Hybrid Postsynthetic Functionalization of Tetraethylenepentamine

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Hybrid post-synthetic functionalization of tetraethylenepentamine onto MIL-101(Cr) for separation of CO2 from CH4 Hyung Chul Yoon, Phani Brahma Somayajulu Rallapalli, Hee Tae Beum, Sang-sup Han, and Jong-Nam Kim Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03382 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Hybrid post-synthetic functionalization of tetraethylenepentamine onto MIL-101(Cr) for separation of CO2 from CH4 Hyung Chul Yoon, ‡* Phani Brahma Somayajulu Rallapalli,‡ Hee Tae Beum, Sang Sup Han, and Jong-Nam Kim* Clean Fuel Laboratory, Korea Institute of Energy Research, Daejeon, 305-343, Republic of Korea.

KEYWORDS tetraethylenepentamine, MIL-101(Cr), adsorption, natural gas sweetening, carbon dioxide, methane

1 Environment ACS Paragon Plus

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ABSTRACT

To remove CO2 from CH4, tetraethylenepentamine was grafted onto coordinatively unsaturated centers of MIL-101(Cr) by post-synthetic functionalization: wet impregnation at 25 °C, followed by grafting, drying, and washing. Compared to MIL-101(Cr), TEPA-MIL-101(Cr) showed 54% higher CO2 adsorption at 1 bar and 98% reduction of CH4 adsorption at 60 bar. The ideal adsorption solution theory (IAST) selectivity of CO2/CH4 for a binary gas mixture of 2% CO2+98% CH4 at 298 K and 60 bar predicted by Toth equation was found to be 11 and 598 for ungrafted and grafted MIL-101(Cr), respectively. Single column breakthrough tests for upgrading 2% CO2+98% CH4 mixture to liquefied quality of natural gas (CO2 < 50 ppm) under various operating conditions including different temperatures and total amount of purge gas at the fixed pressure of 60 bar and temperature of 297 K. At the feed flow rate of 1,000 sccm, the TEPA-MIL-101(Cr) extrudates obtained 0.89 mmol/g CO2 adsorption capacity and nearly 83% of adsorbed CO2 can be removed by regenerating extrudates at 393 K with 79 cm3/gadsorbent of total amount of purge gas.


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Introduction Raw natural gas contains mainly methane and considerable amounts of light and heavy hydrocarbons. The other contaminants are generally acid gases such as CO2 and H2S. The acid gases cause corrosion and fouling of the pipeline and equipment during the transportation and liquefaction of natural gas. In addition, in order to avoid dry ice formation, the CO2 level in the feed natural gas stream before liquefaction should be controlled to below 50 ppmv [1]. Amine-based acid gas removal processes have been employed in the gas industry, but they have many disadvantages including equipment corrosion, amine degradation, high energy requirement, and low efficiency of regeneration. To date, these issues have not been adequately resolved [2]. Currently, adsorptive acid gas removal technologies have attracted significant interest, because of the simplicity of adsorbent regeneration by thermal or pressure variation [3]. However, in order to compete with the liquid amine-based methods for commercial use, the following crucial criteria for the adsorbents must be considered to maximize CH4 recovery: (1) a high CO2 adsorption capacity and substantially low CH4 adsorption capacity, and (2) long-term stability without significant leaching and degradation of amines. Solid hybrid adsorbents, produced by immobilizing liquid amines on the surface of porous solids, have been found to have selectivity comparable to the pristine amines. Amine immobilization on adsorbents is mainly carried out by one of two methods, namely impregnation and grafting. Grafting has been subsequently considered more efficient, as it prevents the leaching of amines by forming covalent bonds between the solid support and amines [4]. Several amine-immobilized adsorbents based on zeolites, periodic mesoporous silica (PMS) materials, mesoporous alumina and activated carbons have been reported for


