Simultaneously carbonized and sulfonated sugarcane bagasse as solid acid catalyst for the esterification of oleic acid with methanol

Simultaneously carbonized and sulfonated sugarcane bagasse as solid acid catalyst for the esterification of oleic acid with methanol

Renewable Energy 130 (2019) 510e523 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Sim...

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Renewable Energy 130 (2019) 510e523

Contents lists available at ScienceDirect

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

Simultaneously carbonized and sulfonated sugarcane bagasse as solid acid catalyst for the esterification of oleic acid with methanol Ken P. Flores a, Jan Laurence O. Omega a, Luis K. Cabatingan a, **, Alchris W. Go 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 b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 January 2018 Received in revised form 18 April 2018 Accepted 21 June 2018 Available online 26 June 2018

Solid acid catalyst was synthesized from sugarcane bagasse, a residue left behind during sugar milling, through a simpler and less energy-intensive one-step process, simultaneous carbonization-sulfonation. A range of synthesis temperature (150, 200, 250  C) and time (4, 6, 8 h) were investigated in the preparation of the catalyst to determine their effects on the catalytic activity and conversion during esterification of oleic acid and methanol. Extensive washing of the freshly synthesized catalyst have significant influence on the performance of the catalyst, as loosely bound acid sites are removed in the process lowering its activity but improving its stability. The catalyst synthesized at 150  C for 8 h, having a sulfonic acid density of 0.59 mmol/g, exhibited the best performance during a 4-h esterification assay using oleic acid and methanol, resulting in an FFA conversion of 46.5% and catalytic activity of 4.62 mmol oleic acid/mmol-SO3H $h. Additionally, the catalyst could be used for at least five 24-h esterification cycles, where an FFA conversion of as high as 87% was achieved. The catalyst retained 76.5 and 86% of its initial catalytic performance and sulfonic acid density, respectively, after the fourth cycle, offering good operational stability. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Solid acid catalyst Sugarcane bagasse Catalytic activity Biodiesel production FFA conversion Carbon-based catalyst

1. Introduction Heterogeneous acid catalysts for biodiesel production have gained increasing attention owing to their significant advantages of eliminating separation, corrosion, toxicity and reduce environmental problems posed by their homogeneous counterparts. In addition, these catalysts can simultaneously catalyze both esterification and transesterification reactions, making them compatible for use with high acid value oils in the production of biodiesel. Traditional solid acid catalysts include zeolites, mesoporous silica, and polystyrene resins. These catalysts generally possess one or more problems of small pore size, low acid density, poor operational stability, bad tolerance to water, and high cost [1e3].

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.K. Cabatingan), [email protected] com, [email protected] (A.W. Go). https://doi.org/10.1016/j.renene.2018.06.093 0960-1481/© 2018 Elsevier Ltd. All rights reserved.

Recently, there has been a growing interest in the use of carbon as the catalyst support material. This led to the emergence of a new class of catalysts derived from natural products having welldefined chemical structures such as sugar, starch, and cellulose. These catalysts can be readily prepared by sulfonation of partially carbonized organic matter using concentrated sulfuric acid. Carbon-based catalysts from glucose [4], sucrose, cellulose, starch [5], and Kraft lignin [6] exhibited high catalytic performance in the esterification of oleic acid (95e96% FFA conversion), outperforming the aforementioned traditional solid acid catalysts. Since these catalysts are derived from major constituents of lignocellulosic biomass, there is a potential for utilizing agricultural residues in the synthesis of such catalysts. In the recent years, several studies have been conducted on using agricultural residues such as coconut shell [7], oil palm trunk [8], rice husk [9], corn straw [10], bamboo [11], spent coffee grounds [12], waste Jatropha curcas seed shells [13], and sugarcane bagasse [14] as the starting material for synthesis of solid acid catalyst (SAC). The catalysts prepared from these materials demonstrated good catalytic performance towards

K.P. Flores et al. / Renewable Energy 130 (2019) 510e523

esterification of high FFA oils, with FFA conversions ranging from 71 to 98%. Esterification reactions involving the use of these catalysts typically require 0.005 to 0.09 w/w of catalyst, a methanol to oil ratio of 1e20, and a reaction time of 2e8 h at 60e110  C [7e14]. Although catalysts synthesized from different biomass residues as starting material and carbon source may have certain advantages of their own, their potential for later local application is hinged on the availability of the raw material in a given locality. In this regard, sugarcane bagasse (SCB) would be a practical candidate as starting material in the Philippine context as it is available locally (4.45 millions tons/year), given that sugarcane is the top agricultural product of the country (22.7 million tons/year) from 2007 to 2016 [15e17]. The utilization of agricultural residues such as SCB in the synthesis of solid acid catalysts (SAC) is attractive owing to the material's low cost and availability, which could potentially lead to a greener process for the production of biodiesel [18]. Moreover, this could lead to the valorization of these waste materials while at the same time addressing waste management concerns. Several studies have been conducted on the use of sulfonated activated carbon from sugarcane bagasse (SAC-SCB) for the (trans) esterification of oils [5,8,14,19]. The catalysts exhibited desirable catalytic performance ranging from 80 to 96% conversion, depending on the catalyst preparation conditions and reaction parameters. The catalytic activity of a sulfonated activated carbon like that of SAC-SCB is attributed to the presence of sulfonic acid (eSO3H) functional groups [20], which are introduced via sulfonation of partially carbonized material. In view of catalyst synthesis, SAC-SCB is typically synthesized via a two-step process involving partial carbonization of crushed SCB at temperatures of 300e800  C for about 0.5e20 h and subsequent sulfonation at temperatures of 120e200  C for a duration of 3e20 h [8,14,19]. Partial carbonization is employed to induce formation of small polycyclic aromatic rings to serve as backbone or support for the active sites [22]. Sulfonation using sulfuric acid is then carried out to introduce the sulfonic acid (eSO3H) functional groups into the polycyclic aromatic rings through covalent attachment by substitution of hydrogen in the CeH bonds of the catalyst structure [14]. Another approach in the preparation of SAC-SCB involves carrying out dilute acid hydrothermal pretreatment step prior to carbonization and sulfonation. However, the synthesized catalyst only had minimal improvement in performance (from 90.3 to 93.2% FFA conversion), and required an additional processing time of 10 h at 200  C for the pretreatment step [21]. Considering the processing conditions (temperature and time) required of the carbonization and sulfonation steps, there are overlaps and could potentially be taken advantaged of by combining the two processing steps. An alternative to the above described preparation schemes is the simultaneous carbonization and sulfonation (SCS) of SCB as proposed by Savaliya and Dholakiya [23] in which both carbonization and sulfonation are carried out in one single step.

