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Chapter 6
Chemical and Adsorptive Characterization of Adsorbents To Capture Greenhouse Gases Under Atmospheric Conditions of Temperature and Pressure B. Delgado,*,1,2,3 R. Lagace,1 S. Godbout,3 J. L. Valverde,4 A. Giroir-Fendler,2 and A. Avalos Ramirez5 1Université
Laval, 2425 rue de l’agriculture, Québec (QC) G1V 0A6 Canada Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, 2 avenue Albert Einstein, Villeurbanne, F-69622, France 3Institut de recherche et de développement en agroenvironnement, 2700 rue Einstein, Québec (QC) G1P 3W8 Canada 4Department of Chemical Engineering, Universidad de Castilla-La Mancha, Avda. Camilo José Cela, 12, 13071 Ciudad Real, Spain 5Centre National en Électrochimie et en Technologies Environnementales, 5230, Boulevard Royal, Shawinigan (QC) G9N 4R6 Canada *E-mail:
[email protected]. 2Université
Methane (CH4) is a greenhouse gas (GHG) with high global warming potential (GWP = 25) and a long atmospheric lifetime (12 years). It is the main GHG emitted by agriculture and among the most emitted by industry and transport. To reduce CH4 emissions, it is necessary to capture it under atmospheric conditions of temperature and pressure at which it is emitted. In this work, several solid adsorbents were developed and tested to capture CH4 emissions under standard conditions in order to show their potential for use in the development of new environmental remediation technologies. The solid materials used in this study were: zeolites, biochars, and a metal-organic framework (MOF). Chemical composition and structural parameters of these compounds were determined in order to correlate their characteristics with adsorption capacities. The adsorbents were chemically analyzed determining their mineral content. The structural characterization consisted on the © 2018 American Chemical Society
surface area with pore volume using N2 adsorption method and crystallinity using XRD. CH4 adsorption capacity of adsorbents was determined at 30ºC with atmospheric pressure diluting pure gas in He as an inert carrier at low concentration. The gas mixture passed through a packed bed reactor to obtain CH4 adsorption capacity and the Freundlich adsorption isotherm was adjusted to experimental data.
Greenhouse Gas Emissions Greenhouse gases (GHGs) capture infrared radiation (IR), allowing them to control the planet’s temperature. Global warming potential (GWP) is the total energy that gases adsorb over a 100-year period. The CO2 GWP is defined as the basis for calculating GWP values for other GHGs, with a reference value of 1 (1). Table 1 shows the atmospheric lifetime and the GWP for different GHGs. GHG concentrations in the atmosphere before the industrial age were relatively low and constant (2). However, over the past 150 years, human activities have generated a significant increase in GHG concentration. From 1750 to 2015, the concentration of CO2 in the atmosphere increased by 144% (from 278 to 400 ppm), while CH4 increased by 264% (from 700 to 1845 ppb) and N2O increased by 121% (from 270 to 328 ppb) (2, 3). In 2014, total GHG emissions estimated by the United Nations Framework Convention on Climate Change (UNFCCC) were 17,690 Mt CO2 equivalent (Mt: megatonnes) (4, 5). Among anthropogenic emissions, the four most emitted GHGs in 2014 were (5): •
•
•
Carbon dioxide (CO2): It is the most emitted GHG in the atmosphere. CO2 emissions account for 77.7% of total GHG emissions with approximately 13,745 Mt CO2 emissions per year. CO2 emission sources include motor vehicles, heterotrophic respiration, fossil fuel combustion, organic decomposition of organic wastes, and chemical reactions. About 1700 Mt of CO2 are removed from the atmosphere each year, captured by plants and algae in a very important stage of the geochemical carbon cycle (photosynthesis) (4, 5). Methane (CH4): It contributes with 2620 Mt CO2 equivalent per year, which corresponds to 14.8% of total GHG emissions (4, 5). CH4 is emitted into the atmosphere during the production and transportation of coal, natural gas, and oil. Other sources of emissions are agricultural activities and anaerobic degradation of organic wastes in municipal landfills (6). Around 80–90% of the CH4 emitted is degraded by microorganisms called methanotrophs, which are important elements in the CH4 cycle (7). Nitrous oxide (N2O): Annual N2O emissions are on the order of 902 Mt CO2 equivalent per year, corresponding to 5.1% of total GHG emissions (4, 5). Agricultural activities are the main source of N2O emissions. 106
•
Other sources of N2O are adipic acid and nitric acid production, fossil fuel combustion, and waste. Fluorinated gases (F gas): They represent 2.4% of total GHG emissions, which corresponds to 425 Mt CO2 equivalent per year (4, 5). Fluorinated gases are emitted primarily by industrial processes, such as refrigeration and the use of a variety of consumer products containing fluorinated gases, such as HFCs (hydrofluorocarbons), PFCs (perfluorocarbons), and SF6 (sulfur hexafluoride).
