Direct sulfonation of cacao shell to synthesize a solid acid catalyst for the esterification of oleic acid with methanol

Direct sulfonation of cacao shell to synthesize a solid acid catalyst for the esterification of oleic acid with methanol

Renewable Energy 152 (2020) 320e330 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Dir...

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Renewable Energy 152 (2020) 320e330

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Direct sulfonation of cacao shell to synthesize a solid acid catalyst for the esterification of oleic acid with methanol Czarina M. Mendaros a, Alchris W. Go b, *, Winston Jose T. Nietes a, Babe Eden Joy O. Gollem a, Luis K. Cabatingan a, ** a b

Department of Chemical Engineering, University of San Carlos, Talamban, Cebu City, 6000, Philippines Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Keelung Road, Taipei City, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 July 2019 Received in revised form 6 December 2019 Accepted 15 January 2020 Available online 20 January 2020

Solid acid catalyst (SAC) was synthesized via direct treatment of cacao shells (CS) with concentrated sulfuric acid at varying temperature (80, 100, 120  C) and time (4, 6, 8 h) settings. The synthesized catalysts were found to have sulfonic acid density ranging from 0.6326 to 0.8500 mmol SO3H/g dry CSSAC and total acid density from 6.0509 to 7.1165 mmol Hþ/g dry CS-SAC. The catalytic activity is dependent on the sulfonic acid density of the catalyst. Catalyst synthesized at 120  C for 6 h showed the highest sulfonic acid density (0.85 mmol SO3H/g catalyst) corresponding to highest catalytic activity (5.73 mmol OA converted/mmol SO3H$h) and a conversion of ~39% after a fixed reaction time of 4 h and carried out at 65  C. Conversions of up to 76% could be achieved after 24 h. The catalyst was reused for 4 cycles and was able to retain 78% of its catalytic activity from the 2nd to the 4th cycle. Direct sulfonation may be an alternative to conventional synthesis process. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Acid density Cacao shell Direct sulfonation Esterification Solid acid catalyst

1. Introduction Biodiesel is a series of mono-alkyl esters of long-chain fatty acids commonly produced via transesterification of refined oils using a base catalyst [1e3] and it is used as an alternative to petrodiesel. Biodiesel production via base-catalyzed transesterification have been studied in much detail in the past year including detailed investigation on mass transfer limitations of the pseudohomogeneous reaction involved [4], reactor configuration [5] and scaling up from laboratory experiments to industrial scale [6]. However, biodiesel is more expensive than petro-diesel owing to the feedstock used, which accounts for 70e90% of the total production cost [7]. As a way of addressing this problem, low-quality oils such as waste cooking oil (WCO) are considered. The main bottleneck with the use of low-quality oils is its high free fatty acid (FFA) content (0.5e20 %w/w), which exceeds the limit (<0.5 %w/w of the oil) [7e9] when using base catalyst. High FFA content results in the consumption of the base catalyst due to saponification,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A.W. Go), [email protected] (L.K. Cabatingan). https://doi.org/10.1016/j.renene.2020.01.066 0960-1481/© 2020 Elsevier Ltd. All rights reserved.

leading to its deactivation and difficulty in product separation. Pretreatment via esterification using acid catalyst, commonly sulfuric acid (H2SO4), is required to reduce the FFA content to below 0.5 %w/ w prior to base-catalyzed transesterification [10]. Unfortunately, the use of mineral acids like H2SO4 have its drawbacks, which includes corrosion and generation of by products during neutralization [6]. For ease of handling of the acid during and after the pre-treatment step, solid acid catalysts (SACs) are considered as alternatives to homogeneous liquid acid catalysts. Solid acid catalysts can be prepared by attaching active functional groups to a solid support via physical or chemical method forming active sites which catalyzes the desired reaction. Solid acid catalysts can be zeolite-non-renewable polymeric- or renewable carbon-based [11]. Addressing the drawbacks of zeolite- and non-renewable polymeric-based SACs owing to their non-renewable nature are carbonbased SACs derived from renewable sources whose supports are sourced from widely available renewable materials and allow for covalent bonding of acid functional groups to the carbon support [12,13]. Carbon-based SACs are synthesized by subjecting the carbonaceous material to pyrolysis to produce the carbon (catalyst support) which is then subjected to sulfonation for the attachment of the active sites. Materials that have been studied as sources of carbon supports for SACs include: refined carbohydrates (glucose, sucrose