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This approach greatly reduces the preparation time as well as the energy requirements of the synthesis of SAC-SCB. A comparison of the different synthesis schemes in the preparation of SAC-SCB is presented in Table 1. In the preparation of SAC-SCB through SCS, a mixture of baggase and sulfuric acid is heated at an elevated temperature under a stream of nitrogen [23]. Through the SCS scheme of synthesizing SAC-SCB the synthesis or preparation could be greatly reduced from almost 30 h to as short as 10 h. Moreover, the main processing steps could be greatly reduced from a synthesis scheme requiring 3 major steps to a one-step synthesis. Furthermore, taking account the fact that sulfuric acid is a dehydrating agent, this would result in the hastening of the carbonization process, whereby removing the inherent hydrogen and oxygen groups present while simultaneously attaching to the carbon backbone. A successful implementation of this approach would result in a cost-effective and more environmentally friendly process, considering the initial investments and capital cost that could be reduced by reducing the required processing steps and the reduction of the energy requirements and emissions entailed in the conventional approach. As of this writing, the effects of time and temperature during SCS on the catalytic activity of SAC-SCB during SCS has not been investigated. Moreover, most of the above mentioned catalyst preparation schemes were typically carried out under oxygen-free atmosphere by continuously purging with nitrogen gas, which would also entail additional cost in the catalyst production. It would be of interest to explore the possibility of preparing SAC-SCB through SCS without the use of purge gas [21,24] to further reduce the preparation cost. In view of the existing gaps in the conditions for the synthesis of SAC-SCB through SCS, this study aims to determine the effects of time and temperature during SCS under minimized oxidative condition on the catalytic performance (FFA conversion) and activity (sulfonic acid activity) of the SAC-SCB in the esterification of oleic acid. Furthermore, the reusability of SAC-SCB in the esterification of oleic acid was also looked into. Moreover, during the course of this study, the importance of incorporating an extensive washing step to produce catalyst of good stability was also elucidated.

2. Material and methods Sugarcane bagasse was collected from a local sugarcane juice vendor in Cebu. Ethanol 99.9 %v/v, methanol 99.8%, n-Hexane 96%, Scharlau, Spain and chemical reagents (Sulfuric acid 95e98 %w/w; hydrochloric acid 36e39 %w/w; barium chloride dihydrate 99.0%; anhydrous sodium carbonate 99.8%; oleic acid AV 195e204, Ajax, Sydney, Australia and sodium chloride 99.5%; magnesium oxide 96e100.5%; sodium hydroxide pellets 97%, Scharlau, Spain) used were obtained through local suppliers.

Table 1 Typical operating conditions for different synthesis methods in the preparation of SAC-SCB. Method of Synthesis

Carbonization-Sulfonation [8,14,21]

Carbonization-Sulfonation with hydrothermal pre-treatment [21]

Simultaneous Carbonization-Sulfonation [23]

No. of steps Pre-treatment Time Pre-treatment Temperature Carbonization Time Sulfonation Time Total synthesis time Carbonization Temperature Sulfonation Temperature Operating Temperature

2 e e

3 10 h 200  C

1 e e

0.5e20 h 3e20 h 3.5e23 h 300e800  C

5h 5e15 h 20e30 h 400e800  C

e e 10 h e

100e200  C 100e800  C

120e200  C 120e800  C

e 180  C

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2.1. Collection, storage, and characterization of sugarcane bagasse

and Equation (5), respectively.

Sugarcane bagasse samples were collected fresh right after pressing of its juice. Collected samples were oven-dried at 60  C for 3 days or until a moisture content of less than 10% w/w was achieved. Dried samples were milled through a 2-mm mesh using a Willey mill, stored at room temperature in a polypropylene box and tightly covered.

%Ash ð%w=wÞ ¼

2.1.1. Particle size distribution About 50 g of milled bagasse was sieved through pre-weighed standard Tyler sieves (850, 450, 250, and 180 mm), stacked in ascending order of mesh size, with the receiver placed at the bottom. The sample was loaded on the topmost sieve and covered. The sieve set was securely mounted on the sieve shaker (Mod.A5911, Intertest Benelux, Netherlands) and operated for 15 min. The sieves containing the samples were weighed and the mean particle size (dm ) was calculated using Equation (1).

mean particle size ðdm Þ ¼

X mi mT

di

(1)

where mi and mT are the mass of the retained samples in a given sieve and the total sample mass, respectively. Diameter di is the average mesh aperture between two sieves. 2.1.2. Proximate components About 1 g of powdered bagasse was weighed (to the nearest 0.0001 g) into pre-fired and pre-weighed 30-mL porcelain crucibles. The crucibles containing the samples were heated in an oven at 105  C for 2 h, transferred to a desiccator and allowed to cool for an hour prior to weighing. This drying procedure was repeated until the change in weight was less than or equal to 0.0005 g. The moisture content (%M), of the sample was calculated using Equation (2).

%M ð%w=wÞ ¼

  ms  mcs;105 C  mc  100% ms

(2)

where, ms is the initial mass of the sample, mc is the mass of the pre-fired crucible and mcs;105 C is the mass of the crucible containing the sample after drying at 105  C. For the determination of volatile matter content, the crucibles were covered with pre-fired crucible lids before being placed into a pre-heated furnace (950  C). With the furnace door open, the crucibles were placed on the outer ledge of the furnace for 2 min, transferred to the edge of the furnace for 3 min, and then moved to the rear of the furnace with the muffle door closed for 6 min. The crucibles were then taken out of the furnace, allowed to cool for 15 min, transferred to a desiccator and allowed to cool for an additional 30 min before weighing. Volatile matter content (%VM) was calculated using Equation (3).

%VM ð%w=wÞ ¼

mcs;105 C  mcs;950 C  100% ms

(3)

where, mcs;950 C is the mass of the covered crucible containing the sample after heating at 950  C. For the determination of ash content, the uncovered crucibles containing the samples used for the determination of volatile matter were placed in the furnace and heated at 750  C for 6 h. After heating, the crucibles were then taken out of the furnace, allowed to cool for 15 min, transferred to a desiccator and allowed to cool for an additional 30 min prior to weighing. Ash content (%Ash) and fixed carbon (%FC) was calculated using Equation (4)

mcs;750 C  mc  100% ms

(4)

%FC ð%w=wÞ ¼ 100  %M  %VM  %Ash

(5)

where mcs;750 C is the weight of the crucible containing the sample after firing at 750  C. 2.2. Simultaneous carbonization and sulfonation of SCB About 3 g of powdered bagasse (644 ± 15 mm) with a moisture content of 8.70 ± 0.20% w/w was weighed and transferred into a Kjeldahl digestion flask, 50 mL concentrated H2SO4 was then added to act as the sulfonating agent. To ensure minimized oxidative conditions, the powdered bagasse was fully immersed in the sulfonating agent. Eight flasks containing the SCB-sulfuric acid mixture were loaded into the Kjehldahl digester for simultaneous carbonization and sulfonation. The digester was heated to a predetermined temperature (150, 200, or 250  C) and held at the set temperature for a duration of 4, 6, or 8 h. After simultaneous carbonization and sulfonation, the flasks containing the mixture were allowed to cool to room temperature. About 180 mL of distilled water was slowly added to each flask with continuous stirring. The solids in the diluted mixture were then recovered via centrifugation or filtration; the recovered solids are then referred to as the solid acid catalyst from sugarcane bagasse (SAC-SCB). For SAC-SCB synthesized at 150  C, solids were recovered via filtration using grade 4 filter paper (pore size ¼ 20e25 mm) and continuously washed with hot water (90  C) until the pH of the washings was above 5. In the case of SAC-SCB synthesized at 200 and 250  C, particles were too fine for filtration, thus requiring the aid of centrifuge for its recovery. Likewise, the collected solids were repeatedly washed with hot water to remove most of the free acids. The collected solids were partially dried overnight in a convection oven (UM500, Memmert, USA) at 80  C for ease of recovery. The partially dried solids were then packed in pre-weighed cellulose extraction thimbles (Whatman 25  80 mm Cat No: 2800258) for subsequent washing with distilled water in a Soxhlet extractor operated under continuous reflux (~100  C) for 12 h. Washing with the aid of a Soxhlet extractor was carried out in order to further remove residual acids, loosely bound active sites [25], and other water-soluble materials. After extraction, the thimbles containing the washed SAC-SCB were oven-dried at 80  C until constant weight. Catalyst yield (Y SAC=SCB ) was calculated using Equation (6)

total SAC­SCB Y SAC=SCB ðw=wÞ ¼ ¼ mSCB

P

mT;SAC  mT mSCB

 (6)

where, mSCB is the total mass of SCB subjected to simultaneous carbonization and sulfonation in a given condition, mT is the mass of the extraction thimble and mT;SAC is the mass of the extraction thimble containing the dried SAC-SCB after washing in a Soxhlet extractor. Collected catalysts were stored in glass vials for further characterization and use. The synthesized solid acid catalysts from sugarcane bagasse (SAC-SCB) were designated as SAC-SCB-tt-TTT, to indicate the sulfonation time (t) and temperature (T). As an example, SAC-SCB-08-150 is used to designate the catalyst synthesized for a duration of 8 h at 150  C. 2.3. Catalyst characterization The synthesized acid catalysts were characterized for its sulfur content and total acid density. Sulfur content was determined by