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capturing CO2 [5]. Among them, metal-organic frameworks (MOFs), formed by the combination of metallic clusters with organic ligands, have attracted particular attention as novel adsorbents because of their large surface area and pore volume [6]. During heating, some MOFs lose their terminal solvent/water molecules connected to the metallic centers to generate coordinatively unsaturated centers (CUSes) or Lewis acid sites [7]. These electron-deficient CUSes enable post-synthetic modification of the MOFs, as an electron-rich molecule (such as organic diamines) can attach to them via the formation of a dative bond. This dative bond strongly binds the amine molecule to the surface of MOFs, a type of functionalization that is impossible in silica materials which lack CUSes [8-10]. MIL-101(Cr) is one such MOF that contains numerous unsaturated chromium sites up to 3 mmol/g [11] Apart from the CUSes, it also possesses bimodal mesoporous cages, high surface area, large pore volume, and high hydrothermal stability; all of which are necessary for adsorbents in natural gas separation [12]. Hwang et al. [13] first reported the grafting of diamines on MIL-101(Cr). Later, Kim et al. [14] and Wang et al. [15] reported the grafting of diethylenetriamine (DETA) and tetraethylenepentamine (TEPA) on MIL-101(Cr), respectively. In these amine-grafted materials, a small increase in CO2 adsorption capacity was observed in the low-pressure range (P < 0.15 bar). However, the overall CO2 adsorption capacities (P ≈ 1 bar) were even less than that of the pristine MIL-101(Cr) [14, 15]. The Chen group has done extensively work on MIL101(Cr) to increase its CO2 adsorption by impregnation using different linear and branched polyethyleneamines (PEI) with various molecular weights, resulting in a high CO2 adsorption by chemisorption (4.1 mmol/g) especially in the low pressure region (P < 0.15 bar) [16-18] However, the PEI-impregnated adsorbents have two main drawbacks:


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(1) leaching when using PEI with low molecular weight [19], and (2) slow diffusion when using PEI with high molecular weight [20]. The Chen group also reported the direct synthesis of amine-functionalized MIL-101(Cr) by using amino-terephthalic acid as a linker, although its CO2 adsorption isotherm did not show any chemisorption and was identical to that of TEPA-grafted MIL-101(Cr) [15, 21]. These disappointing results could be ascribed to: (1) fewer CO2 adsorption sites due to acid-base neutralization between CUSes and other free amine moiety [22], and (2) the blocking of pores with narrow pore openings (1.2–1.6 nm) for mesoporous cages (2.9–3.4 nm) [16]. In order to resolve these problems, substantial amount of amine molecules has to be immobilized at once into the MIL-101(Cr) pores. In the current work, we present a new methodology of post-synthetic amine immobilization into MIL-101(Cr), using incipient wetness (IW) impregnation of a TEPA solution followed by grafting. What is novel here is the methodology to immobilize amines at once into the MIL-101(Cr) pores, thereby minimizing pore blocking and enhancing the CO2 adsorption capacity. Based on the relative volumes of impregnation liquid (Vimp) and the support pores (Vp), the impregnation methods can generally be divided into IW impregnation (Vimp ≤ Vp) and equilibrium adsorption impregnation (Vimp ≥ Vp) [23]. In previous reports, the amine was initially dissolved in excess toluene and reacted with MIL-101(Cr) under reflux conditions for 12 h [13]. In that case, Vimp ≥ Vp, and the impregnation is governed by diffusion. We expect that during their diffusion into the MIL-101(Cr) pores, large amine molecules such as TEPA could initially occupy the pore openings and block additional amine diffusion into the pores, thereby lowering the loading. Therefore, in this study we dissolved TEPA in a minimum amount of toluene,


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and performed IW impregnation which is mainly governed by capillary action. Since the capillary action is much faster than diffusion, a large amount of TEPA can enter the pores at once. The sample was then heated at 80 °C to graft TEPA to the CUSes in MIL101(Cr), followed by drying under nitrogen flow for 3 h to remove the toluene solvent. Subsequent soaking and washing with toluene removed the unreacted TEPA which agglomerated near the pore openings or on the outer surface of MIL-101(Cr), producing the purified grafted TEPA-MIL-101(Cr). Experimental Materials. Chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O, 99.0%), terephthalic acid (H2BDC, 98.0%), acetic acid (ACS Reagent, ≥99.7%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), ammonium fluoride (NH4F, ACS Reagent, ≥98.0%), and tetraethylenepentamine (TEPA, technical grade) were purchased from Sigma-Aldrich. Toluene (extra pure, 99.5%) was purchased from Samchun Pure Chemicals, South Korea. CO2, CH4, N2, He, and O2 were of ultra-high pure grade (99.999%) procured from Korea Special Gas Co., Ltd., South Korea. Preparation of MIL-101(Cr). The synthesis and purification methods for MIL101(Cr) were adapted from reference [24] with minor changes. Briefly, 2.2 g of H2BDC was charged into a 100-ml, Teflon-lined steel bomb. A solution of 5.3 g Cr(NO3)3·9H2O in 65 ml of water was added to H2BDC. The mixture was stirred for 5 min with a spatula. Next, 0.77 ml of acetic acid was added to the mixture, and the bomb was heated in an oven at 493 K for 8 h. After cooling to room temperature, the crude product was filtered and washed several times with water, before drying at 353 K in an air oven. The obtained product was a green powder containing white needle-type crystals of unreacted H2BDC.


