CO 2 Adsorption from Biogas Using Amine-Functionalized MgO

Biogas is a renewable fuel source of methane (CH4), and its utilization as a natural gas substitute or transport fuel has received much interest. However, apart from CH4, biogas also contains carbon dioxide (CO2) which is noncombustible, thus reducing the biogas heating value. +erefore, upgrading biogas by removing CO2 is needed for most biogas applications. In this study, an aminefunctionalized adsorbent for CO2 capture from biogas was developed. Mesoporous MgO was synthesized and functionalized with different tetraethylenepentamine (TEPA) loadings by wet impregnation technique. +e prepared adsorbents (MgO-TEPA) were characterized by X-ray diffraction (XRD) andN2 adsorption-desorption.+e CO2 adsorption performance of the preparedMgO-TEPA was tested using simulated biogas as feed gas stream.+e results show that the CO2 adsorption capacities of the adsorbents increase with increasing TEPA loading. +e optimum TEPA loading is 40 wt.%, which gives the highest CO2 adsorption capacity of 4.98mmol/g. A further increase in TEPA loading to 50 wt.% significantly reduces the CO2 adsorption capacity. Furthermore, the stability and regenerability of the adsorbent with 40% TEPA loading (MgO-TEPA-40) were studied by performing ten adsorption-desorption cycles under simulated biogas and real biogas conditions. After ten adsorption-desorption cycles, MgO-TEPA-40 shows slight decreases of only 5.42 and 5.75% of CO2 adsorption capacity for the simulated biogas and biogas, respectively. +e results demonstrate that MgOTEPA-40 possesses good stability and regenerability which are important for the potential application of this amine-based adsorbent.


Introduction
Due to rising fossil fuel prices, greenhouse gas emissions from fossil fuel combustion, and high energy demands, sustainable and renewable energy sources are needed [1,2].Biogas, considered to be one of the alternative sources of renewable energy, has the potential to supplement the current energy requirements.Biogas produced from anaerobic digestion processes is mainly composed of 50-70% of methane (CH 4 ) and 30-50% of carbon dioxide (CO 2 ) [3].Apart from these two gases, biogas contains smaller amounts of hydrogen sulfide (H 2 S) and trace amounts of ammonia (NH 3 ), nitrogen (N 2 ), hydrogen (H 2 ), carbon monoxide (CO), and oxygen (O 2 ) [4,5].e energy content in terms of calorific value of pure methane is 36 MJ/m 3 (at STP conditions), whereas that of the biogas containing 60-65% CH 4 is approximately 20-25 MJ/m 3 [3,6].erefore, being noncombustible, CO 2 also causes a reduction in the heating value and energy density of biogas on volume basis.In addition, high CO 2 content increases energy demand for compression and transportation of biogas.Biogas can be upgraded to a higher fuel standard by increasing the energy density by reducing major noncombustible gas (CO 2 ) and other impurities.e final product consisting of 95-99% CH 4 and 1-5% CO 2 [7], which is called biomethane, can be used as a substitute for natural gas and a vehicle fuel.erefore, upgrading biogas to biomethane is one of the technologies that receive much attention in the bioenergy industry.
To remove acid gases such as CO 2 and H 2 S from gas streams, several technologies have been developed, including physical absorption, chemical absorption, adsorption, membrane separation, and cryogenic separation [8].Among these technologies, the chemical absorption using liquid amine absorbents is one of the most widely used technologies for separating CO 2 from gas streams [9,10].However, amine scrubbing processes have some disadvantages such as high energy consumption for solvent regeneration, high equipment corrosion rate, solvent degradation, and fouling of the process equipment [11][12][13].As a result, adsorption-based technologies, usually involving solid CO 2 adsorbents, are suggested and studied to overcome those problems [14].Adsorbents for CO 2 capture are mainly porous materials, such as activated carbon [15,16], mesoporous silica [17], zeolites [18,19], and metal-organic frameworks (MOFs) [20,21], which usually have a large surface area in mesopores and micropores.Compared to liquid amine absorption, the solid adsorbents possess significant advantages for energy efficiency.However, there are some existing problems such as low CO 2 adsorption capacities and selectivity.erefore, modification or functionalization of the solid adsorbents by introducing amines into their porous structure for CO 2 adsorption has also attracted great interest [22].
e amine-functionalized adsorbents combine the high CO 2 affinity of amines and porous materials (large pore volume and large surface area) which exhibit advantageous properties such as high CO 2 adsorption rate and high thermal stability.ese adsorbents not only have the potential to reduce the energy consumption compared to the large amount of energy required to heat bulk water for the regeneration of absorbent in the aqueous amine absorbent process but also have high CO 2 adsorption capacity and the ability to reduce the corrosion of equipment caused by high concentrated aqueous amine absorbent [10,23].Several research groups have studied and reported the performances of mesoporous materials functionalized with amines for CO 2 removal.ese materials can be grouped together depending on the preparation methods which are grafting and impregnation with diverse amine species such as aminosilanes, polyethylenimine (PEI), tetraethylenepentamine (TEPA), and diethanolamine (DEA) [24][25][26][27][28][29][30][31].e adsorbents obtained by chemical grafting of amine onto the support are relatively more stable than those obtained by physical impregnation.However, the amount of grafted amine is limited, resulting in a relatively lower amine loading which leads to lower CO 2 adsorption capacity.Unlike the grafting method, the impregnation method possesses several advantages such as easier preparation, higher amine loading, and lower cost [32].
Apart from the solid adsorbents mentioned above, magnesium oxide (MgO) has been widely studied for CO 2 capture and has been reported as a potential adsorbent for CO 2 adsorption [33][34][35].However, pure MgO exhibits fairly small CO 2 adsorption capacity [36].To the best of our knowledge, introducing amines into the porous structure of MgO to form a composite with high CO 2 adsorption capacity has not been reported.erefore, in the present study, a new adsorbent for CO 2 removal from biogas was developed.MgO with mesoporous structure was synthesized and functionalized with tetraethylenepentamine (TEPA) via an impregnation process.
e prepared adsorbents with different amine loadings were investigated for CO 2 adsorption capacity.Multiple adsorption-desorption cyclic stability of the adsorbents was also tested by using simulated biogas and biogas as feed gas streams.

