Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production

Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production

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ARTICLE IN PRESS

JID: JTICE

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Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

Contents lists available at ScienceDirect

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Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production Nisakorn Saengprachum a,b, Somchai Pengprecha c,∗ a b c

International Postgraduate Programs in Environmental Management, Graduate School, Chulalongkorn University, Bangkok, Thailand Center of Excellence on Hazardous Substance Management (HSM), Chulalongkorn University, Bangkok, Thailand Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok, Thailand

a r t i c l e

i n f o

Article history: Received 28 January 2015 Revised 2 June 2015 Accepted 28 June 2015 Available online xxx Keywords: Biodiesel purification Monoglyceride Rice husk ash Silica-alumina

a b s t r a c t In this study, aluminum oxide was coated on extracted silica from rice husks (AO_RHA). First, the silica was produced by heating the rice husk at 700 °C for 6 h following precipitation using NaOH and H2 SO4 (RHA). The coating process was carried out using AlCl3 solution. The effect of stirring times, pH, and calcination conditions were investigated. The characterization of AO_RHA was performed using BET, XRD, FTIR, and ICP techniques. This study demonstrated the influence of the AO_RHA structure on monoglyceride removal. Langmuir and Freundlich adsorption isotherms were used to model the equilibrium isotherms and determine the isotherm constants. The adsorption capacity increased after the excess methanol was removed. In addition, the spent AO_RHA was regenerated and can be efficiently reused up to four times. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Currently, the world energy consumption and environmental concern regarding pollution has been increasing. The appropriate approach to serve this demand is to use biofuels especially biodiesel [1,2]. Because biodiesel is synthesized from natural resource, such as vegetables oils, animal fat, or waste flying oils, the burning of biodiesel produces less greenhouse gases (i.e., sulfur and aromatic polycyclic compounds) [3]. Currently, many countries (i.e., USA, Germany, Italy, and Austria) are already using biodiesel to run diesel engines without modification or as blends with fossil fuel [4–6]. Biodiesel is typically produced via transesterification. In this reaction, triglycerides react with alcohols in the presence of a homogeneous or heterogeneous catalyst to produce a methyl ester and glycerol, as shown in Fig. 1. Homogeneous catalysts, such as sodium or potassium hydroxide, are the preferred method of transesterification due to the resulting high yields [7,8]. However, the reaction of alcohol and sodium or potassium hydroxide generates water. Next, unwanted saponification typically occurs because the water can react with triglycerides and sodium ion or potassium ion to form soap [7,9].

Abbreviation: AO_RHA, aluminum oxide-coated silica. Corresponding author: Tel.: +6622187636; fax: +6622187668. E-mail address: [email protected], [email protected] (S. Pengprecha). ∗

At the end of reaction, the product mixture of biodiesel consists of fatty acid ester, glycerol, excess alcohol, unreacted catalyst, and mono- and diglycerides. These impurities can reduce the performance of the biodiesel and cause many problems in application, as shown in Table 1 [10]. The performance of biodiesel is based on the contaminants and cleanliness of the final product. To pass the American Society for Testing and Materials (ASTM) and the European Standard for biodiesel fuel (EN 14214), two accepted purification methods (i.e., wet washing and dry washing) have been investigated. Wet washing is the traditional purification method but it requires multiple techniques, such as settling, agitating, and filtering. This process generates huge amounts of wastewater that must be treated. Therefore, dry washing using an adsorbent has replaced wet washing for the purification of biodiesel. The dry washing technique typically removes impurities via an adsorption process or membrane technologies. The use of adsorbents and membrane has resulted in a faster purification process without the production of wastewater. The contents of salts and metals such as potassium, sodium, calcium, magnesium and free glycerol recorded were far better than those obtained when water washing was employed [11]. Most of the effective adsorbents are based on silica or silicates [12]. However, trace amounts of unreacted glycerides, such as mono- and diglycerides, remain after the purification process. Magnesol is an adsorbent that exhibits high efficiency for decreasing the amount of glycerides [13]. However, the cost of this adsorbent is higher than other types of adsorbents.

http://dx.doi.org/10.1016/j.jtice.2015.06.037 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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Fig. 1. Transesterification reaction.

