Green and facile synthesis of hierarchically porous carbon monoliths via surface self-assembly on sugarcane bagasse scaffold: Influence of mesoporosity on efficiency of dye adsorption

Journal Pre-proof Green and facile synthesis of hierarchically porous carbon monoliths via surface selfassembly on sugarcane bagasse scaffold: Influence of mesoporosity on efficiency of dye adsorption Ratchadaporn Kueasook, Natthanan Rattanachueskul, Narong Chanlek, Decha Dechtrirat, Waralee Watcharin, Pongsaton Amornpitoksuk, Laemthong Chuenchom PII:

S1387-1811(20)30008-1

DOI:

https://doi.org/10.1016/j.micromeso.2020.110005

Reference:

MICMAT 110005

To appear in:

Microporous and Mesoporous Materials

Received Date: 8 September 2019 Revised Date:

29 November 2019

Accepted Date: 6 January 2020

Please cite this article as: R. Kueasook, N. Rattanachueskul, N. Chanlek, D. Dechtrirat, W. Watcharin, P. Amornpitoksuk, L. Chuenchom, Green and facile synthesis of hierarchically porous carbon monoliths via surface self-assembly on sugarcane bagasse scaffold: Influence of mesoporosity on efficiency of dye adsorption, Microporous and Mesoporous Materials (2020), doi: https://doi.org/10.1016/ j.micromeso.2020.110005. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Inc.

Graphical abstract

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Green and facile synthesis of hierarchically porous carbon monoliths via surface self-assembly on sugarcane bagasse scaffold: Influence of mesoporosity on efficiency of dye adsorption

Ratchadaporn Kueasooka, Natthanan Rattanachueskula, Narong Chanlekb, Decha Dechtriratc,d,e, Waralee Watcharinf, Pongsaton Amornpitoksuka, Laemthong Chuenchoma*

a

Department of Chemistry and Center for Excellence for Innovation in Chemistry, Faculty of

Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand b

Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang,

Nakhon Ratchasima 30000, Thailand c

Department of Material Science, Faculty of Science, Kasetsart University, Bangkok 10900,

Thailand d

Specialized Center of Rubber and Polymer Materials for Agriculture and Industry (RPM),

Faculty of Science, Kasetsart University, Bangkok 10900, Thailand e

f

Laboratory of Organic Synthesis, Chulabhorn Research Institute, Bangkok 10210, Thailand

Faculty of Biotechnology (Agro-Industry), Assumption University, Hua Mak Campus, Bangkok

10240, Thailand

*Corresponding author. Tel.: +66 74 288416; fax: +66 74 558841. Email address: [email protected] (L. Chuenchom)

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Abstract Hierarchically macro- and mesoporous carbon materials (HPCs) were prepared from environmentally friendly phloroglucinol/glyoxylic acid precursors with soft-template F127 using sugarcane bagasse as scaffold via self-assembly surface coating. The sugarcane bagasse scaffold provided macroporosity. Effects of carbon precursor/template ratio as well as slight modification using air treatment on the physicochemical properties of the carbon monoliths were systematically studied and samples were fully characterized. The resulting monoliths prepared using our method exhibited a stable monolithic feature with a hierarchically porous structure and BET surface areas as high as ∼500-645 m2/g, total pore volume 0.2-0.4 cm2/g, mesoporosity contribution as large as 50-60% volume, and mesopore sizes of ∼5 nm. Carbon materials prepared without F127 showed BET surface area and total pore volumes comparable to those of the HPCs but they exhibit much lower degree of mesoporosity (<20% volume). Regardless of the similar chemical properties of all samples, the difference in pore texture largely influenced the adsorption performance of dyes. The selected HPC sample showed the best performance in adsorption of methylene blue dye (MB) with the maximum capacity of ∼100 mg/g, outperforming or being comparable to the monolithic adsorbents reported in literature. Moreover, the monolith shape could be well retained even after complete adsorption. The MB adsorption capacity of the adsorbents is perfectly linearly correlated with the mesoporosity, stressing the importance of mesopores in dye adsorption. The combination of environmentally friendly carbon precursors and sugarcane bagasse as starting materials can efficiently lead to novel monolith carbon adsorbents avoiding the use of toxic chemicals.

Keywords: Hierarchically porous carbon monolith; Mesoporosity; Sugarcane bagasse; Selfassembly; Bulky dyes; Adsorption

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1. Introduction Methylene blue (MB), methyl orange (MO) and rhodamine B (RhB) are among the most widely used synthetic dyes in textile, paper, paint and tanning industries. The aforementioned dyes have been reported to have adverse effects on human health and in ecosystems [1-4]. They are known to be highly toxic, most of them are carcinogenic, and they can cause widespread poisoning via various routes, for example, by inhalation, skin contact, or deposition onto various ecological natural species. Therefore, there is an urgent need to remove those toxic dyes from aquatic systems [5]. Various treatments have been applied to remove these toxic dyes, including coagulation and flocculation [6, 7], oxidation[8-10], electrodegradation [11], ozonation [12], photocatalytic degradation [13], and adsorption[14-16]. Among these, adsorption has been considered to be one of the most attractive strategies for removing toxic dyes from aqueous solutions, because of comparatively cost-effective and simple operation [17]. Porous carbon materials, including activated carbons, carbon nanotubes (CNTs), graphene, graphene oxides (GOs), carbon aerogels, etc., are considered excellent adsorbents due to their large specific surface area, high porosity and chemical stability. Therefore, they are widely used for dye adsorption from wastewater systems. Several studies have reported the use of activated carbons, CNTs, graphene, or GOs as adsorbents for the removal of toxic dyes [18-21]. Although their efficiency in dye removal is satisfactory, their preparation heavily relies on the use of toxic chemicals, corrosive activating agents, and has many complicated steps. Moreover, they are generally in the form of a fine powder. These small particle sizes of activated carbons, CNTs, or unassembled graphene make them exceptionally difficult to collect and reuse, which may eventually cause secondary environmental waste. In contrast, carbon-based materials with a macroscopic bulky shape are easier to handle and collect, with lesser concerns about additional contamination [22-25]. For this reason, to address these problems, porous assemblies of those materials into a monolithic feature together with high adsorption performance have attracted

