PREPARATION OF NITROGEN ENRICHED HIERARCHICALLY NANOPOROUS CARBON FROM POLYBENZOXAZINE FOR METHANE STORAGE Norawit Kaewpornmongkol a and Thanyalak Chaisuwan *,a,b a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Center of Excellence on Petrochemical and Materials Technology, Bangkok, Thailand Keywords: Benzoxazine, Activated carbon, Methane adsorption ABSTRACT The adsorption of methane on a carbon based material, which was successfully obtained by carbonization followed by activation process of polybenzoxazine, has been studied within temperature-pressure range from 308 to 348 K and 0 to 700 psi. The adsorbent characterized by N 2 adsorption-desorption and SEM images to confirm pore structure of the adsorbents exhibited high surface area of 838 m 2 /g. A commercial activated carbon with the surface area of 493 m 2 /g was used to compare the methane adsorption efficiency. An elemental analysis technique was used to measure carbon (C), hydrogen (H) and nitrogen (N). The results indicated that the percentage of C, H, O and N was quite the same between before and after adsorption process. It can be proved that the adsorption of methane is only physical adsorption. For the adsorption test, it was found that the highest amount of methane as much as 25 mmol/g at 700 psi was adsorbed by the adsorbent from the activated carbon derived from polybenzoxazine. On the other hand, the amount of methane adsorbed on the commercial activated carbon was 19 mmol/g at 700 psi. Methane adsorption capacity increases with increasing of both micropore volume and surface area, confirming microporosity with interconnected porous structure plays an important role on methane adsorption capacity. * thanyalak.c@chula.ac.th INTRODUCTION The need for alternative energy sources that replace the petroleum-derived fuels has attracted attention to the study of new clean fuels with lower atmospheric emissions and higher availability than conventional petroleum. Hydrogen and natural gas (NG) are two of the alternative fuels evaluated, which have the common characteristic of being gaseous at room temperature, a fact that introduces difficulties for their transport and storage (Andres et al., 2016). A global climate change has spurred the development of several strategies to reduce atmospheric CO 2. Methane is a potent greenhouse gas, so it must be reduced before emitted into the atmosphere. For this reason there are several technologies for CO 2 and CH 4 capture but CO 2 and CH 4 adsorption are considered to be among the most effective separation processes for these gases (Djeridi et al., 2016). Methane is the main component of natural gas. Many of the previous studies related to ANG have focused on the study of methane adsorption at pressures up to 3500 kpa and room temperature. Previous studies have evaluated the effect of pore size and geometry in Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1
methane storage capacities of porous materials (Andres et al., 2016). A number of researchers have focused on methane adsorption on microporous activated carbons as adsorbents, such as those from the Maxsorb family and AX21 which are endowed with large specific surface areas (Xiaolin et al., 2010). Many porous materials, such as metal-organic frameworks (MOF), activated carbons and zeolites, have been investigated for adsorption process due to its high CH 4 adsorption performance, good adsorbate-adsorbent interaction and nice pore geometry (Andres et al., 2016). Porous carbon is one of materials for CH 4 adsorption because it has high surface area and high total pore volume which can help for physical adsorption, Moreover, its optimal fraction of ultra-micropores (Hailin et al., 2015). However, microporous activated carbons xerogels based on the organic gels are widely used in adsorption and separation technologies, because of their unique pore structure, low cost and abundance of their raw ingredient (Djeridi et al., 2016). The purpose of this work is to study the different porous carbons and the effect of adsorption temperature. EXPERIMENTAL A. Synthesis of Polybenzoxazine Precursors Polybenzoxazine precursors which were synthesized from bisphenol-a, TEPA and formaldehyde with the molar ratio 1:1:4. At the beginning, bisphenol-a was dissolved in DMF and stirred continuously until the solution was clear, followed by adding formaldehyde into bisphenol-a solution under continuous stirring. After that TEPA was dropped into the mixture and stirred continuously for 1 h until the transparent yellow solution appeared. Afterward, the solution was kept in a closed system at 80 C for 2 days to obtain pre-polymer of polybenzoxazine. Pre-polymer precursors were removed solvent by acetone exchanging in shaker bath at room temperature for 3 days, followed by drying in an oven at 80 C for 24 h. Then, resulting organogels were cured at 220 C to obtain the fully cured polybenzoxazine. B. Preparation of carbon adsorbents In preparation of carbon adsorbents, all polymer precursors were carbonized in a quartz tube under nitrogen atmosphere with a nitrogen flow rate of 600 cm 3 /min at pyrolysis temperature of 800 C. For the activated carbons, carbon samples were activated under the CO 2 atmosphere at 900 C for 3 h. C. Characterization of adsorbents The functional groups related to structure of materials were investigated by using FTIR technique. The FT-IR spectra were obtained by using a Thermo Scientific, Nicolet is5 ATR-FTIR spectrometer in the frequency range of 650-4000 cm -1 with 16 scans at a resolution of 4 cm -1. DSC analyzer was carried out by using a NETZCH, DSC 204 F1 Phoenix instrument. Samples were contained in rang 4-8 mg on the aluminum cell and observed from 30 to 300 C. They were operated with heating rate 10 C per minute under nitrogen atmosphere with flow rate 20 ml per minute. BET surface area analysis was obtained with a Quantachrome-Autosorb-1MP. The N 2 adsorption-desorption isotherm measurement was conducted at -196 C. Before measurement, the samples were degassed at 100 C for polymers and 250 C for carbons under vacuum atmosphere. Surface area was Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2
