CO2-imprinted Sustainable Carbon Derived from Sunflower Heads for Highly Effective Capture of CO2 from Flue Gas

In this work, novel and cheap CO2 imprinted adsorbents were synthesized on the surface of activated carbon derived from sunflower heads by surface imprinting technique using ethanedioic acid and acrylamide as template molecule and functional monomer, respectively. To obtain CO2 imprinted adsorbents with optimal properties, we studied the effects of ratio of KOH to sunflower-based activated carbon, carbon dosage and adsorption temperature, and found that the optimal conditions were alkali carbon ratio of 0.75:1, carbon dosage of 0.75 g, and adsorption temperature of 20°C. Under these conditions, maximum CO2 adsorption capacity of the adsorbents can attain 1.71 mmol g–1. The selective adsorption performance of the adsorbents for CO2 was studied in the presence of H2O as well as in simulated flue gas. In addition, the adsorbents after adsorption can be effectively regenerated by N2 purge at 120°C. After five adsorption/desorption experiments, the CO2 adsorption capacity only decreased 11%. The results showed that sustainable and cheap CO2 imprinted adsorbents have good adsorption, selectivity and regeneration properties, which are worth being further studied and applied in CO2 capture.


INTRODUCTION
With the acceleration of global industrialization, carbon dioxide emissions are increasing year by year.The data showed that the annual carbon dioxide emissions from 2015 to 2017 were 400.1, 403.3 and 405.5 ppm, respectively (Yu et al., 2008;Chatterjee et al., 2018;Martins et al., 2018).At present, the amount of carbon dioxide in the atmospheres is nearly 50% higher than in 1750 (Yu et al., 2012;Fang et al., 2017;Sekiguchi et al., 2018).However, there seems to be no sign of a slowdown in the growth of carbon dioxide in the atmosphere.CO 2 emitted by power, transportation and industry accounts for 39%, 23% and 22% of total emissions, respectively (Rojey et al., 2005;Ciuzas et al., 2016;Chen et al., 2019).In the past 50 years, the global warming was mainly caused by CO 2 and expected to continue in this century (Xie et al., 2017;Begum et al., 2018).In view of a large amount of carbon dioxide emitted to the atmosphere, it is urgent to study advanced carbon dioxide adsorption materials for protecting the environment (Lee et al., 2015;Tsay et al., 2016;Chen et al., 2017).
To reduce the amount of carbon dioxide in the atmosphere, carbon capture and storage (CCS) paths provide several advantages (Wang et al., 2018).CCS technology is an environmental-friendly mean for reducing atmospheric carbon dioxide from coal-fired power plants, cement production process, and iron and steel industries (Sahu et al., 2017;Bhatia et al., 2019).Carbon dioxide capture technology can be divided into post-combustion, pre-combustion and eutrophic combustion (Leung et al., 2014).Post-combustion (Huang et al., 2016), pre-combustion (Naderi et al., 2019) and eutrophic combustion (Pan et al., 2015) process routes can easily follow the existing separation process and energy conversion process (Plaza et al., 2007).In the traditional postcombustion process, however, carbon dioxide is separated from flue gases, which is accompanied by high energy consumption (Merkel et al., 2010).In order to solve this problem, scholars developed a series of adsorbents to capture CO 2 (Li et al., 2013), such as zeolite (Zhang et al., 2014), silica (Yang et al., 2013), activated carbon (Li et al., 2010), ordered porous carbon (Chew et al., 2010) and other carbon-containing materials (Wickramaratne et al., 2013).Kim et al. (2019) synthesized the K-Cha zeolite microspheres containing strontium by one-step template-free hydrothermal method, the K-Cha molecular sieves microspheres showed grading characteristics, good adsorption performance for water (316 cm 3 g -1 ) and carbon dioxide (167.0 mg g -1 , 10 bar) at 298 K.And the adsorption selectivity factor of CO 2 /N 2 was 10 ( Kim et al., 2019).Jiang et al. (2019) studied the CO 2 adsorption properties of activated carbon, the commercial adsorbent (i.e., monoethanolamine) and chemical adsorbent (i.e., polyethyleneimine/silica) in order to assess the postcombustion capture advantages for carbon dioxide.The results showed that polyethyleneimine/silica and activated carbon exhibited higher net efficiency of CO 2 capture than monoethanolamine.They found that the regeneration heat of activated carbon was higher than those of monoethanolamine and polyethyleneimine/silica, but it could be reduced through increasing the CO 2 concentration.However, the current materials still have limitations in adsorption properties, adsorption conditions and selectivity (Wang et al., 2011).Imprinting technology (IT) is a biomimetic molecular recognition technology that simulates the recognition mechanism of enzyme-substrate and antibody-antigen molecules in nature (Liu et al., 2008).According to the type of targets, IT can be divided into two categories: molecular imprinting technology (MIT) and ion imprinting technology (Huang et al., 2018).Based on IT, scholars developed a series of functional adsorbents with special recognition and strong adsorption abilities for targets.Generally, the preparation processes of molecular imprinting polymers (MIPs) with recognition ability are shown in Fig. 1 (Wang. et al., 2019): First, select reasonable functional monomers (Zhang et al., 2012) to form pre-polymers with templates by covalent, non-covalent or semi-covalent way (Pan et al., 2010); second, cross-linkers and initiators are used to initiate the chemical polymerization reaction to form stable polymers under certain conditions (Nabavi et al., 2017); Finally, the templates are removed by extraction or acid elution to obtain MIPs with recognition sites for targets (Li et al., 2010;Wang et al., 2010).In addition, choosing appropriate template molecule is extremely significant for its adsorption ability in the preparation of MIPs.Typically, the target material is chosen as a template molecule.However, when the target material is unstable and has low-solubility in the polymerization system, it cannot be used as the template molecule directly.Therefore, the substitution template method is used to prepare MIPs through choosing the template molecule similar to that of target substance (Huang et al., 2019).