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considerable interest [23, 26]. These macroscopic assemblies include granules, beads [27], foams [28], sponges [19, 20], and membranes [29, 30]. Among them, three-dimensional (3D) carbon assemblies, especially foam and sponge types, are most attractive due mainly to their light weight or very low density, so that most such materials prepared to date float on water [23, 31-35]. Although CNT-, graphene-, GO-based 3D monolithic structures possessing interconnected macroporosity (diameter > 50 nm) have shown unique advantages in dye adsorption, the complicated multi-step procedures, high costs and difficulties in scale-up have limited their practical use [32]. In contrast, carbon aerogels (CAs) prepared by polymerization of organic substances, including resorcinol and formaldehyde, can be easily produced in large scale [23, 36]. The resulting highly cross-linked gels are afterwards subjected to critical point drying and carbonization in an inert atmosphere [36]. Despite the scalable process, the preparation of CAs relies on multi-step procedures, and mostly on the use of a large amount of carcinogenic formaldehyde, making the whole production process time-consuming and not green. The preparation of low-cost and environmentally friendly monolithic adsorbents with high dye adsorption performance remains a main research target. Biomass is considered to be one of the most promising precursors for producing carbon materials. The main reasons are abundant availability, renewability and low cost, typical of agricultural wastes [35, 37]. Unfortunately, up to now, the production of biomass-based carbon 3D materials has received little attention compared to the CNT-, graphene- and carbon aerogel based materials [38]. While large pores are commonly present in CNT, carbon aerogel, and graphene based 3D materials, carbon materials with mesopore size between 2-50 nm represent another large distinct class. Mesopores are suitable hosts for bulky molecules, including large-sized dye molecules [39]. These mesopores can facilely be tailored using the self-assembly of macromolecular arrangements of carbon precursors with a soft template to form organic-organic composites, and mesoporous carbons are obtained after subsequent carbonization. Using

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amphiphilic block copolymers as micellar templates, mesopores with different morphologies and sizes can be facilely controlled [40]. However, a unimodal pore systems, whether of macropores (> 50 nm), mesopores (2-50 nm), or micropores (< 2 nm) has inherent performance limitations in various applications, including water remediation [18, 31, 32, 34, 41]. Hierarchical pore texture could then be a solution to these problems. Such hierarchical structures with macropores together with micro- and/or mesopores have been reported [23]. Macropores are desirable for the transport of liquids and gasses, making the smaller pores accessible. Nevertheless, the microporosity of adsorbents limits their practical use because of poor ability to capture large-sized molecules, such as bulky organic dyes (MW>200) [42]. Moreover, they have slow rates of both adsorption and regeneration. To address these shortcomings, carbon materials with hierarchical macro- and mesopore characters would be a good choice. The hierarchically porous carbon materials (HPCs) possess pores of well-defined and interconnected structures, both in the mesopore and macropore regions. Macropores may provide efficient convectional mass transport, although they often present very low adsorption capacity. In the meantime, the mesopores can give rise to high specific surface area and large pore volume, as well as act as active adsorption sites, possessing large adsorption capacity and fast adsorption kinetics towards a wide size-range of adsorbate molecules, including the bulky dyes [23]. Actually, hierarchically porous carbon materials and their composites prepared through the self-assembly surface coating strategy have been reported in a few publications. The macroporous scaffolds varied in type of material, including both inorganic materials, such as cordierite [43, 44], and organic materials, such as chitosan membranes [30] and cigarette filters [45]. However, a small number of reports have presented the use of pure natural precursors as macroporous scaffolds [46, 47]. Even though some of the products were tested for adsorption performance, they were not in the form of monoliths. Moreover, most of them relied on the use of carcinogenic formaldehyde

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and the synthesis methods seemed complicated, time consuming, and induced health and environmental concerns [46-49]. Sugarcane bagasse, the main waste product of the sugar industries, was selected as the carbon source in this study, due to its abundance and suitable structures [50]. It can serve as a scaffold to prepare hierarchically macro- and mesoporous carbon monoliths, since it is mainly composed of cellulose fibers and micrometer sized lignocellulose vessels with intrinsic macroporosity (diameter > 50 nm) [37]. In addition, the surfaces of macropores contain a large amount of hydroxyl functional groups, which can interact with carbon precursors, making this material a potential scaffold for the production of monolithic HPCs. Therefore, in the present work we report a facile and green synthesis of hierarchically macro- and mesoporous carbon monoliths based on the selfassembly of environmentally friendly phloroglucinol/glyoxylic acid precursors with F127 template onto the surface of the intrinsic macropores of sugarcane bagasse. Glyoxylic acid is used as an alternative to carcinogenic formaldehyde usually employed in similar syntheses, as inspired by the work of Ghimbeu and colleagues [51]. Our synthesis method reported here can be regarded as a green and sustainable route, due to the use of environmentally friendly chemicals as well as fewer steps of preparation, which could be suitable for large-scale production. In addition, the effects of preparation parameters on the physicochemical properties, especially the porosity, and on the efficiency in bulky dye adsorption, was thoroughly and systematically investigated and is discussed.

2. Experimental 2.1 Materials All chemicals were used as received without further purification. Triblock copolymer Pluronic F127 (poly(ethylene oxide)-block–poly(propylene oxide)-block–poly(ethylene oxide, PEO106PPO70-PEO106, Mw = 12 600 Da) was purchased from Sigma-Aldrich, phloroglucinol

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(1,3,5-benzentriol,C6H6O3) was purchased from ACROS Organics, glyoxylic acid monohydrate (C2H2O3·H2O) was purchased from Merck KGaA and absolute ethanol (C2H6O) was purchased from RCI Labscan. Sugarcane bagasse (Saccharum officinarum L., cellulose 40-50%, hemicellulose 25-35%, lignin 15-25% and small amounts of minerals and wax (Sun et al., 2004) was obtained from a sugarcane plant in Lopburi province, Thailand, for use as a scaffold. All experiments were performed using deionized (DI) water and all chemicals were AR grade.

