Microporous and Mesoporous Materials 203 (2015) 178–185

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Microporous and Mesoporous Materials

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Tuning hydrochar properties for enhanced mesopore developmentin activated carbon by hydrothermal carbonization

http://dx.doi.org/10.1016/j.micromeso.2014.10.0361387-1811/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Tel.: +65 65162171; fax: +65 67791936.E-mail address: chesmp@nus.edu.sg (M.P. Srinivasan).

Akshay Jain a, Rajasekhar Balasubramanian b, Madapusi P. Srinivasan a,⇑a Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singaporeb Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117585, Singapore

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 September 2014Received in revised form 15 October 2014Accepted 20 October 2014Available online 30 October 2014

Keywords:Hydrothermal carbonizationMesoporous activated carbonBiomassChemical activationHydrothermal conditions

Simple tuning of hydrothermal pre-treatment parameters is shown to result in significant enhancementin the mesopore area of activated carbon obtained from biomass (coconut shell). ZnCl2 content andtemperature were used as parameters to improve the effectiveness of hydrothermal pre-treatment. Withprogressive increase in the treatment temperature, the enhancement of ZnCl2 diffusion was tempered byits reduced solubility. By identifying favorable working ranges for ZnCl2 content and temperature topromote mesopore formation, up to 80% increase in mesopore area is achieved in comparison to theconventional soaking process for formation of mesoporous activated carbon. The enhancement inmesopore area was substantiated by high adsorption capacities for a mixture of phenol red and phenolup to 351 and 264 mg/g, respectively.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

There is a strong need to move towards sustainable andrenewable materials and methods in all aspects of energy andthe environment because of diminishing resources. A considerableamount of work has been done in this direction such as productionof biofuels, solar energy, use of biomass for energy and resourcessuch as activated carbon [1], etc. In this context, activated carbonsfind many applications in water treatment in a range of industriessuch as food [2,3], pharmaceutical, petroleum [4], automobile [5]and nuclear [6,7]. Furthermore, they are of great interest in cataly-sis [8,9], gas storage [10–12], energy storage and supercapacitors[13–19]. Activated carbons with high microporosity are usedextensively for adsorption of small sized pollutants or molecules;however, the treatment of systems containing large molecules(high molecular weight compounds, dyes, etc.) requires adsorbentswith high mesopore content.

Carbons with high mesopore surface areas have utility innumerous applications, for example in chromatography, dyeremoval from water [20,21], electrochemical capacitors [22],bio-catalysis [23,24], electro-catalysis [25], lithium batteries,etc. [21]. To a large extent, synthesis of activated carbons from bio-mass has been carried out using physical (CO2, steam, etc.) andchemical activating agents (typically, ZnCl2 and H3PO4) [26–32]

or a combination of both [33,34]. In general, mesopore contentincreases with increased use of the activating agent. Hu et al.reported that micropore formation is the dominating mechanismat low ZnCl2-to-biomass ratio and pore widening occurs at higherratios [35]. Though up to 95% of ZnCl2 can be recovered post-activation [36–40], extensive usage of chemical activating agentsis still a necessity in order to obtain high mesoporosity [41–49].

Hydrothermal carbonization has gained importance in recentyears for energy and materials processing because of its promisein delivering desirable properties to hydrochar (viz., high concen-tration of oxygenated functional groups (OFG) and low degree ofaromatization) [6,7,50,51]. Fechler et al. reported the hydrother-mal carbonization of glucose under hypersaline conditions todevelop significant porosity (BET surface area of 673 m2/g) [52].However, these and similar processes deliver products with rela-tively low surface areas compared to activated carbons.

The synthesis of highly porous carbon materials based on thehydrochar products [31] is of great importance since hydrocharexhibits special properties in context of activated carbon synthe-sis-particularly presence of OFG which promotes chemical activa-tion [10,17,47,53,54]. Mild processing conditions and simplicityof the hydrothermal carbonization process makes it even moreattractive [4,5,55–59]. OFG formed during hydrothermal treatmentcan increase porosity because they enhance the reactivity withZnCl2, thereby improving its utilization [17,47,60]. Despite thedemonstrated advantages of hydrothermal carbonization, its effec-tiveness in pore creation is highly dependent on the processing

A. Jain et al. / Microporous and Mesoporous Materials 203 (2015) 178–185 179

conditions [5,50,54]. Falco et al. demonstrated the effect ofhydrothermal treatment temperature on surface area of hydrocharderived carbons. An increase in porosity is reported when thehydrothermal carbonization was increased from 180 �C to 240 �Cdue to increase in carbonyl functionalities up till 240 �C. However,further increase in temperature to 280 �C led to fall in porosity[54].