Table 1. Greenhouse Gases Linked to Global Warminga Greenhouse gas
Atmospheric lifetime (years)
Global warming potential (GWP)
CO2
50–200
1
CH4
12
25
N2O
114
298
CFC-11
45
4750
CFC-12
100
10,900
HCFC-22
12
1810
CCl4
26
1400
C2F6
10,000
12,200
SF6
3,200
22,800
Adapted with permission from ref (1). Copyright 2007 Intergovernmental Panel on Climate Change.
a
Anthropogenic sources of GHG emissions can be classified by economic sector (8, 9). The contribution of each sector in 2014 was as follows (5, 8): •
•
•
Energy (61%): In this sector, electricity generation accounts for the largest share of GHG emissions (30.5%). The energy production required for the maintenance and operation of public or private spaces, such as heating, air conditioning, cooling, and cooking, represents about 12.5%. In particular, the use of coal, natural gas, and oil to produce electricity accounts for 65% of GHG emissions in this sector. Industry (7.5%): GHG emissions from industry concern mainly fossil fuel combustion on site for energy self-consumption. This sector also includes emissions from mineral, chemical, and metallurgical processes. Transportation (20.2%): GHG emissions in this sector are related to fossil fuel combustion (road, rail, air, and sea sectors), with gasoline and diesel being the most common fuels. 107
•
•
Agriculture (8.4%): GHG emissions from agriculture are mainly issued from agricultural soil management, livestock farming, enteric fermentation, rice production, and biomass combustion. Waste management (3.0%): GHG emissions from this sector are generated by final disposition of solid organic waste, wastewater management, and waste incineration.
Although land and forest use can also be linked to GHG emissions, they are not taken into account in the list of anthropogenic sources mentioned above. These emissions are generated, for example, during deforestation, land clearing, soil decomposition, or agricultural fires, and they account for about 10% of the total previously listed emissions (5). Human activities have contributed to a significant increase in GHG concentrations over the past 150 years, which are related to environmental issues such as global warming and climate change. The control and quantification of these emissions require technologies that can capture and/or degrade these GHGs (e.g., by adsorption). Adsorption has been used to gas capture because of its high selectivity, high energy efficiency, ease of control, and low capital investment costs.
Adsorbents Adsorbents are solid materials with a large surface area on which the adsorption of gas or liquid molecules occurs. Adsorption is the adhesion of a gas or liquid on the solid surface. The adsorption process includes trapping, separation, and purification of gases or liquids for environmental, industrial, biological, and pharmaceutical applications (10, 11). The most common adsorbents used for GHG capture are zeolites, carbonaceous materials, metal–organic framework (MOFs), and porous organic polymers (POPs). The selection of adsorbent materials depends on factors that affect the adsorption capacity, such as temperature, pressure, humidity, and gas selectivity (12–14).
