Cacao shell-derived solid acid catalyst for esterification of oleic acid with methanol

Cacao shell-derived solid acid catalyst for esterification of oleic acid with methanol

Renewable Energy 138 (2019) 489e501 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Cac...

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Renewable Energy 138 (2019) 489e501

Contents lists available at ScienceDirect

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

Cacao shell-derived solid acid catalyst for esterification of oleic acid with methanol Glorie Mae A. Bureros a, April A. Tanjay a, Dan Elmer S. Cuizon a, Alchris W. Go a, d, *, Luis K. Cabatingan a, **, Ramelito C. Agapay a, b, Yi-Hsu Ju c a

Department of Chemical Engineering, University of San Carlos, Talamban, Cebu City 6000, Philippines Department of Chemical Engineering, National Taiwan University of Science and Technology, No. 43, Keelung Road, Section 4, Daan District, Taipei 106-07, Taiwan c Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, No. 43, Keelung Road, Section 4, Daan District, Taipei 106-07, Taiwan d Visiting Foreign Researcher, National Taiwan University of Science and Technology, Keelung Road, Taipei City, Taiwan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 August 2018 Received in revised form 12 December 2018 Accepted 22 January 2019 Available online 25 January 2019

Cacao shell-derived solid acid catalyst (CS-SAC) was successfully prepared by partial carbonization followed by sulfonation at various temperature (80, 100, 120  C) and time (4, 6, 8 h) settings. The catalysts were analyzed for their acid densities and tested for their activity in the esterification of oleic acid with methanol at 45  C for 4 h, with methanol to oleic acid molar ratio of 7:1 and a catalyst loading of 5 %w/w. It was found that increasing the sulfonation temperature at sulfonation times of 4 and 6 h increased the solid’s total acid density and sulfonic acid density thereby increasing its specific catalytic activity. Prolonging sulfonation time at any temperature, however, had no significant effect. The highest specific catalytic activity of 14.0 mmol FAME$g1 CS-SAC$h1, which corresponds to 79% conversion of oleic acid was observed for SAC sulfonated at 120  C for 4e6 h. Prolonged esterification up to 24 h resulted in high conversion of ~94% even at a low temperature of 45  C. Although the catalyst was later found to decrease in its activity during reuse, its catalytic activity remained stable after the 3rd cycle at a conversion of 48% of the oleic acid. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Cacao shell Oleic acid Esterification Solid acid catalyst Sulfonation

1. Introduction Biodiesel, commonly in the form of fatty acid methyl ester (FAME), is a renewable fuel derived from vegetable oil or animal fat. Compared to petroleum-based diesel, biodiesel leads to lower emissions of carbon monoxide, particulate matter, and unburned hydrocarbon [1]. However, the major drawback of biodiesel is its relatively high price owing to the raw material costs which could constitute over 70% of the total cost of production [2]. Cheaper raw material alternatives, such as waste cooking oil (WCO), have thus been explored. Utilization of WCO as feedstock for biodiesel production would

* Corresponding author. Department of Chemical Engineering, University of San Carlos, Talamban, Cebu City 6000, Philippines. ** Corresponding author. E-mail addresses: [email protected] (A.W. Go), [email protected] (L.K. Cabatingan). https://doi.org/10.1016/j.renene.2019.01.082 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

require a pretreatment step before transesterification because of high free fatty acid (FFA) and moisture content. The presence of high FFA and moisture during transesterification reaction with base catalyst could lead to the undesirable formation of soap. Lepper and Friesenhagen [3] recommended reducing the FFA content via acidcatalyzed esterification reaction wherein FFAs are converted to alkyl esters. Unlike base catalysts, acid catalysts will not react with FFA to form soap. However, reactions employing homogeneous acid catalysts, such as H2SO4, poses inherent challenges in the separation of the catalyst from the reaction mixture and in the purification of the product. This can be circumvented through the use of solid acid catalysts. Studies on solid acid catalysts prepared from agricultural residues have been reported in the literature. Ground bamboo [2], corn straw [4], peanut hulls [5] and coffee residue [6] were carbonized and functionalized with eSO3H groups through sulfonation to produce biomass-derived solid acid catalyst (SAC) that can be used in the esterification of FFA. In the Philippines, about 3000 metric tons of cacao shells are produced annually as a by-product of roasted cacao bean

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production for the chocolate industry. Currently these are discarded as part of the municipal waste. If these could be processed into product of more value, it will not only aid in the disposal of such residue but would also result in a new industry. This would then contribute to achieving zero-waste processes. Valorization of cacao shell as activated carbon was studied by Ahmad et al. [7e9] and Theivarasu et al. [10] for the adsorption of methylene blue dye and 4-nitrophenol. However, no studies have been reported about the utilization of cacao shells as SAC. Cacao shell has high fixed carbon (FC) content of about 23.8% which is comparable to those of other agricultural wastes utilized for the SAC synthesis including ground bamboo (FC ¼ 19.6%), corn straw (19.3%), peanut hulls (21.1%), coffee residue (17.0%) and jatropha seed shells (23.0%) [11]. Fixed carbon in the biomass is a relevant characteristic parameter because it is essentially what remains after carbonization. Solid acid catalyst can be produced by biomass carbonization, followed by sulfonation. During carbonization, volatile matter and other components of the material are released and polycyclic aromatic hydrocarbon sheets are formed [12,13]. The carbonized biomass is then functionalized by sulfonation wherein the sulfonic groups attach to the polycyclic aromatic hydrocarbon sheets [14]. Factors such as carbonization temperature, carbonization time, sulfonation temperature, and sulfonation time have been shown to affect the performance and the textural properties of the synthesized SACs. Wang et al. [14] synthesized SAC from jatropha seed shells and investigated the effect of sulfonation on the textural properties and catalytic activity of the SAC produced. Carbonization was done at 350  C for 5 h followed by sulfonation at 100  C for 8 h. The SAC produced had a surface area of 6.54 m2/g and a sulfur content of 0.06 w/w, which was a significant increase from the <0.01 w/w sulfur content of the biomass prior to sulfonation. In separate studies, Zhou, et al. [2] and Liu et al. [4] studied the effect of carbonization conditions on the catalytic activity and textural properties of biomass-derived SAC. In the study of Zhou et al. [2], bamboo was carbonized at varying temperature of 300e500  C and at varying time of 0.5e5 h. On the other hand, in the study of Liu et al. [4], corn straw was carbonized at 250 to 500  C for 0.5e3 h. Optimum carbonization condition for bamboo-derived SAC and corn straw-derived SAC were found to be 350  C for 2 h and 300  C for 1 h, respectively. The SAC produced at optimum conditions yielded the highest conversions of oleic acid during esterification. Zhou et al. [2] obtained a sulfonic acid density of 1.82 mmol/g for the bamboo-derived SAC whereas Liu et al. [4] determined the total acid density of the corn straw-derived SAC to be 2.64 mmol/g. Both works noted that further increasing carbonization temperature would result in lesser attachment of eSO3H groups during the subsequent sulfonation step. This was attributed to the formation of rigid carbon structures after carbonization at higher temperatures. Zhou et al. [2] also reported that carbonization time of 0.5e3 h had no significant effect on the catalytic activity of the SAC produced. Different temperature and time settings for sulfonation of carbonized biomass were studied by Zhou et al. [2] and Ngaosuwan et al. [6]. Zhou et al. [2] sulfonated carbonized bamboo at 75 to 135  C for 1e5 h whereas Ngaosuwan et al. [6] performed sulfonation at 140 to 200  C for 18 h on spent coffee grounds after carbonization at 600  C and 4 h. Zhou et al. [2] reported that increasing the sulfonation temperature decreased the total acid density of the SAC due to sulfone side reactions that inhibit attachment of sulfonic groups to the carbon sheet. This observation can also be made in the work of Ngaosuwan et al. [6]. Moreover, Zhou, et al. [2] reported that sulfonation time had no significant effect on the catalytic activity of the bamboo-derived SAC. Based on the above-cited studies, the SAC preparation