Table 1 Effects of impurities on biodiesel and engines. Impurity

Effect

Free fatty acids (FFA)

Corrosion Low oxidation stability Hydrolysis (FFA formation) Corrosion Bacteriological growth (filter blockage) Low densities and viscosities Low flash point (transport, storage and use problems) Corrosion of Al and Zn pieces High viscosity Deposits in the injectors (carbon residue) Crystallization Deposits in the injectors (carbon residue) Filter blockage (sulfated ashes) Engine weakening Settling problems Increase aldehydes and acrolein emissions

removal, the effect of contact time and quantity of adsorbent used in each process on the removal efficiency was investigated. 2.1. Materials

Water

Methanol

Glycerides (mono-, di-glyceride)

Metals (soap, catalyst)

Glycerol

Thailand is an agricultural country where large amounts of grain are produced. The production of grain typically results in the production of rice husks. The rice husk could be burned for power generation, and its combustion would create rich husk ash (RHA) [14]. In general, RHA can be produced by incineration in the range of 500– 700 °C, and RHA is rich in silica corresponding to approximately 20% of the ash weight. However, incineration above 800 °C generates crystalline forms [15–17]. It is interesting that RHA can be used in biodiesel purification, but its capacity is less than that of silica gel under the same conditions [18]. Based on these considerations, there have been no investigations on the removal of monoglyceride from crude biodiesel using aluminum oxide-coated silica (AO_RHA) and no information has been reported on the mechanisms of monoglyceride adsorption onto the AO_RHA in the literature. Therefore, the purpose of this study is (i) to evaluate the efficiency of aluminum oxide coated on extracted silica from rice husk ash (AO_RHA) for the removal of monoglyceride from biodiesel production, (ii) to study the preparation of aluminum oxide coated on extracted silica from rice husk ash, and (iii) to characterize AO_RHA to determine the relationship between the adsorption efficiency and the structure.

In a 250-mL flask, 10 g of off-white rice husk ash (Pijit Province, Thailand) and 80 mL of a 2.5 N NaOH solution were added. Then, the mixture was reflexed for 3 h with stirring. Next, the mixture was immediately filtered through a Whatman (No. 40) filter paper, and the residual was washed with 20 mL of boiling water. The filtrate was allowed to cool to room temperature. Then, the pH of the filtrate was adjusted to a pH of 2 using a 2.5 M H2 SO4 solution and adjusted back to a pH of 8.5 with 30% NH4 OH. The mixture was allowed to stand for 3.5 h. Then, the mixture was filtered through a Whatman (No. 40) filter paper, and the residual was dried at 120 °C for 12 h. 6.8 g of pure silica (RHA) was obtained. Finally, the silica was ground and sieved through 100–120 mesh size sieve (125–149 μm) followed by storage in a tightly capped glass bottle for future use [19,20]. In order to prepared the amorphous RHA, 10 g of pure silica (RHA) was immerged in 0.01 M HCl for 3 h in sonicator. The obtained RHA was then washed with DI water until the pH was 7. It was then dried and stored in a tightly capped glass bottle for further use. 2.2. Coating procedure The solution of 1 M AlCl3 was prepared by dissolving AlCl3 .6H2 O in Milli-Q water. Then, 100 mL of 1 M Al (III) solution was adjusted to pH 3, pH 7, pH 9, and pH 12 by adding 4.0 M NaOH to prepare the Al(OH)3 floc suspension, and the mixture was subsequently mixed for 15 min. In a 150 mL glass bottle, 20 g of 100–120 mesh size RHA (120–125 μm) was mixed with 50 mL of the floc suspended Al(OH)3 (pH 3, pH 7, pH 9, and pH 12), and the mixture was placed in an ultrasonicator for 24 h. The sample was washed with deionized water until the pH reached 7, and then, the sample was dried in an oven at 120 °C for 24 h or calcined at 700 °C. The obtained AO_RHA was ground, sieved, and stored in a tightly capped glass bottle for future use. The effect of varying the parameters of the AO_RHA preparation, such as calcination in a muffle furnace at 700 °C for 3 h compared to non-calcination, and stirring times (12 h, 24 h, and 48 h) related to the removal of monoglycerides were also investigated. 2.3. Characterization of adsorbents

2. Experimental procedure Aluminum oxide coating on RHA can be done by adding 4 molar NaOH to a 1 molar AlCl3 solution. In this process, it is more likely that the suspension of aluminum hydroxide with different pH values (i.e., pH 3, pH 7, pH 9, and pH 12) leads to the different formation of AO_RHA. Based on our experiments, the characteristic of AO_RHA and their efficiency for the removal of monoglyceride were investigated. The monoglyceride removal varied with the different forms of AO_RHA. To determine the optimum conditions for monoglyceride

2.3.1. Surface functional group characterization To determine the functional group on the surface of the adsorbent, FTIR spectra of the samples were collected in the range of 400– 4000 cm−1 using an FTIR spectrometer (Nicolet). 2.3.2. Crystallization characterization To analyze the crystallization of the sample, the sample was scanned from 20° to 80° (2 theta) with a step size of 0.020° using a diffractometer (Perkin Elmer) 5000 D and Cu K&1 radiation (a = 40 kV, 30 mA).