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To remove the unreacted H2BDC, a two-stage solvent treatment was applied. The crude product was initially refluxed in DMF overnight, filtered while hot, and then washed with DMF. The material was then dried at 353 K in an air oven. Afterwards, the sample was refluxed in 0.03 M NH4F solution for 8 h, filtered while hot, and washed with hot water. The final product was dried at 393 K in an air oven overnight. Preparation of TEPA-MIL-101(Cr). 550 mg of the prepared MIL-101(Cr) was degassed at 453 K in vacuum for 18 h. The sample was allowed to cool down to 353 K. TEPA (250, 325, 500, or 625 mg) was added to 0.5 mL of toluene to load different wt% amounts of TEPA onto MIL-101(Cr). The mixture was sonicated at room temperature for 15 min, followed by adding the dried MIL-101(Cr) and mixing. The mixture was then dried at 358 K under N2 flow (200 ml/min) for 3 h. Then, 25 ml of toluene was placed in a 50-ml vertex centrifuge tube, and the TEPA-impregnated MIL-101(Cr) was dispersed in the toluene. The mixture was stirred at room temperature for one day. During this time, toluene was removed by centrifugation for 3 times, and each time followed by adding fresh toluene to the sample. After one day, the mixture was dried at 333 K for 3 h in an oven. Instrumentation. The powder X-ray diffraction (PXRD) patterns were measured at ambient temperature with a PHILIPS X’pert MPD diffractometer, in the angle range of 2θ = 2−60° at a scan speed of 0.1°/s with Cu Kα1 (λ = 1.54056 Å) radiation. Fourier transform infrared (FTIR) spectra were collected on a Jasco FTIR-6100 spectrometer at room temperature with 1.0 cm-1 resolution. The thermal stability was investigated with a thermogravimetric analyzer (TGA, Q50 V20.2 Build 27), at the heating rate of 1 K/min under an oxygen atmosphere and in the temperature range of 298−1073 K. SEM image


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was taken with a Hitachi S-4800 system. The elemental (CHN) analysis was performed with a Thermo Scientific FLASH 2000 apparatus. The Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore diameter were determined in a static volumetric gas adsorption system (ASAP 2020, Micromeritics Instrument Corporation, USA) by using the N2 adsorption-desorption isotherm at 77.4 K up to 1 bar of relative pressure. The BET equation, the Barrett–Joyner–Halenda (BJH) desorption, and single point adsorption methods were employed for estimating the surface area, pore diameter, and pore volume, respectively. The pore size distribution (PSD) curve was calculated by the TarazonaNLDFT model for N2 at 77 K, and by DFT model for CO2 at 273 K, using the inbuilt software in ASAP 2020. Equilibrium adsorption measurements. Adsorption isotherms of CO2 and CH4 at pressures up to 1 bar were measured with a BELSORP-mini system (BEL, Japan). The sample was placed in a constant-temperature water bath to maintain the desired temperature. High-pressure CO2 and CH4 adsorption experiments were carried out at 298 K in the pressure range of 0−30 bar in an automated high-pressure gas adsorption system (BELSORP-HP, BEL Japan). Prior to the measurements, the samples were dried at 423 K under vacuum for 18 h. Samples were weighted both before and after drying to obtain the weight difference. The experimental adsorption isotherms are fitted in Freundlich model (equation 1) and Toth model (equation 2). The Percentage of Average Relative Error (D%) between experimental and simulated values was measured by equation 3 as given below.

= () Where P is equilibrium pressure, k and n are Freundlich constants.


