Claims:The following claims specify the scope of the invention:

Claim:
1. For CO2 capture efficiency, the adsorption performance of pore expanded MOF structures comprising:
a) CO2 sorption ability of Ni-MOF-74 analogues by using quantum-chemical calculations and GCMC simulations.
b) Ni-SPP, Ni-BPP and Ni-TPP systems have exhibited excellent CO2 uptake capacities, 8.2, 11.5 and 11.8 mmol/g respectively at 1 bar pressure along with the remarkable selectivity for CO2 over CH4 and N2.
a) The computed isosteric heat of adsorption is found to be 47, 44 and 41 kJ/mol for Ni-SPP, Ni-BPP and Ni-TPP respectively.
2. According to claim 1, the sixth coordination of Ni with water molecule, enhanced the CO2 sorption capacity by 3 mmol/g due to cooperative electrostatic interactions.
3. According to claim 1, the framework can be substantially purged of adsorbed CO2 by exposing it to a removal flow pressure at 1 bar.
4. According to claim 1, the CO2 adsorption-desorption isotherms of hydrated MOFs are well fitted with Langmuir-II adsorption isotherm.
5. According to claim 1, the pore expanded MOFs with CUS sites have exhibited over 80% selectivity for CO2 in presence of CH4 and N2. , Description:Field of Invention
The current invention pertains to design and evaluate the CO2 capture efficiency of new pore-expanded metal−organic frameworks. The whole scheme concentrates on the adsorption efficiency of MOFs with varies pore sizes, functionality of open metal centres and ligands of metal-organic frameworks. By modifying the inorganic node or organic linker via the pore-engineering idea, the incoming CO2 gas molecules are accommodated. Further, the physisorption and diffusion of gas molecules through pores/channels of MOF adsorbents was found to be significant cooperative interactions which reduces the energy needed for gas regeneration process.

Background of the Invention
The current increasing trend of global temperatures, which is 1.5°C higher than the preindustrial period is extremely alarming, because of its negative impacts on ecosystems, biodiversity, food security, cities, tourism etc. across the world (O. Hoegh-Guldberg et al.2018). Intergovernmental Panel on Climate Change (IPCC) efforts to reach the target of zero emissions by 2050 is indeed a challenging task to combat the climate change (weforum.org, Ed. 2019). The steady accumulation of greenhouse gases in the atmosphere, such as CH4, N2O, and CO2, is the primary cause of global warming and as result for the climate change (Qu et al.2017). Among the greenhouse gases, CO2 emission is the highest, accounting for nearly 75%, from fossil fuel-based industries contribute to about 40% emissions of all the global anthropogenic CO2 emissions (PM Eisenberger et al.2009). Extensive research work is going on globally, to address the challenges associated with the traditional combustion methods for the removal of CO2, either by developing alternative cleaner combustion techniques or by with carbon capture, utilization, and storage (CCUS) approaches or by switching over to the production of cleaner renewable energy, (S Sgouridis et al.2019.) However, there is wide gap between the supply from renewable resources and the energy demand. India is the third largest CO2 emitting country in the world and can mitigate ~1000 Mt of CO2 per year if the cost of filtering out CO2 from today’s ~Rs10,000 per ton comes down to minimum of ~Rs500 per ton to meet the target of ~35% reduction in CO2 emissions by 2030 (M Muntean et al. 2018). In CCUS process, the most expensive part is the selective separation and capture of CO2 from the mixture of gases prevail in the flue gas streams and the process account for ~70% of the total cost. This necessitates the scientific community to develop a cost-effective, sustainable CO2 capture technology. Currently, PCC of CO2 using SAs is considered as promising alternative to the conventional liquid amines, owing to its moisture tolerance, regenerability, stability and cost-effective features. MOFs with open metal sites are considered for CO2 adsorption properties in this invention due to their superior adsorption properties.
From literature, there are many liquid amines (CA2810425C) and solid adsorbents (US8841471B2) explored for CO2 capture, but many of the studied materials did not reach the industrial scale. The drawbacks of using liquid amines as CO2 capturing solvents, (regeneration of the solvent, parasitic energy, and solvent degradation) and to overcome the low adsorption capacities, stability of solid adsorbents is a challenging task for researchers to develop fewer energy penalties and low-cost technologies. Adsorption-based processes seem to be more promising. Many factors contribute to the CO2 adsorbents' practicability, including lower energy penalty, waste heat recovery, availability, cost, long life, fast adsorption/desorption kinetics, large adsorption capacity, diffusion, regeneration ability, selectivity, stability, thermal and mechanical strengths, and so on.
In energy and environmental applications, porous materials have been discovered as promising adsorbents, including carbon capture. For instance, US20170113204A1 discloses mechanism of capture of CO2 in metal organic frameworks from air for industrial purposes. The prior art., US8841471B2 discloses the interaction mechanism of open metal sites with CO2. MOFs displays potential aids over other solid adsorbents in terms of have attracted significant interest since their composition (i.e., chemical functionality) and structure (e.g., pore topology and sizes and internal surface areas) in CO2 separation, due to its low vapor pressures, resulting in reduced solvent loss. WO2002088148A1, discloses the design of porous structures with functionality and pore size. In this invention using pore engineering concept is considered to replace Mg with Ni to attain the stability in presence of water and for efficient CO2 capture.
As a result, creating a method /sorbent for selective CO2 capture that is efficient, stable, and cost-effective is a critical field of research. The present invention overcome these problems observed in the prior art by and discloses here in the mechanism of CO2 with solid adsorbents i.e., MOFs is considered for above mentioned drawbacks. The objective of present invention is to ensure new stable sorbents for efficient binding of CO2 in presence of moisture. To estimate the selectivity for CO2 over CH4 and N2 gases to separate the pure component from mixtures in fuel-based industries

