CO2 adsorption study on primary, secondary and tertiary amine functionalized Si-MCM-41

International Journal of Greenhouse Gas Control 51 (2016) 230–238

Contents lists available at ScienceDirect

International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

CO2 adsorption study on primary, secondary and tertiary amine functionalized Si-MCM-41 Sohail Ahmed a , Anita Ramli b,∗ , Suzana Yusup a a b

Department of Chemical Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak, Malaysia

a r t i c l e

i n f o

Article history: Received 2 October 2015 Received in revised form 16 May 2016 Accepted 24 May 2016 Keywords: Carbon dioxide Adsorption process Si-MCM-41 Amine functionalization

a b s t r a c t The present study was conducted to investigate the CO2 adsorption efficiency of siliceous mesoporous material known Si-MCM-41 and impacts of amine loading, amine type, CO2 pressure and adsorption temperatures on CO2 adsorption. Si-MCM-41 was impregnated with monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA). The adsorption study was investigated from very low pressure to 1 bar at three temperatures (i.e. 25, 50 and 75 ◦ C) in pure CO2 atmosphere. Pristine Si-MCM-41 shows maximum CO2 adsorption capacity of 27.78 mg/g at 25 ◦ C and 1 bar pressure. Monoethanolamine functionalized Si-MCM-41 with 10–50 wt.% samples were investigated in order to optimize MEA loading for maximum CO2 adsorption. The 50 wt.% MEA-Si-MCM-41 sample exhibited the highest CO2 adsorption capacity of 39.26 mg/g at 25 ◦ C and 1 bar pressure as compared to other samples. 50 wt.% DEA-Si-MCM41 and 50 wt.% TEA-Si-MCM-41 samples were also investigated for their affinity to capture CO2 at the same conditions. Results show that 50 wt.% MEA-Si-MCM-41 still gives the highest CO2 adsorption capacity while adsorption capacity decreased in order of monoethanolamine, diethanolamine and triethanolamine-functionalized Si-MCM-41. CO2 adsorption capacity of all adsorbents decreases with increasing adsorption temperature above 25 ◦ C. Fourier transform infrared spectroscopy (FTIR) analysis of CO2 -saturated 50 wt.% MEA, DEA and TEA-Si-MCM-41 samples shows the presence of transmission peaks associated to formation of carbamates. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction In the human history, 21st century reflects the highest development the world has ever experienced before. This could only be possible due to energy linked to fossil fuels. Energy needs are exponentially increasing and it is predicted to follow a similar trend in the future due to economic and population growth (IPCC, 2014). Fossil fuels are the dominant energy resources which provide 86% share in the global energy utilization. This large utilization of fossil fuels accounts for 75% carbon dioxide (CO2 ) emissions to the atmosphere from various industries such as fossil fuelled power plants, cement industry, refinery and synthetic ammonia production units (IPCC, 2005). CO2 is the main greenhouse gas (GHG) and the root cause of global warming which influence the climate change (Khan et al., 2015). Apart from CO2 emission from combustion of fossil fuels into atmosphere, CO2 exists as a contaminant in various processes such as natural gas and fuel gas which demands

∗ Corresponding author. E-mail address: anita [email protected] (A. Ramli). http://dx.doi.org/10.1016/j.ijggc.2016.05.021 1750-5836/© 2016 Elsevier Ltd. All rights reserved.

for purification (Tan et al., 2012). Natural gas reserves are commonly contaminated with over 40% CO2 and N2 , and the use of such gas fields is only suitable if the CO2 is removed (D’Alessandro et al., 2010) or reduced to ≤2 mol% according to U.S. pipeline specifications (Bhide et al., 1998). Various processes have been introduced for CO2 removal from gaseous streams such as chemical absorption, membrane-based separation, cryogenic separation and adsorption separation. Chemical absorption process is a well-established and a dominant technology used throughout the world for CO2 removal using aqueous alkanolamines as solvents (Park et al., 2002). However, this process has various serious issues such as; solvent degradation (MacDowell et al., 2010), equipment corrosion, solvent composition kept low (≤30 wt.% MEA) due to constraints of solvent degradation and corrosion (Abu-Zahra et al., 2007), high regeneration energy (70% of total operating costs in a CO2 capture plant) (Idem et al., 2006), low CO2 loading capacity (Rinker et al., 2000), large equipment size (Rochelle, 2009) and environmental concerns due to solvent degradation. Membrane-based separation process is also a mature and commercial technology for CO2 separation. It can be useful for CO2 separation from flue gases of post-combustion