2.2 Preparation of monolithic hierarchical porous carbon materials First, the sugarcane bagasse was collected, washed with distilled water several times to remove dirt, impurities and residual sugar, and then dried in an oven at 100°C for 48 h. The dried raw precursor was then cut into ∼0.5 × 1.0 × 0.2 cm pieces. The resulting monolith sample is here named BG. The average bulk density of BG was ∼0.15 g/cm3. The average macropore sizes (determined using SEM) were in the range of 10 - 100 µm. For a typical preparation method of the hierarchical porous carbon composites, phloroglucinol (1.02 g), glyoxylic acid monohydrate (0.76 g) and Pluronic F127 (1.96 g) were dissolved with stirring at room temperature in absolute ethanol (50 mL) to obtain a clear yellowish solution. 4.0 g of dried BG was mixed with the above solution and the mixture was stirred for 30 min at 300 rpm in a foil-capped 250 mL beaker with a diameter of 7 cm. To ensure homogeneous mixing, the mixture was manually kneaded with a spatula every 30 min, over the 3 h mixing time. During this process, the solution amount in the container was gradually reduced until it was no longer present in the container, indicating that the precursor solution was adsorbed by the BG. Subsequently, the BG monoliths wetted with the solution were transferred into three Petri dishes, then dried at room temperature (RT, 30 ± 2 °C) for 10 h. Then, they were held at 80 °C for 48 h. The obtained materials were pyrolyzed at 800 °C under a nitrogen (N2) atmosphere for 3 h in a tubular furnace. The heating rate from ambient temperature to 600oC was 1 °C/min, and600 °C

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was held for 15 min. From 600 °C to 800 °C, the heating rate was 5 °C/min, and then holding at 800 °C for 3 h. The resulting samples were collected and labeled “HPC(2.4)_800” (HPC = Hierarchically Porous Carbon), “(2.4)” represents the mass ratio of the template (F127):phoroglucinol+gyloxilic acid, while “_800” indicates the pyrolysis temperature in degrees Celsius. The mass ratio of F127: phoroglucinol+gyloxilic acid was also varied to 1.2, while the other synthesis procedures remained the same. This sample is here labeled “HPC(1.2)_800”. As a control experiment, BG was carbonized directly without the coating process and the sample was named “BG_800”. The non-templating sample was prepared with the same procedure as for the HPC samples but without use of F127 template and was labeled “BG_PGF_800”. All the resulting porous carbon materials were collected and labeled as shown above. In order to improve the surface area and porosity, ∼2 g of each sample after the carbonization at 800 °C was then heattreated in a 3 × 15 cm boat crucible placed in a muffle furnace at 400 °C for 1 h in air. All the samples after carbonization at 800 °C and heat-treatment in air were labeled as above but followed by the letter “A”, for instance, HPC(2.4)_800A. In order to confirm good reproducibility, the preparation of all types of samples was replicated at least 3 times.

2.3 Characterization of the resulting materials Scanning electron microscopy (SEM, Quanta 400, FEI) was used to study the surfaces and morphology of the hierarchical porous carbon materials. Fourier transform infrared spectroscopy (FTIR, Spectrum GX, Perkin Elmer, US) was performed with the KBr pellet technique across the wavenumber range 4000 - 400 cm-1. X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, PHI 500 VersaProbe II using Al Ka radiation from Thailand Synchrotron Light Research Institute) was used to determine elemental composition and functional groups. The elemental compositions were also tested by CHNS/O Analyzer (Thermo Scientific, Flash 2000) to confirm the homogeneity of samples. N2 adsorption-desorption analysis (Micromeritics, ASAP 2460) was performed at 77 K.

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The samples were degassed at 120 °C for 16 h before the measurements. From the sorption experiments, specific surface areas (SBET) were estimated using Brauner-Emmett-Teller (BET) isotherm, total pore volume is evaluated at p/p0 of 0.95-0.97, and micropore area and volume were evaluated using t-plot method. The mesopore surface areas were calculated based on t-plot external surface area [total surface areas (BET surface areas) - t-plot micropore areas], while mesopore volumes were determined using total pore volumes – t-plot micropore volumes [52, 53]. The pore size distribution in both micropore and mesopore regions, as well as the average mesopore size was determined using the DFT and Barrett-Joyner-Halenda (BJH) models, respectively. The samples were subject to the measurements in the monolithic form without being ground, as can be seen in Fig. S1. An analysis tube was filled with ∼30-100 mg of the sample. At least three replicate measurements using samples prepared representing different lots were performed. We found only slight differences in the results between different sample weights and different lots of the same sample type. This confirmed the reproducibility of our synthesis procedures.

2.4 Batch adsorption of dyes and adsorbent regeneration The batch adsorption studies employed 3 dyes, namely methylene blue (MB) (representing a cationic dye), methyl orange (MO) (representing an anionic dye), and Rhodamine B (RhB), representing a large molecular-weight dye. Their chemical structures and properties are shown in Table. 1. Kinetics: the adsorption kinetics experiments were performed in 50 mL conical flasks with 30 mg of the carbon material and 25 mL of 40 mg/L dye solution, on a thermostat shaker water bath (Model WB/OB 7-45, WBU 45, Memmert) at 30 ± 2 °C, at the natural (unadjusted) pH of 6.8, for 0-96 h duration.

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Equilibrium isotherm: The adsorption isotherm experiments were performed in 50 mL vials with 30 mg of the hierarchical porous carbon material and 25 mL of 0-850 mg/L dye solution. The mixture was shaken for more than 48 h at 30 ± 2 °C, at the natural pH of 6.8. In both the kinetic and the equilibrium experiments, after complete adsorption the solution was easily separated from the mixture without either centrifugation or filtration, and then was used for further measurements of dye concentration. The concentrations of dye before and after adsorption were measured by a calibrated method, using a UV-Vis spectrophotometer (UV 2600, Shimadzu). The wavenumbers (λ) for the detection of the dyes are listed in Table. 1. The adsorbed amount of dye was calculated with Eq. (1) as follows Qe =

( C 0 − Ce ) V m

(1)

where C0 and Ce are the initial and equilibrium concentrations of dye (mg/L), respectively, m is mass of the composite (g), and V is volume of the reaction solution (L).