K.P. Flores et al. / Renewable Energy 130 (2019) 510e523

Eschka method as described in ASTM D3177-02 [26] with slight modification. The total acid density was determined by following the method used by Savaliya et al. [23]. 2.3.1. Sulfur content and sulfonic acid density About 1 g of SAC-SCB (weighed to the nearest 0.0001 g) was mixed with 3 g of Eschka mixture (composed of 2 parts by weight MgO and 1 part anhydrous Na2CO3) in a 30-mL porcelain crucible and then covered with 1 g Eschka mixture layer on top. The crucible containing the mixture was then heated in a muffle furnace. The temperature inside the furnace was raised to 800  C at a heating rate of 10  C/min and held at the set temperature for 4 h to completely oxidize the SAC-SCB. The crucible was then allowed to cool for an hour. The contents of the cooled crucible were transferred to a 250mL beaker containing 100 mL of hot water and the mixture was thoroughly stirred. The mixture in the beaker was then heated in a water bath (90  C) for 45 min and was stirred occasionally (5-min interval). The solids in the mixture were allowed to settle and then the liquid was decanted to a 600-mL beaker, passing through a grade 4 filter paper (pore size ¼ 20e25 mm). The remaining solids were washed four times with 50 mL hot water, with the washings decanted to the 600-mL beaker and passing through the filter paper in between washings. The collected filtrate was added with 30 mL of 1 M HCl. The solution was then heated to 90  C in a water bath and was slowly added with 15 mL of BaCl2 solution (100 g/L) to precipitate the dissolved sulfates as BaSO4. The solution was allowed to stand in the heating bath for 2.5 h. After heating, the settled solids were filtered through an ashless filter paper (Whatman 42) with the aid of a vacuum pump. The solids were washed with hot water until 10 mL of the collected filtrate was clear when added with 1 drop of 0.1 AgNO3 solution. The ashless filter paper containing the recovered solids was folded and placed in a pre-fired and pre-weighed 30-mL porcelain crucible. The crucible was placed in a furnace and the temperature inside the furnace was then raised from room temperature to 800  C (heating rate of 15  C/min) and held at this temperature for 30 min to burn off the ashless filter paper and dry the recovered solids. Sulfur content (%S), sulfonic acid density (rSO3H ), and extent of sulfonation (xs ) was calculated using Equation (7), Equation (8), and Equation (9), respectively.

%S ðw=wÞ ¼

rSO3H xs ¼



  MW S mc;BaSO4  mc  MW BaSO4

mmol g SAC

%S SAC %S SCB

 100%

(7)

%S 1000 1 mmol SO3 H   100 MW S 1 mmol S

(8)

mSAC  ¼

(9)

where, mSAC is the mass of SAC-SCB sample, mc is the mass of the pre-fired crucible, mc;BaSO4 is the mass of the crucible containing the BaSO4 precipitates, MW S and MW BaSO4 is the molar mass of sulfur and barium sulfate, respectively, and %S SAC and %S SCB is the sulfur content of the SAC-SCB and sugarcane bagasse, respectively. The sulfonic acid density (rSO3H ) was calculated based from the sulfur content (%S) with the assumption that all the sulfur present in the catalyst are in the form of sulfonic acid (eSO3H) moiety. 2.3.2. Total acid density About 0.5 g of SAC-SCB was weighed in a 125-mL Erlenmeyer flask and was added with 50 mL of 2 N NaCl solution. The mixture was shaken at 200 rpm for 24 h (R-2 New Brunswick Scientific, New Jersey, USA) and filtered using a grade 4 filter paper (pore

513

size ¼ 20e25 mm) to remove the solids. A 15-mL aliquot from the filtrate was added with 3 drops of phenolphthalein indicator and then titrated with standardized 0.01 M NaOH solution until endpoint. Total acid density of SAC-SCB (racid ) was calculated using Equation (10).

racid

  V NaOH C NaOH  VV as mmol ¼ mSAC g SAC

(10)

where, V s is the volume of the NaCl solution added to the SAC-SCB, V a is the volume of the aliquot titrated with NaOH solution, V NaOH is the volume of the NaOH solution titrated, C NaOH is the concentration of the titrant NaOH solution. 2.3.3. FTIR Bio-Rad Excalibur FTS 3500 Spectrometer was used to perform FT-IR analysis of selected catalyst samples of washed and unwashed catalyst 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 cm 1. 2.4. Esterification of oleic acid with SAC-SCB In a 250-mL screw-capped Erlenmeyer flask, 30 g of oleic acid was added with 85 mL methanol (methanol to oil molar ratio of 20:1). Oleic acid was selected as the model substrate considering that it is the primary fatty acid found in most vegetable oils [27] and WCO [28]. The flask was loaded in a pre-warmed incubator shaker (New Brunswick Scientific Co. Inc, Model-G25) set at 65  C and 200 rpm for incubation. After an incubation time of 30 min, 3 g of SAC-SCB (10% w/w with respect to the amount of oleic acid) was added to the mixture in the flask to start the esterification reaction. The reaction was kept at the set temperature for 6 h. After the set reaction time, the flask was removed from the shaker and cooled in an ice bath to stop the reaction. The cooled mixture was then filtered through a grade 4 filter paper (pore size ¼ 20e25 mm) and into a 250-mL separation funnel. The remaining oil and solid catalysts were recovered by washing the flask with n-hexane, with the washings filtered through the filter paper to the separation funnel between washings. The collected catalysts (spent SAC-SCB) were rinsed three times with 15 mL of n-hexane to further remove the methyl oleate and unreacted oleic acid from the catalyst surface. The spent SAC-SCB was dried and was analyzed for its sulfur content and total acid density. The mixture in the separation funnel was added with 20 mL of aqueous NaCl solution (5 %w/w) to induce phase separation. The mixture was gently swirled and allowed to settle, forming an organic phase (n-hexane layer) and an aqueous phase. The aqueous phase containing the methanol (lower layer) was discarded. Washing with 20 mL NaCl was done three times to ensure the removal of methanol and leached acids. The n-hexane layer was recovered and transferred into a pre-weighed evaporation flask and concentrated with the aid of a rotary evaporator (VV-micro, Heidolph, England). The recovered oil sample was weighed and was transferred in screw-capped glass vials for analysis of FFA content. Crude yield (%Y crude ), conversion (X FFA ), and FAME yield (Y FAME ) were calculated using Equation (11), Equation (12), and Equation (13), respectively while catalyst activity as catalytic activity (AC ) and sulfonic acid activity (ASO3 H ) were calculated using Equation (14) and Equation (15), respectively.