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and starch) [14e16]; lignocellulosic biomass such as peanut hull [17], wood [17] and Terminalia catappa [18]; agricultural and agroindustrial biomass residues such as sugarcane bagasse [19], spent coffee residue [20], wood chip [17] and coconut shell [21]. Refined carbohydrates are derived from a parent biomass, requiring pretreatment and purification steps adding to the total cost of catalyst synthesis. While lignocellulosic biomass are widely available, these are utilized for several other applications (e.g. food, agriculture, energy), thus, competing with different industries. As an alternative, agricultural and agro-industrial biomass residues are looked into. Synthesis of carbon-based SAC can be done by a twostep carbonization-sulfonation process or by direct sulfonation. Carbonization is a thermal decomposition technique, conducted at a temperature range of 300e600  C for 0.5e4 h [20,22e25]. This is done to break the chemical bonds of the material, rearranging its molecules to form a highly cross-linked, multi-ringed aromatic structure where functional groups, namely, carboxylic acid (eCOOH) and phenolic (eOH) are produced, then sulfonic (eSO3H) acid is easily attached via sulfonation using concentrated sulfuric acid (H2SO4) as an activating agent [17]. Direct sulfonation is done at a temperature range of 20e180  C for 1e10 h wherein simultaneous carbonization and sulfonation happens, using H2SO4 which acts as a dehydrating and sulfonating agent [26e30]. This requires lesser energy compared to that of the two-step carbonization-sulfonation process considering that both employs similar conditions during sulfonation but direct sulfonation avoids the need to carry out carbonization as a pretreatment process. Cacao shell is an example of an agro-industrial biomass residue generated from cocoa bean processing for chocolate production. In the Philippines, ~7000 metric tons (MT) of cacao shells are generated annually [31]. Due to its high carbon content (~50 %w/w), researches conducted using cacao shell involved conversion to activated carbon which were then used as adsorbent [32e34]. An important factor to consider in assessing the potential of a material as solid acid catalyst support is its fixed carbon (FC) content. This component is the remaining carbon of the material after carbonization. Cacao shells have a fixed carbon content of ~24.46 %w/w [22], which is comparable to other residues (11.71e35.2 %w/w) used as raw material for synthesis of solid acid catalysts [35e43]. A study on the synthesis of solid acid catalyst from cacao shell using two-step carbonization-sulfonation has been done by Bureros et al. [22], with resulting catalysts that are comparable with other carbon-based SACs in terms of performance and reusability. Synthesis of CS-SAC via direct sulfonation was carried out in this study to determine whether this method will lead to similar performance as the CS-SAC synthesized using two-step carbonization-sulfonation. This study aimed in particular to investigate the effects of direct sulfonation temperature (80e120  C) and time (4e6 h) on the catalyst yield, sulfur content ratios, acid densities and catalytic activity of the CS-SAC. Acid densities of the spent catalysts were determined to assess the stability of the synthesized catalysts. The reusability of the CS-SAC with the highest observed catalytic activity was also studied. 2. Materials and methods Cacao shells were sourced from Catmon, Cebu, Philippines from a small-scale cacao bean processor to produce chocolate tablets known locally as tablea. Chemical reagents as supplied by (a) Scharlau (Spain) brand: ethanol (99.8 %v⁄v), methanol (99.8 %v⁄v), n-hexane (96 %v⁄v), magnesium oxide (99.9 %w⁄w), silver nitrate, phenolphthalein indicator (98 %v⁄v), sodium chloride (99.9%w/w), anhydrous sodium carbonate (99.8 %w⁄w), sodium hydroxide pellets (97 %w⁄w); (b) Merck (Germany) brand: sulfuric acid (98 %v⁄v), potassium hydroxide (85 %w⁄w); (c) Ajax (Australia) brand:

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hydrochloric acid (36 %w⁄w), barium chloride dehydrate (99 %w⁄w), oleic acid (99 %w⁄w), and potassium hydrogen phthalate (99.8e110.2 %w/w). 2.1. Collection and storage of cacao shell Three batches of cacao shells as received were sorted and representative samples were analyzed for moisture content. Because the moisture content for all batches were found to be less than 10 %w/w, the shells were directly milled using a Wiley mill (Model 4, Thomas Scientific, USA) fitted with 2000 mm aperture screen. The milled cacao shells were then stored in tightly sealed plastic containers until further use. 2.2. Characterization of cacao shell Milled cacao shells were subjected to sieve analysis for determination of the average particle size. The procedure for proximate analysis of cacao shells was adopted from ASTM method D1762-84 (Reapproved 2007) [44] for chemical analysis of wood charcoal. Milled cacao shells were characterized for its total sulfur content, total acid density (rTAD ), carboxylic acid (rCOOH ) and phenolic acid (rOH ) density. Total sulfur content analysis was done through the Eschka method, as described in ASTM D3177-02 [45]. Sulfonic acid density (rSO3H ) was then estimated from the total sulfur content. Total acid density was determined by back-titration method as described by the study of Goertzen et al. [46,47]. The strong acid density (rSAD ) by acid-base titration using the procedure adopted from the study of Wang et al. [48], after allowing the exchange of hydronium ions with 2 N NaCl solution. The phenolic (eOH), and carboxylic acid (eCOOH) density were determined by difference of the other know acid density determined. The same procedures were adopted in the characterization of the produced CS-SAC. 2.3. Simultaneous carbonization-sulfonation of dried cacao shells The simultaneous carbonization-sulfonation of cacao shells was carried out based from the studies by Dholakiya et al. [26] and Flores et al. [49]. Each experimental run was carried out in duplicates. The synthesis was done in a Kjeldhal digester (DK-8S Heating Digester, VELP 241 Scientifica). Approximately 3.5 g of powdered cacao shells was weighed and transferred into a Kjeldhal digestion tube and was mixed with the sulfonating agent, 70 mL of 98%v/v H2SO4, ensuring that the sample was fully immersed to promote homogeneity. Eight tubes containing the cacao shell-acid mixture were mounted in the digester and heated at a rate of 10  C/min to the desired synthesis temperature (80  C, 100  C, 120  C), then was maintained for a particular time (4 h, 6 h, 8 h). After simultaneous carbonization and sulfonation, the resulting mixture was cooled to room temperature. The contents of the tubes were transferred into a 3-L beaker immersed in a water bath. The reaction mixture was diluted with 1 L of distilled water and filtered to recover the solids. To remove the unbound acids, the solids were continuously washed with hot distilled water (90  C) and recovered via centrifugation until the pH of the washing solution was near the pH of the distilled water used for washing. Three to four drops of 0.48 M BaCl2 was added to the washing solution to qualitatively test the presence of sulfate ions (eSO2 4 ). The obtained solids were partially dried in an oven set at 80  C approximately 10 h, to ease the later handling of the solids. A determined amount of the incompletely dried solids were added in a pre-weighed improvised extraction thimble (Whatman No. 42). To leach out the remaining loosely bound sulfate ions, the catalyst was further washed with distilled water in a Soxhlet extractor under continuous reflux (~100  C) for 12 h. The catalyst contained

