The Combination of CO2 Utilization and Solid Sorbent

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 114 (2017) 2460 – 2466

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland

The Combination of CO2 Utilization and Solid Sorbent Preparation in One Step Process Dang Viet Quang, Abdallah Dindi and Mohammad R.M. Abu-Zahra* Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, P.O.Box 54224, Masdar city, Abu Dhabi, United Arab Emirates *Tel.: +97128109181 *Email:[email protected]

Abstract

In this study, a one-step preparation of amino-functionalized mesoporous silica in combination with CO2 utilization was introduced. 3-Aminopropyltriethoxysilane (APTES) solution (30 wt% in water) was first bubbled with CO2 at atmospheric pressure and then mixed with sodium silicate to produce amino-functionalized mesoporous silica. Resulting adsorbent was characterized and tested for CO2 adsorption. The adsorbent has surface area and pore volume of 205.9 m2/g and 0.7 cm3/g respectively. The analysis using Fourier transform infrared spectroscopy revealed the existence of amino-functional groups on silica structure. CO2 adsorption experiments were conducted in pure CO2 using a flow calorimeter. The sorbent reached a maximum CO2 loading of 35.3 mg/g at the adsorption temperature of 50 oC. The success of the proposed one-step process provides a facile method to synthesize solid adsorbent for CO2 capture but contributes an additional option to CO2 utilization. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder under responsibility of the organizing committee of GHGT-13. Peer-review responsibility of the organizing committee of GHGT-13. Keywords: Type your keywords here, separated by semicolons ;

1. Introduction CO2 concentration in the atmosphere has increased sharply in recent year due to increasing anthropogenic CO2 emission. Major source of the anthropogenic CO2 emission is from burning fossil fuel for energy production. To reduce

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1397

Dang Viet Quang et al. / Energy Procedia 114 (2017) 2460 – 2466

CO2 emission, people needs to stop burning fossil fuel, however, it is impossible at the present because the renewable energy sources cannot meet the world demand due to low production capacity and therefore the world economy will continue depending on the fossil fuel energy. Since burning fossil fuel for energy production is not avoidable at the present and near future, cleaner and sustainable measures for energy production has been considered. Among several available options including enhancing power production efficiency, using efficiency, and CO2 capture and storage, the most important approach, which allows the fossil fuel to go is CO2 capture and storage [1]. The concept of this method is to capture CO2 from rich sources and then inject underground for storage. The best places for CO2 storage are geological formations such as depleted oil and gas fields and saline aquafers. One of the most feasible measure for CO2 storage is to inject captured CO2 into depleted oil field for enhancing oil recovery. Extra oil recovery can help offset part of expensive CO2 capture and storage cost. Currently, several CO2 capture technologies have been developed and tested at both lab and larger scale including pre-combustion capture, oxy-fuel combustion capture and post-combustion capture. Among these technologies, postcombustion capture is an utmost important since it can be easily retrofitted to an existing power plant. Retrofit of postcombustion capture process allows existing power plants to continue operating while avoid CO2 emission [2]. There are several methods that are being tested for CO2 post-combustion capture including membrane separation, adsorption using solid sorbent, and absorption using liquid solvent. CO2 capture process based on liquid solvent is a well-known process which have been used for long time for purification of natural gas. This method, however, faces some drawbacks when it is applied to capture CO2 from large sources like a power plant. A number of drawbacks have been pointed out including extensive energy consumption of CO2 capture unit, corrosive problem, and amine degradation in which the large energy requirement for CO2 capture unit is a key problem. High energy consumption is due to the energy that requires to regenerate amine solution. Amine solution, after absorbing CO2 at low temperature is heated to release CO2 and reuse. Amine solution comprising of water up to 70 wt% accounts for high regeneration energy because of its high heat capacity. To avoid water, amine has been impregnated or grafted on solid substrate in form of solid adsorbent. Solid adsorbent with high CO2 adsorption capacity, low heat capacity, low adsorption heat, less corrosion and high stability is expected to lower energy consumption and cost of CO2 capture. Recently, various solid adsorbents have been synthesized and examined for CO2 capture application. Numerous amine monomers and polymers have been impregnated on porous substrates [3-6]. Polyethyleneimine (PEI) based solid sorbent was first introduced by Satyapal et al. since 2001 where PEI was bonded to a high surface area solid polymehtyl methacrylate [7]. Since then various porous substrates including mesoporous silica, alumina, and carbon and many amines ranging from monomer amine such as ethanolamine (MEA), diethanolamine (DEA), and tetraethylenepentamine (TEPA) to polymer amine such PEI have been used to synthesize CO2 adsorbent [4, 8-10]. Mesoporous silica is among most investigated and potential solid supports for CO2 adsorbent due to its high porosity and high surface area with a number of SiOH groups, which can form bonds with amine compounds. Two commonly introduced methods for preparation of amine-silica based adsorbent are impregnation and grafting. In the former method, amine compounds are simply impregnated into pores of substrates where they are physically bonded to substrate by hydrogen bonds. In the later method, amines are chemically grafted on silica substrate through siloxane bonding (O-Si-O) using silane coupling agents [11]. Both impregnation and grafting methods usually involve in 2 step process including synthesizing substrate and functionalization. The most economic silica products that are suitable for CO2 adsorbent, are synthesized from sodium silicate where sodium silicate solution is acidized by an inorganic acid. Silica is then separated from solution, washed, and dried for further processing. In the second step, silica is impregnated or grafted with an amine for CO2 adsorption. Combining these two steps in a one step process would save energy and reduce the cost of CO2 adsorbent. In this study, potential to replace acid in silica synthetic process with CO2 gas and combining amino functionalizing agent in this process have been investigated. 3-Aminopropyltriethoxysilane (APTES) was selected as amino functionalizing agent. To prepare acidizing solution, aqueous APTES solution was brought to pressure with CO2. The CO2 absorbed APTES solution was mixed with sodium silicate to generate amino-functionalized mesoporous silica. The resulting adsorbent later was examined for CO2 adsorption.

