desorption on tetraethylenepentamine-supported surface-modified hydrotalcite

JES-01070; No of Pages 13 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 7 ) XX X–XXX

Available online at www.sciencedirect.com

ScienceDirect www.elsevier.com/locate/jes

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Perspective

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Nutthavich Thouchprasitchaia1 , Nuthapol Pintuyothin1 , Sangobtip Pongstabodee1,2,⁎

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1. Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand 2. Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand

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Optimization of CO2 adsorption capacity and cyclical adsorption/desorption on tetraethylenepentamine-supported surface-modified hydrotalcite

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AR TIC LE I NFO

ABSTR ACT

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Article history:

The objective of this research was to investigate CO2 adsorption capacity of

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Received 1 November 2016

tetraethylenepentamine-functionalized basic-modified calcined hydrotalcite (TEPA/b-cHT) 17

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Accepted 22 February 2017

sorbents at atmospheric pressure formed under varying TEPA loading levels, temperatures, 18

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Available online xxxx

sorbent weight to total gaseous flow rate (W/F) ratios and CO2 concentrations in the influent 19

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Keywords:

Fourier transform infrared spectrometry (FT–IR), thermal gravimetric analysis (TGA), 21

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CO2 capture

Brunauer–Emmet–Teller (BET) analysis of nitrogen (N2) adsorption/desorption and carbon– 22

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Tetraethylenepentamine

hydrogen–nitrogen (CHN) elemental analysis. Moreover, a full 24 factorial design with three 23

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Calcined hydrotalcite

central points at a 95% confidence interval was used to screen important factor(s) on 24

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Optimization

the CO2 adsorption capacity. It revealed that 85.0% variation in the capacity came from 25

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gas. The TEPA/b-cHT sorbents were characterized by means of X-ray diffraction (XRD), 20

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the influence of four main factors and the 15.0% one was from their interactions. A 26

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face-centered central composite design response surface method (FCCCD–RSM) was then 27 when operating with a TEPA loading level of 39%–49% (w/w), temperature of 76–90°C, W/F 29 ratio of 1.7–2.60 g·sec/cm3 and CO2 concentration of 27%–41% (v/v). The model fitted 30 sufficiently the experimental data with an error range of ±1.5%. From cyclical adsorption/ 31 desorption and selectivity at the optimal condition, the 40%TEPA/b-cHT still expressed its 32 © 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. 34

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Introduction

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The global use of fossil fuel to develop local economies worldwide brings about prosperity and a modern life style but with negative effects on the quality of environment and ecosystems, especially as it exceeds sustainable levels. The continuous emission of anthropogenic greenhouse gases,

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effective performance after eight cycles.

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employed to optimize the condition, the maximal capacity of 5.5–6.1 mmol/g was achieved 28

Published by Elsevier B.V. 35

especially carbon dioxide (CO2) from fuel combustion, to the atmosphere is one major cause of the Earth's climate change. The CO2 content in emitted gases varies in relation to the input-material, procedure (Abanades et al., 2005) and operating condition (Kanniche et al., 2010). The more CO2 that is emitted, the more serious the likely climate change will be. Changes in rainfall patterns and the extreme weather events

⁎ Corresponding author. E-mail: [email protected] (Sangobtip Pongstabodee).

http://dx.doi.org/10.1016/j.jes.2017.02.015 1001-0742/© 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

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1.1. Preparation of the sorbents

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The Mg–Al hydrotalcite (hereafter HT) was synthesized via co-precipitation. The mixed hydroxide carbonate solution was first prepared by dissolving NaOH and Na2CO3 in de-ionized water. The mixed nitrate solution (Mg(NO3)2·6H2O and Al(NO3)3·9H2O at a constant Mg/Al molar ratio of 3) was added drop wise slowly into the hydroxide carbonate solution − maintaining the CO2− 3 / (Al + Mg) and OH /(Al + Mg) ratios at 0.67 and 2.25, respectively. The mixture was vigorously stirred for 2 hr to obtain a suspension and then aged at 60°C for 18 hr. After that the suspension was cooled to room temperature and the gel precipitate that formed was harvested by filtration. The gel precipitate was washed with de-ionized water until the elute pH was close to that of de-ionized water. The precipitate was then dried at 100°C overnight. To obtain the high surface mixed metal oxide form, the dried solid was calcined in a muffle furnace under a static air atmosphere at 500°C for 5 hr to yield the cHT. To modify the surface of cHT with basic-alcohol solution, 3 g of cHT was dispersed in 0.5 mol/L KOH in ethanol and stirred at 120 r/min for 30 min. After that, the solid was washed with ethanol several times and dried at 60°C under an atmospheric pressure for 10 hr to obtain the b-cHT. The desired mixed solution of TEPA and ethanol was prepared at a constant mass to volume ratio of 1:2. The 3 g of

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1. Experimental

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astounding that rare search studies in CO2 adsorption on c-HT even though it exhibits a high surface area and high porosity. Many studies in CO2 adsorption capacity are based on sequential univariate analyses in which the interaction between factors is disregarded. Nevertheless, the process has been operated to optimize the adsorption capacity using multivariate analyses. The critical factors were adjusted in accordance with the levels of the other factors. In this study, the CO2 adsorption capacity of synthetic base-modified c-HT with TEPA functionalization (TEPA/b-cHT) at atmospheric pressure was investigated by varying the levels of TEPA loading, adsorption temperature, ratios of weight of sorbent to total gaseous flow rate (W/F ratio) and levels of CO2 content in the influent gas. The CO2 capacity was reported in terms of the amount of CO2 adsorbed in millimole unit per weight of sorbent in gram unit. The optimal condition for CO2 adsorption capacity was also evaluated via a face-centered central composite design response surface method (FCCCD–RSM) based on the derived influent factors and their interactions using a full 24 factorial design with three central points. To study the adsorption/desorption cycle, the sorbent with higher performance at the optimal condition was selected to study the cyclical adsorption/ desorption at least 8 cycles. The selective CO2 adsorption was also mentioned in this work. Characterization of the sorbent was analyzed by means of X-ray diffraction (XRD), Fourier transform infrared spectrometry (FT–IR), thermal gravimetric analysis (TGA), Brunauer–Emmet–Teller (BET) analysis of nitrogen (N2) adsorption/desorption and carbon– hydrogen–nitrogen (CHN) elemental analysis.