conditions to use with cacao shells can be set. It is noted that the effective carbonization temperature ranges from 300 to 400  C and carbonization time of 1e5 h. Thus, a carbonization temperature of 350  C and carbonization time of 1 h may be used with cacao shell. These carbonization conditions were used by Wang et al. [14] with jatropha seed shells which have similar fixed carbon content (23.0%) as cacao shells. In addition, it is noted that the effect of sulfonation conditions at carbonization temperatures of 300e400  C has not been thoroughly studied yet. Noting that biomass-derived SAC yield high catalytic activities at sulfonation temperature range of 80e150  C, sulfonation temperature and sulfonation time to use with carbonized cacao shell may be set at a range of 80e120  C and 4e8 h, respectively. The advantages of SAC compared to other catalyst is that it can be easily recovered from the reaction mixture and then reused. Catalyst reusability can be defined as the number of cycles the catalyst can be reused in a reaction without any significant decrease in the catalytic activity. Before using the SAC for another reaction cycle, SACs are regenerated first by washing and followed by drying. This is done to clean pores of the catalyst. Solvents like hexane and methanol were used for the washing of the biomass-derived SAC. Previous studies [2,5,6] reported a decrease in the catalytic activity after every cycle, the decrease having been attributed to the deactivation of the catalyst due to hydrocarbon deposition on the catalyst, water adsorption, and leaching of sulfonic groups from the catalyst. This work studied the preparation of carbonized cacao shells into SAC for use in the esterification of oleic acid with methanol. Specifically, the effects of the sulfonation temperature and time on the total acid site, sulfur content and on the catalytic activity of the cacao shell-derived solid acid catalyst (CS-SAC) were studied. The catalytic activity of the solid acid catalysts was evaluated through measurements done during the esterification of oleic acid with methanol. The reusability of the catalyst was also investigated.

2. Material and methods 2.1. Materials Roasted cacao shells (CS) were acquired from a cacao beans processor in Jimalalud, Negros Oriental, Philippines and milled using Wiley mill (Model 4, Thomas Scientific, USA) fitted with 2 mm aperture screen. The chemical reagents used included: (a) 95 %w/w oleic acid, 36 %v/v hydrochloric acid, 99 %w/w barium chloride, 99.8e100.2 %w/w potassium hydrogen phthalate, all Ajax (Australia) brand; (b) 99.5 %v/v methanol, 99.7 %w/w ethanol, 96 % w/w n-hexane, magnesium oxide, silver nitrate, sodium sulfate, methyl orange, phenolphthalein, 99.5% w/w sodium chloride, all Scharlau (Spain) brand; (c) 95-97 %v/v sulfuric acid, 85% w/w potassium hydroxide, both Merck (Germany) brands; and (d) 98 %w/ w sodium hydroxide, Qualikems (India) brand.

2.2. Characterization of milled cacao shells 2.2.1. Particle size analysis The particle size analysis procedure was adopted from ASTM C136 [15]. Standard Taylor laboratory test sieves (2 mm, 850 mm, 450 mm, 250 mm, 180 mm) were stacked and about 50 g of milled CS was placed on the top sieve. After placing the cover on top sieved, the whole stack was placed on a mechanical sieve shaker (Model A5911, Intertest Benelux, Netherlands) and shaken for 15 min. Afterwards, the mass of solids retained on each sieve was determined. The mean particle size dn was calculated using Equation (1),

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dn ¼

XmCS;r mCS

  dn

(1)

where mCS;r is the mass of retained particles on each sieve, mCS is the total mass of sample subjected to sieving, and dn is the average screen opening of adjacently stacked sieves. 2.2.2. Proximate analysis Proximate analysis of the cacao shells was carried out in triplicates following the method described in ASTM Standard D1762-84 [16]. Fixed carbon (FC) content of the CS samples was determined by difference after determining the moisture, volatile carbon matter (VCM), and ash contents through gravimetric analysis. Approximately 1 g of the collected cacao shells was weighed (to the nearest 0.1 mg) in a pre-dried crucible using an analytical balance (572 KP, Kern, Germany). The samples were heated and dried at 105 ± 5  C in a convective oven (UM500, Memmert, USA) for 2 h and cooled in a desiccator for 1 h before weighing. The samples were repeatedly heated and weighed at 1-h intervals until the weight of the samples were constant (less than 0.0005 g of mass loss). The moisture content was taken as the weight lost by drying. The percentage of moisture in the sample was calculated using Equation (2),

 %Moisture ¼

mCS  mCS;105 C mCS

  100

(2)

where mCS is the mass of wet sample used, and mCS;105 C is the mass of dried sample after heating at 105  C. The muffle furnace was heated to 950 ± 5  C. The crucibles containing the dried samples were covered and placed in a muffle furnace in the following manner: on the outer ledge of the furnace for 2 min with the furnace door open, on the edge of the furnace for the next 3 min, and then on the rear of the furnace for 6 min with furnace door closed. Afterward, the samples were allowed to cool to room temperature and placed in a desiccator for an hour before weighing. The percentage of volatile matter (VCM) was calculated using Equation (3),

%VCM ¼

  mCS;105 C  mCS;950 C  100 mCS;105 C

(3)

where mCS;950 C is the mass of the sample after heating at 950  C. The lid-covered crucibles containing the samples were placed in the muffle furnace heated to 750 ± 5  C for 6 h. Afterward, the samples were allowed to cool to room temperature, kept in a desiccator for 1 h and then weighed. The percentage of ash was then calculated using Equation (4),

 %Ash ¼

mCS;750 C mCS;105 C

  100

(4)

where mCS;750 C is the mass of residue after burning at 750  C. The fixed carbon percentage in dry basis was calculated using Equation (5),

%FC ¼ 100  %Ash  %VCM

(5)

2.3. Catalyst synthesis and esterification 2.3.1. Carbonization of cacao shells Pre-weighed crucibles (100 mL) were packed with milled CS

491

(particle size: 840.84 ± 67.74 mm) to the brim; the mass of CS was approximately 38 g. The crucibles were then covered and placed inside the muffle furnace (PF3/SPEC, Vecstar, UK). The furnace was then heated to 350  C at a heating rate of 10  C/min and was held at this temperature for an hour. The carbonized cacao shells (CCS) was then cooled, weighed and then stored in a polypropylene bottle. About 2 kg of CCS was prepared for subsequent analyses and sulfonation experiments. The fractional yield of carbonized CS was calculated using Equation (6),