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2. Experimental 2.1. Chemicals 3-Aminopropyltriethoxysilane (APTES, 99 wt%) and sodium silicate (2.5SiO2·Na2O, 26.5 wt% SiO2) were purchased from Sigma-Aldrich and used without further purification. CO2 gas (99.9 %) was supplied by Gulf Industrial Gases CO. L.L.C. 2.2. Adsorbent preparation CO2 absorbed aqueous APTES 30 wt% solution was prepared by bubbling pure CO2 into aqueous APTES 30 wt% solution at atmospheric pressure and room temperature for 24 h using a Buchiglasuster pressure reactor. The CO2 absorbed APTES solution was then withdrawn from pressure reactor and used as an acidic solution for further experiments. The experiment is followed by the rapid addition of sodium silicate solution (29 mL, 8 wt%) into mixing CO2 absorbed APTES solution (20 mL) at room temperature. Upon addition of sodium silicate, solution became gelation and silica hydrogel was obtained. The resulting silica hydrogel was kept still to age at ambient condition for at least two days and at 70 oC in drying oven for one day. The adsorbents were then washed by tap water to remove the excessive NaOH. Before washing, silica hydrogel was broken into small pieces to enhance the mass transfer of the washing process. Washing was finished when the pH of washing water reached about 8. Finally, adsorbents were filtered and dried at 110 oC for 3 h for further characterization and testing.