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have been found frequently. These warning signs threaten ecosystem characteristics, coastal and terrestrial communities (Pachauri et al., 2014), living things. A huge attempt bursts to reduce CO2 content before emitting to the atmosphere. Currently, an aqueous amine scrubbing is used widely in practice. However, this method still has some drawbacks such as a loss of some amine during an operation, a cause of an equipment corrosion, and a high-energy consumption for regeneration. Accessory installation for preventing solvent loss and a high maintenance cost are then required. To overcome these drawbacks, searches focus on a new potential method which expresses not only a high performance of CO2 capture but also a good regeneration-ability/reusability. The chemical CO2 adsorption on a porous material with an immobilized amine is one such approach of gaining interest. Different supports with various anchored amines (Pennline et al., 2008; Lee and Park, 2015; Markewitz et al., 2012; Shakerian et al., 2015; Veneman et al., 2015; Yamada et al., 2014; Yang et al., 2014; Zhang et al., 2013) have been investigated. The performance of CO2 adsorption has been evaluated instinctively with respect to the type of porous support, amine, preparation method, operating mode and composition of the influent. Primary amines have a higher affinity than secondary amines but they need a higher input energy to obtain CO2 desorption. Secondary amines provide a higher adsorption (attractive force) for CO2 than tertiary amines (Jo et al., 2014). Tetraethylenepentamine (TEPA), which is an organic amine consisting of primary and secondary amino-groups, has been used to functionalize solid sorbent surfaces, such as mesoporous ethane-silica nanotubes (Yao et al., 2013), silica (Linneen et al., 2013; Qi et al., 2011; Yue et al., 2006), titanium dioxide (Song et al., 2013), porous sulfur-doped titanate (Song et al., 2015) and cetyltrimethylammonium bromide-modified diatomaceous earth (Pornaroonthama et al., 2015). Of these, TEPA-impregnated solid sorbents showed a high CO2 adsorption capacity with excellent adsorption kinetics and a high durability (Jo et al., 2014; Linneen et al., 2013; Pornaroonthama et al., 2015; Qi et al., 2011; Song et al., 2015; Yao et al., 2013; Yue et al., 2006). Hydrotalcite (HT), called layered double hydroxide, is a class of positively charged lamellar metal hydroxides that is balanced by the presence of anions in the interlayer. Due to its versatile properties, low cost and extensive availability, HT has been widely used in several applications, including as a catalyst for chemical reactions (e.g., catalytic steam cracking of toluene (Zurita et al., 2015), hydrolysis of cellulose (Xu et al., 2015) and carbon disulfide (Li et al., 2016), synthesis of cyclohexanone-formaldehyde resin (Yang et al., 2015), diglycerol dicarbonate (Stewart et al., 2015) and biodiesel (J. Wang et al., 2015; X. Wang et al., 2015; Y.-T. Wang et al., 2015) and dry reforming of methane (Shiratori et al., 2015)) and as an adsorbent for the removal of thiocyanate (Xie et al., 2013), cationic dyes (Miranda et al., 2014) and chromium(VI) ions (Pérez et al., 2015). HT and K-promoted HT have been reported to have a high CO2 adsorption capacity at high partial pressures and high temperatures (Boon et al., 2014; van Selow et al., 2009), but a low capacity at low pressures (Boon et al., 2015). Meanwhile calcined HT (c-HT) has been used as a cationic exchanger (Das et al., 2004), thiocyanate-adsorbent (Li et al., 2008) and methanolysis-catalyst (Xie et al., 2006). It is

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J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 7 ) XXX –XXX

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

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1.2. Characterization of the sorbents

1.4. Experimental design for optimization

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Since the sequential univariate analysis does not inform the significant influence of each main factor nor their interactions on the CO2 sorption capacity, a full 24 factorial design with three central points was then employed varying each factor within the level of the other factors (Table 1). The response was reported in terms of the CO2 adsorption capacity. The matrix of experiments was in a completely randomized mode. The four main factors were the TEPA loading level (% (w/w)), W/F ratio (g·sec/cm3), adsorption temperature (°C) and level of CO2 content in the feed (% (v/v)), noted as factors A, B, C and D, respectively. The natural measurement units of the main factors at the minimum, median and maximum levels were encoded in the levels of dimensionless co-ordinates as a low level (−1), central point (0) and a high level (+1), respectively. Other factors that might affect the response were kept constant throughout the experiments. All tests were run in triplicate and data is shown as the mean value. Based on the full factorial design results, a FCCCD–RSM analysis was subsequently used to optimize the conditions for maximal CO2 adsorption (Table 2). The Design-Expert 7.0 software package (Stat Ease Inc. Minneapolis, USA) was used to analyze the results at a 95% confidence interval, including the analysis of variance (ANOVA), percentage of contribution and the Pareto chart of absolute standardized effects. Statistical significance of the differences in means was accepted at the p < 0.05 level. To elucidate the adequacy of the RSM results, the screened factors were randomly selected to set ten more trials. The concurrence between the experimental data and the estimated one was shown in terms of the error percentage.

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The crystal structures of the synthesized sorbents were examined by XRD on an X-Ray diffractometer (Bruker D8 Advance, Germany), operated at 40 kV and 40 mA using monochromatic CuKα radiation (λ = 1.5406 Å). The scan speed rate was about 0.02°/sec and the XRD profiles were recorded over a 2θ range from 5° to 70°. The functional groups of the samples were analyzed by FT–IR on an FT–IR spectrometer (Perkin-Elmer Spectrum one, USA). The spectrometer was equipped with a mercury– cadmium–telluride detector. The frequency range was 4000– 400 cm− 1 (resolution of 1 cm− 1) with a multi-layer potassium bromide beam splitter. The thermal decomposition of the sorbent's components was evaluated by TGA on a thermographic analyzer (PerkinElmer Pyris-diamond, USA). Each sample (around 10 mg) was placed in a chamber of the analyzer and heated at 10°C/min from room temperature to 800°C under a N2 atmosphere at a flow rate of 10 mL/min. The surface area (SBET), pore volume (Vp) and pore width (Dp) of the sorbents were determined by adsorption/desorption of N2 at −196°C using an accelerated surface area and porosimetry system (ASAP 2020, Micromeritics, USA). The samples were degassed under 1 mm Hg constant pressure at 90°C for 1 hr and heated to and maintained at 115°C for 2 hr before analysis. The TEPA loading of the sorbents was evaluated from the total amount of nitrogen, determined by a CHN elemental analyzer (CE-440, Exeter Analytical Inc., USA). Each sample was weighed (about 2 mg) on aluminum foil, placed in the sample chamber and combusted in pure oxygen under a static condition. The sample gas was analyzed using the thermal conductivity detectors (TCDs) at a constant flow, pressure and temperature.