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

process, where CO2 content is higher than 20% (Favre, 2007). There are various issues related to this technology such as low capacity and poor thermal properties, sensitive to sulphur compounds and other trace elements, reduction in the CO2 permeability in presence of water, continuous use cause plasticization (Scholes et al., 2009), multistage separation (Kenarsari et al., 2013) and lack of simultaneous high permeability and selectivity (Olajire, 2010). Cryogenic process is useful for gas streams which contain high concentration of CO2 , preferably higher than 90%. It involves cooling of the gaseous mixture to a very low temperature (<−73.3 ◦ C) which resulted in freezing or liquefaction of CO2 (Olajire, 2010). This process has various drawbacks such as high energy for refrigeration especially in dilute gas streams, presence of water or heavy hydrocarbons may cause blockage to the heat exchanger (Kargari and Ravanchi, 2012) and pressure drop during operation, process efficiency is reduced due to increasing layer of solid CO2 onto heat exchanger surfaces (Brunetti et al., 2010). Due to the issues in above mentioned processes, adsorption process is considered to be a competitive and viable alternative process for CO2 separation due to various advantages like low energy requirement, cost advantage and ease of applicability over a relatively wide range of temperatures and pressures (Choi et al., 2009; Martunus et al., 2012). Adsorption process can be applied to capture CO2 from various applications specially from flue gases of power plants produced by post-combustion method (Gupta and Ghosh, 2014). In adsorption process, the CO2 present will be adsorbed on the surface or reside in the pores of the selected adsorbent. Subsequently, the regeneration of the adsorbent bed is performed by various techniques (Riboldi and Bolland, 2015). Adsorption process overcomes the shortcomings of chemical absorption, membrane-based separation and cryogenic processes and provides environmental/economical advantages such as no solvent regeneration energy, no use of large quantity of water, no corrosion or solvent degradation issues, use of selective adsorbents can overcome the selectivity and permeability issues, and does not require high refrigeration energy. The key component of the adsorption process is the adsorbent. The success of the adsorption process highly depends on adsorbent which exhibits high adsorption capacity and selectivity to the specified gas (Sayari et al., 2011). Various types of adsorbent materials have been developed for CO2 adsorption separation. Siliceous mesoporous material known as Mobil Composition of Matter No. 41 or Si-MCM-41 first developed by Beck et al. (1992), is a potential candidate to be used as an adsorbent for CO2 separation due to its extraordinary properties such as ordered cylindrical pore system, high surface area (>700 m2 /g), large pore volume (≥0.7 cm3 /g) (Beck et al., 1992). SiMCM-41 has a large number of internal hydroxyl (silanol) groups (∼40–60%) which could ease in modification of its surface properties (Selvam et al., 2001). Amines are basic in nature and used for CO2 capture in chemical absorption process. In their earlier work, Satyapal et al. (2001) have reported on immobilization of amines on solid materials as adsorbents for high CO2 adsorption capacity and selectivity, which is now an active area of research. Amine-based solid adsorbents have potential applications in CO2 removal from open and closed environments. Examples of closed environments are submarines, amphibious vehicles and spacecrafts while open environments are purification of oxygen, carbon monoxide or olefin streams (Leal et al., 1992), industrial gas steams (Leal et al., 1995), space shuttle (Satyapal et al., 2001), gas purification (Belmabkhout et al., 2010) and flue gas of power plants (Xu et al., 2005). The alteration of surface-chemistry of Si-MCM-41 with amino groups can improve its performance for CO2 adsorption. However, it is of a prime importance to study the effect of functionalization with different type of amines in designing the new amine-functionalized adsorbents for CO2 adsorption.

231

This paper focuses on the functionalization of Si-MCM-41 with MEA, DEA and TEA, and the effect of amine loading, effect of variable pressures from very low to 1 bar and effect of adsorption temperatures on CO2 adsorption capacity of the amine-functionalized Si-MCM-41. 2. Materials and methods 2.1. Materials Sodium silicate (NaSiO3 ; SiO2 25.5–28.5%, Na2 O 7.5–8.5%; Merck) as a silica source, hexadecyltrimethylammonium bromide (CTABr, purity ≥ 98%; Sigma Aldrich) as a templating agent, sulphuric acid (H2 SO4 98%, J. T. Baker); deionized water as a solvent for synthesis of Si-MCM-41, monoethanolamine (Purity ≥ 99%; Merck), diethanolamine (Purity ≥ 99%; Merck) and triethanolamine (Purity ≥ 99%; Merck) as functionalizing agents and methanol (Purity 99.9%; Merck) as a solvent for impregnation of amine molecules. 2.2. Synthesis and functionalization of Si-MCM-41 Si-MCM-41 was synthesized using procedure reported by Ramli et al. (2012). In a typical synthesis procedure, 18.70 g of sodium silicate (Na2 SiO3 ) solution was dissolved in 40 g of deionized water to form solution A, while 16.77 g of CTABr was dissolved in 50.23 g of deionized water in a separate beaker to form solution B. Then, both solutions were mixed by gentle stirring and pH of the solution was maintained at pH 10 by adding sulphuric acid in drop-wise. The mixture was stirred for 30 min followed with addition of another 20 g of deionized water to the solution under continuous stirring. The obtained suspension was then transferred into Teflon-lined stainless steel autoclave, sealed and kept at 100 ◦ C under static condition for eight days. The resulting solid product was recovered by filtration, washed several times with copious amount of deionized water and dried at 120 ◦ C for 24 h. The as-synthesized Si-MCM-41 was later calcined in N2 atmosphere at 550 ◦ C for a period of 6 h to remove the organic template. Pristine Si-MCM-41 was functionalized with amines (MEA, DEA and TEA) using procedure as reported by Ramli et al. (2014a). Based on reported method, pristine Si-MCM-41 was functionalized with 10–50 wt.% of monoethanolamine (MEA; primary amine) by wet impregnation method where the required amount of amine was added to 10 g of methanol as solvent and stirred for 15 min for complete dissolution. Then, 2 g of pristine Si-MCM-41 were dispersed into the amine solution. The slurry obtained was stirred vigorously for 30 min, and then dried at 70 ◦ C for 16 h in a vacuum oven at reduced pressure of 700 mmHg. Pristine Si-MCM-41 was also functionalized with 50 wt.% of diethanolamine (DEA; secondary amine) and 50 wt.% of triethanolamine (TEA; tertiary amine) on the basis of the optimum MEA loading to give the maximum CO2 adsorption capacity. The amine functionalized Si-MCM-41 adsorbent is denoted as X Y-Si-MCM-41, where X and Y represent the loading of amine as weight percentage and type of amine impregnated, respectively. 2.3. Characterization N2 adsorption-desorption analysis was performed using a Micromeritics Accelerated Surface Area Porosimetry System (ASAP 2020) volumetric adsorption analyzer at −196 ◦ C and 0.01–1 bar relative pressure. The specific surface area was determined using Brunauer-Emmett-Teller (BET) method (Brunauer et al., 1938) while both the average pore diameters and total pore volumes were calculated from adsorption branches of isotherms by Barrett–Joyner–Halenda (BJH) method (Barrett et al., 1951). Suface

