Zinc Chloride as a Catalyst in Hydrothermal Carbonization of Cocoa Pods Husk Understanding the Effect of Different Carbonization Temperature and ZnCl 2 Concentration

In this study, we have successfully synthesized a high surface area of activated carbons (ACs) from cocoa pod husk (CPH) by using a combination of hydrothermal carbonization (HTC) and chemical activation in the presence of ZnCl 2 as both of catalyst as well as activating agent. During HTC, the effect of HTC temperature (200 and 225 °C) and ZnCl 2 to biomass mass ratio (1:1, 2:1, 3:1) as a catalyst to the characteristics of the obtained hydrochar (HC) and ACs were investigated. The obtained hydrochar (HCs) was then processed by using ZnCl 2 chemical activation with a fixed HC to ZnCl 2 mass ratio of 1:4 and the mixture was pyrolyzed at 600 °C for 1 h under an inert atmosphere. The results showed that the addition of catalyst during HTC presented different AC morphology, where the carbon samples were decorated with carbon microspheres. An increase of surface area was observed, where the ACs synthesized from catalyzed HC at biomass to ZnCl 2 ratio of 1:3 at temperature 200 °C gave a higher surface area of 1954 m 2 /g compared to that of without catalyst (1165 m 2 /g). The catalysis effect was more profound at the HTC temperature of 200 °C compared to 225 °C, as reflected in the significant increase of AC surface area. The addition of a catalyst creates ACs with narrow pore distribution compared to that of synthesized in the absence of a catalyst. It was also observed that the ACs from catalyzed hydrochar possessed higher oxygenated functional groups (OFG) than those without catalysts.


Introduction
Cocoa (Theobroma cacao L.) is a tropical commodity crop from which cocoa beans are obtained.The top 5 cocoa-producing countries include Ivory Coast, Ghana, Indonesia, Nigeria, and Cameroon.In 2020, around 4.7 million tons of cocoa beans were produced globally [1].From cocoa beans, various derivative products can be obtained, including cocoa nibs, cocoa mass, cocoa butter, and cocoa powder [2].The cocoa bean makes up 33% weight of cocoa fruit, leaving 67% of the unused pods, known as cocoa pod husk (CPH) [3].Furthermore, according to Vriesmann et al. [4], ten tons of CPH are generated per ton of dry cocoa beans, resulting in a huge amount of agricultural waste that needs to be managed.On the other hand, CPH contains relatively high cellulose (23.8%), hemicelluloses (8.2%), and lignin (33.4%) [5] which makes it a suitable source for the synthesis of activated carbon (AC) [6].
Conversion of CPH into carbon materials such as hard carbon as well as activated carbon has been attempted by our groups [5,7] as well as several other reports.Our group is the first who investigate the ACs synthesis from CPH by hydrothermal carbonization (HTC) and chemical activation [7].We obtained a surface area of 1694.91 m 2 /g which is larger than the one produced via pyrolysis or other methods.In those works, we investigated the effect of different HTC temperatures on the characteristics of activated carbon produced.Several advantages of HTC are its suitability to treat biomass with high water content, lower operation temperature, and unique properties, such as high oxygenated functional groups of the resulting product [8].At HTC condition (180-250 °C), the water is at the subcritical phase, in which the ion product constant is higher compared to that at room temperature, making an acidic environment that is suitable for a hydrolysis reaction.It is generally accepted that the hydrolysis reaction is the initial step in HTC lignocellulose conversion, followed by dehydration, decarboxylation, polymerization, and aromatization [9].During HTC, several organic acids are also produced as the by-product that can also act as hydrolysis catalysts, making the HTC process usually done without a catalyst addition.
Even so, the introduction of additives in the HTC process is known to be able to influence the properties of hydrochar (HC) such as enhanced degree of carbonization, surface modification, and inclusion of heteroatoms that could be beneficial in the AC synthesis [10].The addition of acids such as citric acid [11], ascorbic acid [12], HCl, and HNO 3 [13] have been reported in the synthesis of hydrochar from various biomass.Furthermore, several metal salts such as CeCl 3 [14][15][16], FeCl 3 [17], CsCl, SnCl 2 [18] have been utilized as HTC catalysts.The utilization of Zinc Chloride ( ZnCl 2 ) as an HTC catalyst has also been reported by several researchers [18][19][20][21][22].However, to the best of the authors' knowledge, the combined effect of temperature and ZnCl 2 (as a catalyst) to biomass mass ratio in the HTC process to the ACs characteristics have never been reported before in CPH conversion.
In this study, we investigated the effect of ZnCl 2 as a catalyst in the HTC process of CPH.The HTC was done at various CPH to ZnCl 2 mass ratios and HTC temperatures.The obtained HCs were subsequently activated by using a chemical activation process with ZnCl 2 as an activating agent.The physicochemical characteristics of the HCs and ACs carbon products were analyzed and compared with the ones without HTC catalysts.We proved that the addition of catalyst and carbonization temperature were closely related.

