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Experimental and Computational Investigation of CO2 Capture on Mix-ligand Metal-organic Framework UiO-66
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 4395 – 4401
The 8th International Conference on Applied Energy – ICAE2016
*
Experimental and computational investigation of CO2 capture on mix-ligand metal-organic framework UiO-66 Qiheng HuangaˈJing Ding a,*ˈXiang Huang aˈXiaolan Wei bˈWeilong Wang a, כ aˊSchool of Engineering, Sun Yat-Sen University, Guangzhou 510006, China b. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
Abstract A mixed-ligand Zr-MOF material has been synthesized as CO2 adsorbents with varous carboxylic acid ligands has for CO2 capture at room temperature. With the purpose of better understanding the thermodynamics and mechanisms of CO2 adsorption, molecular dynamics (MD) simulations have been used to study adsorption kinetics of Zr-MOF adsorbents as well as sorbent structures optimization, and the CO2 adsorption capacity was also simulated by the Monte Carlo (MC) method. Results showed that the CO2 uptake of UiO-66-NH2 under room temperature and pressure reached 134mg/g-sorb. The adsorption capacity of CO2 under low pressure (less than 120kPa) was mainly determined by the pore properties and surface modifications, while the steric hindrance had less influence. The ideal separation factor of UiO-66-NH2-50 were calculated to be 39.7, which showed the excellent CO2/N2 separation performance at 298K. © Published by Elsevier Ltd. This © 2017 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keyword: MOF˗Mix-ligand˗Modified˗Molecular dynamics
1. Introduction Large amounts of greenhouse gases emitting into the atmosphere has led to the serious environmental problems, the substantial harm to natural environment has caused widespread concern around the world.Among them,carbon dioxide is the major contributor [1]. So far, CO2 capture and sequestration (CCS) technology has been proved as one of the most efficient method to face the challenge of climate change and applied in industry for the sustainable development of economy. The traditional commercial adsorbents are not satisfied due to the sysnthsis process cause more energy consumption and have low adsorption capacity, and hence, it is anxious to design a new adsorbent strengthening the CO2
* * Corresponding author. Tel.: +86-020-39332320; fax: +86-020-39332319. E-mail address: [email protected](wang weilong), [email protected](ding jing)
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.933
4396
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
adsorption to improve competition of CCS technology in industry. Metal organic frameworks (MOFs) are new kinds of porous adsorbents with high specific surface area, and the structure can be easily adjusted and modified, which are widely used in adsorption, separation, catalysis process and so on[2]. So far, MOF materials have been synthesized and used in CO2 adsorption. Yaghi synthesized MOF-5 and MOF-177,with the surface area of 3000m2/g,which exhibited the high CO2 capacity of 33.5mmol/g at 298K and 40 bar[3],which exhibited the best adsorption capacity in the literature.Lately, a proved effective method of increasing CO2 capture ability is to introduce amine groups into MOFs, either on the inner surface or in the pore. Couck synthesized the amine functionalized MIL-53(Al) by a direct synthetic method for the purpose of separating CO 2 from CH4[4] However, most MOFs have complex pore nature and surface properties, it is hard to analyze the adsorption mechanism merely by experiments. Therefore, the method of computational chemistry was used to systematically understand the adsorption process, separation performance and diffusion mechanism of MOFs in this paper, which play a important role on the synthesis of materials with high CO2 adsorption performance. The UiO-66 was rencently synthesized and showed high chemical and thermal stability[5], and it was modified by different carboxylic acid ligands an investigation of synthesis of t mixed-ligand Zr-MOF material . The adsorbents are used for capture of CO2 and separation of CO2/N2.The mechanism will be discussed via both experimental and simulation work. 2. Experimental 2.1Adsorbent preparation The synthetic condition of UiO-66 was relatively mild. The UiO -66 was synthesized by a reported method [6].The UiO -66 adsorbents were prepared by a wet impregnation method. Use glacial acetic as a template and the detailed process was as follows: firstly, zirconium tetrachloride, terephthalic acid and glacial acetic in the ratio