232

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

morphology of samples was analyzed by field emission scanning electron microscopy (FESEM). FESEM micrographs were recorded on Zeiss SUPRA 55 VP-FESEM microscope (Germany) operated at 7.00 kV and at magnification of 30K×. Fourier transform infrared spectroscopy (FTIR) analysis of CO2 -saturated adsorbents was carried out on a SHIMADZU 8400S spectrometer in the 400–4000 cm−1 region using KBr pellet technique at ambient conditions. 2.4. Gas adsorption studies Pure CO2 adsorption on the adsorbents was carried out using Rubotherm gravimetric-densimetric gas sorption apparatus. The entire system is a computer-based operating system, using MessPro software for operations. In a typical adsorption experiment, a blank measurement of the empty sample container was carried out at 25 ◦ C in N2 flow of 100 ml/min to determine the mass and volume of sample container (mSC and VSC ). Then, the adsorbent is weighed and placed in the sample container. The cell in which the sample container is housed is then closed and high pressure is applied to check for any leakage in the system. Leak test was performed whenever the chamber is dismantled. The adsorbent was then reactivated in vacuum at 100 ◦ C until there is no more weight loss, from which mass of the sample (mS ) was obtained. After reactivation, the sample was cooled down to the analysis temperature and buoyancy measurement was performed using Helium (He) gas at a flow rate of 100 ml/min. The buoyancy measurement gives the volume of suspension system (VSS ), from which volume of sample (VS ) is obtained. Helium gas is an inert gas that penetrates in all open pores of the materials without being adsorbed. For CO2 adsorption measurement, first the system was evacuated to vacuum. Then, the sample was exposed to pure CO2 at a flow rate of 100 ml/min at the desired temperature. The temperature during adsorption measurements was held constant using a thermostated circulating fluid. The data was received by MessPro software and all the operations were performed by this software. Amount of CO2 adsorbed was calculated using the following equation: mA = m − (mSC + mS ) + (VSC + VS ) × gas (T, P)

(1)

3. Results and discussion 3.1. Characterization of amine-functionalized Si-MCM-41 N2 adsorption-desorption isotherm of pristine Si-MCM-41 has been reported to exhibit a Type IV isotherm according to IUPAC classification with a hyteresis loop of Type H1 which shows that pristine Si-MCM-41 is a mesoporous material with cylindrical pores and has high degree of pore size uniformity. The pristine Si-MCM41 also shows narrow hysteresis loop which signifies that the SiMCM-41 consists of highly uniform mesoporous channels (Ramli et al., 2014b). Functionalization of Si-MCM-41 with MEA changed the shape of the isotherm from Type IV (mesoporous material) to Type III (non-porous material) as the MEA loading increases from 10 to 50 wt.%, which indicates gradual filling of the pores with MEA molecules by increasing loading of MEA until these are fully filled. It is suggested that the pores of Si-MCM-41 are almost completely filled with MEA molecules at 50 wt.% MEA loading which result in pore blockage, thus it became non-porous material (Ramli et al., 2014a). Similar trend of the changes in the N2 adsorption-desorption isotherm is observed with 50 wt.% DEA-Si-MCM-41 and 50 wt.% TEA-Si-MCM-41 which shows that the pores of Si-MCM-41 are almost completely filled with DEA and TEA molecules. Fig. 1 illustrates the N2 adsorption-desorption isotherms of pristine

Table 1 Textural properties of amine-functionalized Si-MCM-41 adsorbents. Sample

Surface area (m2 /g)

Pore volume (cm3 /g)

Pore diameter (nm)

Si-MCM-41 10 wt.% MEA-Si-MCM-41 20 wt.% MEA-Si-MCM-41 30 wt.% MEA-Si-MCM-41 40 wt.% MEA-Si-MCM-41 50 wt.% MEA-Si-MCM-41 50 wt.% DEA-Si-MCM-41 50 wt.% TEA-Si-MCM-41

993 748 81 22 20 19 13 213

1.009 0.590 0.214 0.112 0.102 0.082 0.066 0.170

3.1 2.2 2.1 1.9 – – – –

Si-MCM-41, 50 wt.% MEA-Si-MCM-41, 50 wt.% DEA-Si-MCM-41, and 50 wt.% TEA-Si-MCM-41 for comparison. Isotherm of pristine Si-MCM-41 showed clear changes after impregnation with these amines. Isotherm of 50 wt.% TEA-Si-MCM-41 sample showed higher uptake of N2 compared to other samples, this could be due to space available in pores which formed due to non-linear structure of TEA molecules. Changes in the textural properties of the amine-functionalized Si-MCM-41 adsorbents are presented in Table 1. A significant decrease in specific surface area, pore volume and average pore diameter was observed with increasing loading of 10–50 wt.% MEA over Si-MCM-41. While loading of 50 wt.% DEA and TEA over SiMCM-41 also showed decrease in textural properties. However, sample 50 wt.% TEA-Si-MCM-41 showed higher surface area and pore volume compared to 50 wt.% MEA-Si-MCM-41 and 50 wt.% DEA-Si-MCM-41. This could be explained based on the structure of TEA molecule. As the triethanolamine (TEA) molecule is a larger molecule and non-linear in shape. These molecules do not properly fix in the pores and leave the small spaces available in the pores. Therefore, textural properties of 50 wt.% TEA-Si-MCM-41 are slightly higher compared to other samples. Surface morphology of samples was investigated by FESEM analysis and micrographs are shown in Fig. 2. Micrograph of pristine Si-MCM-41 showed spheriodal shaped particles along with irregular shaped particles and also agglomerates. When pristine Si-MCM-41 was functionalized with 10–50 wt.% of MEA, surface morphology of Si-MCM-41 completely changed. At initial loading of 10 wt.% MEA, particle of Si-MCM-41 started to aggregate. Furthermore, as the weight percentage of MEA increased up to 50 wt.%, particles of Si-MCM-41 gradually agglomerated into different sizes and shapes (Ramli et al., 2014a). In case of 50 wt.% DEA-Si-MCM-41 and 50 wt.% TEA-Si-MCM-41, FESEM micrographs also showed agglomeration of Si-MCM-41 particles in similar trend. This change in morphology can be explained as, at initial loading amine molecules are distributed into the pores initially and then spillover on the external surface that resulted in agglomeration of particles. 3.2. CO2 adsorption on pristine Si-MCM-41 Pristine Si-MCM-41 possesses unique features such as highly ordered uniform hexagonal shaped cylindrical pore arrangement, high surface area, large pore volume, mesopores and large number of silanol groups which are considered as important features for a good adsorbent and expected to facilitate in the CO2 adsorption process. Fig. 3 shows the CO2 adsorption isotherms of pristine Si-MCM-41 at temperatures between 25 and 100 ◦ C and pressures between 0 and 1 bar. These isotherms show that CO2 adsorption using pristine Si-MCM-41 increases with increasing pressure and the maximum CO2 adsorption capacity obtained is only 27.78 mg/g at 25 ◦ C and 1 bar pressure. At low pressure, it shows very low uptake of CO2 which demonstrates its low affinity for CO2 (Ramli et al., 2014c). This behavior of pristine Si-MCM-41 indicates weak

