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Article Cite This: ACS Omega 2019, 4, 7088−7095
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Microwave-Assisted Activation of Waste Cocoa Pod Husk by H3PO4 and KOHComparative Insight into Textural Properties and Pore Development Shiela Marie Villota,† Hanwu Lei,*,† Elmar Villota,† Moriko Qian,† Jeffrey Lavarias,‡ Victorino Taylan,‡ Ireneo Agulto,‡ Wendy Mateo,† Marvin Valentin,‡ and Melba Denson‡
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†
Bioproducts, Sciences, and Engineering Laboratory, Department of Biological Systems Engineering, Washington State University, Richland, Washington 99345-1671, United States ‡ Department of Agricultural and Biosystems Engineering, Central Luzon State University, Science City of Muñoz 3120, Nueva Ecija, Philippines S Supporting Information *
ABSTRACT: This study is motivated to increase the value of cocoa pod husk (CPH), which is currently considered as waste in the cocoa farming, by converting it to activated carbon (AC). The main goal is to comparatively evaluate the impact of H3PO4 and KOH as chemical-activating agents on the resulting textural properties of the obtained carbon from CPH. Furthermore, the effects of the impregnation ratio and precursor’s particle size were also examined. In all cases considered, H3PO4-activated CPH obtained a higher yield and more desirable properties over KOH-activated CPH. Characterization of the physisorptive properties such as Brunauer−Emmett−Teller surface area (SBET), total pore volume (Vtotal), and average pore diameter (Dp) also suggested that H3PO4 is the better activating agent. The highest SBET obtained was 1237.47 m2/g from the 2.00:1.00 impregnation ratio. Vtotal (1.11 cm3/g) was also found to be the highest at this condition. Further analyses on the Dp and pore size distribution revealed that AC obtained from H3PO4 was mesoporous. Proposed porosity development on both activating agents suggest that KOH is a more reactive activating agent for CPH than H3PO4, as evidenced by severe material loss and low structural integrity.
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INTRODUCTION With the increasing trend of consumption of chocolates, the global demand has been exceeding the supply. Worldwide initiatives to intensify cocoa industry has been the answer to deal with this imbalance. In the Philippines, combined efforts of the government, international organizations, and private entrepreneurs have developed a roadmap wherein a production of 100 000 tons of cocoa in single cropping season is aimed. Consequently, this increase in cocoa production will result in an increase in agricultural byproducts and the husk has the highest share as it encompasses 70−75% of fruit’s morphology. Historically, cocoa pod husk (CPH) has never been utilized for further use. It has been a common practice to leave it in the field until degradation, posing considerable impact on the landscape, generating unwanted smell, and causing pollution © 2019 American Chemical Society
on water streams. Waste CPH, therefore, can be a sustainable source of raw materials for processing into a more valuable product such as activated carbon (AC). This study intends to explore the potential use of this biomass by converting it to AC a chemical activation route. Single-step chemical activation of biomass precursors has become more and more popular because of its economic potential and advantage over physical activation in terms of processing. Most cited advantage is the relatively lower process temperature, higher carbon yield, and less activation time.1 Use of microwave energy in many works proves less processing Received: December 26, 2018 Accepted: April 8, 2019 Published: April 19, 2019 7088
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time and more energy efficient compared to conventional heating.2−4 At this point, two of the most commonly reported dehydrating agents for biomass-based carbon activation include H3PO45−8 and KOH,9,10 where the former is considered a weak acid while the latter is considered a strong base. The mechanism of the chemical activation and the characteristics of the evolved product highly depends on the type of activating agents. H3PO4 activation involves two-fold actions: first by depolymerizing cellulose, hemicellulose, and lignin while encouraging the formation of cross-linking between carbon polymers through dehydration and condensation reactions, and second by promoting the formation of phosphate and polyphosphate that bridges and further crosslinks the biopolymer fragments derived from previous action. Phosphate groups promote a dilation process that leaves a matrix of an accessible porous structure after elimination of the acid.11,12 On the other hand, KOH activation involves mainly fragmentation and solubilization of biomass components via a reduction reaction while leaving potassium-containing atoms within the carbon network. These and other potassium compound products that are intercalated within the carbon network, which are eventually removed during pyrolysis and washing, promote the porosity of the carbon structure.12 Although there are many reports on the chemical activation of different biomasses, information on utilization of CPH as the precursor was found to be limited. Current work aims to provide and broaden this information while giving emphasis on microwave-assisted carbonization and comparatively outlining the effects of the H3PO4 and KOH as activating agents. Process conditions such as the impregnation ratio and precursor particle size on the textural and physisorptive properties of the evolved AC were also explored. The authors believe that it is important to have a differentiating insight especially when exploring new precursors such as CPH given that up to this point there is no standardized protocol in chemical activation of biomass, thus this study.