2.3. Adsorbent characterization The surface area, pore volume, and pore sizes were measured by a nitrogen adsorption-desorption method using Quantachrome instruments (Nova Model 25). All samples were degassed at 110 oC for 4 h before analysis. The pore volume and pore size were calculated by the Barrett-Joyner-Halenda (BJH) method using the desorption data. Fourier transform infrared spectroscopic (FTIR) measurements were conducted on a Vertex 80 spectrometer (Bruker) by the KBr pellet method for solid samples and using a Platinum ATR accessory for liquid samples. Adsorbent morphologies were observed on scanning electron microscope (SEM, Quanta 250) and electron transmittance microscope (TEM, Technai G2, 200 kV). The nitrogen content in adsorbent was analyzed on SEM Quanta 250 coupled with an electron dispersive X-ray spectrometer (EDS). The CO2 concentration in APTES solution was analyzed using a TOC Analyzer (Vario TOC Cube, Elementar). 2.4. CO2 adsorption study CO2 adsorption performance of adsorbents was studied on a flow Micro Reaction Calorimeter (URC) provided by Thermal Hazard Technology (UK) as described in a previous publication [12]. Schematic illustration of experimental setup is shown in Figure 1. Typically, an approximately 0.3–0.5 g of adsorbent, which had been activated at 120 oC for 30 min, was fed into analysis cell and mounted on the calorimeter. The flow rate and volume fraction of feed gas are controlled by mass flow controllers (MFC1 and MFC 2). The gas can be directed to moisturizer (A) and then enter a makeup vessel (B) or directly enter makeup vessel by controlling valves (V1, V2, and V3). Before entering the analysis cell or reactor, the gas can be flowed through a desiccant column (C) to remove moisture if needed by controlling valves (V4 and V5). The flow rate of CO2 introducing into reactor was adjusted by MFC3. The valve V6 was used to control the total gas pressure of` the system that was indicated by a transducer (T4). The CO2 concentration in the gas out of reactor was monitored by a CO2 Transmitter Series GMT220 (Vaisala, Finland).

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Dang Viet Quang et al. / Energy Procedia 114 (2017) 2460 – 2466 V7 Gas in

By-pass Gas out

V3 MFC3

MFC1 T1

MFC2

Cap

T3 V1

T2

T4

V6

V5 V2

V4

Shunt

Analysis cell

Reference cell

Flow URC N2

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B

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C

Figure 1. A schematic illustration of the CO2 adsorption experiment Tests were run under the isothermal mode and the adsorption progress was monitored by the variation of power (mW) over time (s). When the power signal stabilized, pure CO2 was introduced into the analysis cell at the rate of 0.5 ml/min and ambient pressure. Since CO2 adsorption is an exothermic process, the power signal increased whenever the adsorption occurred. Test was completed when the power signal came back to initial level and stabilized again. The CO2 loading capacity of adsorbent (mg/g) was calculated by dividing the mass of CO2 adsorbed per the mass of lean adsorbent as described in Eq. 1. ௠ ି௠ ௠௚ ‫ܱܥ‬ଶ ൌ భ బ ‫ͲͲͲͳ כ‬ሺ ሻ (1) ௠బ

௚

where, m0 is the mass of lean adsorbent, which is pre-treated at a desired regeneration temperature in drying oven for 30 min and m1 is the mass of CO2 loaded adsorbent that is the mass difference between the analysis cell containing adsorbent after CO2 adsorption and empty cell.

3. Results and discussion Experimental result revealed that APTES 30 wt% solution absorbed 0.71 mole of CO2/L. CO2 absorbed by APTES solution is based on chemical reaction of amino group with and CO2 to form carbamate. This CO2 absorption is a reversible reaction, CO2 can be desorbed to free amino groups. Figure 2 shows the FTIR spectra of APTES before and after CO2 absorption. The spectra clearly indicate the CO2 absorption of APTES solution with the emergence of new peaks attributed to –COO and N-COO groups in CO2 absorbed spectrum. The emergence of new absorption peak at 950 cm-1 belong to Si-OH group, which confirms that APTES was hydrolyzed when being mixed with water. APTES is sensitive to water, thus it is easily hydrolyzed and self-condensed to form oligomer or small cluster [13, 14]. Absorbed CO2 reacted with NaOH when sodium silicate was added to APTES solution to form silica hydrogel. A representative SEM image of resulting adsorbent exhibited in Figure 3 clearly shows that the adsorbent consists of numerous tiny particles which bonds together to form a porous silicagel structure. Nitrogen adsorption/desorption isotherm and pore size distribution of the adsorbent is presented in Figure 4. The hysteresis loop is of type IV

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0.25

b

CO2 absorbed aqueous APTES 30 wt%

a

aqueous APTES 30 wt% 950 cm-1 (Si-OH)

corresponding to UPAC classification. This confirms that the adsorbent is mesoporous material. Surface area, total pore volume, and average pore size of the adsorbent is 205.9 m2/g, 0.7 cm3/g, and 14.6 nm, respectively.