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in the improvised thimbles were then dried in an oven maintained at 80  C until constant weight. The catalyst yield (YieldCSSAC ) or the amount of CS-SAC produced per unit mass of cacao shell (CS) was determined using Equation (1),

YieldCSSAC ¼

mCSSAC; total ¼ mCS ð1  MÞ

P

  g CS  SAC ðmtþSAC  mt Þ ¼ mCS ð1  MÞ g dry CS (1)

where mCSSAC; total (g) and mCS (g) are the total mass of the catalyst obtained and the milled cacao shell used during synthesis, respectively, while M is the moisture content of the cacao shell used. Moreover, mtþSAC and mt are the mass of the extraction thimbles containing SC-SAC after the last drying process and the mass of the thimble, respectively. The catalyst prepared under the same condition were accumulated and stored in a small airtight plastic container for subsequent analysis and assay of acid density and catalytic activity. The synthesized cacao shell-solid acid catalysts (CSSAC) were then designated as CS-SAC-TTT-t, to indicate the sulfonation temperature (T) and time (t). As an example, CS-SAC-120-6 is used to designate the catalyst synthesized at 120  C for 6 h. 2.4. Determination of catalytic activity

  V A  C A  MM OA g OA ¼ g oil sample msample

X OA

  w C g oleic acid reacted OA0  C OA1 ¼ ¼ w g initial oleic acid C OA0

(2)

where V A (L) is the volume of the NaOH solution used in the

(3)

where C OA0 and C OA1 are the acid values of the oil before and after the methylation reaction, respectively. The catalytic activity over a specified reaction time of 4 h was then calculated using Equation (4) to Equation (6).

"

xFFA mOA

mmolFFAconverted catalyticactivity;A¼ MM ¼ OA g catalyst ,h 1000 mCSSAC t

#

(4) sulfonic acid activity; ASO3 H ¼

A

rSAD

¼

  mmol FFA converted mmol  SO3 H ,h (5)

total acid activity; AH þ ¼

Esterification of oleic acid (OA) with methanol catalyzed using the synthesized CS-SAC was done to determine the activity of the catalyst. Approximately 42.86 g of OA was weighed in a 250-mL screw capped flask. Using a graduated cylinder, 55.40 mL of methanol was measured and added in the flask containing the OA. This is equivalent to a methanol-to-OA molar ratio of 7:1. The flask containing the mixture was closed then loaded to the incubator shaker (Model-G25, New Brunswick Scientific Co. Inc, USA) set at 65  C and 200 rpm. Upon reaching the set temperature of the system, the shaker was turned off and 3 g of the catalyst was loaded to the flask. The reaction was allowed to proceed for 4 h. To stop the reaction, the flasks were quenched in a cooling bath. The reaction mixture was filtered using a Whatman No. 2 filter paper and washed with methanol to recover the spent catalyst. To recover the oil, the mixture obtained after esterification was separated into two phases. Approximately 50 mL of n-hexane and 20 mL of 5 %w/w NaCl was then added to the reaction mixture in a separatory funnel. The funnel containing the mixture was swirled and allowed to settle until two distinct layers were formed. The bottom layer was discarded and further washing with 5 %w/w NaCl, of the remaining mixture in the funnel was done until the pH of the bottom layer was close to neutral. The resulting oil-hexane mixture after washing was weighed. Approximately 10 g aliquot was taken from the oil-phase of the obtained mixture after washing and placed in a pre-weighed evaporating dish. The samples were heated in the oven at 65  C for 2 h to remove the n-hexane in the mixture, then allowed to cool and weighed. Further heating and weighing was done until less than 0.01 g difference of the samples was obtained. About 0.5 g of oil sample was weighed in a 125-mL Erlenmeyer flask. A known volume (25 mL) of neutralized ethanol was transferred to the flask with the aid of a volumetric pipette. The oilethanol mixture was then added with 3 drops of phenolphthalein indicator and titrated against standardized 0.075 M ethanolic NaOH solution until the endpoint. The OA content (C OA ) of the oil was then determined using Equation (2).

