Embedding Graphene Nanoplates into MIL-101(Cr) Pores: Synthesis

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Embedding Graphene Nano-Plates into the MIL-101(Cr) Pores: Synthesis, Characterization, and CO2 Adsorption Studies Sina Pourebrahimi, Mohammad Kazemeini, and Leila Vafajoo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04538 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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Industrial & Engineering Chemistry Research

Embedding Graphene Nano-Plates into the MIL-101(Cr) Pores: Synthesis, Characterization, and CO2 Adsorption Studies Sina Pourebrahimi a, Mohammad Kazemeini a∗, Leila Vafajoo b a b

Chemical and Petroleum Engineering Department, Sharif University of Technology, Tehran, Iran Chemical Engineering Department, Islamic Azad University, South Tehran Branch, Tehran, Iran

Abstract In this research, the equilibrium and dynamic adsorption studies of the CO2 upon the MIL-101(Cr) metal-organic framework (MOF) as well as its GNP hybrid composites; the MIL-101(Cr)/GNP, were performed. First, the hybrid composite samples were synthesized by adding various amounts of GNP in an in-situ manner during the preparation of the MIL-101(Cr). The prepared materials characterized through several physiochemical analyses including the; powder X-ray diffraction (PXRD), adsorption of nitrogen at 77.4K, Fourier transfer infrared (FT-IR) spectroscopy, thermal analysis (DTG) and field emission scanning electron microscopy (FESEM). It was demonstrated that, the synthesized MIL-101(Cr)/GNP possessed a nearly similar crystal structure and morphology compared with those of the virgin sample. Next, the CO2 adsorption studies upon these sorbents were performed through a volumetric adsorption apparatus at 298K and CO2 pressures of up to 40bars using an in-house made rig. It was shown that, the CO2 adsorption capacity enhanced by about 43% (i.e., from 14.38 to 20.62 mmol.g−1) for the hybrid composite containing 10wt% of the GNP compared to the virgin MIL at 298K and 40bars. This enhancement in the CO2 adsorption capacity attributed to the effect of the GNP embedded into the internal MIL-101(Cr) pores giving rise to stronger interactions between the walls of this species and CO2 molecules. Furthermore, increase of the specific surface area as well as total and micropore volumes of the MIL-101(Cr) rationalized to be due to this GNP addition. Ultimately, in order to mechanistically understand the adsorbents’ behaviors, several kinetic and isotherm models understudied. It was revealed that, the FL-PFO and Dual Site Toth relationships outstandingly described the CO2 adsorption upon the sorbents. Keywords: MOF; MIL-101(Cr); CO2 Adsorption; Graphene; MOF Composite

∗

Corresponding author: Tel : +9866165425 E-mail address: [email protected] (M. Kazemeini).

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1. Introduction Global warming considered as the main consequence of the today’s massive greenhouse gas emission widely attracting public and scientific attentions1-3. In addition, the CO2, a well-known major greenhouse gas, released with large quantities from anthropogenic activities annually2-4. Furthermore, after a customary liquid-aminebased scrubbing procedure, the post-combustion CO2 capture through such technologies suffered considerable energy drawbacks3-5. In the two past decades, capture of CO2 utilizing adsorption techniques considered as an operative and viable alternative to replace the traditional CO2 scrubbing processes including the; adsorption, absorption, cryogenic distillation, etc.6,7. Amongst all the effective parameters on the gas adsorption process, choice of a suitable adsorbent considered as the most important one. Furthermore, considering diverse solid-state sorbents currently under development in particular; zeolites, activated carbons, metal oxides and metal–organic frameworks (MOFs) were the most investigated materials for the CO2 capture and storage (CCS) 812 . A novel class of crystalline porous solid-state materials called metal–organic frameworks (MOFs) has recently been investigated to utilize as adsorbent in many studies13-15. This species composed of self-assembled metal ions (metal clusters) and organic linkers (ligands) which linked together through strong coordination12-15. Despite the prominent adsorption properties of MOFs such as; high specific surface area (SBET) and enormously high fraction of void space to total space, the large free volume within their framework makes them typically prone to self-interpenetration (i.e., two frameworks grow and interpenetrated each other) preventing high porosity 13. Hence, for practical application, efforts made to apply appropriate promoters (i.e., porous or layered structures) improving the MOFs stability and adsorption performance. The resulting material will be a hybrid composite adsorbent14, 15. For this purpose, several MOFs were combined with graphitic components. Hence, synthesis, characterization, and adsorption properties of several hybrid materials such as; MIL53(Al)/GNP14, MOF-5(Cu)/GO15, MIL-100(Fe)/GO16, MIL-101(Cr)/GO17, MIL101(Cr)/SWCNT18, and MIL-101(Cr)/MWCNT19 reported to date. MIL-101(Cr) recognized as one of the most promising MOFs widely investigated for many diverse functional applications17-26 including; being used as catalysts or support for them17 as well as sorption of the water vapor reversibly adsorbed28 to name a couple. Moreover, the MIL-101(Cr) might have also been applied as an MOF in mixed2 ACS Paragon Plus Environment