1.5. Cyclical adsorption/desorption and selectivity

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1.3. CO2 adsorption procedure

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The CO2 adsorption capacity of the synthesized sorbents was performed under atmospheric pressure. A desired amount (0.3–0.6 g) of the sorbent was placed between two layers of quartz wool inside a quartz-tube with an inner-diameter of 0.25 in. The operating temperature was controlled by a digital temperature controller equipped with a K-type thermocouple, solid stage relay and dimmer. Prior to evaluation, each sample was pretreated at 110°C for 1 hr under a helium (He) atmosphere at a flow rate of 30 mL/min in order to eliminate any residual gas in the sorbents. The sample was then cooled down to the desired adsorption temperature. The He was then switched to CO2 at the desired concentration (10–30% (v/v)) in He balance at the feed flow rate of 20 mL/min, the feed was routed to the reactor until CO2 saturation of the sorbents. The adsorption temperature was studied over the range of 40–80°C. The influent and effluent gases were removed of any

From the results of optimization, the sorbent which expressed higher performance (40%TEPA/b-cHT) was selected to investigate the cyclical adsorption/desorption. The adsorption condition was done within the optimal operating condition. After CO2 saturation of the sorbent, the CO2 flow was then switched to 20 mL/min He flow and kept at 130°C until there was no detected CO2 in the effluent gas. This was noted as one adsorption/desorption cyclic. To continue the next cycle the He was switched to 30% (v/v) CO2 in He balance and routed to the reactor until CO2 saturation of the sorbent at 80°C, and then followed by the desorption test. At least eight adsorption/desorption cycles were tested. After adsorption/ desorption test, the sorbent was then used to investigate a performance of selective CO2 adsorption. The 30% (v/v) CO2 in H2 was routed to the reactor at 80°C and W/F ratio of 1.8 g·sec/cm3. After saturation of the sorbent, the feed was then switched to 20 mL/min He flow and kept at 130°C until there was no detected CO2 and H2 in the effluent gas. This was

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moisture by a water-trap unit prior to analysis by on-line gas chromatography (GC) using a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with a ShinCarbon packed column and a TCD, with He as the carrier gas. The equilibrium CO2 sorption capacity of the sorbents (mmol CO2/g of sorbent) was evaluated by analysis of the breakthrough curve.

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b-cHT was added to the TEPA-ethanol solution and then stirred at 120 r/min for 30 min. The suspension was heated up to 60°C for 1 hr to obtain a slurry and dried at 90°C in an oven under a static air atmosphere for 10 hr to obtain the sorbent. The sorbent was assigned as x%TEPA/b-cHT, where x was the nominal TEPA loading level.

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Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

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Table 1 – Independent variables and experimental design levels in coded and actual units for the full 24 factorial design with three central points. Standard order

Run order

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Dd

TEPA loading (% (w/w))

Temperature (°C)

W/F ratio (g·sec/cm3)

CO2 concentration (% (v/v))

−1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 0 0 0

−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 0 0 0

−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1 0 0 0

−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1 0 0 0

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t1:25 t1:27 t1:26 t1:28 t1:29 t1:30

TEPA: tetraethylenepentamine; W/F ratio: sorbent weight to total gaseous flow rate ratio. Coded values of (−1), (0) and (+ 1) refer to the actual values of 30%, 35% and 40% (w/w), respectively. b Coded values of (−1), (0) and (+ 1) refer to the actual values of 40, 60 and 80°C, respectively. c Coded values of (−1), (0) and (+1) refer to the actual values of 0.9, 1.35 and 1.8 g·sec/cm3, respectively. d Coded values of (−1), (0) and (+ 1) refer to the actual values of 10%, 30% and 50% (v/v), respectively.

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CO2 adsorption capacity (mmol/g) 2.03 2.31 2.59 3.01 3.27 3.41 3.44 3.58 2.96 4.21 3.78 4.79 5.29 5.36 5.81 5.94 4.87 4.87 4.89

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noted as one cyclical selective CO2 adsorption. The selective CO2 adsorption was tested at least 8 cycles.

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2. Results and discussion

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2.1. Characterization of the sorbents

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Representative XRD profiles of the cHT, b-cHT and respective TEPA/b-cHT assorbents with different TEPA loading levels are shown in Fig. 1. The diffraction patterns of cHT and b-cHT at 11.9°, 23.6° and 34.6° represent HT with crystalline planes of (003), (006) and (012), respectively, which is similar to the values in JCPDS card no. 41-1428. The diffraction peaks at 3.6° and 62.9° represented a Mg(Al)O periclase-type structure with crystalline planes of (200) and (220), respectively, in accord with JCPDS-ICDD 04-0829. The α-Al2O3 peaks were evident at 28.4°, 40.6°, 50.2°, 58.6° and 66.4° for the (012), (104), (113), (024) and (116) crystal planes, respectively. The Al(OH)3 peaks were observed at 18.7° and 20.3° for the planes of (001) and (110, 020), respectively. The AlO(OH) crystal planes of (020), (110) and (120) appeared at 15.7°, 23.6° and 27.0°, respectively. The diffraction peaks at 18.7° and 37.8° were identified to Mg(OH)2 crystalline planes of (001) and (011), respectively, while the peak at 31.4° was the MgCO3 crystalline plane of (104). The profiles of the diffraction peaks were in good agreement with JCPDS 46-1212 for α-Al2O3, JCPDS 12-0457 for Al(OH)3, JCPDS 05-0190 for AlO(OH), JCPDS 76-0667 for Mg(OH)2 and JCPDS 84-2164 for MgCO3, respectively. A change in structural Mg(Al)O periclase to surface hydroxyl HT was appeared after

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modifying with basic-alcohol solution. It was observed a decrease in the peak intensities of the periclase phase. However, there was an insignificant change in the peak intensities of α-Al2O3, Al(OH)3, AlO(OH), Mg(OH)2 and MgCO3. The patterns of the TEPA/b-cHT sorbents with different TEPA loading levels showed no shift in the diffraction peaks of b-cHT, and so there was no deflection of the b-cHT crystal lattice. It was noted that the peaks of HT, Al(OH)3, AlO(OH), Mg(OH)2 and MgCO3 were not observed, suggesting the coverage of TEPA on the surface of those components, as supported by the FT–IR results (see below). Additionally, the intensity of the crystalline peaks for α-Al2O3 and Mg(Al)O periclase phases were decreased with increasing TEPA loading levels. The functional groups on the surface of the sorbents were analyzed from their FT–IR spectra, with representative examples shown in Fig. 2. The spectra of cHT and b-cHT samples expressed a broad vibration frequency bands with a shoulder at 3626 cm−1, centered at 3460 cm−1 and peak at 1630 cm−1, which represent the O–H stretching of the (Mg, Al)–OH surface (Bukka and Miller, 1992), the symmetric O–H stretching of H–bonded adsorbed water molecules (Xue et al., 2007) and the H–O–H bending vibration of adsorbed water molecules (Swiatkowski et al., 2004; Tanaka and White, 1982; Xue et al., 2007), respectively. The bands observed at 1476, 1383 and 870 cm− 1 were attributed to the stretching vibration of carbonate anions (Dávila et al., 2008; Martinez-Gallegos et al., 2006; Roelofs et al., 2002), the symmetry of the carbonate anions lowered from the planar D3h to the C2ν symmetry (Cocheci et al., 2010) and the bending vibration (ν2) of carbonates (Cocheci et al., 2010), respectively. The bands at 785,