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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2.3.3. Surface area and pore diameter characterization To characterize the surface area and pore diameter of the adsorbents, the samples were pretreated and analyzed using a BET method surface area analyzer (using BELSORP analysis program). 2.3.4. Aluminum coated RHA analysis To determine the amount of aluminum, 2000, 1000, 500, 300, and 100 ppm working standard solutions were prepared as described below. The amount of aluminum doped RHA was determined by dissolving 0.2 g of each AO_RHA in 20 mL of 4 M nitric acid digested at 40 °C for 30 min followed by filtration through a 0.45 μm membrane filter. Then, 5 mL of the filtered product was pipetted into a 50-mL volumetric flask, which was filled to the mark Milli-Q water. The solution mixture was serially diluted until the desired concentration was obtained. All of the working standard solutions and samples were measured by inductively coupled argon plasma atomic emission spectrometer (ICP: Perkin Elmer), and a standard curve was constructed by plotting the intensities as function of concentration of the working standard solutions. 2.4. Biodiesel production via base-catalyzed transesterification 400 g of palm cooking oil was added to a 1000 mL round bottom flask equipped with a condenser. The solution containing sodium hydroxide (4.0 g) in 99.9% analytical-grade methanol (115.85 mL) was slowly added to the reaction, and then, the mixture was heated to 65 °C for 1 h. Then, the temperature was increase to 100 °C for 1 h. The reaction mixture was transferred to a separatory funnel to allow the glycerin to separate. Na ion as the catalyst was removed with glycerol during phase separation unit [21]. The upper layer, containing the desired product not yet purified, was removed to a beaker and heated at 90 °C for 10 min to evaporate the excess of methanol. The methyl ester layer was used in the purification process [20, 22]. 2.5. Monoglyceride purification process The purification processes were carried out in a batch experiment in a glass bottle at room temperature. After methanol and the bottom glycerol (i.e., glycerin and catalyst) were removed, the monoglycerides would be removed from the crude biodiesel. 20 g of crude biodiesel was treated with AO_RHA and stirring at 150 rpm in a water bath shaker at room temperature. The optimum conditions, such as contact times (5, 10, 15 and 20 min), various types of AO_RHA (pH 3, pH 7, pH 9, and pH 12), and amount of AO_RHA (3, 5, 10 wt. %), were examined. The adsorption efficiency of AO_RHA was also compared with commercial silica alumina and Magnesol. The adsorbent was removed with a membrane filter (0.45 μm), and the content of residual monoglycerides was analyzed using a GC method. All of the experiments were performed in duplicate. 2.6. Effect of methanol on monoglyceride removal To study the effect of methanol (99.9% analytical-grade) on monoglyceride removal, the molar ratio of biodiesel to methanol was prepared as 1:0, 1:1, 1:2 and 1:3. The purification processes were carried out by adding 0.5 g of AO_RHA to a 150 mL glass bottle that contained 10 g of biodiesel with a stirring rate of 150 rpm for 10 min at room temperature. Then, the adsorbent was removed using a membrane filter (0.45 μm), and the residual monoglyceride content was analyzed using a GC method. All of the experiments were performed in duplicate. 2.7. Adsorption isotherm of AO_RHA on monoglyceride adsorption In the 150 mL glass bottle containing 2, 4, 6, 8, and 10 wt. % of AO_RHA, 10 g of crude biodiesel (0.83% m/m of monoglyceride con-

3

tent) was added. Next, the mixture was immersed in a water bath shaker with an agitation rate of 150 rpm for 10 min. Then, the adsorbent was removed using a membrane filter (0.45 μm), and the residual monoglyceride content was analyzed using a GC method. All of the experiments were performed in duplicate. The Langmuir isotherm equation was calculated using the following equation (Eq. 1):

Ce

  x m

where

x m

=

1 + ab

1

Ce

a

(1)

is the mass of glycerin adsorbed per unit mass of adsorbent

adsorbate ( mg gadsorbent ); a and b are empirical constants; Ce is the equilibrium concentration of adsorbate in the biodiesel after adsorption ( mg L )