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matrix membranes for gas separation29. Besides a noticeable adsorption performance of this material, MIL-101(Cr) exhibited high structural stability under the humid conditions. The stability of MOFs under such criteria was a significant subject matter for gas adsorption studies because it was difficult to remove H2O completely from industrial gas resources such as the flue gas30. In this venue, Karra et al.31 and Dong et al.32 reported a strong stability of the MIL-101(Cr) structure against humid conditions. Also, Wittmann et al.33 reported a strong moisture stability of the MIL-101(Cr) structure with durability of several cycles. This made the MIL-101(Cr) the most promising material so far; for the heat transformation applications like thermally driven heat pumps or adsorption chillers. This water-stability was recently revealed for the MIL-101(Cr)-NH2 where Khutia et al.34 reported water sorption cycle measurements on functionalized MIL-101(Cr) for a heat transformation application. In that study, the water loading capacity and water cycle stability (of 40 adsorption/desorption cycles) of four nitro- or amino-functionalized MIL-101(Cr) materials was assessed for the heat transformation applications. MIL-101(Cr) (with formula Cr3F(H2O)2O[(O2C)-C6H4-(CO2)]3.nH2O (n≈25)) might be generated due to the dicarboxylate groups of the terephthalate units playing linkers’ role to interconnect trimeric building blocks strongly to one another at the same time20. In other words, in the MIL-101(Cr) structure, a tremendous tetrahedral building unit put together by terephthalate linkers and trimeric chromium (III) octahedral clusters. However, in a recent study reported by Zhao et al.35, synthesis of the MIL-101(Cr) preformed in a fluoride-free manner being a noble way of preparing such material. Open-metal (i.e., unsaturated) sites, considered as operative adsorption sites, play a key role in selective adsorption upon the internal surfaces of the MOFs’ channels from a gaseous mixture20. These highly reactive-adsorptive sites might be created through removing guest molecules (such as DMF, H2O, H2BDC, etc.) from the framework of the MOFs resulting in an open structure in the metal cluster part. The outstanding separation performance of MIL-101(Cr) believed to be related to the existence of such adsorptive sites acting as Lewis acid ones17-20. On the other hand, due to existence of some functional groups (such as hydroxyl, carbonyl, epoxide, or alkoxy) usually available on the graphite oxide surfaces14-19, the aforementioned open-metal sites might be occupied hence, deactivated. In other words, due to the electronegative nature of the open-metal sites, these species strongly interacted with oxygen-containing functional groups most probably existing on the graphite oxide surfaces. Hence, it seemed reasonable to incorporate and/or embed porous or layered structures (i.e., possessing 3 ACS Paragon Plus Environment


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less functional group) onto and/or within the MIL-101(Cr) internal structure. Thus, the reduced graphite oxide (rGO) or exfoliated graphite (graphene nano-plates/sheets) indeed might be worthy candidates to synthesize a hybrid composite adsorbent. In this venue, the novel hybrid composites of the graphene nano-plates (GNPs) and MIL-101(Cr) including open-metal sites understudied to obtain a high CO2 adsorption. Initially, synthesis of a rigid metal-organic framework, MIL-101(Cr), based upon the hydrothermal reaction was performed. To synthesis the hybrid composite adsorbents, the graphene nano-plates incorporated onto and/or embedded within its porous structure due to the mixing of the initial materials leading to the MIL. The synthesized hybrid composite sorbents were named; MIL-101(Cr)/GNP. After characterization of this solid sorbents by means of the BET-BJH surface area measurement, powder X-ray diffraction (PXRD), Fourier transfer infrared spectroscopy (FT-IR), thermal analysis (DTG), and field emission scanning electron microscopy (FESEM), the CO2 adsorption capacities of the prepared sorbents investigated through an in-house made volumetric adsorption rig at 298K and pressures of up to 40bars. The CO2 adsorption data collected in both the dynamic and equilibrium manners. Finally, four rather frequently utilized adsorption isotherms including the; Langmuir, Sips, Toth, and Dual Site Toth were used to fit the obtained experimental equilibrium adsorption data. In addition, three of the well-known kinetic models including the; pseudo-first-order (PFO), pseudo-second-order (PSO) as well as fractal like-pseudo first order (FL-PFO) applied to the obtained data in order to shed some lights upon the adsorption behaviors of undertaken systems.

2.

Experimental Section

2.1.

Materials

All chemicals were of analytical reagent (AR) grade and used as received without further purification. Chromium nitrate nonahydrate (Cr(NO ) . 9H O, Sigma-Aldrich, 98%), terephthalic acid (H BDC, E. Merck Inc., 98%), N,N’-dimethylformamide (DMF, Fluka Co., 99%), Hydrofluoric acid (HF, E. Merck Inc., 40V/V%), Ethanol and Methanol (EtOH & MeOH, E. Merck Inc., 99%) were all used for synthesis of the MIL-101(Cr). GNP was purchased from the Research Institute of Petroleum Industry (RIPI). Ordinary deionized water (DI) was used for the reaction and washing solvent. The following health warning reiterated regarding the Hydrofluoric acid (HF). It possessed a number of chemical, physical and toxicological properties making handling 4 ACS Paragon Plus Environment

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of this material especially hazardous. Anhydrous HF is known as a clear, colorless, fuming, and corrosive liquid. All forms of it including the solution or vapor might have caused severe burns to human tissues. Hence, a supply of calcium carbonate or calcium hydroxide to encounter spills ought to be available near the fume hood where the work conducted. 2.2.

