Preparation and CO2 adsorption properties of porous carbon by hydrothermal carbonization of tree leaves

Accepted Manuscript Title: Preparation and CO2 adsorption properties of porous carbon by hydrothermal carbonization of tree leaves Authors: Guangzhi Yang, Shen Song, Jing Li, Zhihong Tang, Jinyu Ye, Junhe Yang PII: DOI: Reference:

S1005-0302(18)30321-9 https://doi.org/10.1016/j.jmst.2018.11.019 JMST 1419

To appear in: Received date: Revised date: Accepted date:

8 August 2017 29 July 2018 12 August 2018

Please cite this article as: Yang G, Song S, Li J, Tang Z, Ye J, Yang J, Preparation and CO2 adsorption properties of porous carbon by hydrothermal carbonization of tree leaves, Journal of Materials Science and amp; Technology (2018), https://doi.org/10.1016/j.jmst.2018.11.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and CO2 adsorption properties of porous carbon by hydrothermal carbonization of tree leaves Guangzhi Yang 1,2,*, Shen Song 1, Jing Li 1, Zhihong Tang 1, Jinyu Ye 1, Junhe Yang 1,2,* 1

School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

2

Shanghai Innovation Institute for Materials, Shanghai 200444, China

IP T

[Receveid 8 August 2017; Received in revised form 29 July 2018; Accepted 12 August 2018] * Corresponding author.

SC R

E-mail address: [email protected] (G.Z. Yang). * Corresponding author.

N

U

E-mail address: [email protected] (J.H. Yang).

Porous carbon materials were prepared by hydrothermal carbonization (HTC) and KOH

A

activation of camphor leaves and camellia leaves. The morphology, pore structure,

M

chemical properties and CO2 capture ability of the porous carbon prepared from the two leaves were compared. The effect of HTC temperature on the structure and CO2

ED

adsorption properties was especially investigated. It was found that HTC temperature had a major effect on the structure of the product and the ability to capture CO2. The

PT

porous carbon materials prepared from camellia leaves at the HTC temperature of 240 °C had the highest proportion of microporous structure, the largest specific surface

CC E

area (up to 1823.77 m2/g) and the maximum CO2 adsorption capacity of 8.30 mmol/g at 25 °C under 0.4 MPa. For all prepared porous carbons, simulation results of isothermal

A

adsorption model showed that Langmuir isotherm model described the adsorption equilibrium data better than Freundlich isotherm model. For porous carbons prepared from camphor leaves, pseudo-first order kinetic model was well fitted with the experimental data. However, for porous carbons prepared from camellia leaves, both pseudo-first and pseudo-second order kinetics model adsorption behaviors were present. The porous carbon materials prepared from tree leaves provided a feasible option for CO2 capture with low cost, environmental friendship and high capture capability.

Key words: Porous carbon; Hydrothermal carbonization; KOH activation; CO2 adsorption 1.

Introduction Global warming has brought considerable harm to environment, and the main factor causing

global warming is greenhouse effect, to which the contribution of CO2 is about 55 %[1]. At present,

IP T

capture and storage technology is the main way to decrease the emission of CO 2. Because of its special physical and chemical properties, CO2 has a wide range of applications in agriculture,

SC R

industry, food, medicine and other fields. Therefore, the development of CO2 capture and storage technology not only has an important significance to reduce the greenhouse effect, but also provides a cheap and easy way to get raw materials for carbon chemistry.

U

At present, the main methods of capture CO2 are liquid absorption[2], membrane separation[3] and solid adsorption[4-7]. The advantage of liquid absorption is the good properties of capturing CO2,

N

but the disadvantage is that the corrosion of the liquid is great, and the energy required for the

A

regeneration of the solvent is large. Because the high cost of preparation of membrane materials, the

M

development of the membrane separation technique is limited. Solid adsorption technology has been widely recognized due to low energy consumption, good stability, flexible operation process

ED

and less corrosion to the equipment. Commonly used solid adsorbents include zeolites[4], mesoporous silica[5], metal-organic frameworks[6], porous polymer materials[7], and porous

PT

carbons[8]. Porous carbon materials have the advantages of light weight, large specific surface area, good chemical and thermal stability, low price and so on, so it has the greatest potential for

CC E

application.

Nowadays, many new porous carbon materials with controlled pore structure, porosity and

pore size have been synthesized by different methods[9,

10]

. Commonly used methods include

A

activation method[11], template method[12] and hydrothermal carbonization (HTC) method[13, 14]. Li et al.[15] prepared porous carbon microspheres directly from waste biomass by HTC technology combined with chemical activation. The prepared porous carbons showed perfect spherical structure under carefully controlled HTC conditions, whereas had large BET specific surface areas (reach up to 1683 and 2439 m2/g) and controllable porous structure. Islam et al.[16] obtained mesoporous activated carbon by the HTC of rattan furniture wastes followed by NaOH activation. The prepared mesoporous activated carbon with a high surface area of 1135 m2/g and an average pore size

distribution of 35.5 Å was an efficient adsorbent for treatment of synthetic dyes in wastewaters. Laginhas et al.[17] obtained activated carbons with high nitrogen content by a combination of HTC with activation. The prepared porous carbon materials had a nitrogen content around 8% (w/w) and a well-developed porous structure with BET surface area and pore volume up to 2130 m2/g and 1.12 cm3/g. In the past, a large number of biomass materials have been used as porous carbon precursors,

IP T

such as rattan[16], wheat straw[18], sawdust[19], palm seed[20], corn stalk[21], cellulose[22], cotton stalk[23], oak leaf[24] and so on. Among these wasted biomass, leaves are sustainable resources without destroying trees. Camphor tree has leaves all the year and is Shanghai’s “city tree”, which

SC R

is widely planted, as well as in many other cities. Camellia tree is evergreen in all seasons and is one of the important greening plants in southern China.

