CHEMICAL ENGINEERING TRANSACTIONS VOL. 52, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-42-6; ISSN 2283-9216 CO2 Adsorption on Polybenzoxazine Grafted Activated Carbon: Effects of Amine Precusor Katipot Inkonga, Pramoch Rangsunvigita,b*, Thanyalak Chaisuwana,b, Santi Kulprathipanjac aThe Petroleum and Petrochemical College, Chulalongkorn University, 254 Soi Chulalongkorn 12, Phayathai Rd., Pathumwan, Bangkok 10330, Thailand bCenter of Excellence on Petrochemical and Materials Technology, 7th floor, Chulalongkorn University Research Building, Soi Chulalongkorn 12, Phayathai Rd., Bangkok 10330, Thailand cUOP, A Honeywell Company, Des Plaines, Illinois, U.S.A. 60017 Pramoch.r@chula.ac.th CO2 adsorption of polybenzoxazine-modified activated carbon prepared by grafting various polybenzoxazine (PBZ) via ring-opening polymerization of different amine precursors was investigated. Benzoxazine monomer (BZ) was synthesized from phenol, paraformaldehyde, and hexamethylenediamie (HMDA) or triethylenetetramine (TETA). The PBZ loading was varied by different benzoxazine monomer solution concentrations from 0.1 to 0.5 g/L. CO2 adsorption isotherms were obtained at 35 °C, 50 °C, and 75 °C. Adsorbents were characterized by TG-DTA, FT-IR, and surface area and pore size analyses. The amount of PBZ grafted on activated carbon was determined by UV-Vis spectroscopy. The results showed that the CO2 adsorption capacity of the PBZ grafted activated carbon was improved due to the synergistic effects between physical and chemical adsorption. When the adsorption temperature increased, the CO2 adsorption capacity of the grafted adsorbent decreased because the physisorption was more dominated than the chemisorption. The decrease in the capacity due to the temperature increase was more pronounced with the unmodified activated carbon. Using TETA resulted in high CO2 adsorption than using HMDA. It was likely that using TETA to synthesize PBZ contained secondary amine and tertiary amine whereas only tertiary amine was found in PBZ synthesized by HMDA. The secondary amine can directly react with CO2, but the tertiary amine has only van der Waals force to attract CO2. CO2 adsorbed onto PBZ/AC was completely desorbed by heating at 120 °C for 24 h. There was no significant change in the CO2 adsorption capacity of the regenerated adsorbents 1. Introduction Nowadays, global warming is one of the most critical issues, which results from the greenhouse gas (GHG) emissions in the atmosphere. Carbon dioxide (CO2) is the major GHG and considered as the dominant contributor to the climate change (Cox et al., 2000,). The main source of CO2 emissions arise from the combustion of fossil fuel. Therefore, CO2 removal from flue gases is considered as the key solution to CO2 emission problem (Keramati and Ghoreyshi, 2013). One of the methods to solve the problem is CO2 capture and storage (CCS). Among the CCS, the simplest is the post-combustion capture. A number of separation technologies could be employed with post-combustion capture are absorption, adsorption, membrane separation, and cryogenic separation (Pires et al., 2011). In recent years, many researches have been focused on the development of adsorption based CO2 capture technologies. A variety of solid adsorbents has been developed and used for CO2 capture. Lu et al. 2008 concluded that the absorbent for competitive CO2 capture by carbon nanotubes, activated carbons, and ceolites. Sayari et al. 2011 concluded the absorbent for CO2 adsorption