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High CO2 Adsorption on Amine-Functionalized Improved Mesoporous Silica Nanotube as an Eco-Friendly Nanocomposite Fatemeh S. Taheri, Ahad Ghaemi, Ali Maleki, and Shahrokh Shahhosseini Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019
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Energy & Fuels
High CO2 Adsorption on Amine-Functionalized Improved Mesoporous Silica Nanotube as an Eco-Friendly Nanocomposite Fatemeh. S. Taheri a, Ahad Ghaemi a,*, Ali Maleki b, Shahrokh Shahhosseini a
a
b
School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology, Tehran, Iran
Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran, Iran * Corresponding author. E-mail:
[email protected]; Fax: +98(21)77240495; Tel: +98(21)77240496
ABSTRACT New improved mesoporous nanocomposite CO2 adsorbents IMSiNTs-PEI (IMP) were synthesized by improved pretreatment of natural Halloysite nanotubes (HNTs) and impregnation of improved mesoporous silica nanotubes (IMSiNTs) with polyethylenimine (PEI). The new improved mesoporous morphology remained the same of HNTs morphology but the specific surface area (340.61 m2/g) and pore volume (0.499 m3/g) after treatment, respectively, were about 7 times and 2.5 times more than those values for HNTs. The results indicate that more active groups were created on the IMSiNTs surface comparing with HNTs. Also an adsorption apparatus was used for analyzing the impact of various range of pressure, temperature and loaded PEI amount on adsorption capacity. The results showed increasing in uptake capacity with increasing pressure and also decreasing in uptake capacity with increasing temperature exhibited that the nature of processes is exothermic. On the other hand, the proper PEI dispersion inside the IMSiNTs could improve the adsorption capacity and the highest adsorption capacity 7.84 mmol/g was obtained at 20℃, 9 bar for IMP-30 PEI. It was also demonstrated that these adsorbents have high thermal stability, well reversibility and stability during adsorption/desorption cycles.
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Keywords: CO2 Capture, Halloysite Nanotubes, Mesoporous Silica, Amine Impregnation.
1. INTRODUCTION Nowadays environmental challenges, such as global warming and climate change, have encouraged researchers to develop and extension effective solutions to CO2 capture and sequestration methods12.
Due to the high use of fossil fuels in the industry and consequently the production of CO2 as a
greenhouse gas, several techniques have been developed to eliminate CO2, which have advantages and restrictions3. The extensive development of nanostructured materials show that the nanomaterials can offer more efficient solutions to challenges ahead of CO2 capture, such as high volume of CO2 capture, energy-efficient and cost-effective4. These compounds have different structures and can be synthesized by various roles in absorption and adsorption as the base, the main source of sorption, particles in the fluid, and so on. So far, various standard methods and a set of nanotechnology, have been proposed for removal of carbon dioxide which include zeolite-based carbon uptake, metal organic frameworks for carbon uptake, polymer membranes for carbon uptake, supported ionic liquid membranes (SILMs), nanotechnology-enabled carbon uptake. First study of nanostructures in the field of adsorption was done by Cuffe et al, to adsorb hightemperature carbon dioxide and transfer it to ultra-thin nanoporous silica5. Mishra et al, controlled the size of pores, surface area, high costs, and adsorption capacity using graphene base nanocomposites coated with nanocrystal iron oxide particles and resistant to high temperatures in carbon dioxide capture6. Also, in the application of silica nanotubes, functionalized with polyethylenimine in carbon dioxide adsorption, adsorbent stability and its recovery capacity was demonstrated by measuring adsorption capacity and adsorption / desorption cycles with the aid of thermogravimetric analyzer (TGA) apparatus7. Kim Wun-gwi et al, used stable silica nanofluids synthesized by sol-gel method to absorb carbon dioxide from the liquid phase8. The higher mass transfer surface and greater stability 2 ACS Paragon Plus Environment