Experimental 2.1 Preparation of CPH
CPH was obtained from a cocoa plantation in Indonesia and was used as a biomass precursor in this study.CPH was repeatedly washed using distilled water to remove impurities, followed by oven-drying at 105 °C for 24 h.The dried CPH was then crushed, ground, and sieved to obtain CPH powder with a size of −30 + 50 mesh (±0.446 mm).The obtained CPH powder was kept dry until used.

Synthesis of hydrochar
Hydrochar was synthesized through high-temperature carbonization (HTC) by adding ZnCl 2 as a catalyst for carbonization.In each experiment, a 4 g of CPH powder was mixed with 80 mL of ZnCl 2 (p.a., Merck, Germany) solution with various ZnCl 2 to CPH mass ratios (1:1, 2:1, 3:1).The mixture was mixed by using a magnetic stirrer for 15 min followed by ultrasonic treatment for another 30 min.The mixture was then hydrothermally carbonized by using a 100 mL of an autoclave with a Teflon liner in an oven at temperatures of (200 and 225 °C) for 24 h.After HTC, the obtained HC was then separated from the liquid product and washed using ethanol (96%, Bratachem, Indonesia) and distilled water repeatedly.The HC was then oven dried at 105 °C for 24 h.As a comparison, a similar experiment was also done by using only distilled water as the HTC medium.The HC sample was coded as HC-x-y, where x is the HTC temperature, and y is the ratio of ZnCl 2 .

Synthesis of activated carbon
Hydrochar obtained from HTC was activated by chemical activation, using the same chemical ( ZnCl 2 ) as activating agent.Synthesis of ACs was done by impregnating HC/ZnCl 2 with a mass ratio of 1:4.The mixture was mixed using a magnetic stirrer for 15 min and left to impregnate for 24 h.The mixture was then oven-dried at 80 °C for 24 h.The pyrolysis process was done by using a tubular furnace at 600 °C for 1 h under an inert N 2 atmosphere.The obtained ACs were washed using 0.1 M HCl solution followed by distilled water until neutral pH.The washed ACs were then dried at 105 °C for 24 h.The dried ACs were then stored and kept dry in a sealed container.The ACs sample are denoted as AC-x-y where x is the carbonization temperature and y is the catalyst ratio during HTC.The commercial AC (Merck, pa, CAS#7440-44-0) was used as a comparison.

Characterization
The obtained HCs and ACs were characterized by several methods.The yield of HCs and ACs was obtained by gravimetric.The scanning electron microscope (SEM, Hitachi SU3500, Japan, and JEOL JSM 6510-A, Japan) was used to observe the morphology of the samples.The functional groups of HCs and ACs were characterized using Fourier Transform Infra-Red Spectroscopy (FTIR, Prestige 21 Shimadzu Instruments) with KBr pellet method (carbon/ KBR ratio is 15/175 mg).The pore characteristics were characterized using an N 2 adsorption-desorption study at 77K (Micromeritics TriStar II).The crystallinity of ACs was characterized using X-ray diffraction (XRD, Bruker D8 Advance, USA).