Synthesis and purification of the MIL-101(Cr)

MIL-101(Cr) was synthesized according to the published work of Ferey et al.20. Briefly, 1.66g of H2BDC, 4g of Cr(NO3)3.9H2O, 2ml of HF acid and 50ml DI water were introduced into a mixer and stirred for 15min. Next, the resulting mixture was transferred into a Teflon-lined stainless steel reactor, capped tightly and heated at 493K for 8h. After synthesis, a detailed treatment applied to remove the unreacted benzene di-carboxylic acid (H2BDC) existing in the MIL’s channels. In this venue, the resulting green precipitate washed completely three times with the DMF (30ml each time at 50ºC), three times with boiling ethanol (30ml each time) and five times with hot DI water (50ml each time at 80ºC) successively, and subsequently immersed in 30ml of the DMF while stirred at 300rpm and 80ºC for 6h. The MIL-101(Cr) powder was separated from the DMF extraction solvent through a careful filtration and soaked in 50ml of methanol. The methanol was replenished twice per day over a five days period. Afterwards, the dark-green microcrystalline precipitate was collected, filtered, and washed thoroughly again with hot DI water for five more times (20ml each time) then dried at 150ºC for 8h under ambient atmosphere. The resulting material denoted as the MIL-101(Cr). The production yield based upon the initial weights was 68%. 2.3.

Synthesis of the MIL-101(Cr)/GNP hybrid composite

A solvothermal reaction procedure was followed to synthesize the MIL-101(Cr)/GNP hybrid composite. Hence, the MIL synthesis was carried out at presence of the GNP. To begin with, 0.3g of graphene nano-plate material dispersed into 30ml of DMF liquid. After 1h of rigorous stirring at 40ºC, 1.66g of H2BDC, 4g of Cr(NO3)3.9H2O, 2ml of HF acid and 50ml DI water were introduced into this well-mixed GNP/DMF suspension and stirred for another 15min. Next, the mixture was transferred into a Teflon-lined stainless steel reactor, capped tightly and heated at 493K for 8h. Afterwards, this vessel was quenched gradually to the ambient temperature and the resulting dark-green crystals collected carefully. To ensure the removal of the residual H2BDC and other impurities, the product purification procedure explained in section 5 ACS Paragon Plus Environment


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(2.2) was applied. The resulting materials were named as the MIL-101(Cr)/GNP. The production yield based upon the initial weights was 69%. It should be noted that, the hybrid composites only differed in their GNP content. It considered worthwhile to state that, the GNP weight loading in the MIL materials’ structure was calculated based upon the initial weights of the solid materials including the; GNP, Cr(NO ) . 9H O, and H BDC. For instance, in the case of the MIL-101(Cr)/GNP 5wt%, the GNP weight loading determined through the following relationship: %GNP=

(.)

(.)( ) . () ( .!!)

. This rational previously utilized in

many similar researches available in the open literature 18, 19, 25. 2.4.

Characterization

The characteristics of the prepared samples identified through applying the; powder Xray diffraction (PXRD) for examining phase structure (through a Philips-1830 diffractometer with Cu-K" radiation source, #=0.154439nm, 40kV, 100mA), field emission scanning electron microscopy (FESEM) to study crystal structure and morphology (through a HITACHI S4160 equipment), BET-BJH for evaluating of the porous media textural features (through a Micromeritics 2020ASAP analyzer at T=77.4K), and Fourier transfer infrared spectroscopy (FT-IR) to determine the functional groups affecting the solid behaviors (through a Bruker Vector 33 spectrometer with KBr beads between 225 to 3725cm-1 equipment). Thermal analysis (DTG) investigating stability of the prepared samples utilizing a TA Q500 instrument heating from 300 to 900K under a nitrogen atmosphere at a rate of 15K/min. 2.5. CO2 adsorption measurements 2.5.1. Adsorption isotherms of CO2 upon the samples In order to determine the CO2 adsorption capacities of the pure GNP, prepared MIL101(Cr), and MIL-101(Cr)/GNP adsorbents, an in-house made volumetric apparatus employed under equilibrium conditions. The schematic of this equipment presented in Figure1. To begin with, 1g of the adsorbent material was charged into the adsorption column. Then, helium gas was passed across the pipes to purge the system as well as rendering the capability of calculating the dead volume of the system. Next, to degas the system and pretreat the sample, valves 5 to 8 opened and others closed. Then, the dynamic vacuuming started allowing the system to be evacuated at temperature of 200°C for 2h. Afterwards, the adsorption column was quenched to room temperature. The CO2 was adsorbed through opening the valves 2, 4, 6 and 7 while closing all other 6 ACS Paragon Plus Environment

