Nadhem Missaoui

Amani Chrouda

Hamza Kahri

Andrew James Gross

Mohammad Rezaei Ardani

Ai Ling Pang

Mohsen Ahmadipour
email: mahmadipour@ukm.edu.my

Pang Ai Ling

PEG-templated synthesis of ultramicroporous n-ZIF-67 nanoparticles with high

Keywords: Gas Adsorption, Equilibrium, Carbon dioxide, Methane, Specific Surface Area, Zeolitic Imidazolate Framework, Selectivity Gas Adsorption, Equilibrium, Carbon dioxide, Methane, Specific Surface Area, Zeolitic Imidazolate Framework, Selectivity

come

Introduction

One of the biggest issues facing the world community is climate change brought on by global warming. The generation and accumulation of greenhouse gases in the atmosphere have mostly been caused by the emission of gases like methane (CH4), carbon dioxide (CO2), and nitrogen oxide (NO2), and especially CO2 [1][2][3]. The major industrial technologies used over the past few decades for separating and capturing CO2 and CH4 are, in general, based on absorption by solvents, membrane separation, and adsorption methods [4][5][6].

Adsorbents such as activated carbon and zeolites are among the most effective materials for gas separation in the chemical industry. Thanks to their large specific surface areas and pore volumes, crystalline porous metalorganic frameworks (MOFs) show great performance for gas adsorption and storage applications. MOFs are comprised of unique networks of metal ions or clusters that are coupled to organic ligands that give rise to unprecedented chemical and structure tunability. Numerous MOFs have been formed and investigated to date for CO2 capture, CH4 storage, and gas mixtures separation [7,8]. MOFs can selectively adsorb gases owing to their unique pore sizes and geometries, as well as functional groups (e.g. via electrostatics) within the pores. Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that have been used extensively and have a tetrahedral network structure that is in many ways similar to zeolites, but with the important difference that they are constructed using transition metals associated with imidazolate ligands [9]. ZIFs, notably ZIF-67, have unique chemical, thermal, and water stabilities, that makes them promising candidates for industrial adsorption applications [10,11]. Thanks to their high specific surface and pore geometries and chemistries, ZIFs are suitable adsorbents for CO2 and CH4 (Table 1) [12][13][14][15][16][17][18][19][20][21][22][23].

Several methods have been reported for the production of ZIF-67 with tunable porosity and morphology, such as sol-gel, solvothermal, microwave/ultrasound-assisted, and surfactant-assisted methods [24][25][26].

These methods permit the preparation of highly crystalline ZIF-67 with different crystallite sizes and porosities with some control, depending on factors such as synthesis time, temperature, and solvent [22,24,27]. Many require reaction times of several hours to days due to slow coordination interactions and discontinuities. For example, ZIF-67 crystals are typically obtained after 12 to 72 h using solvothermal methods at temperatures between 50-200°C [25]. It is therefore desirable to not only discover high-performance adsorbent materials, but also to discover materials that can be easily, economically and ecologically obtained, with a high level of control over crystal size and morphology (including phase purity). Nanocrystalline ZIF-67 with controlled morphology and particle size can be prepared using a polymer or surfactant template [19,23]. The use of a template can prevent the formation of nonuniform agglomerates with large variations in sample size and morphology as well as larger particles [22]. Room temperature methods capable of generating ZIF-67 in a few hours are also desirable [28]. The diverse synthesis possibilities to obtain ZIF-67 with attractive chemical and physical properties has resulted in its use as a great material for catalysis [29], separation [30], and volatile organic compounds (VOCs) adsorption [31] applications.

Nevertheless, the application and advantages of ZIF-67 for gas adsorption and separation have been rarely studied (see Table 1 for a detailed literature summary).

In this study, we report the facile and fast synthesis of ZIF-67 nanoparticles (NPs) in just a few minutes using N, N-dimethylformamide (DMF) as the solvent and polyethylene glycol (PEG) as a soft template. The use of PEG with a mild reaction temperature avoids the need for conventional high temperature solvothermal and microwave methods, while promoting the formation of homogeneous and stable ZIF-67 dispersions. To the best of our knowledge, the use of PEG as a template has not been reported previously for ZIF-67 synthesis. PEG was chosen over other polymer templates since it is low cost and has the capacity to generate a homogenous mixture comprising metallic salts thanks to its attractive solubility properties. During MOF synthesis, the polymer is considered to adsorb onto the metal surface and alter the growth kinetics of colloids, allowing for the uniform growth of hexagonally-shaped particles [19,23]. In these former examples, poly (diallyldimethylammonium chloride) and PEG were used for the production of ZIF-67 and ZIF-8, respectively. Following successful synthesis and characterization of PEG-templated ZIF-67 nanoparticles, we report here the gravimetric adsorption studies of CO2, CH4, and N2 using PEG-templated n-ZIF-67 as well as PEG-templated n-ZIF-8 as adsorbents. The adsorption/desorption isotherms were achieved at temperatures of 273, 298, 323, and 353 K, at low and high pressures, and considered together with Langmuir modelling. Real-time dynamic breakthrough experiments of PEG-templated ZIF-67 for CO2/N2, CO2/CH4 and N2/CH4 mixtures are also reported.

Material and Methods

Synthesis of PEG-templated ZIF-8 nanoparticles (n-ZIF-8)

All the materials in this study were purchased from Sigma-Aldrich and were employed as received, including cobalt nitrate hexahydrate (Co(NO3)•6H2O; 99 %), 2-methylimidazole (C4H6N2: mIm; 99 %), N,Ndimethylformamide (DMF; 99%), polyethylene glycol (PEG, average molecular weight = 20.000 g.mol -1 ), triethylamine (TEA; (C2H5)3N; 99%), and methanol (99 %). The microporous PEG-templated ZIF-8 (Zn(Im)2) nanoparticles were obtained in accordance with the procedures of our recently reported room-temperature method [23]. The ZIF-8 material was activated under vacuum at 423 K for 12 h prior to gas adsorption measurements.

Table 1 summarizes the textural properties of ZIF-8 from our previous study.

Synthesis of PEG-templated ZIF-67 nanoparticles (n-ZIF-67)

ZIF-67 NPs were synthesized in a similar manner to the previous publication with some modification [23], i.e., Co(NO3)2•6H2O and mIm were used as precursors with PEG as a soft template. The choice of PEG was encouraged by its ability to form a homogenous solution with metallic salts thanks to micellar and dispersion effects to promote the uniform growth of particles with reduced particle size and agglomeration. In the first step, the PEG powder (0.4 g) was dispersed in 10 mL of DMF; Co(NO3)2•6H2O (1.4551 g, 5 mmol) was then added with slight stirring for 2 min at 298 K to form the Co-solution. Later, 2-methylimidazole (2.0525 g, 25 mmol) was dispersed in DMF (10 mL); TEA was used as a base and slowly added (6.75 mL) with stirring over 3 min to obtain the linker-solution. After complete dissolution, the Co-solution was mixed with the linker-solution and stirred for 5 min at room temperature. The purple precipitates were filtered, washed with ethanol to remove possible undissolved reagents including PEG, then collected via centrifugation (4000 rpm, 10 min). Finally, the obtained purple powder was dried in a vacuum oven at 423 K for 24 h to achieve the final product, ZIF-67 NP (herein referred to as n-ZIF-67). Scheme 1: Room temperature synthesis reaction to obtain PEG-template ZIF-67 (n-ZIF-67).

Characterization

FTIR spectroscopy was performed using a Perkin Elmer Spectrum Two instrument with attenuated total reflectance (ATR) spectra recorded from 4000 to 400 cm -1 . X-ray powder diffraction spectra (XRD) from 5° to 60° were recorded on a Bruker D8 Discover diffractometer with a scanning rate of 2°/min. The morphology and elemental composition of n-ZIF-67 was determined by transmission electron microscopy (TEM, Philips CM200) combined with an energy-dispersive X-ray spectrometer (EDX). The particle size analyses were determined using Image J software. Thermogravimetric analysis curves (TGA) were measured on a Mettler Toledo STARe apparatus by heating the samples from 298 K up to 973 K at a heating rate of 2°C/min under an air atmosphere. At 77 K, nitrogen adsorption-desorption isotherms (Micrometrics ASAP-2420 instrument; sample degassing at 423 K for 24 h under vacuum) were collected to determine the Brunauer-Emmett-Teller (BET) specific surface area (P/P° ranging from 0.01 to 0.35), total micropore volume (VP), and pore size distribution (t-plot method).

CO2, CH4, and N2 adsorption

Pure component adsorption isotherms of CO2, CH4, and N2 at n-ZIF-67 and n-ZIF-8 were obtained by gravimetric analysis (Autosorb-iQ-MP analyzer) in the range of 0-40 bar at different temperatures: 273, 298, 323 and 353 K.

The samples were degassed at 423 K under a high vacuum for 24 hours prior to the sorption measurement. The CO2 used was of high purity (99.99%) and obtained from Sigma-Aldrich. The gases used were all high purity (99.99%) and obtained from Sigma-Aldrich. Numerous adsorption kinetic models were employed to define the adsorption equilibrium isotherm data. In this work, the Langmuir adsorption kinetics model (Eq. ( 1)) was used to fit the equilibrium isotherm data [32]. This model was used to describe the monolayer pattern of the adsorbed layer on the homogeneous surface. The Langmuir equation can be expressed by the equilibrium adsorption capacity (q, mg/g), maximum adsorption capacity (qm, mg/g), Langmuir isotherm constant or a parameter related to the affinity constant between adsorbate and adsorbent (KL, L/g), and partial pressure (P, bar) as Eq. ( 1). q = q m k L P 1+k L P Eq. ( 1)

Results and discussion

Structural analysis

The FTIR spectrum of the PEG templated n-ZIF-67 recorded between 400 and 4000 cm -1 is shown in Fig. 1. The n-ZIF-67 structure contains the ligand, mIm, which is responsible for most of the band characteristics. Precisely, the bands that appeared between 600 and 1500 cm -1 are ascribed to bending and stretching of the imidazole ring, such as the bands at 1145 and 1308 cm -1 , for the bending signal, and bands at 1415 and 1433 cm -1 , for the stretching vibrations [31,33]. The bands at 994 and 750 cm -1 can be attributed to the bending vibrations of C-N and C-H, respectively, while the band at 694 cm -1 can be attributed to the bending variation of the mIm ring. The absorption band at 1637 cm -1 is due to the C=C stretch mode, whereas the absorption band at 1584 cm -1 is attributed to the stretching mode of C=N. The two small absorption bands at 2929 and 3136 cm -1 can be accredited to the asymmetric stretching modes of the aliphatic C-H and the aromatic ring of mIm, respectively. Interestingly, the Fig. 1. FTIR spectrum of n-ZIF-67.