The essential characteristics of the Langmuir equation can be expressed in terms of a dimensionless separation factor (RL ) as defined in Eq. 2 [23]:

RL =

1 1 + bC0

(2)

where b is an empirical constant; C0 is the initial glycerin concentration. The Freundlich isotherm equation can be expressed using the following equation (Eq. 3):

log

x m

= log K f +

x m is the mgadsorbate ( gadsorbent ); Kf

where

1 n

log Ce

(3)

mass of glycerin adsorbed per unit mass of adsorbent and n are empirical constants; Ce is the equilibrium

concentration of glycerin in the solution after adsorption ( mg L ) The percentage of glycerin removed was calculated according to the following equation (Eq. 4):

Percentage of monoglyceride removed =

C − C  e 0 C0

× 100

(4)

where C0 and Ce ( mg L ) are the biodiesel-phase concentration of initial monoglyceride content and at equilibrium. 2.8. Regeneration process The regeneration process was carried out in methanol. The low polarity monoglyceride was desorbed from AO_RHA by immersing 10 g of spent AO_RHA with 100 mL of methanol in a 150 mL glass bottle, and then, the mixture was placed in an ultrasonicator for 30 min. Next, the AO_RHA was filtrated through a Whatman (number 40) filter paper. Finally, the AO_RHA was dried in an oven at 120 °C for 24 h. 2.9. Monoglyceride analytical method (gas chromatograph method) The monoglyceride content was determined using a CP-3800 Varian gas chromatograph equipped with an on-column injector and a flame ionization detector (FID). A Glycerides Ultimetal, 10 m x 0.32 mm (ID) x 0.1 μm (film thickness) column with retention gap 2 m x 0.53 mm (ID) was used. Hundred milligrams of the biodiesel sample was accurately weighed (± 0.1 mg) into a 10 mL vial. Then, 80 μL of internal standard 1, 100 μL of internal standard 2 and 100 μL of MSTFA were added to the sample vial, which was vigorously shaken and allowed to stand at room temperature for 15–20 min. Approximately 8 mL of heptane was added, and then, 1 μL of the mixture was injected into the gas chromatograph at an oven temperature of 50 °C. After being held for 1 min, the oven was heated at 15 °C/min to 180 °C, at 7 °C/min to 230 °C, and at 10 °C/min to 370 °C (held for 5 min). The flow rate of the helium carrier gas was 4 mL/min. The detector temperature was set to 380 °C, and the total run time was 36 min [24].

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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Fig. 2. FTIR spectra of AO_RHA prepared with different pH conditions and commercial silica alumina.

Fig. 3. Pore size distributions of AO_RHA prepared with different pH conditions, RHA, and commercial silica alumina determined using the BJH method.

3.1. Characteristics of AO_RHA

stretching vibrational brand of Si–O at 1030 cm−1 [26]. The spectra of AO_RHA that was prepared at a higher pH exhibited a slight blueshift (1030 cm−1 ).

pH is the most important factor that influences the Al(OH)3 formation in the coating process [25]. To confirm that aluminum oxide has been coated onto RHA, the infrared spectra of AO_RHA were recorded and are shown in Fig. 2. Similar to those measured from commercial adsorbent, the broad absorption at 800 cm−1 indicates the formation of Si–O–Al. The IR spectra of SiO2 can be observed at

3.1.1. Effect of pH on the pore size distribution The BJH method was used to determine the pore size distribution of AO_RHA, which was prepared with different pH conditions. The results are shown in Fig. 3. The majority of the pore size distributions of the commercial silica alumina are 5 dp/nm while the pore size distribution of RHA was 50 dp/nm. After modification, the distribution

3. Result and discussion

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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Fig. 4. XRD patterns of AO_RHA prepared using different pH conditions, RHA, and commercial silica alumina. Table 2 Value of monoglyceride in biodiesel after treatment with AO_RHA. Condition 2

Surface area (m /g) Pore diameter (nm) Al content (%) %Monoglyceride removal (5 wt. % of AO_RHA)