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ones. Due to some dead volume in the apparatus as well as some gas adsorption on the materials considered, the pressure of the column fell down till equilibrium reached. The equilibrium duration for collecting the CO2 adsorption experimental data was about 3min for each measurement. Using the helium as a probe gas, one was capable to compute the exact pressure drop caused by the gas adsorption in the column. Then, the amount of gas adsorbed calculated through an appropriate equation of state (EOS). Ultimately, the adsorption experimental data obtained at 298K and gas pressures of up to 40bars. It should be noted that the tolerance of pressure range at each run was about ±0.1bars. 2.5.2. Obtaining experimental adsorption kinetic data To investigate the time dependency of the CO2 adsorption upon MIL-101(Cr) and the hybrid composites, pressures of the adsorption column recorded at different intervals (for instance every 30sec) until the adsorption system reached equilibrium at which point, the pressure of the adsorption column became constant. By specifying the initial and instant adsorption column pressure thus, one might have calculated the adsorption amounts of the CO2 at different time intervals leading to kinetic evaluations of the system.

Figure 1: The volumetric apparatus applied for the adsorption equilibrium and kinetic measurements in this work

2.5.3. CO2 adsorption calculations In order to determine precise amounts of the adsorbed CO2, the dead volume measurements were crucial. This included the macro- and meso-pores through which CO2 adsorption might have not taken place. Moreover, this was done by helium 7 ACS Paragon Plus Environment


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injection. The volumes into which the helium might have penetrated included sample cell and gas tank assumed to possess values of V2 and V1, respectively. The latter (i.e., V1) calculated using the initial pressure in the gas tank (P1) and V2 was the total volume calculated at the equilibrium pressure of the entire system (P2). The dead volume was thus obtained through the Eq. (1). $%&'( = $ − $

(1)

After helium injected and degassed at temperature of 200˚C under vacuum, the CO2 injection was performed. In order to determine the mole numbers of CO2 Eq. (2) and Eq. (3) were utilized. + = + =

,- %

(2)

.- /0 , %

(3)

. /0

In which, n1 and n2 represented the initial and equilibrium mole numbers of methane, respectively, V was the volume of the gas tank as well as Z1 and Z2 represented the initial and equilibrium compressibility factors of the CO2 before and after equilibrium. These compressibility factors calculated from the SRK Equation of State provided through Eq. (4). 4,56 ,

1 − 1 + 3

/0

−

6,

46,

7 1 − /0 = 0 /0

(4)

In which, “a’ and ‘b’ were defined through Eq. (5) and Eq. (6), respectively. 9 = 0.42748 F = 0.08664

/ 0> ,>

31 + @A1 − BCD E7

(5)

/0>

(6)

,>

Where, ‘m’ was given by Eq. (7). @ = 0.480 + 1.574I − 0.176I

(7)

In Eq. (5), Eq. (6), and Eq. (7), ‘a’ represented attractive forces between gas molecules, ‘b’ was the volume occupied by the CO2 molecules while ‘ω’ was the acentric factor related to the shape of the CO2 molecules. Thus, the total mole numbers of CO2 entered into the sample cell determined through Eq. (8). 8 ACS Paragon Plus Environment

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+ 0 = + − +

(8)

And finally, the total mole of adsorbed CO2 was calculated by Eq. (9). +4(J&D6K( = + 0 − +%&'(

(9)

Where, nVoid was the amount of gas in the dead volume calculated from Eq. (10). +%&'( =

,LMNO %LMNO

(10)

./0

Here, PVoid was the equilibrium pressure of the system and VVoid was the corresponding dead volume.

3.

Results and Discussion

3.1. Powder X-ray diffraction patterns The powder X-ray diffraction patterns of the MIL-101(Cr) (both simulated and experimental ones) and MIL-101(Cr)/GNP sorbents demonstrated in Figure 2. It ought to be noted that, the simulated PXRD pattern obtained through the open source free software PowderCell produced by the MIT Center for Materials Science and Engineering (MIT CMSE). As it might be seen, the experimental PXRD patterns were in an excellent agreement with the simulated one emphasizing successful preparations of the actual materials. Also, it was revealed that, the major diffraction pattern of the hybrid composite materials were consistent with that of the virgin MIL-101(Cr). The diffraction peaks at 2θ=17.40º, 25.19°, or 27.88º attributed to the recrystallized terephthalic acid were not observed20. It meant that, due to the purification step most of the needle-like unreacted H2BDC molecules were removed from the framework. On the other hand, the main peak around 2θ=24.4º related to the GNP particles as a separate solid phase was not found in the PXRD pattern of the hybrid composites. This indicated that, the MIL-101(Cr)/GNP samples preserved the structural integrity of the MIL101(Cr). In other words, due to the GNP incorporation into and/or embedding within the MIL-101(Cr) pores, no structural destruction occurred. The main reason for such observation was rationalized in terms of the fact that, the MIL-101(Cr) constituted the major component of the hybrid composite while the GNP was only embedded within its crystal framework. The intensity of the main peaks reduced slightly due to the GNP being increased in the MIL-101(Cr) structure. This was related to the reduced scattering contrast between the MIL-101(Cr) internal pores and void channels after the GNP embedded within the MIL’s framework19. Hence, some of the pores were occupied by 9 ACS Paragon Plus Environment


the GNP particles resulting in a slight decrease in the peak intensity. Other additional weak peaks were attributed to the existence of some impurities, despite of the complicated purification step (i.e., explained in section (2.2)) performed on these composite materials.