Hydrothermal pre-treatment of coconut shell in presence ofZnCl2 has been shown to improve chemical activation and signifi-cant improvement in mesopore area by as much as 67% [17,47].However, the factors that led to the observed results and theireffect on material and functional properties were unknown. Thecurrent work addresses the influence of the hydrothermal treat-ment conditions and provides new insight on the interdependencebetween the factors, which will help in designing the hydrother-mal treatment process for increasing mesopore area further. Anappropriate hydrothermal treatment conditions have to be deter-mined for preparation of hydrochar since temperature influences(a) solubility and activity of the chemical activating agent i.e. ZnCl2,(b) induction of OFG on the hydrochar surface, (c) degree ofcarbonization and (d) diffusion phenomena of ZnCl2 and thusextent of biomass dehydration.

In this work, we demonstrate the interdependence of hydro-thermal treatment temperature and ZnCl2 content in influencingcharacteristics of the hydrochar derived from the hydrothermaltreatment. The process conditions can be tuned to preparehydrochars that yielded activated carbons with increased meso-pore area by up to 80% compared to conventionally prepared sam-ples. We show that higher hydrothermal temperature forhydrochar preparation at low ZnCl2:shell ratios favors mesoporeformation, while lower temperatures are preferred when theZnCl2:shell ratios were increased. The reduced mesopore area isattributed to reduced solubility of ZnCl2 at the higher operatingtemperatures and ZnCl2:shell ratios during hydrochar preparationand thus reduced availability of ZnCl2 at the time of activation.Favorable working ranges for the hydrothermal carbonization tem-perature and ZnCl2 content are identified to promote mesoporeformation. Enhancement in mesopore area under favorable hydro-thermal conditions is confirmed by the adsorption capacities for amixture of phenol (264 mg/g) and phenol red (351 mg/g) fromaqueous solutions.

2. Materials and experimental procedure

Coconut (Cocos nucifera) shells, was obtained from Malaysia.Commercial microporous carbon (10–18 mesh, BDH Laboratory,England), commercial mesoporous carbon (Sigma Aldrich), ZnCl2

(>98%, reagent grade, Scharlab), phenol (detached crystals, 99+%,Alfa Aesar), phenol red (Sigma, Germany), sodium hydroxide(Merck, EMSURE, >99%) and hydrochloric acid (37%, Panreac) wereused as received. The coconut shells (after trimming the fibers)were dried at 105 �C for 24 h, crushed using a commercial labora-tory blender (Waring) and then ground and sieved into coarsegranules (10–20 mesh). Mesoporous carbons were prepared byphysico-chemical activation of the precursors with ZnCl2 and CO2.

2.1. Hydrothermal treatment

A Parr 4843 autoclave was charged with 25 g shell and 150 mLwater with the predetermined ZnCl2:shell ratio (1:1, 2:1 and 3:1).The samples were held at the desired temperature (200, 275, 315and 350 �C) for 20 min at the self-generated pressure in the rangeof 12–156 bar varying as a function of temperature and amount ofZnCl2. The hydrothermally treated products (char) were cooled toroom temperature and were dried at 105 �C for 12 h.

2.2. Physico-chemical activation

The activated carbons were prepared using the protocols previ-ously described [17,33–35,47]. Briefly, the precursor was mixedwith ZnCl2 solution with a ZnCl2:shell ratio of 1:1, 2:1 or 3:1 (in caseif ZnCl2 was not added during hydrothermal pre-treatment) anddried at 105 �C for 12 h. The mixture was loaded on to aluminaboats and positioned inside a quartz tube that was placed in afurnace (Carbolite). The temperature was ramped to 800 �C at a rateof 10 �C/min in the presence of N2 at a flow rate of 50 mL/min. N2

was then replaced by CO2 at a flow rate of 40 mL/min for 2 h. Thefurnace was cooled to room temperature in the presence of N2 ata flow rate of 50 mL/min. The products were stirred for 30 min in250 mL hydrochloric acid (about 0.1 mol/L), and washed withabundant distilled water until a pH of 6 was obtained for the rinse.Finally, the activated carbon was dried at 105 �C for 24 h and usedfor analysis and adsorption experiments.