As was mentioned in Section 2.2, n-ZIF-67 NPs were rapidly synthesized in minutes. The material was isolated in about 1 hr with the synthesis process occurring at room temperature and pressure. The method is rapid but also easily scalable, although with the notable downside that DMF and methanol solvents are used. The X-ray powder diffraction method was used for analyzing the chemical composition of as-produced n-ZIF-67 NPs. respectively [36]. The XRD diffraction pattern with Rietveld refinement in Fig. 2 represents the common peaks of ZIF-67 and matches well with the simulated ZIF-67 diffractogram [36,37]. Crystallite sizes were calculated using the Debye-Sherrer equation (Eq. 2) [38][39][40]:

L = Kλ βcosθ Eq. ( 2
)
Where L is the diameter of the crystallites (nm), K is the numerical Scherrer constant (0.93 Å), λ is the X-ray wavelength (1.54 Å), β is the full width at half-maximum intensity (FWHM) in radians, and θ is the Bragg diffraction angle. The crystallite size of n-ZIF-67 NPs was first determined using the Debye-Scherrer equation, equal to about 78 nm. The refined XRD structural parameters, calculated grain sizes, crystallite sizes and strains of n-ZIF-67 are collated in Table 2. Next, we considered the Williamson-Hall (W-H) method to determine the size for comparison with the Scherrer technique and TEM data. In the W-H method [41], the strain induced broadening arising from crystal imperfection and distortions is related by: ) tan( 4

    s Eq. (3)
Here, ε is the root mean square value of the micro-strain,

d d   
. Assuming that the particle size and strain contributions to line broadening are independent of each other and that both have a Cauchy like profile, the observed line breadth is simply the sum of the two:

) sin( 4 ) cos(        D k hkl Eq. ( 4
)
A plot is drawn with ) sin(  along the x-axis and ) cos(  hkl along the y-axis for as-prepared PEG-templated n-ZIF-67 (Fig 3 (a)). The slope and y-intersect of the fitted line represent the strain and the particle size respectively. We can remark that the calculated crystallite size using the W-H technique (DW-H = 91 nm) is larger than that calculated using Scherrer's technique (78 nm). This is due to the presence of strain which also contributes to the broadening of peaks. The grain sizes observed by TEM (see Fig. 3 and discussion below) are larger than those calculated by Scherrer's and W-H techniques. This can be explained by the fact that each particle observed by TEM is formed by several crystallized grains. The morphology of n-ZIF-67 was studied using TEM. According to Fig. 3 (b), the uniform rhombic dodecahedral morphology was observed for ZIF-67 NPs, in agreement with a previous study of ZIF-67 nanocrystals [42]. The perfect crystalline structure of the synthesized n-ZIF-67 NPs was further confirmed by the well-defined rhombic facet and the clearly visible edge of ZIF-67. In addition, the surfaces of the particles appear regular and smooth, and the particles do not appear to be aggregated. Further interpretation of the TEM images using ImageJ software revealed that the particles had a Gaussian distribution, ranging from 30 to 100 nm, (Fig. The EDX analysis data, as shown in Fig. 3 (d), clearly demonstrates the expected elemental characteristic peaks for C (0.26 keV), N (0.53 keV), and Co (1, 6.9 keV), confirming the successful synthesis of ZIF-67 NPs.

Thermal analysis

Thermal stability analysis of n -ZIF-67 NPs was conducted based on TGA curves. Fig. 4 depicts the weight loss rate and weight loss profiles. It is evident that a two-stage weight loss occurred at n-ZIF-8. The first weight loss was detected between 300 and 680 K, related to the removal of physisorbed molecules (e.g. H2O, CO2, and methanol) from the framework, concomitant with several exothermic peaks. The second weight loss at 700 K is followed by a rapid loss and exothermic peaks, which can be associated with the decomposition of the ZIF-67 NPs.

The high thermal stability of ZIF-67 NPs, based on TGA curves, is consistent with previously reported data [34,43,44]. It was found that about 12 wt% of the initial weight of the powder remained after decomposition, which can be attributed to cobalt oxide, CoO, formation.

Nitrogen isotherm and pore size distribution

Considering the TGA analysis data in Section 3.2, n-ZIF-67 NPs were degassed at 423 K prior to N2 isotherm measurements. Fig. 5 displays the pore size distributions and N2 adsorption isotherms for n-ZIF-67. As shown in Fig. 5 (a), the adsorbent showed a N2 rapid uptake at a the very low relative pressure region (10 -5 < P/P° < 10 -2 ).

Based on the IUPAC classification, the sample had a typical type I isotherm [23,35,45] that confirms the microporosity of the material. As shown in Fig. 5, n-ZIF-67 NPs had reversible and reproducible N2 adsorption and desorption isotherms, emphasising a stable material with permanent porosity and good rigidity. The BET method was employed to measure the sample porosity at 0.001 < P/P0 < 0.35. The BET surface area (SBET) of the PEG-templated n-ZIF-67 NPs was 1871 m 2 /g with a micropore volume of 0.86 cm 3 /g, which are higher than previous values reported for ZIF-8 (including PEG-templated ZIF-8) and ZIF-67, as summarized in Table 1, highlighting the attractive adsorption potential of the new material reported here [23,24]. The total pore volume (Vmicro) of n-ZIF-67 was 0.86 cm 3 /g at P/P° = 0.99. Fig. 5 (b) illustrates that the predominant pores of the MOF are 0.64 nm in diameter (ultramicropores). Such ultramicropores are expected to favor the adsorption of CO2 molecules and eventually their preferential adsorption compared to N2 and CH4 molecules, based on geometric considerations (CH4 (0.38 nm) > N2 (0.364 nm) > CO2 (0.33 nm)) [46].

CO2, CH4 and N2 adsorption

The CO2, CH4, and N2 adsorption isotherm performance at n-ZIF-67 and n-ZIF-8, with Langmuir adsorption model fitting, was subsequently investigated at low pressure (0 to 100 KPa/0 to 1 bar) and high pressure (0 to 40,000 KPa/0 to 40 bar) at different temperatures (273, 298, 323, and 253 K) after outgassing at 423 K. Nanoscale ZIF-67, due to its excellent textural properties including large porosity (microporosity and ultramicroporosity) and crystallinity, is an attractive candidate for the adsorption of gases such as CO2, CH4 and N2, and especially the electrophile, CO2, via interactions with its active adsorption sites within its pores [47,23]. The high selectivity of ZIFs for CO2 in gas mixtures, especially industrially important CO2/N2 and CO2/CH4 mixtures, is an attractive point that we endeavored to explore with PEG-templated n-ZIF-67. At low-pressure, between 0 and 100 KPa ) and at different temperatures between 273 and 353 K, the CO2 uptake of PEG-templated n-ZIF-67 increased quasilinearly with increasing pressure (Fig. 5 (a)). At 100 KPa CO2, the uptake did not reach saturation. According to the IUPAC classification, CO2 and CH4 exhibited a non-linear type I isotherm, which is in contrast to the N2 isotherms which showed a linear adsorption behavior (Fig. 5 (a, c, ande)). In contrast, at PEG-templated n-ZIF-8, all of the gas adsorption isotherms exhibited a linear adsorption behavior (Fig. 5 (b, d, andf). The n-ZIF-67 particles showed a higher adsorption capacity for all gases at 100 KPa compared to n-ZIF-8. At 298 K and 100 KPa, the adsorption capacity of CO2 at n-ZIF-67 was 111.89 mg (CO2)/g, which was higher than the 51.5 mg(CO2)/g recorded at PEG-templated n-ZIF-8 samples and ZIF-8 (Table 1) [20,23]. The higher CO2 capture capacity observed for n-ZIF-67 can be attributed, at least in part, to the combination of a higher BET surface area (1871 m 2 /g) and pore volume (0.86 cm 3 /g), and the ultramicropores below 0.7 nm (0.64 nm). Linked to the physical interactions of the free pore sites of ZIF-67 with CO2 molecules, [49,23], n-ZIF-67 adsorbed more CO2 at 273 K (850.52 mg(CO2)/g) than at higher temperatures such as 353 K (425.04 mg (CO2)/g. An analogous behaviour has been observed and defined previously for ZIF-8, ZIF-69, and ZIF-76 [23,49]. Additionally, the n-ZIF-67 material prepared here showed maximum adsorption capacities for CH4 and N2 of 40.72 mg(CH4)/g for CH4 and 23.02 mg(N2)/g for N2 at 298 K and 100 KPa (see Table 1). These adsorption capacities surpassour previous results with PEG-templated n-ZIF-8. This higher adsorption capacity for all three gases compared to n-ZIF-8 highlights the very promising physical characteristics of the newly synthesized n-ZIF-67 material. The adsorption isotherms of CO2, CH4, and N2 at both n-ZIF-67 and n-ZIF-8 at various temperatures (273, 298, 323, and 353 K) and high pressure (0-40 bar) were also considered (Fig. 7). Generally, CO2 and CH4 adsorption increases with elevated pressure and the uptake reaches saturation at both materials. The N2 uptake increases linearly with increasing pressure. The n-ZIF-67 samples showed significantly higher CO2 adsorption capacity compared to CH4 and N2 adsorption, and higher adsorption capacities for all gases compared to n-ZIF-8 [23]. The adsorption capacity of 681 mg (CO2)/g at n-ZIF-67 at 298 K and 40 bar CO2 is the maximum capacity reported for a ZIF-67 and ZIF-8 adsorbent, as far as the authors are concerned (Table 1). The highest adsorption capacity achieved in this study was 846 mg (CO2)/g at 40 bar for n-ZIF-67. This exceptional adsorption capacity at higher pressures further underlines the beneficial physical properties linked to the very large BET surface area and the ultramicropores of the n-ZIF-67 structure. Under the same conditions of 298 K and 40 bar, the adsorption capacities for CH4 on n-ZIF-67 and n-ZIF-8 were 241 and 211.58 mg(CH4)/g, respectively, while the N2 adsorption capacities were 219 and 137 mg(N2)/g, respectively. It is noted that at higher pressures, the N2 isotherms revealed a quasi-linear behavior while the CO2 and CH4 isotherms showed type I behavior.