C OA ¼

titration, C A (mol/L) is the concentration of the titrant (NaOH solution), msample (g) is the mass of the oil sample, and MM OA (282.5 g/mol) is the molar mass of OA. The OA conversion (xFFA ) was then calculated using Equation (3),

A

rTAD

 ¼

mmol FFA converted mmol H þ ,h

 (6)

2.5. Reusability of SC-SAC Eight sets of reactant mixtures were loaded in the incubator shaker to undergo esterification for 24 h to ensure that recycling or reuse was done at the highest possible conversion within a reasonable time. The catalyst with the highest catalytic activity was used to catalyze the reaction. The same procedure was done for the recovery of oil and spent catalyst as described previously. The recovered catalysts were pooled then used for the next cycle. To ensure sufficient amount of catalyst, two sets of reactant mixtures were reduced for every succeeding cycle. The excess spent catalysts were analyzed for its acid density. Percent reduction (% reduction) of the catalytic activity with respect to the first cycle was determined using Equation (7),

% reduction ¼

Ancycle  100 % A1cycle

(7)

where Ancycle is the catalytic activity of the nth cycle and A1cycle is the catalytic activity of the first cycle. 3. Results and discussion The acquired raw cacao shells were milled and characterized in terms of particle size, proximate components and acid site densities. The milled cacao shells used in this study after milling had a mean particle size of 93.15 ± 2.27 mm. The proximate composition (7.40 ± 0.12 %Moisture, 66.78 ± 0.68 %VCM, 23.73 ± 0.56 %FC and 9.50 ± 0.31 %Ash on dry basis (db) of the cacao shells used in this study is close to the range of values reported in literature (9.09e9.45 %Moisture, 67.95e69.28 %VCM, 23.80e24.46 %FC, and 2.05e8.25 %Ash) for cacao shells [22,33,50]. In the synthesis of solid acid catalyst from biomass, an important consideration is the fixed carbon (FC) content because it allows the estimation of the amount of catalyst that can be produced. The FC content of the cacao shell used in this study is close to that of coconut shell (24.40 %w/w) [51], and within the range of values (9.53e35.2 %w/w) of FC contents of the other biomass residues that

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were studied as raw material for the synthesis of solid acid catalysts [35e43]. The sulfur content (0.13 ± 0.01 %w/w, db) of the milled CS used in this study are the same with the values reported by Bureros et al. [22], which was previously reported to be 0.13 ± 0.01 %w/w. The presence of sulfur in the biomass indicates that part of the sulfur making up the total sulfonic group in the synthesized solid acid catalyst is to be accounted as originally present in the biomass. Apart from the fixed carbon and sulfur contents, carbon-based solid acid catalysts are routinely characterized for its acid densities as these quantities affects their catalytic activity in the esterification of oils. These acid sites, namely the sites containing the phenolic (eOH), carboxylic (-COOH) and sulfonic (-SO3H) functional groups, are thought to be formed from the partial carbonization of the biomass and the oxidation of aliphatic groups during the roasting of the cacao beans [52]. Total acid density is the sum of the three acid sites mentioned. The total acid density (0.6976 mmol Hþ/g dry CS) of the raw CS used in this study is considerably smaller in contrast to the value reported by Bureros et al. [22], which is 3.23 mmol Hþ/g dry CS and much lower when compared to CS-SAC from a two-step synthesis process, which had 4.56 mmol Hþ/g dry CS-SAC and 1.48 mmol SO3H/g dry CS-SAC. Discrepancies in the raw CS may be attributed to the difference in origin of the raw material and the manner by which the cacao shells were roasted. Corresponding acid densities of the raw CS in this study are considerably low, thus, needing a sulfonation step to increase the sulfur content as well as the acid densities of the biomass. 3.1. Effect of sulfonation temperature and time on catalyst yield and extent of sulfonation The highest catalyst yield of ~0.73 g CS-SAC/g dry CS is obtained after direct sulfonation at 80  C and 4 h, while the lowest yield of ~0.56 g CS-SAC/g dry CS at 120  C and 8 h. There is a decrease in the catalyst yield as sulfonation temperature is increased from 80  C to 100  C, but further increase in the temperature to 120  C, leads to no significant change in the catalyst yield. At a sulfonation time of 6 h, an 11% decrease is observed from 80  C to 100  C and further increasing the temperature to 120  C only leads to a 5% decrease in the catalyst yield. At 8 h, an increase in temperature from 80  C to 100  C then to 120  C both resulted to a 5% decrease in the catalyst yield. Based on the results obtained, sulfonation temperature has a significant effect (p ¼ 0.0164) on the catalyst yield, while sulfonation time does not have influence (p ¼ 0.1565).