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

Fig. 1. N2 adsorption-desorption isotherms of (a) Si-MCM-41, (b) 50 wt.% MEA-Si-MCM-41, (c) 50 wt.% DEA-Si-MCM-41 and (d) 50 wt.% TEA-Si-MCM-41 * (

233

adsorption,

desorption).

Fig. 2. FESEM micrographs of (a) Si-MCM-41, (b) 50 wt.% MEA-Si-MCM-41, (c) 50 wt.% DEA-Si-MCM-41 and (d) 50 wt.% TEA-Si-MCM-41.

interaction between the siliceous material and CO2 molecules while CO2 adsorption increases with increasing pressure indicates that CO2 physisorption is strongly dependent on pressure. Thus, CO2 adsorption in Si-MCM-41 could only occur via physisorption within

its pores. It also points out that high porosity adsorbent does not guarantee for high CO2 adsorption capacity. Fig. 3 also shows that the CO2 adsorption capacity decreases with increasing temperature from 25 to 100 ◦ C. This trend could be

234

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

45

30

40 35

20 25 °C

15

50 °C

10

75 °C 100 °C

5 0

CO2 uptake (mg/g)

CO2 uptake (mg/g)

25

30 25

25 °C

20

50 °C

15

75 °C

10

0

0.2

0.4

0.6

0.8

5

1

0

Pressure (bar) Fig. 3. Effect of temperature on CO2 adsorption isotherms of pristine Si-MCM-41.

0

0.2

0.4

0.6

0.8

1

Pressure (bar) Fig. 5. Effect of temperature on CO2 adsorption isotherms of 50 wt.% MEA-Si-MCM41.

45

CO2 uptake (mg/g)

40 35 30

Si-MCM-41

25

10 wt.% MEA

20

20 wt.% MEA

15

30 wt.% MEA

10

40 wt.% MEA

5

50 wt.% MEA

0

0

0.2

0.4

0.6

0.8

1

Pressure (bar) Fig. 4. CO2 adsorption isotherms of pristine Si-MCM-41 and 10–50 wt.% MEA-SiMCM-41 samples at 25 ◦ C and 0–1 bar.

attributed to an increase in kinetic energy of the CO2 molecules at elevated temperatures which resulted in further weakening of the interaction between the CO2 molecules and pristine Si-MCM-41. Similar trend in CO2 adsorption capacity with increasing pressure and temperature have been reported by Belmabkhout et al. (2009) in his study using MCM-41. 3.3. CO2 adsorption on MEA-functionalized Si-MCM-41 Fig. 4 shows the effect of 10–50 wt.% MEA loading on pristine Si-MCM-41 for CO2 adsorption capacity at 25 ◦ C and 0–1 bar. Results show that CO2 adsorption capacity increases with increasing in MEA loading. All MEA-Si-MCM-41 samples show abrupt CO2 adsorption when the CO2 pressure was initially increased from 0 to 0.1 bar and then gradually increases with increasing pressure up to 1 bar. Sample 50 wt.% MEA-Si-MCM-41 shows the highest CO2 adsorption capacity of 39.3 mg/g at 25 ◦ C and 1 bar, due to presence of large number of amine functionalities as CO2 -affinity sites. Functionalization of highly ordered Si-MCM-41 material with MEA dispersed the MEA molecules uniformly into the framework as CO2 -affinity sites for CO2 capture. Number of accessible affinity sites for CO2 capture increases with increasing loading of MEA i.e. 10–50 wt.%. The increment in CO2 adsorption capacity is an evidence of successful functionalization of Si-MCM-41 with MEA. Referring the work of Xu et al. (2003), it is suggested that at lower MEA loading, CO2 uptake is attributed to both physical adsorption by condensation and chemical adsorption by MEA, while chemical adsorption became dominant when the pores are filled with 50 wt.% of MEA. An abrupt increase in CO2 adsorption at 0.1 bar and steep slope indicate high affinity of basic amino sites towards acidic CO2 fol-