Zeolites Zeolites are crystalline microporous aluminosilicates consisting of aluminosilicate chains connected by oxygen atoms and alkali metals (M, normally Na, K, and Ca). Zeolites are composed of three-dimensional crystalline structures and high specific surfaces acting as molecular sieves. The content in alumina and silica gives zeolites specific properties, such as acidic and/or basic sites in their structure and the ability to exchange ions (15, 16). Zeolites have been widely used in adsorption, catalysis, and ion exchange. Currently, zeolites are used in new applications such as green chemistry, optics, multifunctional fabrics, medicine, and nanotechnologies (17). 108
Carbon Materials The carbonaceous adsorbents are mainly constituted by carbon atoms and present low polarities. They can contain oxygen and hydrogen in their structure, constituting organic functional groups that may be present on their surface, such as carbonyl, carboxyl, or hydroxyl. Other functional groups can also be grafted on their surface, such as inorganic molecules and metals, which become active centers (14, 18, 19). There are carbonaceous materials with different structures, properties, and applications, such as activated carbons, nanofibers, nanotubes, fullerenes, graphite, or graphene. They can be used for water treatment and gas adsorption (CO2, CH4, H2, N2), catalytic supports, or pharmaceutical applicaions (20, 21). Metal–Organic Framework (MOF) Metal–organic frameworks (MOFs) are hybrid porous compounds. Their structure has coordination bonds between organic and inorganic molecules (Mg, Cu, Ag, Zn, and Cd among others). They present a homogeneous and complex structure, as well as a pore distribution between micropores and mesopores. Their structure can be modified to obtain MOFs with a large specific surface area or other pore geometry, or they can be modified to graft different functional groups (-NH2, -NO2, -OH, = O). The variety of functional groups and the ability to create bonds between organic molecules and metal ions give MOFs very suitable physical properties for use in adsorption, liquid or gas phase separation, catalysis, and chemical sensors (21–23). Porous Organic Polymers (POPs) Porous organic polymers (POPs) are compounds assembled by covalent bonds between light elements (C, O, H, N, B) that form polymeric networks without metals. POPs are conformed by units called organic monomers which can assemble to establish secondary building units (SBUs) (24–26). POPs have large specific surface areas, high pore volume, and low density (21, 27–29). POPs could be classified as amorphous polymers with intrinsic microporosity (PIM) and also structured organic materials (POF), which can generate porosity like COF (Covalent Organic Framework) and PAF (Porous Aromatic Framework) (30).
Adsorption Conditions Methane (CH4) capture using various solid materials (zeolite, carbonaceous materials, MOF, or POP) has been studied for several years. CH4 capture is performed using a PSA (Pressure Swing Adsorption) system at high pressure (P > 3400 kPa) and low temperature (T < 25ºC). The main purpose of these studies is to determine CH4 recovery at high concentration (CH4 > 99 %v/v) in order to be used for energetic applications. However, to our knowledge, the study of the CH4 capture under ambient conditions of low pressure (Ptotal = 101.3 kPa), low temperature (T = 30ºC), and low gas concentration (less than 5000 ppmv) has not 109
been studied. In this work, CH4 adsorption under ambient conditions of pressure and temperature is proposed for the treatment of gaseous emissions with low gas concentrations. The selected adsorbents for CH4 adsorption were commercial zeolites, laboratory-conditioned biochar, and a synthesized MOF. The commercial adsorbents corresponded with zeolitic structure are LTA (Linde Type A), BEA (Beta polymorph A), and FAU (Faujasite). Biochar was obtained by torrefaction of biomass at low temperature (250ºC and 300ºC) and low residence time (60, 90, and 120 min). Biochar samples were codified considering the temperature and time of torrefaction (i.e B300ºC-60 corresponds with biochar (B) torrefied at 300ºC for 60 min). MOF based on Zn and organic ligand (imidazole) was synthesized by solvothermal synthesis (ZIF-8 "Zeolitic imidazolate framework"). Adsorbents were physically and chemically characterized, as described later (Table 2 and Table 3). CH4 adsorption capacity for each adsorbent was evaluated by dynamic adsorption tests under atmospheric conditions of pressure, temperature, and low gas concentration. Prior to CH4 adsorption, samples were outgassed under inert gas flow. CH4 breakthrough curves were performed at 30ºC into a fixed-bed reactor using a gas cell (2 m in length) coupled to a FT-IR spectrometer (Nicolet iS50; Thermo Scientific; USA). The equipment specifications and the experimental protocol for CH4 adsorption have been described by Delgado et al. (31–33) Finally, the classical Freundlich adsorption isotherm was fitted to gas adsorption capacity (11, 34, 35).