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

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b c

Temperature (°C)

W/F ratio (g·sec/cm3)

CO2 concentration (% (v/v))

−1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 0 0 0 0 0 0 0 0 0 0

−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 0 0 −1 +1 0 0 0 0 0 0 0 0

−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1 0 0 0 0 −1 +1 0 0 0 0 0 0

−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1 0 0 0 0 0 0 −1 +1 0 0 0 0

F

TEPA loading (% (w/w))

CO2 adsorption capacity (mmol/g) 3.12 2.73 4.42 4.00 4.16 3.34 4.84 3.92 4.89 3.95 5.16 4.21 5.37 4.69 4.98 4.24 5.80 4.97 5.22 5.79 4.79 6.03 3.58 5.03 5.98 5.94 6.04 5.99

Coded values of (−1), (0) and (+ 1) refer to the actual values of 30%, 40% and 50% (w/w), respectively. Coded values of (−1), (0) and (+ 1) refer to the actual values of 60, 80 and 100°C, respectively. Coded values of (−1), (0) and (+1) refer to the actual values of 0.9, 1.35 and 2.7 g·sec/cm3, respectively. Coded values of (−1), (0) and (+ 1) refer to the actual values of 10%, 30% and 50% (v/v), respectively.

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639, 565 and 446 cm−1 corresponded to the Al–O or Mg–Al–O vibrations inside the brucite-like layer (Prikhod'ko et al., 2001), Mg–Al layered double hydroxide, the stretching and bending vibrations of M–O, M–O–M and O–M–O bonds in brucite-like layer (where M referred to metal component), and the vibrations of the HT octahedral networks, respectively. Comparing the spectra between the b-cHT and the cHT, the adsorption band intensities of the O–H stretching of the (Mg, Al)–OH surface, the symmetric O–H stretching of H–bonded adsorbed water molecules, and the H–O–H bending vibration of adsorbed water molecules at 3612, 3460, and 1630 cm−1, respectively, were increased when modifying the cHT with basic-alcohol solution. These XRD and FT–IR results support that cHT and b-cHT were synthesized in this work. The spectra of the fresh TEPA/b-cHT sorbents exhibited TEPA adsorption bands at 3345 and 3270 cm−1, from the anti-symmetric and symmetric N–H stretching modes of TEPA (Yang et al., 2012), while those at 2930 and 2800 cm−1 corresponded to the anti-symmetric C–H stretching and N–CH2 stretching vibrations, respectively. In addition, the bands of N–H scissoring of NH2 at 1581 cm−1, C–H wagging of CH2 at 1323 cm−1 and C–N stretching of R–N–R at 1110 cm−1 were observed in the spectra of TEPA/b-cHT sorbents. Moreover,

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Run order

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Standard order

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Table 2 – Independent variables and experimental design levels in coded and actual units for the face-centered central composite (FCCC) design with three central points.

E

t2:1 t2:2

the intensity of the peaks that corresponded to TEPA increased with increasing TEPA loading levels. The spectra of the spent TEPA/b-cHT sorbents had a slightly smaller broad band that corresponded to the hydroxyl group, presumably because the hydroxyl group reacted with CO2 to form bicarbonate species. The vibration frequencies of bicarbonate species were also observed as follows. The vibrational modes of anti-symmetric OCO stretching, symmetric OCO stretching, COH bending, C–OH stretching, CO2 out-of-plane deformation, OCO bending and H torsion normally were observed at the frequency of 1630, 1403, 1261, 1009, 833, 703 and 660 cm−1, respectively (Baltrusaitis et al., 2006). The intensity of the band in the range of 1200–1600 cm−1 was increased due to the overlapping vibration of carbamate and bicarbonate species. The bands of alkylammonium carbamate (R-NHCOO− + H3N-R, R: alkyl) were seen as the symmetric CO−2 stretching at 1323 and 1384 cm−1 (Bossa et al., 2008a; Knofel et al., 2009; Robinson et al., 2012), symmetric NH+3 deformation at 1476 cm−1 (Bossa et al., 2008a, 2008b; Robinson et al., 2012), anti-symmetric CO−2 stretching at 1581 cm−1 (Bossa et al., 2008b; Robinson et al., 2012) and NH+3 deformation at 1630 cm−1 (Bossa et al., 2008a), and from the loss of N–H stretching at 3392 cm−1 (Bossa et al., 2008b). The regenerated 40%TEPA/b-cHT sorbent

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391

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R

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O

395

C

394

N

393

revealed intensity peaks corresponding to the carbamate and bicarbonate vibration frequency that were clearly decreased while that for the hydroxyl vibration frequency was slightly increased. The likely reason is that during regeneration at 130°C under a He flow, the carbamate species were decomposed to CO2 while the bicarbonate species were decomposed to CO2 and

U

392

T

E

Fig. 1 – Representative X-ray diffraction (XRD) profiles of the (a) cHT, (b) b-cHT, (c) 30%TEPA/b-cHT, (d) 35%TEPA/b-cHT, (e) 40%TEPA/b-cHT and (f) 50%TEPA/b-cHT sorbents. TEPA/b-cHT: tetraethylenepentamine-functionalized basic-modified calcined hydrotalcite.