Table 1. Average Heating Rates and Total Carbonization Times in Microwave-Assisted Pyrolysis as Affected by Activating Agents and Impregnation Ratios H3PO4-activated AC
impregnation ratio
average heating rate, °C/min
0.50:1.00 1.00:1.00 1.50:1.00 2.00:1.00
58.90 41.20 19.65 7.71
KOH-activated AC
carbonization time, min
average heating rate, °C/min
carbonization time, min
10.64 13.92 25.90 60.37
118.40 46.86 33.72 21.10
6.80 12.60 16.34 24.33
carbonization times. This result was also noted by other researchers on H3PO4-activated biomass and on KOHactivated biomass carbonized under microwave energy.17,18 AC Yield. AC yield is affected both by the extent of the chemical reaction on the particle during chemical activation as well as the heating rate during pyrolysis. Severe chemical reactions due to the chemical type and its concentration can cause significant material loss due to consumption while the high heating rate can lead to undesired burn-off and lead to excessive carbon loss. Figure 1 presents the AC yield by weight percentage as affected by the activating agent, impregnation ratio, and particle size.
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RESULTS Microwave-Assisted Carbonization. Microwave heating is mainly driven by dielectric properties of the material being heated, and because thermal degradation of biomass, biomassactivating agent complex, and excess activating agent involve series of product evolution, the rate of heating is a vital information to look at. In microwave-assisted pyrolysis, microwave energy absorbed by the sample is responsible to cause both volatilization of water and organic gases and increase in temperature of the carbonized and unconverted masses. It can be noted that the H3PO4-biomass complex heats up longer than the KOH-biomass complex as can be seen by comparing the resulting heating rate and total carbonization time (Table 1). This implies that the KOH-biomass complex may have better overall dielectric properties over the H3PO4biomass complex. The metallic byproducts from KOH during activation may have caused this result. Metallic substances when exposed to microwave produce hotspots that can improve heating and heat-induced reactions during carbonization.13,14 Further, Khalil et al. cited that KOH can promote oxidation reactions of carbon on temperature under 700 °C.15 On the other hand, Rosas et al. noted thermal resistance of phosphorus-containing carbon structures in their kinetic study.16 Increasing the impregnation ratio on both chemicals also resulted in heating rate reduction and has led to extended
Figure 1. AC yield as affected by the activating agent, impregnation ratio, and CPH particle size.
Analysis of variance (Table S1) revealed that the interaction of the activating agent and impregnation ratio plays vital role in the obtained AC yield. Table 2 shows mean comparison within cells of the activating agent with respect to the impregnation ratio. The highest AC yield was obtained at H3PO4 and at the Table 2. Mean AC Yield (wt %) as Affected by the Activating Agent and Impregnation Ratio activating agenta impregnation ratio
H3PO4
KOH
0.50:1.00 1.00:1.00 1.50:1.00 2.00:1.00
35.46b 30.83c 36.96ab 39.48a
18.50d 14.74d 7.22e 6.04e
a
Means sharing a similar letter on their superscripts are not significantly different by Fisher’s LSD (α = 0.05).