3. Results and discussion 3.1 Formation of hierarchical porous structures The preparation method introduced in this work is considered simple and suitable for large-scale synthesis of monolithic HPCs. The use of toxic chemicals reported in many previous works, for example evaporated formaldehyde [46] and even activating agents (ZnCl2, KOH, H3PO4) [54] can be avoided in the present work. Furthermore, binders and freeze-drying step generally employed to synthesize a monolithic shape are no longer needed. In the first step, simply mixing phloroglucinol, glyoxylic acid, and triblock copolymer F127 in ethanol resulted in a homogeneous clear yellow solution. Then, BG pieces were introduced into the mixture solution and the suspension was stirred for 30 min. Upon mixing the solution and BG as a scaffold material, the morphological structure of BG with open macropores (Fig. 1A) allows for the facile penetration of

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the carbon precursor (phologlucinol, glyoxilic acid and F127 mixture) into the inner pores by capillary forces, as well as for deposition of the precursor on the outside surfaces. Moreover, the polar functionalities of BG containing electron rich oxygen atoms also enhance the interfacial contact between the scaffold and the precursors through hydrogen bond interactions [37, 55, 56]. During this step, the color of BG specimens became darker and wettability throughout all the specimens was also observed, further confirming the introduction of the precursor solution into the cellular channels of BG. The subsequent evaporation of the solvent (ethanol) induced further coating of the carbon precursor and initiated the self-assembly of phloroglucinol/glyoxylic acid phenolic resins around the F127 template through the formation of micelles of F127. Consequently, upon the solvent evaporation, the micelles were surrounded by and interacted with carbon precursors via hydrogen bonds. This step is known as “evaporation self-assembly (EISA)”, and has been employed to synthesize nanoporous and nanostructured materials in the form of both powders and films [40, 57, 58]. Next, the obtained material was subjected to a further thermal treatment at 80 °C for 48 h in order to complete the cross-linking of glyoxylic acid and phloroglucinol. This changed the material’s color from dark yellow to reddish brown, as well as hardened the monolithic specimens (Fig. 1B). The complete carbonization at 800 °C turned the materials into monolithic porous carbon and completely removed the template, leaving the mesopores on the sugarcane scaffold. The complete removal of the F127 template occurred in the carbonization process. This formed a mesoporous structure and converted the carbon precursor and BG into carbon matrix. Although the resulting black carbon monolith HPC shrunk by ∼75 V% in comparison with the BG scaffold, it retained the monolithic character with sufficient strength (Fig. 1C). We noted that even after the heat treatment in air at 400 °C for 1 h, the monoliths were well retained with only slight weight loss (∼2 wt%). It should also be noted that the carbonized monolith and the monolith after air treatment had similar appearances (Fig. 1D). All the prepared samples had rather similar physical appearance. Furthermore, based on weight analysis, all

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resulting materials show %yield in the range of 19-26 wt% as shown in Table 3. The yields for all samples are similar to those previous reported activated carbons prepared at high temperature (600-900 °C) [59, 60], but there are no needs for binder or other chemicals to form the monoliths in this work. Therefore, due to the use of non-toxic chemicals and abundant natural scaffolds, our proposed procedure seems greener and possible to be scaled-up. The formation mechanism for the synthesis is illustrated in Fig. 2.

3.2 Detailed characterization of HPCs To investigate the morphology of the samples, SEM was employed. The SEM images of pristine sugarcane bagasse (BG) and the monolithic HPC (HPC(2.4)_800A) are shown in Fig. 3A-C. Both samples possess the macropores from the natural xylem and phloem of raw bagasse. Determination of the decrease in the macropore sizes was difficult because the macropore size distribution in the raw bagasse was rather broad (50-110 µm). Nevertheless, the retained macropores indicate that the surface coating, pyrolysis at 800 °C and subsequent air treatment at 400 °C had no effect on the macropore morphology of the raw bagasse. Closer looks using images from the high resolution field emission SEM (FE-SEM) (Fig. 3D-E) on HPC(2.4)_800A reveals that mesopores were present on the scaffold surfaces. Importantly, the presence of both macropores and mesopores results in a hierarchically porous structure. In general, the macropores may facilitate the diffusion of adsorbates into the inner pores, while the mespores act as active adsorption sites. The TEM images of HPC(2.4)_800A (Fig. 3F-G) further confirmed the existence of mesopores. Different pore morphologies can be observed in this sample, combining the disordered mesopores (Fig. 3F) with the tube –like pore structure (Fig. 3G); however, both types of pores had the same average pore diameter of ∼5 nm (Fig. 3F and 3G). To further confirm the existence of the templated mesopores and investigate BET surface area, porosity and pore size distribution, N2 sorption analysis was performed and the results are shown in Fig. 4. The N2

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sorption isotherms of all samples prepared via the templating technique (HPC series before air oxidation) exhibit a type IV isotherm according to IUPAC, with a distinct hysteresis loop, indicating that the mesoporosity contributes to the main fraction of all pore texture in material structure. Table 2 shows the pore textural properties from N2 sorption experiments. The BET surface areas for all samples are in the range 400-600 m2/g. The mesopore volumes of the series of HPCs samples were in the range 0.13-0.25 cm3/g, contributing 45-60 V% of the total pore volume. Moreover, the dominant mesopore sizes were in the range 5.3-5.5 nm, as clearly observed from BJH pore size distributions (Fig. 5, Table 2). The mesopore sizes agree well with the TEM results. The observed mesopore sizes correspond well to a typical size of F127 micelles [40, 57], further indicating successful templating. Mesopore surface areas of HPC(1.2)_800 and HPC(2.4)_800 exceed 100 m2/g, which is considerably high compared to those of conventional microporous activated carbons. In a sharp contrast, BG-800 and BG_PGF_800, which both are the nontemplated samples, show typical isotherms of type I, suggesting that most of pore size is dominated by microporosity (pore size < 2 nm) in these two non-templated samples. Moreover, both BG_800 and BG_PGF_800 show no porosity in the mesopore region (Fig. 5). The origin of microporosity in both these samples is probably from the intrinsic porosity generated from the decomposition of small molecules of the starting materials, generally composed of large fractions of cellulose and hemi-cellulose (BG-800) and the resin polymer in case of BG-PGF-800 [61-63]. Micropore size distribution was studied by DFT model and is shown in Fig. 6. All the samples before air oxidation exhibit small micropores with an average diameter of 0.8 nm. Air oxidation was found to increase SBET, and both micro- and mesopore fractions for the HPC samples. For the HPC series, the major mesopore size (∼5-6 nm) shows no change in the mesopore size distribution after heat treatment, but relatively increased differential pore volume (dD/dV) is observed (Fig. 5). This implies that the heat treatment did not impact the mesopore size of materials. This agrees with the consistent proportion of mesopores before and after heat treatment in Table 2. However,