As discussed prior, according to the adsorption isotherms in Fig. 7, both n-ZIF-67 and n-ZIF-8 adsorbents had higher adsorption capacities for CO2 compared to CH4 and N2. In addition to the smaller size of the molecule that corresponds well with the sub 1 nm micropores and especially the 0.64 nm ultramicropores of n-ZIF-67, CO2 has an important quadrupole moment (13.4×10 -40 C.m 2 ) compared to N2 (4.7×10 -40 C.m 2 ). These factors combined are consistent with better interactions via physisorption of CO2 at the surface of these zeolitic imidazolate frameworks MOFs, and especially n-ZIF-67 [50,51]. The presence of a large quadruple moment in the CO2 molecule can facilitate interactions with the electric field gradient inside pores as well as eventually short-range dipole-quadrupolar interactions. CH4 on the other hand does not have a dipole or quadrupole moment but it does have a weak electric octuplet moment that could play a role in the adsorption of this gas [52].

Another factor to consider is that the three gases possess different electronic properties and therefore polarizabilities The greater polarizability of CH4 molecules: 17.6 × 10 -25 vs. 26.0 × 10 -25 cm 3 for N2 and CH4, respectively can help to explain the better adsorption capacity of CH4 compared to N2 [53].

The adsorption isotherms of CO2, CH4, and N2 for n-ZIF-67 and n-ZIF-8 fitted well to the Langmuir adsorption kinetics model (Eq. ( 1)). Table 3 shows the equation parameters used and the highly correlated correlation coefficient values for the data fitting according to the Langmuir model for n-ZIF-67. The maximum CO2 adsorption capacity (qm) according to the Langmuir adsorption model approach was also higher for n-ZIF-67 compared to n-ZIF-8 (Table 3). The value of the correlation coefficient highlights the slightly more homogeneous nature of the n-ZIF-67 adsorbent surface compared to ZIF-8 where values of R 2 =0.996 were reported [23].

According to the Langmuir isotherm kinetic model, the qm and KL decreased at higher temperature, suggesting an exothermic adsorption process [23,54]. The important characteristics of the Langmuir isotherm, the separation factor (RL), can be represented by Eq. 5 [55].

R L = 1 1+k L P Eq. ( 5
)
Where KL is the Langmuir constant and P (bar) is the pressure. For RL = 0 (KL is very large), irreversible adsorption;

for RL = 1 (KL= 0), linear adsorption; for 0 < RL < 1, favorable and RL < 1 (KL < 0), unfavorable adsorption (that is, desorption). The values of RL for all gases were determined over a wide pressure range and the results are shown in Fig. 8. All the RL values are between 0 and 1, indicating that all three gases (CO2, CH4, N2) are favorably adsorbed at n-ZIF-67 in the studied pressure range from 0 to 40 bar.

Isosteric Heat of Adsorption

The isosteric heat of adsorption (Qst) is an important parameter in gas adsorption studies. It allows us to understand how the adsorbent and adsorbate interact. According to the Langmuir kinetic model, the adsorption enthalpy of CO2 at PEG-templated n-ZIF-67 was estimated from the experimental adsorption isotherms determined at different temperatures. From the Langmuir fitting parameters (Table 3), the adsorption enthalpy was calculated, following the Clausius-Clapeyron equation (Eq. ( 6)) [56] Qst = -R𝑇 2 ( d ln(𝑃) ) Eq. ( 6)

d𝑇
In which: T (K) is the temperature; P is the pressure (bar); R (8.314 J mol -1 K -1 ) is the universal gas constant and Qst is the heat of adsorption in KJ/mol. Based on the general hypothesis that the enthalpy of adsorption is independent of the temperature, the combination of equation 6 offers:

lnP = ( Qst ) + C
Eq. ( 7)

RT

In which: C is the constant.

In this study, the heat/enthalpy of adsorption (Qst) of CO2, CH4, and N2 were determined by linear plotting of the logarithm of the pressure (ln (P)) versus the reverse of the temperature (1/T) by using the equilibrium isotherm data. Fig. 9 (a) shows the heat of adsorption for all gases at zero loading for both n-ZIF-67 and n-ZIF8 frameworks According to the work of Zhou et al. [57], the Qst value for physisorption is lower than 80 KJ/mol, while, it is between 80 and 200 kJ/mol for chemisorption. The heat of adsorption at zero loading for both samples, n-ZIF-67 and n-ZIF-8, are presented in Fig . 9 (a). For the studied gases, the heats of adsorption were higher at n-ZIF-67 compared to n-ZIF-8. This corresponds well with a stronger interaction between the adsorbates and the ultramicropores in n-ZIF-67 framework. The largest relative increase in Qst between the two types of ZIF structures was observed for CO2. The CO2 gas, with its quadrupole moment, is considered especially strongly interacting with the ultramicropores in n-ZIF-67. For n-ZIF-67 and n-ZIF-8, the heats of adsorption at zero loading of CO2 were 34.44 KJ/mol and 27.60 KJ/mol, respectively. The Qst values attained for both adsorbents were less than 80 KJ/mol, indicating that the gas adsorption processes were controlled by physical rather than chemical adsorption.

Fig. 9 (b) shows the variation of the heat of adsorption values at 298 K for CO2, CH4, and N2 at on n-ZIF-67. At low adsorption pressure, the heat of adsorption of CO2 was greater than for either CH4 or N2. With an increase in adsorption uptake, the heat of CO2 adsorption was reduced, owing to a decrease in the number of active adsorption sites as a result of the firm interaction of CO2 with the ultramicroporous structure for n-ZIF-67, the enthalpy of adsorption for CO2 was 34.44 KJ/mol at zero loading and 29.19 KJ/mol at high loading. Consequently, the enthalpy of adsorption of CO2 decreases when the CO2 adsorption capacity increases; this behavior is linked to the surface heterogeneity of the MOF [23]. In the low pressure region, the CO2 gas also showed the highest heat of adsorption, thanks in part to its quadrupole moment. CO2 demonstrated negligible variation in the heat of adsorption with loading, while CH4 displayed a significant decrease in the heat of adsorption from about 26.51

KJ/mol at zero loading to about 24.99 KJ/mol at a loading of ca. 30 mg(CH4)/g. Furthermore, the heat of adsorption for N2 at n-ZIF-67 was independent of the N2 uptake, indicating a weak interaction between N2 and the n-ZIF-67 material. With regard to the other gases, it is evident that the heat of adsorption of CH4 and N2 on n-ZIF-67 was lower than that of CO2. The data globally confirms the favorable selectivity of the n-ZIF-67 adsorbent for CO2/CH4 and CO2/N2, respectively.

Selectivity of CO2/N2, CH4/N2 and CO2/CH4 on ZIF-67 NP

The gas selectivity behavior of PEG-templated n-ZIF-8 for CO2/N2 CO2/CH4 and CH4/N2 at n-ZIF-67 was evaluated and is shown in Fig. 10. First we estimated the selectivity by dividing the adsorption capacity of CO2 by that of N2 or CH4 at each pressure point [13,23,58]. It is evident from Fig. 10 that ZIF-67's selectivity decreased as the pressure increased for CO2 over N2, CO2 over CH4, and CH4 over N2. Especially in the low-pressure region, the adsorption selectivity for CO2 from CO2/N2 and CO2/CH4 mixtures at n-ZIF-67 was consistently higher than that of ZIF-8 materials, as reported in the literature [23,59]. The n-ZIF-67 selectivity towards CO2 at 298 K decreased slowly as the pressure was increased. Because of the low heat of adsorption of CO2 at low pressure, the n-ZIF-67 exhibited lower selectivity towards CO2 (at 298 K) compared, for example, to zeolite 13X (Qst = 49 KJ/mol) [60]. On the other hand, a lower value is probably beneficial in reducing adsorbent regeneration energy requirements [60]. Furthermore, the CO2/N2 selectivity on n-ZIF-67 was maximized in the low-pressure range. As mentioned earlier, CO2 has a smaller molecular size (3.3 Å), higher polarizability (26.3×10 -25 cm 3 ), and a larger quadrupole moment (13.4×10 -40 C.m 2 ) compared to molecular N2 (3.8 Å; 17.6 ×10 -25 cm 3 and 4.7×10 -40 C.m 2 , respectively), that supports the stronger interaction between the active adsorption sites within the ultramicropores of ZIF-67 and the CO2 adsorbate, which is favorable for CO2/N2 separation. Particularly at 0.01, 20, and 40 bar, the CO2/N2 selectivity of n-ZIF-67 was up to 28, 11, and 6, respectively, which was higher compared to report ZIF-8 structures [23,59]. When pressures were low, up to approximately 1 bar, and at 298 K, n-ZIF-67 exhibited a better selectivity for CO2 over CH4, with selectivity ranging from 13, at about 1 bar, to 15, at 0.5 bar. The CO2/N2 selectivity ranges from 6 to 28, whereas the CO2/CH4 selectivity ranges from 4 to 16, which are almost higher than those observed at ZIF-8 [23]. Moreover, with increasing pressure, the selectivity performance decreased. The selectivity for CH4/N2 was very low, nearly close to 1. Overall, these selectivity measurements show a high potential for CO2 separation from CO2/N2 or CO2/CH4 gas mixtures by gas adsorption at higher pressures.

Adsorption/desorption cycles

The adsorption stability of the n-ZIF-67 adsorbent was subsequently evaluated by prolonged cyclic CO2 and CH4 adsorption-desorption cycles (15 cycles of adsorption and regeneration). The adsorbents were heated to 423 K (10 K/min) under N2 gas (50 mL/min) after each adsorption to release any adsorbed gases that remained on the surface of n-ZIF-67. The adsorbent was then cooled to 298 K while being maintained under N2, and the weight was measured. A subsequent CO2 adsorption was performed to continue the cycling test. The percentage ratio of the adsorption capacity of the regenerated adsorbent to the fresh one is defined as the adsorption index (AI) and is determined by the equation as follows:

AI = qn x 100 Eq. ( 8
)
q1

Where, q1 and qn indicate the CO2 and CH4 adsorption capacity of the first cycle and the n th (n = 1-15) cycle, respectively.