2.3. Activated carbon synthesis and classification

Fig. 1 represents the various experiments that have been carriedout in this study. Three categories of samples were prepared: (1)hydrothermal treatment with ZnCl2 (ZHT), (2) hydrothermal treat-ment without ZnCl2 (HT) and (3) soaking with ZnCl2 at 105 �C. Thenomenclature used indicates the sequence of steps employed, ratioof chemical activating agent used and hydrothermal treatmenttemperature. For example, ZHTP-[2]-275 refers to the sampleshydrothermally treated at 275 �C in the presence of ZnCl2 withZnCl2:shell ratio of 2:1 (weight basis) and activated subsequently.HTZP-[2]-275 refers to samples that were hydrothermally pre-treated at 275 �C in the absence of ZnCl2, subsequently hydrocharsoaked in ZnCl2 in a ratio of 2:1 w.r.t. shell and then activated.ZP was prepared by soaking the raw shell in ZnCl2 at 105 �Cfollowed by activation.

2.4. Characterization techniques

Elemental (C, H, N, S) contents were obtained using an elemen-tal analyzer (Vario Macro Cube, Elementar, Germany). Boehm titra-tion [47,61] was carried out for estimation of the OFG which aretypically carboxylic, lactonic and phenolic [62]. Briefly, sampleswere heated to 150 �C and held at that temperature for 24 h inan inert (N2) atmosphere. After cooling to room temperature,1.5 g of the sample was mixed with 50 mL NaOH (0.05 M) andthe solution was agitated by shaking for 24 h. The carbon wasremoved by filtration and 10 mL aliquots of the solution weretitrated with 0.05 M HCl. Nitrogen sorption isotherms wereobtained at 77 K after degassing the carbon sample at 150 �C underN2 atmosphere for 12 h using a gas sorption analyzer (Nova-3000Series, Quantachrome). The surface area was calculated by apply-ing the Brunauer–Emmett–Teller (BET) model to the isotherm dataof the adsorption branch in the relative pressure range p/p0 < 0.3.The pore size distribution was calculated from N2 sorption datausing the nonlocal density functional theory (NLDFT) equilibriummodel for slit pores [17,47]. The total pore volume was estimatedat a relative pressure of about 0.98 and the micropore surface areaand micropore volume were determined using the t-plot method.The mesopore area was then calculated by subtracting the micro-pore area from the BET surface area. The surface morphology ofthe carbons were obtained by FETEM (Field Emission TransmissionElectron Microscopy, model: JEOL JEM-2100F), using an accelerat-ing voltage of 200 kV.

Fig. 1. Classification of activated carbons. Nomenclature: A-[X]-Y ((A: sequence of process; Z-ZnCl2, HT-Hydrothermal treatment, P-Physico-chemical activation); [X]: ratio ofZnCl2 w.r.t raw material; Y: temperature).

180 A. Jain et al. / Microporous and Mesoporous Materials 203 (2015) 178–185

2.5. Adsorption studies

Adsorption experiments of the mixture of phenol red and phe-nol were carried out using batch equilibration. A 50 mL solutioncontaining 500 ppm each of phenol red and phenol was shaken(orbital shaker SH30) with 0.05 g of carbon in Erlenmeyer flasksat room temperature until equilibrium was reached. The solutionwas separated from the adsorbent by centrifugation using a Nuvecentrifuge and analyzed by UV–visible spectrophotometry.

The adsorption at higher concentration levels (500 ppm phenolred and 500 ppm phenol mixture) is adopted in order to obtain thesaturation capacity of the carbons and thus can be compared forthe relative amount of porosity present.