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the pore volume and BET surface area for all samples were significantly increased after heat treatment in air. The heat treatment under air at 400°C is believed to open previously closed template mesopores, or those with small pore windows (likely to be very small micropores) in the carbon matrix, rather than to generate new mesopores. Therefore, this treatment allows access by more N2 molecules to the inner open pores, with an increase in the specific surface area (by approximately 100 m2/g) as well as in total and mesopore volumes for the HPC sample series. Nevertheless, this relatively low temperature treatment had negligible influence on the pore texture properties in the non-templated materials (BG-800A and BG-PGF-800A). BG-800A and BG-PGF-800A are still highly microporous in size range 0.6-0.8 nm and no large contribution from mesoporosity is observed. The increased mesoporosity of the HPC samples after the air treatment has been reported in prior publications, where the air oxidation helped open closed mesopores [57, 64]. It is evident that the heat treatment at 400 °C further increased the mesopore fraction in the HPC series while retaining the monolith feature. In other prior studies, the mesopore fraction has been increased with additional chemicals during the synthesis, for example by using TEOS, and a further multi-step treatment [65, 66]. It is worth mentioning that the harsh high temperature conditions and additional chemicals are avoided in this work. Only a comparatively low temperature treatment was employed instead for a short period, thereby making the process easier to scale up [67]. FTIR spectroscopy was used to provide qualitative information on the chemical functionalities of the samples. Fig. S2 shows the FTIR spectra and Table S1 details the band assignments. All samples before pyrolysis (BG, BG-PGF, HPC(1.2), HPC(2.4)) showed abundant oxygenated functional groups in the FTIR spectra. All the characteristic bands of BG also appear in the spectra of HPC(1.2) and HPC(2.4). Furthermore, the bands at 2935 and 1157 cm-1 are attributed to the C-H and C-O-C asymmertric stretching vibrations in F127, while the bands around 2800 and 1850 cm-1 are due to CH saturated bonds of resin bridges and C=O in anhydrides. This observation indicates that the phloroglucinol/glyoxylic acid resin and F127 have

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been coated onto scaffold. All these bands disappeared after pyrolysis, including the F127 characteristic bands, whose loss indicates that the F127 template was completely removed (HPC(1.2)-800 and HPC(2.4)-800). Furthermore, the C=C peak was observed at 1700-1500 cm-1 for all samples after pyrolysis. The presence of C=C band suggests that carbonization converted resin and bagasse into a carbon matrix. The spectra for all the pyrolysed samples and the ones after the air treatment show very similar patterns, suggesting that the air oxidation had no effect on the functional groups of the pyrolysed samples. XPS analysis was carried out to determine elemental compositions and functionality assignments on the sample surfaces. As can be seen in wide scan XPS in Fig. 7, the presence of C 1s and O 1s peaks can be calculated to weight percentages of each element, as shown in Table. 3. The carbon and oxygen contents in carbonized samples both before and after the air treatment were found to be in the ranges ∼85-91 wt% and ∼815 wt%, respectively. Moreover, the presence of oxygen was further confirmed using elemental analysis and the trend in ratio of O to C contents is in agreement with both XPS and elemental analysis techniques (as shown in Table 3). All samples possessed high carbon contents. The deconvoluted spectra for C 1s and O 1s for HPC(2.4)_800 and HPC(2.4)_800A (Fig. S3) show similar XPS patterns at binding energies of 284.3 , 285.8 and 287.3 eV corresponding to graphitic C=C (with a large contribution), C-O and C=O respectively for C 1s. In high resolution O 1s spectra, the main oxygenated functional groups for these samples were found to be ether and carbonyl groups with binding energies of 531.25 and 532.77 eV, respectively. This confirms no significant change in functional groups on the surfaces, between the samples before and after air oxidation, which agrees well with the FTIR results. In addition, it is interesting to note that acidic oxygenated functional groups (carboxyl) are absent in the samples both after pyrolysis and after air oxidation. All the samples were then found to possess non-acidic character. This neutral character of all the materials agrees with their pHpzc values of ∼6.5 determined by pH drift

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method. This neutral charge is typical of carbon materials with large fraction of sp2 (C=C) character.

3.3 Adsorption of bulky dyes Fig. S4 and Fig. 8 present the change in MB color appearance and kinetic plots of MB adsorption, respectively, by the adsorbents with 40 mg/L MB initial concentration. The adsorption capacities after reaching equilibrium rank order the materials as HPC(2.4)_800A > HPC(1.2)_800A > HPC(2.4)_800 > HPC(1.2)_800 > BG_PGF_800A ∼ BG_800A > BG_PGF_800 ∼ BG_800. It is obvious that all the non-templated samples show a negligible adsorption capacity of < 8 mg/g. It is interesting to note from Fig. S4 that the MB solution became colorless in 48 h for HPC(2.4)-800A. In other words, HPC(2.4)_800A can quickly adsorb MB by more than 60% within 6 h and reached its equilibrium within 48 h. In addition, the monolith character of HPC(2.4)-800A can be retained even after 96 h, proving it has good mechanical stability. In contrast, although BG-800A and BG-PGF-800A possessed higher SBET than HPC(2.4)-800, they showed almost no change in solution color even after adsorption for 96 h. The fragile BG-800A could not even retain its monolith shape. Therefore, the good mechanical stability of the monolith HPC(2.4)-800A is probably due to coated resin. Also, it is noticeable that HPC(2.4)_800A monoliths still floated on the water surface due to the low density (0.019-0.025 g/m2) after complete adsorption at 96 h. This behavior allows one to easily collect the adsorbent after completed adsorption, as there was no sedimentation. We ground HPC(2.4)_800A into a powder, and found that its adsorption of MB reached equilibrium faster than the monolith counterpart (12 h). However, the capacities of MB adsorption between the ground and monolith HPC(2.4)-800A were the same (37 mg/g). This finding proves that despite its monolith feature, the active pores of HPC(2.4)-800A function well, allowing full access by the MB molecules. A detailed evaluation of the kinetics of MB adsorption was based on fitting the kinetic data with a

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pseudo-second-order model (Tables S3-S4). In addition to higher adsorption capacities at equilibrium, the pseudo-second-order rate constants (k2) of the HPC series are obviously higher than those of the non-templated ones (Tables S3-S4). k2 was found to be 7.51×10-3 g mg-1 min-1 for HPC(2.4)-800A . The rate constant found in this work is comparable to those of other monolithic adsorbents reported in the literature under similar conditions (Table S7). This could be attributed to the hierarchical structure of the material. In this case, the presence of macropores ensures fast diffusion of the MB dye to the active adsorption sites. The adsorption isotherms for MB by all the materials are shown in Fig. 9. The isotherm was fit well by the well-known Langmuir model. The details of fitted parameters and the non-linear form of Langmuir isotherm are explained in Tables S5-S6 in the Supporting Information. Maximum capacity estimated from the fitted Langmuir model (Qm) is a direct measure of the adsorption ability at equilibrium for an adsorbent. The Qm values for MB adsorption gave the ranking HPC(2.4)_800A > HPC(2.4)_800 > HPC(1.2)_800A > HPC(1.2)_800 > BG_PGF_800A ∼ BG_PGF_800 ∼ BG_800A ∼ BG_800 . This is similar to the ranking obtained in the kinetic study. It is clear that HPC(2.4)-800A had the best adsorption capacity (101.14 mg/g), while the Qm values are negligible (≤ 4 mg/g) for all the non-templated materials.