In this work, porous carbons were directly fabricated using camphor leaves and camellia leaves

U

as raw materials by carefully controlling the HTC conditions and subsequently chemical activation

N

with KOH. The morphology, pore structure, functional groups and elemental composition of the

A

porous carbon materials prepared from two different leaves were compared. In addition, the CO 2

M

adsorption properties of the two materials as adsorbents were evaluated under a pressure range of 0.1‒0.4 MPa at 25 °C. The HTC temperature, pore structure, CO2 adsorption properties and their

ED

correlations were investigated. The adsorption capacity and the adsorption dynamics were

2. Experimental

CC E

2.1. Materials

PT

investigated, and its possible adsorption isotherm model was proposed.

Camphor leaves and camellia leaves used in this study was obtained locally in University of

A

Shanghai for Science and Technology by collecting fallen leaves. The raw materials were washed several times with running water and then drenched in deionized water using ultrasonic vibration to remove impurities. After that, the samples were dried for 24 h at 105 °C, and then cut into pieces with sizes smaller than 5 mm. All other regents were commercially available and of analytical grade. 2.2. Synthesis of porous carbons

Fig. 1 illustrates the preparation process of porous carbons from two different leaves. Firstly, 3 g treated leaves were completely submerged into 60 ml of deionized water and sealed into an automated stainless-steel hydrothermal reactor with 100 ml volume, and then heated to a certain temperature for HTC for 5 h at 5 °C/min heating rate. After HTC, the reactor was cooled naturally to room temperature. The resulting hydrochars were then obtained as a solid residue after filtration and being washed repeatedly with deionized water and dried at 105 °C for 12 h. Secondly, the

IP T

hydrochars were mixed with KOH solid at a weight ratio of 1:3, stirred in an appropriate amount of deionized water for 12 h and then dried. After dryness, the mixture was placed in an alumina-crucible and heated to 800 °C at 3 °C/min under N2 atmosphere and hold at the

SC R

temperature for 1 h. After been cooled to room temperature, the samples were mixed with 10 wt% HCL solution of exceeded amount, stirred for 12 h, washed repeatedly with deionized water until the washing solution is neutral. Finally, the wet synthesized materials were dried and stored for

U

further use.

N

Five different HTC temperatures were studied, which were 180, 210, 240, 270 and 300 °C.

A

The hydrochars of camphor leaves were named as HTC-Cp-180, HTC-Cp-210, HTC-Cp-240,

M

HTC-Cp-270 and HTC-Cp-300. The final samples of camphor leaves were correspondingly designated as AHTC-Cp-180, AHTC-Cp-210, AHTC-Cp-240, AHTC-Cp-270 and AHTC-Cp-300

ED

according to the HTC temperature. The hydrochars of camellia leaves were named as HTC-Ce-180, HTC-Ce-210, HTC-Ce-240, HTC-Ce-270 and HTC-Ce-300. The final samples of camellia leaves

PT

were correspondingly designated as AHTC-Ce-180, AHTC-Ce-210, AHTC-Ce-240, AHTC-Ce-270 and AHTC-Ce-300 according to the HTC temperature.

CC E

2.3. Characterization

The morphologies and element composition were investigated using an FEI/Philips XL30

A

ESEM FEG field emission scanning electron microscopy (SEM) system equipped with an energy dispersive spectrometer (EDS). The weight loss curves were measured by a thermogravimetric analyzer (Q500 TGA, TA, USA). The surface area and pore volume were determined with a N2 adsorption-desorption isotherm measured at 77 K by using a physisorption analyzer (Micromeritics, Model ASAP 2020, USA) determined by Brunauer-Emmett-Teller (BET) method and t-plot method. The pore size distribution and pore volume were derived from the adsorption branch of isotherm by using the Density-Functional-Theory (DFT) model. Infrared spectra of porous carbons were

recorded on an FTIR spectroscopy (VERTEX. 70 Fourier transform infrared spectrometer, Bruker, USA). Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 F30 S-TWIN (FEI, US) with an accelerating voltage of 300 kV. 2.4. CO2 adsorption

IP T

2.4.1. Test of CO2 adsorption properties CO2 dynamic adsorption experiments were carried out in a fixed column adsorber, and the experimental device is shown in Fig. 2. The entire experimental device consists of a number of

SC R

control valves, a gas mixing module, an adsorption column and a real-time monitoring gas chromatograph.

In the adsorption stage, approximately 1 g of sample was added to the adsorption column for

U

adsorption test. Firstly, the sample in the adsorption column was pretreatment , that is, the sample

N

was implemented at 200 °C with a 25 ml/min feeding rate of He for 2 h for desorption. Secondly,

A

when the temperature in the adsorption column was cooled to the target temperature, the feed gas

M

was switched from He to CO2/He mixture (CO2 volumetric concentration=10%, gas flow rate=25 ml/min) for CO2 adsorption, the concentration of CO2 in the outlet was measured by gas

ED

chromatograph.