%Y crude ðw=wÞ ¼

mproduct  100 % mOA

(11)

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X FFA ðw=wÞ ¼

2.7. Data and statistical analysis

AV i  AV f AV i

Y FAME ðw=wÞ ¼ X FFA 

(12)

MW MO MW OA

(13)

X  mOA AC ðmmol=g$hrÞ ¼ MW FFA OA  m SAC  t e 1000 ASO3 H ðmmol=mmol SO3 H$hrÞ ¼

AC

rSO3H

(14)

(15)

where, mproduct and mOA are the masses of the concentrated product and initial oleic acid, respectively; AV i and AV f is the acid value of the oil before and after esterification, respectively; MW MO and MW OA is the molar mass of the product methyl oleate and oleic acid, respectively; and t e is the esterification time in hours. 2.5. Free fatty acid content and acid value determination FFA content of oil samples was determined via titration as described in AOAC Official Method 940.28 [29]. About 0.5e1 g (accurately weighed to the nearest 0.0001 g) of oil sample was weighed into a 125-mL flask and was added with 25 mL of neutralized ethanol (99%). Neutralization of the alcohol was done by adding 10 drops of phenolphthalein indicator solution to 250 mL of boiled alcohol and titrated with 0.1 M ethanolic NaOH. The oilethanol mixture was added with 2e3 drops of phenolphthalein indicator solution and was titrated with 0.075 M ethanolic NaOH until endpoint. Acid value of the oil sample (AV) was calculated using Equation (16) and free fatty acid content (%FFA) was determined using the calculated acid value (Equation (17)).

 AV

 mg KOH C  V NaOH  MW KOH ¼ NaOH g moil

w  AV MW OA ¼   100% %FFA w 1000 MW KOH

(16)

(17)

where, V NaOH is the volume of the NaOH solution titrated, C NaOH is the concentration of the titrant NaOH solution, moil is the mass of the oil sample, MW OA and MW KOH is the molar mass of oleic acid and KOH, respectively. 2.6. Reusability of SAC-SCB Catalyst reusability was tested using the SAC-SCB with a high catalytic activity, lowest reduction in sulfonic acid groups after esterification and at an esterification time where maximum conversion of oleic acid was achieved. To determine the minimum esterification time, esterification of oleic acid using the selected SAC-SCB was first carried out at 2, 4, 6, 8, 12, and 24 h. Reactions were carried out in duplicates. The reaction condition providing the highest conversion of FFA was adopted for the reusability test. To determine the reusability of the catalyst, the same esterification procedure as described in Section 2.4 was followed. For the first cycle, 6 sets of reaction mixtures were subjected to the esterification reaction. Spent catalysts were washed with n-hexane, dried, and pooled to be used in the next cycle and was analyzed for its sulfur content. For every cycle, the number of prepared reaction mixtures was reduced by one set in order to have enough spent catalysts for sulfur content analysis. SAC-SCB was used for up to 5 reaction cycles.

To aid in the determination of the influence of sulfonation time and temperature on the acid densities and the catalytic activity, two-way analysis of variance (ANOVA) with replication was employed. Microsoft Excel with data analysis package was used for all ANOVA analyses. 3. Results and discussion Sugarcane bagasse used in this study was determined of its proximate components and was found to have the following composition: 17.31 ± 1.02 FC, 79.90 ± 1.06 VM, and 2.79 ± 0.08 ash, in % w/w (dry basis). These results are within values reported in literature (14.95e18.1% FC, 73.78e78.6% VM, 3.3e11.27% ash) [30,31]. In the synthesis of carbon-based SAC, an important consideration is the carbon content of the catalyst support. However, not all carbon remains during the synthesis of the catalyst, considering that the synthesis of SAC-SCB involves the volatilization of volatile organics during partial carbonization. Among the proximate components, special interest is given to the fixed carbon content, as it allows the estimation of the minimum amount of catalyst that could be obtained after catalyst synthesis. Apart from the fixed carbon, of importance is the presence of sulfonic acid groups in carbon-based SAC, and is estimated based on the sulfur content. Sulfur content of SCB was found to be 0.13% w/w (dry basis). This amount is considerably low, requiring a sulfonation step to increase the amount of sulfur in the biomass support. 3.1. Catalyst yield and extent of sulfonation In this study, simultaneous carbonization and sulfonation (SCS) was adopted to dehydrate, devolatilize, carbonize, and incorporate sulfur sites in SCB. Nine types of SAC-SCB were prepared by varying the sulfonation temperature and time. In general, the catalyst yield decreased from ~51 to ~20% (g SAC-SCB/g SCB) when the SCS temperature was increased from 150 to 250  C (Fig. 1). It is interesting to note that the catalyst yield approaches the fixed carbon content (17.3 %w/w) of the raw SCB, implying that as the SCS temperature is increased, the synthesized catalysts become more carbonized, resulting to a more rigid structure. The resulting catalysts could have lower sulfonic acid (eSO3H) densities [14] and consequently lower activities, suggesting that higher sulfonation temperatures beyond 250  C may not be necessary in the synthesis of SAC-SCB. In view of sulfonation time, sulfonation of SCB beyond 4 h did not result in significant effects on the catalyst yield (p > 0.05). These results are comparable to the results reported in a study done by Savaliya et al. [23], where a yield of 25e35% was achieved during the synthesis of SAC-SCB through SCS at 180  C for 10 h. Discrepancies in the catalyst yields may be due to mass losses incurred during the recovery and washing of catalysts as these are carried out differently by different researchers. Unfortunately, most studies involving the synthesis of SAC-SCB, whether direct or indirect sulfonation, do not account for catalyst yield. However, it may be inferred from the results of this study that SCS may be favorable when compared to conventional two-step approaches requiring the separate carbonization and sulfonation steps at high temperatures beyond 250  C and total synthesis time of up to 30 h. Although catalyst yield is an important parameter in view of process performance of the synthesis step, the effectiveness of the sulfonation process would best be evaluated through the successful incorporation of the sulfur sites in SAC-SCB, as the catalytic activity is attributed to sulfonic acid (eSO3H) functional groups [14,21]. In order to assess the extent of sulfonation (x), the prepared catalysts

K.P. Flores et al. / Renewable Energy 130 (2019) 510e523

515

Fig. 1. Yield of SAC-SCB during simultaneous carbonization-sulfonation at various temperature and time.

Fig. 2. Sulfur content ratio (SAC-SCB:SCB) at varying SCS conditions.

were analyzed for their sulfur content, and evaluated by determining the sulfur content ratio of SAC-SCB to raw SCB (Fig. 2). Compared to the initial sulfur content (0.13 %w/w) of SCB, a remarkable increase as high as ~18 times was achieved through SCS. This suggests the successful incorporation of sulfonic acid functional groups into the carbonized SCB. In addition to the introduction of the main functional group, sulfonation also allowed for the formation of weak acid groups such as carboxylic (eCOOH) and hydroxyl (eOH) [8,14,21].

3.2. Characterization of SAC-SCB Apart from process performance during catalyst synthesis, also of importance is the actual activity of the synthesized catalyst, whereby this activity is oftentimes attributed to its acid density (Fig. 3). In view of the effects of sulfonation temperature and time on the total acid density, no significant difference was observed (p > 0.05). However, separately evaluating the strong acid sites (sulfonic acid density) and weak acid sites (weak acid density), it

Fig. 3. Sulfonic acid (SO3H), weak acid, and total acid density of SAC-SCB synthesized at different SCS conditions. SAC-SCB-(Time, hr)-(Temp,  C).