lowed by gradual increase in adsorption. This rapid increase in CO2 adsorption is also an evidence of uniform dispersion of MEA molecules into the pore channels of Si-MCM-41. Whereas, gradual increase in CO2 adsorption could be due to packing of the CO2 molecules in monolayer formation or multilayer formation. Furthermore, diffusion of CO2 may occur either inside the MEA loaded pores or on the external surface because most available exposed amine sites are already occupied with CO2 at low pressure. Results show that increment of CO2 adsorption in MEA functionalized SiMCM-41 samples is not only contribution of MEA or Si-MCM-41 alone but it is a collective contribution of both that is called synergetic effect (Xu et al., 2002). Based on the highest CO2 adsorption capacity at 25 ◦ C, 50 wt.% MEA-Si-MCM-41 is then investigated for the effect of temperature. CO2 adsorption capacity of 50 wt.% MEA-Si-MCM-41 decreases with increasing adsorption temperature above 25 ◦ C (Fig. 5). This is because at elevated temperatures, CO2 molecules are likely to be in an active energized form with higher kinetic energy, therefore, specific molecular orientation for nucleophilic attack may not be favourable which resulted decrease in CO2 adsorption (Bhagiyalakshmi et al., 2010). It is inferred that the 50 wt.% MEA-Si-MCM-41 shows the best CO2 adsorption capacity at optimal adsorption temperature of 25 ◦ C which suggests that it could be more effective CO2 adsorbent for low temperature applications. The interaction between amino sites present in 50 wt.% MEA-SiMCM-41 and CO2 molecules was investigated by FTIR spectroscopy. Prior to analysis, adsorbent 50 wt.% MEA-Si-MCM-41 was exposed to dry CO2 at 25, 50 and 75 ◦ C. Fig. 6 shows the FTIR spectra of CO2 -saturated 50 wt.% MEA-Si-MCM-41 adsorbent at different temperatures which are considerably different than the spectrum of fresh 50 wt.% MEA-Si-MCM-41 adsorbent. A broad peak centered at ∼3370 cm−1 attributed to OH stretching and N H stretching in MEA (Fig. 6a) became narrow and split into two peaks at ∼3413 and ∼3468 cm−1 attributed to NH2 stretching in carbamates are observed (Fig. 4b–d). A peak centered at ∼1660 cm−1 due to OH bending vibration became sharp and split into two peaks at ∼1618 cm−1 and 1638 cm−1 attributed to N H bending in carbamates (Fig. 6b–d). A weak peak attributed to N CO O symmetric stretching in carbamates (Pretsch et al., 2009) is observed at ∼882 cm−1 , while another peak attributed to NCOO sketal vibration is observed at ∼1414 cm−1 which could be due to the formation of ionic carbamate. The presence of these new peaks indicates that CO2 adsorbed on 50 wt.% MEA-Si-MCM-41 through the formation of carbamates/alkylammonium carbamates as previously reported by Liu et al. (2013) and Zhao et al. (2013).

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

235

Fig. 6. FTIR spectra of (a) fresh 50 wt.% MEA-Si-MCM-41 and CO2 -adsorbed 50 wt.% MEA-Si-MCM-41 at (b) 25 ◦ C, (c) 50 ◦ C, and (d) 75 ◦ C. Fig. 8. FTIR spectra of (a) fresh 50 wt.% DEA-Si-MCM-41 and CO2 -adsorbed 50 wt.% DEA-Si-MCM-41 at (b) 25 ◦ C, (c) 50 ◦ C, and (d) 75 ◦ C.

40 CO2 uptake (mg/g)

35 30 25 20

25 °C

15

50 °C

10

75 °C

5 0

0

0.2

0.4 0.6 Pressure (bar)

0.8

1

Fig. 7. Effect of temperature on CO2 adsorption isotherms of 50 wt.% DEA-Si-MCM41.

3.4. CO2 adsorption on 50 wt.% DEA-Si-MCM-41 Based on optimum loading of MEA, Si-MCM-41 was also functionalized with 50 wt.% of DEA in order to study the impact of secondary amine on CO2 adsorption. Compared to MEA molecule, DEA has a larger molecular structure where two alkyl groups are linked to central “N” atom. CO2 adsorption isotherms of 50 wt.% DEA-Si-MCM-41 at temperatures between 25 and 75 ◦ C and 0–1 bar are shown in Fig. 7. 50 wt.% DEA-Si-MCM-41 shows a maximum CO2 adsorption capacity of 35.33 mg/g at 25 ◦ C and 1 bar which is lower than that of 50 wt.% MEA-Si-MCM-41 (39.3 mg/g). CO2 adsorption isotherm also shows a rapid CO2 uptake at low pressure region

(0.1 bar) and steep slope which indicate the strong affinity for CO2 molecules. Whereas, lower CO2 adsorption capacity and decrease in initial CO2 uptake compared to 50 wt.% MEA-Si-MCM-41 could be due to steric hindrance. It can be clearly inferred that the type or structure of an amine molecule influence the CO2 adsorption on the adsorbent. Moreover, CO2 adsorption capacity of 50 wt.% DEA-Si-MCM-41 decreases with increasing temperature above 25 ◦ C in a similar trend that was observed using 50 wt.% MEA-Si-MCM-41. This is because at higher temperatures kinetic energy of CO2 molecules increases, where specific orientation for nucleophilic attack may not be favourable, thus CO2 adsorption capacity decreases (Bhagiyalakshmi et al., 2010). Fig. 8 shows the FTIR spectra of CO2 -saturated 50 wt.% DEASi-MCM-41 at 25, 50 and 75 ◦ C along with the spectrum of fresh 50 wt.% DEA-Si-MCM-41 for comparison. The FTIR spectrum of fresh 50 wt.% DEA-Si-MCM-41 shows various changes after reaction with CO2 at these temperatures. A sharp and narrow peak at ∼3434 cm−1 and a medium intensity peak at ∼1647 cm−1 attributed to N H stretching and N H bending in carbamates were observed respectively (Fig. 8b–d). In addition, a weak peak at ∼882 cm−1 and a medium intensity peak at ∼1413 cm−1 corresponding to N CO O symmetric stretching (Pretsch et al., 2009) and NCOO skeletal vibration were observed respectively, which could be due to the formation of ionic carbamates (Fig. 8b–d). These changes in spectra of CO2 -saturated 50 wt.% DEA-Si-MCM-41 samples are due to the formation of carbamates via chemisorption (Liu et al., 2013; Zhao et al., 2013).

236

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

30

CO2 uptake (mg/g)

25 20 25 °C

15

50 °C

10

75 °C

5 0

0

0.2

0.4 0.6 Pressure (bar)

0.8

1

Fig. 9. Effect of temperature on CO2 adsorption isotherms of 50 wt.% TEA-Si-MCM41.