RHA

pH 3

pH 7

pH 9

pH 12

Commercial

Magnesol

47.70 28.10 – 43

203.20 14.99 2.40 91

113.35 15.13 14.90 79

112.74 14.67 17.45 83

159.85 6.92 16.09 79

510.00 5.60 13.03 69

300.00 3.00 – 47

represents the relative number of pores of different sizes that AO_RHA could provide (in the range of 5–20 dp/nm). This pore size distribution shifted significantly from uncoated RHA (majority in 20–80 nm) toward a smaller distribution similar to the commercial product (approximately 5 nm). In addition, the pore size distribution of AO_RHA produced with pH values of 3 and 12 tended to be smaller than those obtained at a pH of 7 and 9. Therefore, AO_RHA is a mesoporous material because its pore size is in the range of 2–50 nm [25–27]. 3.1.2. Effects of pH on the morphology of AO_RHA Variation in the pH during coating can change the morphology of AO_RHA compared to uncoated RHA and commercial silica alumina. The XRD patterns of uncoated RHA, AO_RHA prepared at various pH values (i.e., pH 3, pH 7, pH 9, and pH 12), and commercial silica alumina are shown in Fig. 4. The XRD patterns of RHA display a crystalline form based on the sharp peaks that were observed. However, the results from coated RHA (AO_RHA) indicate low crystallinity (AO_RHA_pH 9 and AO_RHA_pH 12) and amorphous (AO_RHA_pH 3) characteristics. The XRD patterns obtained from the material prepared at various pH values are different from that of aluminum oxide [27] as follows: pH 3: amorphous Al2 O3 and boehmite (γ -Al2 O3 ) pH 7: amorphous Al2 O3 pH 9 and pH 12: low crystalline boehmite (γ -Al2 O3 )

These results further confirm the Al2 O3 coating formed on the RHA surface with a crystal phase at high pH and a transition to more amorphous phases at lower pH values. 3.1.3. Surface area, pore diameters, and aluminum content The surface area and pore diameter of the AO_RHA and commercial silica alumina are summarized in Table 2. The results indicate that a large amount of aluminum oxide was coated on RHA at pH 7–12. The maximum percentage of aluminum oxide (17.45%), which was higher than that for the commercial product, was achieved at a pH of 9. Coating at all of the pH values decreased the pore sizes to the mesopore range and doubled or even quadrupled the surface area from 47.7 to 100–200 m2 /g, which is closer to 510 m2 /g for commercial silica alumina. A larger surface area was achieved under acidic conditions (pH i.e., 3). This result may be due to the arrangements of the aluminum oxide particles at low pH being fairy random, which results in a loosely packed amorphous structure (Fig. 4) with a large surface area. 3.2. Preliminary monoglyceride removal using AO_RHA prepared with different pH values compared to RHA and commercial silica alumina The monoglyceride removal efficiencies of RHA, AO_RHA prepared at different pH conditions, and commercial silica alumina for biodiesel purification were investigated, and the results are shown in Table 2. In this preliminary study, characteristics, such as surface

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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Table 3 Amount of Al coating on RHA as well as the surface area, pore diameter, and pore volume of the AO_RHA samples. Stirring time (h)

Al coating on RHA (mg/g)

Surface area (m2 /g)

Pore diameter (nm)

Pore volume [cm3 /g]

Uncoated 12 24 48

0 2.45 5.45 5.50

39.24 131.90 137.12 139.12

29.45 25.45 20.24 21.46

0.29 0.83 0.69 0.74

area, pore diameter, and aluminum content, were key factors for selecting the most effective AO_RHA prepared from different pH conditions. For a comparable quantity of adsorbent (5 wt. %), AO_RHA prepared under different pH conditions exhibited better performance for monoglyceride removal than that of RHA, Magnesol, and commercial silica alumina. The maximum monoglyceride removal was obtained using AO_RHA_pH 3. Due to the large difference in the BET surface area of AO_RHA (AO_RHA_pH 3: 203.20 m2 /g, AO_RHA_pH 7: 113.35 m2 /g, AO_RHA_pH 9: 112.74 m2 /g, and AO_RHA_pH 12: 159.89 m2 /g), the surface area is not considered an influential factor affecting monoglyceride removal. A more likely explanation for the high percentage removal by AO_RHA_pH 3 is related to its mesopore structure. It is interesting that the higher aluminum content of AO_RHA_pH 9 results in a slightly better monoglyceride removal performance compared to AO_RHA_pH 7. The results from the preliminary study indicate that AO_RHA prepared at a pH of 3, which exhibited the best removal performance, was selected as most suitable for further studies of the AO_RHA preparation process. The effect of the stirring time and calcination process need to be taken into account. 3.3. AO_RHA preparation process 3.3.1. Effect of stirring time on Al coating on RHA To select the most suitable stirring time required for the coating process, the amount of Al as well as the BET surface area, pore size, and pore volume of the sample were measured at 12, 24, and 48 h, and the results are listed in Table 3. The amount of Al coating on RHA increased from 0 to 2.45 mg of Al/g of RHA in the first 12 h. During the next 12 h, a steady increase in the amount of Al coating to 5.45 mg of Al/g of RHA was observed. Then, the amount of Al coating increased

Table 4 Surface area, pore size, and pore volume of AO_RHA.