At present, scholars have widely studied the preparation methods and applications of MIPs.In order to improve the diffusion kinetics, Xu et al. (2019) used surface imprinting technique to prepare MIPs on the surface of mesoporous silica materials.The kinetic experiments showed that the method could effectively improve the mass transfer rate.Zhao et al. (2014) studied the carbon dioxide separation of molecular imprinted adsorbent from the coal-fired flue gas after desulfurization.The results showed that the NO in flue gas had minor effects on the adsorption performance of the adsorbents for carbon dioxide, and the adsorbents had good adsorption performance and selectivity.With the development of molecular imprinting technology, some new elements have been introduced into molecular imprinted polymers.
Traditional activated carbon has high mechanical strength, fast adsorption speed, high purification degree, and long service life, but the adsorption of gas is affected by its specific surface area and pore size distribution.The traditional activated carbon has a small specific surface area and a wide pore distribution, and the pore volume of the micropores that can effectively capture CO 2 is small, resulting in a low adsorption amount.Compared with biomass-based activated carbon, the pore structure of activated carbon can be effectively adjusted according to the characteristics of CO 2 molecules, resulting in a significant increase in the amount of CO 2 adsorption.Biomass-based carbons are a kind of cheap, sustainable and resourceful material with unique structures (Chen et al., 2016).They have excellent stability and  regeneration performance, and their pore structure is easy to design.The original biochar itself is highly non-porous and exhibits low CO 2 adsorption capacity due to low specific surface area and pore volume (Alabadi et al., 2015).To enhance its adsorption performance, the specific surface area and pore volume can be increased by chemical activation at moderate or high temperatures.Therefore, activated biochar is formed by activation of biomass carbon, and these materials exhibit better CO 2 adsorption capacity (Singh et al., 2017).Many researchers preferred to use a variety of plant residues such as coconut shell (Jain et al., 2015), cornstraw (Chi et al., 2017), rice husks (Kizito et al., 2015) and peanut straw (Shi et al., 2018).Among them, sunflower is a lignin-rich biomass with rich net structures.In the sunflower harvest season, a large number of sunflower heads are discarded.To avoid the waste of resources and relieve the treatment pressure of solid wastes, an activated carbon based on sunflower heads as carbon sources was prepared in this work.Table 1 shows the comparison of adsorption capacity of different activated carbon adsorbents.Singh et al. (2019) describe a simple one-step method that uses porous Dglucose to prepare activated porous carbon spheres to capture carbon dioxide by using a new non-corrosive chemical, i.e., potassium acetate.We use activation methods to control chemical structure, morphology, porosity, and texture characteristics, while Singh works by changing the amount of potassium acetate.The strategies used to synthesize these materials are simple and the total cost of precursors and processes is low.It is speculated that a similar activation strategy can be used to prepare a series of porous activated carbons with enhanced properties and use different carbon precursors for various energy and environmental applications.Active carbon adsorption of CO 2 is a typical physical adsorption.Adsorption refers to the accumulation of components at the phase interface when a fluid is in contact with a porous medium.This article discusses the phenomenon of solid adsorption of gases.The solid matter is called an adsorbent, and the adsorbed gaseous substance is called an adsorbate.An adsorbed phase is formed between the solid adsorbent and the adsorbed gas molecules.For activated carbon adsorption of carbon dioxide, it is caused by the attractive force between the adsorbate carbon dioxide molecules and the adsorbent activated carbon, but this binding force is relatively weak and easy to desorb.
In this work, surface imprinting technology was used to prepare CO 2 -MIPs with highly selective properties for CO 2 using the self-prepared carbon as the carrier that were prepared by using waste biomass sunflower heads as carbon sources.The adsorption properties of carbon dioxide under different conditions were investigated, and the preparation methods and adsorption conditions were optimized.The CO 2 -MIPs were characterized by scanning electron microscopy, nitrogen adsorption/desorption test, fourier transform infrared spectroscopy and thermogravimetric analysis.