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In comparison to the study done by Bureros et al. [22], which employed two-step carbonization-sulfonation to synthesize CSSAC-120-6, the catalyst yield obtained in this study is ~40% higher. This is owing to the omission of the separate carbonization step, where the greatest loss in mass is usually observed. With the higher catalyst yield, direct sulfonation may be a more favorable and cost-effective synthesis method because of the reduced processing step, and is a less energy intensive process. However, further studies may have to be conducted to reduce the required sulfuric acid during direct sulfonation as it would require 27e35 mL of sulfuric acid for each gram of catalyst produced as compared to the conventional 2-step process, where 25 mL per gram of catalyst are required. Aside from the catalyst yield, the extent of sulfonation (quantified as the ratio of the sulfur content of the catalyst and the raw cacao shells) is an important parameter in evaluating the effectiveness of the synthesis step. The extent of successful incorporation of the eSO3H to the biomass as a function of sulfonation temperature and time is depicted in Fig. 1. The sulfur content ratio pertains to the number of times the sulfur content of cacao shell is increased by thermochemical treatment, in this case, direct sulfonation. Relative to the sulfur content (0.13 %w/w) of raw cacao shell, as much as 21 fold increase in the sulfur content is obtained after direct sulfonation using H2SO4. This is proof of the successful incorporation of the sulfonic acid groups onto the surface of the carbon support during sulfonation. Analysis of variance indicated that within the set sulfonation temperature and time investigated, no significant difference (p ¼ 0.06 and p ¼ 0.23, respectively) on the resulting sulfur content ratio of the CS-SAC produced was observed. This is in agreement with the findings of the work done by Bureros et al. [22] wherein there is no significant difference on the total amount of sulfur attached to the CS-SAC after sulfonation, considering that both studies have the same sulfonation temperature and time ranges. The CS-SAC synthesized at various synthesis temperature (80, 100, 120  C) and time (4, 6, 8 h) has an average sulfur content ratio of 19.85. 3.2. Characteristics of CS-SAC as a function of sulfonation temperature and time As mentioned in the previous section, fixed carbon content is an important characteristic of a material to determine its potential as a raw material to serve as catalyst support. Proximate analysis was

Fig. 1. Sulfur content ratio (extent of sulfonation) of CS-SAC after direct sulfonation at various temperature and time.

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Fig. 2. Proximate contents of CS-SAC synthesized at varying sulfonation temperature (80, 100, 120  C) and time (4, 6, 8 h) at a sulfuric acid-to-cacao shell ratio of 20.00 mL/g.

done on the catalysts synthesized at various direct sulfonation temperature and time. Moisture content of the synthesized CSSACs is at ~0.09 ± 0.04 g moisture/g dry CS-SAC. As could be observed from the results presented in Fig. 2, the volatile matter of the CS-SACs is at ~0.50 g VM/g dry CS-SAC excluding the CS-SAC080-4 (VM ¼ 0.5350 g VM/g dry CS-SAC). Apart from the CS-SAC080-6 with an ash content of 0.0462, the rest of the CS-SACs compose of ~0.01 g ash/g dry CS-SAC. As the CS was subjected to sulfonation, the FC content of the catalyst increased. This supports the idea that during carbonization and sulfonation, the decrease in mass of the synthesized catalyst was due to the removal of some of the volatile components and partial dissolution of the ash leaving behind volatiles which would require higher temperatures to volatilize and acid insoluble ashes. The CS-SAC-120-6 had the highest fixed carbon content (49.83%) which is close to the results reported in the studies of Ahmad et al. (~43.2%) [53] and Bureros et al. (49.91%) [22]. Solid acid catalysts are assessed for its activity and stability. As mentioned previously, the presence of the eSO3H groups as well as the weak carboxylic (eCOOH) and phenolic acid groups (eOH) on the synthesized catalysts were determined since catalytic activity are often associated with acid density [54]. Morphological changes, volatilization, oxidation, thermal decomposition and degradation of the biomass occur during simultaneous carbonization and sulfonation, which influences the incorporation of the acid site [49]. The determined acid densities for the synthesized catalyst are shown in Fig. 3. During catalyst synthesis, partial carbonization of the biomass leads to depolymerization and formation of small polycyclic aromatic carbon sheets with phenolic groups. Sulfonation with H2SO4 takes place wherein sulfonic acid is covalently bonded to the polycyclic aromatic carbon sheets as evidenced by the increase in the sulfur content of the carbonaceous material after synthesis [13]. In view of the effects of synthesis condition on the sulfonic acid density (Fig. 3a) increase in temperature from 80 to 120  C and synthesis time of 4e8 h are found to have insignificant influence as have been previously discussed in view of the sulfur content ratio. This is consistent with the finding of Kumar et al. [29] wherein the direct sulfonation time of over 1.5 h in the synthesis of wheat husked-derived SAC did not result in significant increase of the sulfonic acid sites. The highest sulfonic acid density obtained with CS-SAC-120-6 with 0.85 mmol SO3H/g dry CS-SAC. After the sulfonation process, other acid sites were also observed apart from the sulfonic sites. Density of carboxylic acid sites (eCOOH) of the synthesized catalysts increased when the