3.4 Adsorption mechanisms: The influence of porosity It is well known that surface chemistry and pore structure are the decisive factors for adsorption by carbon adsorbents, and both the adsorption mechanism and performance need to be considered. According to FTIR, XPS and pHPZC results discussed above, all the adsorbents tested in the adsorption studies have dominant sp2 character and small amount of oxygen functionality, various adsorption affinities can occur through π-π interactions, hydrogen bonding, hydrophobic, and electrostatic interactions [68]. In our case, Fig. 10 shows the similar molar-adsorption capacities of positively-charged MB (0.35 mmol/g) and negatively-charged MO (0.31 mmol/g) molecules,

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although these two dyes possess opposite charges. In contrast, carboxylic functionalized materials could selectively adsorb positively-charged MB while there was no adsorption affinity toward negatively-charged MO [69-71]. This confirms that electrostatic charge interactions have minor influence on adsorption affinity in the present work. Regardless of surface chemistry, pore textures are likely to govern adsorption performance. Intuitively, one may expect larger adsorption capacity by the adsorbents with higher SBET surface area. Nevertheless, this is not true in our studies. Although the microporous BG-PGF-800 and BGPGF-800A show SBET exceeding 600 m2/g, their Qm values for MB adsorption are only 4 mg/g, much lower than that of HPC(2.4)-800A with similar SBET. The poor correlation with SBET (surface area), as well as the micropore area and MB adsorption capacity is clear in Figs. 11A and 11B. Since the MB molecule is considered large with its largest dimension about 1.70 nm (Table. 1), the MB molecules cannot access the abundant micropores with sizes ≤ 0.8 nm in the nontemplated samples; therefore restriction by pore size does happen, causing slow adsorption kinetics and low capacity for all these materials. In contrast, both adsorption rate and capacity are much improved for the templated monoliths, which reveals a large contribution from mesopores in range of 5-10 nm. This finding is consistent with the previous reports, in which mesopore size of larger than 4 nm allows for the accessibility of large adsorbates. Moreover, it was found that the suitable ratio of pore size of the adsorbent to the molecular size of pollutants should be in the range of 2-6 [15, 72-78]. Interestingly, the linear relationships between MB adsorption capacity and mesopore area and mesopore volume are evident in Figs. 11C and 11D, addressing the importance of mesoporosity. This also confirms by the linear correlation between normalized adsorption capacity by SBET and %O/C (Qm/SBET, Qm/%O/C) and mesopore area in Fig. 11E and 11F. In contrast, it is worth noting that the normalized adsorption capacities by BET surface area for all samples were not affected by the proportion of oxygen content in the samples as shown in Figs. 11G and 11H. The adsorption mechanism related to effects of pore texture is proposed in

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Fig. 12. Macropores of the raw sugarcane bagasse allowed easy penetration of the template F127 and the carbon precursor into the scaffold; therefore, a high mesopore fraction could eventually be attained. Moreover, the sugarcane bagasse has proved to be a good candidate as a scaffold for the preparation of hierarchical porous carbon. The mesopores with a diameter of ∼6 nm prepared in our study can be considered as “effective” pores for adsorption of other large dye molecules. MO and RhB were chosen to prove this concept. The adsorption of MO and RhB by HPC(2.4)_800 reveal Qm values of 0.31 and 0.18 mmol/g, respectively. The MO adsorption capacity is similar to that for MB because of their similar molecular dimensions, regardless of the electrostatic charge. The Qm of RhB is lower than those for MB and MO in mol probably because the size of RhB is almost twice those of MB and MO, so a comparatively small amount of RhB molecules can fit into the effective mesopores (Fig. 10). Similarly, the adsorption of MO and RhB was unsuccessful using the microporous carbons (BG-PGF-800A) with a negligible Qm of about 4 mg/g.

3.5 Comparison of the efficiency in dye adsorption with prior reports To evaluate adsorption capability of dyes by HPC(2.4)-800A, comparison of Qm to those by selected monolithic adsorbents was performed (Table S7). It is obvious that HPC(2.4)_800A exhibits rather high adsorption capacity comparable to other monolithic adsorbents, and also even surpassing many of the other adsorbents. Despite their slightly higher adsorption capabilities (Table S7) under similar conditions, preparation methods of the monolith adsorbents must be considered. Most monolithic adsorbents require multi-step preparation, large amounts of toxic chemicals, for example formaldehyde and a strong acid (HCl), having high costs of production, and therefore making scale-up difficult. Moreover, many studies have used macroporous polymers as scaffolds. In contrast, the preparation of hierarchical porous carbon monoliths reported here employed natural waste, sugarcane bagasse, as the scaffold. More importantly, the current approach required much milder conditions, evidenced by the use of phoroglucinol and

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glyoxylic acid instead of more toxic phenol-formaldehyde. In terms of lower consumption of toxic chemicals usually employed in other preparation approaches, our facile process reported in this work not only reduces time and costs of preparation, but also is considered a greener, more environmentally friendly and efficient preparation method of hierarchically porous carbon monoliths. Interestingly, great reusability of HPC(2.4)_800A adsorbent (Fig. S5) and the incorporation of magnetic properties into the monoliths (Figs. S6 and S7) are demonstrated in the Supplementary Data.