P

R O

O

F

vapor water (Zhang et al., 2016; Zhao et al., 2013), some of which adsorbed on the sorbent due to the memory-effect property of HT. The thermal decomposition behavior of the b-cHT and respective TEPA/b-cHT sorbents along with pure TEPA, as analyzed by TGA, is shown in Fig. 3. The TGA curve of b-cHT revealed a two-stage mass loss. The mass loss in the first stage of around 16% occurred within 215°C, while the second loss of 18% from the first stage mass occurred at 220–380°C. These losses were attributed to the removal of loosely bound water molecules from the pores and the removal of OH groups from the brucite-like interlayer (Sharma et al., 2007), respectively. The thermal behavior of the TEPA/b-cHT sorbents also presented a two-stage mass loss similar to b-cHT, with 18%– 20% loss within 215°C in the first stage, assigned to desorption of bound water molecules from the pores. The second stage at 250–450°C was a loss of about 42%, 49% and 58% mass from the first stage for the 30%TEPA/b-cHT, 40%TEPA/b-cHT and 50%TEPA/b-cHT, respectively, contributed by the loss of hydroxyl groups from the interlayer (Sharma et al., 2007), including the volatilization and decomposition of TEPA (Cao et al., 2013). The thermal behavior of the pure TEPA showed a single stage of mass loss of about 3% of the initial amount within 120°C, due to evaporation of moisture in TEPA, and then of around 97% due to the volatization and decomposition of TEPA at 120–260°C. Note that the thermal decomposition of TEPA was shifted to a higher temperature in the TEPA/b-cHT sorbents due to a strong bonding between TEPA and b-cHT. From these results, the range of operating temperature selected is proper to use in this study. The BET surface area (SBET), pore volume (Vp) and pore width diameter (Dp) of the sorbents are summarized in Table 3, and revealed that each was decreased from 165.2 m2/g, 0.574 cm3/g and 124.9 Å, respectively, for b-cHT to 38.2 m2/g, 0.120 cm3/g and 46.9 Å, respectively, when loading 30% (w/w) TEPA. This was due to the pores of b-cHT becoming filled by TEPA. Increasing the TEPA loading level to 35% and 40% (w/w) further decreased the SBET, Vp and Dp, whilst a 50% (w/w) TEPA loading dramatically decreased the SBET, Vp and Dp by 29-, 26- and

Fig. 2 – Representative FT–IR spectra of the (a) cHT, (b) b-cHT, (c) 30%TEPA/b-cHT, (d) 35%TEPA/b-cHT, (e) 40%TEPA/b-cHT and (f) 50%TEPA/b-cHT sorbents. Solid lines refer to the fresh sorbents and dotted lines to the spent sorbents. The dashed line refers to the regenerated 40%TEPA/b-cHT sorbent. FT–IR: Fourier transform infrared spectrometry; TEPA/b-cHT: tetraethylenepentamine-functionalized basic-modified calcined hydrotalcite.

Fig. 3 – Representative thermal gravimetric analysis (TGA) profiles of the (a) b-cHT, (b) 30%TEPA/b-cHT, (c) 40%TEPA/ b-cHT and (d) 50%TEPA/b-cHT sorbents and (e) pure TEPA. TEPA/b-cHT: tetraethylenepentamine-functionalized basic-modified calcined hydrotalcite.

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436

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t3:1 t3:3 t3:2

Table 3 – Characteristics of the sorbents. Samples

BET surface area (SBET) (m2/g)

Pore volume (Vp) (cm3/g)

Pore width diameter (Dp) (Å)

TEPA loading (% (w/w))

Amine surface density (groups/nm2)

t3:5 t3:6 t3:7 t3:8 t3:9

b-cHT 30%TEPA/b-cHT 35%TEPA/b-cHT 40%TEPA/b-cHT 50%TEPA/b-cHT

165.2 38.2 31.0 21.8 5.7

0.574 0.120 0.061 0.050 0.022

124.9 46.9 38.9 37.0 14.3

28.1 34.6 39.7 48.9

23.4 35.5 58.0 272.9

t3:10 t3:13 t3:12 t3:11

TEPA/b-cHT: tetraethylenepentamine-functionalized basic-modified calcined hydrotalcite; CHN: carbon–hydrogen–nitrogen. TEPA loading was analyzed using a CHN analyzer.

447 448 449 450 451 452 453 454 455 456 457 458 459

R O

O

461

P

446

The CO2 adsorption capacity on the TEPA/b-cHT sorbents at different TEPA loading levels (30%–50% (w/w)), adsorption temperatures (40–100°C), W/F ratios (0.9–2.7 g·sec/cm3) and CO2 concentrations in the feed (10%–50% (v/v)) was initially investigated by sequential univariate analysis, with the results shown in Fig. 4. At 40°C, a W/F ratio of 0.9 g·sec/cm3 and CO2 concentration of 10% (v/v), increasing the TEPA loading level from 30% (w/w) to 40% (w/w) increased the CO2 adsorption capacity 1.14-fold above the 2.03 mmol/g adsorption at 30% (v/v). Since CO2 was captured via reaction with amine active-sites on the sorbent to form carbamate species, then loading more TEPA would increase the amount of amine groups and so CO2 affinity sites on the sorbent, as evidenced in the FT–IR results (Fig. 2). At these lower TEPA loading levels the CO2 adsorption capacity depended on the amount of the amino-groups on the sorbent, which in turn is the amount of TEPA loaded onto the sorbent. Nevertheless, the CO2 adsorption capacity was decreased 1.05-fold to 2.21 mmol/g when the TEPA loading level was increased from 40% to 50% (w/w). This was probably due to the steric hindrance of excess TEPA reducing the amount of

D

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460

E

444

T

443

C

442

E

441

R

440

2.2. CO2 adsorption performance

R

439

8.7-fold, respectively. The likely reason is that the pores of b-cHT were filled progressively by TEPA. Excess TEPA might obstruct CO2 diffusion into the active-sites in the pore, and so reduce the CO2 adsorption level. Among the materials used to prepare the sorbents, only TEPA contained nitrogen atoms and so the total amount of nitrogen was then used to evaluate the amount of TEPA that was actually loaded in the sorbents. From the results (Table 3), the 30%TEPA/b-cHT, 35%TEPA/b-cHT, 40%TEPA/b-cHT and 50%TEPA/b-cHT sorbents were observed to have a TEPA loading level of 28.1%, 34.6%, 39.7% and 48.9%, respectively, which was the nominal loading level. Thus, the TEPA was successfully loaded onto the desired sorbents. Moreover, the amount of TEPA represents the level of amine groups anchored on the surface of sorbent, or the amine surface density. Accordingly, the amine density was derived to be 272.9 groups per nm2 for the 50%TEPA/b-cHT and to decrease with decreasing TEPA loading levels down to 23.4 groups per nm2 for 30%TEPA/b-cHT (Table 3). Note, however, the total amine group density will not be the available amine group density, especially at higher TEPA loading levels, since some of the amine active-sites will concealed by others (steric hindrance), and unavailable for reacting with CO2.