CC E

PT

The CO2 adsorption capacity, Q (mmol/g) of samples is calculated as follows: 𝑄=

𝑃𝑉𝑡 𝑚𝑅𝑇

(1)

𝑡

𝑉𝑡 = ∫ 𝐹𝐶con d𝑡

(2)

0

where, P (Pa) is the pressure during the experiment, Vt (ml) is the adsorption volume of each

A

experiment cycle, m (g) is the mass of the adsorbent, R (kPa L/(K mol)) is the ideal gas constant, T (K) is the temperature, t (min) is the time of each experiment cycle, F (ml/min) is the gas flow rate and Ccon is the concentration ratio of the CO2. 2.4.2. Analysis of CO2 adsorption properties

In order to study the dynamic adsorption properties of CO2 better, to choose the pseudo-first order kinetic model and pseudo-second order kinetic model can best describe dynamic adsorption process. The pseudo-first order kinetic equation is expressed as: 𝑞𝑡 = 𝑞e (1 − e−𝑘1𝑡 )

(3)

K1 (min-1) is the kinetic constant of the pseudo-first order kinetic. The pseudo-second order kinetic equation is expressed as [25]: 𝑞e 2 𝑘2𝑡 1 + 𝑞e 𝑘2 𝑡

SC R

𝑞𝑡 =

IP T

where qt and qe (mmol/g) are adsorption capacities at time t (min) and at equilibrium, respectively.

(4)

where k2 (g/(mmol min)) is the kinetic constant of the pseudo-first order kinetic.

U

For the experimental data after equilibrium, the mathematical model can be used to fit, so the

N

nonlinear Langmuir and Freundlich adsorption models are used to evaluate the properties of the adsorbed CO2 molecules. The Langmuir isothermal model describes the uniform adsorption process

A

of monolayers, the equation is:

𝑞m 𝐾L 𝐶e 1 + 𝐾L 𝐶e

M

𝑞e =

(5)

ED

where qm (mmol/g) and KL are the constants of Langmuir relative to the adsorption and adsorption rate, respectively.

CC E

The equation is:

PT

The Freundlich isothermal model describes the multi-layer inhomogeneous adsorption process.

⁄𝑛

𝑞e = 𝐾F 𝐶e1

(6)

where KF and n are Freundlich constants, KF represents adsorption capacity, and 1/n and n represent

A

the curvature and intensity of adsorption. The parameters and correlation coefficients of the entire isothermal adsorption models were

obtained by means of OriginPro 8 for nonlinear fitting. 3. Results and Discussion 3.1. Characterization of materials

3.1.1. Morphological analysis Fig. 3 shows the SEM images of the hydrochar of camphor leaves and camellia leaves. It is clear that a large number of carbon microspheres with a diameter of 0.9‒7.8 μm appear in the hydrochar of camellia leaves (Fig. 3(d‒f)) compared with that of camphor leaves (Fig. 3(a‒c)). Camellia leave cells contain a lot of cellulose, hemicellulose and lignin. These carbohydrate polymers are composed of monosaccharide, hexose, and acidic polysaccharides. In the process of

IP T

HTC, these basic units formed micron-sized spherical particles after dehydration, decarboxylation, condensation and polymerization[26].

SC R

Fig. 4 shows SEM images of porous carbons prepared from camphor leaves and camellia leaves. It clearly demonstrates the formation of randomly distributed pores and cavities. The different pore sizes and shapes between the particles can be found on the surface of the sample,

U

which may be caused by the pyrolysis of hydrochar of tree leaves and the following reaction during

N

activation.

(7)

K2O + C → 2K + CO

(8)

A

4KOH + C → K2CO3 + K2O + 3H2

M

K2CO3 +2C → 2K+3CO

(9)

ED

In contrast, it can be seen that the porous carbons prepared from the same kind of leaves have different amounts of vias and different sizes of fragments at different HTC temperatures, indicating that the temperature of the HTC greatly affects the cell wall of the hemicellulose, cellulose and

PT

lignin components, which also alter the surface properties of the adsorbent [20]. The morphology of the different samples has irregularly shaped particles with large concave- convex cavity and smooth

CC E

surfaces. By comparing Fig. 4 with Fig. 3, it can be found that obvious morphological changes occurred during the activation process. And it also shows that the porous carbon materials prepared from camellia leaves would not retain their hydrochar spherical morphology after activation[27]. The

A

TEM images of AHTC-Ce-240 are shown in Fig. 5, which shows the presence of irregular holes with sponge-like morphology, demonstrating that there is a large amount of uniform micropores (points in Fig. 5 (b)). 3.1.2. Composition and structure analysis

For further investigation, element analysis of samples by EDS is shown in Table 1. It can be clearly seen from Table 1 that camellia leaves have higher carbon content and lower O/C atomic ratio than camphor leaves. Compared with the leaves and the corresponding activated carbon, the O/C atomic ratio of activated carbon prepared by camphor leaves increase, indicating that C lost more than O in the process of carbonization of camphor leaves and the degree of carbonization of camellia leaves was higher[28]. For the porous carbons prepared by the same kind of leaves, the

IP T

carbon content tends to rise first and then fluctuate between 240 °C and 300 °C as the hydrothermal temperature increases. The O/C is decreased as the hydrothermal temperature increases. The reason may be that more oxidative compounds such as CO and CO2 were further released with

SC R

carbonization at higher temperature[27].