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due to the smaller bubble size, were caused an increase in average absorption rate, overall absorption value and ultimately, absorbed capacity in the nanofluids were several times higher than that of water without nanoparticles. Among various porous supports, amine and polyamine nano-adsorbents based on mesoporous solids have recently been widely considered for CO2 uptake9-12. Following the first report of PEI based on fume silica MCM-4113,14, MCM-4815, HMS16, SBA-1517-19, SBA-1620, SBA-1221, and KIT-622 have been known as an effective promising type of amine supports for CO2 adsorption23. These supports due to the high specific surface area and great pore volume are capable to provide more active sites for the reaction of amines with CO2 molecules24-27. Amine-based solids have characteristics such as higher CO2 adsorption capacity, lower energy requirements, higher resistance to pollutants, and greater stability than amine solutions which has led researchers to study solid solvents compare of solvent-based adsorbents with disadvantages such as corrosion of plants, high energy requirements, amine degradation, foaming 28-29. The choice of most researchers for the support materials in preparation of CO2 adsorbents has been metalorganic frameworks (MOF)30-31, mesoporous carbon32, resins33, mesoporous silica34, aerogel36, macroporous silica36, MCM-4137-38, TiO39, SBA-1540-41. Research has shown that clays, due to the presence of several clay minerals and some impurities in their structure42-48, and on the other hand, due to having features such as low cost, high mechanical and chemical stability, can be used in more advanced applications and in the role of supporting material in solid base adsorbents by changing and improving the structure42,45-46. In the application of nanoclays as effective adsorbents, Pham et al, studied the increase of CO2 adsorption using nanozeolite synthesis. The results showed a very high concentration of nanozeolite at ambient temperature and adsorption capacity dropped by 6.33% after 10 regeneration cycles. This adsorbent due to good potential and adsorption stability, was known as an efficient and economical CO2 adsorbent49. Among the suitable clays for CO2 adsorption, Halloysite has been attracted the attention of 3 ACS Paragon Plus Environment
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researchers
with
its
unique
structure
and
features.
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Halloysite
nanotubes
(HNTs)
(Al2Si2O5(OH)4nH2O), are a kind of dioctahedral 1:1 aluminosilicate mineral clays that are naturally nanotubular with hollow structure which resulted from the multiple rolled layer (their diameter is about 20–50 nm and their length several hundred nanometers) which mined from natural reserves and plenty at countries like Brazil, China, France, America. The Halloysite structure is similar to the chemical structure of kaolinite due to its high chemical stability and specific surface area42. The most recently reported results of CO2 adsorption capacities with amine-functionalized nanoclay adsorbents shows a more practical use of these adsorbents50, especially in industrial and semi-industrial scale because of their recycle ability and high adsorption capacity51-55. Although, the initial preparation of these supports due to multi-stage processes such as calcination and surface development is a little expensive. Atilhan et al. prepared an CO2 adsorbent by modification of montmorillonite nanoclays with different types of amine groups for storing of CO2, CH4 and N2 under elevated pressure at 25 and 50℃ isotherms up to 50 bar. The CO2 uptake reached from 3.4 mmol/g to 7.16 mmol/g at 50 bar and 25 ℃ after modification of nanoclays with Octadecylamine56. Irani et al, investigated the adsorption capacity of CO2 by TEPA functionalized nanosepiolite that was 3.8 mmol/g at 60℃ and 1 vol% of CO2 in N2 with about 1 vol% of H2O57. The present study was conducted to adsorb CO2 using a volumetric apparatus in various operating conditions. We modified the multilayer silica nanotubes with amine to be used as an adsorbent. The results indicate the role of IMSiNTs-PEI in raising of equilibrium uptake of gas molecules. We found that the initial preparation and purification of silica nanotubes and their functionalized with amine groups could maximize the CO2 adsorption capacity. The adsorbents were prepared by the impregnation of various loading amounts (10, 20, 30 and 40 wt%) of PEI on IMSiNTs support. The prepared adsorbent was applied to CO2 adsorption in a new volumetric apparatus, with various pressures higher than atmospheric. To identify the nanocomposite adsorbents behavior, isotherm, kinetic and thermodynamic modeling for CO2 adsorption process data was performed.
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2. EXPERIMENTA 2.1.