of 1:1:30 which were dissolved separately in 50 ml DMF and magnetic stirred for half of an hour. Then, the solution was translated to a 0.1L reactor and then placed in an oven held at 120ć for 24h. The reactor was then removed from the oven and allowed to cool under ambient condition. By filtration and drying, the white crystal of unrefined Uio-66 was obtained. The raw samples were filtered with DMF and anhydrous ethanol for 3h, and then washed with water and dried. The process was repeated twice. then higher purity UiO -66 crystals was obtained. Replacing the half or all of above terephthalic acid with substances of 2- amino terephthalic acid and synthesis method followed the same route of UiO -66. The composites of UiO -66-NH2-50ˈUiO -66NH2 was obtained respectively. 2.2 characterization and CO2 adsorption measurements X-ray power diffraction (XRD) patterns were taken on a Empyrean X-ray diffractometer using Cu Kα radiation.The XRD diffraction patterns were taken in the 2θ range of 5-30eat a scan speed of 1.2°min−1 . ASAP 2020 analyzer was used to measure the Nitrogen adsorption–desorption isotherms at 77 K.The specific surface area was calculated by relative pressures from 0.05 to 0.3 and total pore volume was determined by N2 uptake at the relative pressure of 0.995. Infrared (IR) spectra were recorded at room temperature on a EQUINOX55 spectrometer.Thermo-gravimetric analysis ˄TGA˅was conducted with SDTA851e thermal analyzer. The performances of carbon dioxide capture at room temperature and 1 bar was examined on the TGA/SDTA851e by a gravimetric method. The sequence was listed in detail as follows: Initially, all samples were heated up in pure nitrogen at a heating rate of 10 K/min –100 ◦C and kept for 45 min to remove gases inside. Then the temperature decreased to 25 ◦C, sustained for 30 min to ensure no weight loss.High purity CO2 was introduced to the chamber to conduct adsorption process for 1
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
h. The adsorption data of 50 ◦C and 75 ◦C was also collected with the aim of exploring the impact of temperature on CO2 adsorption. 2.3 computational details In this workˈthe framework of UiO-66-NH2-50 and UiO-66-NH2 was constructed via replacing an H atom in each phenyl ring by an amine group. The entire structures were optimized using the “Forcite” tool in Materials Studio. The framework were energy minimized via Universal Force Field (UFF) as shown in Fig. 1. Atomic charges of UiO-66-NH2-50 and UiO-66-NH2 referred to the work of Zhong et al. [7] , which were calculated by CBAC(Connectivity-Based Atom Contribution). And the charges were reduced to change the electrostatic force with the aim of fitting experimental results [8]. Density functional theory(DFT) calculation were conducted by the Dmol3 module to estimate the charges of CO 2 and N2. Lennard-Jones (LJ) parameters of atoms in CO2 ,N2 and Zr-MOF sorbents were obtained from the Universal Force Field (UFF) and Dreiding Field. Detailed parameters of LJ potential and atomic charges were reported in Supplementary Data. Grand canonical Monte Carlo (GCMC) method was used to help us further understand the thermodynamics and mechanisms during CO2 adsorption. Simulations were carried out at the fixed pressure of 100 kPa and 298 K. While calculating the Lennard-Jones (LJ) 12-6 potential, the cutoff radius was set to 14 Å. The summation method of electrostatic interaction between all the listed atoms was Ewald, and atom based summation method was adopted to calculate the Van der Waals interaction. The simulation for each fixed pressure contains 1 × 107 equilibration steps, followed by 1 × 107 production steps to calculate thermodynamic properties.
Fig.1. Models after geometry UiO-66ˈUiO-66-NH2-50ˈUiO-66-NH2
3. Results and discussion 3.1 XRD and IR analysis Fig. 2 ˄a ˅ shows the X-ray diffraction patterns of all the sorbents.The good agreement has achieved between the pattern of UiO-66 and one reported in literature[6].The similar diffraction patterns of the composites to UiO-66 indicated that structures of the framework remained the same.Fig. 2(b) shows the Fourier transform infrared spectroscopy spectra of the samples: the UiO-66, UiO-66-NH2 and the UiO-66-NH2-50. In the spectrum of the three samples,bands at 1395 cm-1 corresponded to the C-O stretching vibrations in the carboxyl.As compared to UiO-66,the other samples exhibited additional two peaks at 1256 cm-1 , 768 cm-1ˈwhich agree with the result reported in the literature[9].The peaks at 1256 cm-1 , 768 cm-1 were due to C-N, N-H bends .This confirmed the successful grafting of functional group.
