processing of Microporous Metal-Organic Frameworks: A Scalable Route to High-Quality Adsorbents. Science 2018, 5, 30-37. 2. Gelfand, B. S.; Huynh, R. P. S.; Collins, S. P.; Woo, T. K.; Shimizu, G. K. H. Computational and Experimental Assessment of CO2 Uptake in Phosphonate Monoester Metal-Organic Frameworks. Chem. Mater. 2017, 29, 10469-10477. 3. Evans, A.; Luebke, R.; Petit, C. The Use of Metal-Organic Frameworks for CO Purification. J. Mater. Chem. A 2018, 6, 10570-10594. 4. Wales, D. J.; Grand, J.; Ting, V. P.; Burke, R. D.; Edler, K. J.; Bowen, C. R.; Mintova, S.; Burrows, A. D. Gas Sensing Using Porous Materials for Automotive Applications. Chem. Soc. Rev. 2015, 44, 4290-4321.


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Half-Sandwich Iridium- and Rhodium-based Organometallic

Architectures: Rational Design, Synthesis, Characterization, and Applications. Acc. Chem. Res. 2014, 47, 3571-3579. 59. Agarwal, R. A.; Mukherjee, S.; Sanudo, E. C.; Ghosh, S. K.; Bharadwaj, P. K. Gas Adsorption, Magnetism, and Single-Crystal to Single-Crystal Transformation Studies of a Three-Dimensional Mn(II) Porous Coordination Polymer. Cryst. Growth Des. 2014, 14, 5585-5592. 60. Chaudhary, A.; Mohammad, A.; Mobin, S. M. Recent Advances in Single-Crystal-to-SingleCrystal Transformation at the Discrete Molecular Level. Cryst. Growth Des. 2017, 17, 28932910. 61. Huang, S. L.; Hor, T. S. A.; Jin, G. X. Photodriven Single-Crystal-to-Single-Crystal Transformation. Coord. Chem. Rev. 2017, 346, 112-122. 62. Kang, P.; Mai, H. D.; Yoo, H. Cage-like Crystal Packing through Metallocavitands Within a Cobalt Cluster-Based Supramolecular Assembly. Dalton Trans. 2018, 47, 6660-6665. 63. Janczak, J.; Prochowicz, D.; Lewinski, J.; Jimenez, D. F.; Bereta, T.; Lisowski, J. Trinuclear Cage‐Like ZnII Macrocyclic Complexes: Enantiomeric Recognition and Gas Adsorption Properties. Chem. Eur. J. 2016, 22, 598-609.


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64. Coronado, E.; Marqués, M. G.; Espallargas, G. M.; Rey, F.; Yrezábal, I. J. V. Spin-Crossover Modification Through Selective CO2 Sorption. J. Am. Chem. Soc. 2013, 135, 15986-15989. 65. Poater, J.; Gimferrer, M.; Poater, A. Covalent and Ionic Capacity of MOFs To Sorb Small Gas Molecules. Inorg. Chem. 2018, 57, 6981-6990. 66. Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Coinage Metal-N-Heterocyclic Carbene Complexes. Chem. Rev. 2009, 109, 3561-3598. 67. Altmann, P. J.; Pöthig, A. Pillarplexes: A Metal-Organic Class of Supramolecular Hosts. J. Am. Chem. Soc. 2016, 138, 13171-13174. 68. Li, X.; Zhu, L.; Xue, Q.; Chang, X.; Ling, C.; Xing, W. Superior Selective CO2 Adsorption of C3N Pores: GCMC and DFT Simulations. ACS Appl. Mater. Interfaces 2017, 9, 3116131169. 69. Sweatman, M. B.; Quirke, N. J. Gas Adsorption in Active Carbons and the Slit-Pore Model 2: Mixture Adsorption Prediction with DFT and IAST. Phys. Chem. B 2005, 109, 1038910394. 70. Liu, B.; Smit, B. Comparative Molecular Simulation Study of CO2/N2 and CH4/N2 Separation in Zeolites and Metal-Organic Frameworks. Langmuir 2009, 25,5918-5926. 71. Venkataramanan, N. S.; Suvitha, A. Structure, Electronic, Inclusion Complex Formation Behavior and Spectral Properties of Pillarplex. J. Incl. Phenom. Macrocycl. Chem. 2017, 88, 53-67. 72. Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864-B871.


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Table 1. The computed binding energy (ΔE), Basis set superposition error (BSSE) and zero point energy (ZPE) corrected binding energy, Deformation energy of host and guest and change in free energy (ΔG298) for each of the complex system. The energy units are given in the kcal/mol. Gas Molecules

Binding Energy (ΔE)

BSSE Corrected (ΔE)

ZPE Corrected (ΔE)

Strain Energy (Host)

Strain Energy (Guest)

ΔG298

H2

-4.7

-3.5

-3.8

0.4

1.3

1.9

H2O

-17.7

-12.6

-16.8

0.5

1.3

-7.2

H2S

-17.5

-17.0

-16.6

0.2

5.5

-8.9

CO

-13.0

-9.5

-12.1

0.8

0.5

-5.1

CO2

-17.6

-14.2

-16.7

0.4

0.5

-9.0

CS2

-21.9

-22.6

-21.0

0.4

6.6

-10.0

N2

-14.4

-9.8

-13.5

0.2

0.6

-7.6

NO

-13.5

-9.6

-12.5

0.1

0.9

-5.4

NO2

-17.6

-12.2

-16.7

0.6

1.2

-7.6

F2

-22.5

-18.7

-21.5

2.3

14.8

-12.7

Cl2

-20.3

-24.0

-19.4

0.6

15.9

-9.1

Br2

-30.7

-31.4

-29.8

2.3

0.8

-19.6

HF

-15.8

-9.9

-14.9

0.5

0.8

-6.9

HCl

-15.2

-14.1

-14.2

0.4

6.3

-6.8

HBr

-21.7

-19.2

-20.8

0.4

0.7

-12.7


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Figure 1. Optimized geometry of guest encapsulated PPX. The bond length and bond angles of the guest molecules as well as the bond length of Au-Au.


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Figure 2. A graphical representation of (a) polarizability and Au-Au bond length and (b) Change in entropy (∆S298) and enthalpy (∆H298) of each guest incorporated host complex.


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Figure 3. A graphical approach of guest separation in presence of other gas mixture by computing ∆G298 values.


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Figure 4. Noncovalent (NCI) isosurfaces of guest entrapped PPX host with an isosurface value of 0.5 a.u.


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Figure 5. Energy components such as electrostatic, dispersion, orbital interaction and Pauli’s repulsion against the total binding energy of the host-guest complex.


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Figure 6. A two-dimensional view of simulating cells showing the free volume of PPX and the density distribution profiles of gases absorbed on PPX obtained from the GCMC simulation using 2х2х2 supercell at 298 K and 10 bar pressure.


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Figure 7. GCMC simulation of each H2S, CS2, NO2, Br2, HBr and their respective congeners at 298 K temperature and 0.1 10 bar pressure.


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Figure 8. Isosteric heat of adsorption of each host-guest system at 298 K temperature and 0.1 10 bar pressure.


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Figure 9. Ideal adsorption solution theory (IAST) of each host-guest complex at 298 K temperature.


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TOC Graphic


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Sustainable Metallocavitand for Flue Gas-Selective Sorption: A

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