Experimental
2.1.Synthesis of Mesoporous MgO.Mesoporous MgO was synthesized using the sol-gel method as reported in the literature [37].Magnesium nitrate hexahydrate (Mg(NO 3 ) 2 • 6H 2 O) and oxalic acid ((COOH) 2 •2H 2 O), purchased from Sigma-Aldrich, in the molar ratio of 1 : 1, were first dissolved separately in ethanol and used for the synthesis of MgO.
ese solutions were then mixed together with vigorous magnetic stirring to yield a thick white gel and kept stirred for 12 h.Subsequently, the gel product was dried at 100 °C for 24 h.e dried mixture was ground and calcined under atmospheric pressure at 600 °C for 2 h.e chemical reactions in the sol-gel and calcination step can be expressed by equations ( 1) and ( 2), respectively [37]: (1) 2.2.Preparation of Amine-Functionalized Adsorbents.In this study, the amine-functionalized adsorbents were prepared by the wet impregnation method described by Xu et al. [38] with some modification.e desired amount of tetraethylenepentamine (TEPA), obtained from Sigma-Aldrich, was dissolved in 25 ml of methanol under stirring for 15 min.en, 5 g of the synthesized MgO was added to the TEPAmethanol solution, and the resultant slurry was stirred continuously for approximately 30 min.e slurry was then dried at 70 °C for 2 h.e adsorbents impregnated with TEPA were designated as MgO-TEPA-x, where x represents the weight percentage of TEPA in the adsorbents.

Characterization of the Adsorbents.
e crystal structures of the prepared adsorbents were analyzed by powder X-ray diffraction (PXRD) by using a Philips X'Pert MPD diffractometer with Cu-Kα radiation.e N 2 adsorptiondesorption isotherms were measured at −196 °C using a Micromeritics ASAP2060 volumetric analyzer.
e isotherms were used for calculating the surface area, pore volume, and average pore diameter of the adsorbents via the Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method.

CO 2 Adsorption.
e CO 2 adsorption-desorption tests were performed in a fixed-bed reactor of which a simplified flow diagram is given in Figure 1. e reactor was a stainlesssteel adsorption column with an inner diameter of 15 mm and a length of 160 mm.Typically, 5 g of the adsorbent was packed into the adsorption column and supported by quartz wool from both sides.Before each experimental run, the sample was heated to 100 °C in a N 2 (99.99% purity) stream at the flow rate of 40 mL/min for 60 min.After cooling to the adsorption temperature (30 °C), the adsorption tests began by introducing simulated biogas into the adsorption column at a flow rate of 40 mL/min.e simulated biogas was produced by mixing CO 2 (99.99% purity) and N 2 (99.99% purity) from gas cylinders at the desired flow rates to obtain a concentration of 40% CO 2 and 60% N 2 .e adsorbent-bed 2 International Journal of Chemical Engineering temperature was measured by thermocouples and controlled by a temperature controller.After completing the adsorption process, desorption of the adsorbed CO 2 was performed by heating the sample to 150 °C in a N 2 gas stream for 60 min.In order to test the regenerability of the adsorbent, ten cycles of adsorption-desorption were performed on the same sample.In this study, biogas produced from a local swine farm was also used as feed gas to test the CO 2 adsorption performance of the prepared adsorbent using the same experimental procedure.e biogas consists of 38.4% CO 2 and 61.5% CH 4 , which are in the range of the typical biogas compositions found in the literature [39].e concentrations of CO 2 in the feed and treated gas streams were determined by a gas chromatograph equipped with a TCD detector.
e breakthrough curve data obtained from the CO 2 adsorption tests were used for calculating the equilibrium capacity of the adsorbents according to equations (3) and (4) [40]: where t s is the mean residence time (min), C 0 and C t are the inlet and outlet concentrations of CO 2 (vol.%),respectively, q is the equilibrium adsorption capacity of CO 2 (mmol/g), t is the adsorption time (min), Q is the feed volumetric flow rate (ml/min) at standard temperature and pressure (STP), and m is the mass of the adsorbent (g).