Qeq is the gas adsorption capacity at equilibrium (mmol/g); KF is Freundlich isotherm constant, which refers to the ratio of adsorbed gas with respect to the gas present in the total flow rate ((mmol/g)·kPa1/n); P is the gas equilibrium pressure (kPa); and n represents the adsorption intensity (dimensionless). The Freundlich adsorption isotherm was fitted to experimental data by non-linear regression based on the Marquardt-Levenberg algorithm (36). The complete procedure was explained in previous publications (31–33).
Characterization of Adsorbents In order to determine physical-chemical properties of the tested adsorbents, they were characterized using the following techniques: •
•
X-ray diffraction (XRD) determines the crystallinity of the sample using a X-ray diffractometer (Panalytical X’Pert Pro, Netherlands) with a Cu Kα radiation (λ = 1.54184 Å). The sample structure was analyzed using the standard diffraction patterns of the International Center of Diffraction Data (ICDD) (37). Textural properties such as specific surface area, pore volume, and pore size were determined by N2 adsorption at 77 K using a Tristar apparatus 110
•
•
(Micromeritics, Germany). Samples were previously outgassed under vacuum and the parameters were determinded using methods described by Sing and Kowalczyk (38, 39). Thermal stability of adsorbents was evaluated in a thermogravimetric balance (TGA) (TGA-DSC Mettler Toledo; France). The samples were placed in an opened alumina crucible and heated from room temperature to 1000ºC under a synthetic air atmosphere (20% v/v O2 and 80% v/v N2) in order to determine their weight loss. Chemical composition of the samples was determined using an Optical Emission Spectrometer Plasma (ACTIVA-Horiba; Japan), following the standard wet chemical analysis procedure.
Figure 1 presented X-ray diffractograms of tested adsorbents. Zeolites are highly crystalline and were identified using the standard diffraction files that corresponded with the following patterns: PDF 04-009-4861 for LTA, PDF 04-017-1321 for BEA structure, and PDF 04-009-5210 for FAU (32, 33). Biochar obtained by torrefaction of cardboard was analyzed by X-ray diffraction and it was observed that the strongest diffraction peak was at 21.3º, followed by 16º and 23.6º, which corresponded to cellulose. Cellulose presents crystalline and amorphous phases. Cellulose crystallinity is not only affected by the presence of other organic compounds (hemicellulose and lignin) but also by torrefaction conditions (temperature and time). When the temperature and time of torrefaction increased, the obtained biochar became more amorphous (31). Regarding ZIF-8, it was observed that it presented high crystallinity with the characteristic diffraction peaks reported by Venna et al. (40–42)
Figure 1. X-ray diffraction patterns of adsorbents. 111
Table 2 lists the textural properties and thermal stability of samples used as CH4 adsorbents. The surface area of zeolites ranged from 535 to 564 m2/g, while the pore volume ranged from 0.27 to 0.40 cm3/g. FAU presented the largest specific surface area (564 m2/g), total pore volume (0.4 cm3/g), micropore volume (0.27 cm3/g) and pore size (5.0 - 6.5 Å). Furthermore, the zeolites presented similar values to those reported in literature for zeolites with a similar structure (LTA (43), BEA (44), and FAU (43, 45)) (46). Biochar samples are not porous materials as they have a surface area lower than 6 m2/g. In addition, it was observed that torrefaction conditions did not have a high influence over their textural properties. The ZIF-8 presented high surface area (1450 m2/g) and total pore volume (0.70 cm3/g), as well as two pore sizes (3.4 and 11.6 Å) showing similar values to those reported in the literature (47). The MOF presented the highest surface area and pore volume, while biochars presented the lowest. Regarding thermal stability under air atmosphere at 1000ºC, it was observed that zeolites are highly thermally stable with a weight loss lower than 18.8 wt%, while biochar and ZIF-8 presented total weight losses higher than 89.8 and 82 wt% at 1000ºC, respectively.