temperature was increased from 80  C to 120  C. High phenolic hydroxyl functional groups were also noted from the synthesized catalyst with values ranging from 4.8111 to 5.6708 mmol Hþ/g dry CS-SAC. These could have resulted owing to the breakdown or hydrolysis of the lignin network and carbohydrates. However, in view of the total acid density no significant difference (p ¼ 0.10) could be observed among the CS-SAC synthesized at the temperatures and time investigated. Nevertheless, relatively higher total acid densities (6.8291e7.1165 mmol Hþ/g dry CS-SAC) were obtained with CS-SAC-120. These findings are comparable to that of Suganuma et al. [55] who determine a phenolic acid density of 5.6 mmol Hþ/g catalyst and a total acid density of 7.5 mmol Hþ/g catalyst from a cellulose-derived solid acid catalyst.

3.3. Catalytic activity of synthesize CS-SAC Among the three acid moieties, eSO3H is the main functional group in a SAC responsible in catalyzing the desired reaction. In Fig. 4a are shown the catalytic activities of the synthesized CS-SACs. A positive correlation (r ¼ 0.6899) was observed between the sulfonic acid density and amount of OA converted. The highest sulfonic acid activity was ~5.73 mmol OA converted/mmol SO3H$h using the CS-SAC-12-6, which also has the highest sulfonic acid density. Other functional groups (eCOOH and eOH) help improve the catalyst performance. However, no strong correlation could be observed between the weak functional groups, eCOOH (r ¼ 0.1614) and eOH (r ¼ 0.0775), and the amount of OA converted. In view of conversion during the esterification of OA with methanol, using CS-SAC-80, the OA conversion was observed to be around ~0.32e0.33 g OA converted/g initial OA. Consequently, with CS-SAC-100 the conversion was around ~0.26e~0.30 g OA converted/g initial OA. The low OA conversion despite the high total acid density of the catalyst can be attributed to the significant amount of weak acid densities attached to the catalyst which is estimated to be around 4.3780e7.7633 mmol Hþ/g dry CS-SAC. The presence of the eOH and eCOOH functional groups in overabundance can attract water molecules (inherent or reaction byproduct), which could lead to the formation of a water layer on the surface of the catalyst. The presence of such water layer has previously been found to decrease the accessibility of the FFA [25] or may have induced and favored the reverse reaction. At a fixed reaction time of 4 h, the highest OA conversion was recorded to be about ~0.39 g OA converted/g initial OA for CS-SAC-120-6.

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Fig. 3. (a) Sulfonic, (b) Carboxylic, (c) Phenolic, and (d) Total acid density of CS-SAC synthesized under various temperature and time (temperature: 80, 100, 120  C; time: 4, 6, 8 h).

Fig. 4. Effect of sulfonation temperature (80, 100, 120  C) and time (4, 6, 8 h) at a sulfuric acid-to-cacao shell ratio of 20 mL/g on the activity of CS-SAC and percent OA conversion.

3.4. Characteristics of spent catalysts as a function of sulfonation temperature and time It can be inferred from the results presented so far that the catalytic activity of CS-SAC is highly attributed to its sulfonic acid functional groups [29,56]. Though conversion and catalytic activity are response parameters to consider in assessing the performance of the catalyst, the stability of the catalyst must also be considered. The spent catalysts were characterized for its sulfonic acid density after its use in the esterification of OA. The results are presented in Fig. 5.

The stability of the catalyst is as important as its activity since it determines the length of time the catalyst can be used at its designed activity before subjected to regeneration or substitution. It can be observed that the sulfonic acid density of all the CS-SAC decreased after their use in catalyzing an esterification reaction. The decrease in sulfonic acid density may be attributed to the leaching of the components part of the solid matrix where the active sites are attached. After the esterification reaction, washing the catalyst with methanol might have further removed the components of the catalyst matrix where eSO3H groups are attached. In a study by Mo et al. [57], deactivation of sulfonated glucose-based

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solid acid catalyst is attributed to the leaching of sulfonic acid groups due to washing with methanol. The high polarity and hydrophilic characteristics of methanol entails great affinity to the loosely bound sulfonic groups and stabilized by hydrogen bonding or dissolving non-water soluble components of the matrix where the active sites are attached. Thus, the type of solvent used in catalyst washing has an influence on the decrease in active sites. Leaching of sulfonic acid can also be caused by the changes in morphology of the catalyst. During the esterification reaction, the catalyst might have partially collapsed along with the attached sulfonic groups due to continuous shaking of the reaction mixture [58]. For sulfonated solid acid catalysts, leaching of sulfonic acid groups results to poor catalytic activity and stability making it uneconomical to use in biodiesel production. It can be observed that although catalysts synthesized at different temperatures with the same reaction time have comparable sulfonic acid densities, reduction of sulfonic acid density increases as the temperature is lowered. For instance, the percent reduction of sulfonic acid density of CS-SAC-080-6 is 39.31% while for the CS-SAC-120-6 only has 5.99%. At lower synthesis temperature, soft aggregate which are mainly small polycyclic aromatic carbon moieties with sulfonic groups are produced, these are easily removed or leached from the solid matrix when used as catalyst during esterification reactions [59]. Among the catalysts, CS-SAC120-6 had the highest initial acid density (0.85 mmol SO3H/g CS) and the least sulfonic acid reduction (5.99%) suggesting that it has the most stable sulfonic acid groups. Influence of sulfonation temperature was not significant (p ¼ 0.2268) when compared to time (p ¼ 0.0584), however, interaction between the temperature and time was significantly (p < 0.0001) affected the stability of the sulfonic acid sites of the synthesized CS-SACs.