Chemical Reagent
Oxalic acid was purchased from Kangde Chemical Co., Ltd.(Laiyang, China).Acrylamide (AAM), toluene and hydrochloric acid were purchased from China Pharmaceutical Group Chemical Reagent Co., Ltd.Ethylene glycol dimethyl acrylate (EGDMA), azodiisobutyronitrile (AIBN) were purchased from Shanghai Macklin Biochemical Co., Ltd.Acetonitrile (AN) and methanol were purchased from Fuyu Fine Chemical Co., Ltd.(Tianjin, China).The purity of all gases used in this work is higher than 99.99%, which is provided by Qingdao Hausen New Energy Co., Ltd.

Synthesis of Activated Carbon Carbonization
The sunflower heads screened (4 g) were put into tubular furnace and carbonized according to the following heating procedure: the material was purged with N 2 for 30 min at room temperature, heated to 773 K at a heating rate of 5 K min -1 , and kept for 90 min at 773 K. Finally, the material was cooled naturally to room temperature.N 2 protection was used throughout during the carbonization process.

Activation
Carbonized material (1 g) was impregnated in 20 mL distilled water with KOH (0.75 g) at room temperature for 48 h.The impregnated material was dried in oven at 378K.The dried material was activated by the following heating procedure: the material was purged with N 2 for 30 min at room temperature, raised to 773 K at the rate of 10 K min -1 , then raised to 973 K at the rate of 5 K min -1 and maintained for 2 h.The activation process was protected by N 2 .The purpose of activation is to further increase the specific surface area of the materials, and form numerous channels on the basis of the pore structure of the carbonized materials.

Synthesis of CO2-MIPs
MIPs adsorbent was synthesized by the following process: 3 mmol ethylene glycic acid and 12 mmol AAM were dissolved in 80 mL mixed solution of AN and toluene (1:1, v/v).After stirring for 30 min, 20 mmol EGDMA and 0.3 mmol AIBN were added into the mixture.Then, the mixture was dispersed by ultrasonic device for 15 min and purged by N 2 for 10 min to remove oxygen.After that, the mixture was sealed and stirred at 60°C for 12 h.The polymer was washed three times with HCl/methanol (1:9, v/v) solution to remove unreacted substances and then filtered out.The obtained polymer particles were washed with distilled water to neutrality and dried in 60°C oven.