Results and Discussion
3.1.Characterization.Figure 2 shows the XRD patterns of the prepared MgO with different TEPA loadings.e pristine MgO exhibits five well-resolved diffraction peaks at 36.9 °, 42.9 °, 62.3 °, 74.7 °, and 78.6 °. ese are characteristic peaks of MgO, which are consistent with the results of previous reports [35,41,42]. is confirms a high degree of purity and suggests that the obtained MgO samples are highly crystalline.From comparing the diffraction pattern of MgO with those of MgO-TEPA-x with different TEPA loadings; the diffraction angles are nearly identical, indicating that the structure of MgO is preserved after loading the TEPA.However, the diffraction intensity of MgO decreases with increasing TEPA loadings.It has been reported that diffraction intensities are relative to the degrees of pore filling [43].erefore, the incorporation of amine into the pore channels of the support possibly causes the loss of intensity [28]. is indicates that TEPA is loaded into the pore of the MgO support, which is similar to that observed in MCM-41 loaded with polyethylenimine (PEI) [38].
To investigate the porous structure of the prepared samples, N 2 adsorption-desorption isotherms were measured.Figure 3 shows the N 2 adsorption-desorption isotherms of the prepared MgO with different TEPA loadings.According to the International Union of Pure and Applied Chemistry (IUPAC) classification [44], all of the samples display type IV isotherms with the hysteresis loop.e hysteresis loop present in the isotherm is due to the capillary condensation taking place in mesopores.e pristine MgO exhibits a large hysteresis loop compared with the other four TEPA-functionalized samples, indicating the presence of highly porous structure.In addition, it can be seen that the N 2 uptake decreases with increasing TEPA loading.As the TEPA loading increases from 20 to 50 wt.%, the mesopores of MgO are almost completely filled with TEPA molecules at 50 wt.%which results in restricting the access of N 2 into the pores, thus causing MgO-TEPA-50 to become a nonporous material [45].

International Journal of Chemical Engineering
From the N 2 adsorption-desorption isotherms, the surface area, pore volume, and average pore diameter of the MgO samples were calculated according to the BET and BJH method and are listed in Table 1.Loading TEPA onto MgO significantly reduces the surface area and pore volume.For example, when the TEPA loading is 20 wt.%, the surface area and the pore volume decrease from 207 to 118 m 2 /g and from 0.81 to 0.53 cm 3 /g, respectively.A further increase in TEPA loading to 50 wt.%causes a great decrease in the surface area and the pore volume.is is because, when the TEPA loading reaches 50 wt.%or higher, excessive amount of TEPA blocks most of the pores and covers the surface of the adsorbent [46,47].e same trend as the surface area and pore volume is observed for the pore size of the samples.