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Fig. 2(a) XRD patterns and (b) IR spectrum of sorbents.
3.2. N2 adsorption–desorption isotherms Nitrogen adsorption and desorption isotherms are used to get the structural information about the synthesized UiO-66-NH2-50 as shown in Fig. 3.That exhibited permanent microporosity, as evidenced by reversible type I N2 adsorption isotherms.A sharp increase between the relative pressure of 0–0.1 revealed that there were plenty of micropores inside.The parameters of the porous structure calculated from these isotherms and pore size distributions were presented in Table 1. The calculated surface area and pore diameter of UiO-66-NH2-50 were 702 m2/g and 0.45nm respectively.Compared to UiO-66 and UiO-66NH2 reported in literature[6]ˈthe sample show a lower crystallinity.That may contribute to the synthesis conditions and factors of post-processing.
Fig. 3. N2 adsorption–desorption isotherms of sorbents. Table 1 Structural properties
SBET (m2/g)
SLangmuir (m2/g)
Pore diameter (nm)
UiO-66 [6]
1250
1381
0.52
UiO-66-NH2-50
702
936
0.45
UiO-66-NH2 [6]
1084
1196
0.51
3.3 Adsorption ability Fig. 4 illustrates the CO2 adsorption capacities of sorbents at1 bar and 298K.It shows that UiO-66-
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
NH2 have the lagest CO2 adsorption capacity reaching 134mg/g. UiO-66 shows 77.8mg/g of CO2 uptakes which was quite comparable with the earlier reported studies[10]. UiO-66-NH2-50 showed marginally decrease in CO2 sorption capacity compared to UiO-66-NH2 at the same condition.Obviously,the CO2 adsorption is mainly decided by the adsorbent pore properties and the presence of polar functional groups in the low pressure. The impact of space steric hindrance and entrance is smaller.
Fig. 4. Adsorption ability of sorbents at 298K and 1bar˖˄a˅UiO-66˄b˅UiO-66-NH2-50˄c˅UiO-66-NH2
3.4 Adsorption sites The snapshots of the cleaved surface are shown in Fig. 5 to help us get a better view of the favorable CO2 adsorption sites on sorbents.It show that the CO2 adsorption sites change with adsorption quantity, part of the red dot density represented by adsorption of CO 2 molecules appear probability higher position.At low CO2 loading,most CO2 molecules were absorbed near the Regular tetrahedron cage.This was mainly because of the strong force between CO2 and framework atom in the tetrahedron hole.As the CO2 loading increased, CO2 molecules tend to occupy the central areas of the octahedral cage.
Fig. 5. The snapshots of CO2 adsorption sites at 100 kPa of different CO2 loadings
3.5 separation performance of CO2/ N2 As shown in the formula(1)ˈwe calculate the ratio of Henry’s constant of N2 and CO2, then obtains the ideal separation factor. SCO2/N2=HCO2/HN2 (1) Simulated temperature is set to 298 kˈthe uptakes of CO2 and N2 molecules under the 20kpaˈ40Kpaˈ 60kpaˈ80kpa and 100kpa was calculated by GCMC method. Using the reported experimental results by CMARIK[10], compare UIO-66 with UIO-66-NH2 in terms of adsorption data and simulation data.we just discuss the CO2/N2 separation ability on simulation because of lacking UIO-66-NH2 experiment result. We selected the Langmuir equation for fitting the adsorption data. So langmuir model can fit well in the simulation and experiment under the pressure point of each component of the adsorption. Table 2 shows the CO2/N2 Henry’s constant in ZR-MOF, and ideal separation factor ‘S’. As shown in the table, the separation factor is independent to Amino load. Separation coefficient of uio-66-NH2-50 reach 39.7, which is most excellent. The result confirm that adsorbing capacity improve apparently because of modified functional groups, though the adsorption capacity of mixed-ligand MOF materials are less than
4399
4400
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
the material which add functional groups completely, larger than the average of non-modified and completely modified, which make a better CO2/N2 separation performance .