Y CCS ¼

mCCS mCS

(6)

where mCCS and mCS are the mass of carbonized CS obtained and the initial mass of milled CS, respectively. 2.3.2. Sulfonation of carbonized cacao shells CCS samples were sulfonated employing conditions similar to those used in the study of Ngaosuwan et al. [6]. Sulfonation was done in a thermal digester (DK-8S Heating Digester, VELP Scientifica) at a solid-to-acid ratio (SAR) of 1:20 (g sample/mL acid) using concentrated sulfuric acid. The digester was heated at a rate of 15  C/min to three different temperatures (80, 100, 120  C) and sulfonation times (4, 6 and 8 h). After sulfonation, the digestion tubes were cooled in a water bath. The reaction mixture was then diluted and washed with about 300 mL hot distilled water with intermittent settling and decanting of the wash water until the pH of the wash water was neutral. The prepared cacao shell-derived SAC or CS-SAC was then filtered, dried and stored for sulfur content and total acid density (TAD) analyses. The fractional yield of CS-SAC after sulfonation and fractional yield of CS-SAC with respect to the original dry CS were calculated using Equation (7) and Equation (8), respectively,

mCSSAC mCCS

(7)

Y CSSAC=CS ¼ Y CSSAC  Y CCS

(8)

Y CSSAC ¼

where mCSSAC is the mass of recovered CS-SAC. 2.3.3. Esterification of oleic acid with methanol The reaction of oleic acid with methanol in the presence of CSSAC was carried out in screw-capped 250-mL flasks placed in an incubator shaker (New Brunswick Scientific Co. Inc, Model-G25). The esterification conditions were based on the studies cited in Section 1. For the methanol solvent-to-oil-ratio (SOR) of 7:1 (n/n), the works of Liu et al. [4] and Zhou et al. [2] were used as basis while studies by Ngaosuwan et al. [6] and Kastner et al. [5] for the catalyst loading of 5 wt % (1 g SAC/20 g oleic acid). The reaction mixture was incubated at 42.1 ± 2.9  C for 4 h. To halt the reaction, the flask was cooled in an ice bath. The mixture was then filtered to separate the CS-SAC. The recovered CS-SAC was washed with methanol (~80 mL) to recover the adhering product mixture. After washing the spent solid acid catalyst (SSAC) with methanol, the recovered SSAC was then dried in an oven for 12 h at 60  C. The SSAC was then characterized for its TAD and sulfur content. The crude methyl oleate, which is comprised of the filtrate produced after filtering the reaction mixture and washing the spent catalyst, was transferred to a 250-mL separatory funnel and 50 mL of n-hexane was added to dissolve the FFA and FAME. Thirty milliliters of 5% (w/w) NaCl solution was also added to remove leached acid from the reaction product mixture [17]. The n-hexane layer (upper layer), which contains the oil with FAME, was extracted and

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concentrated using a rotary evaporator (VV-Micro, Heidolph, England) until the weight was constant (less than 0.0005 g of mass loss). The oil samples were placed in screw-capped glass vials for later acid value analysis. The percent recovery of oil was calculated using Equation (9),

%recovery ¼

moil recovered  100 moleic acid

(9)

where moil recovered and moleic acid are the mass of recovered oil and the initial mass of oleic acid, respectively. Moreover, the theoretical mass of recovered oil with FAME, moil recovered;th , and the percent difference in the oil recovery were calculated using Equation (10) and Equation (13), respectively,

  MW  FAME þ mFFA;f moil recovered;th ¼ mFFA;f  mFFA;i  MW FFA (10)  mFFA;f ¼ AV f   mFFA;i ¼ AV i 

MW FFA MW KOH

MW FFA MW KOH

%diff ðoil recoveryÞ ¼

  moil recovered

(11)

 moleic acid

(12)



moil recovered  moil recovered;th  100 moil recovered;th (13)

where mFFA;f and mFFA;i are the final and initial mass of FFA, AV f and AV i are the acid values (see Section 2.5) of the oil recovered after esterification and the initial oleic acid, respectively, while MW FAME , MW FFA and MW KOH are the molecular weights of fatty acid methyl ester (FAME), oleic acid as the free fatty acid (FFA) and potassium hydroxide, respectively. 2.4. Characterization of samples Sulfur contents of the CS, CCS, and CS-SAC were determined through Eschka method, as described in ASTM D3177-02 [18]. While the TAD of the cacao shells was determined using acid-base titration adopted from Higson et al. [19]. Furthermore, FT-IR analysis of selected catalyst samples were carried out using Bio-Rad Excalibur FTS 3500 Spectrometer to verify the presence of functional groups. Infrared scans were carried out over the wavenumbers of 2700 to 400 cm1, employing the attenuated total reflectance (ATR) technique with a scanning resolution of 4 cm1. 2.4.1. Sulfur analysis The raw CS, CCS, CS-SAC, and SSAC samples were characterized for its sulfur content. In the analysis of each sample, ~1 g was weighed and mixed thoroughly with 3 g of Eschka mixture (2:1 by weight of MgO and Na2CO3) in a crucible. The crucible was heated gradually in the furnace (PF3/SPEC, Vecstar, UK) to 800  C in an hour, and maintained at this temperature for 30 min. After heating, the crucible was removed from the furnace and the samples were transferred from the crucible into a 200-mL beaker. The samples were mixed with hot water (100 mL) and digested at 95  C for 30e45 min in a water bath while stirring the mixture occasionally. The solution was then filtered using an ordinary filter paper while retaining the insoluble material in the beaker. The insoluble matter was washed thoroughly with hot water through stirring. After washing, the mixture was transferred to the filter paper and washed with hot water while keeping the mixture agitated. During

the washing, the total volume of the filtrate was limited to 250 mL. About 30 mL of HCl solution (1 mL HCl:9 mL water) was added to the filtrate while continuously stirring. The filtrate was heated in a water bath at 95  C for 15 min (PC-220, Corning, UK), 20 mL of 0.48 M BaCl2 solution was then slowly added while continuously stirring. The solution was then allowed to stand in the water bath for at least 3 h. It was then filtered using a fine ashless filter paper (Whatman No. 42) through vacuum filtration. The precipitate was washed repeatedly with hot water until 10 mL of filtrate did not produce a slight opalescence when a drop of 0.1 M AgNO3 solution was added. The wet filter paper with the precipitate was transferred into a pre-fired and pre-weighed crucible. The filter paper was folded loosely allowing air to flow but preventing it from splattering. It was then smoked gradually in the muffle furnace (800 ± 25  C) while preventing it to flame in the furnace for 30 min. The samples were then removed from the furnace and allowed to cool down to ambient temperature before it was transferred to a desiccator. After 30 min, the crucible was then weighed to the nearest 0.1 mg. The sulfur content of the sample, %S, was calculated using Equation (14),

  MW S 3 mBaSO4  MW BaSO4 5  100 %S ¼ 4 msample;S 2

(14)

where mBaSO4 is the mass of precipitated BaSO4, msample;S is the mass of raw CS, CCS, SAC, or SSAC used during sulfur analysis, while MW S and MW BaSO4 are the molecular weights of elemental sulfur and barium sulfate, respectively. Sulfonic acid density of the catalyst, SAD, was calculated using Equation (15),

 SAD ¼ %S 

1 MW S



 

1000 mol SO3 H 1 mol S

 (15)

2.4.2. Total acid density (TAD) analysis A sample (~0.05 g) was weighed and mixed with 0.01M NaOH solution (20 mL). It was then agitated for 1 h under ultrasonic vibration (Ney Tech Ultrasonik). The solution was then titrated using standardized 0.01M HCl solution with the aid of a pH meter (Model 210A, Orion, USA). The TAD of the sample was calculated using Equation (16),

TAD ¼

ðC NaOH V NaOH Þ  ðC HCl V HCl Þ msample;T

(16)

where C NaOH and C HCl are the concentration of NaOH and HCl solutions, VNaOH and VHcl are the volume of NaOH and HCl solutions used, and msample;T is the mass of raw CS, CCS, CS-SAC, or SSAC used during total acid density determination. Total acid density accounts for the eSO3H, eCOOH and eOH acid sites. Thus, sulfonic acid density contributes to the total acid density. Furthermore, the weak acid density (WAD) was calculated using Equation (17).