3.5. CO2 adsorption on 50 wt.% TEA-Si-MCM-41 To study the effect of tertiary amine on Si-MCM-41 for CO2 adsorption, Si-MCM-41 was functionalized with 50 wt.% Triethanolamine (TEA) based on optimum loading of MEA. TEA has larger molecular structure as compared to MEA and DEA with three alkyl groups on central “N” atom. CO2 adsorption behavior of 50 wt.% TEA-Si-MCM-41 at temperatures between 25 and 75 ◦ C and 0–1 bar pressure is shown in Fig. 9. 50 wt.% TEA-Si-MCM-41 shows a maximum CO2 adsorption capacity of 27.92 mg/g at 25 ◦ C and 1 bar which is lower compared to both 50 wt.% MEA-Si-MCM-41 and 50 wt.% DEA-Si-MCM-41 but slightly higher than that was observed using pristine Si-MCM-41. This lower CO2 adsorption could be due to presence of three alkyl groups on the central “N” atom which may result in steric hindrance for interaction with CO2 molecules. CO2 adsorption isotherm of 50 wt.% TEA-Si-MCM-41 obtained at 25 ◦ C clearly shows a fast uptake of CO2 at low pressure region (0.1 bar) and steep slope which indicates a strong affinity to capture CO2 molecules. These results imply that type and structure of amine molecule has strong influence on CO2 adsorption capacity of the amine-functionalized Si-MCM-41 adsorbent. The effect of increasing the adsorption temperature from 25 to 75 ◦ C on the CO2 adsorption capacity of 50 wt.% TEA-Si-MCM-41 is shown in Fig. 9, where the CO2 adsorption capacity decreases to 15.48 mg/g and 9.62 mg/g with increasing temperature to 50 and 75 ◦ C, respectively. This phenomenon can be explained as, at incrased temperatures CO2 molecules have higher kinetic energy, where interaction between CO2 molecules and amino sites is not favourable, thus CO2 adsorption capacity decreased (Bhagiyalakshmi et al., 2010). Fig. 10 shows the FTIR spectra of CO2 -saturated 50 wt.% TEASi-MCM-41 samples along with the spectrum of fresh 50 wt.% TEA-Si-MCM-41 for comparison. FTIR spectra of CO2 -saturated 50 wt.% TEA-Si-MCM-41 samples show various changes which are described as follows. Appearance of a narrow and sharp peak at ∼3413 cm−1 , a medium intensity peak at ∼1638 and a low intensity peak at ∼1560 cm−1 (Fig. 10b–d) are attributed to the carbamates formation. The medium intensity peak observed at ∼1407 cm−1 and a weak intensity peak observed at ∼885 cm−1 are attributed to NCOO skeletal vibration and N CO O symmetric stretching, respectively, which signify the formation of carbamates (Pretsch et al., 2009; Qi et al., 2011). These results provide the evidence of carbamates formation via chemisorption. 3.6. Proposed chemical reactions The interaction between basic amino sites and acidic CO2 molecules yields carbamates. Caplow (1968) proposed a Zwitte-

Fig. 10. FTIR spectra of (a) fresh 50 wt.% TEA-Si-MCM-41 and CO2 -adsorbed 50 wt.% TEA-Si-MCM-41 at (b) 25 ◦ C, (c) 50 ◦ C, and (d) 75 ◦ C.

rion mechanism for the carbamates formation through the reaction between CO2 and primary or secondary amines. This mechanism consists of two steps, where at first the lone pair of electrons on active “N” of amine attack on the “C” of CO2 to form zwitterions, while free amine deprotonates the zwitterion to form carbamate in the second step. The adsorption of CO2 on MEA-Si-MCM-41 material in dry condition is proposed to occur by the formation of carbamate species through zwitterions mechanism because MEA molecule is a primary amine. Two amino groups of MEA-Si-MCM41 surface are involved for adsorption of CO2 molecule in the formation of carbamate species. Carbamate species formed are evidenced in FTIR spectra. The mechanism for the reaction is given below: First step:

RNH2 + CO2  RNH2+ COO−

Second step:

(2)

RNH2+ COO− + RNH2  RNHCOO− + RNH3+

(3)

Diethanolamine (DEA) is a secondary amine which is impregnated on Si-MCM-41 material. Interaction of CO2 molecules with DEA-Si-MCM-41 material involves carbamates formation by the same proposed mechanism. Similarly, two amino groups of DEASi-MCM-41 surface are involved for adsorption of one molecule of CO2 through the formation of carbamate species which are also evidenced from FTIR spectra. The mechanism for CO2 and secondary amine interaction is given below: First step:

R2 NH + CO2  R2 NH + COO−

Second step:

R2 NH + COO− + R2 NH 

R2 NCOO− + R2 NH2+

(4) (5)

Triethanolamine (TEA) is a tertiary amine which is also impregnated on Si-MCM-41. Donaldson and Nguyen (1980) suggested that in CO2 absorption process, tertiary amines do not form carbamates but the reaction can be explained by base catalyzed hydration

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

mechanism. However, the reaction mechanism may be different for CO2 adsorption with tertiary amine. It could be a direct reaction between CO2 molecules and tertiary amine functionalities. The tertiary amine may not under go zwitterion deprotonation mechanism for CO2 interaction or it could be due to electrostatic attraction and van der Walls forces (Ko et al., 2011). The reaction mechanism between CO2 and tertiary amine interaction is given below: First step:

R3 N + CO2  R3 N + COO−

Second step:

R3 N + COO− + R3 N  R2 NCOO− + R4 N +

(6) (7)