Summary of the Invention
Considering the above-mentioned drawbacks in the prior art, the present invention aims to design a new analog of MOF-74 with Ni open metal center, to attain the stability of metal ions in presence of water.

These new MOFs have displayed enhanced CO2 sorption compared to parent MOF-74 and demonstrated excellent selectivity for CO2 over other gases, CH4 and N2 as established by Ideal Adsorbed Solution Theory (IAST).

A further specific objective of the invention is to estimate the moisture tolerance of designed MOFs and its regenerability, stability and cost-effective features while operation itself in a safe and reliable manner.

Brief description of drawings:
Figure 1 Pictorial representation of pore expanded Ni-MOF-74 analogues

Figure 2 a) Cluster model of Ni2 (DOBDC)4, b) Binding configurations of H2O and CO2 at ‘M’ and ‘L’ sites of Ni2(DOBDC)4 cluster, c) Co-adsorption configurations CO2 on pre-adsorbed H2O

Figure 3 Adsorption and desorption isotherms

Figure 4 Hydrated geometries of Ni-SPP, Ni-BPP and Ni-TPP

Figure 5 a) CO2/CH4 selectivity and b) CO2/N2 selectivity

Detailed Description of the Invention
As described above, the present invention targets to design and evaluate the MOFs with open metal sites to determine its CO2 adsorption efficiency and selectivity. The periodic structures of three MOF analogs Ni-SPP, Ni-BPP and Ni-TPP increases pore size from 11, 17, and 23 Å respectively shown in figure 1b. Identified the metal and ligand centers of cluster model is shown in figure 2a. and evaluated the nature and strength of CO2 and H2O binding at these two prominent sites, metal (M-site) and linker (L-site) sites of cluster model shown in figure 2b. and Co-adsorption configurations of CO2 on pre-adsorbed H2O at M and L sites of Ni2(DOBDC)4 cluster is shown in figure 2c. The obtained adsorption-desorption isotherm graphs of CO2 in Ni-SPP, Ni-BPP and Ni-TPP fitted to Langmuir type-II model are shown in figure 3. The effect of the influence of water on CO2 binding interactions in Ni-SPP, Ni-BPP and Ni-TPP is shown in figure 4. The effect of selective adsorption of CO2 in MOF analogues over CH4, N2 gases proved using Ideal Adsorbed Solution Theory shown in figure 5a and 5b.
Infinite systems - Periodic models: In this invention, a series of MOF-74 type analogues are considered to evaluate their gas adsorption performance. The periodic single crystal structures are taken from XRD (J Zheng et al. 2017). Isoreticular M-MOF-74 analogues known as M2 (Ligand)-74, incorporating five metals, (where, M= Ni, and three ligands, (where, Ligand=SPP, BPP and TPP) are chosen to tune the flexibilities of the frameworks using pore engineering concept. The pore size varies from 11, 17 and 23 Å, as shown in the figure1b. A systematic change of the length of conjugated aromatic ligands, 1, 2, and 3 phenylene rings, namely, M-MOF-74-SPP (SPP= 2,5-dihydroxy1,4 benzene dicarboxylic acid, single phenyl with para-COOH), M-MOF-74-BPP (BPP = 3,3’-dioxido-4,4’-biphenyldicarboxylate, bi phenyl with para-COOH), and M-MOF-74-TPP (TPP = 3,3’-dioxido-4,4’-triphenyldicarboxylate, triphenyl with para-COOH). The periodic models with primitive (R3) unit cell of three MOFs (figure 1a) are initially optimized to reduce the computational time and then converted to a conventional hexagonal cell as shown in figure 1b to carry out further GCMC simulations (A Kundu et al.2016).