calculated by using the BET equation. The pore size distributions were carried out based on the Barrett, Joyner and Halenda (BJH) method for mesoporous and the t-plot method for microporous properties. Elemental analyzer (Leco CNH2000) was obtained to determine the composition of samples. D. CH 4 adsorption measurement The volumetric apparatus was used to study methane adsorption on carbon adsorbents. This apparatus consists of a sample holder, a vacuum pump, and pressure transducer. Ultra high purity grade methane (99.99% purity) was used in the adsorption study. A gas reservoir was a high pressure stainless steel reactor and the pressure regulator with 4,000 psi maximum limit was installed to control a gas flow rate into the system. A K-type thermocouple was used for measuring the temperature of gas inside the reactor. The system pressure was measure by pressure transducer in the range of 0 to 3,000 psi with 0.13% error. For each experiment, about 1.0 g of adsorbents was weighed and put into the sample holder. Next, the adsorbent was degassed by using a rotary vacuum pump prior to the methane adsorption. The temperature was controlled at 35 to 75 C within a pressure range of 0-700 psi. The pressures of gas were recorded before and after each gas expansion. RESULTS AND DISCUSSION A. Characterization of BA-tepa Derived Polybenzoxazine Aerogel The functional group identification of benzoxazine precursor and polybenzoxazine was carried out using FT-IR, as shown in Figure 1. Figure 1 FTIR spectra of the benzoxazine precursor (a) and polybenzoxazine (b). The FTIR spectrum of benzoxazine precursor (Figure 1 (a)) demonstrates the asymmetric stretching bands of C-N-C at 1113 cm -1. The adsorption bands at 1257 cm -1 and 1357-1370 cm -1 notice the asymmetric stretching of C-O-C of oxazine and CH 2 wagging of oxazine. The characteristic absorption peaks assigned to the tri-substituted benzene ring and the out- Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3
of-plane bending vibrations of C-H were observed at 1475 and 933 cm -1, respectively. According to Chaisuwan et al, these adsorption bands confirmed that benzoxazine precursor and polybenzoxazine were obtained. In addition, the FTIR spectrum in Figure 4.1 (b) illustrates the significant decreasing of characteristic adsorption peaks after curing of the benzoxazine precursor. Furthermore, the intensity of the characteristic absorption band at 1608 cm -1 referring to tetra-substituted benzene ring after polymerization was increased. Additionally, the thermal behavior of polybenzoxazine and benzoxazine precursor confirmed that the polymerization of benzoxazine occurred. The DSC thermogram of benzoxazine precursor (Figure 2 (a)) shows the exothermic peak from 200-270 C while the exothermic peak of polybenzoxazine disappeared as shown in Figure 2 (b). These obvious results show that the polymerization of benzoxazine precursor by ring-opening of oxazine was taken place (Chaisuwan et al, 2010). Figure 2 DSC thermograms of the benzoxazine precursor (a) and polybenzoxazine after heat treatment at 220 C (fully-cured) (b). B. Characterization of Adsorbents Autosorb 1-MP analyzer was used to investigate the porous characteristics of nitrogen enriched carbons at -196 C. The nitrogen adsorption-desorption isotherms of samples are demonstrated in Figure 3 (a), respectively. The porous carbon samples derived from polybenzoxazine exhibited type I isotherms by IUPAC classification, which is the characteristic of microporous material. The dramatic increase in N 2 adsorption volume in the low pressure range (P/P 0 < 0.1) was observed because of the filling of micropores with N 2 gas. However, the commercial activated carbon showed type IV isotherms as the characteristics of mesoporous material. The isotherm of the activated carbon which have slit-shaped pores demonstrated as the shape of hysteresis loop and the desorption curve of hysteresis contains a slope associated with a force on the hysteresis loop (Alothman et al, 2012). From the pore size distribution graph as shown in Figure 3 (b), the main peak of activated carbon from polybenzoxazine exhibited pore size of sample was lower than 0.8 nm that indicated the ultra-microporous material was obtained. On the other hand, the commercial Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4