Chemicals
Halloysite nanotubes (HNTs) powder was purchased from Aldrich (US). Polyethylenimine (PEI) (average MW~800) was prepared from Sigma- Aldrich Co, hydrocholoric Acid (37%) and methanol were purchased from Merck. 2.2. Preparation of HNTs At first, halloysite nanotubes were calcined for 5 h at 850℃. Then 5 g of calcified HNTs was treated by being soaked in 100 ml of 7 M hydrochloric acid solution for 7 h at 85℃ in a round-bottom flask with stirring under reflux. Then it was washed with deionized water to reach the pH 7 then dried at 70℃ for 12 h to produce high porosity HNTs. 2.3. Preparation of HNTs-amine For preparing the IMSiNTs-PEI sorbents by the wet impregnation route, nanocomposites were prepared with four different loading of 10, 20, 30 and 40 wt% of polyethylenimine. For impregnation of dry HNTs with 10 wt %, 2 gr of PEI and 15 gr of methanol was intermixed and stirred for about 20 min at 60℃ and then 2 gr of prepared mesoporous was added into the mixture solution. The solution continuously stirred for 18 h at 500 rpm in a 100 ml beaker that was sealed with parafilm under previous conditions. The resultant mixture after dried at 60℃ for 8 h, was put into vacuum drying oven at 50℃ to obtain a soft white powder. The same process was done for loadings of 20, 30 and 40 wt% with 0.4, 0.6 and 0.8 gr of PEI, respectively. Prepared samples were denoted IMP-10, IMP-20, IMP-30 and IMP-40. 2.5. Adsorbent Characterization The characterization of HNTs-Amine specimens was carried out by various conventional techniques. The features of surface and morphology of the specimens were obtained using the scanning electron microscopy (SEM/ VEGA-TESCAN). For preparation, the surface of adsorbents was gold-sprayed
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for higher conductivity. Fourier transform infrared (FT-IR) spectra of unmodified and modified samples were recorded on a Shimadzu IR-470 spectrometer by the method of KBr pellet. The X-ray diffraction (XRD) analysis, for solid mesoporous powders was carried out using a X' Pert Pro X-ray diffractometer, worked at 40 kV and 40 mA. Energy dispersive X-ray spectroscopy (EDX) analysis was taken by Numerix DXP-X10P to perform the specimen elemental analysis. N2 adsorption/desorption isotherm were achieved at liquid nitrogen temperature of 77 Kfor ° characterization of the mesoporous materials features. It characterized the features of the mesoporous materials including pore volume, pore size distributions, and specific surface area using of a micromeritics model ASAP 2020. The SBET and the pore size distributions of the porous sorbents were estimated by the Brunauer−Emmett−Teller (BET) and the Barrett–Joyner–Halenda (BJH) methods, respectively. Thermogravimetric analysis (TGA) was taken to evaluate the mesoporous adsorbents thermal stability and was recorded in the range of 25~850℃ under argon flow, which was obtained by BährSTA 504 (Germany) analyzer at a heating rate of 10℃/min. 2.6. CO2 Adsorption/Desorption Measurements CO2 adsorption/desorption capacity of IMSiNTs-PEI nanocomposite was measured in an adsorption setup. 0.5 gr nanocomposite was placed in the device cylindrical compartment and sealed completely. CO2 as a purge gas was injected to the device compartment to determine the amount of gas adsorption. Before the adsorption experiments and in order to remove the moisture and all preadsorbed CO2, prepared adsorbent was preheated under pure N2 gas at 110℃ for 30 min, following by under vacuum for another 40 min. As soon as cooling the temperature of the device to the room temperature, CO2 was injected into the setup adsorbent bed. Experiments were carried out at temperature values of (20, 30, 40 and 50℃) and under pressure (1, 3, 6 and 9 bar) for 100 min. After adsorption and the equilibration for a suitable time, the pressure difference recorded by the device with time was used to measure and estimate the adsorption parameters. Repeatability of CO2 adsorption of samples was 6 ACS Paragon Plus Environment
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obtained 7 times at 110℃ and 30 min under vacuum and in the adsorption device. To evaluate solid-gas adsorption data, a simple and accurate CO2 adsorption apparatus was developed. A schematic of the adsorption setup was shown in Fig. 1. First, in order to ensure the absence of leakage in the connections and removal of existing gases, the nitrogen inert gas was tested. High purity CO2 gas from the cylinder before entrance to the reactor was preheated by passing through preheater. The mixing reservoir in the path caused the stability of the CO2 pressure and temperature, and then the stable gas was transferred to the adsorbent reactor. The heat required for the reactor is supplied by an electric heater and the temperature and pressure changes of CO2 are instantaneously recorded by the computer.