MIL-101(Cr) Simulated Pattern MIL-101(Cr)/GNP 15wt%

Intensity (a.u.)

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MIL-101(Cr)/GNP 10wt%

MIL-101(Cr)/GNP 5wt% MIL-101(Cr)

0

5

10

15

20

25

30

2 Theta (°°) Figure 2: The PXRD patterns of the MIL-101(Cr), and its hybrid composites with GNP prepared in this work

3.2. Nitrogen adsorption-desorption isotherms In order to examine textural characteristics of the parent and hybrid composite materials, nitrogen adsorption-desorption isotherms on these samples were determined at 77.4 K (Figure 3). It was observed that, N2 adsorption isotherms of the two hybrid composite samples (i.e., MIL-101(Cr)/GNP 5wt% and 10wt%) were reversible and did not exhibit any hysteresis loop due to desorption process. Moreover, these materials included a compatible microporous structure and the other two samples (i.e., MIL101(Cr) and MIL-101(Cr)/GNP 15wt%) indicated some kind of hysteresis loop due to the desorption step hence, a mesoporous structure of these latter materials was signaled. The obtained isotherms therefore, might have been classified as type-I according to the IUPAC classification. The major adsorption of N2 in the adsorption isotherms occurred at low relative pressure (i.e., P/P0≈0.08). It was reported that the MIL-101(Cr) had two types of microporous cages; the smaller one possessed pentagonal windows with diameter of 1.2nm while the larger cage contained pentagonal-hexagonal windows with diameter in the range of 1.45-1.60nm. Hence, a secondary adsorption step in the range of 0.1C=C) groups were observed at 1976 cm-1 and 1103 cm-1, respectively. The band corresponding to hydroxyl group (–OH stretching, 3435cm-1) was relatively prominent compared to the insigniﬁcant ratios of anti-symmetric and symmetric =CH vibrations (2921 and 2853cm-1) in case of the hybrid composite 12 ACS Paragon Plus Environment

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adsorbent22. As it was expected from previous results, majority of peaks were related to the MIL-101(Cr) structure, emphasizing that, the integrity of the MIL-101(Cr) structure was kept intact through the hybridization process. sym =CH

O-C-O >C=C

3725

3225

2725

2225

1725

1225

725

470

740 600

1976

2921

3435

(c)

1103

2853

(b)

Aromatic Rings >C=O O−C=O Cr-O

̶OH

1640 1450 1402 Aromatic Rings

anti-sym =CH

(a)

Intensity (a.u.)

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225

(Cm-1)

Wave Number Figure 5: The FT-IR spectra of the virgin GNP, synthesized MIL-101(Cr), and MIL-101(Cr)/GNP 10wt% in this research

3.4. Thermal analysis Figure 6 demonstrated the derivative thermogravimetric (DTG) curves for evaluating the thermal stability of the prepared MIL species. These DTG curves indicated the sharp peaks below 373K attributed to desorption of physically adsorbed water. The peaks between 373 and 573K corresponded to the loss of the coordinated water molecules36. Then, the sharp peaks occurred at about 640K assigned to the removal of OH/F groups36. More importantly, the beginning of the decomposition of the MIL101(Cr) structure was revealed in this display. After that, sharp peaks occurred at about 673K, followed by other sharp ones at about 758K. The occurrence of these sharp peaks attributed to the decomposition of the organic ligand (H2BDC). It was noticed that, the shapes of the peaks representing decomposition of the organic ligands in the composites were slightly shifted from those of the MIL-101(Cr). This later observation was ascribed to changes in their chemical environment due to the incorporation and/or embedding of the GNP. In other word, the composites and the pure MIL-101(Cr) samples had similar thermal stability indicating that, the introduction of low amounts of the GNP did not change thermal stability of the MIL units considerably. 13 ACS Paragon Plus Environment


0.5

Derivative of Weight Loss (%/K)

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MIL-101(Cr)

0.45

MIL-101(Cr)/GNP 15wt.%

0.4


0.35


0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 300

450

600

750

900

Temperature (K) Figure 6: DTG of the MIL-101(Cr) and its hybrid composites with different amounts of GNP prepared in this work

3.5. The FESEM images The morphology of the MIL-101(Cr)/GNP 10wt% hybrid composite obtained through the FESEM images provided in Figure 7 (-a and -b). These images showed texture variation of the hybrid material compared to the virgin MIL-101(Cr) 17. Figure 7-a (at the left hand side) exhibited the compactness of the MIL-101(Cr)/GNP structure. The hybrid composite material included dense arrangement of graphene particles embedded within the MIL-101(Cr) pores. It was concluded that, graphene did not destruct the octahedral crystals of the MIL-101(Cr). On the other hand, the presence of graphene particles in the internal pores of the hybrid composite material might have lowered distortion forces affecting the framework structure resulting in higher mechanical and thermal stabilities compared to those of the virgin one. Furthermore, the well-admixed GNPs with the MIL-101(Cr) framework considered as a consequence of strong interactions between the functional groups existing at the GNPs inner layer surfaces and external surfaces of the MIL’s metal clusters. On the other hand, according to the Figure 7-b (at the right hand side), and through comparison between these two images (7-a, and 7-b), it seemed there were two types of morphologies (i.e., of octahedral (7-a) and needle (7-b) shapes), emphasizing that there were indeed some impurities existed in the hybrid composite. These results further emphasized the obtained PXRD and FTIR data discussed earlier in this paper.