The adsorption stability of n-ZIF-67 can be considered as its recyclability. The n-ZIF-67 adsorbent was used/reused over 15 cycles of successive CO2 adsorption and desorption under the specified condition of 298 K and 40 bar of CO2 gas (Fig. 12 (a)). It was stated earlier that the CO2 adsorption capacity of as-prepared n-ZIF-67 was 681 mg (CO2)/g (100%). In the second cycle, this value dropped to 680.2 mg (CO2)/g (99.88%). After each cycle, the CO2 adsorption capacity of n-ZIF-67 was very slightly reduced. The capacity was 679.6 mg (CO2)/g by the end of the 15 th cycle (99.79%). This reduction should be related to the material loss as a result of the recycling process. Nevertheless, the CO2 adsorption capacity after 15 cycles was still higher than 99%, which confirms the high reusability of the n-ZIF-67 adsorbent. Moreover, the CO2 adsorption and desorption cycle curves were similar, suggesting the strong stability of n-ZIF-67 and appropriateness of this material for CO2 adsorption.

The extended cyclic CH4 adsorption on n-ZIF-67 adsorbent was also conducted, and the results are shown in Fig.

12 (b). The results show that the adsorption capacity only decreased from 100% to 99.91% by the end of the 15 th cycle, signifying that most CH4 molecules can also be successfully desorbed throughout the regeneration operation.

Conclusion

In summary, PEG-templated hexagonal n-ZIF-67 nanoparticles were successfully synthesized for the first time via a simple and rapid protocol at room temperature. The adsorbent benefits from a very high BET surface of 1891 m 2 /g, a large pore volume of 0.86 cm 3 /g, and an average ultramicropore size of 6.40 Å that is complimentary in size to CO2 and favorable for its adsorption. The other gases, CH4 and N2, adsorb much less efficiently. n-ZIF-67

shows excellent performance not only for CO2 adsorption but also for CO2/N2, CO2/CH4 and CH4/N2 separations physical trapping) and polar interactions between the gas and the adsorbent. Adsorption-desorption cycling tests for CO2 and CH4 gas adsorption revealed excellent adsorbent stability with more than ca. 99.9% of the adsorption capacity remaining after 15 cycles. PEG-templated n-ZIF-8 that is obtained from a simple synthesis with low cost materials has the potential to be considered for economical production on a larger scale and is thus a promising physisorbent for CO2 capture and separation.

Introduction

One of the biggest issues facing the world community is climate change brought on by global warming. The generation and accumulation of greenhouse gases in the atmosphere have mostly been caused by the emission of gases like methane (CH4), carbon dioxide (CO2), and nitrogen oxide (NO2), and especially CO2 [1][2][3]. The major industrial technologies used over the past few decades for separating and capturing CO2 and CH4 are, in general, based on absorption by solvents, membrane separation, and adsorption methods [4][5][6].

Adsorbents such as activated carbon and zeolites are among the most effective materials for gas separation in the chemical industry. Thanks to their large specific surface areas and pore volumes, crystalline porous metalorganic frameworks (MOFs) show great performance for gas adsorption and storage applications. MOFs are comprised of unique networks of metal ions or clusters that are coupled to organic ligands that give rise to unprecedented chemical and structure tunability. Numerous MOFs have been formed and investigated to date for CO2 capture, CH4 storage, and gas mixtures separation [7,8]. MOFs can selectively adsorb gases owing to their unique pore sizes and geometries, as well as functional groups (e.g. via electrostatics) within the pores. Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that have been used extensively and have a tetrahedral network structure that is in many ways similar to zeolites, but with the important difference that they are constructed using transition metals associated with imidazolate ligands [9]. ZIFs, notably ZIF-67, have unique chemical, thermal, and water stabilities, that makes them promising candidates for industrial adsorption applications [10,11]. Thanks to their high specific surface and pore geometries and chemistries, ZIFs are suitable adsorbents for CO2 and CH4

(Table 1) [12][13][14][15][16][17][18][19][20][21][22][23].

Several methods have been reported for the production of ZIF-67 with tunable porosity and morphology, such as sol-gel, solvothermal, microwave/ultrasound-assisted, and surfactant-assisted methods [24][25][26].

These methods permit the preparation of highly crystalline ZIF-67 with different crystallite sizes and porosities with some control, depending on factors such as synthesis time, temperature, and solvent [22,24,27]. Many require reaction times of several hours to days due to slow coordination interactions and discontinuities. For example, ZIF-67 crystals are typically obtained after 12 to 72 h using solvothermal methods at temperatures between 50-200°C [25]. It is therefore desirable to not only discover high-performance adsorbent materials, but also to discover materials that can be easily, economically and ecologically obtained, with a high level of control over crystal size and morphology (including phase purity). Nanocrystalline ZIF-67 with controlled morphology and particle size can be prepared using a polymer or surfactant template [19,23]. The use of a template can prevent the formation of nonuniform agglomerates with large variations in sample size and morphology as well as larger particles [22]. Room temperature methods capable of generating ZIF-67 in a few hours are also desirable [28]. The diverse synthesis possibilities to obtain ZIF-67 with attractive chemical and physical properties has resulted in its use as a great material for catalysis [29], separation [30], and volatile organic compounds (VOCs) adsorption [31] applications.

Nevertheless, the application and advantages of ZIF-67 for gas adsorption and separation have been rarely studied (see Table 1 for a detailed literature summary).

In this study, we report the facile and fast synthesis of ZIF-67 nanoparticles (NPs) in just a few minutes using N, N-dimethylformamide (DMF) as the solvent and polyethylene glycol (PEG) as a soft template. The use of PEG with a mild reaction temperature avoids the need for conventional high temperature solvothermal and microwave methods, while promoting the formation of homogeneous and stable ZIF-67 dispersions. To the best of our knowledge, the use of PEG as a template has not been reported previously for ZIF-67 synthesis. PEG was chosen over other polymer templates since it is low cost and has the capacity to generate a homogenous mixture comprising metallic salts thanks to its attractive solubility properties. During MOF synthesis, the polymer is considered to adsorb onto the metal surface and alter the growth kinetics of colloids, allowing for the uniform growth of hexagonally-shaped particles [19,23]. In these former examples, poly (diallyldimethylammonium chloride) and PEG were used for the production of ZIF-67 and ZIF-8, respectively. Following successful synthesis and characterization of PEG-templated ZIF-67 nanoparticles, we report here the gravimetric adsorption studies of CO2, CH4, and N2 using PEG-templated n-ZIF-67 as well as PEG-templated n-ZIF-8 as adsorbents. The adsorption/desorption isotherms were achieved at temperatures of 273, 298, 323, and 353 K, at low and high pressures, and considered together with Langmuir modelling. Real-time dynamic breakthrough experiments of PEG-templated ZIF-67 for CO2/N2, CO2/CH4 and N2/CH4 mixtures are also reported.

Material and Methods

Synthesis of PEG-templated ZIF-8 nanoparticles (n-ZIF-8)

All the materials in this study were purchased from Sigma-Aldrich and were employed as received, including cobalt nitrate hexahydrate (Co(NO3)•6H2O; 99 %), 2-methylimidazole (C4H6N2: mIm; 99 %), N,Ndimethylformamide (DMF; 99%), polyethylene glycol (PEG, average molecular weight = 20.000 g.mol -1 ), triethylamine (TEA; (C2H5)3N; 99%), and methanol (99 %). The microporous PEG-templated ZIF-8 (Zn(Im)2)

nanoparticles were obtained in accordance with the procedures of our recently reported room-temperature method [23]. The ZIF-8 material was activated under vacuum at 423 K for 12 h prior to gas adsorption measurements.

Table 1 summarizes the textural properties of ZIF-8 from our previous study.

Synthesis of PEG-templated ZIF-67 nanoparticles (n-ZIF-67)

ZIF-67 NPs were synthesized in a similar manner to the previous publication with some modification [23], i.e., Co(NO3)2•6H2O and mIm were used as precursors with PEG as a soft template. The choice of PEG was encouraged by its ability to form a homogenous solution with metallic salts thanks to micellar and dispersion effects to promote the uniform growth of particles with reduced particle size and agglomeration. In the first step, the PEG powder (0.4 g) was dispersed in 10 mL of DMF; Co(NO3)2•6H2O (1.4551 g, 5 mmol) was then added with slight stirring for 2 min at 298 K to form the Co-solution. Later, 2-methylimidazole (2.0525 g, 25 mmol) was dispersed in DMF (10 mL); TEA was used as a base and slowly added (6.75 mL) with stirring over 3 min to obtain the linker-solution. After complete dissolution, the Co-solution was mixed with the linker-solution and stirred for 5 min at room temperature. The purple precipitates were filtered, washed with ethanol to remove possible undissolved reagents including PEG, then collected via centrifugation (4000 rpm, 10 min). Finally, the obtained purple powder was dried in a vacuum oven at 423 K for 24 h to achieve the final product, ZIF-67 NP (herein referred to as n-ZIF-67).

Scheme 1: Room temperature synthesis reaction to obtain PEG-template ZIF-67 (n-ZIF-67).

Characterization

FTIR spectroscopy was performed using a Perkin Elmer Spectrum Two instrument with attenuated total reflectance (ATR) spectra recorded from 4000 to 400 cm -1 . X-ray powder diffraction spectra (XRD) from 5° to 60° were recorded on a Bruker D8 Discover diffractometer with a scanning rate of 2°/min. The morphology and elemental composition of n-ZIF-67 was determined by transmission electron microscopy (TEM, Philips CM200) combined with an energy-dispersive X-ray spectrometer (EDX). The particle size analyses were determined using Image J software. Thermogravimetric analysis curves (TGA) were measured on a Mettler Toledo STARe apparatus by heating the samples from 298 K up to 973 K at a heating rate of 2°C/min under an air atmosphere. At 77 K, nitrogen adsorption-desorption isotherms (Micrometrics ASAP-2420 instrument; sample degassing at 423 K for 24 h under vacuum) were collected to determine the Brunauer-Emmett-Teller (BET) specific surface area (P/P° ranging from 0.01 to 0.35), total micropore volume (VP), and pore size distribution (t-plot method).

CO2, CH4, and N2 adsorption

Pure component adsorption isotherms of CO2, CH4, and N2 at n-ZIF-67 and n-ZIF-8 were obtained by gravimetric analysis (Autosorb-iQ-MP analyzer) in the range of 0-40 bar at different temperatures: 273, 298, 323 and 353 K.