For further study the composition of raw material, the camphor leaves and camellia leaves were characterized by thermogravimetric analysis as shown in Fig. 6. When the temperature is

U

below 160 °C, camphor leaves and camellia leaves have a small amount of weight loss, which may

N

be caused by oil substances pyrolysis. When the temperature is between 160 °C and 300 °C, the TG

A

of camellia leaves curve shows a decreasing trend. The DTG curve of camellia leaves first appears a

M

shoulder peak around 240 °C, and then there is a weight loss peak around 300 °C. The reason is that when the temperature is greater than 160 °C, the hemicellulose begins to decompose, then when the

ED

temperature is higher than 240 °C, the cellulose begins to hydrolyze[29]. However, compared with camphor leaves, it can be found that at 300 °C camellia leaves have a larger amount and faster rate

PT

of weight loss. This may be caused by the different composition in the raw material, and the camellia leaves have more cellulose and hemicellulose[30]. When the temperature is higher than

CC E

300 °C, in the DTG curve of camellia leaves, a shoulder peak around 340 °C appears, followed by an obvious weight loss peak around 380 °C, but the camphor leaves are relatively gentle weight loss. This may be caused by the end of the decomposition of cellulose and hemicellulose, and then lignin

A

begins a lot amount of decomposition. When the temperature is higher than 400 °C, the weight loss rate of the two raw materials gradually decreases, and the carbon material is further decomposed. Combined with the SEM picture of hydrothermal products, the difference in cellulose, hemicellulose and lignin content has an important influence on the morphology and structure of hydrothermal carbonization products. The adsorption isotherm of all the samples exhibited a combined Type 1 and Type 4 according to the IUPAC (International Union of Pure and Applied Chemistry). Type 1 isotherm with a narrow

knee at low relative pressure (P/P0 < 0.1) can be associated with microporous structure, while Type 4 isotherm with a hysteresis loop at relative pressure around 0.8 is usually associated with the filling and emptying of mesoporous by capillary condensation. The nitrogen adsorption/desorption isotherms of the porous carbons at -196 °C are shown in Fig. 7. The BET surface area, pore volume and average pore size of the prepared samples are listed in Table 2. It can be seen from Fig. 7 that the adsorption isotherms of all samples are combined with Type 1 and Type 4, that is to say, the

IP T

porosity of the sample is mainly composed of micropores and mesoporous structures. In the range of low relative pressure (P/P0 < 0.1), microporous pores dominate the pore size distribution as the isotherm rises sharply. With the increase of temperature from 180 to 240 °C, the BET surface area

SC R

increases. However, as the temperature increases from 240 to 300 °C, the specific surface area gradually decreases, and carbon content changes in the same law (Table 1), probably due to the content of carbon that affects the chemical activation process[31]. Therefore, AHTC-Cp-240 and

U

AHTC-Ce-240 show the largest specific surface area and highest total pore volume of all the carbon

N

samples, suggesting that the HTC treatment at 240 °C followed by chemical activation with KOH at

A

800 °C is the optimized experimental conditions. The porous carbons prepared from camellia leaves

M

have higher specific surface area, total pore volume, micropore surface area and micropore pore volume compared with that of the camphor leaves. The reason is that the HTC process produces

ED

special rough surfaces, functional groups and carbon contents, which affects subsequent activation reactions to increase surface area and porosity[32].

PT

Qualitative analysis of functional groups of the porous carbons was carried out by FTIR spectroscopy as shown in Fig. 8. In the peak of the fingerprint band, the vibration absorption peak

CC E

of C=O at 1700 cm-1 is wide, indicating the presence of acid, ketone and aldehydes. The absorption peak at 1600 cm-1 reveals the presence of aromatic C=C with a benzene skeleton. The peak at 1400 cm-1 illustrates the presence of C=C conjugated olefins. The peak at 1047 cm-1 is due to the C-O

A

stretching vibration of cellulose and hemicellulose. A C-N stretching vibration peak appears at 1000 cm-1. Deformation vibration peak of aromatic hydrocarbon C-H in mono substituted benzene ring at 886 cm-1 and 802 cm-1. All of these peaks correspond to the functional groups present in each sample. As the hydrothermal temperature increases, additional peaks appear at 3600 cm -1 and 2896 cm-1 from AHTC-Ce-240, which corresponds to the -OH of carboxylic acids and the C-H contraction of alkanes. The band near 2896 cm-1 may be caused by the C-H stretching of CH2 and CH3 from the cellulose-hydrolyzed oligomers[33].