Adsorption Experiment Basic Experiments
In this paper, the performance of adsorbent was evaluated by a fixed bed dynamic adsorption device.The adsorption test device is shown in Fig. 2. The prepared adsorbent (0.5 g) was filled into the glass adsorption column (10 mm diameter; 5 mm inner diameter).At the bottom of the column, there is a gas distribution orifice plate with a pore diameter of about 100 µm.Before adsorption, the CO 2 -MIPs were treated by the N 2 flow of 500 mL min -1 (about 1 h) at 98°C to release CO 2 adsorbed on the adsorbent.After the experiment started, 10% CO 2 was kept in the adsorption column at 25°C at a flow rate of 10 mL min -1 , and the gas coming out was mixed with N 2 (90 mL min -1 ).Desorption experiments were performed after adsorption experiments.During the adsorption process, the gas flow rate was controlled by mass flow controller (D08-3D/ZM), and the CO 2 concentration was measured by portable infrared CO 2 analyzer (GXH-3010E).In addition, desorption experiments were carried out under 120°C in N 2 flow to remove adsorbed CO 2 .
In order to reuse the adsorbent and collect the captured carbon dioxide, we carried out desorption experiments.After CO 2 adsorption becomes saturated, the material is desorbed under high temperature and N 2 purging to ensure multiple utilization.CO 2 desorbed by this method is collected together with N 2 .In the desorption process, we use mass flowmeter to control the flow rate of N 2 .
The adsorption capacity of CO 2 (q b ) was calculated according to the following equation: where, t (min) is the adsorption time, C outlet (% vol) is the concentration of CO 2 after CO 2 -MIPs adsorption, C inlet (% vol) is the initial concentration of CO 2 , V (mL min -1 ) is the flow of gas into the adsorbent; and m (g), the mass of the adsorbent.

Contrast Experiments
In the six groups of contrast experiments as shown in Table 2, the research factors are as follows: alkali-carbon ratio, dosage of the carrier, adsorption temperature, desorption temperature, concentration, water and selectivity.By comparing the adsorption curves of six group experiments, the optimum operation conditions were obtained.

Characterization
The pattern of the MIPs was characterized by Japanese electron JEOL-7500F field emission scanning electron microscopy (FESEM).The surface properties and chemical structures of the adsorbents were studied by Brookvertex

Structural Characterization
Fig. 3 shows the FT-IR spectra of the carrier and CO 2 -MIPs.The broad bands near 3440 cm -1 are the stretching vibration of OH group in -COOH and -C-OH.The absorption peaks near 1637 cm -1 are caused by the superposition of the deformed vibration of N-H bond and the stretching vibration of C=O.The absorption peaks at 1384 cm -1 and 1097 cm -1 are the stretching vibration of C-N.The infrared spectra of the two adsorbents are nearly the same, indicating that their chemical groups are basically similar.
Fig. 4 shows the nitrogen adsorption-desorption isotherms and pore size distributions of the carrier and CO 2 -MIPs.According to Brunauer-Deming-Teller classification, the curves belong to type I isotherm, indicating that CO 2 -MIPs were mainly composed of microporous structure.With the increase of relative pressure, the adsorption curve exhibited hysteresis.From the pore distribution curve, the pore size distribution of CO 2 -MIPs was concentrated.The specific surface area, pore volume and pore diameter are shown in Table 3.The specific surface area of CO 2 -MIPs was higher than that of the carrier.Pore volume and pore size of CO 2 -MIPs were smaller than those of carrier.This shows that the macropores of the carrier decreased after molecular imprinted polymerization, and its micropores and mesopores increased, leading to the decrease in average pore size.
Fig. 5 shows the SEM images of the carrier and CO 2 -MIPs.Compared with the SEM image of the carrier, the CO 2 -MIPs obviously exhibits more pores, illustrating that surface imprinting technology played the role in creating more adsorption cavities.Combined with the structure analysis of BET measurement, it can be concluded that the micropore and mesopore of CO 2 imprinted activated carbon increased significantly, which is beneficial to CO 2 adsorption in theory.Fig. 6 shows the thermogravimetric analyses of the carrier and CO 2 -MIPs.It can be seen from the diagram that the weight of CO 2 -MIPs exhibits less decline in comparison with that of the carrier during heating from room temperature to 800°C.The weight of the carrier decreases by 3.44% from room temperature to 100°C, which was caused by desorption of H 2 O that was physically adsorbed on the surface of the carrier, while the weight of CO 2 -MIPs decreases by 2.94% before 158°C.Under N 2 protection, when the temperature rises from 158°C to 800°C, the weight loss of the carrier is 10.4%, while that of the CO 2 -MIPs is only 7.55%.According to the above experimental results, it can be found that the stability of CO 2 -MIPs is better than that of the carrier, indicating that the imprinting process is conducive to the formation of structurally stable adsorbent.