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could be observed that at lower temperatures of 150  C, the acid sites are predominantly of the sulfonic acid group (71e77%), which decreases to as low as 47% when the temperature is increased to 250  C. High sulfonic acid densities (0.57e0.67 mmol/g SAC-SCB) are obtained when synthesis was carried out at 150  C, which generally decreases as temperature is increased. In a related study, Lou et al. [14] observed that during sulfonation, the introduction of eSO3H brought about a corresponding decrease in CeH bonds. This suggests that sulfonation is achieved through covalent attachment of eSO3H in the carbon framework by substitution of hydrogen in the CeH bonds [14]. Increasing the sulfonation temperature leads to the graphitization of the biomass and cross-linking of carbon sheets, as the CeH bonds are eliminated prior to attachment of the sulfonic acid groups, resulting in a more rigid carbon framework, which in effect results to a low sulfonic acid density [14,21]. At this point, it can be inferred that in the SCS of SCB, the temperature should not go beyond 150  C. This coincides with the study of Lou et al. [14], whereby 150  C was determined to be the optimum sulfonation temperature in a two-step preparation of SAC-SCB, which had a sulfonic acid density of 1.06 mmol/g. Varying the sulfonation time did not significantly influence the sulfonic acid density of the synthesized catalysts. Although an increase in sulfonic acid density is observed during synthesis at 200  C from 4 to 6 h, statistical analysis indicated that there is no significant difference (p > 0.05) between the synthesized catalysts. However, the increase and decrease in the different acid sites is influenced by various factors during sulfonation which includes devolatilization, thermal decomposition, and changes in morphology. In various studies, sulfonation of carbon-based materials resulted to sulfonic acid densities ranging from 0.48 to 1.83 mmol/g, depending on the material and the sulfonation conditions [8,14,25]. In view of SAC-SCB prepared via conventional process, SAC-SCB having sulfonic acid densities ranging from 1.06 to 1.54 mmol/g were found to have good catalytic performance (>90% conversion) in the esterification of FFA [14,21]. Meanwhile, Savaliya et al. [23] reported a sulfonic and total acid density of 0.90 and 1.90 mmol/g, respectively, for the SAC-SCB synthesized via SCS at 180  C for 12 h. The difference in the determined acid density is largely dependent on the washing step after the synthesis process, as sulfur-containing moieties such as polycyclic aromatic hydrocarbons (PAH) may have been removed during the process of washing [25]. However, there is no consensus as to how the washing step is carried out. Typically, synthesized catalyst is filtered and continuously washed with hot water (~80  C) until the pH of the washings reach neutral [19,21], or in some cases, the washings are tested for presence of sulfate ions through the addition of barium chloride [8,14,23]. Despite these attempts to eliminate residual acids, these approaches face challenges in view of achieving a good consistency and repeatability owing to the mode of contact and exposure during washing. In light of this, filtered catalysts recovered after washing with hot water (~95  C) were compared with catalysts exhaustively washed using a Soxhlet extractor operated under reflux (~100  C) for 12 h (Fig. 4). As could be observed, even after washing with hot water until neutral pH, a decrease in sulfonic acid density was still observed after exhaustive washing using Soxhlet extractor. This will potentially lead to a decrease in the actual performance and activity of the catalyst. Moreover, from these results, it could be inferred that an effective washing process should be developed so not to jeopardize the characterization and later assessment of the catalyst performance and activity.

Fig. 4. Sulfonic aicd density of SAC-SCB* (washed until neutral pH) and SAC-SCB (exhaustively washed using Soxhlet extractor for 12 h) synthesized at 150  C over various sulfonation times (4,6,8 h).

3.3. Catalytic activity of SAC-SCB Catalytic activity of a SAC is oftentimes attributed to the presence of the sulfonic acid group. However, the actual activity is best determined through its performance when used for its intended application. In view of biodiesel production and in the reduction of FFA in low quality oils, esterification of oleic acid was carried out with the synthesized catalysts to determine its catalytic performance (FFA conversion) and activity (sulfonic acid activity). The sulfonic acid activity of SAC-SCB was calculated to determine the FFA conversion brought about by its sulfonic acid functional groups. This provides a more objective comparison of the synthesized catalysts, considering that the catalysts synthesized were of varying sulfonic acid densities. As presented in Fig. 5, FFA conversion reached as high as ~90%, translating to a catalytic activity of 6.6e7.5 mmol FFA/mmol SO3H$h with the catalyst (SAC-SCB*) prepared at 150  C without being exhaustively washed in a Soxhlet extractor. The conversion and activity significantly decreased to as low as 44e53% and ~4.5 mmol FFA/mmol SO3H$h, respectively, when the synthesized catalysts were subjected to exhaustive washing prior to use in the esterification of oleic acid at 45 ± 5  C for 6 h. The decrease can be attributed to the relatively lower sulfonic acid density, which resulted from the removal of sulfur-containing moieties during extensive washing (Figs. 5 and 6). To further support the presence of sulfonic groups and their removal during washing, FTIR spectra of the catalysts washed with hot water and extensively washed with an aid of a Soxhlet apparatus was obtained and presented in Fig. 7. As could be observed, bands at 1030, 1056 and 1111 cm1 indicates the presence of sulfur groups in the form of sulfoxide or sulfone (S]O) [11,14,23], while bands at 1350 and 600 cm1 indicates that these sulfur groups are that of the sufonic acid group (eSO3H) [12]. Catalysts obtained after exhaustive washing resulted in the decrease in the intensities of these bands suggesting that some of these groups have been removed. Furthermore, observed bands for aromatic C]C (1600e1700 cm1) and CeH (680e850 cm1) are also observed to have decreased suggesting that the sulfonic groups that were removed are those attached to aromatic rings, which are expected to form during carbonization and serves as the support for the active moiety. The decrease in the band intensities suggests that these groups or a portion of these might have been solubilized and washed out during the washing process. Another mechanism by which the sulfonic acid groups might have been removed could be through hydrolysis, where aromatic C]C bending at 1600 cm1 is observed after exhaustive washing. Some of the carboxylic moiety's might have also been solubilized considering that band

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517

Fig. 5. Catalytic performance (a) and activity (b) of SAC-SCB* (washed until neutral pH) and SAC-SCB (exhaustively washed using Soxhlet extractor for 12 h) synthesized at 150  C at various sulfonation times (4,6,8 h).

Fig. 6. Sulfonic acid density of fresh and spent catalyst of SAC-SCB* (washed until neutral pH), (a) and SAC-SCB (exhaustively washed using Soxhlet extractor for 12 h, (b) synthesized at 150  C over various sulfonation times (4,6,8 h).

Fig. 7. FTIR spectra of SAC-SCB synthesized at 150  C and 8 h.