WAD ¼ TAD  SAD

(17)

2.5. Acid value (AV) analysis and catalytic activity determination The acid value (mg KOH/g oil sample) was determined via titration with ethanolic KOH (0.1 M) according to ISO 660 [20] methods. Approximately 0.2 g of oil sample was weighed in a 100mL flask. About 50 mL of ethanol containing 0.5 mL of

G.M.A. Bureros et al. / Renewable Energy 138 (2019) 489e501

phenolphthalein indicator was heated to boiling. The ethanol solution was then added to the sample while the temperature of the ethanol was still above 70  C. The mixture was neutralized with standardized 0.1M KOH until the endpoint of titration was reached as indicated by a slight but definite color change that persists for at least 15 s. The acid values (AV) of the initial oleic acid and final unreacted oleic acid after the reaction were calculated using Equation (18),

AV sample ¼

V KOH C KOH MW KOH 1000  msample;AV

(18)

where V KOH is the volume of KOH solution used, MW KOH is the molecular weight of KOH, cKOH is the concentration of KOH solution, in mol$L1, of the standard volumetric potassium hydroxide solution, msample;AV is the mass, in g, of sample used for AV analysis of initial oleic acid sample and oil recovered after esterification, respectively. The percent FFA conversion, %X FFA , and FAME yield, Y FAME , were calculated using Equation (19) and Equation (20), respectively.

%X FFA

AV initial  AV final ¼  100 AV initial 

Y FAME ¼ %X FFA 

MW FAME MW FFA

ðY FAME ÞðmFFA;i Þ MW FAME t e

msample

following the same procedures. The recovered catalysts were then pooled for the next reaction cycle. The esterification parameters were kept constant and the SAC was reused up to five cycles of esterification. To keep the SOR and catalyst loading constant, these parameters were proportioned with the actual amount of CS-SAC recovered after every reaction cycle. The spent catalysts for each cycle were pooled and analyzed for its sulfonic and total acid densities employing the same methods described in Section 2.3. The catalytic activities of the spent catalysts were determined following the procedure mentioned in Section 2.4. The percent difference of specific catalytic activity (% diff SCA) between the first reaction cycle and nth reaction cycles was calculated using Equation (24),

%diff SCA ¼

SCA1  SCAn  100 SCA1

(24)

where SCA1 and SCAn are, respectively, the specific catalytic activity at the first and nth esterification cycle. 3. Results and discussion

(19)

 (20)

The specific catalytic activity (SCA), and the relative catalytic activities, specific sulfonic activity (SSA) and specific acid activity (SAAÞ, were calculated using Equation (21), Equation (22), and Equation (23), respectively,

SCA ¼

493

(21)

SSA ¼

SCA SAD

(22)

SAA ¼

SCA TAD

(23)

where mFFA;i and msample are, respectively, the mass of oleic acid and CS-SAC used; MW FAME and MW FFA are the molecular weights of the fatty acid methyl ester (as methyl oleate) produced and the free fatty acid (as oleic acid), respectively; and t e is the esterification time. 2.6. Reusability of CS-SAC The method for examining the reusability of the CS-SAC which demonstrated the highest catalytic activity was adopted from Wang et al. [14]. 2.6.1. Optimum esterification time The catalyst that was observed to have the highest catalytic activity was used for esterification at various time (1, 2, 4, 6, 8, 12 and 24 h). This was done in order to determine the esterification time that gives the highest FFA conversion within the time range investigated. The optimum esterification time was then used in the reusability study of the catalyst. 2.6.2. Reusability test For the first esterification cycle, eight sets of the same amount of reactants underwent esterification. The catalysts were recovered

3.1. Raw and processed cacao shell characteristics and yields The raw CS were milled and the particle size distribution of the milled CS was determined through sieve analysis. The mean particle size of the milled CS was calculated to be 840.84 ± 7.74 mm. As mentioned in Section 1, the fixed carbon (FC) content is a characteristic parameter that can be used as basis in assessing the potential of a biomass to be valorized as acid catalyst support. The FC value essentially indicates the amount of carbon that remains after carbonization. Proximate analysis of the raw CS was thus performed to determine its fixed carbon content, as well as its moisture, volatile combustible matter (VCM), and ash contents. The results obtained in this study are presented in Table 1 along with results reported in previous studies. Partial carbonization of the raw CS at 387 ± 17.71  C for 1 h resulted in a fractional yield of ~0.48 g CCS$g1 dry CS. This implies that during carbonization, mass losses were incurred due to the release of moisture and volatile matter as a result of the breaking of chemical bonds within the carbon structure [22]. Proximate analysis was also performed on the carbonized cacao shells (CCS) in order to determine its composition (Table 1). The moisture content of CS obtained from this study closely matches with the result obtained by Pereira et al. [21], whereas the VCM and FC contents of CS obtained from this study are close to the values reported by Jenkins & Ebeling [11]. With respect to the other agricultural wastes that have been valorized as acid catalyst support, the FC content of raw cacao shells obtained from this study is close to that of Jatropha curcas waste seed shells (FC ¼ 23.0%) as reported by Wang et al. [14]. The FC content of the CCS (49.91%) is larger than that of the raw CS (24.46%). This can be attributed to the mass losses during carbonization. As mentioned previously, the mass losses are due to the release of moisture and other volatile matter during carbonization. The FC content of CCS obtained in this study is close to that obtained by Ahmad et al. [7]. Additionally, the raw CS was analyzed for its sulfur density and total acid density and were found to have 0.04 mmol S g1 dry CS and 3.24 mmol Hþ$g1 dry CS, respectively. From these acid densities, the weak acid density, which is composed of functional groups eOH and eCOOH, was calculated to be 3.20 mmol weak acids$g1 dry CS. The CCS was also analyzed for its sulfur and total acid density and was found to have 0.04 mmol S g1 CCS and 1.66 mmol Hþ$g1

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Table 1 Proximate composition of CS and CCS compared with those reported in literature. Composition (%w/w)

Moisture VCM Ash Fixed carbon

CS Sample

CCS Sample

This study

Jenkins & Ebeling [11]

Pereira et al. [21]

This study

Ahmad et al. [7]

9.09±0.26 69.28±1.97 6.27±0.10 24.46±1.75

Not reported 67.95 8.25 23.80

9.45±0.92 Not reported 2.05±0.25 Not reported

5.57±0.05 38.23±0.24 11.86±0.13 49.91±0.18

9.00 Not reported 22.00 43.2

CCS, respectively. Also, the weak acid density was calculated to be 1.62 mmol weak acids$g1 CCS. Further discussion on the changes in the sulfur and weak acid content of the raw CS during carbonization is presented in Section 3.2. During sulfonation of the CCS, the SAC yield with respect to the raw CS decreased to ~39%. In other words, there is further decrease to ~61% on the mass of the dry CS due to further degradation of the biomass and carbonization. Although sulfonation was carried out at various temperatures (80, 100, 120  C) and times (4, 6, 8 h), these conditions did not have a significant effect on the mass loss during sulfonation (p > 0.05). After the raw CS underwent carbonization followed by sulfonation, the overall CS-SAC yield is 39% (g CSSAC$g1 dry CS).