3.2. CO2 adsorption 3.2.1. Adsorption pressure study The CO2 adsorption capacity (qe) of porous carbons were studied at 25 °C under the pressure of 0.1 MPa, as shown in Fig. 9. For the porous carbons prepared from two kinds of raw materials, with the raise of HTC temperature from 180 °C to 240 °C, the amount of adsorbed CO2 increases to

IP T

the maximum, mainly due to the increase in pore volume and specific surface area. However, when the temperature of HTC continues to rise (270‒300 °C), the adsorption capacity decreases, mostly

SC R

owing to the decrease in pore volume and specific surface area, and the decrease in oxygen-containing functional groups on the surface of the carbon material. As shown in Table 3, with the increase of pressure, the adsorption capacity of each sample increases gradually. Based on

U

the preliminary study of the material, the different adsorption capacity is mainly due to the difference in the number and type of functional groups of porous carbons (Fig. 8), as well as the

N

difference in specific surface area, micropores and total volume (Table 2). Therefore,

A

AHTC-Ce-240, which has the largest number of functional groups, the largest microporous

M

distribution, surface area and total pore volume, shows the highest adsorption capacity of CO2 in all

ED

samples. 3.2.2. Dynamic adsorption study

PT

Fig. 10 shows the dynamic adsorption curves and kinetic model curves of the porous carbon prepared from camphor leaves at different pressures. It is clear that the pseudo-second order model

CC E

underestimated the CO2 uptake up to 60 min, after which it continuously overestimated CO2 uptake until the process approached equilibrium. In addition, pseudo-first order kinetic model is in good fitted with the experimental data during the whole experiment process. At other temperatures, the

A

porous carbons prepared from camphor leaves conform to the same law. Fig. 11 shows the dynamic adsorption curves and kinetic model curves of the porous carbon

prepared from camellia leaves at different pressures and temperature. It can be found that the five kinds of porous carbon materials can rapidly adsorb to 95% of the saturated adsorption capacity in 10 min under the pressure of 0.1 MPa and 0.2 MPa, and then reach the saturated value of adsorption. When the pressure is 0.3 MPa and 0.4 MPa, the five kinds of porous carbon materials can adsorb to

95% of the saturated adsorption capacity in about 40 min, then reach the saturated saturation value. It is clear that the adsorption isotherms become steep as the pressure increases, the adsorption equilibrium time is longer, and the effect of CO2 adsorption is different due to the different porosity[8]. The pseudo-second order adsorption kinetic model is lower than the experimental adsorption data before 60 min, and thereafter it is higher than the experimental adsorption data and finally approached. Tables 4‒8 show the kinetic parameters of the prepared carbon materials at

IP T

different pressures. It can be found that the pseudo-first order kinetic model can be more close to the experimental data when the porous carbon materials were prepared at relatively low HTC temperature (AHTC-Ce-180/210), based on the higher R2 value and the lower error value. It

SC R

explains that the pseudo-first order kinetic model is better than the pseudo-second order kinetic model to express the adsorption experiment of CO2 experiment. When the porous carbon materials prepared at a relatively high HTC temperature (AHTC-Ce-240/270/300), under low pressure (0.1‒

U

0.2 MPa), pseudo-first order kinetic model is closer to the experimental data, indicating that the rate

N

of CO2 diffusion to the adsorbent surface is faster than that of the surface chemical reaction. While

A

in the high-pressure conditions, the pseudo-second order kinetic model is closer to the experimental

M

data, based on the higher R2 value and lower error value. It explains that the pseudo-second order kinetic model is better than the pseudo-first order kinetic model to express the adsorption

ED

experiment of CO2 experiment. The reason is that as the hydrothermal temperature increases, the number of functional groups on the surface increases (Fig. 8), and while the adsorption process is

PT

under high-pressure conditions, the rich surface functional groups can promote the adsorption of CO2.

CC E

3.2.3. Isothermal adsorption study The CO2 adsorption data (at 25 °C under 0.4 MPa), the Langmuir and the Freundlich model

A

predictive equilibrium curves are shown in Figs. 12 and 13. Comparing Fig. 13 with 12, Langmuir model can be found to be closer to the actual experimental results than Freundlich model. Table 9 shows the parameters of Langmuir model and Freundlich model. It can be found that the correlation coefficient (R2) of Langmuir adsorption model is larger than that of Freundlich adsorption isotherm model, respectively, which shows that Langmuir model can describe the experimental results better. Therefore, Langmuir model can better describe the isothermal adsorption process, and the

parameter qm represents the maximum single layer adsorption capacity, together with the surface area and porosity, indicating that the mechanism of CO2 absorption is mainly physical adsorption. 4. Conclusion In this study, major microporous leaf-based porous carbons were successfully prepared by combining HTC and KOH activation, also proved to be a promising material for CO2 capture. The

IP T

morphology, pore structure, functional groups and elemental composition of the porous carbon materials prepared from two different leaves were compared. The effect of HTC temperature on the

SC R

prepared products was investigated, and the temperature was found to be of great influence. The AHTC-Ce-240 exhibited the largest specific surface area (up to 1823.77 m2/g), the highest total pore volume (1.07 cm3/g) and mircroporsity (0.70 cm3/g). And for CO2 adsorption, AHTC-Ce-240

U

had a high capacity of 8.30 mmol/g at 25 °C under the pressure of 0.4 MPa. For the porous carbons prepared from camphor leaves and camellia leaves, Langmuir isotherm model described the

N

equilibrium data better than Freundlich isotherm model, and the main mechanism of CO2 absorption

A

is physical adsorption. For the porous carbons prepared from camphor leaves, pseudo-first order

M

kinetic model was in good fitted with the experimental data. However, for the products prepared from camellia leaves, both pseudo-first and pseudo-second order kinetics model adsorption

ED

behaviors were present. Overall, porous carbon materials derived from tree leaves have the advantages of low cost, long-term stability of CO2 capture and easy regeneration. Thus, they may

PT

have a potential use in carbon capture and storage.