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presented in Figure S2. Interaction between the activating agent and impregnation ratio was found to cause significant surface area differences among treatment units. Increasing impregnation ratio directly increases SBET on both activating agents, although H3PO4-activated ACs generally have significantly higher SBET than KOH-activated ACs. The result can be accounted to the level of interaction between the activator and CPH, that ishigher impregnation ratio promotes a higher degree of dehydration reaction thereby developing more surface area in the process. Chemical activation with H3PO4 at impregnation ratios 1.50:1.00 and 2.00:1.00 can be considered the best among the treatments considered. On the other hand, increasing impregnation ratio in KOH activation from 1.50:1.00 to 2.00:1.00 caused a significant drop in SBET. This drop can be due to the consumption of the biomass substrate and resulted in widening the meso- and microporous areas to external surfaces. KOH, as can be seen from the scanning electron microscopy (SEM) micrograph (Figure 5c), had caused severe loss of substrate material, indicating a high degree of reaction. Smic and Smes represent the area from the SBET that represents the microporous and mesoporous ranges, respectively. Figure 2
2.00:1.00 ratio while lowest was at KOH at the same ratio. It was also found that increasing the impregnation ratio promoted a higher yield on H3PO4 activation. In KOH activation, a consistent decline in yield with respect to increasing impregnation ratio was observed. Such trend hinted severe reaction of KOH with the biomass even at the low concentration. This comparatively lower yield against H3PO4 may be attributed to a much higher degree of reaction and faster heating rate promoted by KOH. Noted to be a strong base caused higher degree of activation reactions while the metallic potassium-containing reaction products may have induced microwave heating and led to much higher heating rates compared to H3PO4. Higher heating rates and higher degree of reaction resulted to high carbon loss and therefore a comparatively lower yield. Furthermore, main effects are also worth noting by looking at Fisher’s LSD groupings within each factor (Figure S1). H3PO4-activated CPH obtained superiorly higher AC yield across all conditions compared to KOH-activated CPH. It has been noted that H3PO4 activation favors the conversion of aliphatic to aromatic compounds resulting to higher yields of char.19 The lowest impregnation ratio (0.50:1.00) was found to obtain highest yield while the rest was found to not significantly differ from each other. Tracing to the total carbonization time, the lowest ratio was subjected to relatively short pyrolysis time because of rapid heating and therefore minimal losses due to carbon burn-off can be incurred. A higher impregnation ratio resulted to slow heating rates and prolonged heating, increasing the risk of carbon burn-off and results to a lower carbon yield. Lastly, the smallest particle size, 0.25 mm, resulted to a significantly lower yield compared to larger 1.00 and 2.00 mm diameter. Both particle sizes, 1.00 and 2.00 mm, resulted to similar yields. A feedstock particle consumes easily when subjected to severe reactions and fast heating rates. Smaller particle feedstock has the risk of being consumed to very fine particles during activation and completely burned off during carbonization. Surface Area. The Brunauer−Emmett−Teller (BET) method remains to be the most widely used procedure for evaluating the surface area of porous and finely divided materials.20,21 Table 3 shows the SBET of the derived AC as
Figure 2. AC surface area as affected by the activating agent and impregnation ratio.
shows the micropore and mesopore areas of the produced AC from both activating agents as affected by the impregnation ratio and particle size. The highest micropore area of the AC produced with a value of 482.42 m2/g is produced using KOH at 1.5:1.0 ratio and at 1.0 mm particle size while the lowest is 125.63 m2/g when using H3PO4 at the 0.5:1.0 ratio and the 0.25 mm CPH particle size. The results implied that the porous area from the H3PO4-activated AC is less microporous than that of KOH-activated AC. Extended heating and high impregnation ratio can result in the burn-off of the carbon structures and the widening of micropores to meso- and macropores.22,23 Pore Volume. Pore volume is the integrated contribution of all internal void spaces, regardless of pore width. Vtotal is simply the volume accounts on all the porous spaces within a particle or agglomerate. Porosity is defined as the ratio of the total pore volume to the volume of the particle or agglomerate. It can be considered as a supportive property for the surface area−high pore volume implies a high internal surface area (SBET) and vice versa. Table 4 shows the mean pore volume of the AC produced as affected by an activating agent and impregnation ratio. Analysis of variance is shown in Table S3 while comparison among treatment means is presented in Figure S3. As expected, the
Table 3. Mean BET Surface Area (m2/g) of Derived ACs as Affected by the Activating Agent and Impregnation Ratio activating agenta impregnation ratio
H3PO4
KOH
0.50:1.00 1.00:1.00 1.50:1.00 2.00:1.00
325.63c 673.61b 1015.88a 1139.17a
348.83c 406.08c 494.51c 344.65c
a
Means sharing a similar letter on their superscripts are not significantly different by Fisher’s LSD (α = 0.05).