Results and discussion 3.1 Yield of HCs and ACs
The effect of the ZnCl 2 catalyst on the HCs yield is presented in Fig. 1 (a).An increase of catalyst to CPH ratio from HC-0 to HC-3 lowered the HC yield from 63.16 to 53.6% and 57.4% to 49.8% for HTC temperature from 200 to 225 °C respectively.The ZnCl 2 is known as a Lewis acid that could act as a catalyst during the HTC process.The catalyst would accelerate the reaction of cellulose, hemicelluloses, and lignin decomposition into gaseous products such as CO and CO 2 , thus decreasing the yield of solid products.Furthermore, the HTC-225 also showed a lower yield compared to HTC-200 due to a higher devolatilization rate that resulted in more gaseous products at a higher HTC temperature [23].Similar trends have been previously reported [6,24,25].
The effect of the HTC catalyst on the yield of ACs is presented in Fig. 1 (b).An increase in the HTC catalyst ratio also shows decreasing the ACs yield.It is known that during chemical activation, ZnCl 2 would inhibit tar formation and promote the decomposition of precursor to volatile gases that result in a pores formation [26].An increase in ZnCl 2 ratio during the HTC process facilitated more lignocellulose decomposition [27], and facilitated the chemical activation, producing more pores which contributed to the lower yield.

Functional groups of HC and AC
Observation of functional groups in the CPH, HC, and AC are presented in Fig. 1 (c), (d).Several notable peaks from the CPH spectrum are the O-H stretching around 3000-3600 cm −1 due to absorbed moisture in CPH, C-H vibrational stretching around 2800-3100 cm −1 , the C=O stretching at 1760 cm −1 and C=C stretching at ~1600 cm −1 of hemicelluloses structure, and C-O stretching at ~1100 cm −1 from the glycosidic bond of cellulose structure, and peaks around 700-900 cm −1 that indicates lignin's C-H vibration [28,29].Upon carbonization, the chemical transformation of CPH to hydrochar could be observed clearly with changes of several notable peaks as shown in Fig. 1 (c).The C=O stretching was dominant in hydrochar than that of CPH showing the presence of aromatization and decomposition of dehydration product.Lower C-O stretching was due to the decomposition of lignocellulosic biomass resulting in lower C-O intensity in hydrochar.Spectra of C-H and O-H were similar however with lower intensity in hydrochar than CPH [7].
In the presence of ZnCl 2 , the peaks of C=O and C=C were strengthened, indicating the stronger aromatization and decomposition of dehydration products in the presence of a catalyst, which agrees well with Li et al. [19].Bands of O-H, C-H, and C-O were somewhat similar for all HCs.
The observation of functional groups of the AC samples is presented in Fig. 1 (d).It could be observed that the FTIR spectra of both samples are different from their respective HCs spectra.This indicates complete transformation of HC to AC after chemical activation.Several peaks are still present in the AC samples, namely the O-H stretching (300-3600 cm −1 ) due to the presence absorbed water and phenolic functional groups, C=C stretching (~1570 cm −1 ) of aromatic carbon ring structures, and C-O stretching (1150 cm −1 ) that indicates carboxylic acid, lactone, and carbonyl functional groups [30,31].The C-H peak around 3000 cm −1 is no longer present in the AC samples due to removal of C-H structures during activation [32].So do C=O stretching around 1760 cm −1 was disappear.Furthermore, it is notable that the AC-3 sample has higher oxygenated functional group content compared to AC-0, as reflected in the sharper peaks of O-H and C-O stretching.We hypothesize that this trend could happen due to the presence of ZnCl 2 during the HTC process has helped more complete conversion of CPH to carbon structures, with more volatiles already released during the HTC process, compared with one without catalyst.Thus, during chemical activation, the more functional groups could be preserved in the AC.