Fig. 1 Setup for CO2 adsorption measurement
3. RESULTS AND DISCUSSION Elemental quantification to identify the bare and modified HNTs composition has been accomplished by SEM and EDX mapping. The SEM analysis indicates the surface morphology of open-ended 7 ACS Paragon Plus Environment
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cylindrical hollowed HNTs samples before and after modifications. Fig. 2a, shows that the average length of the HNTs tubes are 0.4~1.6 μm, with an external diameter of (50~90) nm and internal diameter of (20~50) nm (Fig. 2d). The diffraction reflection of HNTs at 2θ = 12.0° relative to the basal spacing of 0.74 nm, which indicated this sample is halloysite-7Å. Adsorbed water and crystal water were removed by calcination and acid leaching of HNTs. But these treatments caused obvious shortening of the IMSiNTs length compare with the HNTs. Also two types of pores were formed, the bare HNTs tubs and newly formed mesoporous in the range of 2~50 nm, which the pore size distribution results substantiated it. The obtained results are compatible with the research findings of Shu and Niu42,58. Fig 2.c exhibits the tubes were impregnated with 30% PEI which PEI chain filled into the inner tubes or wrapped around them.
(a)
(d)
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(b)
(e)
(c)
(f)
Fig. 2 SEM images of (a) HNTs, (b) IMSiNTs, (c) IMP-30; EDX patterns of (d) HNTs, (e) IMSiNTs, (f) IMP-30. The presence of the main elements of O, Al and Si in the pristine HNTs in addition to, N and C after modification of HNTs with PEI in the composition of HNTs–PEI was authenticated by the EDX analysis. The EDX spectrum illustrated the presence of O and Si as mainly content in IMSiNTs and a little Al resulted from the leaching of alumina after acid treatment. Also the presence of nitrogen with 3 other elements in elemental map of IMP-30 demonstrates the impregnation of HNTS with PEI and interaction between them (Fig. 2d, e and f). In Fig. 3, two peaks at 3693 and 3622 cm-1 were observed. These peaks attributed to the inner surface Al-OH stretching bands that was vanished after calcination and acid-leaching and illustrated 9 ACS Paragon Plus Environment
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the ruination of octahedral structure of HNTs. The peaks at 910, 536 and 468 cm-1 attributed to the Al-OH, Al-O-Si and Si-O-Si bending vibrations, respectively that deformed in IMSiNTs. The wide range of Si-O stretching band at 1031 cm-1 transferred to 1068 cm-1 which may be accompanied by the replacement of H atoms with electronegative differences in aluminum atoms during the acid treatment process. The terminal Si-OH disfiguration band was observed at 929 cm-1 band42,55. After calcination and acid-leaching, the intensify of band Si-O-Si band amplified at 468 cm-1 which might be attributed to the leaching of Al3+ and some amorphous silica. The new bands resulted of PEI impregnation at 1520 and 1465 cm-1 represented vibrational bending –NH2 symmetric and asymmetric. Existence of stretching vibration of –CH2 in PEI chain at 2925, 2881 cm-1 confirmed that the PEI loading within IMSiNTs was doing correct. The reaction between the terminal Si-OH and – NH groups is the main reason for vanishing of the band at 929 cm-1 for IMSiNTs. The severity of – NH vibration is varied by changing the PEI concentration within IMSiNTs because of the CO2 adsorption during the synthesis process. The final product had C=O vibration at 2310 cm-1 which was necessary to recover it before adsorption42,55.
Fig. 3 FTIR spectra of HNTs, IMSiNTs, and nanocomposites. Fig. 4, indicated the XRD patterns of raw Halloysite and modified samples after heating at 850℃ and acid leaching. These treatments made disappearance of diffraction reflection of Halloysite. By 10 ACS Paragon Plus Environment
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creating an amorphous structure, the wider reflections became a substitute of the Halloysite reflection at 2
and 22° that exhibited the Halloysite was modified. Since the amorphous reflection of HNTs, 𝜃
modified HNTs and amine-impregnated HNTs are in the same position. The XRD patterns in Fig 3 showed the broader amorphous reflection after amine impregnation that will be broader with increasing the amine loading51.