(1)

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=

(2)

( ) ( )

where q is the amount adsorbed, qsat is the saturated amount adsorbed, P is the equilibrium pressure, b is the equilibrium constant and n is the parameter which indicates the heterogeneity of the adsorbent. % =

∑

(3)

The equilibrium selectivity is due to the variable affinity of the adsorbent towards different components in the gas mixture, and is an important parameter for assessing the separation ability and the regeneration performance of the adsorbent. The ideal adsorption solution theory (IAST) selectivity of CO2/CH4 (Sads) predicted from Toth model parameters of the high-pressure sorption isotherms was calculated by using the following equation 4. ads =

1

"

× "

(4)

where q1 and q2 represent the quantities of adsorbed CO2 and CH4, and p1 and p2 the corresponding partial pressures, respectively. The isosteric heat of adsorption (Qst) form single isotherm measured at single temperature was predicted by the following equation 5 using Toth model parameters.

#$% = ln (

)

+

) * +

+

* )+ *

(,)

- + /" + 01

(5)

where Psat is saturated adsorbate pressure, b & n are Toth model parameters, θ is the coverage (θ) = q/qsat, λp is latent heat of vaporization of the adsorbate, R is universal gas constant and T is the adsorption temperature.


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Dynamic adsorption measurements. The dynamic adsorption experiments were carried out in a single column (1 cm in diameter and 33 cm long) breakthrough setup, which is shown in the breakthrough measurement setup is shown in Figure 1. The weight of the TEPA-MIL-101(Cr) adsorbent packed in the column was 19 g. Initially, the adsorbent was regenerated under 200 sccm of N2 at 373 K overnight, a procedure that was also carried out after each set of experiments. A binary gas mixture (2% CO2 and 98% CH4) was used for the breakthrough studies. The breakthrough point for CO2 was set to 50 ppm, which satisfies the specifications for liquefied natural gas (LNG). The feed and purge flows were controlled by mass flow controllers (No. 5850, Brook Instruments, USA). The temperatures at the bottom, middle, and upper portions of the column were measured during the adsorption experiments by T-type thermocouples. The concentration of CO2 in the product gas was analyzed by a CO2 analyzer (Infrared Gas Analyzer, Fuji Electric Co. Ltd., Japan). The amount of purge gas per gram of adsorbent can be defined as: #"2345 =

%×6789 :;

(6)

where Vpurge is the total volume of the purge gas, mads is the total mass of the loaded MIL101(Cr), and t is the total purging time. A binary gas mixture of 2% CO2+98% CH4 was used to examine the efficacy of TEPA-MIL101(Cr) for acid gas removal via the pressure temperature swing adsorption (PTSA) process. The PTSA breakthrough experiments at room temperature consist of repeating the following steps: (a) Backfill the column with pure CH4 until Pmax = 60 bar; (b) Conduct adsorption until the CO2 concentration is up to 50 ppm; (c) Depressurize until Pmin = 1 bar; (d) Increase the temperature of the column;


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(e) Purge with pure CH4; (f) Cool the column down to room temperature; (g) Repeat step (a).

Results and Discussion Characterization. Figure 2 shows the powder X-ray diffraction (PXRD) spectra of MIL101(Cr) and TEPA-MIL-101(Cr) (500 mg TEPA + 550 mg MIL-101(Cr)). The former is in good agreement with a previous report [11]. Because of scattering from TEPA-MIL-101(Cr), the PXRD peak intensities were reduced after grafting. Broadening and shifting of the peaks towards the lower angle were also observed, and attributed to the expansion of the MIL-101 lattice after the grafting of amine molecules. Similar observations had been made in mesoporous silica composite materials [25]. Figure S1 depicts the Fourier transform infrared (FTIR) spectra. TEPA-MIL-101(Cr) shows additional peaks at 2810 cm-1 due to the C-H stretching vibration, and at 2939 and 3272 cm-1 due to the N-H stretching vibrations, all of which are absent in MIL101(Cr). These results of TEPA-MIL-101(Cr) are in good agreement with a previous report [16]. Figure 3 and the inset show the N2 sorption isotherms and PSD calculated by non-local density functional theory (NLDFT) from the N2 adsorption data at 77 K, respectively, for MIL101(Cr) and TEPA-MIL-101(Cr) (500 mg TEPA + 550 mg MIL-101(Cr)). After grafting, the Brunauer–Emmett–Teller (BET) surface area and pore volume of MIL-101(Cr) were reduced from 3104.11 to 823.50 m2/g and from 1.8 to 0.49 cm3/g, respectively. NLDFT was applied for the PSD analysis, because the classical BJH method based on Kelvin equation could only be applied to mesopore size analysis, as it fails to describe the micropores and even narrow mesopores properly. The other classical methods, such as Dubinin-Radushkesvich (D-R), Horth