Morphology of HC and AC
Observation of the CPH and HC morphology is presented in Fig. 2 Fig. 2 (a) showed a well-defined cell wall structure of CPH that was decomposed after HTC.At HC-200-0, a resemblance of CPH structure is still observable, but with more irregular morphology and destroyed cell walls.With the addition of ZnCl 2 catalyst (HC-200-3; Fig. 2 (c)) and an increase in HTC temperature (HC-225-0; Fig. 2 (d)), the cell wall structure could not be observed in the HC samples.Furthermore, on the HC-225-3 sample (Fig. 2 (e)) some cavities are observed on the surface, indicating initial pore formation after HTC.
Observation of AC samples' morphology using SEM is presented in Fig. 3.It shows that the AC-200-0 and AC-225-0 exhibited a similar morphology, in the form of a boulder shape with irregular cavities observed on the ACs surface.This morphology is commonly found on ACs from ZnCl 2 chemical activation due to ZnCl 2 evaporation during the activation process [33].On the other hand, the AC-200-3 and AC-225-3 samples possessed different surface morphology, compared to the one without catalyst.It could be seen that agglomerates of carbon microspheres are decorating the ACs surface with random macro cavities.As previously explained, the presence of ZnCl 2 catalyst during the HTC process help the hydrolysis of the glycosidic bond of cellulose and hemicelluloses and resulted in abundant sugar monomers in the solution [6].The monomers would further be transformed via dehydration, condensation, and polymerization into a spherical carbon structure [34].Similar carbon microspheres structure due to catalyzed HTC has also been reported by previous researchers [6,18].

Pore characteristics
The pore characteristics of HC samples are presented in Fig. 4 and Table 1.It shows that the HC isotherm curves exhibited a transient between type II and IV isotherm with H 3 type hysteresis loop that indicates the presence of macropores in the AC structure.Furthermore, from the BJH pore distribution curve (Fig. 4 (b)), it could be observed that the HC still possessed a wide mesopore distribution, with an average pore diameter of around 15 nm (Table 1).Furthermore, the effect of HTC temperature and catalyst concentration to the surface area and pore diameter of HCs were not significantly different, as seen in Table 1.This is also supported by relatively similar HC morphology in Fig. 2.
Observation of the nitrogen adsorption and desorption isotherm of ACs is presented in Fig. 5 (a), (b).It shows that all samples exhibited a type IV isotherm with H 2 type hysteresis loop, according to IUPAC classification that shows all ACs samples exhibited complex mesoporous structures [35], except for AC-200-3 and AC 225-0.The type IV isotherm typically shows a hysteresis loop around relative pressure of 0.4 to 1.0 due to capillary condensation of nitrogen in the mesopores of AC structures during the desorption process.The step loops around 0.4-0.6 demonstrates these ACs samples possess uniform mesoporous structures [36].This result is in accordance with the BJH pore distribution curve, presented in Fig. 5 (e), (f), with more uniformly distributed pores around 2-5 nm.The AC-200-3 sample exhibited a typical type I isotherm that indicates a more dominant micropores structure of AC, as also reflected in the BJH distribution that is dominantly for pores <4 nm.A higher concentration of ZnCl 2 seems tends to increase the surface area by decreasing pore size.Furthermore, the AC-225-0 sample exhibited a transient between type II and IV isotherm with H 3 type hysteresis loop that indicates the presence of macropores in the AC structure which are not completely filled with the pore condensate [37].This statement is also in line with the wide pore distribution that is observed in the BJH analysis.The commercial AC as shown in Fig. 5 (c) exhibited a type IV isotherm with a type H 4 hysteresis loop.The more pronounced uptake at low P/P 0 was associated with the filling of the micropores [37].
It is known that there are several roles of ZnCl 2 during the chemical activation process [38].From room temperature to 160 °C the evaporation of water and dehydration of biomass by ZnCl 2 occurred.During an increase of temperature to 280 °C, ZnCl 2 catalytically assists the cellulose degradation through hydrolysis, oxidative degradation, and further dehydration, along with swelling of the biomass structure that led to mesoporous structures.From temperature 280-400 °C further pore formation and aromatization of carbon structure occurs, and further increase of temperature lead to decomposition and vaporization of ZnCl 2 and pore development via formation spaces between carbon layers.At a high ZnCl 2 ratio as activating agent, it is also known that pore widening is the dominant mechanism during the activation, resulting in wide distribution of pore size, as obtained in AC-0.Similar results have been reported by previous researchers [39,40].On the other hand, HTC with ZnCl 2 catalyst affects the pore development in the AC.As previously mentioned, the AC-1 to AC-3 possessed pores ranging from 2-10 nm with narrower distribution than AC-0.
The pore characteristics of AC samples are presented in Table 2.It could be seen that all AC samples in this study possessed higher surface area compared to commercial AC (Merck).Furthermore, it could be seen that the use of a catalyst during HTC process gave an effect on the surface area of AC samples, with the highest surface area of 1954 and 1885 m 2 /g obtained for AC-200-3 and AC-225-2 samples respectively, compared with AC-200-0 (1165 m 2 /g) and AC-225-0 (1694 m 2 /g).Compared to AC-225 samples, the difference in surface area for AC-200-0 and AC-200-3 is more notable with an increase of surface area almost two times (~790 m 2 /g) compared to the one without any catalyst.As previously mentioned, during HTC, ZnCl 2 could act as a Lewis acid that catalyzes the scission of glycosidic bonds in cellulose structure starting from a temperature of 200 °C [27].Without a catalyst, the HTC temperature was the only factor that influence the hydrolysis of CPH and resulted in different pore structures.Higher carbonization temperature is known to be more beneficial in the pore formation of the HC [11], while the initial pore formation in HC is essential to promote a more porous structure of the AC [41].This could explain the observation of a higher surface area of AC-225-0 compared to AC-200-0.Furthermore, for both temperatures, various catalyst ratios gave a relatively close surface area of around 1750-1950 m 2 /g.Thus, it could be seen that the effect of catalyst addition is more profound at a lower HTC temperature of 200 °C.
These results obtained were in line with previous studies.MacDermid-Watts et al. [17] reported an increase of 200 m 2 /g surface area of AC sample without catalyst and with FeCl 3 catalyst in the synthesis of AC from corn distiller's fibers.A similar result was also reported by Wu et al. [42] during synthesizing AC from lignin, where an increase of surface area up to 300 m 2 /g was observed with the addition of ZnCl 2 during HTC.Susanti et al. [16] also reported an increase of AC surface area of ~300 m 2 /g for sample obtained from CeCl 3 catalyzed HTC combined with KOH chemical activation, compared to one without catalyst.