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K.P. Flores et al. / Renewable Energy 130 (2019) 510e523

intensity of carboxyl groups (C]O, 1795 and OeC, 1200 to 1350 cm1) also decreased. In a related study by Mo et al. [25], sulfonated D-glucose was washed with methanol for 24 h at 60  C prior to use in esterification were found to have lower activities than the unwashed catalysts by 12e20%. Moreover, the authors confirmed the leaching of catalytic species by using the recovered methanol from the washing in the esterification of acetic acid. It was found that the methanol from the washings had active agents in catalyzing the reaction. Deactivation of the catalyst is primarily caused by leaching out of polar sulfonic groups to gain better stability through hydrogen bonding with polar solvents such as alcohol or water [25]. Leaching of the active sites poses a challenge in the effective use of SAC in biodiesel production as this would lead to poor catalytic performance and stability, in view of catalyst reuse, and contamination of the products, which defeats the primary purpose of employing SAC. In relation to this, the sulfonic acid densities of SAC-SCB* and SAC-SCB after esterification of oleic acid with methanol were also looked into (Fig. 6). Following the use of the synthesized catalysts in the esterification of oleic acid with methanol, a reduction in sulfonic acid density was observed in both SAC-SCB* and SAC-SCB. However, the reduction in sulfonic acid density of the catalysts which were extensively washed (SAC-SCB) was not as pronounced when compared to those that were only washed until a neutral pH. This suggests that exhaustive washing with an aid of a Sohxlet extractor is an effective means of removing weakly attached sulfurcontaining groups. Moreover, it could be observed that from Fig. 6b, sulfonation at prolonged time of 8 h resulted in lesser amounts of sulfonic acids removed, with 93% of the original sulfonic groups retained after the reaction. This also suggests better stability of the synthesized catalysts. Comparing SAC-SCB* and SAC-SCB sulfonated for 6 and 8 h, the spent catalysts resulted in similar sulfonic acid densities (0.48e0.54 mmol/g), further suggesting the need to ensure the removal of loosely bound acid groups prior to catalyst evaluation and use. As such, washing by Soxhlet extraction was employed in the preparation of the catalysts synthesized at higher temperatures. The performance and activity of SAC-SCB prepared at varying SCS conditions are presented in Fig. 8. As could be observed, FFA conversion is positively correlated (r ¼ 0.75) with the catalysts' sulfonic acid densities. The highest conversion of 54% was achieved at an SCS temperature of 150  C for 4 h, where sulfonic acid density was also found to be the highest. At an SCS time of 4 h, increasing the temperature from 150 to 200  C resulted in the decrease of FFA

conversion from ~54% to ~44%. Further increasing the sulfonation temperature to 250  C caused a significant drop in FFA conversion to ~7%. A similar behavior was exhibited by the catalysts synthesized for a duration of 8 h. In contrast, these observations could not be observed from the catalytic performance of the catalysts prepared at an SCS time of 6 h. Nevertheless, the conversions brought about by these catalysts (synthesized for 6 h) are still in agreement with their corresponding sulfonic acid densities. Similar to what has been observed on the sulfonic acid densities of SAC-SCB, the effect of SCS time on FFA conversion is not as pronounced as those brought about by SCS temperature. At 150  C, increasing the SCS time from 4 to 6 h resulted to a decrease in FFA conversion from 54 to 44%. Prolonging the SCS further to 8 h did not have a significant effect on the FFA conversion (p > 0.05). At an SCS temperature of 200  C, increasing the SCS time from 4 to 6 h resulted in a catalyst rendering higher conversion (~50%) of oleic acid during esterification. However, a prolonged SCS time of 8 h resulted in a catalyst of poor catalytic performance (~38% conversion). On the other hand, at 250  C, varying the SCS time did not have a considerable effect on the FFA conversion (p > 0.05). In view of catalytic activity (Fig. 8b), an abrupt decrease in activity was observed when the temperature during SCS was increased to 250  C. As for sulfonation time, it was found to generally not have an influence in the catalytic activity of the synthesized catalysts. However, at 200  C, peculiar differences could be observed where an increase in SCS time from 4 to 6 h resulted in a decrease in catalytic activity from 5.78 to 4.62 mmol FFA/mmol SO3H$h, which then again increased to 5.38 mmol FFA/ mmol SO3H$h when the SCS time was increased further to 8 h. Catalytic activity provides an idea on the ability of the available active sites in catalyzing the esterification reaction. However, this is greatly influenced by various factors which includes catalyst morphology (surface area, pore size, and particle size) and distribution of active sites. Moreover, active sites were only estimated based on the total sulfur content, which may not be in its active form as attached on the catalyst. For instance, although the catalysts synthesized at 250  C have sulfonic acid densities comparable to SAC-SCB-4-200 and SAC-SCB-8-200 (Fig. 3), their performances are considerably lower. This could indicate differences in the catalyst structure and sulfur moiety between the two groups of catalysts. The higher SCS temperature may have produced catalysts which are more rigid in structure and less flexible due to the stacking of carbon sheets, which presumably may have prevented large reactant molecules from coming into contact with the eSO3H

Fig. 8. Effect of SCS conditions on (a) performance and (b) sulfonic acid activity of SAC-SCB.

K.P. Flores et al. / Renewable Energy 130 (2019) 510e523

groups [14]. Moreover, these catalysts have significantly lower performances as compared to those synthesized at 150  C, despite having about twice the weak acid density. Having a high weak acid density (eCOOH) could enhance the catalytic performance of the catalysts as it tends to attract methanol or by enhancing the oleic acid-methanol interaction. However, this could also imply the presence of high amounts of eOH groups, which could potentially attract water molecules formed during the reaction, preventing hydrophobic reactants such as that of oleic acid from accessing the main functional groups [14]. Nevertheless, the above results further support the premise that the catalytic activity of SAC-SCB is attributed to its sulfonic acid density [8,14,21]. Both catalytic activity and conversion are important response parameters to consider in the selection of catalyst. However, stability of the catalyst should also be taken into consideration. In view of catalyst stability, the spent catalysts were reanalyzed of their sulfur content after its use in the esterification of oleic acid (Fig. 9). Analysis of sulfur content reveals a decrease in sulfonic acid density of all the SAC-SCB's following their use in catalyzing the esterification reaction of oleic acid with methanol. Among the catalysts, SAC-SCB-04-150 and SAC-SCB-06-200 had the highest reduction in sulfonic acid density (41.3 and 37.9%, respectively). In the case of SAC-SCB-06-200, its sulfonic acid density after the reaction is close to those of the other catalysts synthesized at 200  C, despite having a relatively greater initial sulfonic acid density. Therefore, if these catalysts were reused, they can be expected to have comparable activities. In relation to this, if the catalysts synthesized at 150  C were to be reused, SAC-SCB-04-150 would have the lowest activity in the group. Thus in view of a good balance in conversion, stability, and activity, SAC-SCB-08-150 is the most promising among the catalysts synthesized. 3.4. Reusability of SAC-SCB As for the reusability study, a suitable candidate among the synthesized catalysts was chosen for use in 5 successive esterification cycles. In terms of catalytic activity, the catalysts synthesized at 150 and 200  C showed potential (Fig. 8b). However, in view of stability, SAC-SCB-6-150 and SAC-SCB-8-150 were found to be superior, as the rest of the catalysts considered were found to have relatively greater reduction in sulfonic acid densities after one cycle of use (Fig. 9).