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and Kawazoe (HK), and Saito and Foley (S-F) methods could describe micropore filling, but are not applicable for mesopore size analysis. Overall, none of these single methods could explain both the micro- and mesopore regions in a single adsorption isotherm. In addition, these classic methods do not describe the pore wall-fluid interactions, instead they assume that the pore fluid has similar thermodynamic properties (shifts in critical point, freezing point, triple point, etc.) as the bulk fluid [26]. To accurately describe thermodynamic properties of the pore fluid, statistical mechanics should be applied to connect the macroscopic properties to the molecular level behavior. Examples include density functional theory (DFT), NLDFT, molecular simulations (Monte Carlo simulation), and molecular dynamics (MD). Among them, NLDFT gives more accurate predictions of fluid density profile in narrow pores [26]. As shown in the inset in Figure 3, MIL-101(Cr) contained two major peaks in the ranges of 2.16−2.45 and 2.9–3.17 nm, which are close to the mesoporous cage sizes of 2.9 and 3.4 nm calculated from its crystal structure [12]. It also contained two minor peaks within 2.45−2.8 nm. In comparison, the PSD of TEPAMIL-101(Cr) showed a great reduction of pore volume in the range of 2.16–2.66 nm. The peak at around 3 nm disappeared, and an additional peak emerged at around 1.8 nm. The super tetrahedra of MIL-101(Cr) are microporous and have a diameter of 0.86 nm. However, the N2 adsorption method could not obtain the PSD below 1 nm. The reason might be that filling of the pores in the 0.5–1 nm diameter range with N2 usually occurs at very low relative pressures (10-7 to 10-5). At such low pressures, N2 diffusion and adsorption are both very slow, because the diatomic N2 molecule has a quadrupole moment, leading to specific fluid-wall interactions [27, 28]. To overcome this problem, CO2 adsorption was measured for both MIL-101(Cr) and TEPAMIL-101(Cr) (500 mg TEPA + 550 mg MIL-101(Cr)) at 273 K, and the PSD was calculated using DFT and plotted in Figure 4.


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CO2 can access the ultra-micropores, because its higher absolute pressure at 273 K (P0 = 26200 Torr) allows it to diffuse very quickly to reach equilibrium much faster than N2 [29]. CO2 adsorption was measured at 273 K to determine the PSD values for pores smaller than 1 nm using density functional theory (DFT). In Figure 4, MIL-101(Cr) showed several PSD peaks in the range of 0.47–0.89 nm, but the major peak was within 0.55–0.59 nm. It should be noted that the former can also be due to secondary pores generated by the agglomeration of unreacted terephthalic acid particles, whose removal from inside the pores of MIL-101(Cr) framework is difficult [12]. TEPA-MIL-101(Cr) showed a major peak at 0.35–0.37 nm. Due to the grafting of TEPA on the CUSes, the pore size was reduced and ultra-micropores were formed. Results from both the NLDFT study for N2 and the DFT study for CO2 clearly reveal that grafting TEPA reduced the pore size and available surface area of MIL-101(Cr). The thermogravimetric analysis (TGA) results in Figure S2 show that TEPA-MIL-101(Cr) is stable at least up to 423 K. TEPA-MIL-101(Cr) shows two weight-loss steps. In the first step, 10.7% weight loss occurred between 25 and 100°C, corresponding to the loss of moisture or adsorbed atmospheric gases. The second step is a continuous weight loss of 37.5% between 150 and 430°C until the framework decomposition, which indicates the slow departure of TEPA from the MIL-101 pores due to the former’s high boiling point (340°C). The sample amount of TEPA-MIL-101(Cr) in the TGA experiment was 20.75 mg. Correspondingly, the absolute weight loss in the first and second steps was 2.22 and 6.95 mg. The latter means 0.335 g TEPA (i.e. 59%) was present in 1 g of MIL-101(Cr). In the scanning electron microscopy (SEM) images (Figure S3), the octahedral cubic symmetry and morphology of MIL-101(Cr) remained the same after the grafting of TEPA. Figure S4, Figure S5, and Figure S6 represent the results of energy-dispersive X-ray