N C O

438

U

437

F

t3:4

Fig. 4 – CO2 adsorption capacity as a function of (a) the nominal TEPA loading level at an operating temperature of 40°C, W/F ratio of 0.9 g·sec/cm3 and CO2 concentration of 10% (v/v); (b) the operating temperature for the 40%TEPA/b-cHT sorbent at a W/F ratio of 0.9 g·sec/cm3 and CO2 concentration of 10% (v/v); (c) the W/F ratio for the 40%TEPA/b-cHT sorbent at 80°C and CO2 concentration of 10% (v/v); and (d) the CO2 concentration in the feed stream for the 40%TEPA/b-cHT sorbent at a W/F ratio of 1.8 g·sec/cm3 and operating temperature of 80°C. Data are shown as the mean ± 1SD, derived from three repeats. W/F ratio: sorbent weight to total gaseous flow rate ratio; SD: standard deviation. TEPA/b-cHT: tetraethylenepentamine-functionalized basic-modified calcined hydrotalcite. Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481

8

496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532

536 537 538 539

C

F

495

E

494

R

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R

492

O

491

C

490

N

489

U

488

O

From the results of CO2 adsorption performance in Section 3.2, not only the significant factors and their interaction, but also optimal condition for CO2 adsorption was still not determined. Thus, the sets of experimental matrix of a full 24 factorial design with three central points were designed to

487

R O

535 Q6

486

P

2.3. Experimental design for optimization

485

D

534

484

evaluate the significant ones. Based on the derived factorial design, the FCCCD–RSM analysis was then employed to optimize the condition for maximal adsorption. To visualize the main and interaction effects on the response, a Pareto chart at a 95% confidence interval was then plotted in Fig. 5. The t-value of | Effect| was calculated by dividing the numerical effect with its associated standard error (Anderson and Whitcomb, 2007). If the t-value of | Effect| was higher than the t-value limit (2.7765), the effect had a significant influence on the response. The t-value for the TEPA loading level-temperature interaction (AB interaction) and TEPA loading level-temperature-CO2 concentration interaction (ABD interaction) were lower than the t-value limit, implying that they had no significant effect on the CO2 adsorption capacity. The height of the vertical bar implies the level of influence on the capacity. The influence of the main factors on the capacity was then arranged in order of the height as CO2 concentration in the feed > W/F ratio ⋙ temperature > amount of TEPA loading. In addition, the chart showed the direction of influence on the capacity. A blue bar corresponds to an antagonistic effect whereas an orange bar represents a synergistic effect on the CO2 adsorption capacity. The interaction of TEPA loading-W/F ratio, TEPA loading-W/F ratio-CO2 concentration, and temperatureW/F ratio expressed a negative effect while each main factor and the other interactions exhibited a positive effect on the capacity. Increase in the level of main factors tends to arise a magnitude of the response, confirming the results in Section 3.2. The ANOVA at a 95% confidence interval was constructed to identify which factors and interactions had a significant effect on the response, as shown in Table 4. From contribution percentage (related to the total sum of squares), the four main factors contributed some 85.0% of the variation in the CO2 adsorption capacity, while their interactions accounted for 15.0%. This evidenced that the influence of interaction on the response should be considered in maximal adsorption condition. Their relative importance was ranked as the level of CO2

T

533

available amine groups plus the sorbent pores being almost totally filled with TEPA and so preventing CO2 access into the active-sites (Cao et al., 2013; Jo et al., 2015; J. Wang et al., 2015; X. Wang et al., 2015; Y.-T. Wang et al., 2015; Yu et al., 2012), the latter being as evidenced by the BET results (Table 3). Based on the adsorption capacity, a TEPA loading level of 40% (w/w) was selected for further study. The influence of the adsorption temperature on the CO2 capacity of the 40%TEPA/b-cHT adorbent at a W/F ratio of 0.9 g·sec/cm3 and CO2 concentration of 10% (v/v) was then evaluated. The CO2 adsorption capacity was increased around 1.23- and 1.31-fold when increasing the temperature from 40 to 60 and 80°C, respectively. At higher temperatures, CO2 molecules have a higher kinetic energy and so a reduced diffusion resistance (Heydari-Gorji et al., 2011; Xu et al., 2002; Yue et al., 2006), allowing more CO2 to reacted with the CO2 affinity sites in the sorbent. Further increasing the temperature from 80 to 100°C, however, slightly decreased (1.1-fold) the CO2 adsorption capacity to 2.92 mmol/g due to the thermodynamic limitation (Qi et al., 2011; Xie et al., 2016) of the exothermic reaction between CO2 and the amine group in TEPA (Goel et al., 2015; Heydari-Gorji et al., 2011). Typically, the behavior of chemisorption was first increased and then reduced by raising the temperature. From the temperature profile, a temperature of 80°C was selected for further study. Next, the CO2 adsorption capacity of the 40%TEPA/b-cHT sorbent at 80°C and CO2 concentration of 10% (v/v) was evaluated when varying the W/F ratios. The capacity increased around 1.2-fold when increasing the W/F ratio from 0.9 to 1.8 g·sec/cm3, since this would promote a longer retention time of CO2 in the reactor, and so an adequate time for diffusion of CO2 molecules into to the affinity sites (Yue et al., 2006). However, the capacity did not change visibly when increasing the W/F ratio from 1.8 to 2.7 g·sec/cm3, which likely corresponds to elimination of the external diffusion limit. W/F ratio of 1.8 g·sec/cm3 was then selected for further study. Finally, the influence of the CO2 concentration on the adsorption capacity of the 40%TEPA//b-cHT sorbent at 80°C and a W/F ratio of 1.8 g·sec/cm3 was evaluated (Fig. 4d). The adsorption capacity increased 1.58- and 1.64-fold (from 3.6 mmol/g at 10% (v/v) CO2) when increasing the CO2 concentration to 20% and 30% (v/v), respectively. This reflects that the increased level of CO2 provides more opportunities for CO2 molecules to interact with available amine active-sites in the sorbent per unit time. Moreover, a higher CO2 concentration shifts the chemical equilibrium towards carbamate formation or increased the reaction rate (Goel et al., 2015). Further increasing the CO2 content from 30% to 50% (v/v) did not change the adsorption capacity, which is due to the available affinity sites on the sorbent being limited and already saturated.

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E

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J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 7 ) XXX –XXX

Fig. 5 – Pareto chart of the four factors and their interactions for the full 24 factorial design. Factors A, B, C and D are defined in Table 1.