3.2. Extent of sulfonation achieved

40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Thermochemical treatment of CS (a) Sulfur content ratio

Sulfur acid density of processed CS Sulfur density of raw CS

WACR ¼

Weak acid density of processed CS Weak acid density of raw CS

(25)

(26)

Similarly, a weak acid content ratio (WACR) is also calculated using Equation (26). Through these ratios, the number of times by which component density (i.e., the proportion of sulfur or weak acid component to the actual mass of solid) is increased by thermochemical treatment becomes evident (see Fig. 1). For the sulfur content ratio, there is a significant increase in the sulfur content after sulfonation with H2SO4 (Fig. 2a). This suggests that the incorporation of eSO3H groups to the surface of CCS during sulfonation was successful. Analysis of variance revealed that there is no significant difference (p > 0.05) on the total amount of sulfur attached to the CCS after sulfonation at the temperature (80120  C) and treatment time (4-8 h) settings investigated. This indicates that within these ranges, sulfonation temperature and time do not significantly affect the degree of incorporation of sulfonic groups on the CCS structure. Looking at the weak acid content ratio, sulfonation increased the weak acid content present in the CCS. Sulfonation time was found to have no significant effect (p > 0.05) on the weak acid content ratio, whereas temperature was found to significantly increase the weak acid content. It may be that as sulfonation introduces eSO3H to the CCS, weak acids (eCOOH and eOH) are at the same time formed due to oxidation of hydrocarbons [5,24,25]. 3.3. Sulfur and total acid density of CCS as a function of sulfonation temperature and time As mentioned in the previous section, sulfonation temperature

Weak acid content ratio

Sulfur content ratio

It is noted that carbonization resulted to a ~52% loss in mass of the cacao shells due to the dehydration and degradation of the raw CS. However, carbonized cacao shells (CCS) was found to have a sulfur density of 0.04 mmol S g1 CCS which is similar to that of CS. This means that during carbonization, a constant proportion of sulfur is lost with the mass removed. During biomass carbonization, chemical bonds are known to break as the temperature is increased leading to the removal of functional groups (eOH and eCOOH) [23]. This could be the reason why the weak acid density of CCS is (1.62 mmol weak acids$g1 CCS) is lower than that of the raw CS (3.20 mmol weak acids$g1 dry CS). Of interest is the extent by which incorporation of eSO3H to CCS is achieved through sulfonation. To verify this, a quantity referred to as sulfur content ratio (SCR) is defined as follows:

SCR ¼

1.00 0.80 0.60 0.40 0.20 0.00

Thermochemical treatment of CS (b) Weak acid content ratio

Fig. 1. (a) Sulfur (strong acid), and (b) Weak acid content ratio of CS after carbonization (CCS) and after sulfonation (CS-SAC) under various temperatures and time (e.g. 80-4 means sulfonation at 80  C at 4 h). Error bars are standard deviations of triplicate analysis.

1.60 1.50 1.40 1.30 1.20 1.10 1.00 4

6

8

Total acid density (mmol H+·g-1 CS-SAC)

Sulfur acid density (mmol S·g-1 CS-SAC)

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5.00 4.50 4.00 3.50 3.00 2.50 4

Sulfonation time (h) (a) 80 °C

100 °C

120 °C

495

6

80 °C

100 °C

Fig. 2. (a) Sulfur and (b) Total acid density of CS-SAC synthesized under various sulfonation conditions (temperature: 80, 100, deviations of triplicate analysis.

and time had no significant effect on the net addition of the amount of sulfur. However, it should be taken into account that during sulfonation, further degradation of the biomass, carbonization, and changes in the biomass structure or morphology can occur. In view of the degradation, there is further decrease to ~61% on the mass of the dry CS during sulfonation. Although sulfonation was carried out at various temperatures (80, 100, 120  C) and times (4, 6, 8 h), these conditions did not have a significant effect on the mass loss during sulfonation (p > 0.05). Due to further degradation of biomass, an increase in the sulfur and acid density was observed (Fig. 2). Partial carbonization of biomass at low temperatures (200400  C) [13,26] theoretically causes the release of volatile matter and other functional groups which then leads to the formation of small polycyclic aromatic hydrocarbon sheets. The removal of these volatile constituents is further supported by the FTIR spectra (Fig. 3) obtained, where a sharp decrease in the observed peak at 1250 cm1 and 1650 are observed after carbonization, which corresponds to the decrease in the available sulfoxides, carboxyl [6,27], and amino groups, probably from the degradation and volatilization of proteins and organic acids. This is consistent with the observed decrease in weak acid sites and total available sulfur as previously discussed. Furthermore, sharp distinct peaks were observed in the carbonized cacao shell at 1500 to 1600 cm1, indicating the formation of aromatic groups [2,6,28]. The formation of these groups supports the idea that aromatic sheets are formed, which in turn would allow the conducive incorporation of the eSO3H groups to the carbonized biomass during sulfonation with H2SO4 [13,29]. The presence of unsaturated carbon bonds from aromatic groups may explain the observed increase in the sulfur and acid density after sulfonation. Both sulfonic and total acid density significantly increased as sulfonation temperature was increased from 80 to 120  C at a sulfonation time of 4 h and 6 h. Apparently, at these sulfonation times, the introduction of sulfonic groups to the carbon structure is more effective as sulfonation temperature is increased. Spectra obtained from FTIR analysis of the sulfonated CCS were observed to exhibit bands at 1050, 1350 and 600 cm1 indicates that these sulfur groups are that of the sufonic acid group (eSO3H) [2,6,27,30]. Furthermore, observed bands for aromatic C]C (1500e1600 cm1) were also observed to have decreased suggesting that sulfonic groups were attached to aromatic rings. The highest sulfur density (1.48 mmol SO3H$g1 CS-SAC) and total acid density (4.56 mmol Hþ$g1 CS-SAC) was obtained after sulfonation at 120  C for 6 h. However, at a temperature of 120  C and for a prolonged time of 8 h in the presence of concentrated