Investigation on Preparation Conditions
The ratio of alkali to carbon is an important factor in the preparation of sunflower-based activated carbon, which has a great influence on the pore structure of activated carbon.Fig. 7 is the adsorption curves of CO 2 on different carriers prepared using different alkali-carbon ratios.With the increase  of the ratio of alkali to carbon, the CO 2 adsorption amount of sunflower-based activated carbon increases first and then decreases.When the ratios of alkali to carbon are 0.25:1, 0.5:1, 0.75:1 and 1:1, the adsorption capacities were 1.28, 1.31, 1.45 and 0.90 mmol g -1 , respectively.The maximum adsorption capacity was obtained when the ratio of alkali to carbon was 0.75:1.The results show that low alkali-carbon ratio leads to inadequate activation and less pore structure, while high alkali-carbon ratio leads to excessive activation and the damage of pore structure, both of which are not conducive to improve adsorption capacity of the sunflowerbased activated carbon for CO 2 .
Fig. 8 is the CO 2 adsorption curves of CO 2 -MIPs prepared with different dosages of the carriers in the preparation process.In order to enhance the interaction between CO 2 and the carrier, a large number of CO 2 trapping sites were created on the surface of the carrier through MIT to obtain CO 2 -MIPs.When the dosages of the carriers were 0, 0.5, 0.75 and 1 g, the adsorption capacities of CO 2 -MIPs reached 0.76, 1.03, 1.39 and 1.29 mmol g -1 , respectively.The best adsorption capacity was obtained when the dosage of the carrier was 0.75 g.When the dosage of the carrier is too low, the accumulation of active sites led to the decrease of utilization rate of adsorption sites.When the adding amount is too high, the removal efficiency is also low, which may be due to the average adsorption sites in per gram CO 2 -MIPs decrease.

Effects of Operation Conditions
Fig. 9 shows the adsorption curves of CO 2 -MIPs for CO 2 at different temperatures.With the increase of temperature, the adsorption capacity of CO 2 on CO 2 -MIPs decreases gradually, indicating that the CO 2 adsorption is a weak interaction, which is significantly affected by temperature.The best CO 2 adsorption temperature of CO 2 -MIPs was achieved at 25°C.The reason is that the binding affinity of CO 2 -MIPs for CO 2 gradually decreases as the temperature increases, leading to that some adsorbed CO 2 was desorbed  from the adsorbent.Therefore, the temperature increase promotes CO 2 -MIPs to quickly reach the adsorption equilibrium with comparatively lower CO 2 adsorption saturation capacity.
In Fig. 10, the CO 2 adsorption rate depends considerably on CO 2 concentration.When the CO 2 concentrations in the flue gas are 1%, 4%, 7% and 10%, the CO 2 adsorption capacities of the CO 2 -MIPs are 2.52, 2.06, 1.71 and 1.39 mmol g -1 , respectively.The results showed that the lower concentration of CO 2 in the feed gas had the better adsorption effect.
Fig. 11 shows the adsorption curves of CO 2 -MIPs regenerated at different temperatures.When the regeneration temperature of CO 2 -MIPs was 120°C, the CO 2 adsorption capacity of the regenerated CO 2 -MIPs reached the maximum, indicating that the regenerated CO 2 -MIPs at 120°C exhibited the best reusability.From the above, the optimum desorption temperature is 120°C.Fig. 12 shows the variation of saturated adsorption capacity of CO 2 -MIPs during cyclic adsorption/desorption experiments.The saturated adsorption capacity of CO 2 -MIPs after first regeneration exhibits a slight decrease, which is due to the incomplete regeneration of the partial adsorption sites of CO 2 -MIPs at 120°C.In the subsequent cyclic experiments, the CO 2 adsorption capacity tends to be stable.After 5 cycles, the CO 2 adsorption capacity of the regenerated CO 2 -MIPs has a little decrease.The reason is that part of CO 2 adsorbed in the micropore of the adsorbent has strong chemical interaction with the adsorbent, which makes it difficult to desorb, resulting in the decrease of the adsorption capacity of CO 2 -MIPs.
Fig. 13 shows the CO 2 adsorption curves under the relative humidity of 70% in comparison with dry gas.The CO 2 adsorption capacities of CO 2 -MIPs in the presence and absence of H 2 O vapor were 1.40 and 1.44 mmol g -1 , respectively, illustrating that humidity has no remarkable    influence on the CO 2 adsorption capacity of CO 2 -MIPs.The results can be explained by the surface properties of CO 2 -MIPs and the mechanism of CO 2 adsorption.There are a large amount of amino active sites on the surface of CO 2 -MIPs, which play an important role in increasing the interaction between CO 2 and CO 2 -MIPs.In theory, amide groups can interact with water molecules.However, hydrogen bonds between water molecules weaken the interaction between amide groups and water molecules.In addition, CO 2 molecule is non-polar, and the hydrophobic group has little effect on the interaction between CO 2 molecule and amide group.Compared with the activated carbon aerogel amine adsorbent, when water is involved in the reaction, the carbamate formed by the reaction of carbon dioxide and amino acid will continue to react with carbon dioxide and water molecules, thus forming bicarbonate.The amino group in organic amines will also react with CO 2 and water molecules to form bicarbonate, which will increase the amount of CO 2 adsorption.