4. Conclusion Our previous work shows that pristine Si-MCM-41 showed a maximum CO2 adsorption capacity of 27.78 mg/g at 25 ◦ C and 1 bar while the adsorption capacity decreases with increasing temperature. Functionalization of pristine Si-MCM-41 with MEA, DEA and TEA drastically affect the CO2 adsorption performance due to their strong affinity towards CO2 . By functionalizing with 10–50 wt.% MEA, Si-MCM-41 showed substantial increase in CO2 adsorption capacity with increasing MEA loading. 50 wt.% MEASi-MCM-41 showed highest CO2 adsorption capacity of 39.3 mg/g at 25 ◦ C and 1 bar. Based on maximum CO2 adsorption capacity shown by 50 wt.% MEA-Si-MCM-41, 50 wt.% DEA-Si-MCM-41 and 50 wt.% TEA-Si-MCM-41 were investigated for their affinity towards CO2 . The CO2 adsorption capacity decreases in the order of 50 wt.% MEA-Si-MCM-41 > 50 wt.% DEA-Si-MCM-41 > 50 wt.% TEASi-MCM-41 owing to an increase in steric hindrance which is related to structure of amine molecule. Effect of temperature shows that CO2 adsorption capacity of 50 wt.% MEA-Si-MCM-41, 50 wt.% DEA-Si-MCM-41 and 50 wt.% TEA-Si-MCM-41 samples decreases with increaseing temperature. However, these materials showed effective CO2 adsorption at very low pressure (0.1 bar) due to strong CO2 affinity and uniform dispersion of amine functionalities in the framework of Si-MCM-41. It can be inferred that reactivity of these amines decreases in order of MEA (primary) > DEA (secondary) > TEA (tertiary) due to steric hinderance. FTIR analysis of CO2 -saturated amine-functionalized Si-MCM-41 samples presented evidences for carbamates formation at all range of studied temperatures, which confirmed that CO2 molecules are adsorbed via chemisorption. Acknowledgements The authors would like to thank Malaysian Ministry of Higher Education (MOHE) for the research funding under Fundamental Research Grant Scheme (FRGS/1/2012/UTP/02/02). The authors also would like to thank Universiti Teknologi PETRONAS (UTP) for the Graduate Assistantship awarded to Mr. Sohail Ahmed. References Abu-Zahra, M.R.M., Schneiders, L.H.J., Niederer, J.P.M., Feron, P.H.M., Versteeg, G.F., 2007. CO2 capture from power plants: part II. A parametric study of the technical performance based on monoethanolamine. Int. J. Greenh. Gas Control 1, 135–142. Barrett, E.P., Joyner, L.G., Halenda, P.P., 1951. The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 73, 373–380. Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T., Schmitt, K.D., Chu, C.T.W., Olson, D.H., Sheppard, E.W., 1992. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 114, 10834–10843. Belmabkhout, Y., Serna-Guerrero, R., Sayari, A., 2009. Adsorption of CO2 from dry gases on MCM-41 silica at ambient temperature and high pressure. 1: Pure CO2 adsorption. Chem. Eng. Sci. 64, 3721–3728. Belmabkhout, Y., Serna-Guerrero, R., Sayari, A., 2010. Adsorption of CO2 -containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for gas purification. Ind. Eng. Chem. Res. 49, 359–365.

237

Bhagiyalakshmi, M., Yun, L., Anuradha, R., Jang, H., 2010. Synthesis of chloropropylamine grafted mesoporous MCM-41, MCM-48 and SBA-15 from rice husk ash: their application to CO2 chemisorption. J. Porous Mater. 17, 475–484. Bhide, B.D., Voskericyan, A., Stern, S.A., 1998. Hybrid processes for the removal of acid gases from natural gas. J. Membr. Sci. 140, 27–49. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 309–319. Brunetti, A., Scura, F., Barbieri, G., Drioli, E., 2010. Membrane technologies for CO2 separation. J. Membr. Sci. 359, 115–125. Caplow, M., 1968. Kinetics of carbamate formation and breakdown. J. Am. Chem. Soc. 90, 6795–6803. Choi, S., Drese, J.H., Jones, C.W., 2009. Adsorbent materials for carbon dioxide capture from large anthropogenic point sources. ChemSusChem 2, 796–854. D’Alessandro, D.M., Smit, B., Long, J.R., 2010. Carbon dioxide capture: prospects for new materials. Angew. Chem. Int. Ed. 49, 6058–6082. Donaldson, T.L., Nguyen, Y.N., 1980. Carbon dioxide reaction kinetics and transport in aqueous amine membranes. Ind. Eng. Chem. Fundam. 19, 260–266. Favre, E., 2007. Carbon dioxide recovery from post-combustion processes: can gas permeation membranes compete with absorption? J. Membr. Sci. 294, 50–59. Gupta, T., Ghosh, R., 2014. Rotating bed adsorber system for carbon dioxide capture from flue gas. Int. J. Greenh. Gas Control 32, 172–188. IPCC, 2005. IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. IPCC, 2014. Climate change 2014: mitigation of climate change. In: Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Idem, R., Wilson, M., Tontiwachwuthikul, P., Chakma, A., Veawab, A., Aroonwilas, A., Gelowitz, D., 2006. Pilot plant studies of the CO2 capture performance of aqueous MEA and mixed MEA/MDEA solvents at the University of Regina CO2 capture technology development plant and the boundary dam CO2 capture demonstration plant. Ind. Eng. Chem. Res. 45, 2414–2420. Kargari, A., Ravanchi, M.T., 2012. Carbon dioxide: capturing and utilization. In: Dr Guoxiang, L. (Ed.), Greenhouse Gases-Capturing, Utilization and Reduction. In Tech, p. 338. Kenarsari, S.D., Yang, D., Jiang, G., Zhang, S., Wang, J., Russell, A.G., Wei, Q., Fan, M., 2013. Review of recent advances in carbon dioxide separation and capture. RSC Adv. 3, 22739–22773. Khan, A.A., Halder, G.N., Saha, A.K., 2015. Carbon dioxide capture characteristics from flue gas using aqueous 2-amino-2-methyl-1-propanol (AMP) and monoethanolamine (MEA) solutions in packed bed absorption and regeneration columns. Int. J. Greenh. Gas Control 32, 15–23. Ko, Y.G., Shin, S.S., Choi, U.S., 2011. Primary, secondary, and tertiary amines for CO2 capture: designing for mesoporous CO2 adsorbents. J. Colloid Interface Sci. 361, 594–602. Leal, O., Bolivar, C., Sepulveda, G., Molleja, G., Martinez, G., Esparragoza, L., 1992. Carbon dioxide adsorbent and method for producing the adsorbent. US Patent 5087597A. Leal, O., Bolivar, C., Ovalles, C., Garcia, J.J., Espidel, Y., 1995. Reversible adsorption of carbon dioxide on amine surface-bonded silica gel. Inorg. Chim. Acta 240, 183–189. Liu, Z.-l., Teng, Y., Zhang, K., Cao, Y., Pan, W.-p., 2013. CO2 adsorption properties and thermal stability of different amine-impregnated MCM-41 materials. J. Fuel Chem. Technol. 41, 469–475. MacDowell, N., Florin, N., Buchard, A., Hallett, J., Galindo, A., Jackson, G., Adjiman, C.S., Williams, C.K., Shah, N., Fennell, P., 2010. An overview of CO2 capture technologies. Energy Environ. Sci. 3, 1645–1669. Martunus, Helwani, Z., Wiheeb, A.D., Kim, J., Othman, M.R., 2012. Improved carbon dioxide capture using metal reinforced hydrotalcite under wet conditions. Int. J. Greenh. Gas Control 7, 127–136. Olajire, A.A., 2010. CO2 capture and separation technologies for end-of-pipe applications—a review. Energy 35, 2610–2628. Park, S.H., Lee, K.B., Hyun, J.C., Kim, S.H., 2002. Correlation and prediction of the solubility of carbon dioxide in aqueous alkanolamine and mixed alkanolamine solutions. Ind. Eng. Chem. Res. 41, 1658–1665. Pretsch, E., Buhlmann, P., Affolter, C., Pretsch, E., Bhuhlmann, P., Affolter, C., 2009. Structure Determination of Organic Compounds—Tables of Spectral Data, 4th ed. Springer, Berlin. Qi, G., Wang, Y., Estevez, L., Duan, X., Anako, N., Park, A.-H.A., Li, W., Jones, C.W., Giannelis, E.P., 2011. High efficiency nanocomposite sorbents for CO2 capture based on amine-functionalized mesoporous capsules. Energy Environ. Sci. 4, 444–452. Ramli, A., Ahmed, S., Yusup, S., 2012. Effect of synthesis duration on the physicochemical properties of siliceous mesoporous molecular sieve (Si-MMS). Deff. Diff. Forum 326–328, 647–653. Ramli, A., Ahmed, S., Yusup, S., 2014a. Effect of monoethanolamine loading on the physicochemical properties of amine-functionalized Si-MCM-41. Sains Malays. 43, 253–259. Ramli, A., Ahmed, S., Yusup, S., 2014b. Synthesis and functionalization of Si-MCM-41 with amines. In: Aliofkhzraei, M. (Ed.), Handbook of Functionalized Nanometerials. Nova Science Publishers Inc., New York, pp. 125–155.