Table 2. Textural Properties and Thermal Stabilitya Adsorbent
Surface area m2/g
Vμpores cm3/g
Pore volume cm3/g
Pore size Å
Weight lossb wt%
Zeolites LTA
539
0.27
0.19
5
18.8
BEA
535
0.4
0.18
5.0 – 6.1
3.5
FAU
564
0.4
0.27
5.0 – 6.5
7.5
Biochar B250°C-90
4
9.0·10-3
93.5
B250 °C-120
3.7
8.5·10-3
94.6
B300 °C-60
6
2.3·10-2
92.4
B300 °C-90
4.1
1.4·10-2
89.8
B300 °C-120
3
5.7·10-3
91.3 MOF
ZIF-8 a
1450
0.7
Adapted with permission from ref (33).
0.63 b
3.4 – 11.6
Weight loss (wt%) at 1000ºC.
112
82
Table 3. Ultimate Analysis (wt%) and Mineral Content (mg/kg)a Ultimate analysis C
N
H
b
S
Mineral content Oc
Si/Al
wt%
Si
Al
Ca
Fe
K
Mg
Na
ppm
ppm
wt%
ppm
ppm
ppm
wt%
Zeolites LTA
1.2
8.04
6500
4400
13500
2.1
BEA
33.2
0.02
200
200
200
0.04
FAU
1.4
1.18
3800
1300
12200
11.54
Biochar
113
B250°C-90
54.2
0.5
6.4
0.9
31.6
245
880
0.84
220
190
–
0.26
B250°C-120
61.8
0.8
5.8
0.8
25.4
160
1260
1.07
430
260
17
0.33 0.19
B300°C-60
55.4
0.7
7.4
1
27.9
155
830
0.63
360
150
–
B300°C-90
56.7
0.8
4.7
0.7
26.4
190
1470
1.29
560
320
36
0.41
B300°C-120
60.4
0.9
3.8
0.6
25.5
185
1850
1.50
440
330
50
0.44
MOF ZIF-8 a
42.1
24.6
Adapted with permission from ref (33).
4.4 b
Ash content was considered.
c
Calculated by difference.
Chemical composition of adsorbents are presented in Table 3. Zeolites are alumino-silicates connected by oxygen atoms. They contain metal cations to balance the charges generated by the Si/Al ratio. The metal content of zeolites has an effect on physical properties (11, 48) and CH4 adsorption capacity (45, 49). BEA presented the highest Si/Al ratio (33.2) and LTA the lowest (1.20). Metals presented in the highest concentration in zeolites are Ca (LTA 8.04 wt% and FAU 1.18 wt%) and Na (FAU 11.54 wt% and LTA 2.10 wt%). In the case of biochars, the ultimate analysis showed high carbon content, which increased with temperature and residence time of torrefaction due to the degradation of volatile biomass. Hydrogen and oxygen content decreased with temperature and residence time as a consequence of the biochar dehydration, thus making biochar more hydrophobic. In biochar, the minerals presented in higher concentration were: Na, Al, Ca, and Mg. This was in accordance with data reported in the literature (50). The chemical composition of ZIF-8 was: 14.20 wt% of Zn, 42.1 wt% of C, 24.6 wt% of N, and 4.4 wt% of H. Regarding the chemical composition, biochar presented the highest carbon content and zeolites the highest mineral content.