Fig. 6. CO2 adsorption isotherms measures at 298K on the samples Zr-MOF
Fig. 7. N2 adsorption isotherms measures at 298K on the samples Zr-MOF
Table 2 Henry constants and ideal separation factor of Zr-MOF
HCO2
HN2
SCO2/N2
UiO-66
3.640
0.139
26.2
UiO-66-NH2-50
7.306
0.184
39.7
UiO-66-NH2
8.268
0.255
32.4
4. conclusion In our study, CO2 capture performance of the sorbents at room temperature (298 K) and 1 bar was investigated. Models of the framework and TEPA were built and energy minimized by molecular dynamic method. The simulated adsorption isotherms were compared to experimental results and further validated the accuracy of the force field parameters. UiO-66-NH2 has the best CO2 adsorption capacity up to 3.02mmol/g at1 bar and 298K. At the initial stage, the CO2 molecules were absorbed mainly in the tetrahedron cages of unit cell, and then started to spread to the center of the octahedron cage. CO2 and N2 adsorption isotherm data of the three Zr-MOFs was accurately fitting by Langmuir equation. UiO-66NH2-50 has the best CO2/N2 separation performance of 39.7 among three adsorbents at 298K. Acknowledgements This work was supported by the funding of Science and Technology Program of Guang Zhou (201510010248), Nature Science Foundation of China (51436009), Nature Science Foundation of China (U1507113), and Science and Technology Planning Project of Guang Dong Province (2015A010106006). Reference
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
[1] U. Desideri, A. Paolucci, Performance modelling of a carbon dioxide removal system for power plants, Energ Convers. Manage. 40 (1999) 1899–1915. [2] N. Mohaghegh, M. Tasviri, E. Rahimi, M.R. Gholami, Comparative studies on Ag3PO4/BiPO4–metal-organic framework– graphene-based nanocomposites for photocatalysis application, Appl. Surf. Sci. 351 (2015) 216–224. [3] S.S.Y. Chui, S.M.F. Lo, J.P.H. Charmant, A.G. Orpen, I.D. Williams, Science 283 (1999) 1148–1150. [4] S. Couck, J.F. Denayer, G.V. Baron, T. Rémy, J. Gascon, F. Kapteijn, An amine-functionalized MIL-53 metal-organic framework with large separation power for CO2 and CH4, J. Am. Chem. Soc. 131 (2009) 6326–6327. [5] J.H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K.P. Lillerud, J. Am. Chem. Soc. 130 (2008) 13850–13851. [6] Y.T.Huang. UiO - 66 type MOFs materials synthesis and gas adsorption performance research[D]. South China university of technology.2012 [7] Wong-Foy A G, Matzger A J, Yaghi O M. Exceptional H2 Saturation Uptake in Microporous Metal-Organic Frameworks[J]. Journal of the American Chemical Society, 2006, 128(11):3494-3495. [8] Xu Q, Zhong C. A General Approach for Estimating Framework Charges in Metal−Organic Frameworks[J]. Journal of Physical Chemistry C, 2010, 114(11):5035-5042. [9] W.W.Zhao. UiO - 66 type of MOFs application in high performance liquid chromatography[D]. Beijing university of technology,2015. [10] Cmarik G E, Min K, Cohen S M, et al. Tuning the adsorption properties of UiO-66 via ligand functionalization[J]. Langmuir, 2012, 28(44):15606-15613
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ScienceDirect Energy Procedia 105 (2017) 4395 – 4401
The 8th International Conference on Applied Energy – ICAE2016
*
Experimental and computational investigation of CO2 capture on mix-ligand metal-organic framework UiO-66 Qiheng HuangaˈJing Ding a,*ˈXiang Huang aˈXiaolan Wei bˈWeilong Wang a, כ aˊSchool of Engineering, Sun Yat-Sen University, Guangzhou 510006, China b. School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
Abstract A mixed-ligand Zr-MOF material has been synthesized as CO2 adsorbents with varous carboxylic acid ligands has for CO2 capture at room temperature. With the purpose of better understanding the thermodynamics and mechanisms of CO2 adsorption, molecular dynamics (MD) simulations have been used to study adsorption kinetics of Zr-MOF adsorbents as well as sorbent structures optimization, and the CO2 adsorption capacity was also simulated by the Monte Carlo (MC) method. Results showed that the CO2 uptake of UiO-66-NH2 under room temperature and pressure reached 134mg/g-sorb. The adsorption capacity of CO2 under low pressure (less than 120kPa) was mainly determined by the pore properties and surface modifications, while the steric hindrance had less influence. The ideal separation factor of UiO-66-NH2-50 were calculated to be 39.7, which showed the excellent CO2/N2 separation performance at 298K. © Published by Elsevier Ltd. This © 2017 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keyword: MOF˗Mix-ligand˗Modified˗Molecular dynamics