519

Furthermore, SAC-SCB-6-150 and SAC-SCB-8-150 have the advantage with regard to catalyst preparation, owing to their relatively lower synthesis energy requirement. Statistical tests would dictate that there is no significant difference between SAC-SCB-6-150 and SACSCB-8-150 in terms of activity (p > 0.05) or stability with reference to reduction of sulfonic acid density (p > 0.05). Nevertheless, SACSCB-8-150 was selected for the reusability study owing to its slight advantage over SAC-SCB-6-150 in both aspects. Prior to the reusability test, SAC-SCB-08-150 was used in the esterification of oleic acid with methanol over a period of 24 h to determine the highest possible conversion achievable at the selected esterifiction conditions (Fig. 10a). As could be observed in Fig. 10a, the conversion increased with increasing esterification time, having a conversion of 85% after 24 h. Although much higher conversions could have been achieved, prolonged reaction times beyond 24 h may not be practical for industrial applications. Nevertheless, these results provide an idea on the stability of the catalyst over a 24-h reaction period. In Fig. 10b, the sulfur density only slightly decreased from 0.59 to 0.56 mmol SO3H/g SAC but eventually reached a stable sulfur density of ~0.53 mmol SO3H/g SAC. These results further suggest the importance of washing to ensure no loosely bound sulfur-containing moieties are present in the final catalyst to be used. For stability, in view of catalyst reusability, each esterification cycle was conducted for 24 h (Fig. 11a). Initially, conversion decreased by ~17% after the first cycle. However, the reduction in performance diminished in each successive cycle. After the third cycle, the catalytic performance of SAC-SCB-08-150 remained essentially stable at ~67% (p > 0.05), resulting to a 76.5% retention of the original performance. Meanwhile, Savaliya et al. reported a retention of ~87% of the original performance of SAC-SCB synthesized through SCS, after 4 cycles of use [23]. The relatively higher stability of the catalyst reported in literature may be due the employed reactivation step, whereby the recovered catalysts were washed with methylene dichloride and dried at 80  C for 10 h prior to use in the subsequent cycle [23]. Deactivation of the catalyst may be caused by several factors which includes catalyst poisoning, whereby sulfonate esters are formed, rendering the available sulfur sites inactive. This potentially occurs by esterification of the catalyst's sulfonic acid groups in an alcohol rich environment, such as that of esterification oleic acid with methanol [32]. Another possibility would be the clogging of pores by the reactants or products,

Fig. 9. Sulfonic acid density of unused and spent SAC-SCB.

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Fig. 10. (a) Performance and (b) sulfonic acid density of SAC-SCB-08-150 over a 24-h esterification period.

Fig. 11. (a) Performance and (b) sulfonic acid density of SAC-SCB-08-150 in successive 24-h esterification cycles.

resulting in hindered access to the active sites. Although washing of the catalyst prior to its reuse was carried out, there was no means of determining the adequacy of the washing. A similar behavior to the catalytic performance was observed in the sulfonic acid density of SAC-SCB-8-150 (Fig. 10b). The sulfonic acid density, initially at 0.59 mmol/g, remained unchanged (p > 0.05) at ~0.51 mmol/g after the third cycle. Ultimately, 86% of the initial sulfonic acid density of SAC-SCB-8-150 was retained after 5 cycles of use. This is in agreement with the results of the study by Mo et al. [25], whereby ~80% of the initial active sites of a sulfonated D-glucose pre-washed with methanol was retained after 8 cycles of use. Apart from washing with water after catalyst synthesis, washing with methanol may also be necessary to ensure complete removal of loosely attached active sites. 3.5. Comparison of SAC-SCB with other carbon-based solid acid catalysts Summarized in Table 2 are some of the biomass-derived solid acid catalysts reported in literature. Among those listed, the highest

specific catalytic activity (AC) and sulfonic acid activity (ASO3H) was exhibited by the SAC derived from waste Jatropha curcas seed shells. This was followed by the catalyst synthesized from spent coffee grounds (SCG) and the bamboo-derived catalyst, which had relatively high activities owing to their use in short reaction periods of 2e4 h. The relatively greater activity exhibited by the SAC derived from waste Jatropha curcas seed shells was also due to short reaction time of 2 h, in addition to the minimal amount of the catalyst used (0.005 w/w), and its relatively high sulfonic acid density of 2.0 mmol/g. However, as what has been observed in this study, a high sulfonic acid density may not always result to a high sulfonic acid activity. On one hand, The SAC synthesized from corn straw had a relatively lower sulfonic acid activity of 5.08 mmol/mmol$h despite having the highest sulfonic acid density of 2.44 mmol/g. On the other hand, SAC from SCG had the second highest sulfonic acid activity of 55.09 mmol/mmol$h despite having the lowest sulfonic acid density of 0.45 mmol/g. These results further support the idea that the catalytic activity of a SAC is also greatly influenced by other factors such as catalyst morphology (surface area, pore size, and particle size) and availability of active sites. Unfortunately, this

K.P. Flores et al. / Renewable Energy 130 (2019) 510e523

521

Table 2 Solid acid catalysts derived from agricultural waste residues. Catalyst Source

SCc T ( C)/t (h)

ECd T ( C)/t (h)

Substrate

Sugarcane bagasse rSO3Ha: 1.06 racidb: 3.69

Ck: 375/0.5 Sl: 150/15 C: 350/5 S: 100/8

80/6 80/2

C: 300/1 S: 80/4

60/4

Oleic acid w/we: 0.05 Oleic acid w/w: 0.005 Oleic Acid w/w: 0.07

Bamboo rSO3H: 1.82 racid: 1.74

C: 350/2 S: 105/2

90/2

Oil Palm Trunk rSO3H: 1.41 racid: 5.31

C: 400/4 S: 150/15 65/5

Spent coffee grounds rSO3H: 0.45 racid: 0.99

C: 600/4 S: 200/18 60/4

Sugarcane bagasse rSO3H: 0.59 racid: 0.79

S: 150/8 (soxhlet)m S: 150/8

Waste Jatropha curcas seed shells rSO3H: 2.0 racid: n.s. Corn Straw rSO3H: 2.44 racid: 2.64

Sugarcane bagasse rSO3H: 0.70 racid: 1.12 a b c d e f g h i j k l m

45 ± 5/24 45 ± 5/6

Alcohol

Methanol n/nf: 10 Methanol n/n: 1 Methanol n/n: 7 Oleic acid w/w: 0.06 Ethanol n/n: 7 Palmitic Acid w/w: 0.09 Caprylic Acid w/w: 0.05 Oleic acid w/w: 0.10

Methanol 18 Methanol 3 Methanol 20 Oleic acid w/w: 0.10 Methanol 20

Cycles/ Washing

XFFA (%)g

8 n-hexane

95 (85.5)h 11.21

10.57

4 methanol

95.7 (95.1) 98

12.39

5.08

[10]

29.03

15.95

[11]

7.70

5.46

[8]

24.79

55.09

[12]

1.26

2.14

This study

5.26

7.49

e 5 ethanol

n/n: 6 n-hexane n/n: 5 methanol n/n: 5 n-hexane n/n: e

98.4 (27.8) 88.8 (~79.5) 71.5 (>20) 85.1 (66.6) 89.1

ACi

ASO3Hj Ref [14]

338.76 169.38 [13]

SO3H density as mmol eSO3H per g catalyst. Total acid density as mmol acid/g catalyst. Catalyst synthesis conditions (temperature and time). Esterification temperature and time. Catalyst loading as g catalyst per g oil/substrate. Alcohol to oil ratio as mol alcohol per mol oil. Percent FFA conversion. FFA conversion after several cycles of use. Catalytic activity as mmol FAME per g catalyst per hour. Sulfonic acid activity as mmol FAME per mmol SO3H per hour. Carbonization temperature and time. Sulfonation temperature and time. Catalyst is exhaustively washed using Soxhlet apparatus.