Selectivity
Under the optimal operating conditions, we compared the adsorption capacity of CO 2 -MIPs in the atmosphere of 10% CO 2 and simulated flue gas, including CO 2 (10%), N 2 (89.92%),SO 2 (350 mg m -3 ), NO (500 mg m -3 ).As shown in Fig. 14, the CO 2 adsorption capacity of CO 2 -MIPs in simulated flue gas is to some extent lower than that in N 2 diluted CO 2 , but the difference is within acceptable level.Apparently, the basic and acid groups loaded on the surface of carbon materials can actually uptake other gaseous species, and the porous structure of activated carbon is also conducive to other competitive adsorptions.

CONCLUSION
Porous carbon materials were prepared by using abandoned sunflower heads as a carbon source, and cheap CO 2 -MIPs were prepared by using sunflower-based activated carbon as carrier.The CO 2 adsorption performance (1) CO 2 adsorption performance of the carrier can be improved by changing the dosage of activator (KOH), and the optimum ratio of alkali to carbon was 0.75:1.(2) By changing dosage of the carrier in the preparation process of CO 2 -MIPs, we found the CO 2 -MIPs had the best adsorption capacity when the dosage of the carrier was 0.75 g. ( 3) The effect of temperature on the MIPs adsorbent is remarkable.With the increase of temperature, the adsorption capacity of CO 2 on adsorbents decreases gradually.25°C is the best adsorption temperature for CO 2 -MIPs.(4) The optimum desorption temperature was 120°C, and the CO 2 adsorption capacity of regenerated CO 2 -MIPs can reach 1.91 mmol g -1 .During the five adsorption/desorption cycles of CO 2 -MIPs, the adsorption capacity has a slight decrease.(5) While water vapor has no remarkable influence on CO 2 adsorption capacity of CO 2 -MIPs, the competitive adsorbates in simulated flue gas may to some extent weaken CO 2 adsorption.As a whole, CO 2 -MIPs developed in this work have ideal adsorption performance and are worth further studying and integrating into industrial treatment unit.
spectrometer (FT-IR).The thermal stability of the adsorbents was analyzed by using Netzschtg 209F3 thermogravimetric analyzer (TG) at the heating rate of 10°C min -1 in nitrogen atmosphere.Mac ASAP 2020 automatic physical adsorption analyzer was used to measure the specific surface area and pore size distribution of the MIPs with nitrogen.

Fig. 7 .
Fig. 7.The adsorption curves under different ratio of alkali to carbon.

Fig. 8 .
Fig. 8.The adsorption curves of CO 2 -MIPs prepared with different dosages of the carrier.

Fig. 10 .
Fig. 10.The adsorption curves obtained under different content of CO 2 .

Fig. 13 .
Fig. 13.The adsorption curves obtained with or without vapor.

Fig. 14 .
Fig. 14.The adsorption curves under the conditions of simulated flue gas.
of CO 2 -MIPs was investigated.The following conclusions were drawn:

Table 1 .
Comparison of different adsorbents.

Table 2 .
Operation conditions of experiments.