The samples were degassed at 423 K under a high vacuum for 24 hours prior to the sorption measurement. The CO2 used was of high purity (99.99%) and obtained from Sigma-Aldrich. The gases used were all high purity (99.99%) and obtained from Sigma-Aldrich. Numerous adsorption kinetic models were employed to define the adsorption equilibrium isotherm data. In this work, the Langmuir adsorption kinetics model (Eq. ( 1)) was used to fit the equilibrium isotherm data [32]. This model was used to describe the monolayer pattern of the adsorbed layer on the homogeneous surface. The Langmuir equation can be expressed by the equilibrium adsorption capacity (q, mg/g), maximum adsorption capacity (qm, mg/g), Langmuir isotherm constant or a parameter related to the affinity constant between adsorbate and adsorbent (KL, L/g), and partial pressure (P, bar) as Eq. ( 1). q = q m k L P 1+k L P Eq. ( 1)

Results and discussion

Structural analysis

The FTIR spectrum of the PEG templated n-ZIF-67 recorded between 400 and 4000 cm -1 is shown in Fig. 1. The n-ZIF-67 structure contains the ligand, mIm, which is responsible for most of the band characteristics. Precisely, the bands that appeared between 600 and 1500 cm -1 are ascribed to bending and stretching of the imidazole ring, such as the bands at 1145 and 1308 cm -1 , for the bending signal, and bands at 1415 and 1433 cm -1 , for the stretching vibrations [31,33]. The bands at 994 and 750 cm -1 can be attributed to the bending vibrations of C-N and C-H, respectively, while the band at 694 cm -1 can be attributed to the bending variation of the mIm ring. The absorption band at 1637 cm -1 is due to the C=C stretch mode, whereas the absorption band at 1584 cm -1 is attributed to the stretching mode of C=N. The two small absorption bands at 2929 and 3136 cm -1 can be accredited to the asymmetric stretching modes of the aliphatic C-H and the aromatic ring of mIm, respectively. Interestingly, the Co=N stretching vibration band is observed at 424 cm -1 [34], suggesting that the cobalt ions are linked chemically with nitrogen atoms of the methylimidazole (mIm) groups to shape the imidazolate. The FTIR results confirmed the successful synthesis of pure ZIF-67 [35]. As was mentioned in Section 2.2, n-ZIF-67 NPs were rapidly synthesized in minutes. The material was isolated in about 1 hr with the synthesis process occurring at room temperature and pressure. The method is rapid but also easily scalable, although with the notable downside that DMF and methanol solvents are used. The X-ray powder diffraction method was used for analyzing the chemical composition of as-produced n-ZIF-67 NPs. The sharp diffraction peaks at 2θ values of 7.4°, 10.4°, 12.8°, 14.7°, 16.5°, 18.0° and 19.5°, 22.1°, 24.5°, 26.7°, and 29.6° are assigned to the ZIF-67 planes of (011), (002), (112), (022), (013), ( 222), (114), (233), ( 134) and (044), respectively [36]. The XRD diffraction pattern with Rietveld refinement in Fig. 2 represents the common peaks of ZIF-67 and matches well with the simulated ZIF-67 diffractogram [36,37]. Crystallite sizes were calculated using the Debye-Sherrer equation (Eq. 2) [38][39][40]:

L = Kλ βcosθ Eq. (2)
Where L is the diameter of the crystallites (nm), K is the numerical Scherrer constant (0.93 Å), λ is the X-ray wavelength (1.54 Å), β is the full width at half-maximum intensity (FWHM) in radians, and θ is the Bragg diffraction angle. The crystallite size of n-ZIF-67 NPs was first determined using the Debye-Scherrer equation, equal to about 78 nm. The refined XRD structural parameters, calculated grain sizes, crystallite sizes and strains of n-ZIF-67 are collated in Table 2. Next, we considered the Williamson-Hall (W-H) method to determine the size for comparison with the Scherrer technique and TEM data. In the W-H method [41], the strain induced broadening arising from crystal imperfection and distortions is related by: ) tan( 4

    s Eq. (3)
Here, ε is the root mean square value of the micro-strain, d d  



. Assuming that the particle size and strain contributions to line broadening are independent of each other and that both have a Cauchy like profile, the observed line breadth is simply the sum of the two:

) sin( 4 ) cos(        D k hkl Eq. ( 4
)
A plot is drawn with ) sin(  along the x-axis and ) cos(  hkl along the y-axis for as-prepared PEG-templated n-ZIF-67 (Fig 3 (a)). The slope and y-intersect of the fitted line represent the strain and the particle size respectively. We can remark that the calculated crystallite size using the W-H technique (DW-H = 91 nm) is larger than that calculated using Scherrer's technique (78 nm). This is due to the presence of strain which also contributes to the broadening of peaks. The grain sizes observed by TEM (see Fig. 3 and discussion below) are larger than those calculated by Scherrer's and W-H techniques. This can be explained by the fact that each particle observed by TEM is formed by several crystallized grains. The morphology of n-ZIF-67 was studied using TEM. According to Fig. 3 (b), the uniform rhombic dodecahedral morphology was observed for ZIF-67 NPs, in agreement with a previous study of ZIF-67 nanocrystals [42]. The perfect crystalline structure of the synthesized n-ZIF-67 NPs was further confirmed by the well-defined rhombic facet and the clearly visible edge of ZIF-67. In addition, the surfaces of the particles appear regular and smooth, and the particles do not appear to be aggregated. Further interpretation of the TEM images using ImageJ software revealed that the particles had a Gaussian distribution, ranging from 30 to 100 nm, (Fig. The EDX analysis data, as shown in Fig. 3 (d), clearly demonstrates the expected elemental characteristic peaks for C (0.26 keV), N (0.53 keV), and Co (1, 6.9 keV), confirming the successful synthesis of ZIF-67 NPs.

0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

Thermal analysis

Thermal stability analysis of n -ZIF-67 NPs was conducted based on TGA curves. Fig. 4 depicts the weight loss rate and weight loss profiles. It is evident that a two-stage weight loss occurred at n-ZIF-8. The first weight loss was detected between 300 and 680 K, related to the removal of physisorbed molecules (e.g. H2O, CO2, and methanol) from the framework, concomitant with several exothermic peaks. The second weight loss at 700 K is followed by a rapid loss and exothermic peaks, which can be associated with the decomposition of the ZIF-67 NPs.

The high thermal stability of ZIF-67 NPs, based on TGA curves, is consistent with previously reported data [34,43,44]. It was found that about 12 wt% of the initial weight of the powder remained after decomposition, which can be attributed to cobalt oxide, CoO, formation.

Nitrogen isotherm and pore size distribution

Considering the TGA analysis data in Section 3.2, n-ZIF-67 NPs were degassed at 423 K prior to N2 isotherm measurements. Fig. 5 displays the pore size distributions and N2 adsorption isotherms for n-ZIF-67. As shown in Fig. 5 (a), the adsorbent showed a N2 rapid uptake at a the very low relative pressure region (10 -5 < P/P° < 10 -2 ).

Based on the IUPAC classification, the sample had a typical type I isotherm [23,35,45] that confirms the microporosity of the material. As shown in Fig. 5, n-ZIF-67 NPs had reversible and reproducible N2 adsorption and desorption isotherms, emphasising a stable material with permanent porosity and good rigidity. The BET method was employed to measure the sample porosity at 0.001 < P/P0 < 0.35. The BET surface area (SBET) of the PEG-templated n-ZIF-67 NPs was 1871 m 2 /g with a micropore volume of 0.86 cm 3 /g, which are higher than previous values reported for ZIF-8 (including PEG-templated ZIF-8) and ZIF-67, as summarized in Table 1, highlighting the attractive adsorption potential of the new material reported here [23,24]. The total pore volume (Vmicro) of n-ZIF-67 was 0.86 cm 3 /g at P/P° = 0.99.

CO2, CH4 and N2 adsorption

The CO2, CH4, and N2 adsorption isotherm performance at n-ZIF-67 and n-ZIF-8, with Langmuir adsorption model fitting, was subsequently investigated at low pressure (0 to 100 KPa/0 to 1 bar) and high pressure (0 to 40,000 KPa/0 to 40 bar) at different temperatures (273, 298, 323, and 253 K) after outgassing at 423 K. Nanoscale ZIF-67, due to its excellent textural properties including large porosity (microporosity and ultramicroporosity) and crystallinity, is an attractive candidate for the adsorption of gases such as CO2, CH4 and N2, and especially the electrophile, CO2, via interactions with its active adsorption sites within its pores [47,23]. The high selectivity of ZIFs for CO2 in gas mixtures, especially industrially important CO2/N2 and CO2/CH4 mixtures, is an attractive point that we endeavored to explore with PEG-templated n-ZIF-67. At low-pressure, between 0 and 100 KPa ) and at different temperatures between 273 and 353 K, the CO2 uptake of PEG-templated n-ZIF-67 increased quasilinearly with increasing pressure (Fig. 5 (a)). At 100 KPa CO2, the uptake did not reach saturation. According to the IUPAC classification, CO2 and CH4 exhibited a non-linear type I isotherm, which is in contrast to the N2 isotherms which showed a linear adsorption behavior (Fig. 5 (a, c, ande)). In contrast, at PEG-templated n-ZIF-8, all of the gas adsorption isotherms exhibited a linear adsorption behavior (Fig. 5 (b, d, andf). The n-ZIF-67 particles showed a higher adsorption capacity for all gases at 100 KPa compared to n-ZIF-8. At 298 K and 100 KPa, the adsorption capacity of CO2 at n-ZIF-67 was 111.89 mg (CO2)/g, which was higher than the 51.5 mg(CO2)/g recorded at PEG-templated n-ZIF-8 samples and ZIF-8 (Table 1) [20,23]. The higher CO2 capture capacity observed for n-ZIF-67 can be attributed, at least in part, to the combination of a higher BET surface area (1871 m 2 /g) and pore volume (0.86 cm 3 /g), and the ultramicropores below 0.7 nm (0.64 nm). Linked to the physical interactions of the free pore sites of ZIF-67 with CO2 molecules, [49,23], n-ZIF-67 adsorbed more CO2 at 273 K (850.52 mg(CO2)/g) than at higher temperatures such as 353 K (425.04 mg (CO2)/g. An analogous behaviour has been observed and defined previously for ZIF-8, ZIF-69, and ZIF-76 [23,49]. Additionally, the n-ZIF-67 material prepared here showed maximum adsorption capacities for CH4 and N2 of 40.72 mg(CH4)/g for CH4 and 23.02 mg(N2)/g for N2 at 298 K and 100 KPa (see Table 1). These adsorption capacities surpassour previous results with PEG-templated n-ZIF-8. This higher adsorption capacity for all three gases compared to n-ZIF-8 highlights the very promising physical characteristics of the newly synthesized n-ZIF-67 material. The adsorption isotherms of CO2, CH4, and N2 at both n-ZIF-67 and n-ZIF-8 at various temperatures (273, 298, 323, and 353 K) and high pressure (0-40 bar) were also considered (Fig. 7). Generally, CO2 and CH4 adsorption increases with elevated pressure and the uptake reaches saturation at both materials. The N2 uptake increases linearly with increasing pressure. The n-ZIF-67 samples showed significantly higher CO2 adsorption capacity compared to CH4 and N2 adsorption, and higher adsorption capacities for all gases compared to n-ZIF-8 [23]. The adsorption capacity of 681 mg (CO2)/g at n-ZIF-67 at 298 K and 40 bar CO2 is the maximum capacity reported for a ZIF-67 and ZIF-8 adsorbent, as far as the authors are concerned (Table 1). The highest adsorption capacity achieved in this study was 846 mg (CO2)/g at 40 bar for n-ZIF-67. This exceptional adsorption capacity at higher pressures further underlines the beneficial physical properties linked to the very large BET surface area and the ultramicropores of the n-ZIF-67 structure. Under the same conditions of 298 K and 40 bar, the adsorption capacities for CH4 on n-ZIF-67 and n-ZIF-8 were 241 and 211.58 mg(CH4)/g, respectively, while the N2 adsorption capacities were 219 and 137 mg(N2)/g, respectively. It is noted that at higher pressures, the N2 isotherms revealed a quasi-linear behavior while the CO2 and CH4 isotherms showed type I behavior.