could not be looked into, as the relevant data are not reported in most of the studies. For this reason, and in addition to the varying reaction parameters, an objective comparison between the catalysts with reference to performance and activity is difficult. In view of catalyst reusability, the SAC derived from Jatropha curcas seed shells exhibited the highest stability, being able to retain almost all of its initial performance after 4 cycles of use. This was followed by the SCB and oil palm trunk-derived catalysts, which retained ~90% of their original performance after 8 and 6 cycles of use, respectively. Meanwhile, SAC-SCB-08-150 offered better stability (78% retention) as compared to both SCG and bamboo-derived catalysts, which retained only ~28% of their initial performance after 5 cycles. Although SAC-SCB-08-150 exhibited the lowest catalytic activity, its main advantage is on the required synthesis conditions. Other biomass-derived catalysts were prepared via the conventional two-step approach of carbonization and subsequent sulfonation, with total synthesis times ranging from 4 to 22 h and energy-intensive conditions requiring a synthesis temperature of up to 600  C. Meanwhile, SAC-SCB-08-150 can be prepared under a milder and less energy-intensive process at 150  C for 8 h. Biomass-derived SACs prepared through SCS are presented in Table 3. The peanut shell-derived catalyst showed the highest specific activity and sulfonic acid activity owing to its high sulfonic acid density of 2.13 mmol/g, minimal amount of catalyst used (0.04 w/w), and a short reaction time of 3 h. However, the high activity could also be due to the high esterification temperature of 85  C. Meanwhile, SAC-SCB-08-150 was used in a 6-h esterification reaction with the lowest temperature of 45 ± 5  C. Despite this, SACSCB-08-150 (washed until neutral pH) exhibited higher sulfonic acid activity (7.49 mmol/g$h) as compared to that of the SAC-SCB reported in the study of Savaliya et al. [23] (6.92 mmol/g$h), which was observed at a relatively higher reaction time and temperature of 11 h and 65  C, respectively.

With regards to catalyst synthesis, the peanut shell-derived catalyst had the advantage of requiring the lowest temperature of 85  C, while the catalyst derived from X. sorbifolia bunge hulls required the least amount of preparation time. Meanwhile, SACSCB-08-150 have the advantage over the SCB-derived and the catalyst synthesized from red liquor solids with reference to both synthesis time and temperature. In view of catalyst reusability, the peanut shell-derived catalyst offered the highest stability, being able to retain 97% of its original performance even at the 6th cycle of reusing the catalyst. Although SAC-SCB-08-150 exhibited lower performance, it had a relatively higher retention (78%) as compared to the catalysts derived from red liquor solids and X. sorbifolia, bunge hulls, which retained 2.35 and ~70% retention of initial performance, respectively. Furthermore, it should be noted that the stability test of SAC-SCB-08-150 was conducted at 24-h esterification cycles, which is higher than all of the listed catalysts. Overall, based on the results from this study, the catalyst prepared from low-value biomass, sugarcane bagasse, possessed good potential for biodiesel production, mainly owing to its good stability and relatively low synthesis requirements. Despite showing relatively lower catalytic performance as compared to most of the synthesized catalysts such as those form Jatropha curcas shells, the local availability of sugarcane bagasse provides a practical advantage. Thus, solid acid catalyst derived from sugarcane bagasse would be more practical for biodiesel production in the local context.

4. Conclusions In this study, solid acid catalyst derived from sugarcane bagasse (SAC-SCB) was prepared through simultaneous carbonizationsulfonation (SCS). In general, increasing the SCS temperature from 150 to 250  C resulted in a decrease in the catalytic performance. In

522

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Table 3 Biomass-derived solid acid catalysts synthesized through simultaneous carbonization-sulfonation. Catalyst Source

SCSc T ( C)/t (h)

Sugarcane bagasse rSO3Ha: 0.90 racidb: 180/10 1.9 Red Liquor Solids rSO3H: 1.15 racid: 1.52 200/12

ECd T ( C)/t Substrate and Catalyst (h) Loading 65/11 65/4

Bunge hulls (X. sorbifolia) rSO3H: 0.86 racid: 1.71 Peanut Shell rSO3H: 2.13 racid: 3.53

150/1

70/5

85/3

85/3

Distillers' Grain rSO3H: 0.89 racid: 1.89

20/1.4

117/n.s.

Wheat Husk rSO3H: 0.30 racid: 0.76

90/1.5

65/3.5

Coconut meal rSO3H: 0.28 racid: 3.38

100/1

65e70/8

Sugarcane bagasse rSO3H: 0.59 racid: 0.79 Sugarcane bagasse rSO3H: 0.70 racid: 1.12

150/8 (soxhlet)k 150/8

45 ± 5/24

a b c d e f g h i j k

45 ± 5/6

Soapstock oil w/we: 0.05

Alcohol

Methanol n/nf: 15 Oleic acid w/w: 0.05 Methanol n/n: 55 Soybean soapstock w/w: Methanol n/n: 0.07 9 Oleic acid w/w: 0.04 Methanol n/n: 10 Acetic acid w/w: 0.02 n-Butanol n/n: 1.5 Rice Bran Oil Fatty Acid w/w: Methanol n/n: 0.08 20 Waste palm oil w/w: 0.05 Methanol n/n: 12 Oleic acid w/w: 0.10 Methanol n/n: 20 Oleic acid w/w: 0.10 Methanol n/n: 20

Cycles/Washing

XFFA (%)g

ACi

ASO3Hj Ref

4/Methylene dichloride 3/Conc. H2SO4

97.2 (84.3)h 85 (<2)

6.26

6.92

4/n-Butanol

97.2 (~67.8) 98 (>95)

6/5/Ethanol e 4/n-Hexane 5/n-Hexane No Washing

97.5 (50.7) 81.3 () 92.7 (>80) 85.1 (66.6) 89.1

[23]

15.04 13.11 [32] 9.83

11.26 [33]

28.9

13.63 [34]

e

e

[24]

e

e

[35]

1.55

3.48

[36]

1.26

2.14

This study

5.26

7.49

eSO3H density as mmol eSO3H per g catalyst. Total acid density as mmol acid/g catalyst. Simultaneous carbonization and sulfonation temperature and time. Esterification temperature and time. Catalyst loading as g catalyst per g oil/substrate. Alcohol to oil ratio as mol alcohol per mol oil. Percent FFA conversion. FFA conversion after several cycles of use. Catalytic activity as mmol FAME per g catalyst per hour. Sulfonic acid activity as mmol FAME per mmol SO3H per hour. Catalyst is exhaustively washed using Soxhlet apparatus.

contrast, SCS time was found to have no significant effect on the catalytic performance. The determination of catalyst performance should include conversion, catalytic activity, and reusability. However, a careful design of the assessment of these parameters should be looked into as these are affected by degree of washing and should be taken into consideration to avoid misleading results. The performance of the catalyst in terms of oleic acid conversion was observed to be greatly dependent on the available sulfur sites. Ultimately, the catalyst synthesized at 150  C for 8 h provides a good balance between activity and stability, owing to its high sulfonic acid density of 0.59 mmol/g and catalytic activity (sulfonic acid activity) of 4.62 mmol/mmol$h, as well as good retention of sulfur sites. The catalyst exhibited good stability, being able to retain 76.5% of its original performance (87% FFA conversion) and 86% of its initial sulfonic acid density (0.59 mmol/g) after being used for 5 24-h esterification cycles. Declaration of conflict of interest None. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin, Synthesis of biodiesel via acid catalysis, Ind. Eng. Chem. Res. 44 (2005) 5353e5363, https://doi.org/10.1021/ie049157g. [2] A.A. Kiss, A.C. Dimian, G. Rothenberg, Solid acid catalysts for biodiesel production e-Towards sustainable energy, Adv. Synth. Catal. 348 (2006) 75e81, https://doi.org/10.1002/adsc.200505160. [3] M.G. Kulkarni, R. Gopinath, L.C. Meher, A.K. Dalai, Solid acid catalyzed biodiesel production by simultaneous esterification and transesterification,

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