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spectroscopy (EDS) elemental analysis and EDS mapping for both MIL-101(Cr) and TEPAMIL-101(Cr), confirming the presence of N after grafting. The CHN analysis results (Table 1) reveal that the percentages of carbon, hydrogen, and nitrogen all increased in TEPA-MIL101(Cr) compared to MIL-101(Cr). TEPA-MIL-101(Cr) contained 12.71% nitrogen, which is equivalent to 0.343 g of TEPA per gram of MIL-101(Cr). In theory, MIL-101(Cr) contains 3 mmol/g CUS [13], meaning that 3 mmol (0.567 g) of TEPA is required to achieve 100% grafting on the CUSes. In the present work, the actual grafted amount of TEPA (0.343 g, 1.81 mmol) is equivalent to a grafting ratio of 60.5%, which is slightly greater than that obtained from the TGA results (59%). Similarly, the grafting ratio calculated from CHN analysis for samples impregnated with 250, 325, and 625 mg TEPA was 42.6%, 51.9%, and 62.03% respectively.

CO2 and CH4 adsorption studies. Figure 5 shows the effect of grafting ratio on the CO2 adsorption capacity of various TEPA-impregnated MIL-101(Cr) samples. The CO2 adsorption isotherms were measured at 298 K in the pressure range of 0−1 bar. The CO2 adsorption capacity obtained for 42.6%, 51.9%, 60.5%, and 62.03% grafting ratio was 2.9, 2.8, 3.4, and 3.3 mmol/g, respectively. The capacity was not further enhanced after reaching the grafting ratio of 60.5%. Hence, an equal wt% amount of TEPA was the optimum for loading onto MIL-101(Cr) to maximize the CO2 adsorption capacity. Note that the discussions hereinafter are only for TEPA-MIL-101(Cr) prepared with this optimum grafting ratio. Figure 6(a) shows the CO2 and CH4 adsorption isotherms at 298 K in the pressure range of 0−1 bar. The CO2 adsorption capacities at 1 bar for non-purified and purified (i.e., after toluene soaking and washing) TEPA-MIL-101(Cr), and pristine MIL-101(Cr)


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were 2.5, 3.4, and 2.2 mmol/g, respectively. Although non-purified TEPA-MIL-101(Cr) contains more TEPA, it has a lower CO2 adsorption capacity because the unreacted TEPA could block the pores. The capacity of purified TEPA-MIL-101(Cr) is 3.5 times higher in the low-pressure regime (0–0.15 bar) than MIL-101, whereas the previous reports failed to show such results. At 1 bar, the overall CO2 adsorption capacity of TEPA-MIL-101(Cr) is 54% higher than those of MIL-101(Cr), DETA-MIL-101(Cr) (0.69 mmol/g) [14], TEPA-MIL-101(Cr) (2.75 mmol/g) [15], and the well-studied triamine grafted poreexpanded mesoporous silica TRI-PE-MCM-41 (2.5 mmol/g) [30]. The steps of soaking and washing in/with toluene after grafting minimize the pore blocking by removing unreacted TEPA, while the grafted TEPA molecules remain on the CUSes within the MIL-101(Cr) framework. TEPA contains two primary and three secondary amine sites. In general, the primary amines are more reactive than secondary amines. Hence, out of the two primary amine sites, one site will participate in the dative bond formation with the CUS center of the MIL-101(Cr) framework, and the reaming four amine sites (1 primary + 3 secondary) are available for CO2 adsorption. Due to their higher affinity, the primary amine sites can strongly hold the CO2 molecules, and more energy would be required for CO2 desorption [31]. The CH4 adsorption capacities for MIL-101(Cr) and TEPA-MIL101(Cr) were found to be 0.44 and 0.15 mmol/g respectively at 1 bar, corresponding to a 65.9% reduction after TEPA grafting. Figure 6(b) illustrates the experimental high-pressure adsorption isotherms of CO2 and CH4 for both materials in the pressure range of 0–30 bar at 298 K. The isotherms were fitted with Freundlich and Toth equations, and the parameters are given in Table S1. The high-pressure adsorption capacity of CH4 on TEPA-MIL-101(Cr) is of special interest, as it is a key to achieve