Samples

Surface area (m2 /g)

Pore diameter (nm)

Pore volume (cm3 /g)

Uncalcined Calcined at 700 °C for 3 h

119.79 128.67

10.48 26.10

0.31 0.84

slowly to 5.50 mg of Al/g of RHA in the following 24 h period. Similar trends were observed in the surface area, pore size, and pore volume where the value changed dramatically during the first 24 h followed by a steady change during the rest of the time. Therefore, a stirring time of 24 h was selected for further experiments. 3.3.2. Effect of calcination and lack of calcination on the AO_RHA structure The BET surface area, pore size, and pore volume of AO_RHA before and after calcination are reported in Table 4. Calcination increased the surface area from 119.79 m2 /g to 128.67 m2 /g at 700 °C, which corresponds to an increase in the pore diameter from 10.48 to 26.10 nm. The BJH pore-size distributions of pre- and post-calcination AO_RHA are shown in Figs. 5a and 5b. The pore diameter increased after calcination [28], which is due to the water contained in AO_RHA structures being released resulting in an increase in the pore size and surface area of AO_RHA [29]. These results indicate that further removal experiments should be conducted with AO_RHA prepared by stirring RHA with AlCl3 at a pH of 3 with 24 h of stirring followed by calcination at 700 °C. 3.4. Monoglyceride removal using AO_RHA 3.4.1. Effect of contact time on monoglyceride removal using AO_RHA The effect of the contact time on monoglyceride removal was evaluated, and the results are shown in Fig. 6. The amount of AO_RHA was selected to be 5% wt. The average monoglyceride content decreased sharply from 1.04% m/m (0%) to 0.75% m/m (27.88%) in the first 5 min followed by a slow decline to 0.67% m/m (35.57%), 0.65% m/m, (37.5%), and 0.64% m/m (38.46%) after 10, 15, and 20 min, respectively. AO_RHA can remove a large portion of the monoglycerides from biodiesel, and a contact time of 10 min is the most practical because little removal occurs after this time.

Fig. 5. BJH pore-size distributions of uncalcined (a) and calcined AO_RHA at 700 °C (b).

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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Fig. 6. Effect of contact time on monoglyceride removal using AO_RHA.

Fig. 7. Relationship between AO_RHA quantity used and monoglyceride removal (room temperature, agitation rate 150 rpm, 10 min).

3.4.2. Effect of adsorbent quantity In this study, the production of biodiesel from palm cooking oil through base catalyzed transesterification was investigated. After separating the glycerol, the crude biodiesel consisted of mono-, di-, and triglycerides. Then, the crude biodiesel was purified using various quantities of AO_RHA. The effect of the adsorbent quantity on the monoglyceride removal is shown in Fig. 7. For a comparable quantity of AO_RHA (3 wt. %, 5 wt. %, and 10 wt. % with 100 mesh size (150 μm)), the removed monoglyceride content was 0.98% m/m to 0.34% m/m, 0.20% m/m, and 0.17% m/m, respectively. A large monoglyceride removal was obtained for 5 wt. % and slightly decreased for the larger quantities. Nevertheless, AO_RHA did not exhibit ability to adsorb di- and triglycerides. The high percentage monoglyceride removal by AO_RHA is related to the molecular weight of the monoglyceride (MW = 356.55) being less than that of the diglyceride (MW = 621) and triglyceride (MW = 640). A more likely explanation for the high percentage monoglyceride removal by AO_RHA may be due to monoglyceride containing two polar functional groups (i.e., OH). Therefore, monoglyceride can diffuse in the pores of the mesoporous structure of AO_RHA. In addition, the AO_RHA structure consists of Si and Al, which may facilitate monoglyceride adsorption. The properties of the purified biodiesel were determined to be within the EN 14214 specification (i.e., acid number, viscosity, flash point and free and total glycerin content) as can be seen in Table 5. 3.4.3. Comparison of monoglyceride adsorption by amorphous structure of AO_RHA and RHA After coating RHA with Al2 O3 , the morphology of RHA changed from crystalline to amorphous, which increased the adsorption capacity of the adsorbent for monoglyceride. To confirm that the