affected by activating agent and impregnation ratio. The particle size is not expected to affect the result of the surface area and other textural properties (pore volume and pore diameter) because the method of analysis of the said properties is irrelevant to the size of the particle being analyzed, so instead, pore sizes were treated as replicates during the analysis. Each cell on the table represents the mean of three replicates. Analysis of variance on the BET surface area is presented at Table S2 while comparison among means is 7090
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Table 4. Total Pore Volume (cm3/g) of the Derived AC as Affected by the Impregnation Ratio and Activating Agent
that H3PO4-activated AC is mostly mesoporous while KOHactivated is microporous. To verify this, further analysis on average pore diameter and pore size distribution were worked out and are discussed in the following section. Average Pore Diameter. Both the size and distribution of micropores, mesopores, and macropores determine the adsorptive properties of porous materials. For instance, small pore size will not trap large adsorbate molecules and large pores may not be able to retain small adsorbates, whether they are charged, polar molecules or uncharged, non-polar compounds.24 Average pore diameter was also estimated from eq 1.25
activating agenta impregnation ratio
H3PO4
KOH
0.50:1.00 1.00:1.00 1.50:1.00 2.00:1.00
0.236d 0.552c 0.837b 1.062a
0.184d 0.207d 0.258d 0.176d
a
Means sharing a similar letter on their superscripts are not significantly different by Fisher’s LSD (α = 0.05).
result of the ANOVA shows the same trends as the surface area. Interaction of activating agents with the impregnation ratio mainly affects the resulting pore volume. Generally, the total pore volume increases with the impregnation ratio used a very similar trend with the SBET. H3PO4-impregnated CPH at 2.00:1.00 ratio produced the highest pore volume of 1.060 cm3/g while the KOH-impregnated CPH at 0.50:1.00 ratio produced the lowest pore volume of 0.168 cm3/g. Resulting trends on pore volume can be accounted in a way similar to the resulting SBET. In addition to pore volume, AC’s mesoporous and microporous profiles are also important because these characteristics outline its applications (e.g., liquid vs gaseous adsorption). Vmic accounts for the microporous portion of the total pore volume. Similarly, with Smic and Vtotal, Vmic is typically calculated by the t-plot method and is part of the report of the physisorption analysis. Figure 3 shows the
Dp =
4Vtotal × 1000 SBET
(1)
Table 5 shows the means of the estimated pore diameter of the produced ACs as affected by the activating agent and Table 5. Mean Pore Diameter of the Derived AC as Affected by the Activating Agent activating agent
pore diametera
H3PO4 KOH
3.228a 2.190b
a
Means sharing a similar letter on their superscripts are not significantly different by Fisher’s LSD (α = 0.05).
impregnation ratio. Analysis of variance (Table S4) revealed that the only factor that affects the resulting AC pore diameter is the type of activation agent used on the CPH with H3PO4activated CPH having significantly larger diameter than KOHactivated precursors. Figure S4 shows the result of the mean comparison between activating agents. Comparing the results of the main properties with other similar works (Table 6) show our work obtained comparable results. In general, it is evident that the H3PO4-activated biomass offers a higher BET surface area and pore volume than that of the KOH-activated biomass. In terms of average pore diameter, both activating agents seem to yield mesoporous activated carbons. Pore Size Distribution. Pore size distributions are obtained from N2 isotherms produced by the physisorption analysis following the Barrett−Joyner−Halenda (BJH) method.17,26,27 In the BJH method, specific pore volume dV/dDp is plotted against pore diameter (Dp) and the cumulative frequency of the given pore size range is defined by the area under the curve. In this analysis, AC having the most desirable SBET and Vtotal was considered from both chemicalsthese are AC activated at 2.0:1.0 impregnation ratio from the H3PO4activated group and AC activated at the 1.5:1.0 KOH-activated group. Pore size distribution and the corresponding N2 isotherm of the respective AC are shown in Figure 4. The figure is marked with a broken line at the 2 nm mark to indicate the critical diameter that identifies mesopores from micropores. In H3PO4-activated AC, it can be observed that the area that falls in the mesoporous range (area to the right of the broken line) is more dominant than the microporous range, implying that AC on this group is mainly mesoporous. This is supported by its sorption isotherm (inset figure) which clearly shows a Type IVa isotherm which is a property of mesoporous adsorbents.28 On the other hand, KOH-activated AC pore size distribution revealed its microporous nature. By definition, microporous ranges from 2 nm and below, and as
Figure 3. AC pore volume as affected by the activating agent, impregnation ratio, and CPH particle size.