As discussed prior, according to the adsorption isotherms in Fig. 7, both n-ZIF-67 and n-ZIF-8 adsorbents had higher adsorption capacities for CO2 compared to CH4 and N2. In addition to the smaller size of the molecule that corresponds well with the sub 1 nm micropores and especially the 0.64 nm ultramicropores of n-ZIF-67, CO2 has an important quadrupole moment (13.4×10 -40 C.m 2 ) compared to N2 (4.7×10 -40 C.m 2 ). These factors combined are consistent with better interactions via physisorption of CO2 at the surface of these zeolitic imidazolate frameworks MOFs, and especially n-ZIF-67 [50,51]. The presence of a large quadruple moment in the CO2 molecule can facilitate interactions with the electric field gradient inside pores as well as eventually short-range dipole-quadrupolar interactions. CH4 on the other hand does not have a dipole or quadrupole moment but it does have a weak electric octuplet moment that could play a role in the adsorption of this gas [52].

Another factor to consider is that the three gases possess different electronic properties and therefore polarizabilities The greater polarizability of CH4 molecules: 17.6 × 10 -25 vs. 26.0 × 10 -25 cm 3 for N2 and CH4, respectively can help to explain the better adsorption capacity of CH4 compared to N2 [53].

The adsorption isotherms of CO2, CH4, and N2 for n-ZIF-67 and n-ZIF-8 fitted well to the Langmuir adsorption kinetics model (Eq. ( 1)). Table 3 shows the equation parameters used and the highly correlated correlation coefficient values for the data fitting according to the Langmuir model for n-ZIF-67. The maximum CO2 adsorption capacity (qm) according to the Langmuir adsorption model approach was also higher for n-ZIF-67 compared to n-ZIF-8 (Table 3). The value of the correlation coefficient highlights the slightly more homogeneous nature of the n-ZIF-67 adsorbent surface compared to ZIF-8 where values of R 2 =0.996 were reported [23].

According to the Langmuir isotherm kinetic model, the qm and KL decreased at higher temperature, suggesting an exothermic adsorption process [23,54]. The important characteristics of the Langmuir isotherm, the separation factor (RL), can be represented by Eq. 5 [55].

R L = 1 1+k L P Eq. ( 5
)
Where KL is the Langmuir constant and P (bar) is the pressure. For RL = 0 (KL is very large), irreversible adsorption;

for RL = 1 (KL= 0), linear adsorption; for 0 < RL < 1, favorable and RL < 1 (KL < 0), unfavorable adsorption (that is, desorption). The values of RL for all gases were determined over a wide pressure range and the results are shown in Fig. 8. All the RL values are between 0 and 1, indicating that all three gases (CO2, CH4, N2) are favorably adsorbed at n-ZIF-67 in the studied pressure range from 0 to 40 bar.

Isosteric Heat of Adsorption

The isosteric heat of adsorption (Qst) is an important parameter in gas adsorption studies. It allows us to understand how the adsorbent and adsorbate interact. According to the Langmuir kinetic model, the adsorption enthalpy of CO2 at PEG-templated n-ZIF-67 was estimated from the experimental adsorption isotherms determined at different temperatures. From the Langmuir fitting parameters (Table 3), the adsorption enthalpy was calculated, following the Clausius-Clapeyron equation (Eq. ( 6)) [56] Qst = -R𝑇 2 ( d ln(𝑃) ) Eq. ( 6)

d𝑇

In which: T (K) is the temperature; P is the pressure (bar); R (8.314 J mol -1 K -1 ) is the universal gas constant and Qst is the heat of adsorption in KJ/mol. Based on the general hypothesis that the enthalpy of adsorption is independent of the temperature, the combination of equation 6 offers: lnP = ( Qst ) + C Eq. ( 7)

RT

In which: C is the constant.

In this study, the heat/enthalpy of adsorption (Qst) of CO2, CH4, and N2 were determined by linear plotting of the logarithm of the pressure (ln (P)) versus the reverse of the temperature (1/T) by using the equilibrium isotherm data. Fig. 9 (a) shows the heat of adsorption for all gases at zero loading for both n-ZIF-67 and n-ZIF8 frameworks According to the work of Zhou et al. [57], the Qst value for physisorption is lower than 80 KJ/mol, while, it is between 80 and 200 kJ/mol for chemisorption. The heat of adsorption at zero loading for both samples, n-ZIF-67 and n-ZIF-8, are presented in Fig . 9 (a). For the studied gases, the heats of adsorption were higher at n-ZIF-67 compared to n-ZIF-8. This corresponds well with a stronger interaction between the adsorbates and the ultramicropores in n-ZIF-67 framework. The largest relative increase in Qst between the two types of ZIF structures was observed for CO2. The CO2 gas, with its quadrupole moment, is considered especially strongly interacting with the ultramicropores in n-ZIF-67. For n-ZIF-67 and n-ZIF-8, the heats of adsorption at zero loading of CO2 were 34.44 KJ/mol and 27.60 KJ/mol, respectively. The Qst values attained for both adsorbents were less than 80 KJ/mol, indicating that the gas adsorption processes were controlled by physical rather than chemical adsorption.

Fig. 9 (b) shows the variation of the heat of adsorption values at 298 K for CO2, CH4, and N2 at on n-ZIF-67. At low adsorption pressure, the heat of adsorption of CO2 was greater than for either CH4 or N2. With an increase in adsorption uptake, the heat of CO2 adsorption was reduced, owing to a decrease in the number of active adsorption sites as a result of the firm interaction of CO2 with the ultramicroporous structure for n-ZIF-67, the enthalpy of adsorption for CO2 was 34.44 KJ/mol at zero loading and 29.19 KJ/mol at high loading. Consequently, the enthalpy of adsorption of CO2 decreases when the CO2 adsorption capacity increases; this behavior is linked to the surface heterogeneity of the MOF [23]. In the low pressure region, the CO2 gas also showed the highest heat of adsorption, thanks in part to its quadrupole moment. CO2 demonstrated negligible variation in the heat of adsorption with loading, while CH4 displayed a significant decrease in the heat of adsorption from about 26.51

KJ/mol at zero loading to about 24.99 KJ/mol at a loading of ca. 30 mg(CH4)/g. Furthermore, the heat of adsorption for N2 at n-ZIF-67 was independent of the N2 uptake, indicating a weak interaction between N2 and the n-ZIF-67 material. With regard to the other gases, it is evident that the heat of adsorption of CH4 and N2 on n-ZIF-67 was lower than that of CO2. The data globally confirms the favorable selectivity of the n-ZIF-67 adsorbent for CO2/CH4 and CO2/N2, respectively. The gas selectivity behavior of PEG-templated n-ZIF-8 for CO2/N2 CO2/CH4 and CH4/N2 at n-ZIF-67 was evaluated and is shown in Fig. 10. First we estimated the selectivity by dividing the adsorption capacity of CO2 by that of N2 or CH4 at each pressure point [13,23,58]. It is evident from Fig. 10 that ZIF-67's selectivity decreased as the pressure increased for CO2 over N2, CO2 over CH4, and CH4 over N2. Especially in the low-pressure region, the adsorption selectivity for CO2 from CO2/N2 and CO2/CH4 mixtures at n-ZIF-67 was consistently higher than that of ZIF-8 materials, as reported in the literature [23,59]. The n-ZIF-67 selectivity towards CO2 at 298 K decreased slowly as the pressure was increased. Because of the low heat of adsorption of CO2 at low pressure, the n-ZIF-67 exhibited lower selectivity towards CO2 (at 298 K) compared, for example, to zeolite 13X (Qst = 49 KJ/mol) [60]. On the other hand, a lower value is probably beneficial in reducing adsorbent regeneration energy requirements [60]. Furthermore, the CO2/N2 selectivity on n-ZIF-67 was maximized in the low-pressure range. As mentioned earlier, CO2 has a smaller molecular size (3.3 Å), higher polarizability (26.3×10 -25 cm 3 ), and a larger quadrupole moment (13.4×10 -40 C.m 2 ) compared to molecular N2 (3.8 Å; 17.6 ×10 -25 cm 3 and 4.7×10 -40 C.m 2 , respectively), that supports the stronger interaction between the active adsorption sites within the ultramicropores of ZIF-67 and the CO2 adsorbate, which is favorable for CO2/N2 separation. Particularly at 0.01, 20, and 40 bar, the CO2/N2 selectivity of n-ZIF-67 was up to 28, 11, and 6, respectively, which was higher compared to report ZIF-8 structures [23,59]. When pressures were low, up to approximately 1 bar, and at 298 K, n-ZIF-67 exhibited a better selectivity for CO2 over CH4, with selectivity ranging from 13, at about 1 bar, to 15, at 0.5 bar. The CO2/N2 selectivity ranges from 6 to 28, whereas the CO2/CH4 selectivity ranges from 4 to 16, which are almost higher than those observed at ZIF-8 [23]. Moreover, with increasing pressure, the selectivity performance decreased. The selectivity for CH4/N2 was very low, nearly close to 1. Overall, these selectivity measurements show a high potential for CO2 separation from CO2/N2 or CO2/CH4 gas mixtures by gas adsorption at higher pressures.