Finite systems - Cluster models: For understanding the gas-sorbate interactions at the molecular level, a small fragment, which is a repetitive unit of large-sized MOF is a suitable model to carry out the calculations (H Kim et al. 2019). Hence a cluster model of Ni2 (2,5-dihydroxy1,4 benzenedicarboxyalic acid)4 or Ni2(DOBDC)4 as shown in Figure 2, is chosen in this Invention. The key structural characteristic of this model is that it has a nickel paddlewheel with a Ni−Ni distance of 2.362 Å and each nickel atom is coordinated with four oxygen atoms of organic linkers. Potential binding sites for both H2O and CO2 are identified as site M, L shown in the Figure 2b.
DFT calculations: Ab initio calculations are performed using the Cambridge Sequential Total Energy Package (CASTEP) module of Materials Studio (SJ Clark et al. 2005) to optimize the structures of SPP, BPP and TPP. Generally, flue gases emitting from the most of industries comprises water vapor along with other gases. Moreover, coordinatively unsaturated site (CUS) of MOFs are more susceptible for binding with water molecules. Therefore, it is important to investigate both the hydrated and pristine MOF structures i.e., MOF with pre-bound water molecule to Ni atom as sixth coordinate and without the water molecule. Optimizations using the GGA, PBE functional (JP Perdew et al. 1996) with PBE+TS dispersion correction (A Tkatchenko et al. 2009, N Marom et al. 2011). BFGS algorithm (DC Liu et al.1989) is used in the optimization procedures until the force on every atom reached to 0.03 eV/Å and self-consistent energy reached to 1.0e-6 eV/atom. The energy cut-off of the plane-wave basis set to 340 eV and ultrasoft pseudopotential (D Vanderbilt et al. 1990) using at gamma 𝝉-point. All calculations include spin-polarization, with high-spin state for pristine and with low-spin state for hydrated structures.
GCMC simulations have been carried out by using Sorption Module of Materials Studio 2019, to estimate the CO2 adsorption capacity of Ni-MOF-74 analogs. The chemical potential, volume, and temperature are all kept constant in the Grand Canonical ensemble (H Eslami et al. 2007). Fugacities and quantum effects of CO2 adsorption are investigated. CO2 adsorption isotherms are obtained at 298 K for both pristine and hydrated MOFs at 1 bar pressures, using Metropolis algorithm and UFF force field (AK Rappe et al.1992) Ewald summation is used to calculate the long-range electrostatic interactions within the accuracy of 0.001 kcal/mol and van der Waals interactions with cut off 12.5 Å are described by Lennard-Jones potentials (TEIII Cheatham et al.1995).The simulations include 1 x105 cycle equilibration steps and 1 x 106 cycles production steps for each interval of pressure. In addition to CO2 adsorption capacity, the selectivity of CO2 in presence of CH4 and N2 has also been investigated. The CO2 uptake capacities are calculated by the equation (1). The selective adsorption behavior of CO2 in binary mixtures is established by ideal adsorbed solution theory (IAST) (DW Hand et al. 1985) in equation (2). The IAST approach has already been proven to be accurate for adsorption of a wide range of gas combinations in various solid adsorbents, as well as CO2 capture inside metal–organic frameworks.