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(b)

(a)

Figure 7: The FESEM images of the hybrid composite adsorbent MIL-101(Cr)/GNP 10wt% prepared in this work

3.6. CO2 adsorption studies 3.6.1. CO2 equilibrium adsorption experimental data

Obtained CO2 equilibrium adsorption experimental data upon the MIL-101(Cr) and its hybrid composite represented in Figure 8. It was observed that, the CO2 adsorption capacities upon the sorbents considerably enhanced with pressure at lower values (i.e., P≤ 10 bars). This was related to the adsorption of gas molecules onto the micropores. The aforementioned adsorption capacities of the MIL-101(Cr)/GNP were higher than those of the virgin material throughout the pressure range studied. For instance, the maximum CO2 adsorption capacity upon the MIL-101(Cr) and its hybrid composite containing 10wt% GNP (happened to be the optimum hybrid composite) were 14.38mmol.g-1 and 20.62mmol.g-1 at 298K and 40bars, respectively. As discussed earlier, enhancement of the specific surface area, total and micropore spaces considered as the main consequence of the GNP embedding into the MIL-101(Cr) framework. Furthermore, in the case of the hybrid composite adsorbent, the transition pressure (i.e., where 90% of the adsorption sites would be occupied by the adsorbate molecules) was higher than that of the GNP free sample. It meant that, there existed more reactive adsorption sites on the hybrid composite compared with those of the MIL-101(Cr). This was directly attributed to a larger specific surface area of the former compared to the latter. 3.6.2. CO2 adsorption comparison with literature

Table 2 represented a comparison of CO2 adsorption capacities of the virgin GNP, different MIL-101(Cr), and their hybrid composites with some graphitic components (e.g., the graphite oxide, carbon nano tubes and GNP) at 298K and 10, 25, and 40bars. It was revealed that, the MIL-101(Cr)/GNP synthesized in this work had the highest 15 ACS Paragon Plus Environment


CO2 adsorption capacities amongst the adsorbents listed in Table 2 at corresponding operation conditions. Furthermore, the present work was the only one providing measurements of the CO2 adsorption upon the GNP made composite at 40bars. It was indicated that this value was still on the rise from its corresponding 25bar amount (which was already the highest between the three materials compared). This emphasized the role of the GNP as a very promising material to improve the virgin MOF’s CO2 adsorption capability. Moreover, this seemed to be mainly attributed to enhancement in the reactive capability of the composite prepared in this work (i.e., shown through its BET surface area values) being more than doubled compared to the CNT made composite. It is noteworthy that, the BET values were rather close for the GNP and GO composited MOF materials and their corresponding CO2 adsorption were rather close as well. CO2 adsorption amount (mmol.gr-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

21 18 15

transition pressure

12 9

transition pressure

6 MIL-101(Cr)@GNP 3 MIL-101(Cr) 0 0

5

10

15

20

25

30

35

40

Pressure (bar) Figure 8: The CO2 adsorption isotherms of the parent and the optimum hybrid composite adsorbent synthesized in this research at 298K and up to 40bars Table 2: The CO2 adsorption capacities of some MIL-101(Cr) and their hybrid composites compared with some graphitic components Material

SBET (m2.g-1)

Vtotal (cm3.g-1)

MIL-101(Cr) MIL-101(Cr)/GNP 10wt% GNP MIL-101(Cr) MIL-101(Cr)@CNT 10wt%

2486 3032 750 1270 1243

1.12 1.41 1.47 1.56 1.47

CO2 adsorption capacity (mmol.g-1) 10bars 25bars 40bars 298K 298K 298K 11.23 13.18 14.38 16.54 19.28 20.62 9.14 14.27 16.59 1.95 3.12 2.45 4.31 -


Reference

This work This work This work [19] [19]

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MIL-101(Cr) MIL-101(Cr)@GO 5wt%

2670 2950

1.28 1.42

11.48 13.20

14.60 18.65

-

[25] [25]