on flue gas treatment including activated carbon, zeolites, mesoporous silica, carbon nanotube, metal-organic frameworks (2011,). However, these adsorbents have limitation of their own. To overcome the limitation, the amine-functionalized adsorbents DOI: 10.3303/CET1652018 Please cite this article as: Inkong K., Rangsunvigit P., Chaisuwan T., Kulprathipanja S., 2016, Co2 adsorption on polybenzoxazine grafted activated carbon: effects of amine precusor, Chemical Engineering Transactions, 52, 103-108 DOI:10.3303/CET1652018 103 have been investigated. Knowles et al. (2005) used aminosilane grafted on hexagonal mesoporous silica (HMS) and amorphous silica (S2). CO2 adsorption capacity of all aminopropyl-functionalized silica materials exhibited higher adsorption capacity than silica (S2 and HMS), and the aminopropyl groups indeed enhanced the CO2 adsorption capacity. Changa et al. (2009) used various aminosilanes grafted on SBA-15. The grafted aminosilanes on SBA-15 exhibited high adsorption capacity for CO2 under moist as well as dry conditions. CO2 adsorption capacity of the samples was found to be in the order of tri- > di- > mono. Then, Ko et al. (2011) studied the effect of amine type for CO2 capture by using primary (1°), secondary (2°), and tertiary (3°) amine grafted on surface SBA-15. The CO2 adsorption capacity for the amine type follows the sequence: 1°amine >2°amine >3°amine. Xu et al. (2003) used a polyethyleneimine (PEI) impregnated on MCM-41. At the PEI loading of 50 wt% in MCM-41, the CO2 adsorption capacity was 30 time higher than the MCM-41. PBZ is a very attractive polymer for the CO2 adsorption due to its high affinity with CO2. Because of different amines and large amount of amines, PBZ can adsorb CO2 via different mechanisms and enhances CO2 adsorption performance (Ishida and Agag, 2011). In this work, Activated carbon (AC) was functionalized with BZ via ring-opening polymerization to enhance CO2 adsorption capacity. BZ was synthesized from phenol, paraformaldehyde, and HMDA or TETA. Adsorption temperature, type of amine, and loading of PBZ were also investigated to find the optimum condition for CO2 adsorption. 2. Experimental 2.1 Benzoxazine Monomer Syntheses The synthesis of the BZ from HMDA (98 %, Aldrich Co., LLC) or TETA (technical grade, 60 %, Aldrich Co., LLC), phenol (99 %, Aldrich Co., LLC) and ρ-formaldehyde (95 %, Aldrich Co., LLC) was achieved with the 1:2:4 molar ratio. The synthesis of BZ started by first dissolving HMDA or TETA in dioxane (99.95 %, RCL Labsan Limited) in a glass bottle and stirring until a clear solution was obtained. Phenol and ρ-formaldehyde were also dissolved in dioxane, and stirred until a clear solution was obtained before slowly dropping the HMDA or TETA solution into the mixture. The mixture was continuously stirred in an ice bath under 10 °C for 1 h until a transparent yellow viscous liquid was obtained. 2.2 Adsorbent Preparation AC (supported by Carbokarn Co., Ltd.) was ground and sieved to obtain a particle size of 20 - 40 mesh. Then, the AC was dried at 120 ºC for 24 h. The BZ solution (0.1 - 0.5 g/L) was added to AC. The solid to liquid ratio was 1 g of ACs to 20 mL of BZ solution. The AC together with the BZ solution was stirred at 180 rpm and 80 °C for 2 h under reflux condition, before filtering and polymerization to obtain PBZ grafted AC. 2.3 Characterization The structure of BZ was characterized by Nicolet/Nexus 670 FTIR instrument. Perkin-Elmer/DSC 7 was used to determine the cure temperature