Methane Adsorption The adsorption system consisted of a packed bed reactor where the operating conditions of gas adsorption were fixed. CH4 breakthrough curves were carried out in order to obtain the gas adsorption capacity of the adsorbent. The adsorption temperature was 30ºC and the total pressure of gases was 101.3 kPa; the CH4 partial pressure ranged from 0.50 kPa to 0.4 kPa (icons in Figure 2). The Freundlich isotherm was fitted to experimental data of CH4 adsorption (solid line in Figure 2). In Figure 2, it can be observed that for all materials, the adsorption capacity increased with CH4 partial pressure. The highest CH4 adsorption capacity for each adsorbent was obtained at 0.4 kPa of CH4 and at a temperature of 30ºC. Among the different materials tested, biochar presented the highest adsorption capacity, followed by ZIF-8 and zeolites. CH4 adsorption capacity of biochars was 0.322 mmol of CH4/g B300ºC-60 and 0.354 mmol CH4/g B300ºC-90. Biochars had a surface area lower than 6 m2/g; thus, they were not porous adsorbents. However, they had a carbon content higher than 55.4 wt.%, which may interact with CH4. For biochars, CH4 adsorption capacity increased with torrefaction time, because the carbon content increased and the moisture decreased. Thus, B300ºC-90 with a carbon content of 56.7 wt%C, presented a higher CH4 adsorption capacity than B300ºC-60 (55.4 wt%C) (51–53). Biochar was followed by ZIF-8, which presented a CH4 adsorption capacity of 0.110 mmol of CH4/g adsorbent. ZIF-8 had the highest surface area (1450 m2/g) and pore volume (0.63 cm3/g), and also high carbon content (42.1 wt% C). Then, zeolites with BEA and FAU structures presented high CH4 adsorption capacities, with 0.041 mmol CH4/g for BEA and 0.040 mmol CH4/g for FAU, respectively. Among zeolites, FAU had the highest surface area (564 m2/g), micropore volume (0.27 cm3/g), and higher pore size which also affects the CH4 adsorption capacity (44, 54, 55). Furthermore, the extra 114
framework cations also have an effect on the zeolite properties and adsorption process because at low pressures, the adsorption process is based on adsorbate and adsorbent interactions (48, 56). Thus, the adsorbents with higher carbon content presented the higher CH4 adsorption capacity because they have more affinity for CH4. Furthermore, for zeolites, it was observed that adsorption capacity increased with the surface area and pore volume, as explained previously in the literature (31–33).
Figure 2. CH4 Adsorption isotherms at 30ºC and total pressure of 101.3 kPa. Icons represents the experimental data. Solid lines corresponded with Freundlich isotherm model. (Adapted with permission from ref. (33).)
In addition, the experimental data were modeled according to the Freundlich isotherm (Equation 1) using nonlinear regression (Delgado et al. (32, 33)). The calculated Freundlich and statistical parameters are presented in Table 4. KF values were ranged from 0.080 to 0.899 mmol CH4/g adsorbent· kPa1/n. The lowest value of KF corresponded to LTA and the highest to biochar B300ºC-90, which presented the lowest and highest adsorption capacities, respectively. 1/n values were around unity, ranged from 0.982 to 1.066 for B300ºC-60 and ZIF-8, respectively. In order to validate the model and parameter’s significances, the F-test and the t-test were used at a 95% confidence level. As Fcalculated was higher than the F-test value, the Freundlich model is valid and represents the experimental data. Similarly, tcalculated was higher than the t-test value, thus the parameters are statistical meaningful confirming a complete statistical meaningfulness for models and parameters. 115
Table 4. Freundlich Isotherm Parametersa Adsorbent Zeolites
LTA
BEA
FAU
Biochar
B300°C-60
B300°C-90
MOF
ZIF-8
Freundlich
t-test
KF
0.080
48.7
2.37
1/n
1.008
60.6
2.37
KF
0.106
37.2
2.45
1/n
1.040
47.0
2.45
KF
0.108
15.8
2.37
1/n
1.033
20.0
2.37
KF
0.802
40.7
2.37
1/n
0.982
49.6
2.37
KF
0.899
79.0
2.37
1/n
1.017
99.0
2.37
KF
0.285
17.7
2.45
1/n
1.066
22.8
2.45
KF, Freundlich isotherm constant (dimensionless).