Absorbance

0.20

-1

-

1564 cm (COO )

0.15

b -1

1084 cm (C-N)

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1308 cm (N-COO ) -1

0.05

-

1433 cm (COO ) -1 1485 cm (COO )

1800

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1400

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Wavenumber (cm )

Figure 2. FTIR spectra of aqueous APTES 30 wt% before (a) and after (b) CO2 absorption

Figure 3. Representative SEM image of adsorbent

To verify the presence of organic moieties in adsorbent structure, FTIR spectrum of the adsorbent was collected and is shown in Figure 5. Vibration bands at 3300-3400, 1061, 792, and 463 cm-1 observed on the spectra of adsorbents are characteristic vibrations of the major bonds in silica structure. The vibration at 3300-3400 cm-1 is assigned to O-H bond in the sample (water or O-H group attached to the silica structure). The peaks at 1101, 792 and 463 cm-1 belong to asymmetric stretching, symmetric stretching and bending of Si-O-Si bond [15]. The absorption band at 1638 cm-1 could be corresponding to the deformation vibration of adsorbed water. Peaks at 2931, 2880, and 1474 cm-1 are attributed to the vibrations of C-H bond. The vibration at 695 cm-1 is attributed to Si-C bond. Peaks at 3293 and 1561 cm-1 are corresponding symmetric stretching of N-H bonding and the deformation of NH2 groups in adsorbents [1618]. The existence of Si-C, C-H, and N-H bonds in adsorbent structure indicated that APTES hydrolytic products were successfully condensed with silica when CO2 absorbed APTES was mixed with sodium silica. This further confirms the success of the proposed one-step process in combination with CO2 utilization to prepare amino-functionalized adsorbent.

Si-O-Si

0.015

400

0.010

3

Pore volume (cm /g⋅ nm)

300

3

Quantity adsorbed (cm /g STP)

1.0

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N-H

C-H N-H

200

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0.000 0

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0.6 100

0 0.0

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Figure 4. Nitrogen adsorption/desorption isotherms and pore size distribution

0.4 4000

3500

3000

2500

2000

1500

1000

500

-1 Wavenumber (cm )

Figure 5. FTIR spectra of resulting adsorbents


CO2 adsorption performance of the adsorbent was investigated and results is displayed in figure 6. It indicated that the CO2 loading of the adsorbent changed as a function of adsorption temperature. It slightly increased from 30 to 50 o C and decreased with further increasing from 50 to 90 oC. The CO2 loading reached a maximum at 50 oC with 35.3 mg CO2/g adsorbent. Lower CO2 loading at higher adsorption temperature is likely due to exothermal reaction between CO2 and amine that reversibly shifts to desorption side as temperature increased. This allows adsorbent to be thermally regenerated and suitable for temperature swing CO2 adsorption process.

CO2 loading (mg/g)

40

30

20 20

40

60

80

100

o

Temperature ( C)

Figure 6. CO2 adsorption performance of the adsorbents CO2 capture and storage is an expensive way to reduce carbon dioxide emission, it is hard to be realized without significant subsidiary from government. One of the best way to boost CO2 capture is to combine it with CO2 utilization or convert captured CO2 to a valuable products that could be commercialized. The revenue from selling those products may help compensate a part of CO2 capture cost. In this process, CO2 was used in place of H2SO4, a high corrosive, toxic, and expensive acid. The expense saving from H2SO2 avoidance can offset part of CO2 capture process or contribute to reduce the cost of CO2 adsorbent.

4. Conclusion This study demonstrates that amino-functionalized adsorbent can be synthesized by a one-step method using CO2 as an acidic agent. The replacement of high corrosive, toxic, and expensive H2SO4 with CO2 may contribute to reduce the cost of CO2 adsorbent and in the end, the cost of CO2 capture. Resulting adsorbent has relatively high surface area, pore volume, and large pore size. The adsorbent shows good CO2 adsorption performance, however, CO2 loading is little low and therefore further work needs to be done to improve the CO2 adsorption capacity of resulting adsorbent.

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