CC E

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China

(Nos. U1760119, 51472160 and U1560108), the Shanghai Nature Science Foundation (No.

A

16ZR1423400) and the Science and Technology Commission of Shanghai Municipality (Nos. 15JC1490700 and 16JC1402200).

References [1] A.A. Lacis, G.A. Schmidt, D. Rind, R.A. Ruedy, Science 330 (2010) 356-359. [2] X. Luo, Y. Guo, F. Ding, H. Zhao, G. Cui, H. Li, C. Wang, Angew. Chem. Int. Ed. 53 (2014)

7053-7057. [3] X. Gao, X. Zou, H. Ma, S. Meng, G. Zhu, Adv. Mater. 26 (2014) 3644-3648. [4] T.D. Pham, M.R. Hudson, C.M. Brown, R.F. Lobo, Chemsuschem 7 (2015) 3031-3038. [5] M.A. Alkhabbaz, P. Bollini, G.S. Foo, C. Sievers, C.W. Jones, J. Am. Chem. Soc. 136 (2014) 13170-13173. [6] R.L. Siegelman, T.M. Mcdonald, M.I. Gonzalez, J.D. Martell, P.J. Milner, J.A. Mason, A.H.

IP T

Berger, A.S. Bhown, J.R. Long, J. Am. Chem. Soc. 139 (2017) 10526- 10538. [7] J.W. Yoon, T.U. Yoon, E.J. Kim, A.R. Kim, T.S. Jung, S.S. Han, Y.S. Bae, J. Hazard. Mater. 341 (2017) 321-327.

SC R

[8] W. Hao, E. Björkman, M. Lilliestråle, N. Hedin, Appl. Energy 112 (2013) 526-532.

[9] X. Huang, X. Zhan, C. Wen, F. Xu, L. Luo, J. Mater. Sci. Technol. 34 (2018) 855-863. [10] K. Wei, K.O. Kim, K.H. Song, C.Y. Kang, J.S. Lee, M. Gopiraman, I.S. Kim, J. Mater. Sci.

U

Technol. 33 (2017) 424-431.

N

[11] M. Shoaib, H.M. Al-Swaidan, Asi J. Chem. 26 (2014) 5295-5297.

A

[12] Y.Q. Shi, J. Zhu, X.Q. Liu, J.C. Geng, L.B. Sun, ACS Appl. Mater. Interfaces 6 (2014)

M

20340-20349.

[13] B. Ledesma, M. Olivares-Marín, A. Álvarez-Murillo, S. Roman, J.M.V. Nabais, J. Supercrit.

ED

Fluids 138 (2018) 187-192.

[14] M.T. Reza, S.R. Poulson, S. Román, C.J. Coronella, J. Anal. Appl. Pyrolysis, 131 (2018)

PT

85-92.

[15] K. Li, S. Liu, T. Shu, L. Yan, H. Guo, Y. Dai, X. Luo, S. Luo, Mater. Chem. Phys. 181 (2016)

CC E

518-528.

[16] M.A. Islam, M.J. Ahmed, W.A. Khanday, M. Asif, B.H. Hameed, Ecotoxicol. Environ. Saf. 138 (2017) 279-285.

A

[17] C. Laginhas, J.M.V. Nabais, M.M. Titirici, Microporous Mesoporous Mater. 226 (2016) 125-132.

[18] H. Mao, D. Zhou, Z. Hashisho, S. Wang, H. Chen, H. Wang, M.J. Lashaki, RSC Adv. 5 (2015) 36051-36058. [19] K. Malwade, D. Lataye, V. Mhaisalkar, S. Kurwadkar, D. Ramirez, Int. J. Environ. Sci. Technol. 13 (2016) 2107-2116. [20] M.A. Islam, I.A.W. Tan, A. Benhouria, M. Asif, B.H. Hameed, Chem. Eng. J. 270 (2015)

187-195. [21] Y. Li, Y. Li, L. Li, X. Shi, Z. Wang, Adv. Powder Technol. 27 (2016) 684-691. [22] Suhas, V.K. Gupta, P.J.M. Carrott, R. Singh, M. Chaudhary, S. Kushwaha, Bioresour. Technol. 216 (2016) 1066-1076. [23] A.N.A. Elhendawy, A.J. Alexander, R.J. Andrews, G. Forrest, J. Anal. Appl. Pyrolysis 82 (2008) 272-278.

IP T

[24] B. Hu, S.H. Yu, K. Wang, L. Liu, X.W. Xu, Dalton Trans. 40 (2008) 5414-5423. [25] Y.S. Ho, G. Mckay, Process. Biochem. 34 (1999) 451-465.

[26] M.M. Titirici, R.J. White, C. Falco, M. Sevilla, Energy Environ. Sci. 5 (2012) 6796-6822.

SC R

[27] M. Sevilla, A.B. Fuertes, R. Mokaya, Energy Environ. Sci. 4 (2011) 1400-1410.

[28] Y. Gao, H.P. Chen, J. Wang, T. Shi, H.P. Yang, X.H. Wang, J. Fuel Chem. Technol. 39 (2011) 893-900.

U

[29] Y. Wang, H.H. Wang, F. Zhu, J. Zhan, Sci. Silvae Sin. 48 (2012) 98-106.