1. Introduction Large amounts of greenhouse gases emitting into the atmosphere has led to the serious environmental problems, the substantial harm to natural environment has caused widespread concern around the world.Among them,carbon dioxide is the major contributor [1]. So far, CO2 capture and sequestration (CCS) technology has been proved as one of the most efficient method to face the challenge of climate change and applied in industry for the sustainable development of economy. The traditional commercial adsorbents are not satisfied due to the sysnthsis process cause more energy consumption and have low adsorption capacity, and hence, it is anxious to design a new adsorbent strengthening the CO2
* * Corresponding author. Tel.: +86-020-39332320; fax: +86-020-39332319. E-mail address: [email protected](wang weilong), [email protected](ding jing)
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.933
4396
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
adsorption to improve competition of CCS technology in industry. Metal organic frameworks (MOFs) are new kinds of porous adsorbents with high specific surface area, and the structure can be easily adjusted and modified, which are widely used in adsorption, separation, catalysis process and so on[2]. So far, MOF materials have been synthesized and used in CO2 adsorption. Yaghi synthesized MOF-5 and MOF-177,with the surface area of 3000m2/g,which exhibited the high CO2 capacity of 33.5mmol/g at 298K and 40 bar[3],which exhibited the best adsorption capacity in the literature.Lately, a proved effective method of increasing CO2 capture ability is to introduce amine groups into MOFs, either on the inner surface or in the pore. Couck synthesized the amine functionalized MIL-53(Al) by a direct synthetic method for the purpose of separating CO 2 from CH4[4] However, most MOFs have complex pore nature and surface properties, it is hard to analyze the adsorption mechanism merely by experiments. Therefore, the method of computational chemistry was used to systematically understand the adsorption process, separation performance and diffusion mechanism of MOFs in this paper, which play a important role on the synthesis of materials with high CO2 adsorption performance. The UiO-66 was rencently synthesized and showed high chemical and thermal stability[5], and it was modified by different carboxylic acid ligands an investigation of synthesis of t mixed-ligand Zr-MOF material . The adsorbents are used for capture of CO2 and separation of CO2/N2.The mechanism will be discussed via both experimental and simulation work. 2. Experimental 2.1Adsorbent preparation The synthetic condition of UiO-66 was relatively mild. The UiO -66 was synthesized by a reported method [6].The UiO -66 adsorbents were prepared by a wet impregnation method. Use glacial acetic as a template and the detailed process was as follows: firstly, zirconium tetrachloride, terephthalic acid and glacial acetic in the ratio of 1:1:30 which were dissolved separately in 50 ml DMF and magnetic stirred for half of an hour. Then, the solution was translated to a 0.1L reactor and then placed in an oven held at 120ć for 24h. The reactor was then removed from the oven and allowed to cool under ambient condition. By filtration and drying, the white crystal of unrefined Uio-66 was obtained. The raw samples were filtered with DMF and anhydrous ethanol for 3h, and then washed with water and dried. The process was repeated twice. then higher purity UiO -66 crystals was obtained. Replacing the half or all of above terephthalic acid with substances of 2- amino terephthalic acid and synthesis method followed the same route of UiO -66. The composites of UiO -66-NH2-50ˈUiO -66NH2 was obtained respectively. 