3. Results and discussions

3.1. Elemental analysis of hydrochar precursors

The Van Krevelen diagram (Fig. 2) obtained from elementalanalysis (Table S1, Supporting Information) that shows the changein H/C and O/C ratios due to hydrothermal treatment offers anunderstanding of the elemental reactions that had occurred [62].The decrease in the H/C and O/C ratios suggest that dehydrationreactions dominated at all the temperatures studied, indicatingthat an acidic environment is present [58,63,64]. The HT process(hydrothermal treatment in the absence of ZnCl2) resulted ingreater extent of dehydration with increase in hydrothermal treat-ment temperature from 200 �C to 350 �C [57]. The greater extent ofdecrease in the elemental ratios observed for ZHT (hydrothermaltreatment in the presence of ZnCl2) underscores the role of ZnCl2

in promoting dehydration. Indeed, hydrothermal treatment at

200 �C with ZnCl2 (ZHT-[2]-200) showed better dehydration thanthe sample hydrothermally treated at 275 �C in the absence ofZnCl2 (HT-275). ZHT-[2]-275 has the highest extent of dehydrationalong with low degree of aromatization which is advantageous foractivated carbon synthesis [10]. As expected, higher temperaturecauses a decrease in H/C and O/C ratios which indicates the higherdegree of condensation [62].

3.2. Boehm titration

Table 1 shows the estimated quantities of OFG in hydrocharsprepared at different temperatures in the absence and presenceof ZnCl2. Hydrothermal pre-treatment resulted in increase in OFGcontent compared to raw shell irrespective of the hydrothermalconditions employed. Sevilla et al. reported the chemical transfor-mation (hydrolysis, isomerization, tautomerization, hydration,dehydration and cyclization [65]) of cellulose when hydrother-mally carbonized above 210 �C [62]. However, we observed anincrease in OFG even at 200 �C due to the presence of hemicellu-lose in shell which hydrolyzes at lower temperatures [51]. Theincrease in OFG content with hydrothermal temperature isobserved up to 275 �C followed by a decrease that may be due tothe decomposition of OFG at higher temperatures [66]. Inclusionof ZnCl2 in hydrothermal treatment resulted in increase in theOFG content and the largest increase was observed at 200 �C. Thisis consistent with the report of Amarasekara et al. that ZnCl2 low-ers the activation energy for the breakdown of glycosidic linkage ofcellulose due to the formation of zinc-oxygen coordination andthus facilitates breakdown at the lower temperature [60,67]. High-est OFG content in the hydrochar was observed at 275 �C in thepresence of ZnCl2 which is consistent with the highest extent ofdehydration inferred for this sample from the elemental ratios.

Fig. 2. Van Krevelen diagram demonstrating (H/C) vs. (O/C) atomic ratios for char obtained by hydrothermal treatment in the presence and absence of ZnCl2.

Table 1Surface acidity (OFG) (meq/g) of the hydrochar precursors used for activated carbonsynthesis (OFG content in raw coconut shell = 1.04 meq/g).

Hydrothermal temperature 200 �C 275 �C 315 �C 350 �C

HT 1.2 1.46 1.35 1.35ZHT-[2] 1.5 1.59 1.51 1.51

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

800

1000

Volu

me

adso

rbed

(cm

3 /g)

Relative Pressure (P/P0)

ZHTP-[2]-200 ZHTP-[2]-275 ZHTP-[2]-315 ZHTP-[2]-350

Fig. 3. Nitrogen sorption isotherms of ZHTP carbons at 77 K.

A. Jain et al. / Microporous and Mesoporous Materials 203 (2015) 178–185 181

3.3. BET surface area and mesoporosity

3.3.1. Physico-chemical activation of hydrothermally derived chars inthe presence of ZnCl2: effect of hydrothermal temperature

Table 2 and N2 sorption isotherms in Fig. 3 shows the increasein mesopore area with increase in hydrothermal treatment tem-perature from 200 �C to 275 �C. The enhanced diffusion and exten-sive dehydration at the higher temperature led to increase in OFGcontent that in turn facilitated incorporation of zinc ions andresulted in efficient chemical activation [17,47,60,67]. However,mesopore area decreased significantly when hydrothermal treat-ment was carried out at 350 �C due to reduced solubility of ZnCl2

at the processing conditions and consequent underutilization ofZnCl2. This inference is consistent with other works which reportedthe precipitation of salt under subcritical and supercritical condi-tions of water [68–71]. Harris et al. also reported a reduction inthe dielectric constant of water at elevated temperatures and pres-sures (up to 2 Kbar) which implies that the charge on the zinc ionswas less shielded, thereby leading to the reduction in solubility

Table 2Porosity data of carbons prepared with ZnCl2:shell ratio = 2.