4. Conclusion Hierarchically porous carbon materials (HPCs) were successfully produced from environmentally friendly phloroglucinol/glyoxylic acid precursors with soft-template F127 using sugarcane bagasse as scaffold by self-assembly method and pyrolysis under inert atmosphere, followed by facile heat treatment. It is worth noting that no toxic chemicals were employed, making the process environmentally friendly and easy for scale-up. The HPCs presented a hierarchically macro- and mesoporous structure with a monolithic appearance, and good mechanical stability. HPCs were tested for the adsorption of bulky dyes. When used to adsorb MB, the selected carbon monolith showed a great maximum adsorption capacity of 101 mg/g, outperforming other monoliths in the literature, also with a good recyclability. The MB removal efficiency by the carbon monoliths was predominantly attributed to mesoporosity. Moreover, the method illustrated here is easily scalable and can be conveniently functionalized, for example by adding magnetic properties into the monoliths to enable manipulations in industrial operations.

Acknowledgements This work was funded by Development and Promotion of Science Technology Talents (DPST) Research Grant (Grant No. 017/2559). R. Kueasook thanks the Faculty of Science Research Fund,

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Prince of Songkla University, for research assistantship fellowship (Contract No. 1-2558-02-002). The Graduate School, the Center of Excellence in Nanotechnology for Energy (CENE) at Prince of Songkla University and the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Thailand are acknowledged for partial support. The authors are indebted to Dr. Seppo Karrila for proofreading the manuscript.

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Table 1 Chemical structure, molecular size, molecular weight, and wavenumber of UV adsorption for dyes.

Molecular size (nm)

Molecular weight (g/mol)

Methylene blue (MB)

1.70×0.76×0.33

319.85

644

Cation

Methyl orange (MO)

1.31×0.55×0.18

327

465

Anion

Rhodamine B (RhB)

1.59×1.18×0.56

478

554

Cation

Dye

Chemical structure

UV adsorption Change (nm)

33

Table 2 Pore textural properties measured by nitrogen sorption at 77K. Surface area (m2/g) Sample

Template/ Carbon precusors

BG_800

% Mesopore (by f g volume) BJH DFT

Mesob

Totalc

Microd

Mesoe

421.60 ± 48.13 384.24 ± 39.05

37.36 ± 9.08

0.180 ± 0.028

0.150 ± 0.021

0.030 ± 0.007

16.67

-

0.89

476.79 ± 51.85 416.13 ± 37.49

60.66 ± 14.36

0.210 ± 0.034

0.159 ± 0.026

0.051 ± 0.008

24.29

-

0.93

618.56 ± 61.24 575.36 ± 53.89

43.20 ± 7.35

0.269 ± 0.034

0.224 ± 0.025

0.045 ± 0.009

16.61

-

0.81

638.78 ± 57.36 584.78 ± 49.58

54.00 ± 7.78

0.257 ± 0.033

0.224 ± 0.023

0.033 ± 0.010

12.73

-

0.68

492.76 ± 62.58 367.56 ± 43.29 125.21 ± 19.29

0.273 ± 0.027

0.143 ± 0.019

0.130 ± 0.008

47.48

5.5

0.86

606.49 ± 49.96 448.61 ± 39.11 157.87 ± 10.85

0.325 ± 0.033

0.175 ± 0.022

0.150 ± 0.011

46.05

5.5

0.89

514.00 ± 53.78 332.36 ± 34.12 181.64 ± 19.66

0.329 ± 0.025

0.133 ± 0.018

0.196 ± 0.007

59.64

5.4

0.86

644.01 ± 57.25 415.29 ± 42.08 228.72 ± 15.17

0.410 ± 0.032

0.165 ± 0.019

0.245 ± 0.013

59.65

5.4

0.86

BET (SBET)

Microa

Pore size (nm)

Pore Volume (cm3/g)

0 BG_800A BG_PGF_800 0 BG_PGF_800A HPC(1.2)_800 1.2 HPC(1.2)_800A HPC(2.4)_800 2.4 HPC(2.4)_800A a

t-plot micropore area [micropore surface areas]

b

t-plot external surface area [mesopore surface area] was calculated by total surface area (BET surface area) - t-plot micropore area]

c

total pore volume

d

t-plot micropore volume

e

difference of total pore volume and micropore volume [mesopore volume] was determined by total pore volume - t-plot micropore volume

f

BJH adsorption average pore diameter

g

DFT pore size

34

Table 3 The yield through the preparation process and the compositions from XPS and elemental analysis for all samples.

Sample

%yielda (wt%)

XPS %C

%O wt% 90.68 89.05 90.19 86.05 91.56

at% 7.16 8.45 7.55 10.85 10.07

wt% 9.32 10.95 9.81 13.95 8.44

O/C×100 wt% 10.28 12.30 10.88 16.21 9.22

%C wt% 71.63±0.52 61.93±1.08 75.68±0.11 70.63±0.89 81.33±0.36

Elemental analysis %H %O wt% wt% 1.42±0.43 4.99±018 1.31±0.03 7.82±0.22 1.37±0.05 6.29±0.17 1.90±0.07 12.80±0.12 1.21±0.04 4.89±0.14

O/C×100 wt% 6.97 12.63 8.31 18.12 6.01

BG_800 BG_800A BG_PGF_800 BG_PGF_800A HPC(1.2)_800

19.12±3.45 19.09±3.38 26.08±1.73 25.54±2.18 23.03±2.89

at% 92.84 91.55 92.45 89.15 89.93

HPC(1.2)_800A

22.60±3.45

88.12

84.78

11.88

15.22

17.95

72.90±0.30

1.58±0.01

7.63±0.19

10.47

HPC(2.4)_800

19.72±2.61

93.78

91.88

6.22

8.12

8.84

79.37±0.19

1.21±0.04

2.80±0.10

3.53

HPC(2.4)_800A

19.70±2.53

90.14

87.28

9.86

12.72

14.57

75.13±0.27

1.64±0.03

9.99±0.30

13.30

a

%yield =

[weight of resulting sample / weight of the as-prepared material before carbonization (pure BG or pure BG+carbon precursors)] × 100.