Table 5 Quality of purified biodiesel. Property

Method

Limitation of B100

Purified biodiesel

Free glycerin (% m/m) Total glycerin (% m/m) Acid number (mg KOH/g) Viscosity at 40 °C (mm2 /s) Methanol content (% m/m) Flash point (°C)

EN 14214 EN 14214 EN 14214 EN 14214 EN 14110 EN 14214

0.02 max 0.25 max 0.5 max 3.5–5.0 0.2 max 101 min

0 0.06 0.05 4.34 0.1 > 120

amorphous structure synergistically aided in monoglyceride removal along with Al2 O3 , amorphous RHA was studied for the removal of monoglyceride under the same conditions as the AO_RHA preparation. Amorphous AO_RHA (5 wt. %) exhibits a better performance than amorphous RHA with up to 91% monoglyceride removal (Fig. 8). 3.4.4. Effect of methanol for monoglyceride removal Because methanol exerts a negative influence on the removal of monoglycerides from crude biodiesel using AO_RHA, the effect of varying the ratio of biodiesel to methanol on the adsorption of monoglyceride is shown in Table 6. The viscosity of the mixed biodiesel was much lower than the original biodiesel. In addition, the percentage of monoglyceride removal in the unmixed biodiesel for a biodiesel: methanol ratio of 1:1, 1:2, and 1:3 was 52%, 39%, 26%, and 26%, respectively. The maximum monoglyceride removal using AO_RHA occurred when methanol was removed. Because monoglyceride contains 2 hydroxyl groups (–OH), monoglycerides tend to dissolve in methanol instead of adsorbing on the AO_RHA. Therefore, the adsorption of monoglyceride onto AO_RHA is reduced by methanol.

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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Fig. 8. Comparison of monoglyceride adsorption by amorphous structure of AO_RHA and RHA.

Table 6 Effect of methanol on monoglyceride removal. Ratio of biodiesel: methanol (mole: mole)

Viscosity at 40 °C (mm2 /s)

% Monoglyceride removal

1:0 1:1 1:2 1:3

4.34 3.46 3.02 2.81

52 39 26 26

The adsorption isotherms of monoglyceride on AO_RHA can be explained by the Freundlich and Langmuir adsorption isotherms [30]. Linear plots of 1/(x/m) as a function of 1/Ce were obtained to identify the Langmuir adsorption models. The Langmuir isotherm constants (a) and (b) were calculated from the intercept and slope of the linear plot. The values of a and b are 303 and 1.27, respectively, and these values are related to adsorption capacity and the rate of adsorption. The RL value provides an indication of the adsorption behavior (i.e., it can be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0)). The results indicate that the RL values for monoglyceride adsorbing onto AO_RHA are 0.44 which is less than 1 and greater than zero, indicating favorable adsorption under the conditions used in this study.

In addition, linear plots of log (x/m) as a function of log Ce were obtained to identify the Freundlich adsorption, and Kf and n are determined from the intercept and slope, respectively. The Kf value, which was 10–10 , indicates the extent of adsorption, and the n value (1.04) indicates the favorability of the adsorption process [31,32] noted that an n value between 1 and 10 represents favorable adsorption conditions. Therefore, the n values of the Freundlich adsorption isotherm obtained here indicate that adsorption is favorable. In addition, the correlation coefficients indicated that the adsorption of monoglyceride on AO_RHA under these conditions was fitted to Langmuir equations with R2 values greater than the R2 value of the Freundlich adsorption isotherm (Table 7). 3.5. Adsorption mechanism of monoglyceride onto AO_RHA Based on the XRD pattern, FTIR spectra, BET, and ICP analysis, when Al was incorporated into the framework of silanol, the material had the form shown in Fig. 9. The XRD pattern of AO_RHA displays a strong γ -Al2 O3 presence, which is an octahedral type. This result is confirmed by the Al–O–Si stretching frequency located at 800 cm−1 . The best monoglyceride removal was obtained using AO_RHA prepared at a pH 3, and this material has an amorphous structure with Al as the center due to the lower Al content compared to the other

Table 7 Langmuir and Freundlich isotherm constants. Adsorbate

Mono glyceride

Freundlich parameters

Langmuir parameters

Intercept log Kf

Kf (l/g)

Slope 1/n

a (mg/g)

b (g/mg)

R2

RL

R2

–2.38

4.18 × 10– 3

0.96

0.88

303

1.27

0.90

0.44

Fig. 9. Structure of AO_RHA.