Activated carbons Mesopore surface area (m2/g)

ZP-[2] 383 ± 17ZHTP-[2]-200 490 ± 2ZHTP-[2]-275 640 ± 53ZHTP-[2]-315 390 ± 16ZHTP-[2]-350 95 ± 3

[70]. As a specific illustration it can be seen that, though ZP-[2]and ZHTP-[2]-350 underwent the same activation, the mesoporearea in the latter is significantly smaller due to reduced solubilityof ZnCl2 at these hydrothermal conditions.

3.3.2. Effect of ZnCl2:shell ratio under different hydrothermaltreatment temperatures

The effect of ZnCl2:shell ratios was investigated by employingdifferent ratios of 1:1, 2:1 and 3:1 under a range of temperatures

BET surface area (m2/g) Total pore volume (cm3/g)

1421 ± 75 0.98 ± 0.041673 ± 37 1.17 ± 0.011652 ± 132 1.29 ± 0.011655 ± 19 1.08 ± 0.031113 ± 47 0.63 ± 0.02

Fig. 4. Favorable working ranges of hydrothermal treatment conditions forenhancement of mesopore area. Percentages refer to change in mesopore arearelative to activated carbons prepared by conventional soaking at same ZnCl2:shellratios.

Table 4Porosity data of carbons prepared with ZnCl2 addition after hydrothermal treatment.

Sample Mesopore surfacearea (m2/g)

BET surfacearea (m2/g)

Total porevolume (cm3/g)

HTZP-[2]-200 432 ± 4 1549 ± 94 1.00 ± 0.13HTZP-[2]-275 521 ± 10 1744 ± 112 1.22 ± 0.04HTZP-[2]-315 531 ± 13 1877 ± 14 1.306 ± 0.02HTZP-[2]-350 531 ± 19 1806 ± 253 1.384 ± 0.16

182 A. Jain et al. / Microporous and Mesoporous Materials 203 (2015) 178–185

(200 �C, 275 �C, 315 �C and 350 �C). The percentage enhancementin mesopore area (relative to conventionally processed samples)observed due to hydrothermal treatment for a ZnCl2:shell ratio of1:1 (up to 80%) (Table S2, Supporting information) was more thanthat observed at higher ratios (67% increase for 2:1 (Table 2) and25% increase for 3:1 (Table S3, Supporting information)). Theprogressive decrease in the impact of hydrothermal treatmentwith increasing ZnCl2:shell ratio may be due to reduced propor-tional availability of OFG on the hydrochars. The information inTables 2, S2 and S3 is condensed in Table 3 to clearly indicatethe importance of using the appropriate amount of ZnCl2 atdifferent hydrothermal temperature conditions.

The mesopore area increases when the temperature is increasedfrom 200 �C to 275 �C, but the extent of increase decreases whenthe ZnCl2:shell ratio is increased. This may be due to two reasons:(a) reduced OFG content relative to ZnCl2 at higher ZnCl2:shellratios leading to reduced utilization of ZnCl2; (b) reduced availabil-ity of ZnCl2 at higher ratios due to diminished solubility. Theimpact of reduced solubility at higher treatment temperaturesand ZnCl2:shell ratios is clearly seen in the form of decreasedmesopore areas when hydrothermal treatment temperature isincreased from 315 �C to 350 �C. Therefore, it can be concluded thatmore effective chemical activation and higher porosity requires theright combination of temperature and amount of activating agent.Clearly, higher ZnCl2:shell ratio is not necessarily favorable forhydrothermal treatment at higher temperature.

3.3.3. Favorable working range of hydrothermal treatment forenhancement in mesopore area

As discussed earlier, higher hydrothermal treatment tempera-ture increases the diffusivity and extent of dehydration but atthe same time results in reduced solubility which renders ZnCl2

less effective and thus adversely affects mesopore development.Fig. 4 demarcates the effective region of hydrothermal treatment(function of temperature and ZnCl2 content) for the synthesis ofdesired hydrochar in context of activated carbon production. Thisregion in the plot denotes the conditions under which hydrother-mal treatment can be carried out with the assurance that mesoporearea will increase relative to that obtained by conventional pre-treatment. The percentage denote the change in mesopore areaof carbons relative to that obtained for carbons prepared in theabsence of hydrothermal treatment. The plot shows that highertemperatures coupled with lower ZnCl2:shell ratios favor increasedmesopore formation and likewise for higher ZnCl2:shell ratios atlower hydrothermal treatment temperatures.