a
tcalculated
((mmol/g)·kPa1/n);
Fcalculated
F-test
16744
4.74
11317
5.14
1794
4.74
11417
4.74
44430
4.74
2603
5.14
1/n, adsorption intensity
In Figure 2, the experimental and predicted (Freundlich model) adsorption capacities are compared showing the good fit between them. The Freundlich isotherm is suitable for describing the CH4 adsorption for zeolites, biochar, and MOF at low pressures (Table 4 and Figure 2). 1/n represents an adsorption intensity being higher than unity in most cases. This means that non-ideal adsorption behavior was observed at low partial pressures (Henry’s Law if 1/n = 1). Otherwise, the Freundlich isotherm can predict adsorption at low pressure for both heterogeneous and multilayer adsorption (34, 35). The Freundlich isotherm (with 1/n > 1) suggests cooperative adsorption, where the adsorption surface is not homogeneous and adsorbed gas has an effect on the adsorption of other gas molecules (57).
Perspectives of GHG Capture Under Atmospheric Conditions For several years, CO2 and CH4 capture were studied by systems that use high pressure (P > 3400 kPa) and low temperature (T < 25 °C) for energetic purposes. On the other hand, there is an acknowledged lack in the control of GHG emissions under ambient conditions for low energy consumption technologies. The increase of CH4 and N2O emissions by agriculture and the industrial sector, and the necessity to control and detect these emissions, represent a major challenge for global climate action especially for nations who compromised the United Nations Framework Convention on Climate Change in 2015 and signed the Paris Agreement Measuring. In 2017, the Conference of the Parties (COP23) highlighted the urgency to reduce GHG emissions to achieve climate neutrality by 2050. Thus, the capture of CH4 and other GHGs under ambient conditions 116
would help to deliver on climate commitments and mitigate the impacts of climate change. Furthermore, with GHG emissions at low concentrations, it will favor the development of different economic activities. Since great quantities of GHGs are emitted under atmospheric conditions, it is necessary to develop new high performing technologies to capture these gases without consuming high amounts of energy. These will be decisive in the application of these technologies by many GHG emitters, who find that available technologies are excessively expensive.
Conclusion The adsorption of CH4 under atmospheric conditions can be performed using different adsorbent solids. Among the different materials used for CH4 adsorption at 30ºC and partial pressures lower than 0.4 kPa, different physical and chemical properties affect their CH4 adsorption capacity. Physical properties such as a specific surface, volume, or pore size are crucial to promoting interactions between the gas and the adsorbent surface. Furthermore, at a low pressure of CH4, the adsorption capacity of the materials is strongly influenced by the chemical composition of the adsorbents. Among the tested adsorbents, the biochar presented the highest adsorption capacity (0.354 mmol CH4/g B300ºC-90), followed by ZIF-8 (0.109 mmol CH4/g ZIF-8); whereas, commercial zeolites presented the lowest adsorption capacity (0.041 mmol CH4/g BEA and 0.040 mmol CH4/g FAU). The Freundlich isotherm can be used to correctly predict the CH4 adsorption capacity under atmospheric conditions for a variety of solid adsorbents.
Acknowledgments The authors gratefully acknowledge the Agricultural Greenhouse Gases Program (AGGP-AAC), Campus France for “Eiffel scholarships (812591L),” and Rhône-Alpes Region (CMIRA 2014-Accueil Doc 1400856201) which supported the research project. Dr. Avalos Ramirez specially thanks the Research Program for College Researchers (FRQ) and Discovery Grants Program (NSERC) for complementary funding. The first author would like to give special thanks to “Centre National en Électrochimie et en Technologies Environnementales”, as well as “Universidad de Castilla la Mancha” for their technological support.
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