N

[30] E. Mészáros, E. Jakab, G. Várhegyi, P. Tóvári, J. Therm. Anal. Calorim. 88 (2007) 477-482.

A

[31] J. Wang, S. Kaskel, J. Mater. Chem. 22 (2012) 23710-23725.

M

[32] I.A.W. Tan, A.L. Ahmad, B.H. Hameed, J. Hazard. Mater. 153 (2008) 709-717. [33] S.H. Jung, S. Kim, A.Y. Chung, H.T. Kim, J.H. So, J. Ryu, H.C. Park, C.H. Kim, Appl. Clay

A

CC E

PT

ED

Sci. 95 (2014) 60-66.

Table list: Table 1 Element analysis of samples by EDS C (at.%)

N (at.%)

O (at.%)

O/C

Camphor leaves

71.31

11.82

17.71

0.25

Camellia leaves

89.78

1.62

8.60

0.10

AHTC-Cp-180

60.10

10.49

29.41

0.49

AHTC-Cp-210

63.62

15.47

20.91

0.33

AHTC-Cp-240

65.56

13.98

20.40

0.31

AHTC-Cp-270

66.11

14.23

19.62

0.30

AHTC-Cp-300

62.35

16.75

20.90

0.34

AHTC-Ce-180

80.11

2.61

17.30

0.22

AHTC-Ce-210

87.59

1.26

11.16

0.13

AHTC-Ce-240

90.53

2.15

7.32

0.08

AHTC-Ce-270

89.99

2.87

7.14

0.08

AHTC-Ce-300

92.14

2.35

5.51

0.06

M

A

N

U

SC R

IP T

Samples

Table 2 BET characteristics of the prepared porous carbons SBET (m2/g)a 514.98 773.63 1633.71 1322.58 1350.79 774.81 1050.29 1823.77 1482.09 1175.43

ED

Samples

Vtotal (cm3/g)b 0.37 0.50 0.98 0.90 0.79 0.99 1.07 1.07 0.88 0.67

Smicro (m2/g)c 192.65 601.72 1083.57 1061.31 1128.14 266.53 389.79 1307.43 959.99 738.74

Vmicro (cm3/g)c 0.11 0.32 0.58 0.56 0.59 0.14 0.21 0.70 0.51 0.40

Dp (nm)d 2.84 2.59 2.41 2.71 2.35 5.05 4.07 2.17 2.36 2.29

A

CC E

PT

AHTC-Cp-180 AHTC-Cp-210 AHTC-Cp-240 AHTC-Cp-270 AHTC-Cp-300 AHTC-Ce-180 AHTC-Ce-210 AHTC-Ce-240 AHTC-Ce-270 AHTC-Ce-300 Notes: a BET surface area b Total pore volume measure at P/P0 = 0.99. c Micropore surface area and micropore volume estimated d < 2 nm by DFT method d Average pore diameter

Table 3 CO2 adsorption properties (mmol/g) of the prepared porous carbon materials 0.2

0.3

0.4

0.12 0.33 0.80 0.48 0.33 0.18 0.56 0.92 0.68 0.53

1.24 1.43 2.19 1.51 1.22 1.61 1.89 2.44 1.8 1.63

1.94 2.26 4.05 3.25 2.64 2.76 3.77 6.77 5.96 5.02

2.45 2.68 6.63 4.09 3.50 3.05 6.19 8.3 7.12 6.51

IP T

0.1

SC R

CO2 pressure (MPa) AHTC-Cp-180 AHTC-Cp-210 AHTC-Cp-240 AHTC-Cp-270 AHTC-Cp-300 AHTC-Ce-180 AHTC-Ce-210 AHTC-Ce-240 AHTC-Ce-270 AHTC-Ce-300

Table 4 Adsorption kinetic parameters of AHTC-Ce-180 at different pressures

k1 (min ) qe (mmol/g) R2 Error (%) k2 (g/(mmol min)) qe (mmol/g) R2 Error%

M

Pseudofirst order

0.1 0.9098 0.18 0.9981 0.00 0.9176 0.18 0.9717 0.00

1.65 0.9748 0.38

3.13 0.9498 0.88

2.82 0.9436 0.96

CC E

PT

ED

Pseudosecond order

0.4 0.4430 3.3 0.9926 0.42 0.2900

U

-1

Pressure （MPa） 0.2 0.3 0.5924 0.6378 1.61 2.76 0.9922 0.9920 0.17 0.30 0.7919 0.5136

N

Parameters

A

Kinetic model

Table 5 Adsorption kinetic parameters of AHTC-Ce-210 at different pressures

A

Kinetic model Pseudofirst order Pseudosecond order

Parameters -1

k1 (min ) qe (mmol/g) R2 Error (%) k2 (g/(mmol min)) qe (mmol/g) R2 Error%

0.1 0.5374 0.56 0.9889 0.08

Pressure （MPa） 0.2 0.3 0.6046 0.5027 1.90 3.77 0.9878 0.9945 0.27 0.38

0.4 0.2586 6.06 0.9906 1.37

0.9176

0.7919

0.5136

0.2900

0.58 0.9240 0.25

1.94 0.9187 0.84

3.86 0.9392 1.55

6.38 0.9537 2.19

Table 6 Adsorption kinetic parameters of AHTC-Ce-240 at different pressures

-1

k1(min ) qe (mmol/g) R2 Error (%) k2 (g/(mmol min)) qe (mmol/g) R2 Error (%)