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577

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t4:1 t4:3 t4:2

Table 4 – Analysis of variance (ANOVA) of the significant terms for the full 24 factorial design with three central points. Source

Sum of squares

DF

Mean square

F-value

P-value

t4:5 t4:6 t4:7 t4:8 t4:9 t4:10 t4:11 t4:12 t4:13 t4:14 t4:15 t4:16 t4:17 t4:18 t4:19 t4:20 t4:21 t4:22 t4:23 t4:24 t4:25 t4:26 t4:27

Model A B C D AC AD BC BD CD ABC ACD BCD ABCD Curvature Residual Lack of fit Pure error Total

24.010 0.830 1.150 7.060 13.520 0.330 0.180 0.066 0.073 0.600 0.008 0.130 0.042 0.027 2.550 0.003 0.003 0.000 26.56

13 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4 2 2 18

1.850 0.830 1.150 7.060 13.520 0.330 0.180 0.066 0.073 0.600 0.008 0.130 0.042 0.027 2.550 0.001 0.001 0.000

2632.23 1187.48 1637.23 10,061.12 19,271.29 465.67 253.07 94.15 104.02 849.53 11.86 185.33 60.16 38.15 3640.49

<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0006 0.0005 <0.0001 0.0262 0.0002 0.0015 0.0035 <0.0001

Model summary statistics 0.9999 R2 0.9995 Adj. R2

t4:28 t4:30 t4:29 t4:32 t4:31 t4:33

The factor codes (A, B, C and D) are defined in Table 1. DF refers to degree of freedom. P-value is based on the 95% confidence interval.

583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608

0.00 0.00

O

P

R O

3.14 4.32 26.60 50.90 1.23 0.67 0.25 0.28 2.24 0.03 0.49 0.16 0.10 9.61

D E T

C

E

582

R

581

0.0896

Percent contribution

0.66 0.9938

content in the feed at 50.9%, followed by the W/F ratio at 26.6%, with the temperature and amount of TEPA loading being much lower. The level of influence of the interactions between these factors was relatively low, and ranged from 2.24% for the CO2 concentration-W/F ratio interaction down to 0.03% for the TEPA loading level-temperature-W/F ratio interaction. This is similar to that in a Pareto chart (Fig. 5). If the P-value of any effect was higher than 0.05, the effect had no a significant influence on the response. All effect shown in Table 4 had a significant influence on the response, whilst the TEPA loading level-temperature interaction and TEPA loading level-temperature-CO2 concentration interaction (not presented in Table 4), had no a significant effect on the capacity. The lack of fit exhibited a probability of 0.0896, indicating that all significant terms on the response were considered in the model. The significant model terms had a model F-value of 2632.23, which implied that there was a < 0.01% chance that this was due to noise. The adjusted R2 (Adj. R2) was employed to recompense for adding variables to the model, where the addition or subtraction of the additional variables increases or decreases the Adj. R2. As there are more independent variables in the model and the sample size is not very large, the R2 will increase and the Adj. R2 may be smaller than the R2. However, from Table 4, the value of Adj. R2 (0.9995), which was very close to that of R2 (0.9999) and that of Pred. R2 (0.9938), implying that the requisite terms were included in the model. The coefficient of variation (CV) of 0.66 indicated that only 0.66% of the data points were dispersed around the mean. However, it can be seen in Table 4 that a significance of curvature was noted, the factors and the response did not show a linear-relationship. When changing the level of each

R

580

CV% Pred. R2

N C O

579

U

578

10.16

F

t4:4

main factor from a lower level through the central point to higher level, the average response value did not correspond to the response value at the central point for the factors studied. This indicated that the second-order model was strongly adequate to evaluate the CO2 adsorption capacity in the design space. Due to a presentation of curvature in the design space, a response surface analysis would be required to achieve a better perception. It was noticeable that the order of the influence level of the main factors in the ANOVA table and the Pareto chart were in accord with the results of the CO2 adsorption performance. However, the addition of adequate quadratic terms are required to improve the accuracy due to the presence of curvature in the design space, the FCCCD–RSM analysis was then employed to fabricate a matrix of the experiments, as shown in Table 2. The relationship between the response and the factors was then modeled. The CO2 adsorption capacity (Qe), in terms of the coded factors, could be expressed by Eq. (1): Qe ¼ þ5:65–0:37A þ 0:23B þ 0:23C þ 0:47D−0:17 BC−0:26BD−1:39D2

609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627

ð1Þ

where A, B, C and D refer to the main factors as defined in Table 1; BC and BD refer to the interactions of the main factors; and D2 refers to the quadratic terms of the main factors. From the statistics of the model as shown in Table 5, the magnitude of Adj. R2 was about 0.9170. This elucidated that around 91.7% of the response variability could be explained by the RSM-model. Therefore, it can be said that the model

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

629 628 630 631 632 633 634 635 636

10 t5:1

J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 7 ) XXX –XXX

Table 5 – The statistics of response surface method (RSM) model and its validation.

t5:3 t5:2

t5:4 t5:5 t5:6

Model summary statistics 2

Adj. R

0.9170

r

t5:7

0.9688

Adeq. precision

24.171

Validation of the RSM-model

t5:8

A

B

C

D

t5:9

TEPA loading (wt.%)

Temperature (°C)

W/F ratio (g·sec/cm3)

CO2 concentration (% (v/v))

Estimated

Experimental

t5:10 t5:11 t5:12 t5:13 t5:14 t5:15 t5:16 t5:17 t5:18 t5:19

45.0 40.0 40.0 35.0 25.0 35.0 45.0 40.0 25.0 35.0

80.0 90.0 70.0 50.0 79.0 90.0 50.0 45.0 65.0 70.0

2.00 1.60 1.40 1.20 1.80 1.00 1.20 1.60 1.40 1.80

30.0 30.0 30.0 25.0 15.0 35.0 25.0 30.0 30.0 25.0

5.51 5.73 5.39 4.84 5.05 5.81 4.49 5.13 5.88 5.48

5.43 5.67 5.42 4.88 5.03 5.76 4.45 5.15 5.91 5.53

646 647 648 649 650 651 652 653 654 655

F O

R O P

665

E

D

2.4. Cyclical adsorption/desorption and selectivity

T

645

C

644

E

643

R

642

Fig. 6 – Plot of experimental (actual) versus predicted adsorption capacity. Factors A, B, C and D are defined in Table 1.

657 658 659 660 661 662 663 664

A potential sorbent should have not only a high adsorp- 666 tion performance but also a good durability. Therefore, the 667

R

641

656

O

640

RSM-model to evaluate the optimization capacity and provide the feasible operating regions for CO2 adsorption capacity. The contour plot was then used, as shown in Fig. 7. The lines of constant capacity were then connected to form response contours. The optimal operating region for maximal CO2 adsorption capacity (5.5–6.1 mmol/g) was at the shaded portion, representing a TEPA loading level of 39%–49% (w/w), temperature of 76–90°C, W/F ratio of 1.7–2.60 g·sec/cm3 and a CO2 concentration of 27%–41% (v/v).