238

S. Ahmed et al. / International Journal of Greenhouse Gas Control 51 (2016) 230–238

Ramli, A., Ahmed, S., Yusup, S., 2014c. Adsorption behaviour of Si-MCM-41 for CO2 : effect of pressure and temperature on adsorption. Chem. Eng. Trans. 39, 271–276. Riboldi, L., Bolland, O., 2015. Evaluating pressure swing adsorption as a CO2 separation technique in coal-fired power plants. Int. J. Greenh. Gas Control 39, 1–16. Rinker, E.B., Ashour, S.S., Sandall, O.C., 2000. Absorption of carbon dioxide into aqueous blends of diethanolamine and methyldiethanolamine. Ind. Eng. Chem. Res. 39, 4346–4356. Rochelle, G.T., 2009. Amine scrubbing for capture. Science 325, 1652–1654. Satyapal, S., Filburn, T., Trela, J., Strange, J., 2001. Performance and properties of a solid amine sorbent for carbon dioxide removal in space life support applications. Energy Fuels 15, 250–255. Sayari, A., Belmabkhout, Y., Serna-Guerrrero, R., 2011. Flue gas treatment via CO2 adsorption. Chem. Eng. J. 171, 760–774. Scholes, C.A., Kentish, S.E., Stevens, G.W., 2009. The effect of condensable minor components on the gas separation performance of polymeric membranes for carbon dioxide capture. Energy Procedia 1, 311–317. Selvam, P., Bhatia, S.K., Sonwane, C.G., 2001. Recent advances in processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves. Ind. Eng. Chem. Res. 40, 3237–3261.

Tan, L.S., Lau, K.K., Bustam, M.A., Shariff, A.M., 2012. Removal of high concentration CO2 from natural gas at elevated pressure via absorption process in packed column. J. Nat. Gas Chem. 21, 7–10. Xu, X., Song, C., Andresen, J.M., Miller, B.G., Scaroni, A.W., 2002. Novel polyethylenimine-modified mesoporous molecular sieve of MCM-41 type as high-capacity adsorbent for CO2 capture. Energy Fuels 16, 1463–1469. Xu, X., Song, C., Andresen, J.M., Miller, B.G., Scaroni, A.W., 2003. Preparation and characterization of novel CO2 molecular basket adsorbents based on polymer-modified mesoporous molecular sieve MCM-41. Microporous Mesoporous Mater. 62, 29–45. Xu, X., Song, C., Miller, B.G., Scaroni, A.W., 2005. Adsorption separation of carbon dioxide from flue gas of natural gas-fired boiler by a novel nanoporous molecular basket adsorbent. Fuel Process. Technol. 86, 1457–1472. Zhao, A., Samanta, A., Sarkar, P., Gupta, R., 2013. Carbon dioxide adsorption on amine-impregnated mesoporous SBA-15 sorbents: experimental and kinetics study. Ind. Eng. Chem. Res. 52, 6480–6491.