Conclusion

In summary, PEG-templated hexagonal n-ZIF-67 nanoparticles were successfully synthesized for the first time via a simple and rapid protocol at room temperature. The adsorbent benefits from a very high BET surface of 1891 m 2 /g, a large pore volume of 0.86 cm 3 /g, and an average ultramicropore size of 6.40 Å that is complimentary in size to CO2 and favorable for its adsorption. The other gases, CH4 and N2, adsorb much less efficiently. n-ZIF-67

shows excellent performance not only for CO2 adsorption but also for CO2/N2, CO2/CH4 and CH4/N2 separations physical trapping) and polar interactions between the gas and the adsorbent. Adsorption-desorption cycling tests for CO2 and CH4 gas adsorption revealed excellent adsorbent stability with more than ca. 99.9% of the adsorption capacity remaining after 15 cycles. PEG-templated n-ZIF-8 that is obtained from a simple synthesis with low cost materials has the potential to be considered for economical production on a larger scale and is thus a promising physisorbent for CO2 capture and separation.

Fig. 2 :

 2 Fig.2: Rietveld plots of the XRD data for the PEG templated n-ZIF-67 (Co(mIm)2 with the observed (black line), calculated (solid line), calculated Bragg reflection positions (vertical purple line), and the difference pattern between observed and calculated profiles (blue line, bottom).

3

 3

  (c)), close to the size of the crystallites indicated via the Williamson-Hall data and the Debye-Scherrer equation (about 78 nm).

Fig. 3 .

 3 Fig.3. (a) strain graph of the cubic phase of n-ZIF-67 NPs (b) TEM image (c) TEM-based particle size distribution and (d) EDX spectrum

Fig. 4 .

 4 Fig.4. Thermal stability analysis of PEG-templated n-ZIF-67 (heating rate of 2°C/min; in air).

Fig. 5 .

 5 Fig. 5. (a) N2 adsorption-desorption isotherms for n-ZIF-67, and (b) the corresponding pore size distribution.

Fig. 6 .Fig. 7 .

 67 Fig. 6. CO2, CH4, and N2 adsorption isotherms with Langmuir adsorption kinetic model fitting of n-ZIF-67 (a, c, e) versus n-ZIF-8 (b, d, f) at different temperatures (273, 298, 323, and 353K) and low pressure (0-1 bar)

Fig. 8 .

 8 Fig. 8. Separation factor (RL) plot against pressure for gas adsorption at n-ZIF-67.

Fig. 9 (

 9 Fig. 9 (b) shows Qst as a function of the CO2 uptake. The Qst value is associated with different forces depending on the nature of the interaction between adsorbent-adsorbate, such as Van der Waals forces, dipole-dipole and dipole-quadrupole interactions, and chemical bonds.

Fig. 9 .

 9 Fig. 9. (a) Comparison of the isosteric heats of adsorption at zero loading at 298 K for CO2, CH4 and N2 at n-ZIF-67 and n-ZIF-8; (b) Variation in the heat of adsorption with loading at n-ZIF-67.

Fig. 10 .Fig. 11 .

 1011 Fig.10. CO2/N2, CO2/CH4 and CH4/N2 selectivity's for n-ZIF-67 at 298 K.

Fig. 12 :

 12 Fig. 12: CO2 and CH4 adsorption cycles of ZIF-67 NPs adsorbents.

from 1 :

 1 1 ratio mixtures at different pressures from 1 to 20 bar, taking into account both the adsorption isotherm data set and breakthrough experiments. The selectivity for CH4 over N2 was very low, emphasizing the main impact of this material for CO2 capture. Comparative adsorption experiments revealed consistently better adsorption and separation performance for the PEG-templated n-ZIF-67 compared to PEG-templated n-ZIF-8 or other ZIFs in the literature. The adsorption enthalpy data (Qst = 34.44 KJ/mol) obtained from highly correlated Langmuir model fitting revealed that CO2 adsorption at n-ZIF-67 was driven by physical rather than chemical interactions at zero loading, consistent with the important role of the high specific surface area ultramicroporous structure (e.g.

Fig. 1 .

 1 Fig.1. FTIR spectrum of n-ZIF-67.

Fig. 2 :

 2 Fig.2: Rietveld plots of the XRD data for the PEG templated n-ZIF-67 (Co(mIm)2 with the observed (black line), calculated (solid line), calculated Bragg reflection positions (vertical purple line), and the difference pattern between observed and calculated profiles (blue line, bottom).

  3 (c)), close to the size of the crystallites indicated via the Williamson-Hall data and the Debye-Scherrer equation (about 78 nm).

Fig. 3 .

 3 Fig.3. (a) strain graph of the cubic phase of n-ZIF-67 NPs (b) TEM image (c) TEM-based particle size distribution and (d) EDX spectrum

Fig. 4 .

 4 Fig.4. Thermal stability analysis of PEG-templated n-ZIF-67 (heating rate of 2°C/min; in air).

Fig. 5 (Fig. 5 .

 55 Fig. 5. (a) N2 adsorption-desorption isotherms for n-ZIF-67, and (b) the corresponding pore size distribution.

Fig. 6 .Fig. 7 .

 67 Fig. 6. CO2, CH4, and N2 adsorption isotherms with Langmuir adsorption kinetic model fitting of n-ZIF-67 (a, c, e) versus n-ZIF-8 (b, d, f) at different temperatures (273, 298, 323, and 353K) and low pressure (0-1 bar)

Fig. 8 .

 8 Fig. 8. Separation factor (RL) plot against pressure for gas adsorption at n-ZIF-67.

Fig. 9 (

 9 Fig. 9 (b) shows Qst as a function of the CO2 uptake. The Qst value is associated with different forces depending on the nature of the interaction between adsorbent-adsorbate, such as Van der Waals forces, dipole-dipole and dipole-quadrupole interactions, and chemical bonds.

1 Fig. 9 .

 19 Fig. 9. (a) Comparison of the isosteric heats of adsorption at zero loading at 298 K for CO2, CH4 and N2 at n-ZIF-67 and n-ZIF-8; (b) Variation in the heat of adsorption with loading at n-ZIF-67.

Fig. 10 .Fig. 11 .

 1011 Fig.10. CO2/N2, CO2/CH4 and CH4/N2 selectivity's for n-ZIF-67 at 298 K.

from 1 :

 1 1 ratio mixtures at different pressures from 1 to 20 bar, taking into account both the adsorption isotherm data set and breakthrough experiments. The selectivity for CH4 over N2 was very low, emphasizing the main impact of this material for CO2 capture. Comparative adsorption experiments revealed consistently better adsorption and separation performance for the PEG-templated n-ZIF-67 compared to PEG-templated n-ZIF-8 or other ZIFs in the literature. The adsorption enthalpy data (Qst = 34.44 KJ/mol) obtained from highly correlated Langmuir model fitting revealed that CO2 adsorption at n-ZIF-67 was driven by physical rather than chemical interactions at zero loading, consistent with the important role of the high specific surface area ultramicroporous structure (e.g.

Table 1 .

 1 Comparison of the CO2 and CH4 adsorption parameters diverse MOFs and ZIFs

ZIFs/ MOFs Soft template BET surface area Particle Size Pore volume

  Cu3(BTC)2 with BTC: benzene-1,3,5-tricarboxylic acid; c Zn(bIm)(nIm) with bIm: benzimidazole and nIm: 2-Nitroimidazole; d Zn(cbIm)(nIm) with cbIM: 5-chlorobenzimidazole; e Zn2(dhtp)(H2O)2.8H2O with dhtp: 2,5dihydroxyterephthalate; f Al4(OH)8(BTEC) with BTEC: 1,2 ,4,5-benzenetetracarboxylic acid ; g Zn(almeIm)2 with almeIm: 4-methylimidazole-5carbaldehyde; h Zn(bIm)2 with bIm: benzimidazole; i Zn(bIm)2; j Zn(dcIm)2 with dcIm: 4,5-dichloroimidazole; j Zn(mIm)2 with mIm: 2methylimidazolate; k PDADMAC: poly(diallyldimethylammonium chloride) ; l Co(mIm)2 ; m PEG as polyethylene glycol (average molecular weight = 20.000 g mol -1 )

						Pore diameter	CO2 adsorption	CH4 adsorption	CO2/CH4 Selectivity	Ref
			m 2 /g	(nm)	cm 3 /g	(nm)	(mg/g)	(mg/g)
	Zr-MOF a	none	1433	63	0.63	0.88
	HKUST-1 b none	1571	-	0.79		558.8 (298 K, 15	73.6 (298 K, 15 bar) -		[13]
							bar)
	ZIF-68 c , 69 d	none	1090, 950	-	-	1.03, 0.78 61.6-79.2 (298 K, 102.4 (298 K, 1 bar) 5.5-5.6 1bar)	[14]
	Mg-MOF-74 e none	1174	5000-25000	0.648	1.02	360.8 (298 K, 1 bar) 16.0 (298 K, 1 bar)	-	[15]
	MIL-120 f	none	-	-	-	-	211.8 (303 K, 10	28.8 (303 K, 10 bar)	-	[16]
	ZIF-93 g	none	864	-	0.46	1.79	bar) 407 (298 K, 60 bar)	-	-	[17]
	ZIF-11 h	none	-	-	0.46	1.49	269.3 (298 K, 40	-	-	[17]
	ZIF-7 i	none	-	-	0.21	0.75	bar) 89.8 (298 K, 40 bar)	-	-	[17]
	ZIF-71 j	none	1025	1000-	-	1.65	356.4 (298 K, 45	-	-	[18]
	ZIF-8 j	PDADMA	1264	2000 57	0.51	-	bar) 347.6(298 K, 40	-	-	[19]
		C k					bar)
	ZIF-8	none	1475		0.70	-	36.1(298 K, 1 bar)	-	-	[20]
	ZIF-8	none	1502	90	0.54	0.7-1.2	469.9 (298 K, 40	-	-	[21]
	ZIF-67 l	none	1478	500-	0.66	bar) 0.64-1.04 513.9 (298 K, 50	-	-	[22]
				1000			bar)
	n-ZIF-8	PEG m	1694	150	0.67	0.787	51.5 (298 K, 1 bar) 36.0 (298 K, 1 bar)
	n-ZIF-8	PEG	1694	150	0.67	0.787	547.0 (298 K, 40 bar)	211.58 (298 K, 40 bar)	3.00-8.06	[23]
	n-ZIF-67	PEG	1891	66	0.86	0.64	72.0 (298 K, 1bar) 49.0 (298 K, 1 bar)		This
	n-ZIF-67	PEG	1891	66	0.86	0.64	681 (298 K, 40 bar) 241 (298 K, 40 bar) 3.67-16.05	work