CO 2 Adsorption Behaviors.
e effect of TEPA loading on the CO 2 adsorption performance of MgO-TEPA was investigated using simulated biogas as the feed gas.Figures 4  and 5 show the breakthrough curves and the adsorption capacities (mmol of CO 2 /g of adsorbent), respectively.As can be seen, the CO 2 adsorption of the pristine MgO reaches the saturation point within only 12 min, resulting in a low adsorption capacity of 0.95 mmol/g.Increasing TEPA loading within a suitable range increases the breakthrough time and adsorption capacity of the adsorbents, which is consistent with the results of previous reports [48,49].is is because, as the loading increases, more amine active sites for CO 2 adsorption are provided.When the TEPA loading increases to 40 wt.% (MgO-TEPA-40), the adsorbent shows the maximum breakthrough time and adsorption capacity of 34 min and 4.98 mmol/g, respectively.However, as the TEPA loading increases from 40 to 50 wt.%, the breakthrough time and the adsorption capacity significantly drop from 34 to 28 min and from 4.98 to 4.26 mmol/g, respectively.Too high loadings may cause the TEPA molecules to aggregate in the pore or coat the surface of the adsorbent, which hinder the diffusion of CO 2 to react with the active sites in the pores [48,50].e above results suggest that the optimum TEPA loading for MgO modification is 40 wt.%. e most common mechanism for CO 2 capture in supported amines under dry conditions involves the formation of carbamates between two amine groups [51,52].Two types of these amine groups exist within TEPA including primary (R 1 -NH 2 ) and secondary (R 1 -NH-R 2 )   e reactions of CO 2 with the primary and secondary amines in TEPA can be represented as in equations ( 5)-( 7) [52]: In order to explore the degree of utilization of amine groups in TEPA by CO 2 , the amine efficiency was defined as the molar ratio of the adsorbed CO 2 to all amine groups present in the adsorbents [48].e number of amine groups was calculated from the molar amount of nitrogen atoms in TEPA.e molecular weight of TEPA is 189.3 g/mol, and each TEPA molecule contains 5 amine groups.erefore, the amine efficiency was calculated and is presented in Figure 5.It can be seen that the amine efficiency decreases with an increase in TEPA loading.e highest amine efficiency of 0.54 is obtained at 20 wt.% TEPA loading, suggesting that two moles of amine groups reacts with one mole of CO 2 .At low TEPA loading, the amine chains are more dispersible, making the adsorption sites easily available.However, as the amine loading increases, amines would begin to conglomerate within the pores.is leads to poor distribution of amine sites, resulting in a reduction in the number of the accessible amine sites for CO 2 sorption, although the potential amine sites are more for higher TEPA loading.With a further increase in TEPA loading to 50 wt.%, the amine efficiency decreases significantly to 0.32, indicating that some of amine sites are unoccupied.
e CO 2 adsorption capacities of MgO and MgO-TEPA-40 in this study and those of amine-impregnated adsorbents reported in the literature are compared in Table 2.As can be seen, various supports and amines are used for adsorbent preparation.e comparison shows that the CO 2 adsorption capacity of MgO-TEPA-40 is higher than those previously reported in the literature.

Regenerability of the Adsorbents.
In industrial applications, the stability and regenerability of CO 2 adsorbents are critical parameters for long-term operation.In this study, the cyclic CO 2 adsorption capacity of MgO-TEPA-40 was investigated by performing ten CO 2 adsorption-desorption cycles using simulated biogas and real biogas as feed gas streams.Figure 6 shows the cyclic CO 2 adsorption capacities of the adsorbent in ten consecutive runs.For the simulated biogas, the CO 2 adsorption capacity decreases slightly with increasing number of adsorption-desorption cycle.After ten cycles, the CO 2 adsorption capacity decreases from 4.98 to 4.71 mmol/g, which is a decrease of only 5.42%.e CO 2 adsorption capacity for the biogas exhibits the same trend, decreasing from 4.87 to 4.59 mmol/g after ten cycles, meaning the capacity loss of only 5.75%.ese capacity drops are lower than those of some amine-based adsorbents reported in the literature [49,65].e loss of adsorption capacity during the adsorption-desorption cycles could be due to the volatilization of the impregnated TEPA [51,66].

Conclusions
Mesoporous MgO was synthesized and functionalized by impregnation with different tetraethylenepentamine (TEPA) loadings.e prepared MgO-TEPA was used as an adsorbent for CO 2 separation from simulated biogas.e CO 2 adsorption capacity of the adsorbents was found to increase with increasing TEPA loading.MgO-TEPA-40 with 40 wt.%TEPA exhibits the best CO 2 adsorption performance, with the CO 2 adsorption capacity of 4.98 mmol/g.However, a further increase in TEPA loading to 50 wt.%causes a significant reduction of CO 2 adsorption capacity.e stability

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Table 1 :
Surface area, pore volume, and average pore diameter of MgO-TEPA-x.

Table 2 :
Comparison of CO 2 adsorption capacity of amine-impregnated adsorbents.MgO-TEPA-40 were examined by performing ten consecutive CO 2 adsorption-desorption runs under simulated biogas and real biogas conditions.MgO-TEPA-40 shows slight decreases of only 5.42 and 5.75% of CO 2 adsorption capacity for the simulated biogas and biogas, respectively, after ten cycles.ese results indicate that MgO-TEPA-40 has a stable CO 2 adsorption capacity and good regenerability both in simulated biogas and biogas, which are beneficial properties for practical applications as CO 2 adsorbent.