Pseudofirst order Pseudosecond order

0.1 0.8446 0.91 0.9909 0.12 1.8901

Pressure （MPa） 0.2 0.3 1.0324 0.4003 2.36 6.62 0.9980 0.9333 0.13 2.40 1.0017 0.1064

0.4 0.2580 8.19 0.9754 2.17 0.0531

0.93 0.9278 0.28

2.39 0.9706 0.45

8.61 0.9798 2.71

6.87 0.9937 0.94

IP T

Parameters

SC R

Kinetic model

-1

k1 (min ) qe (mmol/g) R2 Error (%) k2 (g/(mmol min)) qe (mmol/g) R2 Error (%)

0.4 0.4939 7.00 0.9695 1.62 0.1337

0.68 0.9278 0.55

1.84 0.9768 0.54

7.21 0.9917 1.05

M

Pseudofirst order

0.1 0.2107 0.64 0.9723 0.46 0.4862

Pressure （MPa） 0.2 0.3 0.3487 0.3881 1.77 5.55 0.9803 0.9161 0.38 2.28 0.3484 0.1225

N

Parameters

A

Kinetic model

U

Table 7 Adsorption kinetic parameters of AHTC-Ce-270 at different pressures

CC E

PT

ED

Pseudosecond order

5.77 0.9941 0.78

Table 8 Adsorption kinetic parameters of AHTC-Ce-300 at different pressures

A

Kinetic model Pseudofirst order Pseudosecond order

Parameters -1

k1 (min ) qe (mmol/g) R2 Error (%) k2 (g/(mmol min)) qe (mmol/g) R2 Error (%)

0.1 0.1127 0.48 0.9414 0.44 0.3355

Pressure （MPa） 0.2 0.3 0.1993 0.2300 2.26 5.03 0.9350 0.9170 1.53 3.86 0.1366 0.0767

0.53 0.8218 0.47

2.42 0.7848 1.90

5.31 0.9840 1.21

0.4 0.1971 6.43 0.9726 3.03 0.0482 6.85 0.9843 1.57

Table 9 Parameters of Langmuir model and Freundlich model Langmuir R2

KF

n

R2

0.04 0.00 0.04 0.03 0.01 0.05 0.00 0.02 0.01 0.01

0.9812 0.9999 0.9423 0.9787 0.9920 0.9994 0.9999 0.9470 0.9590 0.9732

0.45 0.61 1.01 0.64 0.48 0.65 0.57 1.11 0.82 0.69

1.26 1.11 1.40 1.37 1.46 1.18 1.72 1.49 1.60 1.65

0.9151 0.9278 0.9249 0.9598 0.9717 0.8625 0.9999 0.9246 0.9189 0.9353

N A M ED PT CC E A

IP T

KL

U

AHTC-Cp-180 AHTC-Cp-210 AHTC-Cp-240 AHTC-Cp-270 AHTC-Cp-300 AHTC-Ce-180 AHTC-Ce-210 AHTC-Ce-240 AHTC-Ce-270 AHTC-Ce-300

qm (mmol/g) 2.73 3.13 7.86 5.71 5.05 3.22 9.01 9.39 7.78 7.35

SC R

Samples

Freundlich

IP T

Figure list:

M

A

N

U

SC R

Fig. 1. Schematic of porous carbons preparation

A

CC E

PT

ED

Fig. 2. Schematic diagram of the CO2 adsorption instrument

IP T SC R U N A M

Fig.3. SEM images of the hydrochars: HTC-Cp-180 (a), HTC-Cp-240 (b), HTC-Cp-300 (c),

A

CC E

PT

ED

HTC-Ce-180 (d), HTC-Ce-240 (e), HTC-Ce-300 (f)

IP T SC R U N A M

Fig.4. SEM images of the synthesized porous carbons: AHTC-Cp-180 (a), AHTC-Cp-240 (b),

CC E

PT

ED

AHTC-Cp-300 (c), AHTC-Ce-180 (d), AHTC-Ce-240 (e) and AHTC-Ce-300 (f)

A

Fig.5. TEM images of AHTC-Ce-240: (a) 20 nm, (b) 5 nm

M

A

N

U

SC R

IP T

Fig.6. TG/DTG curve of the camphor leaves and camellia leaves

A

CC E

PT

porous carbons at -196 °C

ED

Fig.7. N2 adsorption-desorption isotherms (a, b) and pore size distribution(c, d) of the prepared

Fig.8. FTIR spectra of AHTC-Ce-180 (a), AHTC-Ce-210 (b), AHTC-Ce-240 (c), AHTC-Ce-270 (d) and AHTC-Ce-300 (e)

SC R

IP T

Fig.9. CO2 adsorption properties of the prepared porous carbons materials at 0.1 MPa and 25 °C

A

CC E

PT

ED

M

A

N

U

Fig.10. Kinetic model and actual dynamic adsorption curves of AHTC-Cp-240

Fig.11. Kinetic model and actual dynamic adsorption curves of the porous carbons prepared from camellia leaves at different pressures

SC R

IP T

Fig.12. CO2 adsorption data and the Langmuir model predictive equilibrium curves

A

CC E

PT

ED

M

A

N

U

Fig.13. CO2 adsorption data and Freundlich model predictive equilibrium curves