Fig. 4 XRD patterns of HNTs, IMSiNTs, and adsorbents nanocomposites Table. 1 Porous textures of HNTs, IMSiNTs and as-prepared adsorbents. Adsorbent
BET Surface area (m2/g)
Total pore volume (cm3/g)
HNTS
49.24
0.270
IMSiNTs
340.61
0.499
IMP-10
74.39
0.415
IMP-20
55.56
0.390
IMP-30
33.61
0.310
IMP-40
16.94
0.210
Fig. 5a&b showed the nitrogen adsorption-desorption isotherms of HNTs, IMSiNTs and the nanocomposites (IMP-10, IMP-20, IMP-30, IMP-40) and their porous parameters which were obtained from a Quantachrome Autosorb automated adsorption apparatus were given in Table 1. The 11 ACS Paragon Plus Environment
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obvious hysteretic loops at the range of 0.2 < P 0) the process is possible and spontaneous or 0
0
impossible and non-spontaneous respectively. The sign of S represents the randomness of the 0
adsorbent organization at the interface of gas/solid when the adsorption process takes place, which in the case of S > 0 , it is more random and it is less random at S D-R > Temkin61,62. The summary of previous studies on various mesoporous silica adsorbents functionalized by amine were indicated in Table 5 where the as-prepared adsorbents properties like silica support, type and percent (wt %) of amine, optimum temperature of CO2 adsorption, BET surface area and pore volume were compared.
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8 7 6 q [ mmol/g ]
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5 4 3
Exp. Langmuer Freundlich Dubinin Radushkevich Temkin Hill
2 1 0
1
2
3
4
5
6
7
8
9
PCO2 [ Bar ]
Fig. 11 Comparison of isotherms and CO2 adsorption experimental data at 20℃ Table. 4 the isotherms models of CO2 adsorption for IMP-30 Models
Parameter
20℃
30℃
40℃
Langmuir qe qm k L P (1 k L P )
qm KL R2
28.14 0.001 0.993
26.50 0.001 0.994
10.18 0.001 0.994
Freundlich
kF n R2
0.968 0.862 0.994
0.916 0.663 0.996
1.018 0.621 0.994
qm Beta E R2
0.209 2.935 0.413 0.978
0.186 3.156 0.398 0.989
0.137 2.852 0.419 0.991
A B R2
1.325 2.783 0.917
1.175 2.555 0.925
1.285 1.872 0.929
qe kF P
1
n
Dubinin Radushkevich qe qm e λω
2
Temkin q e = B* ln AT + B* ln C e
Where qe (mmol/g) is the value of CO2 adsorption capacity, qm (mmol/g) is the of CO2 adsorption maximum value, P (bar) is the equilibrium pressure, k L (1/bar) is the Langmuir isotherm constant, k F [(mmol/g)(1/bar)1/n] & n is Freundlich isotherm constant,
(mol2.J-2) is the Dubinin Radushkevich 𝜆
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(D-R) constant, 𝜔(J.mol-1) is the Polanyi potential (equal to RT ln((1 P)1 ) , A(L/mol) is the Temkin isotherm constant, B RT .bT1 ; bT (J/mol). By matching the experimental data with the above mentioned models and calculating the correlation coefficient (R2), the validity of the models from its proximity to unit value was determined that represented the best data fitting towards the special isotherm model61.
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Table. 5 The findings summary of CO2 uptake capacity with the mesoporous support-amine functionalized by the others. Adsorbent
Support
Amine (wt %)
T (℃)
MCM-41-PEI 50
MCM-41
PEI (50)
PE-MCM-41
MCM-41
MCM-41-NH2
SBET (m2/g)
Pore volume (cm3/g)
support
ads
75
1042
PEI (55)
75
MCM-41
NH2
MBS-1
MCM-41
MCM-48-PEI 50
Ref.
support
ads
4
0.85
0.01
22
570