C

639

appears satisfactory. Additionally the magnitude of the correlation coefficient (r) was about 0.9688, revealing a very strong relationship between the response and independent factors. The adequate precision (Adeq. precision) signal to noise ratio of 24.171 was greater than 4, indicating that the model was effective for estimating the response values. Moreover a plot of the actual capacity against the estimated response was performed, as shown in Fig. 6. All the plotted points fell along an imaginary 45-degree straight line, indicating the experimental results were close to the estimated capacity, suggesting that the model is acceptable. The distance of each data from the straight line corresponded to its deviation from the related experimental value. This evidenced that the model fitted sufficiently the experimental data. To verify the RSM-model, set ten more experiments were randomly done, as shown in the bottom part of Table 5. The estimated capacity was close to the experimental capacity with an error range of ± 1.5%. The RSM-model was acceptable and satisfied. This implied a potential of the

1.47 1.06 −0.55 −0.82 0.40 0.87 0.90 −0.39 −0.51 −0.90

N

638

Error (%)

U

637

CO2 adsorption capacity (mmol/g)

Fig. 7 – Contour plot of the four main factors (A, B, C and D) for face-centered central composite design response surface method (FCCCD–RSM) analysis. Factors A, B, C and D are defined in Table 1.

Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015

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681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707

Fig. 8 – (a) Cyclical CO2 adsorption/desorption and (b) selective CO2 adsorption over the 40%TEPA/b-cHT sorbent. TEPA: tetraethylenepentamine.

Acknowledgments

758 757

This research was supported by the Rachadapisek Sompote Fund for Postdoctoral Fellowship, Chulalongkorn University; the Annual Government Statement of Expenditure; the Thailand Research Fund (No. IRG5780001), Chulalongkorn University and Faculty of Science of Chulalongkorn University. The authors would like to thank the Department of Chemical

759

F

680

O

679

710

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The CO2 adsorption capacity of synthetic base-modified c-HT with TEPA functionalization at atmospheric pressure was studied by varying the levels of TEPA loading, adsorption temperature, ratios of weight of sorbent to total gaseous flow rate (W/F ratio) and levels of CO2 content in the influent gas. From the results of one-variable-at-a-time, it revealed that increase the level of TEPA loading from 30% (w/w) to 40% (w/w) increased the capacity about 1.14-fold at 40°C, a W/F ratio of 0.9 g·sec/cm3 and CO2 concentration of 10% (v/v). This was due to more CO2 affinity sites on the sorbent, as evidenced in the FT–IR results. Further increase the TEPA loading level to 50% (w/w), the capacity was dropped due to the steric hindrance of excess TEPA and obstruction of CO2 diffusion into the active-sites, as evidenced by CHN elemental analysis and the BET results. The CO2 adsorption capacity of the 40%TEPA/ b-cHT sorbent was increased when increasing the temperature from 40 to 80°C due to reducing diffusion resistance. Further increasing the temperature to 100°C, the capacity was dropped slightly due to the thermodynamic limitation of the exothermic reaction. The CO2 sorption capacity was increased with increasing W/F ratios until reaching the plateau of no external diffusion limit. Finally, increasing the CO2 concentration increased the CO2 sorption capacity by providing more opportunities for CO2 molecules to interact with the available amine active-sites, up to the point of saturation when further increases in CO2 concentration had no marked effect. The sets of experimental matrix of a full 24 factorial design with three central points were designed to determine a significant factor and its interaction. These four main factors contributed about 85.0% of the variation in CO2 sorption capacity, while their interactions contributed 15.0%. Increase in the level of main factors tends to arise a magnitude of the response, confirming the results of one-variable-at-a-time. The FCCCD–RSM analysis was then employed to optimize the conditions for maximal CO2 adsorption (5.5–6.1 mmol/g). The optimal operating region was represented at TEPA loading level of 39%–49% (w/w), temperature of 76–90°C, W/F ratio of 1.7–2.60 g·sec/cm3 and a CO2 concentration of 27%–41% (v/v). The estimated capacity was close to the experimental capacity with an error range of ±1.5%. The RSM-model was acceptable and satisfactory. To test cyclical adsorption/desorption and selectivity at the optimal condition, the 40%TEPA/b-cHT sorbent was selected. The sorbent still expressed its effective performance after eight cycles of adsorption/desorption and eight cycles of selectivity. This could be said that the synthesized TEPA-functionalized surface-modified HT sorbent is one candidate of potential sorbents for CO2 adsorption application.

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40%TEPA/b-cHT sorbent was used to test cyclical adsorption/ desorption. The adsorption condition was done in the optimal operating region. A CO2 concentration of 30% (v/v) in He balance was routed to the reactor at 80°C, a W/F ratio of 1.8 g·sec/cm3. After adsorption saturation, the feed was then switched to the He flow at 130°C until no detected CO2 in the effluent. This stage was noted as desorption. The adsorption/desorption test was done at least 8 cycles, as shown in Fig. 8. Carbamate and bicarbonate species were formed on the sorbent surface during the adsorption process, evidenced in Fig. 2. Carbamate species were decomposed to CO2 during the desorption process. The CO2 was then flushed out of the column in the He flow. Bicarbonate species were decomposed to CO2 and water (Zhang et al., 2016; Zhao et al., 2013). The CO2 was then flushed out while some of the water was likely to be adsorbed on the sorbent. The probable reason to explain is due to the memoryeffect property of reconstructing back into the original HT structure (Allen et al., 1989), as evidenced in intensity of the hydroxyl vibration peak of the regenerated sorbent in Fig. 2e. After eight adsorption/desorption cycles, the adsorption capacity decreased slightly from 6.1 to 5.8 mmol/g. Since the hydrogen (H2) is one of the most promising carbon free clean energy carrier and prospective to be used as an alternative fuel in the future. The H2-rich stream composes of 40%–70% H2, 5%–30% CO2 and the others. To separate CO2 from the H2-rich stream, the selective CO2 adsorption over 40%TEPA/b-cHT sorbent was then applied here at the optimal condition except switching to 30% (v/v) CO2 in H2 balance. After saturation of the sorbent, the feed was then switched to 20 mL/min He flow and kept at 130°C until there was no detected CO2 and H2 in the effluent gas. This was noted as one cyclical selective CO2 adsorption. The selective adsorption was tested at least 8 cycles, as shown in Fig. 8. The selective adsorption capacity was decreased slightly around 1.05% from the 9th cycle to the 16th cycle while H2 adsorption capacity was less than 0.02 mmol/g in each cycle. This could be said that the 40%TEPA/b-cHT was an effective sorbent for selective CO2 adsorption. Thus, the synthesized TEPA-functionalized surface-modified HT sorbent is a candidate of potential sorbents for CO2 adsorption application.

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Please cite this article as: Thouchprasitchaia, N., et al., Optimization of CO2 adsorption capacity and cyclical adsorption/ desorption on tetraethylenepentamine-supported ..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.02.015