activated carbon showed pore size about 1.2-2 nm, 2.5-3 nm, and more than 3.5 nm that possessed both less micropore and much mesopore. The textural characteristics of the resulting carbons are shown in table 1. After activation process, the activated carbon from polybenzoxazine obviously exhibited a higher surface area from 133 m 2 /g to 838 m 2 /g owing to the generation of fine micropores (pore less than 1 nm) during the high temperature activation process. The results notice that the activation process can increase micropore volume of the samples because CO 2 can react with carbon and generate new pores. Figure 3 N 2 adsorption-desorption isotherms of nitrogen enriched carbons (a) and Pore size distributions of nitrogen enriched carbons (b). To ensure that nitrogen enriched carbons were obtained from one step synthesis, the nitrogen contents in all samples were investigated by using the elemental analyzer and the results are summarized in table 2. Polybenzoxazine had 11.18 % of nitrogen. After pyrolysis process, nitrogen groups were still observed. Table 1 Textural properties of nanoporous carbon derived from polybenzoxazine Sample name S BET (m 2 /g) S meso b (m 2 /g) S micro c (m 2 /g) V total d (cm 3 /g) V meso b (cm 3 /g) V micro c (cm 3 /g) C-800 133 39 115 0.10 0.02 0.07 AC-800 838 417 820 0.46 0.15 0.42 Commercial AC 493 531 204 0.67 0.64 0.10 a obtained from multi point BET method b obtained from BJH method c obtained from t-plot method d obtained from total pore volume calculated from nitrogen adsorption at P/P 0 =0.995 Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5
However, the amount of nitrogen was lower because some of nitrogen atoms were eliminated during carbonization process (Ma et al, 2016; Zhang et al, 2014). Obviously, the amount of oxygen was higher after the activation process because we used CO 2 gas during the activation process to generate functional group on surface of carbons. Table 2 Elemental compositions of all samples Sample name Elemental composition (wt.%) C H N O PBZ-TEPA 65.00 6.43 11.18 17.39 C-800 b 84.60 0.95 8.35 6.10 C-800 a 82.10 1.54 7.90 8.46 AC-800 b 86.40 1.00 5.74 6.86 AC-800 a 84.20 1.29 5.50 9.01 Commercial AC 65.70 2.25 0.67 31.38 b obtained from before the adsorption a obtained from after the adsorption C. Methane adsorption of Activated Carbons According to the elemental analysis result, the nitrogen contents almost did not change after the adsorption process. It can be proved that the CH 4 adsorption performance is depended on only physical property of carbons. In this work, the effect of pore structure was investigated to obtain the suitable condition for nitrogen-enriched carbon for this application. The CH 4 adsorption of all samples was measured in the temperature at 35, 50, and 75 C. The regeneration process was also a crucial one for the adsorption process. The results of CH 4 adsorption capacity in various pressures from 0 to 700 psi were shown in Figure 4. From the result, it can be noticed that the activated carbons showed more CH 4 adsorption than non-activated carbons. AC-800 showed the higher CH 4 capacity up to 25.1 mmol/g as compared with C-800 (23.6 mmol/g) because the activation process can actually create many micropores that are suitable pore size to adsorb gas as exhibited in table 1. AC-800 has higher micropore volume, higher surface area, and interconnected porous structure. In case of different adsorbents, AC-800 also demonstrated the higher CH 4 uptake than commercial activated carbon (19.5 mmol/g), corresponding to the pore size distribution graph and textural properties as presented in Figure 3 (b) and table 1. AC-800 containing many micropores has higher surface area and micropore volume. On the other hand, commercial activated carbon which has many mesopores showed lower surface area and micropore volume (Andres et al, 2016). It can be inferred that a carbon adsorbent with high surface area and high micropore volume, leading to have more suitable methane adsorption and the activation process can produce the volume of micropores. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6
Figure 4 CH 4 adsorption of porous carbons at 35 C. CONCLUSIONS Nitrogen enriched interconnected microporous carbon was prepared from polybenzoxazine precursors, which could be proved by FT-IR and DSC techniques. However, it can be proved that methane capture is only physical adsorption. AC-800 demonstrated the highest CH 4 adsorption capacity at 25.1 mmol/g because it has suitable properties to adsorb CH 4 gas including high surface area, high micropore volume. Furthermore, nitrogen enriched carbon that prepared from polybenzoxazine has interconnected porous structure which is better than commercial activated carbons when using at a wide range of pressures. ACKNOWLEDGEMENTS This research is financially supported by the Petroleum and Petrochemical College, Center of Excellence on Petrochemucals and Materials Technology, Chulalongkorn University, Thailand. REFERENCES Alothman, Z. A. (2012) A Review: Fundamental Aspects of Silicate Mesoporous Materials. Materials, 12, 2874-2902. Andres A. García Blanco a, A. F. V. a, Sophia A. Korili b, Antonio Gil b, Karim Sapag a,* (2016) A comparative study of several microporous materials to store methane by adsorption. Microporous and Mesoporous Materials, 224, 323-331. Chaisuwan T., T. K., Supanan Luangsukrerk, Sujitra Wongkasemjit (2010) Removal of heavy metals from model wastewater by using polybenzoxazine aerogel. Desalination, 256, 108 114. Djeridi W. a, N. Ben Mansour b, A. Ouederni a, P.L. Llewellyn c, L. El Mir b,d (2016) Study of methane and carbon dioxide adsorption capacity by synthetic nanoporous carbon based on pyrogallol-formaldehyde. International journal of hydrogen energy, 1-8. Hailin Cong b, Meirong Zhang a,b,1, Yanli Chen a, Kai Chen a, Yajuan Hao a, Yunfeng Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 7
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