of benzoxazine monomer. The amounts of PBZ impregnated on AC were determined by a Shimuszu/UV-1800 UV-Vis Spectrophotometer (at λmax = 277 nm). Thermal stability of adsorbents was investigated by TG-DTA (Perkin-Elmer/Pyris Diamond). Surface area of adsorbents was measured by a surface area and pore size analysis (Quantachrom/Autosorb1-MP). Figure 1: schematic of experimental setup 2.4 CO2 Adsorption Measurement One gram of an adsorbent was loaded into the adsorption chamber. He gas (99.999 %, Praxair Inc.) was used to measure the system volume by expansion principle. The adsorption processes were carried out using high 104 purity CO2 gas (99.99 %, Praxair Inc.). Effects of adsorption temperature were investigated by varying the temperature from 35 to 75 °C within a pressure range of 0 - 1.1 atm. 2.5 Regeneration After adsorption, the regeneration of spent adsorbent was carried out by taking the adsorbent out of the reactor for heating at 120 °C for 24 h. to remove adsorbed CO2 and volatile components. These adsorption/desorption cycles were repeated at least three times. 3. Results and Discussion 3.1 Benzoxazine Monomer Characterizations The spectra of BZ(TETA) and BZ(HMDA) are shown in Figure 2(a) The asymmetric stretching of C–O–C at 1,254 and 1,224 cm−1, the asymmetric stretching of C–N–C at 1,119-1,118 cm−1 and the CH2 wagging of oxazine at 1,367 - 1,332 cm−1 can be observed. Additionally, the stretching of ortho-substituted benzene ring observed at 754 cm−1and out of plane bending vibrations of C–H at 924 - 910 cm−1 are clearly present, indicating that BZ is incorporated into the structure (Thayalak et al., 2010). The curing temperature of BZ was investigated by DSC. Figure 2(b) shows the DSC thermogram of BZ. The DSC thermogram of BZ(TETA) shows the exotherm peak starts at 152 °C with a maximum peak at 184 °C, while the exothermic peak of BZ(HMDA) is starting at 124 °C with the maximum peak at 196 °C, attributed to the polybenzoxazine by ring- opening polymerization. After BZ is fully cured, the exothermic peak disappears, suggesting that the BZ is completely polymerized. Wavenumber (cm -1 ) 150025003500 1000200030004000 % T ra n sm it ta n c e BZ(TETA) BZ(HMDA) Temperature(oC) 40 60 80 100 120 140 160 180 200 220 H e a t fl o w (m W ) (e x o u p ) BZ(HMDA) BZ(TETA) Figure 2: (a) FTIR spectra of BZ(TETA) and BA(HMDA) and (b) DSC thermogram of the BZ(TETA) and BZ(HDMA) 3.2 Adsorbent Characterization Table 1: Amounts of PBZ impregnated on AC and surface area of absorbent Initial concentration of BZ solution (g/L) PBZ impregnated on AC (wt% PBZ) Surface area (m2/g) AC - - 1,022 BZ (TETA) 0.1 0.13 964 0.3 0.49 958 0.5 0.54 939 BZ (HMDA) 0.1 0.10 956 0.3 0.38 945 0.5 0.87 928 The amount of PBZ grafted on the AC is shown in Table 1. Interestingly, the higher concentration of the BZ solution increases the amount of grafted BZ. Figure 3(a) shows the TGA thermograms of PBZ(TETA), PBZ(HMDA), AC, and grafted AC. The AC shows about 7 wt% loss between 100 °C and 250 °C, corresponding to the removal of volatile and moisture. The thermograms of both PBZ(TETA) and PBZ(HMDA) start to lose weight at 250 °C, and the maximum weight loss can be observed between 250 °C and 700 °C, where the decomposition products, like a combination of benzene derivatives, amines, phenolic compounds, (a) (b) 105 and mannich base compounds, were reported (Hemvichian et al., 2002). The thermograms of modified AC show the weight loss in two steps. The first step is around 100 °C, which