Adsorption isotherm models usually suggested a description of the interactions between adsorbate and adsorbent materials as well as played an important role in optimization of such systems. Therefore, the experimentally obtained equilibrium adsorption data in this work were modeled utilizing the Langmuir, Sips, Toth, and Dual Site Toth (DST) isotherms to estimate adsorption capacity of the CO2 molecules upon the MIL species at different pressures (Table 3). It ought to be reiterated that, in these models, qe (mmol.gr-1) represented the equilibrium adsorbed CO2 moles per unit mass of the adsorbent, P (bar) was the equilibrium pressure of CO2 in the gas phase, b (bar-1) was the affinity constant, qm (mmol.gr-1) was the maximum adsorption capacity of CO2 upon the sorbent, and n and t were the surface heterogeneity parameters. In case of the Dual Site Toth equation, A and B were representative of two different kinds of adsorption sites possessing their own properties such as affinity and heterogeneity parameters. The results of modeling were tabulated in Table 3. The determining coefficients (R2) characterizing the percentage of variability in the dependent variable were employed to analyze the fitting degree of the isotherm and kinetic equations for the obtained experimental CO2 adsorption data. This value varied from 0 to 1. Considering the values of this item, it was a foregone conclusion that, the Dual Site Toth equation described the experimental data better than the other three models undertaken (Figure 9). This model superiority was attributed to the point that, it considered the adsorbent surface heterogeneous (i.e., with a various distribution of the adsorption sites) possessing different adsorption affinities and energies. Moreover, the Spearman’s correlation coefficient (rs) assessing the global correlation and dispersion of its relative errors utilized in the current work evaluating its statistical acceptance. Eq. (11) represented this error function: QJ = 1 −

! ∑Z N[-ASTUV,N 5S>XY,N E

(11)

\(\5 )

Where, n was the number of the experimental data, ]K^_ (mmol/g) and ]`4a (mmol/g) were the amounts of the experimental and calculated adsorbed CO2 upon the surfaces through the adsorption isotherm model undertaken. It ought to be noted that, as the value of the corresponding rs became closer to 1, the relative error of this equation lowered hence, it became more precise. Thus, based upon these error indicators’ values, 17 ACS Paragon Plus Environment


presented in Table 3, the Dual Site Toth model was chosen as the best one describing adsorption of CO2 on the prepared composite materials in this research. Table 3: Adjustable parameters for adsorption isotherm equations at 298K studied in this research Isotherm model

Adsorbent

qm (mmol.gr-1)

b (bar-1)

t

n

R2

rs

Langmuir d. e b = bc . f + d. e Sips

MIL-101(Cr)

16.67

1.715

-

-

0.932

0.944


23.58

1.698

-

-

0.922

0.931

MIL-101(Cr)

15.71

1.376

-

1.21

0.957

0.965


22.27

1.113

-

1.34

0.946

0.967

MIL-101(Cr)

15.33

1.108

1.53

-

0.981

0.994


22.05

0.872

1.67

-

0.978

0.991

R2

rs

b = bc.

(d. e)

fh g fh g

f + (d. e) Toth

b

d. e

= bc.

(f + (d. e)i ) Dual-Site Toth

b = bcj . + bck .

fh i

qmA

qmB

bA

bB

tA

tB

nA

nB

MIL-101(Cr)

4.96

9.93

0.681

0.998

1.49

1.62

-

-

0.986

0.997


7.18

14.36

0.554

0.814

1.58

1.73

-

-

0.983

0.995

dj .e

fh (f(dj .e)ij ) ij

dk .e

f

h (f(dk .e)ik ) ik

CO2 adsorption amount (mmol.g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

20 16 12 8 Dual Site Toth Equation 4

MIL-101(Cr) MIL-101(Cr)/GNP

0 0

10

20

30

40

Pressure (bar) Figure 9: The dual Site Toth adsorption isotherm data fitted to the MIL-101(Cr) and hybrid composite material MIL-101(Cr)/GNP 10wt% at 298K determined in this research

3.6.3. CO2 adsorption kinetics

Kinetic investigations always provided significant information about the mechanism of a given process. Hence, to examine the mechanism of adsorption in this research, the CO2 adsorptions on MIL-101(Cr) and MIL-101(Cr)/GNP10wt% were studied as a 18 ACS Paragon Plus Environment

Page 19 of 24

function of time and the obtained results demonstrated in Figure 10. Actually, this figure indicated the transient CO2 adsorption profiles upon both the MIL-101(Cr) and MIL-101(Cr)/GNP 10wt% sorbents. As revealed, the CO2 sorption was rather rapid during the first 20sec then after steadily continued to increase till the equilibrium achieved. Most probably, in the first 10-20sec; the CO2 molecules contacted external surfaces of the sorbents. Afterwards, the CO2 molecules diffused into the MIL’s pores while getting adsorbed within its internal surfaces. In other words, initially, the reactive-adsorptive sites on the external surfaces of the sorbent led to a rather intensive adsorption. Then, a slow diffusion process followed up. CO2 adsorption amount (mmol.gr-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21 18

diffusion into MIL-101(Cr)/GNP 10wt% pores

15 dissusion into MIL-101(Cr) pores

12 9 6 3 0 0

20

40

60

80

100

120

Time (sec)

Figure 10: The CO2 adsorption kinetic experimental data on the parent (filled circular) and hybrid composite (open diamond) adsorbents prepared in this research