3.3.4. Hydrothermal treatment without ZnCl2 followed by physico-chemical activation with ZnCl2: effect of hydrothermal temperature

The effect of hydrothermal temperature in the absence of ZnCl2

needs to be investigated in order to determine the distinct andadvantageous role of ZnCl2 when incorporated during hydrother-mal treatment. The significance of ZnCl2 addition during hydro-thermal treatment can be perceived upon comparison of samplescoded ZHTP-[2] from Table 2 and HTZP-[2] (Carbon derived fromhydrothermal pre-treatment in the absence ZnCl2 followed byphysico-chemical activation with ZnCl2) from Table 4. ZnCl2

Table 3Percentage change in mesopore area with change in hydrothermal treatmenttemperature (detailed porosity data can be found in Tables S2 and S3, SupportingInformation).

Temperaturechange

Mesopore area(%) (ratio –1:1)

Mesopore area(%) (ratio – 2:1)

Mesopore area(%) (ratio – 3:1)

200 �C ? 275 �C +36 +31 +14275 �C ? 315 �C +26 �39 �39315 �C ? 350 �C �18 �75 �76

incorporation during hydrothermal treatment is more effectivefor mesopore generation compared to post addition of ZnCl2 athydrothermal temperatures of 200 and 275 �C. Fig. 5(a) and (b)also shows that pores in the range of 15–50 Å increased with thehydrothermal temperature from 200 �C to 275 �C when ZnCl2

was incorporated during hydrothermal treatment. This is due tomore effective dehydration in the presence of ZnCl2 giving rise tohigher OFG content and better activation.

In contrast, the addition of ZnCl2 during hydrothermal treatmentis not effective when the hydrothermal treatment is carried out at315 �C and 350 �C due to reduced solubility of ZnCl2 at highertemperatures as discussed earlier. Likewise, there is a decrease inpores in the mesoporous range at 315 �C followed by a drastic fallat 350 �C (Fig. 5(a) and (b)). ZHTP carbons generate prevalently mes-opores in the range of 2–4 nm. In particular, ZHTP-[2]-200 andZHTP-[2]-275 possess higher differential volume in the range ofbroader pore size distribution compared to ZHTP-[2]-315 andZHTP-[2]-350 which is also substantiated by the mesopore volume.The trend of pore size distribution with change in hydrothermaltemperature is also consistent with the literature where theyreported an increase in differential pore volume towards the widerpore size range of 1.5–2.5 nm with increase in temperature [54]. Onthe other hand, in the case of HTZP (Fig. 5(c) and (d)), samplesthat underwent hydrothermal treatment at 315 �C and 350 �C(HTZP-[2]-315 and HTZP-[2]-350) have greater porosity in themesoporous range (20–50 Å) compared to HTZP-[2]-200 andHTZP-[2]-275. This indicates that ZHT is a better precursor (in termsof energy and quality) for chemical activation, which is substanti-ated by the presence of higher amount of OFG as well.

3.4. Surface morphology analysis

FETEM images of the activated carbon (Fig. 6) shows heteroge-neous/roughened morphologies for samples that underwent

0 20 40 60 800.00

0.01

0.02

0.03

0.04

0.05

0.06

0 40 80 120 160 2000.0

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0 20 40 60 80 1000.00

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0 40 80 120 160 2000.2

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dV(d

)(cc/

Å/g

)

Size(Å)

ZHTP-[2]-200 ZHTP-[2]-275ZHTP-[2]-315 ZHTP-[2]-350

(a)

Por e

volu

me

(cc/

g)

Size(Å)

ZHTP-[2]-200 ZHTP-[2]-275ZHTP-[2]-315 ZHTP-[2]-350

(d)(c)

d V(d

)( cc/

Å/g

)

Size(Å)

HTZP-[2]-200 HTZP-[2]-275HTZP-[2]-315 HTZP-[2]-350

(b)

Pore

vol u

me

(cc/

g)

Size(Å)

HTZP-[2]-200 HTZP-[2]-275HTZP-[2]-315 HTZP-[2]-350

Fig. 5. NLDFT pore size distribution and cumulative pore volume of ZHTP-[2] (a and b) and HTZP-[2] (c and d).