2.2 characterization and CO2 adsorption measurements X-ray power diffraction (XRD) patterns were taken on a Empyrean X-ray diffractometer using Cu Kα radiation.The XRD diffraction patterns were taken in the 2θ range of 5-30eat a scan speed of 1.2°min−1 . ASAP 2020 analyzer was used to measure the Nitrogen adsorption–desorption isotherms at 77 K.The specific surface area was calculated by relative pressures from 0.05 to 0.3 and total pore volume was determined by N2 uptake at the relative pressure of 0.995. Infrared (IR) spectra were recorded at room temperature on a EQUINOX55 spectrometer.Thermo-gravimetric analysis ˄TGA˅was conducted with SDTA851e thermal analyzer. The performances of carbon dioxide capture at room temperature and 1 bar was examined on the TGA/SDTA851e by a gravimetric method. The sequence was listed in detail as follows: Initially, all samples were heated up in pure nitrogen at a heating rate of 10 K/min –100 ◦C and kept for 45 min to remove gases inside. Then the temperature decreased to 25 ◦C, sustained for 30 min to ensure no weight loss.High purity CO2 was introduced to the chamber to conduct adsorption process for 1
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
h. The adsorption data of 50 ◦C and 75 ◦C was also collected with the aim of exploring the impact of temperature on CO2 adsorption. 2.3 computational details In this workˈthe framework of UiO-66-NH2-50 and UiO-66-NH2 was constructed via replacing an H atom in each phenyl ring by an amine group. The entire structures were optimized using the “Forcite” tool in Materials Studio. The framework were energy minimized via Universal Force Field (UFF) as shown in Fig. 1. Atomic charges of UiO-66-NH2-50 and UiO-66-NH2 referred to the work of Zhong et al. [7] , which were calculated by CBAC(Connectivity-Based Atom Contribution). And the charges were reduced to change the electrostatic force with the aim of fitting experimental results [8]. Density functional theory(DFT) calculation were conducted by the Dmol3 module to estimate the charges of CO 2 and N2. Lennard-Jones (LJ) parameters of atoms in CO2 ,N2 and Zr-MOF sorbents were obtained from the Universal Force Field (UFF) and Dreiding Field. Detailed parameters of LJ potential and atomic charges were reported in Supplementary Data. Grand canonical Monte Carlo (GCMC) method was used to help us further understand the thermodynamics and mechanisms during CO2 adsorption. Simulations were carried out at the fixed pressure of 100 kPa and 298 K. While calculating the Lennard-Jones (LJ) 12-6 potential, the cutoff radius was set to 14 Å. The summation method of electrostatic interaction between all the listed atoms was Ewald, and atom based summation method was adopted to calculate the Van der Waals interaction. The simulation for each fixed pressure contains 1 × 107 equilibration steps, followed by 1 × 107 production steps to calculate thermodynamic properties.
Fig.1. Models after geometry UiO-66ˈUiO-66-NH2-50ˈUiO-66-NH2
3. Results and discussion 3.1 XRD and IR analysis Fig. 2 ˄a ˅ shows the X-ray diffraction patterns of all the sorbents.The good agreement has achieved between the pattern of UiO-66 and one reported in literature[6].The similar diffraction patterns of the composites to UiO-66 indicated that structures of the framework remained the same.Fig. 2(b) shows the Fourier transform infrared spectroscopy spectra of the samples: the UiO-66, UiO-66-NH2 and the UiO-66-NH2-50. In the spectrum of the three samples,bands at 1395 cm-1 corresponded to the C-O stretching vibrations in the carboxyl.As compared to UiO-66,the other samples exhibited additional two peaks at 1256 cm-1 , 768 cm-1ˈwhich agree with the result reported in the literature[9].The peaks at 1256 cm-1 , 768 cm-1 were due to C-N, N-H bends .This confirmed the successful grafting of functional group.
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Fig. 2(a) XRD patterns and (b) IR spectrum of sorbents.
3.2. N2 adsorption–desorption isotherms Nitrogen adsorption and desorption isotherms are used to get the structural information about the synthesized UiO-66-NH2-50 as shown in Fig. 3.That exhibited permanent microporosity, as evidenced by reversible type I N2 adsorption isotherms.A sharp increase between the relative pressure of 0–0.1 revealed that there were plenty of micropores inside.The parameters of the porous structure calculated from these isotherms and pore size distributions were presented in Table 1. The calculated surface area and pore diameter of UiO-66-NH2-50 were 702 m2/g and 0.45nm respectively.Compared to UiO-66 and UiO-66NH2 reported in literature[6]ˈthe sample show a lower crystallinity.That may contribute to the synthesis conditions and factors of post-processing.