8

Sulfonation time (h) (b) 120 °C 120  C;

time: 4, 6, 8 h) Error bars are standard

H2SO4, further carbonization may have occurred resulting in a more rigid carbon structure [24]. In effect, the introduction of sulfonic groups on the carbon structure is hindered and a decrease in the sulfonic and total acid density is observed. Generally, based on the results obtained, the sulfur and total acid density increased as the sulfonation temperature was increased from 80 to 100  C for a sulfonation time of 4e8 h (Fig. 3). For CS-SAC synthesized at 120  C, both characteristics also increased as the sulfonation time was increased from 4 to 6 h. These results are in agreement with the study conducted by Zhou et al. [2] on sulfonation of bamboo, where sulfonation beyond 2 h at 105  C did not result in a further increase in acid density. Moreover, sulfonation beyond 105  C for 3 h resulted in a decrease in the acid density [2]. The increase in sulfonic and total acid sites are found to be positively correlated (r ¼ 0.9189). High acid densities are often associated with a high catalytic activity; however, the actual performance of the catalyst is also influenced by its morphology resulting from the thermochemical (carbonization and/or sulfonation) treatment. Thus, it is imperative that the catalytic activity of CS-SAC be evaluated through actual application of the catalyst, in this case, in the esterification of oleic acid. 3.4. Effect of sulfonation time and temperature on the FFA conversion The various sulfonation conditions used in this study led to different sulfonic and acid densities (Fig. 2). In order to evaluate the performance of the catalyst with respect to their sulfonic and acid densities, esterification of oleic acid at 42.1 ± 2.9  C for 4 h was performed using the prepared catalysts. The effect of the different sulfonation conditions on the catalyst’s ability to cause the conversion of oleic acid during esterification is presented in Fig. 4. With catalysts obtained from a sulfonation time of 4 h, the FFA conversion significantly increased, from 63 to 78% as the sulfonation temperature was increased from 80 to 120  C (Fig. 4a). Likewise, the FFA conversion obtained from esterification using CS-SAC sulfonated for 6 h also increased, from 69 to 79%, as the sulfonation temperature was increased from 80 to 120  C (Fig. 4b). This increase can be attributed to the significant increase in the sulfonic and acid density at these conditions (Fig. 2). Theoretically, the presence of weak acid groups could enhance the catalytic performance of the catalyst by acting as active sites for the attachment of the reactants [25]. This is possible because of the strong affinity between the polar moiety of the reactants and the eOH groups attached to the surface of the CS-SAC [2]. With the possible adsorption of the reactants to the surface of the catalyst,

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0.035 Aromatic C=C Bending

Aromatic C-H Bending

0.03

0.02 0.015 0.01 Carboxylic O-C

Sulfoxide or Sulfone S=O

0.005 Sulfonic S=O

2650

2400

2150

1900

1650

1400

1150

900

Wavenumber (cm-1) Raw Cacao Shell

Absormance Intensity (AU)

0.025 Sulfone or Sulfonic S=O

Carboxylic C=O

Carbonized Cacao Shell

650

0

400 -0.005

CS-SAC 4H-120C

Fig. 3. FT-IR spectra of raw cacao shell, carbonized cacao shell and CS-SAC synthesized at 120  C for 4 h.

the reaction could have been enhanced. For the CS-SAC sulfonated for 8 h at the three different temperatures, no significant differences in FFA conversions were observed (Fig. 4c). This corresponds also to the insignificant differences in the sulfur and acid density of the CS-SAC synthesized at these conditions. Thus, for a prolonged sulfonation time of 8 h, sulfonation temperature was found to have no significant effect on the catalyst’s activity. This may also be attributed to the changes in the morphology of the CS-SAC as it is sulfonated at a prolonged time. Although sulfonic and weak acid sites are present in the catalyst, these sites may not be accessible to the reactants during esterification because the carbon structure may have collapsed. With the CS-SAC synthesized at a sulfonation temperature of 80  C, the FFA conversion significantly increased, from 63 to 69%, as the sulfonation time was increased from 4 to 6 h (Fig. 4d). However, as the sulfonation time was increased to 8 h at the same sulfonation temperature, the increase in the FFA conversion was not significant. Similarly, at a sulfonation temperature of 100  C, the FFA conversion generally increased, from 63 to 74%, as the sulfonation time was increased from 4 to 6 h (Fig. 4e). On the other hand, as the sulfonation time was increased to 8 h at this temperature, the decrease in the FFA conversion was not significant. The increase in the FFA conversion for CS-SAC sulfonated at 80 and 100  C as sulfonation time was increased from 4 to 6 h coincides with the increase in the sulfur and total acid density of the CS-SAC synthesized at these conditions. Hence, for CS-SAC sulfonated at 80 and 100  C, the effect of sulfonation time was only significant within the 4e6-h range. The highest FFA conversion of 79% was obtained using the CSSAC sulfonated at 120  C for 6 h. However, the increase in the FFA conversion from 78 to 79% as sulfonation time was increased from 4 to 6 h was not significantly different (p > 0.05). This can also be supported by the insignificant increase in the sulfonic and acid densities of these catalysts. Increasing the sulfonation time to 8 h only resulted in a decrease in the FFA conversion to 73% (Fig. 4f). As mentioned previously, a prolonged sulfonation time may have changed the morphology of the catalyst, hence, a decrease in the FFA conversion was observed.

Although the CS-SAC sulfonated at 120  C for 6 h led to the highest FFA conversion, as mentioned previously, Tukey HSD test revealed that this value is not significantly different (p > 0.05) with the FFA conversion obtained using the CS-SAC sulfonated at 120  C for 4 h. This means that at a higher sulfonation temperature of 120  C, sulfonation time can be shortened to 4 h while still obtaining a high FFA conversion. This process is more energy efficient in the sulfonation step. Hence, the ability of the catalyst to effect conversion is not solely dependent on the presence of acid sites (eSO3H, eCOOH, and eOH) but maybe also on the morphology of the catalyst. This is what can be inferred from the results obtained in this study. The specific catalytic activity (SCA) defined as the amount of FAME (in mmol) produced per gram of catalyst per hour was also calculated using Equation (21). Using this quantity as the response variable, a similar trend on the effect of sulfonation time and temperature was observed as when using FFA conversion as the response variable. The highest observed specific catalytic activity (14.04 mmol FAME$g1 SAC$h1) was with SAC sulfonated at 120  C and 6 h. 3.5. Characterization of spent catalysts Spent catalysts recovered after esterification of oleic acid were analyzed for their sulfonic and total acid densities in order to determine the stability of the catalysts during esterification of oleic acid with methanol. Presented in Fig. 5 are the sulfonic and weak acid densities of the catalysts before and after esterification. Both the sulfonic (eSO3H) and weak acid (eOH and eCOOH) densities generally decreased after esterification (Fig. 5). Comparing the decrease in the content of these functional groups after esterification, it can be observed that the decrease in the eSO3H density is higher than the decrease in the weak acid groups. This indicates that, compared with the weak acids (eOH and eCOOH), more eSO3H groups were removed from the surface of the catalyst. The leaching of eSO3H groups can be attributed to the solvent used for the washing of the catalyst. According to Mo et al. [31], methanol attracts the hydrophilic sulfonic groups due to its

100.00

100.00

80.00

80.00

% XFFA

% XFFA

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60.00 40.00

60.00 40.00 20.00

20.00

0.00

0.00 80

100

4

120

100.00

80.00

80.00

% XFFA

% XFFA

100.00

60.00 40.00

6

8

Sulfonation time (h) (d)

Sulfonation temperature (°C) (a)

20.00

60.00 40.00 20.00

0.00

0.00 80

100

120

4

Sulfonation temperature (°C) (b)

100.00

100.00

80.00

80.00

60.00 40.00 20.00

6

8

Sulfonation time (h) (e)