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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9

Fig. 10. Mechanism of monoglyceride adsorption onto AO_RHA.

Fig. 11. Quality of the purified biodiesel over recycled adsorbent.

materials. In this particular case, the monoglyceride may be attached to the oxygen atoms surrounding the octahedral or orthoborate layers forming bidentate attachments. This adsorption could be considered a monolayer adsorption, which is consistent with the Langmuir equation, and the interaction between AO_RHA and monoglyceride is most likely a hydrogen bond, which is confirmed by the results shown in Fig. 10. 3.6. Regeneration process The regenerative process was performed with methanol. The high polarity of methanol can desorb the monoglyceride from AO_RHA.

The efficiency of the regenerated AO_RHA was compared to the original AO_RHA. However, the efficiency of the regenerated AO_RHA decreased. Therefore, the regenerated AO_RHA can only be reused approximately four times (Fig. 11). 4. Conclusion AO_RHA is an effective adsorbent for removal of less polar monoglycerides from crude biodiesel. The most effective monoglyceride removal was observed for amorphous AO_RHA, which was prepared using a 1 M AlCl3 solution at a pH of 3 with stirring for 24 h followed by calcination at 700 °C. The maximum

Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037

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monoglyceride removal occurred at a stirring rate of 150 rpm for 10 min. The attractive interaction between AO_RHA and the monoglyceride molecule is most likely a hydrogen bond. The adsorption isotherm for the monoglyceride onto AO_RHA resulted in a good fit to the Langmuir model and, AO_RHA could be regenerated and effectively reused four times. Acknowledgments The authors are grateful to a Petroleum Authority of Thailand research grant for supporting this project. We also wish to thank Assoc. Prof. Dr. Nongnuj Meangsin of the Department of Chemistry, Faculty of Science, Chulalongkorn University, for her editorial comments. References [1] Wang YD, Al-Shemmeri T, Eames P, McMullan J, Hewitt N, Huang Y, et al. An experimental investigation of the performance and gaseous exhaust emissions of a diesel engine using blends of a vegetable oil. Appl Therm Eng 2006;26(14– 15):1684–91. [2] Altın R, Çetinkaya S, Yücesu HS. The potential of using vegetable oil fuels as fuel for diesel engines. Energy Conv Manage 2001;42(5):529–38. [3] Masjuki HH, Kalam MA, Syazly M, Mahlia TMI, Rahman AH, Redzuan M, et al. Experimental evaluation of an unmodified diesel engine using biodiesel with fuel additive. In: The 1st International Forum on Strategic Technology; 2006. p. 18–20. [4] Balat M, Balat H. Progress in biodiesel processing. Appl Energy 2010;87(6):1815– 35. [5] Balat M, Balat H. A critical review of bio-diesel as a vehicular fuel. Energy Conv Manage 2008;49(10):2727–41. [6] Demirbas A. Biodiesel production via non-catalytic SCF method and biodiesel fuel characteristics. Energy Conv Manage 2006;47(15–16):2271–82. [7] Felizardo P, Neiva Correia MJ, Raposo I, Mendes JF, Berkemeier R, Bordado JM. Production of biodiesel from waste frying oils. Waste Manage 2006;26(5):487– 94. [8] Uzun BB, Kılıç M, Özbay N, Pütün AE, Pütün E. Biodiesel production from waste frying oils: optimization of reaction parameters and determination of fuel properties. Energy 2012;44(1):347–51. [9] Leung DYC, Guo Y. Transesterification of neat and used frying oil: optimization for biodiesel production. Fuel Process Technol 2006;87(10):883–90. [10] Berrios M, Skelton RL. Comparison of purification methods for biodiesel. Chem Eng J 2008;144(3):459–65. [11] Wang Y, Wang X, Liu Y, Ou S, Tan Y, Tang S. Refining of biodiesel by ceramic membrane separation. Fuel Process Technol 2009;90(3):422–7. [12] Predojevic´ ZJ. The production of biodiesel from waste frying oils: a comparison of different purification steps. Fuel 2008;87(17–18):3522–8.

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Please cite this article as: N. Saengprachum, S. Pengprecha, Preparation and characterization of aluminum oxide coated extracted silica from rice husk ash for monoglyceride removal in crude biodiesel production, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.037