is from the removal of volatile and moisture. The second step of the impregnated AC is around 250 °C, which is from the PBZ degradation. Figure 3(b) shows the nitrogen adsorption-desorption isotherms of the adsorbents, which exhibit type I isotherm according to the IUPAC classification. This indicates that the adsorbents exhibit both meso- and microporosity (Pevida et al., 2008). Surface areas of the adsorbents are shown in Table 1. The results show that, after grafting of PBZ on the AC, the BET surface area of the samples confirms that PBZ is successfully introduced on the support (Xu et al., 2003). Temperature ( o C) 100 300 500 700200 400 600 800 % w e ig h t lo ss 0 20 40 60 80 100 AC PBZ(TETA) 0.13 wt% PBZ(TETA)/AC 0.49 wt% PBZ(TETA)/AC 0.54 wt% PBZ(TETA)/AC PBZ(HMDA) 0.10 wt% PBZ(HMDA)/AC 0.38 wt% PBZ(HMDA)/AC 0.87 wt % PBZ(HMDA)/AC Relative Pressure (P/P0) .1 .3 .5 .7 .90.0 .2 .4 .6 .8 1.0 N 2 A d s o rb e d v o lu m e ( c m 3 /g @ S T P ) 0 100 200 300 400 AC 0.13 wt% PBZ(TETA)/AC 0.49 wt% PBZ(TETA)/AC 0.54 wt% PBZ(TETA)/AC 0.10 wt% PBZ(HMDA)/AC 0.38 wt% PBZ(HMDA)/AC 0.87 wt% PBZ(HMDA)/AC Figure 3: (a) TGA thermograms of PBZ, AC, and modified AC and (b) Nitrogen adsorption/desorption isotherms of AC and modified AC at -196 °C 3.3 CO2 Adsorption i) Effect of PBZ loading on CO2 adsorption Pressure(atm) 0.0 .2 .4 .6 .8 1.0 1.2 C ap ac it y (m m o l/ g -a d so b at e ) 0.0 .5 1.0 1.5 2.0 2.5 3.0 AC 0.10 wt% PBZ(HMA)/AC 0.38 wt% PBZ(HMDA)/AC 0.86 wt% PBZ(HMDA)/AC 0.13 wt% PBZ(TETA)/AC 0.49 wt% PBZ(TETA)/AC 0.53 wt% PBZ(TETA)/AC Pressure(atm) 0.0 .2 .4 .6 .8 1.0 1.2 C a p a c it y (m m o l/ g -a d so b a te ) 0.0 .2 .4 .6 .8 1.0 1.2 1.4 AC 0.10 wt% PBZ(HMDA)/AC 0.38 wt% PBZ(HMDA)/AC 0.87 wt% PBZ(HMDA)/AC 0.13 wt% PBZ(TETA)/AC 0.49 wt% PBZ(TETA)/AC 0.54 wt% PBZ(TETA)/AC Figure 4: Adsorption isotherms of AC and modified AC at (a) 35 °C and (b) 75 °C Table 2: CO2 adsorption capacity of adsorbents at 35 °C, 50°C and 75°C and 1 atm Adsorbent CO2 adsorption capacity (mmol/g adsorbent) 35 °C 50 °C 75 °C Activated Carbon (AC) 2.12  0.04 1.61  0.01 1.02  0.01 0.10 wt% PBZ(HMDA)/AC 2.22  0.14 1.63  0.09 0.99  0.06 0.38 wt% PBZ(HMDA)/AC 2.37  0.01 1.66  0.06 0.98  0.06 0.87 wt% PBZ(HMDA)/AC 2.07  0.02 1.48  0.15 0.96  0.07 0.13 wt% PBZ(TETA)/AC 2.26  0.01 1.72  0.02 0.99  0.03 0.49 wt% PBZ(TETA)/AC 2.46  0.01 1.86  0.04 1.09  0.02 0.53 wt% PBZ(TETA)/AC 1.94  0.02 1.53  0.02 0.92  0.01 The influence of PBZ loading on the CO2 adsorption of PBZ grafted AC was investigated in the range of 35 - 75 °C and up to 1.1 atm. Figure 4 shows the adsorption isotherms at 35 °C and 75 °C. As observed, all adsorbents show low adsorption capacity at the low pressures. In other words, the adsorption capacity (b) (b) (a) (a) 106 increases at higher pressures. The CO2 adsorption capacity at 35 °C, 50 °C and 75 °C and 1 atm is shown in Table 2. The CO2 adsorption capacity of both PBZ(TETA) and PBZ(HMDA) grafted AC at 35 °C, 50 °C, and 75 °C shows the same trend. After grafting PBZ(TETA) or PBZ(HMDA) on the AC, the adsorption capacity increases with the increase of the amount of PBZ. The CO2 adsorption capacity reaches the maximum capacity with the 0.49 wt% and 0.38 wt% of PBZ(TETA) and PBZ(HMDA), respectively. However, the adsorption capacity decreases when the amount of grafted was higher than 0.49 wt% PBZ(TETA) or 0.38 wt% PBZ(HMDA). The results show that the CO2 adsorption capacity of 0.13, 0.49 wt% PBZ(TETA)/AC and 0.10, 0.38 wt% PBZ(HMDA) is higher than that of the AC, whereas the