% XFFA

% XFFA

497

60.00 40.00 20.00

0.00 80

100

120

Sulfonation temperature (°C) (c)

0.00 4

6

8

Sulfonation time (h) (f)

Fig. 4. Effect of sulfonation temperature at various sulfonation time of (a) 4 h, (b) 6 h, and (c) 8 h, and effect of sulfonation time at various sulfonation temperatures of (d) 80  C, (e) 100  C, and (f) 120  C on the percent conversion of oleic acid during the esterification at 42.1 ± 2.9  C for 4 h with methanol to oleic acid molar ratio of 7:1 and a catalyst loading of 5%w/w with respect to the oleic acid. Error bars are standard deviations of triplicate runs.

polarity thus resulting to the leaching of eSO3H groups. Another factor that may have contributed to the decrease in the eSO3H acid density can be attributed to the structure of the catalyst, wherein eSO3H groups were loosely bonded to the surface of the carbon structure. Leaching of acid sites could be an indication that the catalyst is unstable. It can also be observed that catalysts that had high acid densities, i.e., those synthesized at higher sulfonation temperatures, had a higher decrease in both acid densities as compared to the catalysts synthesized at lower sulfonation temperatures. On the other hand, for catalysts synthesized at the prolonged time, there was no notable effect on the decrease in the total acid density. 3.6. Reusability study Among the advantages of using solid acid catalyst are its ease of recovery from the reaction mixture and its reusability. In this study, the catalytic activity of the CS-SAC sulfonated at 120  C for 4 h was investigated over four cycles of recovery and reuse. After every cycle, the catalyst was recovered, washed with hexane and dried at 60  C overnight. The washing agent used in the reusability study is hexane instead of methanol for reasons as mentioned in Section 3.5. Due to the polarity of methanol, a large amount of eSO3H were leached out, especially for the catalyst synthesized at higher sulfonation temperature of 120  C. It is noted, however, that during the reaction, leaching may have already occurred due to the presence of

methanol and water in the reaction medium. After a reaction time of 4 h, it was found out that with methanol as the washing agent, about 0.58 mmol SO3H$g1 SAC was removed. Hexane, on the other hand, is non-polar and was found to cause a lower degree of leaching during washing of a bagasse-derived SAC [24]. In order to compare the degree of leaching with respect to the washing agent, the spent catalyst recovered from the first run, at a longer reaction time of 24 h, was washed with hexane, and it was found out that the ~0.39 mmol SO3H$g1 SAC was removed. This confirms that even with prior leaching during the reaction, a lesser amount of eSO3H can be leached from the catalyst’s surface with hexane as the washing agent. The percent FFA conversion obtained up to 4 cycles is presented in Fig. 6. The percent FFA conversion obtained using the fresh catalyst is 93% as the esterification time reached 24 h. After the first run, the sulfonic and weak acid densities decreased from 1.39 to 1.00 mmol SO3H$g1 SAC and 3.02 to 2.33 mmol weak acids$g1 SAC, respectively (Fig. 7). As mentioned previously, after 4 h of reaction time, ~0.58 mmol SO3H$g1 SAC was removed with methanol as washing agent, whereas, with hexane as the washing agent, only ~0.39 mmol SO3H$g1 SAC was removed even at a longer reaction time of 24 h. Thus, lesser eSO3H groups were leached out when hexane was used as the washing agent. On the other hand, the decrease in weak acids employing different washing agents was not notable; however, taking into account the reaction time, after only 4 h of reaction time, ~0.70 mmol weak acids$g1 SAC was removed

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Sulfonic acid density (mmol SO3H·g-1 sample)

1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 80-4

80-6

80-8

100-4

100-6

100-8

120-4

120-6

120-8

120-6

120-8

Sulfonation Temperature - Time (°C - h) (a) SAC

SSAC

Weak acid density (mmol H+ ·g-1 sample)

3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 80-4

80-6

80-8

100-4

100-6

100-8

120-4

Sulfonation Temperature - Time (°C - h) (b) SAC

SSAC

Fig. 5. (a) Sulfonic and (b) Weak acid densities of SAC (sulfonated at various temperature and time) and SSAC (recovered after esterification at 42.1 ± 2.9  C for 4 h with methanol to oleic acid molar ratio of 7:1 and a catalyst loading of 5%w/w with respect to the oleic acid). Error bars are standard deviations of triplicate runs.

when methanol was used, while after a longer reaction time of 24 h, a close amount of weak acids, ~0.69 mmol weak acids$g1 SAC, was removed when hexane was used. The spent catalyst from the first run was used for the first recycle and obtained 74% FFA conversion. After the second recycle, the FFA conversion dropped to 56%, which can be attributed also to the decrease in both the sulfonic and total acid densities. These densities dropped to 0.94 mmol SO3H$g1 SAC and 3.06 mmol Hþ$g1 SAC, respectively. The percent FFA conversion obtained in the third and fourth recycles was constant at 48%. This can be supported also by the insignificant differences in their acid densities. From the results obtained, it can be deduced that the synthesized catalyst decreases in activity during the first three cycles of repeated use and then exhibits constant activity in succeeding cycles. This decrease in activity may be attributed to the loosely attached eSO3H groups to the carbon structure which resulted to leaching of these sites. Since eSO3H, eOH, and eCOOH are strong Brønsted sites, these sites tend to adsorb water from the reaction media. This may have hindered the reactants from interacting with the acidic sites resulting in a decrease in the catalytic activity. This is

made evident with the decrease in the specific acidic activity from 0.63 to 0.55 mmol FAME$mmol1 Hþ$h1. Deactivation due to deposition of reactants also may have contributed to the decreased activity of the catalyst. Changes in the morphology of the catalysts as it is being reused up to the fourth recycle may have happened, wherein the carbon structure collapsed as the catalyst was repeatedly used for esterification. The sulfonic and acid sites may not be accessible to the reactants at this point due to the decreased pore size of the carbon structure. Nevertheless, the conversion and acid densities approach to a constant value after the 3rd reuse, which may imply that the catalyst could potentially be further reused at a constant activity after most of the loosely bounded active sites have been removed. 3.7. Comparison of catalytic activity of CS-SAC with other biomassderived SAC The highest specific catalytic activity (14.0 mmol FAME$g1 SAC$h1) was achieved by the CS-SAC sulfonated at 120  C for 6 h. To compare the results of this study with other biomass-derived

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499

100.00 90.00 80.00

% XFFA

70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00 1

0

2 Recycle number

3

4

1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0

1

2

3

4

Recycle number (a) SAC

SSAC

Weak acid desnity (mmol weak acids·g-1 sample)

Sulfonic acid density (mmol SO3H·g-1 sample)

Fig. 6. Reusability study of CS-SAC (sulfonated at 120  C for 4 h) during esterification at 42.1 ± 2.9  C for 24 h with methanol to oleic acid molar ratio of 7:1 and a catalyst loading of 5%w/w with respect to the oleic acid. Error bars are standard deviations of triplicate runs.

3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 0

1

2

3

4

Recycle number (b) SAC

SSAC

Fig. 7. Catalyst’s (a) sulfonic and (b) weak acid density after each cycle of esterification during the esterification at 42.1 ± 2.9  C for 24 h with methanol to oleic acid molar ratio of 7:1 and a catalyst loading of 5% w/w with respect to the oleic acid. Error bars are standard deviations of triplicate runs.