Fig. 3. N2 adsorption–desorption isotherms of sorbents. Table 1 Structural properties
SBET (m2/g)
SLangmuir (m2/g)
Pore diameter (nm)
UiO-66 [6]
1250
1381
0.52
UiO-66-NH2-50
702
936
0.45
UiO-66-NH2 [6]
1084
1196
0.51
3.3 Adsorption ability Fig. 4 illustrates the CO2 adsorption capacities of sorbents at1 bar and 298K.It shows that UiO-66-
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
NH2 have the lagest CO2 adsorption capacity reaching 134mg/g. UiO-66 shows 77.8mg/g of CO2 uptakes which was quite comparable with the earlier reported studies[10]. UiO-66-NH2-50 showed marginally decrease in CO2 sorption capacity compared to UiO-66-NH2 at the same condition.Obviously,the CO2 adsorption is mainly decided by the adsorbent pore properties and the presence of polar functional groups in the low pressure. The impact of space steric hindrance and entrance is smaller.
Fig. 4. Adsorption ability of sorbents at 298K and 1bar˖˄a˅UiO-66˄b˅UiO-66-NH2-50˄c˅UiO-66-NH2
3.4 Adsorption sites The snapshots of the cleaved surface are shown in Fig. 5 to help us get a better view of the favorable CO2 adsorption sites on sorbents.It show that the CO2 adsorption sites change with adsorption quantity, part of the red dot density represented by adsorption of CO 2 molecules appear probability higher position.At low CO2 loading,most CO2 molecules were absorbed near the Regular tetrahedron cage.This was mainly because of the strong force between CO2 and framework atom in the tetrahedron hole.As the CO2 loading increased, CO2 molecules tend to occupy the central areas of the octahedral cage.
Fig. 5. The snapshots of CO2 adsorption sites at 100 kPa of different CO2 loadings
3.5 separation performance of CO2/ N2 As shown in the formula(1)ˈwe calculate the ratio of Henry’s constant of N2 and CO2, then obtains the ideal separation factor. SCO2/N2=HCO2/HN2 (1) Simulated temperature is set to 298 kˈthe uptakes of CO2 and N2 molecules under the 20kpaˈ40Kpaˈ 60kpaˈ80kpa and 100kpa was calculated by GCMC method. Using the reported experimental results by CMARIK[10], compare UIO-66 with UIO-66-NH2 in terms of adsorption data and simulation data.we just discuss the CO2/N2 separation ability on simulation because of lacking UIO-66-NH2 experiment result. We selected the Langmuir equation for fitting the adsorption data. So langmuir model can fit well in the simulation and experiment under the pressure point of each component of the adsorption. Table 2 shows the CO2/N2 Henry’s constant in ZR-MOF, and ideal separation factor ‘S’. As shown in the table, the separation factor is independent to Amino load. Separation coefficient of uio-66-NH2-50 reach 39.7, which is most excellent. The result confirm that adsorbing capacity improve apparently because of modified functional groups, though the adsorption capacity of mixed-ligand MOF materials are less than
4399
4400
Qiheng Huang et al. / Energy Procedia 105 (2017) 4395 – 4401
the material which add functional groups completely, larger than the average of non-modified and completely modified, which make a better CO2/N2 separation performance .
Fig. 6. CO2 adsorption isotherms measures at 298K on the samples Zr-MOF
Fig. 7. N2 adsorption isotherms measures at 298K on the samples Zr-MOF
Table 2 Henry constants and ideal separation factor of Zr-MOF
HCO2
HN2
SCO2/N2
UiO-66
3.640
0.139
26.2
UiO-66-NH2-50
7.306
0.184
39.7
UiO-66-NH2
8.268
0.255
32.4
4. conclusion In our study, CO2 capture performance of the sorbents at room temperature (298 K) and 1 bar was investigated. Models of the framework and TEPA were built and energy minimized by molecular dynamic method. The simulated adsorption isotherms were compared to experimental results and further validated the accuracy of the force field parameters. UiO-66-NH2 has the best CO2 adsorption capacity up to 3.02mmol/g at1 bar and 298K. At the initial stage, the CO2 molecules were absorbed mainly in the tetrahedron cages of unit cell, and then started to spread to the center of the octahedron cage. CO2 and N2 adsorption isotherm data of the three Zr-MOFs was accurately fitting by Langmuir equation. UiO-66NH2-50 has the best CO2/N2 separation performance of 39.7 among three adsorbents at 298K. Acknowledgements This work was supported by the funding of Science and Technology Program of Guang Zhou (201510010248), Nature Science Foundation of China (51436009), Nature Science Foundation of China (U1507113), and Science and Technology Planning Project of Guang Dong Province (2015A010106006). Reference
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