surface area of PBZ grafted on the AC decreases (as shown in Table 2). The increase in the CO2 adsorption capacity of the grafted AC is because of the affinity between CO2, a Lewis acid, and -NH- group, a Lewis base, in PBZ structure. Therefore, a combination of physical and chemical adsorption may result in the significant increase in the CO2 adsorption. The adsorption capacity of 0.54 wt% PBZ(TETA)/AC and 0.87 wt%PBZ(HMDA) has lower capacity than the AC may be due to PBZ blocks the pore size of AC, substantiated by the decrease in the surface area. ii) Effect of amine type on CO2 adsorption Table 2 shows the effect of amine precursor to synthesize BZ. The results show that the CO2 adsorption capacity of PBZ(TETA) is higher than PBZ(HMDA) because of the different amine type in PBZ after polymerization. It was likely that using TETA to synthesize PBZ contains 2° amine and 3° amine. The 2° amine can react directly with CO2 to produce carbamates through the zwitterionic mechanism and also adsorb CO2 by electrostatic or van der Waals forces. In contrast, using HMDA contained only 3° amine. The 3° amine cannot react with CO2 directly without H2O. In this study, the CO2 adsorption test was carried out under dry condition; therefore, CO2 is adsorbed on tertiary amine through electrostatic or van der Waals forces only (Ko et al., 2011) iii) Effect of temperature on CO2 adsorption Figures 6(a) and 6(b) show the effects of adsorption temperature on the CO2 adsorption of PBZ-grafted AC and AC. The corresponding CO2 adsorption capacity is listed in Table 2. It can be observed that the CO2 adsorption capacity decreases with the increase in the operating temperature. It is likely that PBZ, a thermosetting polymer is produced from BZ by curing reaction, has a three dimensional structure that cannot be changed. Hence, when the adsorption temperature increases, PBZ cannot move to adsorb CO2. As the result, CO2 adsorption capacity decreases. It further implies that the physisorption is more dominant than the chemisorption or the adsorbent is predominantly determined by the thermodynamic effect rather than the kinetic effect. In the contrary using PEI, which is a thermoplastic, as the temperature is increased, PEI becomes more flexible and CO2-affinity sites are more exposed to CO2; thus, CO2 adsorption capacity increases (Wang et al., 2012). Another reason for the decrease in the capacity is explained by the interaction between CO2 and tertiary amine. Because the 3° amine attaches with CO2 by van der Waals forces, the van der Waals forces can be broken easily by increasing the temperature. 3.4 CO2 Adsorption after Regeneration Figure 5 shows the CO2 adsorption isotherms at 50 °C of regenerated samples of 0.49 wt% PBZ(TETA)/AC and 0.38 wt%PBZ(HMDA)/AC. During the three cycles, the CO2 adsorption capacity of the 0.49 wt% PBZ(TETA)/AC and 0.38 wt% PBZ(HMDA)/AC is not significantly changed, confirming the reproducibility and stability of the material. Pressure(atm) 0.0 .2 .4 .6 .8 1.0 1.2 C ap ac it y (m m o l/ g -a d so b at e ) 0.0 .5 1.0 1.5 2.0 2.5 Fresh 1 st 2 nd 3 rd Pressure(atm) 0.0 .2 .4 .6 .8 1.0 1.2 C a p a c it y (m m o l/ g -a d so b a te ) 0.0 .5 1.0 1.5 2.0 Fresh 1 st 2 nd 3 rd Figure 5: CO2 adsorption isotherms at 50 °C of (a) 0.49 wt% PBZ(TETA)/AC and regenerate sample and (b) 0.49 wt% PBZ(HMDA)/AC and regenerate sample (b) (a) 107 4. Conclusions The grafting of PBZ improved the CO2 adsorption capacity due to the synergistic effects between physical and chemical