Here, in Eq. 1, molecular weight is for the framework, where in Eq. 2, q1 and q2 are the amount of adsorbed component 1 and 2, the partial pressures of components 1 and 2 are p1 and p2, respectively.

The optimized structures of pristine Ni-SPP, Ni-BPP and Ni-TPP chosen in this invention are shown in figure 1 and the corresponding optimized hydrated structures with CO2 loading are shown in figure 4. GCMC simulations for all three MOFs have been carried out at 1 bar pressure and 298 K temperature. In force-field-based GCMC simulations, atomic charges of MOFs structural atoms play a decisive role in predicting accurately the solid-gas interactions which are mainly driven by electrostatic potential difference. The atomic charges are taken from quantum mechanical molecular electrostatic potentials ESP (J Zheng et al.2020). Since the ab initio-based ESP charges are found to describe the electrostatic interactions between gas molecules and adsorbents more accurately.

Figure 3 shows the adsorption-desorption isotherms of CO2 at 298 K and 1 bar pressure. The Langmuir model is used to fit the isotherms; all three MOs exhibit Langmuir type-II ads-/des-sorption isotherms. For all three MOFs, desorption curves are almost completely coinciding with the corresponding adsorption curves, indicating the rigidity of the pore structure. The steep slope for Ni-SPP indicates faster CO2 uptake at lower pressure of 0.1 bar. Nevertheless, for Ni-BPP and Ni-TPP, it is gradually gets saturated at partial pressure of 0.6 bar. Apart from the knowledge of CO2 uptake capacity, selectivity of the MOFs is essential for evaluating the efficiency of the adsorbent. As shown in figure 5 all three MOFs exhibit good selectivity for CO2 over CH4 and N2.

The major interactions prominently visible between CO2 and the framework can be summarized as follows. The most favorable binding sites for CO2 in pristine MOFs is with the metal center showing a tilted angle of 136.39. The oxygen atom of CO2 is interacting with coordinatively unsaturated sites of pristine MOF. However, the carbon atom of CO2 interacts with oxygen of O (H2O) in hydrated MOFs as shown in Figure 4. CO2 loading in the hydrated structures can be visualized in Figure 5.3. As clear from this figure at each hydrated metal site, two CO2 molecules interact similarly in all the three MOF structures. Therefore, enhanced CO2 uptake observed in Ni-TPP is attributed to other binding sites (vide infra). The strength of the CO2 binding is increased by hydration of CUS site, and this effect can be clearly seen from increased isosteric heats values of hydrated MOF structures. Further to compare the influence of water coordination to Ni atom, CO2 uptake and Qst are calculated for pristine and hydrated MOFs.

For molecular level understanding of CO2 binding to MOFs framework, Ni2 (2,5-dioxido-1,4-benzenedicarboxylate)4 (J Borycz et al. 2014) cluster models have been prepared and optimized using DMOl3 module (B Delley et al. 2000) of Materials Studio. The PBE functional with DNP (B Delley et al. 1990) basis set is used to evaluate the nature and strength of CO2 and H2O binding (K Tan et al. 2015) at two prominent sites, metal (M-site) and linker (L-site) sites of the cluster models. The results showed that the unsaturated metal site is the primary binding site, whereas the aromatic ring of linker is the secondary binding site. PBE+TS dispersion correction functional has been included to account dispersion dominated interactions by doing single point calculations (A Tkatchenko et al.2012) The binding energies (BEs) were calculated by the following equations (3-5)

+ 3)
+ 4)
+ + 5)
Here in Eq. 3, EBE(H2O) represents the total energy of cluster+H2O complex, E(cluster) , E(H2O) are the energies of individual cluster and free H2O molecule respectively Similarly, in equations 4, 5 are the BEs of CO2, co-adsorption of CO2. The geometries are depicted in Figures 2b with their binding energies. As evident from the Figure 2b, 2c binding of H2O and CO2 at metal center is stronger than at linker site. It is impressive to see the enhanced binding of CO2 in presence of water molecule, which again substantiate the improved sorption properties of hydrated MOFs than the pristine MOFs.
5 Claims & 5 Figures