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high CO2/CH4 selectivity for commercial use. The obtained CO2 and CH4 adsorption capacities for MIL-101(Cr) (26.31 and 8.89 mmol/g, respectively) are close to reported values [32]. The CO2 adsorption capacity of the functionalized TEPA-MIL-101(Cr) is 10.5 mmol/g, nearly 60% less than that of MIL-101(Cr). However, it also showed excess adsorption (negative adsorption) for CH4 at high pressure. To estimate the absolute adsorption capacity of CH4, the isotherm data were fitted and extrapolated to 60 bar using the Toth equation, as suggested by a previous report [33]. In our present work, the obtained average relative error (D%), as shown in Table S1 for Toth equation fit on TEPA-MIL-101(Cr) was 1.88%, suggesting that the fit is satisfactory [34, 35]. The CH4 adsorption capacities at 60 bar for TEPA-MIL-101(Cr) and pristine MIL-101(Cr) were found to be 0.29 and 13.71 mmol/g, respectively, corresponding to a reduction of nearly 98% after TEPA grafting, presumably due to the molecular sieving effect of the adsorbent resulting from the different kinetic diameter between CH4 (0.38 nm) and CO2 (0.33 nm). The excess (negative) adsorption usually occurs at very high pressures. But in the case of TEPAMIL-101(Cr), a trend of decreasing methane adsorption appeared above 8.9 bar, and negative adsorption values were observed at over 27.5 bar in the powder form (Figure S7). In the extruded form, negative adsorption was also observed at over 1 bar (Figure S8). These results are mainly due to the errors in the mass balance calculations when using the commercial adsorption unit, which used helium to measure the void free space. Helium is an inert gas with a small kinetic diameter (0.26 nm), therefore it can access the finest pores of the adsorbent. When using methane with a larger kinetic diameter (0.38 nm), in contrast, the TEPA grafting greatly reduces the porosity of MIL-101 (as evidenced by the PSD curves shown in Figure 4) and hinders the diffusion of methane into the pores. As a result, a larger void free space was calculated using


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Energy & Fuels

helium than the amount of methane in the system, resulting in excess or negative adsorption values. Similar results due to diffusional hindrance of methane into narrow pores have been previously reported in zeolites and carbon materials [36, 37]. The isosteric heat of adsorption (Qst) of CO2 and CH4 at 298 K on MIL-101(Cr) and TEPA-MIL-101(Cr) was estimated using the Toth isosteric heat equation suggested by Whittaker et al. [38] and plotted in Figure 7. The Qst at zero coverage are also given in Table S1. The Qst of CO2 and CH4 adsorption on MIL-101(Cr) was found to be 34.08 and 19.5 kJ/mol, respectively. These values are similar to 32.5 kJ/mol obtained for CO2 and 18 kJ/mol obtained for CH4 [39]. TEPA-MIL-101(Cr) has relatively higher Qst (47.95 kJ/mol) for CO2 adsorption because of the strong interaction between CO2 and TEPA. Meanwhile, its Qst of CH4 adsorption was found to be 6.28 kJ/mol, which is lower than that of MIL-101(Cr). The Qst of the CH4 adsorption on TEPA-MIL-101(Cr) showed an increasing trend with loading because the n parameter in Toth equation is greater than 1 [38] Except for this part, the value of Qst decreased with increasing loading of CO2 and CH4. Figure 8 shows the IAST selectivity of CO2/CH4 with 0.5–5% CO2 at 298 K and 60 bar predicted by Toth equation and the values are tabulated in Table 2. The IAST described by Myers and Prausnitz [40] offers a conclusive strategy to estimate the adsorption selectivity for gas mixtures from the single-component isotherm data. Previous reports have discussed the accuracy of IAST selectivity to assess adsorption of gas mixtures in various zeolites and MOF materials [41-43]. Because this method does not require experimentation with the mixtures, it is particularly profitable for engineering applications. The selectivity of TEPA-MIL-101(Cr) is always higher than that of MIL-


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101(Cr) under the same conditions, indicating the former’s high affinity towards CO2 over CH4. The calculated CO2/CH4 selectivities of ungrafted and grafted MIL-101(Cr) for a binary gas mixture of 2% CO2+98% CH4 are 11 and 598, respectively. Breakthrough studies for adsorption of CO2 and CH4. The ability of TEPA-MIL101(Cr) to upgrade natural gas from pipeline quality (2% of CO2) to that suitable for liquefaction (

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