In order to simulate the adsorption kinetics, three well-known kinetic models including the; pseudo-first-order (PFO), pseudo-second-order (PSO), and fractal like-pseudo first order (FL-PFO) were utilized to fit the experimental data. The obtained results were displayed in Table 4. Based upon the obtained correlation coefficient (R2) and the Spearman’s correlation coefficient (rs) values and closeness of the qe,cal and qe,exp values, it was concluded that, the adsorption process of CO2 upon the sorbents was described best through the fractal like-pseudo first order (FL-PFO) kinetic model (Figure 11). Accordingly, prediction of the adsorption kinetics through this equation revealed that, the rate constants of these adsorption systems were time independent. This was attributed to the existence of different adsorptive sites for adsorption (i.e., considering heterogeneous surfaces). This further suggested that, a shifting of preferred adsorptive sites as time passed by might have occurred. 19 ACS Paragon Plus Environment


Table 4: Adjustable parameters for adsorption kinetic equations at 298K studied in this research Kinetic model

Adsorbent

qe,cal (mmol.gr-1)

k1 (min-1)

k2 (min-1)

l

R2

rs

Pseudo-First-Order bi = bm . Af − mno(−pf i)E

MIL-101(Cr)

14.790

0.008

-

-

0.964

0.974


20.684

0.067

-

-

0.959

0.968

MIL-101(Cr)

14.521

-

0.006

-

0.971

0.982


22.270

-

0.035

-

0.968

0.980

MIL-101(Cr)

14.381

0.005

-

1.951

0.997

0.999


20.263

0.011

-

1.693

0.995

0.998

Pseudo-Second-Order pq bqm i bi = f + pq bm i Fractal Like-Pseudo-FirstOrder bi = bm . Af − mno(−pf il )E

CO2 adsorption amount, qt (mmol.gr-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

21 18 15 12 9 FL-PFO equation

6

MIL-101(Cr)

3 MIL-101(Cr)@GNP

0 0

20

40

60

80

100

120

Time (sec) Figure 11: Effect of the contact time on the adsorption of CO2 upon the MIL-101(Cr) and MIL-101(Cr)/GNP 10wt%. The symbols showed the experimental data while the dashed-lines indicated the predicted values by the FL-PFO model

4. Conclusion In this research, graphene nano-plates embedded into the prepared MIL-101(Cr) pores to synthesize a hybrid composite metal-organic framework in an in-situ manner. The optimum composite (MIL-101(Cr)/GNP 10wt%) indicated to be a very promising adsorbent for the CO2 adsorption. This material was physically well characterized and mechanistically understudied. In the first part of this work, the samples were characterized by means of the PXRD, BET-BJH, FT-IR, DTG, and FESEM analyses. In this venue, the optimum hybrid 20 ACS Paragon Plus Environment

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composite sorbent exhibited higher specific surface area as well as enhanced total and micropore volumes compared to those of the virgin material. In the second part of this study, the CO2 adsorption capacities upon the virgin and hybrid composite materials were undertaken. For this purpose, the equilibrium adsorption data obtained experimentally through in-house made equipment. It was revealed that, the CO2 adsorption upon the composite was higher compared to that of the parent MIL-101(Cr). In particular, the optimum hybrid composite indicated the maximum CO2 adsorption of 20.63mmol.gr-1 at 298K and pressure of 40bars. This was an increase of 43% for the CO2 adsorption capacity compared to that of the GNP free sample. This enhancement attributed to several factors of which some major ones provided below: Increase in the specific surface area (from 2486 to 3032 m2.gr-1), total volume (from 1.12 to 1.41cm3.gr-1) and micropore volume (from 0.42 to 0.69cm3.gr-1) through embedding of the GNP into the MIL’s pores. This led towards enhancement in the number of CO2 adsorptive sites. Enhanced surface dispersive forces of the MIL-101(Cr)/GNP due to embedding the optimum amount of dense array of atoms of the GNP within the virgin MIL. This caused an increase in the CO2 sorption affinity towards the hybrid composite surface. Presence of more unsaturated metal sites (e.g., Cr3+ open-metal sites) vs. the saturated ones (e.g., GO (graphene oxide)) upon the hybrid composite structure. This provided more active open-metal sites giving rise to a better CO2 sorption. In the last part of this work, in order to provide a mechanistic view point of the understudied process, the adsorption modeling of the CO2 upon the prepared materials was further undertaken through initially seeking an appropriate adsorption isotherm. Amongst several models considered, the Dual Site Toth isotherm provided a satisfying fit towards this adsorption data. This led to the conclusion that, the nature of the sorbents surfaces considered was heterogeneous. Then in a final move in this direction, the kinetics of the CO2 adsorption upon the prepared MOFs was understudied. Considering several kinetic models, it was revealed that the FL-PFO equation outstandingly described this adsorption system with a goodness of fit of above 99%. In this venue, the adsorption constants were revealed to be time independent being a 21 ACS Paragon Plus Environment


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Page 22 of 24

consequence of the aforementioned surface heterogeneity of the materials prepared. Ultimately, the values of these constants determined for the process at hand using the Genetic Algorithm tool. These types of data shall be instrumental in any future optimizations of the aforementioned systems.

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CO2 uptake (mmol.gr-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

21 18 15 12 9

Toth Equation

6

MIL-101(Cr)@GNP

3

MIL-101(Cr)

0 0

5

10

15 20 25 Pressure (bar)

30


35

40

Embedding Graphene Nanoplates into MIL-101(Cr) Pores: Synthesis

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