breakdown into micropore and mesopore portions of the total pore volume of the AC produced as affected by the activating agent, impregnation ratio, and particle size. The highest Vmic (0.238 cm3 /g) was produced on 1.5:1.0 ratio KOHimpregnated CPH while the lowest (0.065 cm3/g) was produced by using a 0.5:1.0 ratio H3PO4-impregnated CPH. Vmeso, on the other hand, accounts for the integrated volume due to mesopores from the total pore volume and is quantified by simply subtracting Vmic from the Vtotal. The calculated mesopore volume of H3PO4 is significantly higher than that in KOH. The highest mesopore volume was found using H3PO4 (1.044 cm3/g) at 2.00:1.00 ratio. Furthermore, an increasing trend can be noted in the mesopore volume as the concentration increases using H3PO4. Mesopore volume measured on KOH-activated AC, on the other hand, showed no significant differences across all the conditions used in the study. Results on the micropore and mesopore volume suggest 7091
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following textural and pore development mechanism with respect to activating agent can be outlined. At least three stages during chemical activation is proposed as shown in Figure 6. Stage I is the initiation of the activation process which involves development of primary pores and cavities. This starts immediately upon contact of the activating agent and usually done at normal room temperature. The concentration of the activating agent plays vital role in the rate of diffusion and coverage of the precursor particle surface. The particle size dictates the initial accessible surface so higher surface area of the particle (smaller particle size) is usually preferred. Stage II is the pore network development. At this stage, the primary pores widen due to consumption and internal walls offer new sites for activation (secondary pores). Depending on the reactivity or dehydrating strength of the activating agent, both primary and secondary pores can widen into mesopores or into macropores as this stage progresses. Stage II proceeds until the activating agent or the precursor is exhausted. It is at this stage that the porosity of the evolved AC is developed. The third and last stage (Stage III) of the chemical activation is the completion stage wherein exhaustion of the activating agents or the precursor occurs. The impregnation ratio directly affects the resulting activated region on a precursor particle. In the practical context of chemical activation, the adequate impregnation ratio should be the balance amount of the activating agent to the biomass precursor that will yield to the desired product properties. Reactivity of the chemical activator to the biomass substrate mainly affects this balance. Practically, depth of the activated region in line with relevant textural property can be indices whether the impregnation ratio is adequate or not, which are most likely designed based on the desired application of the evolved AC. SEM micrographs seen in Figure 5 show the external and activated regions of the carbon during activation and microwave-assisted pyrolysis for H3PO4 and KOH. For H3PO4-activation, there is a low to a mild degree of activation severity wherein increasing the impregnation ratio results in better textural results (e.g. increasing SBET). It can also be observed that using H3PO4, the particle core noticeably remains intact and the activation was limited up to some depth with respect to the surface. In contrast, KOH-activated AC shows large and deep cavities, which depicts that the core of the particle is accessed and consumed resulting in low structural integrity particles. Relatively low yield further supports this claim. Given that the range of the impregnation ratio was similar with both activating agents, the result suggests that KOH is a stronger dehydrating agent over H3PO4.
Table 6. Textural Properties Comparison with Other Related Worksa textural properties
biomass cocoa pods (this work) Acacia mangium wood29 poplar sawdust30 date palm stem31 olive residue32 Willow33 Scots pine bark33 banana fruit bunch34 Delonix regia pods34
pore diameter (nm)
activating agent
BET surface area (m2/g)
pore volume (cm3/g)
H3PO4
1237.47
1.11
3.23
KOH H3PO4
494.51 1161.29
0.26 0.57
2.19 1.96
KOH H3PO4
167.68 1133.30
0.09 0.44
2.17 NR
KOH H3PO4
761.20 1100.00
0.34 1.15
NR 4.16
KOH H3PO4 KOH H3PO4 KOH H3PO4
947.00 771.00 1390.00 376.00 1594.00 339.00
0.85 0.57 0.61 0.19 0.57 0.13
3.60 0.81 1.27 1.50−2.00 1.50−2.00 1.50−2.00
KOH H3PO4
1056.00 15.37
0.40 0.46
1.50−2.00 57.00
KOH H3PO4
1.04 22.29
NR 0.26
NR 61.70
KOH
0.01
0.35
NR
a
NR-not reported.
can be seen in the plot, the curve lies mostly in that region. The accompanying sorption isotherm supports this result as it can be observed that it exhibits type I(b) isotherms which are typically found with materials having pore size distributions over a broader range including wider micropores and possibly narrow mesopores (