35

List of Figures and Tables Fig. 1. Photos of (A) sugarcane bagasse (BG), (B) composite HPC(2.4), (C) HPC(2.4)_800, and (D) HPC(2.4)_800A. Fig. 2. Schematic illustration of the preparation steps for all HPCs by self-assembly methods. Fig. 3. SEM and TEM analysis. (A) SEM image at 500× of sugarcane bagasse (sectional areas of the vessels), (B-C) SEM images at 500×-800× of HPC(2.4)_800A (sectional areas of the vessels), (D-E) FE-SEM image at 300000x-600000x of HPC(2.4)_800A and (F-G) TEM image at 500× of HPC(2.4)_800A. Fig. 4. Nitrogen adsorption-desorption isotherms of (A-D) samples before air oxidation, (E-H) samples after air oxidation. Fig. 5. Mesopore size distributions of all samples using BJH model. Fig. 6. Micropore sizes of all samples calculated by DFT method. Fig. 7. Survey XPS spectra for all samples. Fig. 8. Effect of contact time on the methylene blue (MB) adsorption over (A) samples before air oxidation and (B) samples after air oxidation. Reaction conditions: Initial concentration 40 ppm, temperature 30±2°C and adsorbent dosage 0.03 g/25 mL. Fig. 9. Adsorption isotherms of methylene blue (MB) onto all monoliths (A) all samples before air oxidation, and (B) all samples after air oxidation. Reaction conditions: initial concentration 40-850 ppm, temperature 30±2°C and adsorbent dosage 0.03 g/25 mL. Fig.10. Adsorption isotherm for methylene blue (MB), methyl orange (MO) and Rhodamine B (RhB) on HPC(2.4)_800A. Fig. 11. Correlation of adsorption affinity towards MB dye checked with various parameters: (A) surface area, (B) micropore area, (C) mesopore area, (D) mesopore volume, and (E-F) the correlation of mesopore area with normalized Qm by both SBET and %O/C respectively and (G-H) the correlation of the normalized Qm by SBET with O/C ratio in at% and wt% respectively. Fig. 12. Schematic illustration of the effect of pore morphology on the adsorption performance of MB.

Table 1 Chemical structure, molecular size, molecular weight, and wavenumber of UV adsorption for dyes. Table 2 Pore textural properties measured by nitrogen sorption at 77K. Table 3 The yield through the preparation process and the compositions from XPS and elemental analysis for all samples.

36

Fig. 1. Photos of (A) sugarcane bagasse (BG), (B) composite HPC(2.4), (C) HPC(2.4)_800, and (D) HPC(2.4)_800A.

37

Fig. 2. Schematic illustration of the preparation steps for all HPCs by self-assembly methods.

38

Fig. 3. SEM and TEM analysis. (A) SEM image at 500× of sugarcane bagasse (sectional areas of the vessels), (B-C) SEM images at 500×-800× of HPC(2.4)_800A (sectional areas of the vessels), (D-E) FE-SEM image at 300000x-600000x of HPC(2.4)_800A and (F-G) TEM image at 500× of HPC(2.4)_800A.

39

Fig. 4. Nitrogen adsorption-desorption isotherms of (A-D) samples before air oxidation, (E-H) samples after air oxidation.

40

Fig. 5. Mesopore size distributions of all samples using BJH model.

41

Fig. 6. Micropore sizes of all samples calculated by DFT method.

42

Fig. 7. Survey XPS spectra for all samples. .

43

Fig. 8. Effect of contact time on the methylene blue (MB) adsorption over (A) samples before air oxidation and (B) samples after air oxidation. Reaction conditions: Initial concentration 40 ppm, temperature 30±2°C and adsorbent dosage 0.03 g/25 mL.

44

Fig. 9. Adsorption isotherms of methylene blue (MB) onto all monoliths (A) all samples before air oxidation, and (B) all samples after air oxidation. Reaction conditions: initial concentration 40-850 ppm, temperature 30±2°C and adsorbent dosage 0.03 g/25 mL.

45

Fig.10. Adsorption isotherm for methylene blue (MB), methyl orange (MO) and Rhodamine B (RhB) on HPC(2.4)_800A.

46

Fig. 11. Correlation of adsorption affinity towards MB dye checked with various parameters: (A) surface area, (B) micropore area, (C) mesopore area, (D) mesopore volume, and (E-F) the correlation of mesopore area with normalized Qm by both SBET and %O/C respectively and (G-H) the correlation of the normalized Qm by SBET with O/C ratio in at% and wt% respectively.

47

Fig. 12. Schematic illustration of the effect of pore morphology on the adsorption performance of MB

Green and facile synthesis of hierarchically porous carbon monoliths via surface self-assembly on sugarcane bagasse scaffold: Influence of mesoporosity on efficiency of dye adsorption Ratchadaporn Kueasooka, Natthanan Rattanachueskula, Narong Chanlekb, Decha Dechtriratc,d,e, Waralee Watcharinf, Pongsaton Amornpitoksuka, Laemthong Chuenchoma* a

Department of Chemistry and Center for Excellence for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand b

Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang,

Nakhon Ratchasima 30000, Thailand c Department of Material Science, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand d Specialized Center of Rubber and Polymer Materials for Agriculture and Industry (RPM), Faculty of Science, Kasetsart University, Bangkok 10900, Thailand e Laboratory of Organic Synthesis, Chulabhorn Research Institute, Bangkok 10210, Thailand f Faculty of Biotechnology (Agro-Industry), Assumption University, Hua Mak Campus, Bangkok 10240, Thailand * Corresponding author. Tel.: +66 74 288416; fax: +66 74 558841. Email address: [email protected] (L. Chuenchom)

Highlights •

Porous carbon monoliths were facilely prepared by an environmentally method.

•

Sugarcane bagasse as a scaffold to serve macroporosity.

•

No toxic or corrosive activating chemicals and binders are needed.

•

Stable porous carbon monoliths possess hierarchical macro- and mesoporosity.

•

The mesoporosity is directly correlated to methylene blue adsorption efficiency.

Author Contributions Section Author Ratchadaporn Kueasook

Contributions - Conducted experiments - Data analysis - Drafted the manuscript - Prepared the manuscript - Revised the manuscript

Natthanan Rattanachueskul

-

Conducted experiments Data analysis Drafted the manuscript Prepared the manuscript Revised the manuscript

Narong Chanlek

-

Conducted XPS measurements and data interpretation Revised the manuscript

Decha Dechtrirat

-

Proof-read the manuscript

Waralee Watcharin

-

Proof-read the manuscript

Pongsaton Amornpitoksuk

-

Proof-read the manuscript

Laemthong Chuenchom

-

Designed experiments Conducted experiments Data analysis Drafted the manuscript Prepared the manuscript Revised the manuscript Corresponding author

Declaration of Interest Statement

All the authors herein declare that there are no conflicts of interest.