Fig. 6. FETEM images of (a) ZHTP-[2]-200, (b) ZHTP-[2]-275, (c) ZHTP-[2]-315, (d) ZHTP-[2]-350.

A. Jain et al. / Microporous and Mesoporous Materials 203 (2015) 178–185 183

chemical activation with ZnCl2 (a–d). The degree of heterogeneity isgreater for ZHTP-[2]-275 than for ZHTP-[2]-200 which is consistentwith the higher mesopore area obtained when ZnCl2 is incorporatedduring hydrothermal pre-treatment at 275 �C. In contrast, the

morphology is comparatively smooth for ZHTP-[2]-315 andZHTP-[2]-350 than for ZHTP-[2]-200 and ZHTP-[2]-275 which isindicative of the deleterious effect of higher hydrothermaltemperature on mesopore development.

Table 5Adsorption capacity for phenol red and phenol (mg/g) on mesoporous activated carbons (BET surface areas and mesoporosity are presented in Table S4, Supporting Information).

Sample ZP-[2] ZHTP-[2]-200 ZHTP-[2]-275 ZHTP-[2]-315 ZHTP-[2]-350 Commercial microporous Commercial mesoporous

Phenol red adsorbed (mg/g) 263 306 351 294 153 95 119Phenol adsorbed (mg/g) 221 254 264 217 132 103 51OFG (meq/g) 0.3 0.68 0.62 0.5 0.35 0.3 0.25

184 A. Jain et al. / Microporous and Mesoporous Materials 203 (2015) 178–185

3.5. Adsorption of phenol and phenol red mixture on carbon

Table 5 presents the adsorption capacities of various activatedcarbons for the removal of the mixture of phenol red and phenol.The highest and lowest adsorption capacities for phenol red andphenol were observed when the hydrothermal treatment temper-atures were 275 �C and 350 �C, respectively, which is consistentwith the mesopore areas of activated carbons obtained from hyd-rochars produced at these conditions. Furthermore, the higheradsorption capacity of ZHTP-[2]-200 and ZHTP-[2]-275 for phenolred (which is an anionic dye, and therefore OFG content is notfavorable) even in the presence of higher OFG content shows theimportance of higher mesopore area which dominated over OFGcontent. Particularly, the difference in adsorption capacitiesbetween ZHTP-[2]-275 and ZHTP-[2]-315 for phenol emphasizesthe importance of mesopores in a competitive environmentpossibly due to blockage of smaller pores by the larger molecules.Similar observations were reported by Takahashi et al. during HRPimmobilization [72] and Zhuang et al. for dye adsorption [20]. Thehigher uptake capacity of mesoporous carbons can be validated bythe higher surface areas of the synthesized carbons compared tocommercial mesoporous and microporous carbons.

4. Conclusions

Hydrothermal pre-treatment conditions used in convertingbiomass (coconut shell) to the hydrochar precursor are shown tohave significant impact on the mesopore area of the resultantactivated carbons. The diffusion and solubility of ZnCl2 used inhydrothermal pre-treatment were optimized for formation ofmesopores by application of appropriate hydrothermal treatmentconditions (ZnCl2 content and temperature). Advantages derivedfrom higher hydrothermal treatment temperatures are temperedby the reduced availability of the ZnCl2 due to reduced solubility.By factoring in these contrasting contributions, a favorableworking range that considers energy requirement (hydrothermaltreatment temperature) and resource usage (ZnCl2:shell ratio) isproposed and is shown to produce activated carbon with up to80% increase in mesopore area compared to conventionallyprocessed biomass. The enhancement in mesopore area is substan-tiated by the high adsorption capacities for a mixture of phenol redand phenol up to 351 and 264 mg/g, respectively.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgement

The financial support received from NUS, Singapore andNational Environment Agency (Environmental TechnologyResearch Programme) of Singapore for the pursuit of this projectare gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.micromeso.2014.10.036.

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