SAC, a summary of the catalytic activity of different agricultural residues valorized as a catalyst support in the synthesis of SAC for the production of FAAE, or biodiesel is presented in Table 2. In view of their SCA, Jatropha seed shells and rice husk evidently obtained high SCA with respect to the other catalysts reported in the literature. For the Jatropha seed shell-derived SAC, the high SCA can be attributed to the high sulfonic acid density obtained by this catalyst. While for the rice husk-derived SAC, the high SCA can be attributed to the prolonged reaction time of up to 9 h. In addition, esterification reaction of both SAC was done at a higher temperature of 80  C resulting in an enhanced reaction thereby increasing the catalytic activity. Meanwhile, the specific catalytic activity obtained from this study, which is 14.0 mmol FAME$g1 SAC$h1, is slightly higher to that of corn straw and bagasse-derived SAC, which are 11.8 and 12.5 mmol FAME$g1 SAC$h1, respectively. Comparing corn strawderived SAC with the CS-SAC, although both were evaluated at almost similar esterification conditions except for their reaction time, wherein CS-SAC was performed at a lower reaction temperature, CS-SAC achieved a slightly higher catalytic activity than corn straw-derived SAC. In view of their acid densities, corn strawderived SAC had a slightly higher SO3H density than CS-SAC, however, CS-SAC obtained a considerably higher weak acid density (3.08 mmol weak acids$g1 SAC) than corn straw-derived SAC (0.2 mmol weak acids$g1 SAC). Thus, the higher catalytic activity of CS-SAC as compared with the corn straw-derived SAC can be attributed, predominantly, by the presence of weak acid groups

(eOH and eCOOH). On the other hand, comparing bagasse-derived SAC with CS-SAC, the former consumed larger amount of methanol and the reaction was performed at a higher temperature. Despite these advantages, a slightly higher catalytic activity was still obtained by the CS-SAC than bagasse-derived SAC. This can be attributed to the higher SO3H and weak acid density of CS-SAC compared to bagasse-derived SAC. The catalytic activity can also be expressed in terms specific sulfonic activity (SSA) (mmol of FAME$mmol1 SO3H $h1). Jatropha seed shell-derived SAC still has the highest activity with 164.5 mmol FAME$mmol1 SO3H $h1. Thus, among the catalysts compared, it is the most efficient biomass-derived SAC. On the other hand, corn straw-derived SAC obtained the lowest SSA, thus, in order to effect a high conversion using this catalyst, the amount of catalyst to be used in the reaction should be substantial. While for the bagasse-derived SAC and CS-SAC, the former obtained a slightly higher SSA than CS-SAC, thus, in terms of their SSA, bagasse-derived SAC is more efficient. With respect to the reusability study performed in this work, the FFA conversion obtained increased to 93% as the reaction time was prolonged to 24 h. With the increase in the reaction time, it was observed that the specific catalytic activity of CS-SAC declined to 2.7 mmol FAME$g1 SAC$h1. Comparing the number of recycles and the FFA conversion after the final reuse of the different catalysts, it can be observed that CS-SAC had the least stable performance as a result of the low SCA achieved. Among the catalysts being compared in Table 2, Jatropha waste

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Table 2 Catalytic activity of different agricultural residues valorized as SAC in the esterification of oleic acid. Raw material

Carbonization (T(C)/t(h))

Sulfonation (T(C)/t(h))

SO3H/ TADa

Alcohol SOR CL (w/w)c T(C)/t (h) XFFA (n/n)b

Cacao shells

400/1

120/6

1.48/4.56

MeOH 7:1

0.05

42/4

0.79 14.0

f Cacao shells Corn straw

400/1 350/1

120/4 80/4

1.39/4.41 2.44/2.64

MeOH 7:1 MeOH 7:1

0.05 0.07

42/24 60/4

0.93 0.93

100/8 150/15 150/8

2.00/1.06/3.69 -/ 5.25

MeOH 1:1 MeOH 10:1 MeOH 5:1

0.005 0.05 0.003

80/2 80/5 80/9

0.91 0.91 0.87

Jatropha Seed shells 350/5 Bagasse 375/0.5 Rice husk 450/15 a b c d e f

SCAd

SSAe 9.1

Recycles/washing/XFFA, final reuse Ref

-/ Methanol/2.7 1.9 5/n-hexane/0.48 11.9 4.9 -/ Methanol/328.9 164.5 4/Methanol/0.91 11.0 10.3 8/n-hexane/0.81 129.8 e 10/Ethanol/0.71

This study This study [4] [14] [24] [32]

SO3H/TAD are sulfonic and total acid density of the SAC. SOR is the solvent-to-oil ratio expressed as moles solvent/mole oil (n/n). CL is the catalyst loading which is the mass of SAC used per weight substrate (w/w). SCA is the specific catalytic activity expressed in (mmol FAME$g1 SAC-$h1). SSA is the specific sulfonic activity expressed in (mmol FAME$mmol1 Hþ$h1). Conditions and results of the reusability study of cacao shells.

seed shells-derived SAC is the most efficient catalyst followed by rice husk-derived SAC. Both catalyst only used a minimal amount of catalyst and smaller volume of methanol. But a higher reaction temperature and a higher reaction time was used in the evaluation of the activity of rice-husk derived SAC. Then, cacao shell-derived and bagasse-derived SAC have comparable performance but bagasse-derived SAC consumed a larger volume of methanol and a higher reaction temperature. Lastly, corn straw-derived SAC is the least efficient among these catalysts and requires a substantial amount of catalyst to effect a high conversion. From these results, it can be validated that cacao shells can be a potential catalyst support for the synthesis of SAC intended for the esterification of oleic acid with methanol. Although Jatropha waste seed shells-derived SAC achieved a better performance than cacao shell-derived SAC, it is noted, that considering the availability of raw material, waste cacao shells are more abundant in the local context than Jatropha seed shells. 4. Conclusions Cacao shell-derived solid acid catalyst was successfully prepared by the sulfonation of partially carbonized cacao shells at 350  C for 1 h, with concentrated H2SO4. The resulting catalyst was analyzed of its acid density and employed as catalyst in the esterification of oleic acid and methanol with for the intended purpose of reducing free fatty acids in low quality oil for biodiesel production. The following are the main findings of the study: ii. The eSO3H and acid densities were found to increase as the sulfonation temperature was increased from 80 to 120  C at sulfonation times of 4 and 6 h. iii. Prolonged time of 8 h, sulfonation temperature was found to have no significant effect on the sulfonic and acid densities. iv. The FFA conversion was also found to exhibit the same responses as with the acid densities in relations to the effects of sulfonation temperature and time. v. At the highest sulfonation temperature (120  C) investigated, it was found out that the FFA conversions of the catalysts sulfonated for 4 and 6 h had the highest acid density of FFA conversions of 78 and 79%, respectively, at an esterification condition of ~45  C with an FFA to methanol molar ratio of 1:7. vi. In view of energy conservation, catalyst synthesis time may be shortened to 4 h. vii. A high conversion of ~94% can be obtained at an esterification time of 24 h.

viii. Loosely attached active sites are removed during first few cycles of the esterification reaction with the FFA conversion decreasing to 74% in the first reuse and then down to a constant conversion of 48% after the third recycle.

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