For weak acid density (Fig. 5b), it decreased after esterification. It can be observed that the decrease in eSO3H functional groups (Fig. 5a and c) is greater than the reduction in weak acid density with the greatest recorded to be only about 32% (Fig. 5b and d) relative to that of sulfonic acid density which is estimated to be about 84%. These data suggest that more eSO3H were removed from the catalyst surface during the esterification reaction than the other functional groups (eCOOH and eOH). The catalyst CS-SAC120-6 exhibited the least reduction in total acid density with only a ~2.4% decrease. In terms of weak acid density, CS-SAC-120-6 and CS-SAC-120-8 have small decrease after esterification reaction with percent reduction in the range of 1.8e2.0%. Both the sulfonation temperature and time have significant effect (p ¼ < 0.0001) on the total and weak acid density retained in the spent CS-SACs. 3.5. Reusability of SC-SAC The CS-SAC-120-6 showed the highest catalytic activity and better retention of sulfonic sites, thus, it is used for subsequent reusability test. To assess the stability of the CS-SAC, OA conversion was determined over four cycles. As shown in Fig. 6a, a decrease of the OA conversion from ~0.77 to 0.60 was obtained after 4 cycles. On first cycle of using the recovered catalyst, a decrease of ~10.6% in the OA conversion was observed. The second reuse (cycle 2) resulted to another ~10.5% decrease in activity relative to the fresh catalyst, which corresponds to an overall decrease in activity by ~21%. Beyond the second cycle, the OA conversion stabilized at 0.60. Since the functional groups attached in the catalyst support is responsible for catalyzing the reaction, analysis of the acid density was also done after every cycle. Although eSO3H is the main functional group involved, eCOOH and e OH groups also influence

Fig. 5. Sulfonic acid density (a) and weak acid density (b) of CS-SAC synthesized under various temperature and time (temperature: 80, 100, 120  C; time: 4, 6, 8 h) and sulfonic acid density (c) and weak acid density (d) of spent CS-SAC recovered after esterification.

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327

Fig. 6. Reusability study of CS-SAC sulfonated at 120  C for 6 h (Esterification conditions: T ¼ 65  C, t ¼ 24 h, catalyst loading ¼ 7 %w/w of OA, MOR 7:1).

the catalysis process within the reaction, thus, changes in the total acid density after each cycle was investigated. As can be observed from Fig. 6b the total acid density decreased during the first two cycles and maintained at ~6.50 mmol Hþ/gcatalyst$h on the succeeding cycles. From this, it can be said that the decrease in OA conversion may be attributed to the decrease in the acid densities caused by the leaching of the functional groups which are responsible for catalyzing the reaction. In this study, ~78% of the catalytic performance was retained after four cycles, higher than the values reported by Bureros et al. [22], where ~52% of the catalytic performance was retained after the same amount of cycles. Higher stability may be due to incorporation of exhaustive washing with water using a sohxlet extractor for 12 h right after catalyst synthesis. The exhaustive washing step ensures removing majority of the loosely bound active cites [49], which may have resulted in the lesser decrease in the total acid density. For instance, the CS-SAC synthesized via direct sulfonation only has an 18% decrease on the total acid density when compared to the CS-SAC synthesized via two-step carbonization-sulfonation that were not exhaustively washed prior to its use, which had a decrease of 38% [22].

as well as the catalytic activity of the CS-SAC is comparable to that of SCB-SAC. With regards catalyst reusability, the SCB-SAC prepared via twostep synthesis offered the highest stability which retained ~90% of its original performance after 8 cycles of use. This was followed by the catalyst synthesized from peanut shell, being able to retain 95% of its initial performance after 6 cycles. Comparing SCB-SAC and CSSAC wherein both catalysts are synthesized through direct sulfonation, it is noted that both exhibit the same degree of stability which retained 78% of their original performance after 4 cycles of use without regeneration. The CS-SAC prepared through direct sulfonation has higher retention (78%) than the CS-SAC prepared via two-step synthesis (52%) considering that both are being reused for 4 cycles and each cycle is being run for 24 h. Despite having lower catalytic activity compared to other biomass-derived SAC through direct sulfonation, solid acid catalyst derived from cacao shell has comparable stability. Considering that direct sulfonation has lesser synthesis time and energy requirement than the twostep synthesis, CS-SAC synthesized via direct sulfonation may be more practical to adopt for industrial use, for geographical locations where these are abundant.