adsorption. The increase in the adsorption temperature reduced the CO2 adsorption capacity due to the strong influence of physisorption. Using TETA as a amine precusor resulted in the higher CO2 adsorption capacity than using HMDA as the precusor to synthesize BZ because of the different amine types in the PBZ structure. The results also showed that the CO2 adsorption capacity depended on the amount of PBZ loading, and the 0.49 wt% PBZ(TETA) or 0.38 wt% PBZ(HMDA) loadings were the optimum amounts. The decrease in the adsorption capacity was observed when the AC was loaded with PBZ(TETA) or PBZ(HMDA) higher than 0.54 wt% or 0.86 wt% because of the pore filling effect. The CO2 adsorption capacity of the regenerated adsorbents showed no significant change in the capacity. Acknowledgements The authors would like to sincerely thank The 90th Anniversary of Chulalongkorn University Fund and Grant for International Integration: Chula Research Scholar, Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University, Thailand; The Petroleum and Petrochemical College, Chulalongkorn University, Thailand; Center of Excellence on Petrochemical and Materials Technology, Thailand; and UOP, A Honeywell Company, USA, for providing support for this research work. References Chaisuwan T., Komalwanich T., Luangsukrerk S., Wongkasemjit S., 2010, Removal of heavy metals from model wastewater by using polybenzoxazine aerogel, Desalination, 256, 108–114. Changa F.Y., Chao K.J., Cheng H.H., Tan C.S., 2009, Adsorption of CO2 onto amine-grafted mesoporous silicas, Separation and Purification Technology, 70, 87-95. Cox P.M, Betts R.A., Jones C.D., Spall S.A., Totterdell I.J., 2000, Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model, Nature, 408,184-187. Hemvichian K., Laobuthee A., Chirachanchai S., Ishida H., 2002, Thermal decomposition processes in polybenzoxazine model dimers investigated by TGA–FTIR and GC–MS, Polymer Degradation and Stability, 76, 1-15. Ishida H., Agag T., 2011, Handbook of Benzoxazine Resins, Elsevier, New York. Keramati M., Ghoreyshi A.A., 2013, Improving CO2 adsorption onto activated carbon through functionalization by chitosan and triethylenetetramine, Physica E. 57, 161-168. Knowles G.P., Graham J.V., Delaney S.W., Chaffee A.L., 2005, Aminopropyl-functionalized mesoporous silicas as CO2 adsorbents, Fuel Processing Technology, 86, 1435-1448. Ko Y.G., Shin S.S., Choi U.S., 2011, Primary, secondary, and tertiary amines for CO2 capture: Designing for mesoporous CO2 adsorbents, Journal of Colloid and Interface Science, 362, 594-602. Lu C., Bai H., Wu B., Su F., Hwang J.F., 2008, Comparative study of CO2 capture by carbon nanotubes, activated carbons, and zeolites, Energy and Fuels, 22, 3050-3056. Pevida C., Plaza M.G., Arias B., Fermoso J., Rubiera F., Pis J.J., 2008, Surface modification of activated carbons for CO2 capture, Applied Surface Science, 25, 7165-7172. Pires J.C.M., Martins F.G., Alvim-Ferraz M.C.M., Simões M., 2011, Recent developments on carbon capture and storage: An overview, Chemical Engineering Research and Design. 89, 1446-1460. Sayari A., Belmabkhouta Y., Serna-Guerrero R., Flue gas treatment via CO2 adsorption, 2011, Chemical Engineering Journal, 171, 760-774. Wang J., Chen H., Zhou H., Liu X., Qiao W., Long D., Ling L., 2012, Carbon dioxide capture using polyethylenimine-loaded mesoporous carbons, Journal of Environmental Sciences, 25 124-132. 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 and Mesoporous Materials, 62, 29-45. 108