Crystallinity of AC
The XRD diffractograms of AC samples are presented in Fig. 6.It shows that there are two broad peaks observed in all AC samples, indicating the amorphous nature of the AC samples.The broad peaks centered around 26.5° and 43° correspond to the (002) and (100) planes of graphitic carbon structure, respectively.Furthermore, there are some ZnO peaks observed in the AC-0 samples that might come from an imperfect washing process.The ZnCl 2 was impregnated into the biomass and hydrolyzed to H[ZnCl 2 (OH)] during the impregnation step.During activation at 600 °C, the H[ZnCl 2 (OH)] was decomposed into ZnO and HCl.
A comparison of this work with previous studies under similar synthesis methods (HTC and pyrolysis) is presented in Table 3 [20,[41][42][43].It could be observed that the AC synthesized from CPH with the combination of  [44,45].The high surface area AC obtained in this study make the AC from CPH with the combination of ZnCl 2 catalyzed HTC and ZnCl 2 chemical activation a prospective material for various application such as adsorbent [46], catalyst support [47], and supercapacitor electrode [48].

Conclusion
In this study, the effect of HTC temperature and ZnCl 2 as a catalyst during HTC process on the characteristic of AC was investigated.ZnCl 2 could act as a Lewis acid catalyst that assists the hydrolysis process during HTC, as indicated in the FTIR analysis and the decrease of HC yield with the increase of catalyst concentration.Further conversion of HC to AC was done by using chemical activation at the same HC:ZnCl 2 ratio of 1:4 and heating at 600 °C for 1 h.It could be observed that different morphology of AC was obtained for AC samples with and without catalyst, whereas with ZnCl 2 catalyst, the AC had a porous structure decorated with carbon microsphere.Furthermore, the effect of the catalyst on the AC surface area is more profound at HTC temperature of 200 °C as shown in the high difference in the AC-200-3 surface area of 1954 m 2 /g compared to one without catalyst (1165 m 2 /g).The increase in the surface area could come from the initial porous structure of catalyzed HC could help in the development

Fig. 1
Fig. 1 Percentage of (a) HCs yield, (b) ACs samples and FTIR spectra of (c) CPH and HCs, (d) ACs samples at various catalyst ratios and HTC temperature

Table 2
Pore characteristics of AC samples

Table 3
Comparison of the surface area of various AC reported in the literature