356.1 (273, 9.8 bar) 57.6 (273, 9.8 bar) 2.2-3.6 [12] a Zr6O4(OH)4(BDC)6 with BDC: 1,4-dicarboxybenzene; b

Table 2 :

 2 Refined structural parameters and calculated grain sizes, crystallite sizes and strains of n-ZIF-

67 n-ZIF-67 [Co (mIm)2] Cubic

	Space group	I -4 3 m
	Cell parameters	a=b=c = 17.0545 (1) Å
	Cell volume	4873.139 (2) Å 3
		Rp = 0.339
	Discrepancy factors (%)	RF = 0.914 Rwp = 0.53
		χ 2 = 1.94
	Bragg R factor	1.34
	DTEM (nm)	~ 30 -100
	Dsch (nm)	78
	DW-H (nm)	~ 91
	Strain (ε)	0.000325

Table 3

 3 Equation parameters for the Langmuir isotherm model for PEG-templated n-ZIF-67

	Adsorbate	T(K)	Q m (mg.g -1 )	k L (L. g -1 )	R 2
		273	854.48	1.98	0.998
		298	690.39	1.75	0.999
	CO2	323	551.40	1.62	0.999
		353	429.83	1.48	0.998
		273	269.67	1.46	0.999
		298	245.71	1.38	0.999
	CH4	323	211.02	1.29	0.999
		353	171.22	1.14	0.999
		273	259.12	1.22	0.999
		298	229.32	1.16	0.999
	N2	323	188.51	1.09	0.999
		353	142.98	0.98	0.999

Table 1 .

 1 Comparison of the CO2 and CH4 adsorption parameters diverse MOFs and ZIFs Zr6O4(OH)4(BDC)6 with BDC: 1,4-dicarboxybenzene; b Cu3(BTC)2 with BTC: benzene-1,3,5-tricarboxylic acid; c Zn(bIm)(nIm) with bIm: benzimidazole and nIm: 2-Nitroimidazole; d Zn(cbIm)(nIm) with cbIM: 5-chlorobenzimidazole; e Zn2(dhtp)(H2O)2.8H2O with dhtp: 2,5dihydroxyterephthalate; f Al4(OH)8(BTEC) with BTEC: 1,2 ,4,5-benzenetetracarboxylic acid ; g Zn(almeIm)2 with almeIm: 4-methylimidazole-5carbaldehyde; h Zn(bIm)2 with bIm: benzimidazole; i Zn(bIm)2; j Zn(dcIm)2 with dcIm: 4,5-dichloroimidazole; j Zn(mIm)2 with mIm: 2methylimidazolate; k PDADMAC: poly(diallyldimethylammonium chloride) ; l Co(mIm)2 ; m PEG as polyethylene glycol (average molecular weight = 20.000 g mol -1 )

	ZIFs/ MOFs	Soft template	BET surface area	Particle Size	Pore volume	Pore diameter	CO2 adsorption	CH4 adsorption	CO2/CH4 Selectivity	Ref
			m 2 /g	(nm)	cm 3 /g	(nm)	(mg/g)	(mg/g)
	Zr-MOF a 356.HKUST-1 b none none 1433 63 0.63 0.88 1571 -0.79 558.8 (298 K, 15	73.6 (298 K, 15 bar) -		[13]
							bar)
	ZIF-68 c , 69 d	none	1090, 950	-	-	1.03, 0.78 61.6-79.2 (298 K, 102.4 (298 K, 1 bar) 5.5-5.6 1bar)	[14]
	Mg-MOF-74 e none	1174	5000-25000	0.648	1.02	360.8 (298 K, 1 bar) 16.0 (298 K, 1 bar)	-	[15]
	MIL-120 f	none	-	-	-	-	211.8 (303 K, 10	28.8 (303 K, 10 bar)	-	[16]
	ZIF-93 g	none	864	-	0.46	1.79	bar) 407 (298 K, 60 bar)	-	-	[17]
	ZIF-11 h	none	-	-	0.46	1.49	269.3 (298 K, 40	-	-	[17]
	ZIF-7 i	none	-	-	0.21	0.75	bar) 89.8 (298 K, 40 bar)	-	-	[17]
	ZIF-71 j	none	1025	1000-	-	1.65	356.4 (298 K, 45	-	-	[18]
	ZIF-8 j	PDADMA	1264	2000 57	0.51	-	bar) 347.6(298 K, 40	-	-	[19]
		C k					bar)
	ZIF-8	none	1475		0.70	-	36.1(298 K, 1 bar)	-	-	[20]
	ZIF-8	none	1502	90	0.54	0.7-1.2	469.9 (298 K, 40	-	-	[21]
	ZIF-67 l	none	1478	500-	0.66	bar) 0.64-1.04 513.9 (298 K, 50	-	-	[22]
				1000			bar)
	n-ZIF-8	PEG m	1694	150	0.67	0.787	51.5 (298 K, 1 bar) 36.0 (298 K, 1 bar)
	n-ZIF-8	PEG	1694	150	0.67	0.787	547.0 (298 K, 40 bar)	211.58 (298 K, 40 bar)	3.00-8.06	[23]
	n-ZIF-67	PEG	1891	66	0.86	0.64	72.0 (298 K, 1bar) 49.0 (298 K, 1 bar)		This
	n-ZIF-67	PEG	1891	66	0.86	0.64	681 (298 K, 40 bar) 241 (298 K, 40 bar) 3.67-16.05	work

1 (273, 9.8 bar) 57.6 (273, 9.8 bar) 2.2-3.6 [12] a

Table 2 :

 2 Refined structural parameters and calculated grain sizes, crystallite sizes and strains of n-ZIF-67

	n-ZIF-67 [Co (mIm)2]	Cubic
	Space group	I -4 3 m
	Cell parameters	a=b=c = 17.0545 (1) Å
	Cell volume	4873.139 (2) Å 3
		Rp = 0.339
	Discrepancy factors (%)	RF = 0.914 Rwp = 0.53
		χ 2 = 1.94
	Bragg R factor	1.34
	DTEM (nm)	~ 30 -100
	Dsch (nm)	78
	DW-H (nm)	~ 91
	Strain (ε)	0.000325

Table 3

 3 Equation parameters for the Langmuir isotherm model for PEG-templated n-ZIF-67

	Adsorbate	T(K)	Q m (mg.g -1 )	k L (L. g -1 )	R 2
		273	854.48	1.98	0.998
		298	690.39	1.75	0.999
	CO2	323	551.40	1.62	0.999
		353	429.83	1.48	0.998
		273	269.67	1.46	0.999
		298	245.71	1.38	0.999
	CH4	323	211.02	1.29	0.999
		353	171.22	1.14	0.999
		273	259.12	1.22	0.999
		298	229.32	1.16	0.999
	N2	323	188.51	1.09	0.999
		353	142.98	0.98	0.999

Acknowledgment

Financial support for this study was provided by Universiti Kebangsaan Malaysia (UKM), under Geran Universiti Penyelidikan (GGPM-2022-067), ANR under reference ANR-20-CE05-0006, and the Deanship of Scientific Research at Majmaah University, Saudi Arabia. The authors would like to thank the Ministry of High Education and Research Fund of Tunisia. Acknowledgment Financial support for this study was provided by Universiti Kebangsaan Malaysia (UKM), under Geran Universiti Penyelidikan (GGPM-2022-067), ANR under reference ANR-20-CE05-0006, and the Deanship of Scientific Research at Majmaah University, Saudi Arabia. The authors would like to thank the Ministry of High Education and Research Fund of Tunisia.

Data availability

No data was used for the research described in the article

Data availability

No data was used for the research described in the article

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Declarations of interest: NONE

Conflict of Interest

Author Contributions Statement

Nadhem Missaoui, Amani Chrouda: Formal analysis, Resources, Methodology, Writing -review& editing. Hamza Kahri, Andrew J Gross, Investigation, Writing-original draft. Mohammad Rezaei Ardani: Investigation, Pang Ai Ling: Conceptualization, Writing -review& editing. Mohsen Ahmadipour: Supervision, Funding acquisition, Conceptualization, Writing -review& editing.

Adsorption/desorption cycles

The adsorption stability of the n-ZIF-67 adsorbent was subsequently evaluated by prolonged cyclic CO2 and CH4 adsorption-desorption cycles (15 cycles of adsorption and regeneration). The adsorbents were heated to 423 K (10 K/min) under N2 gas (50 mL/min) after each adsorption to release any adsorbed gases that remained on the surface of n-ZIF-67. The adsorbent was then cooled to 298 K while being maintained under N2, and the weight was measured. A subsequent CO2 adsorption was performed to continue the cycling test. The percentage ratio of the adsorption capacity of the regenerated adsorbent to the fresh one is defined as the adsorption index (AI) and is determined by the equation as follows:

Eq. ( 8)

Where, q1 and qn indicate the CO2 and CH4 adsorption capacity of the first cycle and the n th (n = 1-15) cycle, respectively.

The adsorption stability of n-ZIF-67 can be considered as its recyclability. The n-ZIF-67 adsorbent was used/reused over 15 cycles of successive CO2 adsorption and desorption under the specified condition of 298 K and 40 bar of CO2 gas (Fig. 12 (a)). It was stated earlier that the CO2 adsorption capacity of as-prepared n-ZIF-67 was 681 mg (CO2)/g (100%). In the second cycle, this value dropped to 680.2 mg (CO2)/g (99.88%). After each cycle, the CO2 adsorption capacity of n-ZIF-67 was very slightly reduced. The capacity was 679.6 mg (CO2)/g by the end of the 15 th cycle (99.79%). This reduction should be related to the material loss as a result of the recycling process. Nevertheless, the CO2 adsorption capacity after 15 cycles was still higher than 99%, which confirms the high reusability of the n-ZIF-67 adsorbent. Moreover, the CO2 adsorption and desorption cycle curves were similar, suggesting the strong stability of n-ZIF-67 and appropriateness of this material for CO2 adsorption.

The extended cyclic CH4 adsorption on n-ZIF-67 adsorbent was also conducted, and the results are shown in Fig.

12 (b). The results show that the adsorption capacity only decreased from 100% to 99.91% by the end of the 15 th cycle, signifying that most CH4 molecules can also be successfully desorbed throughout the regeneration operation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.