3.6. Comparison of CS-SAC with other carbon-based solid acid catalyst

4. Conclusions

In Table 1 are reported results of different agricultural residues used as a carbon precursor in the synthesis of solid acid catalyst for the esterification of OA with methanol which serves as a comparison with the results of this study. Among the carbon-based solid acid catalyst employed in the esterification of OA, SAC derived from the de-oiled Jatropha curcas seed cake waste exhibits the highest catalytic activity, based on the amount of catalyst and sulfonic acid sites. The peanut shell-derived SAC has the highest catalytic activity of all the SAC synthesized via direct sulfonation. The high catalytic activity from the result of both studies may be attributed to the higher esterification temperature (80e85  C) employed. Moreover, the high catalytic activity of peanut shell-derived SAC, although synthesized at the lowest synthesis temperature (85  C) and shortest time (3 h), is mainly due to its relatively higher sulfonic acid density. Comparing sugarcane bagasse (SCB)-derived SAC synthesized via direct sulfonaiton with the CS-SAC in this study, although esterification with SCB-SAC had higher catalyst loading (10.0 %w/w) and the same reaction time (24 h), the OA conversion

Synthesis of the cacao shell-derived solid acid catalyst was done via direct sulfonation. Sulfonation temperature and time have minimal influence on the sulfonic sites and catalytic activity of fresh catalyst but greatly influences the stability and retention of active sites. The presence of sulfonic groups is positively correlated with the catalytic activity exhibited during esterification. Catalyst synthesized at 120  C for 6 h showed highest sulfonic acid sites (0.85 mmol SO3H/g dry CS-SAC) and highest catalytic activity (~5.8 mmol OA converted/mmol SO3H$h). The highest OA conversion achieved during esterification at 65  C for 24 h with a methanol-to-oil mole ratio of 7:1 and a catalyst loading of 7% w/w of OA was 0.77. After the 3rd use only 78% of the catalytic activity and 82% of the acid density were retained, but remained the same thereafter, implying good stability of the catalyst. Among the different acid moieties present in the catalyst, sulfonic acid greatly influences the activity of the catalyst. The intermediate washing steps right after sulfonation process affects the acid density and improves stability of the synthesized catalyst.

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Table 1 Carbon-based solid acid catalyst synthesized using two-step carbonization-sulfonation or direct sulfonation for the esterification of oleic acid. Biomass

Method of Synthesis Synthesis Conditions Acid T ( C)/t(h) Density

rHh Two-step

Corn straw

Direct sulfonation Two-step

de-oiled Jatropha curcas Two-step seed cake waste Peanut shell Direct sulfonation Cacao shell Two-step Cacao shell Cacao shell a b c d e f g h i j k l m n o p

Direct sulfonation Direct sulfonation

rSO3Hi

Catalytic Activity (Fresh Catalyst)

T ( C) t(h) Acj

A Hk

ASO3Hl

Washing

Recycles Activity Retention XFFA R nm

Ref

R %n

Fresho Recycledp

3.69 1.06

5.0

10:1

80

5

12.88

3.49 12.15

n-hexane 8

90

0.91

0.81

[59]

0.79 0.59 2.64 2.44

10.0 7.0

20:1 7:1

45 60

24 4

1.26 12.39

1.59 2.14 4.69 5.08

n-hexane 4 Methanol e

78 e

0.85 0.98

0.66 e

[49] [24]

e

2.00

0.5

1:1

80

2

339.82 e

169.91 Methanol 4

97

0.96

0.95

[60]

3.53 2.13 4.56 1.48

4.0 5.0

10:1 7:1

85 42

3 24

28.91 12.74

8.19 13.57 2.79 8.61

e 6 Methanol 4

95 52

0.98 0.93

0.95 0.48

[27] [22]

7.12 0.85 7.12 0.85

7.0 7.0

7:1 7:1

65 65

4 24

4.93 2.27

0.69 5.80 0.32 2.67

e Methanol 4

78

0.39 0.77

e 0.60

This study This study

Catalyst loading based on weight of OA. Methanol-to-OA ratio. Esterification conditions. Carbonization temperature. Carbonization time. Sulfonation temperature. Sulfonation time. total acid density (mmol Hþ/gcatalyst). sulfonic acid density (mmol SO3H/gcatalyst). catalytic activity (mmol FFA reacted/gcatalyst∙h). catalytic activity (mmol FFA reacted/mmol Hþ∙h). catalytic activity (mmol FFA reacted/mmol SO3H∙ ∙h). number of cycles the catalyst was used in the reusability test. percent of the initial catalytic activity retained on the last cycle of the reusability test. fractional FFA conversion of fresh catalyst. OA conversion after several cycles of use.

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Sugarcane bagasse

Tcd/tce: 375/0.5 Tsf/tsg: 150/15 Tsf/tsg: 150/8 Tcd/tce: 350/1 Tsf/tsg: 80/4 Tcd/tce: 350/5 Tsf/tsg: 100/8 Tsd/tse: 85/3 Tcd/tce: 387/1 Tsf/tsg: 120/6 Tsf/tsg: 120/6 Tsf/tsg: 120/6

CLa (%w/w) MORb (n:n) ECc

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